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ISTANBUL TECHNICAL UNIVERSITY FACULTY OF AERONAUTICS AND ASTRONAUTICS
GRADUATION PROJECT
JUNE, 2018
NUMERICAL ANALYSIS OF BIRD STRIKE DAMAGE ON COMPOSITE
STRUCTURE USING ABAQUS/EXPLICIT
Thesis Advisor: Prof. Dr. Zahit MECİTOĞLU
Koray BAŞ
Department of Aeronautical Engineering
Anabilim Dalı : Herhangi Mühendislik, Bilim
Programı : Herhangi Program
ii
JUNE 2018
ISTANBUL TECHNICAL UNIVERSITY FACULTY OF AERONAUTICS AND ASTRONAUTICS
NUMERICAL ANALYSIS OF BIRD STRIKE DAMAGE ON COMPOSITE
STRUCTURE USING ABAQUS/EXPLICIT
GRADUATION PROJECT
Koray BAŞ
110120183
Department of Aeronautıcal Engineering
Anabilim Dalı : Herhangi Mühendislik, Bilim
Programı : Herhangi Program
Thesis Advisor: Prof. Dr. Zahit MECİTOĞLU
iii
Thesis Advisor : Prof. Dr. Zahit MECİTOĞLU ..............................
Istanbul Technical University
Jury Members : Prof. Dr. Vedat Ziya DOĞAN .............................
Istanbul Technical University
Asst. Prof. Demet BALKAN ..............................
Istanbul Technical University
Koray BAŞ,student of ITU Faculty of Aeronautics and Astronauticsstudent ID
110120183, successfully defended the graduation entitled “NUMERICAL
ANALYSIS OF BIRD STRIKE DAMAGE ON COMPOSITE STRUCTURE
USING ABAQUS/EXPLICIT”, which he prepared after fulfilling the requirements
specified in the associated legislations, before the jury whose signatures are below.
Date of Submission : 28 May 2018
Date of Defense : 13 June 2018
1
To my family,
2
FOREWORD
For the foreword, 1 line spacing must be set. The foreword, written as a first page of
the thesis must not exceed 2 pages.
The acknowledgements must be given in this section.
After the foreword text, name of the author (right-aligned), and the date (as month
and year) must be written (left-aligned). These two expressions must be in the same
line.The foreword is written with 1 line spacing.
June 2018
Koray BAŞ
Astronautical Engineer
3
4
TABLE OF CONTENTS
Page
TABLE OF CONTENTS ........................................................................................... 4 ABBREVIATIONS .................................................................................................... 5 SUMMARY ................................................................................................................. 8
ÖZET ........................................................................................................................... 9
1. INTRODUCTION ................................................................................................ 12 1.1 Purpose of Thesis ............................................................................................. 14
1.2 Literature Review ............................................................................................. 14 1.3 Hypothesis ........................................................................................................ 20
2. BIRD STRIKE PROBLEM................................................................................. 22 2.1 The Importance of Bird Strike in Aviation....................................................... 22
2.2 Statistical Analysis of Reported Accidents ...................................................... 22
3. SIMULATIN OF BIRD STRIKE ....................................................................... 29 3.1 Simulation Steps ............................................................................................... 29 3.2 Problem Description and Modeling of Bird Geometry .................................... 30 3.3 Aircraft Wing Modeling ................................................................................... 33
3.4 Aircraft Wing Meshing..................................................................................... 34
4. SOLUTION TECHNIQUES ............................................................................... 36 4.1 Lagrange Solution Technique........................................................................... 36 4.2 Eulerian Solution Technique ............................................................................ 38
4.3 ALE (Arbitrary Lagrangian Eulerian) Solution Technique ................. 39 4.4 SPH (Smoothed Particle Hydrodynamics) Solution Technique .......... 42
4.5 Comparison of the Solution Teqniques ................................................ 43
5. BIRD STRIKE ANALYSIS BY USING SPH METHOD ................................ 45 5.1 Metallic Plate Bird Strike Analysis .................................................................. 45
6. CONCLUSIONS AND RECOMMENDATIONS ............................................. 45 References ................................................................................................................. 46
5
ABBREVIATIONS
ALE : Arbitrary Lagrangian Eulerian
CPU : Computer Precessor Unit
EASA : Europian Aviation Safety Agency
FAA : Backpropagation
FAR : Federal Aviation Administration
FEM : Finite Element Method
ICAO : International Civil Aviation Organization
JAR : Joint Aviation Requirements NATO : North Atlantic Treaty Organization
SPH : Smooth Particle Hydrodynamics STANAG : Standardization Agreement
US : United States
6
LIST OF TABLES
Page
Table 3.1 : FAA Bird Strike Conditions. ................................................................... 32
Table 4.1 : Comparision between Lagrangian, Eulerian, and SPH Method ............. 45
7
LIST OF FIGURES
Page
Figure 1 1: Aircraft Components Exposed to the Risk of Bird Strike ..................... 14
Figure 1 2: Multi-material Bird Model ................ 17Error! Bookmark not defined.
