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I INDIAN INSTITUTE OF TECHNOLOGY GANDHINAGAR

DISCIPLINE OF MECHANICAL ENGINEERING

FLOW MODELLING FOR HIGH ALTITUDE AIRSHIPS

FINAL REPORT

ON

BTech Project

Semester 8/AY 2013-2014

by

Ritu Gavasane

Final Year Undergraduate| Mechanical Engineering

SUBMITTED TO

Professor Murali Damodaran

BTech Project Supervisor

Discipline of Mechanical Engineering

24 April 2014

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ABSTRACT

CFD modeling of flow past complete airship with finned control surfaces is attempted in this work to set

the basis for the development of an integrated aerodynamics, flight dynamics and control model using

high fidelity simulation techniques. A computational simulation was initially done on the GNV Rao profile

of airship for various angles of attack. A similar computational simulation is done to find the aerodynamic

characteristics and validate the wind tunnel tests of the scaled ZHIYUAN-1 airship using STAR CCM+

for various angles of attack. Conclusions are drawn on the aerodynamic characteristics of general layout

of airship configurations.

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CONTENTS

Introduction

A. GNV Rao Airship 4

4

1. Computational setup 4

2. Results and Discussion 7

3. Conclusion 9

B. ZHUIYUAN-1 Airship

1. Introduction

10

2. Wind Tunnel Facilities 10 3. Computational setup 11

4. Results and Discussion 16

5. Comparison between Forced and Free transition study

17

6. Conclusion 19

Acknowledgements 20

References 20

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Introduction

This study presents a computational investigation on the external aerodynamics of a three dimensional

Airship with finned control surfaces. Use of CFD tools for simulating such problems has gained a wide

impetus. The first section of this work attempts to simulate the external aerodynamics over an Airship

which uses GNV Rao envelope profile. Such an airship was developed at IIT Bombay within its Program

on Airship Design and Development (PADD) which was launched in 2001. The second section of the

project involves modelling of ZHIYUAN-I airship with fins including free and forced transition

modelling. The results obtained in this study are validated with the experimental data obtained from

literature. Also a comparison is drawn between free and forced transition study.

A. GNV Rao Airship

I. Computational setup

A numerical investigation on the external aerodynamics was done initially for GNVR Hull only for angles

of attack ranging from 00 to 100. Later on, fins were attached to the hull to examine the effects of the fins

on the force coefficients and lift and drag produced. Figure (1) shows the Airship developed at IIT Bombay

under its PADD program and fig (2) shows the GNVR shape selected as the hull geometry. The shape

consists of a combination of three sections, with a fineness ratio of 3.05. The front section is elliptic, the

mid section is an arc of a circle and the end section is parabolic. This geometry was revolved to create a

three dimensional hull of the Airship. Figure (3) shows the three dimensional geometry of the Airship

hull.

Figure1 Airship Developed at IIT Bombay Figure 2 GNV Rao Envelope Profile

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Figure 3 Three Dimensional Hull

A computational domain was then created around the geometry and appropriate boundary conditions were

imposed on the domain. On the Airship surface, no slip boundary conditions were imposed. Figure (4)

shows the computational domain used for the study. The velocity inflow boundary conditions were

imposed at radial distance of 10 times the chord length. The rest of the surfaces were given outflow as the

boundary conditions.

Figure 4 Computational Domain

Following are the Navier-Stokes equations solved in the discretized computational domain in Star CCM+

solver in integral form using finite volume formulation.

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Continuity Equation:

(

.

+ .

.

) = 0 (1)

Momentum Equations:

.

.

+ = (. )

.

+

.

(2)

The finite volume approach on which CFD calculations are based are made over a collection of discrete

grid points called Mesh. Grid with a base size of 5 mm with a volume cell count of 37,605 was chosen for

simulations. A structured trimmer mesh was generated in the domain to perform the computation. Prism

layers were inserted near the surface of the hull to capture near wall effects. Figure (5) shows the meshed

domain. Refinement of mesh in the wake region of the Airship was done to capture the vorticity arising

out of turbulence. An overlapping mesh structure was created around the Airship geometry to enable

changing the angle of attack of the Airship.

