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

[Mighty Mira]

Originally Posted by trebuchet03

I'm not sure I follow that? Why would that make the wake size smaller? Intuitively, I'm thinking of a steep angled rear end compared to a gently sloping rear end - the steep one has a pretty bad wake issue... Maybe I'm just not understanding what you said?

I'm referring to the area of suction behind (the area below boundary layer separation) as "wake". Probably the wrong choice of words.

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.

[trebuchet03]

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 But for us, we're not sure how much of an effect it will have at our relative low velocities

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.


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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?

[Mighty Mira]

Originally Posted by trebuchet03

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?

Go with some sort of thick tape, and cut it in a zigzag pattern as per here. I'm guessing that a material store will have these sorts of scissors. As to where to find 1mm thick tape, not sure exactly.

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.

[Jim Dunlop]

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.

[DRW]

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.

[trebuchet03]

Originally Posted by DRW

To summarize, it seems like turbulators help when there is unwanted airflow separation. A successful turbulator is placed a certain distance in front of the point of separation.

Technically - they should be placed just before the transition. So it's correct to say before separation, but it's the zone before separation you're attempting to fix (and thus prevent separation).

[Snax]

Originally Posted by trebuchet03

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.

This sort of thing can be easily seen in commercial airliner flaps. At full extension, they essentially become multiple airfoils working together as one virtual airfoil. The same thing is seen in rear wings of racecars like F1.

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....

[Mighty Mira]

Originally Posted by DRW

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.

From what I can see (from the picture of the sphere with boundary layer tripping), there is still a minimum angle that must be maintained or the flow will still separate. i.e., no turbulator is going to help the back of a semi (or similar shape). The boundary layer on the sphere separated at 45 degrees. Note that on the back of the lancer the flow still appears to be separating. (Although that's trusting that the CFD is a true picture, which it isn't necessarily.)

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.

[trebuchet03]

Originally Posted by Snax

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?

All good questions - we intend to investigate

Originally Posted by Snax

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....

I am not aware of equations for such a thing - there very well could be though. The reason, Turbulent flow isn't fully understood (especially in a mathematical sense) - Turbulence is a chaotic system. Complicating things more - we're dealing with curved surfaces There are, however, relationships for laminar flow, and fairly accurate equations in the transition zone. But once you get to high Reynolds numbers, small changes drastically change the outcome.

no turbulator is going to help the back of a semi (or similar shape).

Absolutely right At least, there no research that supports this. Really, a vortex generator fixes an aerodynamic flaw in the design. Think of it as a band aid - the cure being a costly redesign or potentially difficult modification that many/most are not willing to do.

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.

Awesome Are you involved with heat exchanger design? Regurgitating class lectures - turbulence enhances heat transfer (higher Re# = better) - I'm sure you knew that already. As for the "N" - is that a reference to the Nusselt number (I imagine that, and perhaps the prandtl number are used often in your field )?

[Mighty Mira]

Originally Posted by trebuchet03

Absolutely right At least, there no research that supports this. Really, a vortex generator fixes an aerodynamic flaw in the design. Think of it as a band aid - the cure being a costly redesign or potentially difficult modification that many/most are not willing to do.

Thinking about it some more... aren't VGs and turbulators separate things? i.e. you'd use VGs if you are generating large vortexes at the corners of the car (e.g. back of a BMW). In something a bit rounder but with a rake of 20 to 45 degrees back, you'd want to trip the boundary layer with a turbulator so as to decrease suction at the back.

And with something like the Lancer, they could have achieved the same thing with a small ridge on the roof of the car somewhere.