Figure 1 3: Numerical and Experimental Shape after the Impact … ........................ 19 Figure 1 4: Bird strike simulation on composite plate with the (a) Lagrangian and (b)
Eulerian impactor models .................................................................................. 20
Figure 2-1: Number of Reported Bird Strikes by Year ............................................ 24
Figure 2-2: Time of Occurence ................................................................................. 25
Figure 2-3: Phase of Flight ...................................................................................... 25 Figure 2-4: Aircraft Components Damaged by Birds ............................................... 26
Figure 2-5: Effect on Flight ..................................................................................... 26
Figure 2-6: Action Taken After Bird Strike on Departure ........................................27
Figure 2-7: Damage Categories ............................................................................... 28 Figure 2-8: Number of Reported Bird Strikes to Commercial Aircraft by Height
Above Ground Level ............................................................................. 28
Figure 2-9: Number of Reported Bird Strikes to General Aviation Aircraft by Height
Above Ground Level ......................................................................................... 29
Figure 3-1: Flowchart for Bird Strike Analysis Procedure ....................................... 30
Figure 3-2: Bird Strike Experiment Set-up .............................................................. 31 Figure 3-3: Different Substitude Bird Impactor Geometries .................................... 31
Figure 3-4: Bird dimensions that is used in the analysis ..........................................33 Figure 3-5: F-16 Wing model that is used in this thesis ........................................... 34
Figure 3-6: Aircraft Wing Mesh ............................................................................... 35 Figure 3-7: Aircraft Leading Edge Mesh .................................................................. 36
Figure 4-2: Bird strike simulation on rigid plate with Lagrangian impactor model . 37
Figure 4-3: Eulerian Modeling Method for Soft Body Projectile… ......................... 39
Figure 4-4: Bird strike simulation on rigid plate with Eulerian impactor model ...... 40
Figure 4-5: ALE Modeling Method for Soft Body Projectile … .............................. 41 Figure 4-6: Bird strike simulation on rigid plate with ALE impactor model ........... 42
Figure 4-7: Comparison of the approaches on mesh movement … .......................... 42
Figure 4-8: SPH Modeling Method for Soft Body Projectile ...................................43
Figure 4-9: Bird strike simulation on rigid plate with SPH impactor model … ....... 44
8
NUMERICAL ANALYSIS OF BIRD STRIKE DAMAGE ON COMPOSITE
STRUCTURE USING ABAQUS/EXPLICIT
SUMMARY
As a result of aerial vehicles collisions with birds in the air, there are many accidents
that cause high cost and sometimes losing of life. In the aviation sector, bird impact
is considered an important problem that causes material damage and threatens flight
safety. Birdstrikes on aircraft is a major threat to human life and there is a need for
devolop structures which have high resistance towards these structures. It is
imperative that today's designed and manufactured aviation structures comply with
safe flight and landing requirements. In order to satisfy these requirements, the
behavior of structural parts against bird impact is investigated by using the finite
element method and / or tests. Through the obtained results, it is aimed to improve
the design process and to produce more durable and safe structures. But the high cost
of testing and of the "trial-and-error method" to increase the number of tests,
manufacturers are forced both time and financially.
According to the Federal Aviation Administration(FAA)'s Federal Aviation
Regulation(FAR), European Aviation Safety Agency(EASA)'s Joint Aviation
Requirements(JAR) and North Atlantic Treaty Organization(NATO)'s
Standardization Agreement(STANAG) on damage tolerance and fatigue evaluation
of structure, an airplane must be capable of successfully completing the flight during
which likely structural damage might occur as a result of impact with a bird which is
according to regulation, at cruise velocity at sea level or 0.85 cruise velocity at 8000
feet.
The focus of the current study is on the numerical modeling and simulation of high
velocity impact loads from soft body projectiles on composite structures with
ABAQUS/explicit. Solution techniques such as Lagrangian, Eulerian, ALE(Arbitrary
Lagrangian Eulerian), and SPH(Smooth Particle Hydrodynamics) are studied then it
is decided which solution technique is more suitable for such a bird strike on a wing
leading edge. As a result, SPH(Smooth Particle Hydrodynamics) method is chosen to
to use in this bird strike analysis because of good representation of splashing
behaviour and lower computational cost than the others which also have good
representation of splashing behaviour. At first step of the analysis, the impact on flat
plate is studied in experiment and simulation, which allows for the validation of the
modeling methods. As a second step, the bird impact on a composite wing leading
edge is treated.
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NUMERICAL ANALYSIS OF BIRD STRIKE DAMAGE ON COMPOSITE
STRUCTURE USING ABAQUS/EXPLICIT
ÖZET
Hava araçlarının havadaki kuşlarla çarpışmaları sonucunda, yüksek maliyete ve
bazen de yaşam kaybına neden olan çok sayıda kaza vardır. Havacılık sektöründe,
kuş etkisinin maddi hasara neden olan ve uçuş güvenliğini tehdit eden önemli bir
sorun olduğu düşünülmektedir. Uçaklara kuş çarpması, insan hayatı için büyük bir
tehdittir ve bunlara karşı yüksek direnç gösteren yapılara ihtiyaç vardır. Günümüzün
tasarlanan ve üretilen hava yapılarının güvenli uçuş ve iniş şartlarına uygun olması
zorunludur. Bu gereklilikleri karşılamak için, sonlu elemanlar metodu ve / veya
testleri kullanılarak yapısal etkilerin kuş etkisine karşı davranışı incelenir. Elde
edilen sonuçlarla, tasarım sürecinin iyileştirilmesi ve daha sağlam ve güvenli
yapıların üretilmesi amaçlanmaktadır. Ancak test maliyetlerinin yüksek olması ve
“deneme-yanılma yönteminin” test sayısını arttırması, üreticileri hem zaman hem de
maddi olarak zorlamaktadır.
Federal Havacılık İdaresi (FAA) Federal Havacılık Tüzüğü (FAR), Avrupa Havacılık
Güvenliği Ajansı (EASA) 'nın Ortak Havacılık Şartları (JAR) ve Kuzey Atlantik
Antlaşması Örgütü (NATO)' nun hasar toleransı ve yapının yorulma değerlendirmesi
üzerine olan Standardizasyon Anlaşması (STANAG)’na göre bir uçağın, deniz
seviyesinde seyir hızında veya 8000 feet'de 0.85 seyir hızında, regülasyona göre olan
bir kuşla çarpma sonucunda meydana gelebilecek olası yapısal hasarların meydana
gelebileceği uçuşu başarıyla tamamlayabilmelidir.
Federal Havacılık İdaresi (FAA), meydana gelen kazalar sonucunda 1990'lı yılların
başından itibaren kuş çarpması sebebiyle meydana gelen kazaları detaylı olarak takip
ederek, raporlamaya başlamıştır. Bu raporlarda; kazalara karışan kuşların
büyüklükleri, kazaların yerden ne kadar yükseklikte olduğu, günün hangi zaman
diliminde oldukları, uçuşun hangi aşamasında gerçekleşdikleri gibi çeşitli
istatistiklere ulaşılabilmektedir. Federal Havacılık İdaresi (FAA)’nin bu kuş çarpması
veritabanındaki bilgilere ulaşılarak, bu istatistiklerin grafiksel olarak yorumları
yapılmıştır.
