Isentropic means there's no effects from friction, boundary layer, or turbulence. Airflow through a fixed duct is mostly isentropic, airflow through a duct with a changing wall IE the floor of a car and the road, the flow is NOT isentropic, thus isentropic assumptions are not correct. The road surface is rough textured and the sun shining on it creates a boundary layer of air over the road. All of these factors increase the turbulence the air experiences in the duct. Furthermore the turbulence of the tire is added to this turbulence.