Aerodynamic Drag - Introduction

Introduction

The pressure distribution over the body surface exerts normal forces which, summed and projected into the freestream direction, represent the drag force due to pressure . The nature of these normal forces combines shock wave effects, vortex system generation effects and wake viscous mechanisms all together.

When the viscosity effect over the pressure distribution is considered separately, the remaining drag force is called pressure (or form) drag. In the absence of viscosity, the pressure forces on the vehicle cancel each other and, hence, the drag is zero. Pressure drag is the dominant component in the case of vehicles with regions of separated flow, in which the pressure recovery is fairly ineffective.

The friction drag force, which is a tangential force on the aircraft surface, depends substantially on boundary layer configuration and viscosity. The calculated friction drag utilizes the x-projection of the viscous stress tensor evaluated on each discretized body surface.

The sum of friction drag and pressure (form) drag is called viscous drag. This drag component takes into account the influence of viscosity. In a thermodynamic perspective, viscous effects represent irreversible phenomena and, therefore, they create entropy. The calculated viscous drag use entropy changes to accurately predict the drag force.

When the airplane produces lift, another drag component comes in. Induced drag, symbolized, comes about due to a modification on the pressure distribution due to the trailing vortex system that accompanies the lift production. Induced drag tends to be the most important component for airplanes during take-off or landing flight. Other drag component, namely wave drag, comes about from shock waves in transonic and supersonic flight speeds. The shock waves induce changes in the boundary layer and pressure distribution over the body surface. It is worth noting that not only viscous effects but also shock waves induce irreversible phenomena and, as a consequence, they can be measured through entropy changes along the domain as well. The figure below is a summary of the various aspects previously discussed.