Originally Posted by kennybobby
What does all this mean? It looks like tufts are shaking and moving in the non-vg portion. In vg they are jumping up and shaking more. Don't you want smooth laminar flow at or near the surface? Would turbulent flow at the surface be a "good thing"? Do you not want a boundary layer at the surface? Please explain.
Good questions. I'm having a hard time with flowing one thought to another in this explanation, I'm sorry. Please try to follow as best you can.
Thought 1: The pressure gradient across a boundary layer is constant (from the surface to the top of the boundary layer). That means the pressure at the surface is equal to the static pressure of the streamtube adjacent at that location.
Thought 2: The flow at the surface NOT behind a VG is NOT flowing at the desired 60+ mph. I say 60+ because 60 should be the minimum velocity unless you are pulling the flow along with you. If the flow behind your vehicle is slower than freestream, you are grabbing that air and sucking it along with you (we don't want that).
Thought 3: Think back to thought 1. Pressure at the surface is equal to pressure at the streamtube adjacent. If we have pulled the flow down to the surface to test velocity, we have also pulled the streamtube to the surface. This has a positive effect by either: stretching the streamtube at that location (top location stays put and the bottom comes down) which would increase static pressure, or bring the entire streamtube down which will stretch adjacent streamtubes which will have relatively equivalent effect.
Thought 4: BAD - More skin friction... but should be relatively small.
Bringing it together:
Pulling the air down to the surface to as close to the freestream will increase the pressure at the surface. This is good for two reasons, it increases the pressure on the glass, but it also increases the pressure in the separated wake - a synergy effect.
MAKESHIFT VG DATA:
FPD change 0.6405 to 0.6286 (Cd change of 0.268 to 0.263, not TOO much) with makeshift VGs that were "eyeball'd" relative to tuft streamlines. Size, shape, location, and AoA were ALL eyeballed with a possible positive difference being realized. However please note these numbers are not validated with statistical analysis. I have some work to do.