Raporlar incelendiğinde uçağın ana elemanlarından jet motorları ve kanatların en çok
darbeye maruz kaldığı yapılar olduğu görülmektedir. Kuş çarpması kazalarının gün
geçtikçe arttığı da raporlar incelendiğinde görülmektedir. Bunun sonucu olarak
uçağın bu kazaya en çok maruz kalan parçalarının tasarım ve üretim kısmında kuş
çarpmasına dayanıklı olacak şekilde yapılması gerekmektedir. Uçak tasarımlarının,
10
dünya genelinde uygulanan belirli yönetmelikler çerçevesinde, belirli standartlara
sahip olması ve gerekli onaylarının alınması gerekmektedir. Bu onayların alınması
için yapılan testlerde, kuş cinslerinin çeşitliliği ve uçak üzerinde çarpmaya maruz
kalabilecek elemanların çok olması test maliyetlerinin yüksek olmasına sebebiyet
vermektedir.
Test maliyetlerinin yüksek olması, insanları bu testleri bilgisayar üzerinde yaparak
maliyet yükünden kurtulmaya iterek aynı zamanda bu testlerde kullanılan
hayvanların varlılarını sürdürmesini sağlamıştır. Kuş elemanının seçimi ve simüle
edilmesi üzerine dünya çapında araştırmalar yapılmış olup, bu tez kapsamında ilgili
makalelerden yararlanılmıştır.
Bu tezin konusu uçak kanadının hücum kenarı üzerine kuş çarpmasının sonlu
elemanlar yöntemini kullanarak çözümü üzerinedir. Bu problemin ilk adımı en uygun
analiz yöntemini belirlemek olmalıdır. Öyleyse, sonlu elemanlar yönteminin
incelenmesinin gerekli olduğu, önceki çalışmaların incelenerek problemin en uygun
ve en gerçekçi şekilde simüle edilebilmesidir. Geçmiş çalışmalar ve ilgili makaleler
incelendiğinde, kuş çarpması olayını incelemek için dört yöntem kullanılmaktadır.
Bunlar, Lagrangian yöntemi, Euler yöntemi, ALE ve SPH (Pürüzsüz Parçacık
Hidrodinamiği) yöntemidir.
Lagrangian yöntemi genellikle rijit cisim analizi üzerinde kullanılırken Euler
yöntemi ise akışkan analizlerinde kullanılmaktadır. Akışkan-katı etkileşimi
probleminin analizi için ise ALE yöntemi kullanılır. Kuş yapısı çarpma esnasında su
gibi davrandığı ve parçacıkların vücut ile hareket ettiği için SPH (Pürüzsüz Parçacık
Hidrodinamiği) metodu bu tarz problemler için daha uygun olmaktadır.
Bu çalışmanın odak noktası, ABAQUS / Explicit ile kompozit yapılarda yüksek hızlı
kuş çarpma yüklerinin sayısal modellenmesi ve simülasyonu üzerinedir. Lagrangian,
Eulerian, ALE (Arbitrary Lagrangian Eulerian) ve SPH (Pürüzsüz Parçacık
Hidrodinamiği) gibi çözüm teknikleri üzerinde çalışılmış, daha sonra kanadın hücum
kenarında böyle bir kuş çarpması için hangi çözüm tekniğinin daha uygun olduğuna
karar verilmiştir. Sonuç olarak, bu kuş çarpması analizinde SPH (Pürüzsüz Parçacık
Hidrodinamiği) yöntemi, sıçrayan davranışların iyi bir şekilde temsil edilmesi ve
daha az hesaplama maliyetinin yanı sıra sıçrayan davranışların iyi bir şekilde temsil
edildiği diğerlerine göre daha düşük bir hesaplama maliyetinin olması sebebiyle
seçilmiştir.
Nümerik kuş çarpması analizlerinde tipik kuş şekilleri, dairesel silindir, yarı küresel
silindir, küre ve elipsoid gibi bir dizi ilkel geometriyi içerir. Kuş malzemesinin
modellenmesi ile ilgili bazı basitleştirilmiş varsayımlar da vardır. Bununla birlikte,
konuyla ilgili literatür, çarpma yükü analizlerinin doğruluğu üzerine mermi şekli
veya kuş modeli materyalinin etkisini sistematik olarak araştırmak için
karşılaştırmalı bir çalışma içermemektedir.
11
Analizin ilk aşamasında, kuş çarpması analizi öncelikle düz bir plaka üzerinde test
edilmiştir. Literatür çalışması kapsamında olan önceki çalışmalardaki test ve numerik
sonuçlarla karşılaştırılıp modellenmiş olan kuşun gerçekci sonuçlar verip vermediği
gözlenmiştir. Uygun kuş goemetrisi belirlendikten sonra analiz yapılacak olan uçak
kanadı modellenmiştir. Uçak F-16 seçilmiş olup, uçak askeri bir uçak olduğundan
dolayı net çizimler ve bilgilere ulaşılamadığından elde olan bilgilere dayanarak
gerçeğe yakın bir şekilde modellenmiştir. Kanat modellenmesinde CATIA V5R21
programı kullanılmıştır. Kanat modellendikten ve uygun kuş geometrisi yapılan
çalışmalar sonucunda elde edildikten sonra, modellenen kanadın hücum kenarı
üzerinde kuş çarpması analizi gerçekleştirilmiştir.
12
1. INTRODUCTION
Foreign material impact is a serious problem for aircraft structures in general. Bird
strike is one of those, which causes significant loss of money and human life.
Because bird strike causes about 90% percent of the aircraft accidents, people have
been trying to protect and strengthen aircrafts from bird strike since early 1970s.
Pilots may face from two to five birdstrikes among their carrier since in-service
knowledge indicates that birdstrike occasions are common incident in aviation. The
first known bird strike accident was recorded in 1908 in the Dayton / Ohio / North
Carolina region of the United States as the murder of a bird by Orville Wright who is
one of the co-founders of the first plane which have a steam engine. It can be given
as an example of the accident that there was no loss of life by forced landing in
Hudson River in New York city of United States of America in January 2009, which
was registered as one of the biggest known accidents. The first bird strike to a jet-
powered aircraft was in Germany in 27 August 1939. The first test flight of a jet-
powered aircraft was on 24 August 1939. Three days later, during the second flight, a
loss of thrust was experienced after a bird strike. Birdstrike events postures
significant threats to civilian and military aircrafts as they lead to fatal to basic
aircraft components. Fuselage, engines, wings, windshield, nose/radom are most
common aircraft components stricken by birds according to reports. Figure 1-1
shows those components of an aircraft that have a risk of a birdstrike.
