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My understanding is that with the sphere and the trip wire, the trip wire trips the boundary layer into turbulence (energizes it), and as a result, the boundary layer can sustain a steeper angle than the prius-like 17 degrees or so before separating. For the same length car, you might have a more effective boattail that way, since the area of suction would be smaller. i.e. On one hand (prius style), you have a larger area of suction behind the vehicle, and on the more extreme rake + VGs or turbulator tape or whatever you use to trip the boundary layer, you have less area of suction but more energy lost to trip the boundary layer. Do you get what I'm saying? Think VW beetle but with boundary layer tripping somewhere on top, and a small wing near the very bottom, to get the air leaving in a horizontal direction. |
Mighty, I had an interesting conversation with the South East ASME district leader (at least, afterwards - I was told that was his position) - and in speaking with him with respect to HPV fairings, it was recommended (among other things) to explore completely turbulent flow - tripping with some sort of speed bump on the leading edge...
In the next couple months, we'll be modeling that to see what happens - as turbulence is chaotic, there's good chances that it will help, hurt and/or do nothing :p But for us, we're not sure how much of an effect it will have at our relative low velocities :p On the polar opposite side of things, it was also recommended that we check out deHavilland slots to maintain laminar flow (and provide necessary ventilation). deHavilland slots basically suck air into the foil just before the transition point - I believe this basically "resets" the BL, or at least decreases the thickness. ----- Any good ideas on how to trip the BL? I was thinking of just experimenting with a triangular shape -like a FedEx Triangle mailer sort of shape. Any suggestions? |
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I suspect that a lot of hatchbacks where the hatch is roughly 30 degrees or so might conceivably have good airflow with the addition of such turbulator tape. However, I have to wonder why the front windshield wipers aren't tripping the boundary layer. Or maybe the energy they add is too diffuse by the time the air gets to the rear of the car. Or the top of the windshield for that matter. |
Thank you for the nice write-up. It's interesting for me to see how N(Re), viscosity, turbulence, etc. apply to aerodynamics. In my field, we mostly think of turbulence in terms of how it affects mixing in a pipe (i.e. flushing out "dead legs") and heat transfer. Also we only care about Newtonian fluids for the most part.
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To summarise, it seems like turbulators help when there is unwanted airflow separation. A successfull turbulator is placed a certain distance in front of the point of separation.
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As for a boundary layer trip, I think any shape that does not promote laminar flow over it would be effective. I think the more significant question is the height of the trip from the surface. How thick does the trip layer need to be for best efficiency? There is obviously a sweet spot where too much depth unnecessarily accelerates more than just boundary layer air. It will remain attached further back, but it ends up being wasted energy. Likewise, too little depth won't accelerate the boundary layer enough and it will detach before the trailing edge. I suspect there is a formula that can explain this relationship fairly accurately - or at least ballpark a decent starting point based on some general assumptions. Intuitively, I 'get it'. But mathmatically, er.... |
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I suspect that this angle will vary depending on how much the boundary layer is energized (turbulator height) and how far back the boundary layer was energized. |
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And with something like the Lancer, they could have achieved the same thing with a small ridge on the roof of the car somewhere. |
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