(a) Meshed Hull

(b) Prism Layer at the walls of Airship

Four equations of Continuity, x momentum, y momentum, z momentum are sufficient to solve for four

dependent variables, the three components of velocity, u, v, w, and pressure. All the simulations were

done at an inflow velocity of 20 m/s and the Reynolds number is 6106. All the computations assume

fully turbulent model. The freestream turbulence level is 0.1 %. The calculations of turbulent flows over

solid bodies involve the solution of continuity and momentum equations along with some model of

turbulence. For turbulence modeling, we chose the Spalart-Allmarus model. The SA model is effective to

capture near wall effects and provides a superior accuracy than the standard k- model for wall-bounded and adverse pressure gradients flows in boundary layers. Simulations were performed for different angles

of attack () ranging from 00 to 100. The quantities of interest in our studies are the coefficients of drag (CD), lift (CL), and pitching moment (CM) experienced by the hull. The reference area and reference length

are taken to be 6.218 m2 and 5 m, respectively, to compute the aerodynamic coefficients.

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After computations were done for the only hull case, four fins with the Airfoil section of NACA 0012

were attached to the geometry as shown in the fig (6). Other computational setup and meshing parameters

were set similar to previous case and the reference area in this case was taken to be 6.612 m2.

Figure 6 Hull with fins

II. Results and Discussion

After the external aerodynamics over finned airship and hull was simulated, the trend in variation of lift,

drag and moment coefficients with the variation in angles of attack was observed. Figure (7) shows the

variation of aerodynamics coefficients with various angles of attack for finned airship and hull.

(a) (a) Coefficient of Lift

(b) Coefficient of Drag (b) (c) Coefficient of Moment

Figure 7 Aerodynamic coefficients vs angle of attack

It can be seen from the aerodynamic coefficients profiles that attachment of fins increses the lift from 0.03

to 0.1. Drag and Moment profiles do not vary much on attachment of fins.

Figures (8) and (9) show the computational results for finned airship and hull computed at angle of attack

=100. The skin friction and velocity streamline show that flow reamins attached to the hull for most of the times and flow separation over the airship occurs only at the end of the hull and fins.

CL

W

CD

W CM

W

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(a) Velocity Profile (b) Wall Shear Stress Profile

(c) Velocity Streamlines (d) Skin Friction Lines

Figure 8 Computational results for Hull

(a) Velocity Profile (b) Wall Shear Stress Profile

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(c) Velocity Streamlines (d) Skin Friction Lines

Figure 9 Computational results for Finned Airship

III. Conclusion

For both finned airship and hull, the Lift, Drag and Moment Coefficients were observed to increase as the

angle of attack in increased. Attaching the fins increased Lift Coefficient from 0.03 to 0.1 at = 100. Stall has not occurred in the range of = 00 to 100. Flow separation was observed at the very end of hull and tail of fins.

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B. ZHIYUAN-1 Airship

I. Introduction

An autonomous control fuel-powered airship known as ZHIYUAN-1 was tested in the wind tunnel after

making a scaled model of the airship at the school of Aeronautics and Astronautics of Shanghai Jiao Tong

University, China, in 2009. This airship shown in Fig. 10 serves both as a reference configuration for

theoretical investigations and as a flying test platform for studies in aerodynamic, flight mechanics and

control, aeroelasticity, structural design. The aerodynamic characteristics of the airship are very important

for the designs of control system and propulsion system. The conventional configuration layout of the

airship consists of the hull, fins, and gondola. For these reasons, the wind-tunnel tests of scale ZHIYUAN-

1 airship were performed. The dimensions of the real and scale ZHIYUAN-1 airship as the experimental

model in the wind-tunnel tests are shown in Table 1.

Table 1 Dimensions of the real and scale ZHIYUAN-1 airship

Real airship Scale model

Length, m 25.0 1.8286

Maximum diameter, m 7.576 0.5543

Fineness ratio of the hull 3.3 3.3

Volume of the hull, m3 750 0.2935

Surface area, m2 480.388 2.5701

Location of maximum diameter, m 9.840 0.7197

Moment center, m 12.001 0.8778

Reference area, m2 82.544 0.4416

Reference length, m 25 1.8286

Volume Reynolds number 1.89.3 106 2.58 106

This work is organized as follows: The test facilities and wind tunnel models are described in Sec. II. In

Sec. III, computational investigations on scale ZHIYUAN-1 airship simulated on CFD software are

presented. In final section, we draw some comparison between CFD and experimental results and

conclusions on aerodynamic characteristics of general layout airship configurations.