13
Figure 1-1: Aircraft Components Exposed to the Risk of Bird Strike
An aircraft has to satisfy “continued safe flight and landing” requirements . These
requirements are verified by institutaions such as Federal Aviation Administration
(FAA), Europian Aviation Safety Agency (EASA), International Civil Aviation
Organization (ICAO). An aircraft gets validation for airworthiness by performing
the required tests by these intitutations. These tests are mainly based on the bird
strike tests usually performed during the design process. Practical tests needs huge
amount of money and time. In the past there were no other opportunity than
performing practical bird strike tests, but nowadays in order to save time and money,
computer-based simulaitons are used. In case of failed tests, producer has to change
or re-manufacture the first design, so computer and software technologies which can
design and model the birdstrike cases are developed to avoid these loss of time and
money. After testing the first desing, using computer softwares and validating it,
final desing can be produced and tested manually. Reduced design time and safer
designs can be accomplished this way.
14
1.1 Purpose of Thesis
The main purpose of this thesis is to analyse a bird strike on wing structure of a
military type of aircraft using Abaqus/Explicit. Explicit finite element method is used
to analyze this type of short duration highly nonlinear impact problem. Within the scope
of this thesis study, detailed information about the bird crash and the related
regulations will be reached and an analysis and evaluation of the bird crash events
will be presented.
The advantages and disadvantages of available solution techniques such as
Lagrangian, Eulerian, ALE (Arbitrary Lagrangian Eulerian) and SPH (Smooth Particle
Hydrodynamics) methods used in bird strike analyzes will be examined and
compared.
In addition, soft body impactor which is a bird in this study, is going to be modeled
according to literature review. Cylindrical with hemi-spherical ends bird geometry will
be modeled by using SPH (Smooth Particle Hydrodynamics) solution technique.
The SPH method will be modeled again from a previous work on bird strike and the
bird strike phenomenon will be examined on a wing leading edge which will be
taken as an example after the validity of the parameters is proved. In this study, bird
strike analysis on a wing leading edge will be carried out, the effect of the change of
solution net density and the material thickness will be examined. Strain-strain
amounts on the sash will be compared with the material properties and a comment
will be tried to be made.
1.2 Literature Review
In this section, previous researches about bird strike are carried out and necessary
information related to the subject of this research are given. Researches about bird
strikes has been started since 1970s and it is still a current subject to work on.
Experimental equations are traditionally used to analyze bird strike problems to
determine the thickness of structural components required to resist bird strikes.
However, the airworthiness requirements have changed slightly and experimental
equations have not adequately met today’s highly optimized complex aircraft
15
structures. Many researchers have focused on bird strike research using computers
and related softwares in the last few decades.
In previous studies on bird impact, studies have been made on thickness, material,
speed and bird shape to characterize the resilience of aircraft structural parts against
bird impact. Studies that began in the 1970s were generally recorded as experimental
works because of the inadequate numerical calculation conditions in these years. The
striking of a rigid structural element with a hydrodynamically accepted bird is first
discussed by Barber (1975). On a circular plate, the birds of different sizes are
bombarded in the experimental environment, revealing the time-dependent variation
of the pressure change on the plate. Small and large birds were studied different
conditions of experimental tests where the birds impact spesific points on the rigid
plate. Different bird velocities when the impact happens have been observed. He has
studied in detail the formation of shock waves, the normalization of the pressure and
the decrease of the pressure. [1]
In 1976 Barber stated that as aircraft speed increase, consequences of bird/aircraft
strikehas also increased. They have been tried to change the flight paths of aircraft to
reduce the probability of the impact. As a result of this work it is reduced that the
probability of the crush but not totally eliminated. After the experimental and
analytical investigations leaded in The Bird Impact Loading Program, he figured out
that birds behave as fluid during impact. Until this time it is the most significant
conclusion of the investigation. It is stated that modeling bird as rougly right circular
cylindirical fluid geometry successfully predicts impact and steady flow pressures,
and pressure durations. He also work on bird orientation at impact as striking birds
with different angle of attacks. [2]
In 1979 Barber showed that during the initial impact a shock propagates into the
projectile and he examined that the pressure in the shock compressed region is
initially very high and is uniform across the impact area. He showed that there are
four phases of fluid behaviour during a bird impact; the shock phase of initaial
impact, shock pressure decay, steady flow, and termination. Steady flow pressures
16
are indepedendent of bird mass but depend in a predictable way on impact velocity,
impact angle, and bird material properties. [3]
Because of the difficulty of creating and reproducing tests made with a real bird,
many researchers have experimented with bird models produced from different
materials. For the first time Allcock and Collin (1969) have showed that materials
like wax, foam, and emulsion can be used instead of using real birds. They also
investigated bird strikes on blades of the gas turbines. They compared the damages
on different materials such as steel and aluminium alloy, also worked on blade angle
effects. [4]
Since the 1980s, progresses in computer technology have led to reduction of
computer prices and the development of finite element softwares. At this point,
researchers have had the opportunity to analyse impact tests on computer based
softwares and compare them with experimantal tests.
In the past, many studies have been conducted to develop founder models for birds to
improve numerical simulation results. Some authors have attempted to model a bird
with a defined failure strain and a simple elastoplastic material, and some have
pointed to the limitations of this simplified approach. [5] [6] Zhang and Li proposed
a method for determining the material constants of a rate-sensitive, tensile stiffening
model based on the nonlinear least squares and penalty function method. [7] An
optimization/search method to identify bird material constants for a given constructor
model was developed by Wang and his friends. [8] Bai discussed different elastic-
plastic formation models and compared numerical simulations with experiments. [9]
Their numerical simulation results using the calibrated model parameters give similar
results to experimantal ones.
