presentation on lift sphere

8/7/2019 presentation on lift sphere

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Chapter 11: Flow over bodies;

Lift and Drag

Eric G. PatersonDepartment of Mechanical and Nuclear Engineering

The Pennsylvania State University

Spring 2005


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Chapter 11: Flow over bodies; lift and dragME33 : Fluid Flow 2

Note to Instructors

These slides were developed1, during the spring semester 2005, as a teaching aid

for the undergraduate Fluid Mechanics course (ME33: Fluid Flow) in the Department of

Mechanical and Nuclear Engineering at Penn State University. This course had two

sections, one taught by myself and one taught by Prof. John Cimbala. While we gave

common homework and exams, we independently developed lecture notes. This was

also the first semester that Fluid Mechanics: Fundamentals and Applications was

used at PSU. My section had 93 students and was held in a classroom with a computer,

projector, and blackboard. While slides have been developed for each chapter ofFluidMechanics: Fundamentals and Applications, I used a combination of blackboard and

electronic presentation. In the student evaluations of my course, there were both positive

and negative comments on the use of electronic presentation. Therefore, these slides

should only be integrated into your lectures with careful consideration of your teaching

style and course objectives.

Eric Paterson

Penn State, University Park

August 2005

1 These slides were originally prepared using the LaTeX typesetting system (http://www.tug.org/)and the beamer class (http://latex-beamer.sourceforge.net/ ), but were translated to PowerPoint forwider dissemination by McGraw-Hill.


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Objectives

Have an intuitive understanding of the variousphysical phenomena such as drag, friction andpressure drag, drag reduction, and lift.

Calculate the drag force associated with flow

over common geometries.Understand the effects of flow regime on thedrag coefficients associated with flow overcylinders and spheres

Understand the fundamentals of flow overairfoils, and calculate the drag and lift forcesacting on airfoils.


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Motivation


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Motivation


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External Flow

Bodies and vehicles in motion, or with flow over them,experience fluid-dynamic forces and moments.

Examples include: aircraft, automobiles, buildings,

ships, submarines, turbomachines.

These problems are often classified as External Flows.Fuel economy, speed, acceleration, maneuverability,

stability, and control are directly related to the

aerodynamic/hydrodynamic forces and moments.

General 6DOF motion of vehicles is described by 6equations for the linear (surge, heave, sway) and

angular (roll, pitch, yaw) momentum.


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Fluid Dynamic Forces and Moments

Ships in waves present one of the most

difficult 6DOF problems.

Airplane in level steady flight: drag =

thrust and lift = weight.


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Drag and Lift

Fluid dynamic forces aredue to pressure and

viscous forces acting on

the body surface.

Drag: component

parallel to flow direction.

Lift: component normal

to flow direction.


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Drag and Lift

Lift and drag forces can be found byintegrating pressure and wall-shear stress.


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Drag and Lift

In addition to geometry, lift FL and drag FD forcesare a function of density V and velocity V.

Dimensional analysis gives 2 dimensionless

parameters: lift and drag coefficients.

Area A can be frontal area (drag applications),

planform area (wing aerodynamics), or wetted-surface area (ship hydrodynamics).


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Example: Automobile Drag

Scion XB Porsche 911

CD = 1.0, A = 25 ft2, CDA = 25ft

2 CD = 0.28, A = 10 ft2, CDA = 2.8ft

2

Drag force FD=1/2VV2(CDA) will be ~ 10 times larger for Scion XB

Source is large CD and large projected area

Power consumption P = FDV =1/2VV3(CDA) for both scales with V

3!


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Drag and Lift

For applications such as tapered wings,CL and CD may be a function of span

location. For these applications, a local

CL,xand CD,xare introduced and the totallift and drag is determined by integration

over the span L


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Lofting a Tapered Wing


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Friction and Pressure Drag

Fluid dynamic forces arecomprised of pressure andfriction effects.

