Vehicle Aerodynamics Flashcards
What are the Pros and Cons of Hatchbacks?
Hatchback: Large separation (at the roof line) but maximum internal volume (good for short cars). Not the best drag coefficient because of the large separation.
What are the Pros and Cons of Saloons?
Saloon: Attached flow along rear window and the boot lid. Separation at rear of boot lid. Can have vortices (but not as strong as fastback). Decent use of space through classical boot. If well designed this can give very low drag values.
What are the Pros and Cons of Fast Backs?
Fastback: Attached flow along the rear window but possibly with strong vortices. Not good for volume (boot space), but good to excellent drag coefficient. Can have a lot of lift though, which may require spoilers in performance cars.
What is optimal fastback rear angle
20 degrees, above 30 degrees flow becomes detached
What aerodynamic detailing can be applied to the rear of a hatchback car to improve the drag?
There is usually a degree of boattailing to reduce the base area and recover some pressure. This means that the roofline drops slightly before the rear window and that the sides of the car are brought in. In addition a distinct separation lip can help to slightly improve the base pressure (and thus reduce drag):
WHat does boat tailing at the side do?
Increases base pressure and reduces base area
Explain the use of a lip at the rear of the car
Increase in pressure offset increased base area
If sepration cant be avoided best to sart it with a well defiined edge
How is seperation prevented on the A pillars and on the hood
On the A pillars seperation is prevented by ensuring a smoth surface without steps and sufficiently large radius of curvature
On the hood seperation is prevented by have an aerodymaically and sufficiently rounded nose shape
Expalin the flow in the underbody
Significant proportion of drag
least streamlined area
could introudce cladding but this complicates maintenacne may introduce overheating and incurs additional cost
one way is to decrease flow spped in the underbody area
this can be done with the use of a spoiler
also move the stagnation point at front downwards by careful design of the nose
How can wheel drag be reduced
drag is proportional to exposed area
thinner wheels produce less drag
use of side skirts or small front spoilers underneath the car
Explain windshiel angle
Modern cars are swept considerably
Seperation bubble does not contribute much to drag
most modern cars keep wipers behind a deflector confirming seperation bubble less of a problem than wiper
Explain the effect of gurney flaps
the device is a simple right angle strip that is added to the trailing edge of a wing
the effect is to increase effective camber
as the flap also creates a thick trailing edge (relaxes kutta condition) overall aerodynamics are realtively complex and quite subtle
however main effect is to increase lift
Explain crosswind effects
On windy days, flow direction relative to the car is not always head on.
This produces an asymetric flow over the car leading to considerable sideforce and yawning moments.
Streamlined cars suffer more from this problem since they act like aerofoils
occcurence of flow seperation which increases the drag
Explain the effect of tail height
When roof height an car length are kept constant varying the height of the boot changes the angle of the rear window
by increaseng the height this angle is minimum, which is benificail for the flow along the window but at the same time the base area is increased (which increases drag)
therefore there is an optimum
