Quote:
Originally Posted by RedDevil
Theory
- 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.
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I don't doubt your observations and I'm mulling over why I think you're experiencing the negative MPG.
However, I think you have turbulent and laminar flow flip-flopped. Laminar flow will separate early while the turbulent air tends to 'stick' a little longer. Its hard to think about and conceptualize... [been sitting here backspacing for about 5 minutes trying to think of a good way to explain it]
The usual example is the golf ball but I don't like that. With turbulent air, the momentum is broken and skin drag increases. The particles have vectors in all directions which means SOME of them have vectors in the direction we WANT it to go. In laminar flow, the air is saying "No sir, me and my buddies are going this way."... However since they are all going in that direction, there is less air bouncing off of the surface causing less skin friction. Finesse is needed to keep laminar flow as long as needed and then trip the flow to turbulent so it will stick to the converging tail.
Vortices are such an interesting subject matter. The low pressure region in the center wants to suck the air inward, while the centrifugal force wants to pull it apart... while the whole time, the shear forces within caused by speed differentials cause eddies and reverse vortices (can you tell I geek over vortices?
). Not sure why, and it's a high level subject, but the center of the vortex DOES have an affinity to a surface. I doubt this is evident in the dirty tails of our vehicles...
Ok, sorry so long. Have a great night!
-Ryan