McCallum and Constantinou of the BAE systems worked on the effect of bird shape
during bird strike. Explicit Finite element solver (LSTC Ls-Dyna) was used. First,
Arbitrary Lagrange Eulerian (ALE) and Smooth Particle Hydrodynamics (SPH)
techniques are used to analyze the traditional bird shape that is effective on a square
flat panel then the results were compared. After selection of appropriate solution
17
techniques, the multi-material bird model was modeled. The multi-material bird
model is given in Figure__. Finally, modeling the multi-material bird model may
have an important consequence for damage initiation and failure of the target. In this
study, the difference in stagnation pressures, von-Mises stress and displacement of
the panel are shown. [10]
Figure 1-2: Multi-material Bird Model
Airoldi and Cacchione (2006) evaluated the numerical performances of bird models
with different material characterisations and shapes using Lagrangian approach. The
approach has been applied to analyse bird impacts in idealised conditions considering
the normal impact on a rigid target. Lagrangian approach has been found suitable to
perform a large number of analyses focusing on the impact loading parameters
obtained by bird models of different shapes and accepting different material
characterisations. [11]
The main goal of Guida’s study was to develop a new leading edge structure that is
made of innovative materials and/or technologies satisfies the classical leading edge
configuration such as performance and weight. Using finite element analysis and the
experimental tests was the way Guida worked on. During his entire research, three
different finite element approaches have been performed: Lagrangian, ALE, SPH.
The lagrangian approach may be preferred by him because of its better
approximation to the experimental results. On the other hand, the behaviour of the
18
bird deformation appears to be more realistic in SPH approach than the other
approaches. [12]
In order to reduce the experimental costs, Guida (2009) studied on designing a
leading edge by using the finite element methods. In this study Smooth Particle
Hydrodynamics (SPH) was used for modeling the bird whereas a classical finite
element approach acepted. Greatly similar results between the experimental test and
the SPH bird numerical model were obtained. Figure 3 shows the comparision of the
test and simulation. [13]
Figure 1-3: Numerical and Experimental Shape after the Impact
Guida and Marulo (2008) studied on sandwich structure for a wing leading edge.
They used Lagrangian and ALE methods to model the bird structure in this research.
In Guida’s other research (2010) he compared Lagrangian and Smooth Particle
Hydrodynamics (SPH) on a wing leading edge which is made of sandwich
honeycomb material. In another reaserch of Guida’s (2011), the results of a
numerical campaign aimed at designing and analysing a novel ribless tailplane of a
C27J aircraft is approved by an experimental birdstrike test. Advanced numerical
simulation techniques can significantly help to design safer and more efficient
aircraft structures capable of withstanding a birdstrike is shown by these results. [14]
[15] [16]
19
Heimbs (2011) examined the effects of collisions at high speeds on boom composite
plots, along with the phenomenon of bird hitting. Lagrangian and Eulerian impactor
models have been compared in this study. [17] This comparision is given in Figure 4.
Figure 1-4: Bird strike simulation on composite plate with the (a) Lagrangian and (b) Eulerian
impactor models
Ubels (2003) investigated several composite leading edge designs for a bird strike
and these designs are based on a novel application of composite materials with high
energy-absorbing characteristics: the tensor-skin concept. [18] Furtermore Reglero
(2010) examined bird impact events on aluminum foam composite composite wing
structures. [19]
Heimbs (2011) observed how composite leading edge reacts to a bird strike using
Abaqus/Ecplicit finite element software. Bird strike on a rigid plate analyses were
performed firstly in order to validate the bird model then this validated bird model
was used to analyse a bird strike on a composite wing leading edge. It is appeared
that final simulation results correlate with the experimental data in this study. [20]
McCarthy et al have been developed a material model for a glass-based fibre metal
liminate suitable for use in explicit finite element simulations of bird strike in their
first part of a two part paper. [21] In Part 2 of this paper, the developed material
model is used to simulate composite wing leading edge. Experimental tests and
numerical simulation results are compared in the second part of this paper. This
comparision showed that the SPH method verified to be very useful for modeling
bird strike. [22]
20
Kim et al (2011) investigated soft impact damage assessment of composite fan stage
assemblies. They used ALE and SPH techniques to model soft impact damage. They
stated that, ‘in this study, a modeling approach for capturing damage due to bird
strike on composite fan blades and a fan assambly was developed. [23]
Hanssen (2004 ) studied on numerical model for bird strike of aluminium foam-
based sandwich panels using LS-DYNA finite element sofware. The bird was
modeled by using Lagrangian Eulerian (ALE) approach but Lagrangian approach
was used for the sandwich panel. The numerical model of the affected sandwich
panel was verified by empirical tests. [24]
Salem (2011) investigated experiments about bird strike on a flat plate using
different solution techniques for bird modeling, Lagrangian, ALE and SPH. [25]
Nishikawa (2010) studied high-velocity collisions with soft-body interactions. [26]
Sebastian Heimbs (2017) investigated bird strike on a helicopter searchlight which is
made of aluminium alloy using validated SPH bird model on his previous research.
[27] Another bird strike related study was guided by Goyal et al (2013). This paper
mainly focused on using SPH method and compare the results against Lagrangian
method. In conclusion, they showed that SPH method is suitable for bird strike
events within 10% error. [28]
1.3 Hypothesis
This thesis is designed to investigate the phenomenon of bird strike and to try to
reduce the time and cost problems faced by the aviation sector in the validation
process. Considering this purpose, it is planned to give a preliminary information
within the scope of Chapter 2 about what is the bird impact phenomenon first and
then statistical information is given and commented on these statistical data.
In Chapter 3, it is described that bird strike simulation steps and bird geometry. Then
it is showed that how the wing and the bird was modeled.
21
In Chapter 4, available solution techniques were described as Lagrangian, Eulerian,
ALE (Arbitrary Lagrangian Eulerian), and SPH (Smooth Particle Hydrodynamics).
Advantages and disadvantages about these solution techniques were discussed and
the suitable solution technique was chosen.
In Chapter 5, bird strike analysis by using SPH method was carried out. Firstly, Bird
strike on a flat plate was performed and the bird geometry was validated. After the
bird geometry validated according to previous studies and experiments, bird strike
analysis on a wing leading edge is investigated.
