While pressure is the force acting perpendicular to the surface, wall shear stress is the force exerted by the air as it slides across the surface, parallel to it.

The following images contain some details:

• regarding the separation on the wheels, you can observe the effect of the flow behind them. There is separation at the rear, resulting in low friction,

• you can see the effect of the mirror, which influences the flow around the car's body,

• there is stagnation at the front, which indicates low surface friction.

Drag, or air resistance, is the force exerted by the wind on an object in the direction of the airflow. It consists of pressure drag (acting perpendicular to the surface) and friction drag (slinding over the surface). In most cases, pressure drag is dominant.

The drag coefficient is a dimensionless quantity that represents the aerodynamic resistance of an object moving through a fluid. It is defined as follows:

The image below illustrates typical C_D values (from NASA and Wikipedia). More streamlined objects will have a low C_D, while less streamlined objects will have a high C_D.

The C_D provides a measure of the aerodynamic efficiency of an object. By approximation, the actual drag force F_D on the object, as well as the power P required to propel it, vary as a function of the wind speed according the following formulas:

Underpressure zones are areas where the pressure is lower than that of the surrounding air. These zones are often caused by abrupt changes in shape, especially those resulting from sudden changes in cross-sectional or surface geometry. Smoothing these areas can help reduce drag.

The following images provide insight into which features of your object are causing drag by displaying 'clouds' of low pressure.

The red clouds here indicate the sources of drag, and technically, this is an iso-surface for the total pressure coefficient with a value of 0. Anything inside this volume represents a low-pressure bubble that causes drag, generating resistance and slowing your car down.