Quote:
Originally Posted by ConnClark
A thicker boundary layer is NOT more stable around a curve.
You don't trip the boundary layer to thicken it. You trip it to re-energize it. Also trucks and vans have long flat surfaces along the roof in sides for only one reason, to increase cargo volume. Trucks and vans also have a reason for a sharp taper on the end, to decrease length. Do not confuse design features to curtail cost and fit in size regulations as rules for aerodynamics.
|
As said, new to all of this and just trying to suck it all in.
As this is all relative, with regard to a moving vehicle or wing section there are two boundaries to the layer, that on the surface which is stationary and that adjacent to the freestream, which is at 99% of freestream velocity.
In the Laminar condition, it is thin and there is a consistant gradient of change in velocity, in the turbulent boundary layer, it is thicker and there is pressure and velocity variations throughout the layer, but still generally follows the velocity gradient.
Laminar is low pressure high velocity whereas turbulent is higher pressure lower velocity, when the slope changes after the point of maximum camber, the laminar flow seperates and tumbles into major turbulance, it does not have the volume of air or the pressure to fill the gap created, so the freestream tumbles in to fill the void. In the same circumstance the turbulent boundary layer has a higher pressure and greater volume of air and hence can maintain contact with the surface for a longer distance after the point of max camber. This effect I imagine would allow it to maintain attachment for a longer distance around a curve.
I'm still trying to understand the distinction that people are trying to place on the energizing/re energizing term, when you mix freestream with the slower moving surface air, it creater turbulence, it is no longer freestream and the boundary layer becomes thicker.
For a short distance you will have some higher velocities closer to the surface, but the layer will reorganize itself quickly to it's normal state.
What is the difference between a 2" boundary layer at the end of a long vehicle and a 2" boundary layer at the end of a short vehicle that has been tripped artificially?
I wasn't suggesting that semi trucks were built long for the purpose of a thicker boundary layer, just that because they are longer the boundary layer is thicker, hence aerodynamic principles need to be applied in a different way.