Question

In flow over blunt bodies such as a cylinder, how does the pressure drag differ from the friction drag?

Short answer

#tag_title# Short Answer #tag_content# The main difference between pressure drag and friction drag in the context of flow over blunt bodies is that pressure drag is typically more substantial for blunt bodies due to the early flow separation and larger pressure difference between the front and rear surfaces. In contrast, friction drag is less significant in blunt bodies, as pressure drag is the dominant force in these scenarios.

Step-by-step solution

Understand Pressure Drag

Pressure drag, also known as form drag, is the drag force resulting from the pressure differences on the front and rear surfaces of an object moving through a fluid. These pressure differences occur due to the separation of flow around the object, which creates a downstream wake of lower pressure. This wake produces a force on the object in the opposite direction of its motion, contributing to the overall drag.

Understand Friction Drag

Friction drag, also known as skin friction or viscous drag, is the force caused by the viscous resistance of the fluid against the object's surface. It is due to the shear stress generated by the fluid particles sliding past the object's surface. Friction drag is dependent on factors such as the fluid's viscosity, the object's surface roughness, and its shape.

Difference in Pressure Drag for Blunt Bodies

In the case of flow over blunt bodies, such as a cylinder, the pressure drag is significantly higher compared to that of streamlined objects, like an airfoil. This is because the flow separation occurs earlier for blunt bodies, causing a larger wake and a more significant pressure difference between the front and rear surfaces. The result is a higher pressure drag for blunt bodies.

Difference in Friction Drag for Blunt Bodies

The friction drag in flow over blunt bodies might be less pronounced than in streamlined bodies, depending on the specific geometry and flow conditions. However, in many situations, friction drag is relatively smaller in magnitude compared to pressure drag for blunt bodies, because pressure drag is the dominant and more significant force in these scenarios.

Compare Pressure Drag and Friction Drag for Blunt Bodies

For a blunt body like a cylinder, the pressure drag is typically more substantial than the friction drag. This is due to the flow separation and formation of a large low-pressure wake downstream, leading to a greater pressure difference between the front and rear surfaces of the object. In contrast, friction drag is usually less significant in blunt bodies compared to streamlined objects, where minimizing pressure drag is a primary design goal.