Chapter 11: Flow over bodies; Lift and Drag - PowerPoint PPT Presentation

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Chapter 11: Flow over bodies; Lift and Drag

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Thickness and camber influences pressure distribution (and load distribution) ... Camber and thickness shown to have large impact on flow field. ... – PowerPoint PPT presentation

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Title: Chapter 11: Flow over bodies; Lift and Drag


1
Chapter 11 Flow over bodiesLift and Drag
  • Eric G. Paterson
  • Department of Mechanical and Nuclear Engineering
  • The Pennsylvania State University
  • Spring 2005

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 of Fluid Mechanics
    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.sourcef
orge.net/), but were translated to PowerPoint for
wider dissemination by McGraw-Hill.
3
Objectives
  • Have an intuitive understanding of the various
    physical phenomena such as drag, friction and
    pressure drag, drag reduction, and lift.
  • Calculate the drag force associated with flow
    over common geometries.
  • Understand the effects of flow regime on the drag
    coefficients associated with flow over cylinders
    and spheres
  • Understand the fundamentals of flow over
    airfoils, and calculate the drag and lift forces
    acting on airfoils.

4
Motivation
5
Motivation
6
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 6
    equations for the linear (surge, heave, sway) and
    angular (roll, pitch, yaw) momentum.

7
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.
8
Drag and Lift
  • Fluid dynamic forces are due to pressure and
    viscous forces acting on the body surface.
  • Drag component parallel to flow direction.
  • Lift component normal to flow direction.

9
Drag and Lift
  • Lift and drag forces can be found by integrating
    pressure and wall-shear stress.

10
Drag and Lift
  • In addition to geometry, lift FL and drag FD
    forces are a function of density ? 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).

11
Example Automobile Drag
Scion XB
Porsche 911
CD 1.0, A 25 ft2, CDA 25ft2
CD 0.28, A 10 ft2, CDA 2.8ft2
  • Drag force FD1/2?V2(CDA) will be 10 times
    larger for Scion XB
  • Source is large CD and large projected area
  • Power consumption P FDV 1/2?V3(CDA) for both
    scales with V3!

12
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,x and CD,x are
    introduced and the total lift and drag is
    determined by integration over the span L

13
Lofting a Tapered Wing
14
Friction and Pressure Drag
  • Fluid dynamic forces are comprised of pressure
    and friction effects.
  • Often useful to decompose,
  • FD FD,friction FD,pressure
  • CD CD,friction CD,pressure
  • This forms the basis of ship model testing where
    it is assumed that
  • CD,pressure f(Fr)
  • CD,friction f(Re)

Friction drag
Pressure drag
Friction pressure drag
15
Streamlining
  • Streamlining reduces drag by 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
    separation and vortex shedding can excite
    structural modes.

16
Streamlining
17
Streamlining via Active Flow Control
  • Pneumatic controls for blowing air from slots
    reduces drag, improves fuel economy for heavy
    trucks (Dr. Robert Englar, Georgia Tech Research
    Institute).

18
CD of Common Geometries
  • For many geometries, total drag CD is constant
    for Re gt 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
    the success of doing this.

19
CD of Common Geometries
20
CD of Common Geometries
21
CD of Common Geometries
22
Flat Plate Drag
  • Drag on flat plate is solely due to friction
    created by laminar, transitional, and turbulent
    boundary layers.

23
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
24
Effect of Roughness
  • Similar to Moody Chart for pipe flow
  • Laminar flow unaffected by roughness
  • Turbulent flow significantly affected Cf can
    increase by 7x for a given Re

25
Cylinder and Sphere Drag
26
Cylinder and Sphere Drag
  • Flow is strong function of Re.
  • Wake narrows for turbulent flow since TBL
    (turbulent boundary layer) is more resistant to
    separation due to adverse pressure gradient.
  • ?sep,lam 80º
  • ?sep,lam 140º

27
Effect of Surface Roughness
28
Lift
  • Lift is the net force (due to pressure and
    viscous forces) perpendicular to flow direction.
  • Lift coefficient
  • Abc is the planform area

29
Computing Lift
  • Potential-flow approximation gives accurate 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/pan
    el2/
  • Kutta condition required at trailing edge fixes
    stagnation pt at TE.

30
Effect of Angle of Attack
  • Thin-foil theory shows that CL2?? for ? lt ?stall
  • Therefore, lift increases linearly with ?
  • Objective for most applications is to achieve
    maximum CL/CD ratio.
  • CD determined from wind-tunnel or CFD (BLE or
    NSE).
  • CL/CD increases (up to order 100) until stall.

31
Effect of Foil Shape
  • Thickness and camber influences pressure
    distribution (and load distribution) and location
    of flow separation.
  • Foil database compiled by Selig
    (UIUC)http//www.aae.uiuc.edu/m-selig/ads.html

32
Effect of Foil Shape
  • Figures from NPS airfoil java applet.
  • Color contours of pressure field
  • Streamlines through velocity field
  • Plot of surface pressure
  • Camber and thickness shown to have large impact
    on flow field.

33
End Effects of Wing Tips
  • Tip vortex created by leakage of flow from
    high-pressure side to low-pressure side of wing.
  • Tip vortices from heavy aircraft persist far
    downstream and pose danger to light aircraft.
    Also sets takeoff and landing separation at busy
    airports.

34
End Effects of Wing Tips
  • Tip effects can be reduced by attaching endplates
    or winglets.
  • Trade-off between reducing induced drag and
    increasing friction drag.
  • Wing-tip feathers on some birds serve the same
    function.

35
Lift Generated by Spinning
Superposition of Uniform stream Doublet Vortex
36
Lift Generated by Spinning
  • CL strongly depends on rate of 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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