22
2. BIRD STRIKE PROBLEM
2.1 The Importance of Bird Strike in Aviation
As a result of the collision of an aircraft with foreign objects, material and life-lost
accidents occur. This phenomenon, which is called as a foreign material impact in
the field of aviation, is examined in two different sub-sections. One of these
subsections, solid body impact; a piece of metal that has fallen from another aircraft
or a stone which is on the runway is often struck by the force of the wind on the
airstrip and hit by the aircraft. Another sub-title, which is the subject of examination
of this thesis at the same time, is an accident that occurs when one or more birds
collide with aircrafts that may lead to loss of human and bird life. The main
difference between these two collision states is that the solid body reacts rigidly
during the collision, while the bird usually behaves in the direction of fragmentation
during bird collision. These two behaviors have different effects on the structural
elements of the aerial vehicle. Bird strike is one of the most important problems
encountered in civilian and military aviation areas. Guida (2008) stated that, it is
estimated that around 36,000 birds have been hit every year worldwide. [12]
2.2 Statistical Analysis of Reported Accidents
With the awareness of the importance of the accidents caused by bird strikes,
reporting and researches have accelerated in this regard. The FAA, EASA and ICAO
are involved in the reporting of bird crash accidents. From these institutions, the
FAA reports all bird crashes from 1990 onwards to the day-to-day, and also
publishes them on the website at the same time. This database of accidents includes
the type of aircraft, the area of use, the size of the accident, the economic effects, the
part of the aircraft in which the accident occurred, the type of bird, the time zone at
which the accident occurred, and many detailed information.
As you can see in the charts below, many detailed information can be accessed from
these accident records published by the FAA. The values obtained from the database
are interpreted on the graphs.
23
Figure 2-1: Number of Reported Bird Strikes by Year
Looking at the number of bird strikes by years, the increase in the number of
accidents is clearly visible, as seen in Figure 2.1. Considering the accidents
happening only in the USA, the number of accidents recorded in 2000 was 5730, but
in 2012 this value increased to 12983. From Figure 2.1 it can be seen that bird strike
accidents are increasing day by day and the aviation industry is becoming more
influential both economically and in terms of loss of human life. In addition,
although the graph in Figure 2.1 is based solely on the US, it can be interpreted that
the world-wide distribution is also the same.
24
Figure 2-2: Time of Occurence
When examining the accidents in the FAA database, the time zone at which the
accidents took place can also be accessed in detail. As can be seen from the graphic
in Figure 2-2, 64182 of them were in daytime, 30654 of them in the night, 4365 of
them in dusk while the remaining 3414 of them were in dawn. Percentages of them
are also shown in the Figure 2-2.
Figure 2-3: Phase of Flight
25
As it can be seen from Figure 2-3, most of the accidents take place during the lower
steps of landing and departure of the aircraft. Approach, landing roll, departure and
take-off run constitute approximately 93% of these accidents. So the bird strike
problem occurs mostly in lower attitudes.
Figure 2-4: Aircraft Components Damaged by Birds
Distribution of the aircraft components damadeg by birds shown in Figure 2-4. Wing
takes %14 of the accidents of all. It shows that wing is one of the major component
of the aircraft which has the possibility of bird strike.
Figure 2-5: Effect on Flight
26
Damage resulting from the bird impact is affecting the flight plans of the aircraft in
various forms. As can be seen from the graphic in Figure 2.5, 87668 of the aircraft
that were reported as the result of bird strikes continued to their flights without any
problem. Here, aircraft parts used in the aviation industry can be conceived as the
result of this accident-resistant design. However, although they are designed against
the condition of bird strike, flights can be affected by cancellation of departure,
precautious landing, motor stop etc. due to bird strike.
Figure 2-6: Action Taken After Bird Strike on Departure
Figure 2-6 shows that the action taken after bird stike occurs on departure. It can
cause fuel jettision, fuel burn or overweight landing. It can be seen that %50 percent
of the strikes end up with fuel jettision.
27
Figure 2-7: Damage Categories
It is obvious that bird strike may cause different kind and importance of damages.
Figure 2-7 shows the consequences after bird strike. These consequences can be list
as minor damage, uncertain damage, substantial damage or it can be destroyed by
bird strike. But percentage of the destruction is so small that it can never be called.
Figure 2-8: Number of Reported Bird Strikes to Commercial Aircraft by Height Above Ground
Level
28
When the graph in Figure 2.8 is examined, it can be seen that the 57711 accidents,
which is over 70% of the accidents that took place, appeared in a range up to an
altitude of 500 feet (152.4 m). In other words, it is known that this interval is an
altitude that can be used for aircraft generally during landing and departure. As the
altitude of the plane increases from the ground, it is understood that there is a serious
decrease in the number of accidents. This decrease in the number of accidents is
correlated with the birds' natural habitat. In other words, these data are used as a very
important resource in the prevention of bird attacks.
Figure 2-9: Number of Reported Bird Strikes to General Aviation Aircraft by Height Above
Ground Level
As it can be seen in the Figure 2-9, it has a similar tendency as commercial aircrafts.
As the altitude increases, the number of accidents decreases. Most of the accidents
occur under 500 feet (152.4 m).
29
3. SIMULATIN OF BIRD STRIKE
3.1 Simulation Steps
In this section, the steps for the analysis of a bird strike that have to be followed is going to be
described. The analysis of the problem of bird strike can be divided into four main parts. First,
the problem should be defined and related aviation standards should be defined. In this context,
the impact location, bird shape and weight, impact speed should be clearly assessed. Secondly,
the appropriate solution should be chosen. In addition, required material model for the soft
impactor must be selected. Thirdly, material models should be determined according to metallic
and non-metallic aircraft structures. Finally, the bird simulation is going to be performed.[34]
Figure 3-1 shows that the flowchart for suggested procedure of bird strike analysis.
Figure 3-1: Flowchart for Bird Strike Analysis Procedure
30
3.2 Problem Description and Modeling of Bird Geometry
Dead bird or chicken corpses are used in the bird strike certification tests. However, in the
experimental tests, various materials are used which have specific geometric shapes reflecting
of bird body. Considering the variability of bird species and the variations in the impact, the
bird model must have a certain degree of similarity to that used in experimental tests.
Figure 3-2: Bird Strike Experiment Set-up
One of the most important parts of the bird impact analysis is determining the appropriate bird
model. Model designation includes bird geometry and material selection. There are certain
geometric shapes commonly used in bird geometry. These are cylindrical, cylindrical
hemispherical ends, ellipsoidal and spherical shapes. In particular, the cylindrical hemispherical
ends and ellipsoidal shaped bodies yielded closer results to the actual bird body in the tests.
Figure 3.2 shows the bird model geometric shapes that Heimbs (2010) indicated in his paper.
