Quote:
Originally Posted by freebeard
Viscid/inviscid isn't black and white, it's a gradient.
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I don't think that is right. No air is inviscid. None. Inviscid fluid is imaginary - it doesn't exist.
Inviscid airflow is used only to greatly simplify the model of airflow that can be used. That is, it's a 'model trick'.
Wikipedia puts it well (I can give more formal references if anyone wishes. Anderson's books on aerodynamics are good in this regard):
Inviscid flow is the flow of an inviscid fluid, in which the viscosity of the fluid is equal to zero. When viscous forces are neglected, such as the case of inviscid flow, the Navier-Stokes equation can be simplified to a form known as the Euler equation. Using the Euler equation, many fluid dynamics problems involving low viscosity are easily solved, however, the assumed negligible viscosity is no longer valid in the region of fluid near a solid boundary.
In other words, if we're interested in pressures acting on the surface of the body (ie a solid boundary) - and with car aero, that is
all that we are interested in - then using inviscid flow models is of no / very little use to us.