Thread: Biz Intro
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Old 12-10-2008, 10:17 PM   #2 (permalink)
Mech & Aero Engineer
Join Date: May 2008
Location: Garland, Texas
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I took at look at their video. Sorry, but I smell some snake oil. First, allow me to explain how a vortex generator is supposed to reduce drag: a turbulent boundary layer will stay attached to a surface beyond the point where a laminar layer will detach. A wake is flow that has detached and is tumbling. Let's say you have a 25 degree downward slope on the back of your car. If the boundary layer upstream of this downward surface turn is laminar (consisting of non-mixing layers) it will detach a short distance from the turn due to an adverse pressure gradient and leave a fairly tall wake. A turbulent boundary layer will stay attached further along the slope and perhaps make it all the way to the back edge, leaving a shorter wake. So for this case, turbulence generators upstream of the downslope would probably reduce the form drag by reducing the area of the low pressure wake. Skin friction drag actually goes up with a turbulent boundary layer - but form drag is the dominant factor for cars. So turbulence in the flow can increase the degree to which the flow will stay attached to the surface with an adverse pressure gradient, but it is not boundary layer superglue! If there is a 90 degree turn along the surface (like the back edge of a pick-up truck cab) there is no way the flow is going to stay attached no matter how much turbulent mixing there is. I would doubt the effectiveness of a vortex generator for any surface turn greater than 30 degrees. Rounding all the square edges to a four-inch radius for a bus or truck (basic streamlining) I believe would be a much better approach than trying any vortex voodoo. A four percent decrease is probably within the error band of any mileage measurements. The tow tank tests done at Texas Tech on heavy truck models looked nothing like either one of aeroserve's flow depictions. Inflatable boat-tails (or Kamm backs) on the back end of trucks and buses do work, but they have not gained acceptance for some reason (the idea has been around since the 1930's).
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