II. Wind Tunnel Test

Wind tunnel test facilities

The experiment was performed at the 3.2 m wind tunnel at lowspeed research institute of China

Aerodynamics Research & Development Center. The 3.2 m wind tunnel is a single-return continuous-

flow tunnel with a dual closed/open test section. The wind tunnel consists of the tunnel body; power supply

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system; measuring, control and processing system; and model support system. Test section dimensions

are 3.2 m 5 m

Experiment Method and Condition

The integral forces and moments were measured using the general force measure method. The model is

supported by wire and stern frame. The forces and moments were obtained through a six component strain

balance. The angle of attack and sideslip angle were obtained using the sensor of angle of attack and

sideslip angle. The scale of the model used in _3:2 m wind tunnel is 1:13.7, leading to a model length of

L = 1:8286 m. The onset flow velocity is 60.39 m/s and volume Reynolds number of Rev =2.58 106. The

turbulence level is 0.1%, and the temperature is 25 0C. The range of angle of attack is -300 to 300

Experimental Model

The layout of the experimental airship model is classical. It has a gondola and four mutually perpendicular

rear fin surfaces, each incorporating an aerodynamic-flap-type control surface. The geometry parameters

are shown in Table 1. The airships geometry configuration is defined as follows.

Figure 10 Remote and autonomous control fuel-cell-powered ZHIYUAN-

1 airship.

III. Computational Setup

1. Hull Configuration

The contour of the hull configuration of experimental model is shown in Fig. 11. The X is the coordinate

in the axial direction, the R is the coordinate in the radial direction.

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Figure 11 Hull configuration of the experimental model.

2. Fin and Control Surface Configuration

The layout of the fins is used for this airship model. The airfoil section of the fins is NACA0010. The

parameters of the fins are shown in Table 2 and Fig. 12. Fig. 13 shows the scaled model of ZHIYUAN-1

airship that was used for wind tunnel tests.

Figure 12 Configuration of fin

Figure 13 Scaled model of ZHIYUAN-1 airship

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Table 2 Parameters for fin

Parameters Values, m

Root chord b0 0.1617

Tip chord b1 0.0936

Semispan h 0.1504

Leading edge sweep angle 400

Coordinates of fins based on hull nose

A (1.5618, 0.1748)

B (1.6655, 0.1368)

C (1.7233, 0.1368)

D (1.7496, 0.2872)

E (1.7174, 0.2872)

F (1.6560, 0.2872)

Computational Setup

The curve shown in Fig. 11 was revolved to make the hull of the scaled ZHIYUAN-1 Airship and fins as

shown in Fig. 12 were attached to the hull to get the geometry as shown in Fig. 14. For forced transition

study, a strip was inserted on the hull at x/c=0.52 position as shown in Fig. 14 (b).

(a) Airship geometry for free transition case (b) Airship geometry for forced transition case

Figure 14

A computational domain was then created around the geometry and appropriate boundary conditions were

imposed on the domain. On the Airship surface, no slip boundary conditions were imposed. Figure 15

shows the computational domain used for the study. In accordance with the wind tunnel tests, a similar

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cylindrical domain was constructed. The velocity inflow boundary conditions were imposed on the

circular wall at a distance of 5 m from the Airship. The rest of the surfaces were given freestream as the

boundary conditions.

Figure 15 Computational Domain

The finite volume approach on which CFD calculations are based are made over a collection of discrete

grid points called Mesh. Grid with a base size of 5 mm with a volume cell count of 37,605 was chosen

for simulations. A structured trimmer mesh was generated in the domain to perform the computation.

Prism layers were inserted near the surface of the hull to capture near wall effects. Figure 16 shows the

meshed domain. Refinement of mesh in the wake region of the Airship was done to capture the vorticity

arising out of turbulence. An overlapping mesh structure was created around the Airship geometry to

enable changing the angle of attack of the Airship.

