Aerodynamics Flashcards
Principals of fluid dynamics are…
Central to the understanding of how objects behave in air and water.
- gases and liquids are fluids with similar mechanical behaviours
- the effects of a fluid depend on both the flow of the fluid and the object in the fluid
- the relative velocity of the object with respect to the fluid, viscosity of the fluid and the shape/form of the object influence the magnitude of the acting forces
Laminar flow…
Is characterised by smooth, parallel layers of fluid.
Turbulent flow…
Is characterised by mixing of adjacent fluid layers.
Effects of turbulence?
- Turbulent flow takes energy away from an object (all moving objects possess kinetic energy).
- Caused by pressure differential at ends of an object
- Where fluid and surface meet there is high pressure
- At opposite end of object there is often turbulence and therefore low pressure
- Force vector is directed from area of high pressure to area of low pressure. This is drag force
Form drag (profile drag)
Major contributor to overall drag during most human and projectile motions
Form of object, frontal surface area and velocity affect drag forces:
Fd = kAv^2
Flow divergence
By shaping objects with a longer leading edge, fluid particles diverge earlier and strike the objects surface at a larger angle of incidence (angle relative to object surface)
Dimpling on a golf ball…
Traps air molecules to allow an accumulation of air at the front of the ball; oncoming air molecules to diverge from laminar flow earlier to all reduce drag compared to when air diverges nearer the ball surface or after a collision with it.
Dimpling also makes the boundary layer more turbulent, increasing its velocity and thus decreasing the pressure. This keeps air in contact for longer, reducing magnitude of the vacuum behind object, therefore reducing turbulence.
Adding a tapered tail to an object <> …
Promotes laminar flow across the object when compared to an object without a tail.
Surface drag
Also called skin friction or viscous drag
Layer of fluid particles adjacent to surface slowed because of shear stress.
Next adjacent layer affected by particles adjacent to the surface, etc.
Area of affected particles called boundary layer
Force of body on fluid creates reaction force opposing motion
Surface drag reduced by?
Reducing the amount of fluid-catching projections on an objects surface
Decreasing area of surface in contact with air
What is the Bernoulli Principal?
Regions of relative high-velocity flow are associated with regions of relative low pressure, opposite is true for regions of low-velocity flow
Early separation from object creates…
vacuum and therefore turbulence
More turbulent boundary layer:
Increased particle speed = lower pressure
Dimpling makes the boundary layer more turbulent, increasing its velocity and thus decreasing pressure. Keeps air in contact longer (later flow separation), reducing magnitude of the vacuum and therefore reducing turbulence.
Lift
Explained with Bernoulli physics
Faster movement over wing causes lower pressure than under wing, therefore lift force upward
Lift
Explained with Newton physics
Downward momentum of air requires upward momentum of another mass; equal and opposite reaction