Often useful to decompose,

FD = FD,friction + FD,pressureCD = CD,friction + CD,pressure

This forms the basis of shipmodel testing where it is

assumed thatCD,pressure = f(Fr)

CD,friction = f(Re)

Friction drag

Pressure drag

Friction & pressure drag


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Streamlining

Streamlining reduces dragby reducing FD,pressure, at

the cost of increasing

wetted surface area and

FD,friction

.

Goal is to eliminate flow

separation and minimize

total drag FD

Also improves structural

acoustics since separationand vortex shedding can

excite structural modes.


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Streamlining


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Streamlining via Active Flow Control

Pneumatic controlsfor blowing air from

slots: reduces drag,

improves fueleconomy for heavy

trucks (Dr. Robert

Englar, Georgia

Tech ResearchInstitute).


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CD of Common Geometries

For many geometries, total drag CDis constant forRe > 104

CD can be very dependent upon

orientation of body.

As a crude approximation,superposition can be used to add

CD from various components of a

system to obtain overall drag.

However, there is no mathematical

reason (e.g., linear PDE's) for thesuccess of doing this.


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Flat Plate Drag

Drag on flat plate is solely due to friction createdby laminar, transitional, and turbulent boundarylayers.


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Flat Plate Drag

Local friction coefficient

Laminar:

Turbulent:

Average friction coefficient

Laminar:

Turbulent:

For some cases, plate is long enough for turbulent flow,but not long enough to neglect laminar portion


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Effect of Roughness

Similar to MoodyChart for pipe flow

Laminar flow

unaffected by

roughness

Turbulent flow

significantly affected:

Cfcan increase by 7xfor a given Re


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Cylinder and Sphere Drag


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Cylinder and Sphere Drag

Flow is strong function ofRe.

Wake narrows for

turbulent flow since TBL

(turbulent boundary layer)is more resistant to

separation due to

adverse pressure

gradient.

Usep,lam 80

Usep,lam 140


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Effect of Surface Roughness


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Lift

Lift is the net force(due to pressure andviscous forces)perpendicular to flow

direction.Lift coefficient

A=bc is the planformarea


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Computing Lift

Potential-flow approximation givesaccurate CL for angles of attack below

stall: boundary layer can be neglected.

Thin-foil theory: superposition of uniform

stream and vortices on mean camber line.

Java-applet panel codes available online:http://www.aa.nps.navy.mil/~jones/online

_tools/panel2/

Kutta condition required at trailing edge:

fixes stagnation pt at TE.


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Effect of Angle of Attack

Thin-foil theory shows thatCL2TE forE < EstallTherefore, lift increaseslinearly with E

Objective for mostapplications is to achievemaximum CL/CD ratio.

CD determined from wind-

tunnel or CFD (BLE or NSE).CL/CD increases (up to order100) until stall.


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Effect of Foil Shape

Thickness andcamber influencespressure distribution(and load distribution)

and location of flowseparation.

Foil databasecompiled by Selig(UIUC)http://www.aae.uiuc.edu/m-selig/ads.html


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Effect of Foil Shape

Figures from NPS airfoil java

applet.

Color contours ofpressure field

Streamlines throughvelocity field

Plot of surface pressure

Camber and thickness shownto have large impact on flowfield.


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End Effects of Wing Tips

Tip vortex created byleakage of flow from high-

pressure side to low-

pressure side of wing.

Tip vortices from heavyaircraft persist far

downstream and pose

danger to light aircraft.

Also sets takeoff and

landing separation atbusy airports.


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End Effects of Wing Tips

Tip effects can bereduced by attaching

endplates orwinglets.

Trade-off between

reducing induceddrag and increasing

friction drag.

Wing-tip feathers onsome birds serve the

same function.


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Lift Generated by Spinning

Superposition of Uniform stream + Doublet + Vortex


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Lift Generated by Spinning

CL strongly depends on rateof rotation.

The effect of rate of rotation

on CD is small.

Baseball, golf, soccer,

tennis players utilize spin.

Lift generated by rotation is

called The Magnus Effect.

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