[30]
Figure 3-3: Different Substitude Bird Impactor Geometries
Federal Aviation Administration (FAA) have some regulations for the bird strike test condition.
According to FAA’s Issue Paper G-1, bird strike test condition is given by the parameters in Table
3-1.
31
Table 3-1: FAA Bird Strike Conditions
Test Condition Bird Weight Impact Speed 14 CFR
Airplane 4.0 lb
(1.8 kg)
VC at sea level
§ 25.631
The bird strike he requirement is specified in § 25.631 [35] as;
(a) The aircraft must be capable of continued safe flight and landing during which likely structural
damage or system failure occurs as a result of –
(1) In airplane mode, impact with a 4-pound bird when the velocity of the aircraft relative to the
bird along the aircraft’s flight path is equal to Vc at sea level or 0.85Vc at 8,000 ft, whichever is
more critical;
(2) In VTOL/conversion mode, impact with a 2.2 pound bird at Vcon or VH (whichever is less) at
altitude up to 8,000ft.
(b) Compliance must be shown by tests or by analysis based on tests carried out on sufficiently
representative structures of similar design.
where, VC is cruise speed and VH indicates the hover speed.
32
In this thesis, bird strike on a wing leading edge is investigated for a fighter aircraft and bird
impactor geometry is selected as cylinder with hemispherical ends. Bird impactor dimensions
are given in Figure 3.3.
Figure 3-4: Bird dimensions that is used in the analysis
33
3.3 Aircraft Wing Modeling
In this thesis, bird strike simulation is performed on a wing leading edge. F-16 fighter is chosen
to be used. CATIA V5R21 software is used to model the wing. Modeled wing is shown in
Figure 3-5.
Figure 3-5: F-16 Wing model that is used in this thesis
Lack of the information about military based aircrafts has led this study to make some
assumptions. NACA 64A204 airfoil is used to model the wing. 8 spars and 3 ribs were used to
model this geometry. Spars were extended inside the fuselage in order to fix wing with
fuselage.
34
3.4 Aircraft Wing Meshing
After modeling the aircraft wing structure, HyperMesh 14.0 sofware were used to mesh the
geometry. Element size has been set as 10 mm and quad meshes used only. Final mesh
geometry is shown in the Figure 3.6.
Figure 3-6: Aircraft Wing Mesh
There are triangular surfaces on the leading edge so it was a problem to mesh these areas as
quad elements. Because of this problem mesh style has been set as ‘map as triangle’ on these
triangular faces. Figure 3-7 shows that how triangular areas of wing leading edge elements
mapped.
35
Figure 3-7: Aircraft Leading Edge Mesh
36
4. SOLUTION TECHNIQUES
Selecting the method to be used in the analysis is one of the most serious issues in the
simulation of the bird impact problem. In this section it will be examined what the basic finite
element solution techniques used for non-rigid bodies during collision simulation are. It will
also show applicability to the bird strike problem, taking into account the advantages and
disadvantages of the finite element methods mentioned. There are basically four finite element
approaches that can be used in the simulation of the bird impact problem. These approaches
are; Lagrangian Solution Technique, Eulerian Solution Technique, Arbitrary Lagrangian-
Eulerian Solution Technique (ALE) and Smooth Particle Hydrodynamics Solution Technique
(SPH). The main difference between these techniques is the solution networking approach.
4.1 Lagrange Solution Technique
Generally Lagrange modeling method is the most commonly used method of finite element
software. In the Langrangian method, each nodal point located on the solution network is fixed
on the element, and when the element is in motion and deformed, the material on the element
remains fixed with the node points. As the boundary conditions used during solution
networking collide with the material boundaries, the boundary conditions are always clearly
expressed in this method. This method is generally used in rigid body analysis.
The main problem of this approach is the deterioration of the network of solutions. Large
amounts of deformations on the object lead to unrealistic results and cause the analysis to fail.
In addition, breaks in the form of extreme deformations of the solid object increase the time for
analyzing the resulting material erosion excessively. Solution network created by Lagrangian
approach is shown in the Figure 4.1
37
Figure 4-1: Lagrangian Modeling Method for Soft Body Projectile
The Lagrangian method lies behind the fact that it is a very good finite element method for the
analysis of solid bodies, numerically containing formulations for solid bodies in its formulation.
In contrast to rigid bodies, due to large displacement and fragmentation in the bird simulation,
the formulation is beginning to require time to reach resolution, and the results are deviating
from the expected values. [29] The element deformation in a Lagrangian bird model is shown
in the Figure 4.2.
Figure 4-2: Bird strike simulation on rigid plate with Lagrangian impactor model
As a result, Lagrangian approach is not suitable to use in the analysis of bird impact because of
the high resolution time and calculation burden as well as the distance from the results, even
when trying to use it with additional solution methods.
38
4.2 Eulerian Solution Technique
Heimbs stated that, ‘The major limitation of the Lagrangian model with respect to the flow
behavior is excessive mesh distortion, hence reduced time step. A promising alternative is the
Eulerian modeling technique, which is mostly applied to the simulation of fluid behavior.’ [30]
In the Euler method, the solution network basically represents the control volume. The Euler
method can be used as an alternative to the problem of solution network distortion on objects
exhibiting fluid behavior in the Langarnge method and the small computational steps involved
in each computation. In the Euler method, the solution network is constant in the space
environment and the material passes through this constant solution network space. [31] Figure
4.3 shows that the solution network created by Eulerian approach.
.
Figure 4-3: Eulerian Modeling Method for Soft Body Projectile
The solution is fixed in space with the solution of the network and the problem of the solution
of the Lagrange method is overcome due to the problem of network corruption and small
calculation intervals. In addition, due to excessive shape changes due to program errors do not
occur. In general, the Euler approach is used in liquid materials and flow processes. In the
Euler approach, each solution network element fixed in space has a net volume, and each
element may be filled with fluid locally.
The main problem of this approach is that the boundary conditions don’t contain the object
clearly. Depending on the size of the selected solution, the post-analysis visualization software
39
may choose the outer boundary conditions to be too coarse. Moreover, the behavior of the
material becomes more difficult because the nodes are not fixed on the element. Longer
simulation times are needed because more complex digital calculations are required to achieve
the material behavior that is easily obtained in the Lagrange method. The element deformation
in a Lagrangian bird model is shown in the Figure 4.4.
