Originally Posted by ryannoe
RedDevil - unless you have introduced a new surface, I have my doubts that it is laminar at your rear bumper. Laminar flow is more likely to separate from the vehicle than turbulent flow (which I suspect is what you have). I'd like to hear what happens. Don't forget to post it for us!
To be precise: my point is that the airflow on the side bumper is
usually turbulent unless that bumper is hit by strong sidewind.
I don't know for sure as I have no way to see what actually happens.
I just deduced what happens from the bits of information I have:
- The 2Gen Insight suffers from side wind effects; under some conditions I can feel a constant but erratical series of jerks, esp. when moving slightly with the wind instead of against it.
- Fuel consumption is much higher on side wind impact than when driving straight against the wind. At 6 Bft I noticed the FC went up 30% compared to driving straight against it driving on a gently sweeping highway in a completely flat and almost treeless plane, steady pace etc.
- Airflow on a plate that leans into the wind tends to be laminar.
- Airflow moving along a plate is unstable and tends to get turbulent.
- Rounded edges can make an airflow passing along it follow the curve. This is the Coanda effect. A laminar flow may follow the curve completely.
- The Coanda effect is inherently instable and tends to enlarge any instabilities in the airflow until the turbulence allows it to break loose.
The radius of the bend is strongly related to the instability; if it is too sharp the flow cannot follow it and breaks loose.
- Turbulent air won't follow an inward curve but rather move on in the same direction.
- Vortices are more or less stable by themselves and folllow a curved edge, especially the part of the vortex that is already moving in that direction.
That may explain why vortices help in the Mitsubishi case; it allows part of the air to curve an edge too sharp for laminar or turbulent air to follow.
When hit by a strong sidewind the airflow curves around the edge of the rear bumper, creating a low pressure area at the side and rear of the bumper, pulling it back.
Straight against the wind the air is turbulent and ignores the corner when it moves by.
In between the air will sometimes curve round and sometimes go straight. Both conditions are unstable and likely to induce the other. This produces the shaking sensation.
Breaking or preventing the Coanda effect on the corner should enhance stability and yield better MPG.
One way is to sharpen the trailing edge (Prius II, Chevy Volt etc.)
Another is inducing turbulence by adding a ridge or crease or sticking on zigzag tape just before the corner. The latter is my plan.
Nobody that I know of has done this, however Cobb has removed his rear bumper and claims that improves side wind stability and MPG.
The uncovered bodywork falls back slightly with a sharp cliff and has sharp edges.
This is no way as much proof as I would like, but it does fit.
Both my car and me are quite new to this game. There are some more mods that I want to do and some testing gear to get and use. Especially the rear wheel well covers, as I suspect that will decrease turbulence and so increase the effect of the edge curve cure.
Once that is sorted out and the conditions are right (I need strong and stable southwestern wind, like 6 or 7 Bft) then sure I will get out the word.
Meanwhile, if there's anyone out there with a wind tunnel and an Insight, I'd be over the moon if you could park it slightly askew in that tunnel and test it with and without zigzag tape, ridges and the like. Well, who knows?