Considering a vehicle traveling at a known velocity, a wake is formed behind it. Into this, vortices can be seen rotation on both clockwise and anticlockwise directions. The wake circulation is given by:

Wake circulation equaiton.

However analysing the back portion of car, more precisely, where the flow separates from the body:

Big vortices formed after separation has the same size as two the boundary layer thickness at separation point.

One assumption which can be made is that the big vortices formed after separation and far from the body have the same order of magnitude as two times the boundary laywer thicknesss at the separation point. In addition the circulation (Tau) has the order of magnitude similar to two times pi and a path “a” and vS. This occur because the velocity at separation point “SP” has an order of magnitude proportional to the circulation. Hence, it can be written that:

The size of the vortex are strictly related to the magnitude order of the boundary layer thickness. Thus it is allowed to write that:

The size of the vortex is proportional to two time the boundary layer thickness at the separation point SP.

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