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Figure 16 Meshed domain

Figure 17 Meshed geometry

Four equations of Continuity, x momentum, y momentum, z momentum are sufficient to solve for four

dependent variables, the three components of velocity, u, v, w, and pressure. All the simulations were

done at an inflow velocity of 60.39 m/s and the Reynolds number is 2.58106. All the computations

assume turbulence alongwith transition model. The freestream turbulence level is 0.1 %. The calculations

of transition flows over solid bodies involve the solution of continuity and momentum equations and the

-Re- transition model along with some model of turbulence. For turbulence modeling, we chose the K- model as it is a pre requisite to transition modelling in STAR CCM+. Simulations were performed for different angles of attack () ranging from -300 to 300. The quantities of interest in our studies are the coefficients of drag (CD), lift (CL), and pitching moment (CM) experienced by the hull. The reference area

and reference length are taken to be 0.414 m2 and 1.82 m, respectively, to compute the aerodynamic

coefficients. After computations were done for the free transition case, the similar process was repeated

for forced transition case on the geometry with strip on the hull to force transition to occur at the strip.

Other computational setup and meshing parameters were set similar to previous case.

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IV. Results and Discussion

Free Transition

Figure 18 shows the flow field patterns for free transition case obtained at =300. Figure 19 shows the aerodynamic force coefficients obtained for the case alongwith the experimental values.

(a) Velocity profile (b) Skin friction coefficient profile

(c) Wall Shear Stress for = 300

Figure 18 Flow field patterns for free transition case

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Coefficient of Lift Coefficient of Drag Coefficient of Drag

Figure 19 Aerodynamic coefficients for free transition case

Forced Transition

Figure 20 shows the aerodynamic force coefficients obtained for the case along with the experimental

values.

Coefficient of Lift Coefficient of Drag Coefficient of Drag

Figure 20 Aerodynamic coefficients for forced transition case

V. Comparison between Forced and Free transition study

Figure 21 Comparison of flow fields for free and forced transition including the wall shear stress and

skin friction line profiles.

-50 0 50

-1

-0.5

0

0.5

1

CL

CFD Experimental -40 -20 0 20 40

0.00

0.10

0.20

0.30

0.40

CD

CFD Experimental

-50 0 50

-0.1

-0.05

0

0.05

0.1

CM

CFD Experimental

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Free transition: Wall shear stress Forced transition: Wall shear stress

Free transition: Skin friction

Forced transition: Skin friction

Figure 21 Comparison of flow fields for free and forced transition

Figure 22 shows the comparative plots for aerodynamic coefficients for free and forced transition

case.

Coefficient of Lift Coefficient of Drag

-40 -20 0 20 40

-0.60-0.50-0.40-0.30-0.20-0.100.000.100.200.300.400.500.60

CL

Free transition Forced transition

-40 -20 0 20 40-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

CD

Free transition Forced transition

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Coefficient of Moment

Figure 22 Comparison of Aerodynamic coefficients for free and forced transition

VI. Conclusion 1. All the computationally calculated force coefficients in both free and forced transition case

studied agree well with the experimental wind tunnel tests.

2. In case of forced transition, flow separation was observed at the position of strips on the airship hull.

3. The CFD results of free and forced transition did not differ much for the assumed point of transition that was positioned at x/c=0.52.

4. Maintaining laminar flow or delaying the location of transition point is very useful in reducing the drag on the airship.

5. The free transition data has been used to verify the accuracy of computational software.

-40 -20 0 20 40

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

CM

Free transition Forced transition

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Acknowledgements

I sincerely thank Akshay Kanoria and Rachit Prasad, graduate students at IIT Gandhinagar for their

valuable guidance in this project. I also thank IIT Gandhinagar for giving me an opportunity to carry out

this work. I also thank the HPCL staff who cooperated me while carrying out this project.

References

[1] Gawale A., Pant R.S., Design, fabrication and flight testing of remotely controlled airship.

[2] Suman S.,Lakshmipathy S.,Pant R.S., Evaluation of Assumed-Transition-Point Criterion in Context of Reynolds-Averaged Simulations Around Lighter-Than-Air Vehicles, Journal of Aircraft Vol. 50, No. 2, MarchApril 2013.

[3] Xiao-liang Wang, Gong-yi Fu, Deng-ping Duan,Experimental Investigations on Aerodynamic Characteristics of the ZHIYUAN-1 Airship, Journal of Aircraft Vol. 47, No. 4, JulyAugust 2010

[4] STAR CCM + User Guide 8.04