Aerodynamics John Chen

8/18/2019 Aerodynamics John Chen

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• What do we want from aerodynamics?

• Basic ideas from 2D

• Differences between 2D and 3D

• The importance of unsteadiness

Outline


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• Aero

• Air

• Dynamics

What is aerodynamics?


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What do we want from

aerodynamics?

•Predict forces and moments on abody due to motions relative to fluid


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Four forces on an airplane

NASA Glenn Research center

What is the most important part that identifies thetype, performance, and purpose of an airplane?


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2D Airfoil


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Vectors• Direction

•Magnitude


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Newton’s 1st lawapplied to airplanes

• “Every object persists inits state of rest or uniform

motion in a straight lineunless it is compelled tochange that state byforces impressed on it.”

If Thrust == Drag, airplane holds constant airspeedIf Thrust , airspeed , then dragWhen Drag == thrust, airplane holds a new, higher constant airspeed

Isaac Newton(1643-1727)


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Newton’s 2nd lawapplied to airplanes

• Force = mass * acceleration

Excess Thrust = Thrust - DragForces on the fluid causes acceleration

Isaac Newton(1643-1727)


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Lift

• What cause it?

• How to explain it?

Air and motion

Newton’s lawsBernoulli’s principle


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Motion effects on lift (1)• Change in momentum

FF

F1

F2

Lift (wing goes up) Sir Isaac Newton

Force causes a change in velocity which in turn generates another force.

For every action , there is an equal reaction.

Conservation of momentum (m * v)


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Motion effects on lift (2)• Pressure difference

FF

Lift (wing goes up) Issac Newton

Faster flow = lower relative pressure

Streamlines


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• The speed of a fluid is directlyrelated to pressure

Conservation of energy(Energy: the capacity for doing work)


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Motion effects on circulation (3)

• Circulation

Conservation of angular momentum

Kutta-Joukowski lift theorem

http://hyperphysics.phy-astr.gsu.edu/


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Stall and Separation!


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What causes Separation?

H i g h e r P r e s s u r e

L o w e

r

P r e s s u r e

H i g h e r P r e s s u r e

L o w e

r

P r e s s u

r e

R e v e r s e d F l o w R e g i o n


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L o w e r P r e

s s u r e


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Camber in Actual Airfoils


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Factors that affect Lift• Air• Mass, viscosity, compressibility

• Motion

• velocity and inclination to flow

• Object• Shape and size

Governed by Newton’s laws and Bernouli’s equation


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Separation!• Pressure

TextLow pressure

Low pressure


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Aside: Boundary Layer


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Friction Drag• Turbulence vs. Laminar Boundary Layers

Recrit

Transition

t0 t1

Viscosity


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Drag• Pressure / Form Drag

• Delayed separation in Turbulent flow (less drag)

• Early separation in laminar flow (more drag)

• Friction Drag

• Low for Laminar flows

• High for turbulent flows


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Reynolds Number

• Predication of laminarvs. turbulent flow

• Defines dynamicsimilarity

Inertia force

Viscous force

Re =Viscosity

Fluid Velocity *Length=

Blood flow in brain: ~100Blood flow in aorta: ~1000

Typical pitch in Major League Baseball:200,000Person swimming: 4000,000Blue Whale: 300,000,000A large ship: 5000,000,000


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Trailing edge vertices


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Downwash Introduces Drag

Flow from the lower side (higher pressure) wants to ‘leak’ to the low

pressure side of the wing.

Starts from wing tip

Vorticity from trailing edge


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Vorticity


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Lifting Body Problems


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Ramifications of Downwash

Introduce dragReduce lift


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Aspect Ratio


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Oswarld’s efficiency


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Lift Distribution• For level flight, minimum induced drag occur

when the lift distribution is elliptical

• Either the wing shape is elliptical, or

• The incidence angle produces an ellipticallift


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Other 3D Effects


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• Dave Willis: “so what?! we got all this lift /drag etc. Garbage!”


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FastAero Bat flight simulation, Dave Willis et al.


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Tatjana Hubel et al.


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Conclusion• Physical conservation principles to gain

insights into fluid flow

• Trailing edge vortices play important role

• Reduces lift and increases drag

• Unsteady effects are complicated due tovorticity distributions and added massacceleration effects.


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Acknowledgment• David Willis (U Ma. Lowell)

• for his slides from last year and many

pretty pictures

• Tatjana Hubel (Brown)• for her suggestions on this presentation


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Aerodynamics John Chen

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