Quote:
Originally Posted by jakobnev
aerohead: What's your take on rounded rear ends(think beetle and TT) being worse for Cd that straight and sharp edges? (basically any modern hatch exept the new scirocco (which has a Cd of 0.34)
To me it certainly looks like the round edges direct the air in behind the car and the straight sharp edges let the air flow past the volume or air that moves with the car.
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jakobnev,your's is a good question and also one of the most difficult to answer,so I'm going to let Dr.Hucho explain it as he did in his SAE Paper from 1976 which involved his work at Volkswagen AG.And the caveat is that you should not make generalities about this kind of complex flow.
-------------------------------------------------------------------------- The short answer is that fastbacks and square-backs do well with rounding,notchbacks do not.
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A good fastback will have the top and side flow re-joining at similar angles,velocities,and pressures.No longitudinal vortices will form,the flow will separate at the slowest velocity,highest pressure,into the smallest wake of greatest base pressure and lowest drag.Curvature of the C-Pillar buttress and body sides was demonstrated to be beneficial. The separation point CAN move around a bit,and from Kamm's/Fachsenfeld's research,it's best just to make a sharp chop.
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A square-back typically has top and side flow of similar velocity and pressure and as these flows separate at the back of the body they form a wake of homogenous turbulence,free of longitudinal vorticity,there is no lift.
Curvature along the roof is good.It's okay at the back,although it's ther same situation with separation point,and a chop is preferred
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The notch-back coupe or sedan,if the C-pillar is rounded improperly,can direct the side flow into the highly cambered top flow at the backlite centerline which will create counter-rotating longitudinal vortices of high drag.
-------------------------------------------------------------------------- The original basic Jetta form had over Cd 0.40.
Pushing the notcback up to create a wagon type roof drop the drag to Cd 0.38.
Raising the basic form trunklid 150mm got to Cd 0.37.
Changing to a 10-degree fastback got the same car to Cd 0.34.
At 28-degrees fastback angle,the flow changed back to squareback flow with full wake.