Short Answer:
The difference between pressure drag and friction drag lies in how they are created. Pressure drag is caused by the shape of the object and the pressure difference between its front and back, while friction drag is caused by the fluid’s viscosity rubbing against the object’s surface.
Pressure drag is more important in blunt bodies where flow separates easily, and friction drag is more dominant in streamlined or smooth surfaces. Both types of drag act against the motion of the object and must be considered while designing civil structures exposed to fluid flow.
Detailed Explanation:
Difference Between Pressure Drag and Friction Drag
In fluid mechanics, drag is the resistance a fluid exerts on an object moving through it or when a fluid flows around a stationary object. This total drag is mainly divided into two types—pressure drag and friction drag. Understanding the difference between them is important for engineers to reduce energy losses and ensure safety and performance in designs.
Pressure Drag
Pressure drag, also known as form drag, occurs due to the difference in pressure between the front and back of a body placed in a fluid flow. When fluid hits the front of the object, it creates high pressure. As the fluid moves around the object, it may separate from the surface, especially in blunt or non-streamlined shapes. This separation creates a low-pressure wake behind the object.
The greater the pressure difference between the front and back, the more the object is pulled backward by the fluid—this is pressure drag.
Key Points of Pressure Drag:
- Caused mainly by flow separation and wake formation.
- Depends on shape of the object (more in blunt bodies like flat plates, cylinders).
- More dominant at high velocities where flow becomes turbulent.
- Affects structures like bridge piers, poles, towers, and chimneys.
Engineers try to reduce pressure drag by using streamlined designs to keep the flow attached to the surface for longer and avoid early separation.
Friction Drag
Friction drag, also known as skin friction, is caused by the viscosity of the fluid as it flows along the surface of the body. As fluid particles touch the surface, they experience resistance due to adhesion and shear forces. This slows down the fluid in a thin layer called the boundary layer.
The smoother and longer the surface, the more the fluid rubs against it, and hence more friction drag is produced. But friction drag is generally smaller than pressure drag in blunt objects.
Key Points of Friction Drag:
- Caused by viscous shear forces near the object’s surface.
- More noticeable in streamlined shapes, like airfoils or smooth pipes.
- Affects long surfaces more than short or blunt ones.
- Increases with surface roughness and fluid viscosity.
To reduce friction drag, surfaces are made smooth and polished, and sometimes laminar flow is maintained for as long as possible before it turns turbulent.
Summary of the Difference
- Origin: Pressure drag is due to pressure difference; friction drag is due to shear stress.
- Dominance: Pressure drag dominates in blunt bodies; friction drag dominates in streamlined shapes.
- Dependence: Pressure drag depends on shape; friction drag depends on surface and viscosity.
Both types of drag act in the opposite direction to fluid motion and are considered while designing efficient, safe structures in civil engineering.
Conclusion:
Pressure drag and friction drag are two different components of total fluid resistance. Pressure drag comes from the pressure difference around an object due to shape and flow separation, while friction drag results from the fluid’s viscosity rubbing against the surface. Knowing the difference helps engineers design shapes and surfaces that minimize resistance, improve stability, and enhance performance in structures exposed to wind, water, or any moving fluid.