Figure 4-4: Bird strike simulation on rigid plate with Eulerian impactor model
Given the difficulties mentioned above, the Euler technique in rigid body analysis requires
more time and computational power as it requires much more computation per element than the
Lagrange technique and requires a much more detailed solution network for the same result.
For this reason, this method is not efficient when it is thought that both fluid and rigid body
analysis is done in bird strike analysis.
4.3 ALE (Arbitrary Lagrangian Eulerian) Solution Technique
In the classic Euler approach, the solution network reflects a fixed area in space, and the area to
be computed should cover the area where the material is likely to be found, except for the
environment in which the material is located. For this reason, the classical Euler approach
requires more computation than the Lagrange approach. In addition, in order to approach the
40
same result with Lagrange approach, solution network elements are needed in a smaller
structure. Given all these disadvantages of the classical Euler method, the ALE method allows
much more efficient analysis. The fixed solution network in the classical Euler method has a
movable or expandable structure if required by the ALE (Arbitrary Lagrangian-Eulerian)
method. The location of the solution network in the ALE approach is updated to match the
location of the bird element in space. In this way, the computational times are drastically
reduced when the network of updateable solutions requires much less solution networking than
the conventional method. But it should not be forgotten that the solution network elements on
the birds that are disintegrated on the rigid plate are very important. The solution network
elements in this region should be determined according to the desired accuracy of the analysis
result. [30], [31] Figure 4.5 shows the solution network created by ALE (Arbitrary
Lagrangian-Eulerian) approach.
Figure 4-5: ALE Modeling Method for Soft Body Projectile
In conclusion, the ALE approach allows for more efficient fluid-solid body analysis by taking
good aspects of the Eulerian and Lagrangian approaches. On the other hand, it is trying to get
rid of the disadvantages of Euler and Lagrange methods. The disadvantage of the ALE method
is that the user has to be experienced while determining the solution network volume. Figure
4.6 shows the element deformation in a Lagrangian bird model.
41
Figure 4.6: Bird strike simulation on rigid plate with ALE impactor model
Jenq (2006) plotted the distorted mesh and its related background void mesh using Lagrangian,
Eulerian, and ALE (Arbitrary Lagrangian-Eulerian) approaches and compared them in the same
plot. It is shown in the Figure 4.7.
Figure 4-7 : Comparison of the approaches on mesh movement
42
4.4 SPH (Smoothed Particle Hydrodynamics) Solution Technique
In addition to methods such as Lagrange, Euler and ALE, SPH (Smooth Particules
Hydrodynamics) method has been developed in order to get rid of solution network problems
and to make more efficient analyzes. Initially developed for the calculations of astrophysical
collisions at hypersonic speeds in the 1970s, fluid-rigid interaction problems from the
beginning of the 1990s, collision simulations, analyzes of fragile and bendable structures, and
analyzes subjected to high deformation. The bird collision problem, which occurs in large
diameter deformation, is suitable for use with the SPH method. [32]
Figure 4-8: SPH Modeling Method for Soft Body Projectile
In addition to methods such as Lagrangian, Eulerian and ALE, SPH (Smooth Particules
Hydrodynamics) method has been developed in order to get rid of solution network problems
and to make more efficient analyzes. Initially developed for the calculations of astrophysical
collisions at hypersonic speeds in the 1970s, fluid-rigid interaction problems from the
beginning of the 1990s, collision simulations, analyzes of fragile and bendable structures, and
analyzes subjected to high deformation. SPH method is suitable for use with the bird strike
problem with large deformation. Thanks to the meshless structure of the SPH method, there are
no solution network problems resulting from large deformations. The conventional solid
Lagrange solution significantly reduces the step time in the elements, which are deformed
compared to the solution network and is fixed. According to the Euler method, the SPH method
requires far fewer elements. It is also easy to follow the deformation behavior of each particle
as in the Lagrange method. [22] [30] [33]
43
Figure 4-9: Bird strike simulation on rigid plate with SPH impactor model
On the other hand, the SPH method also have some disadvantages. There is a high memory and
CPU requirement for the calculation, and this problem is inherited by parallel multiprocessor
computers. Another disadvantage is that, when the boundary conditions are determined, the
relative value of the particles deviates from the true value when the boundary conditions are
exceeded.
4.5 Comparison of the Solution Teqniques
Lagrange, Euler, ALE and SPH finite element methods investigated with advantages and
disadvantages in this chapter will be compared with one another in this section and the method
to be used within the scope of this thesis will be selected. Of these four methods, the Lagrange
method is generally used for rigid body analysis. Euler method is usually used for fluid
analysis. However, these methods do not suffice in terms of the presence of both liquid and
solid elements in the problem of bird striking, and results of these methods are not realistic. The
two other methods examined in this case, ALE and SPH methods, give more feasible results.
On the other hand, in the SPH method to be used as an analysis method in this thesis study,
birds will be expressed as particle and rigid plate will be expressed according to Lagrange
solution network method. With the SPH method, the greatest amount of deformations in the
44
bird element are represented most appropriately and the closest results to the actual conditions
are approached.
Heimbs (2010) made a table about advantages and disadvanges about of bird modeling
methods. [30]
Table 4-1: Comparision between Lagrangian, Eulerian, and SPH Method
Literature studies comparing the results of the Lagrange, Euler, ALE and SPH methods with
the experimental tests show that the Lagrange method gives realistic results when large
deformations can not be avoided, whereas in cases where deformations occur, calculation errors
and high computational costs are found. The Euler method, on the other hand, is not suitable
for bird impact analysis due to the high error rate of the results of relative impact strength,
which requires high computational cost. The ALE method may not be preferred due to the high
computational costs associated with the SPH method and the consequences of severe shape
changes in the Euler control volume. The SPH method is suitable for use in bird impact
analysis due to its high stability, low computational cost and compatibility with experimental
tests.
45
5. BIRD STRIKE ANALYSIS BY USING SPH METHOD
5.1 Metallic Plate Bird Strike Analysis
Bird Geometry Validation
In order to validate the bird geometry which is going to be used in this thesis, it is used in a bird
strike analysis on a flat plate, and compared the results with an experimental test and a
numerical solution found in literature research.
6. CONCLUSIONS AND RECOMMENDATIONS
46
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