Turbulence - Friend or Foe? |Here's a lot of Aero info, not spread in several threads

[GasSavers_RoadWarrior]

Originally Posted by sonyhome

If you have it handy, link it... We gotta be careful planes use those to attach flow and avoid bubbles that disrupt plane stability, and not to improve MPG. I dunno if one implies the other? BTW, added http://aerodyn.org/Drag link to my previous post.

Yes, sometimes you've gotta think upside down and backwards on what applies to planes to apply it to cars, because you don't really want high lift.

Unfortunately what I thought I posted over there seems to have been eaten by one of the hacks or server crashes.

Anyway, I think it was the mentioning of use of turbulation devices to pre-empt flow detachment and formation of large vortices, and to cause small vortices that would entrain the bulk airflow round sharper angles than it would normally follow without separation. I determined that a minimum 1/4 inch feature was needed at the typical Reynolds numbers we see at highway speed in a typical size car, for it to have significant effect on airflow. The ideal spot for such turbulation devices is supposed to be in the rear 3rd of a body. I did some rough back of envelope figuring that seemed to suggest that about 2 inches of surface is the minimum that flow will reattach to at "our" reynolds numbers, so placing of turbulation strips should be a minimum two inches from changes in curvature, unless you're using them to separate flow.

Bear in mind that trucks by virtue of length are in a different calss of Reynolds numbers than cars, so that devices which work on trucks may need to be scaled differently to be effective on cars and vice verse i.e. quarter inch strips won't do crap on a semi-trailer, they may be getting towards marginal on full size extended vans. Also they would have to be placed further than 2 inch from the trailing edge.

The most effect is seen apparently with groups of 3 strips spaced their own height apart.

However, I have come to consider a slightly different application approach more theoretically desirable. Because the angle at which the air departs the vehicle body determines the "angle of attack" as it were of the body shape as an airfoil, you want to get that angle as shallow as possible, near horizontal. There is of course though a balance between doing this and adding too much base drag. One needs to keep the angle of attack as neutral or negative as possible otherwise the lift forces are actually pulling backwards on the body, giving induced drag. Keeping the "trailing edge" higher than the "leading edge" is one reason why it's beneficial to have a clean edge separation off a trunk or stub-trunk at about half the height of the vehicle at the rear. However, this doesn't fix everything because of a outflow from the roof centerline that will happen due to air at higher pressure trying to take the easiest escape route rather than going all the way over the top. This leads to similar phenomena as tip vortices on aircraft wings. This flow will attempt to wrap over the sides of the vehicle, and will cause a net downflow effect on air departing the vehicle at the rear. This is actually not good, Kammbacks get talked up, but to be truly efficient they should kick the air back up before separation. There is a balance however between where base drag reduction is good and where induced drag becomes a problem.

Anyway..... turbulation strips could be doing 3 things at once... Placed at a shallow angle to the airflow, 15-30 degrees in groups of 3 they would i) act to keep flow attached as turbulators, ii) enhanced lower drag action by virtue of delta wing swirl effect exploited by other turbulation devices, and iii) direct airflow away from the vehicle with an upward component at the rear, helping to kill downwash from the "tip vortex" effect and reduce induced drag. These would be most effective in this manner on the lower half of the vehicle on the sides. With vehicles like vans I think you'd be better off having them straight on the top half of the side and angled towards the centerline on the roof.