Short Answer:

Pressure drag is the resistance experienced by a body moving through a fluid due to the difference in pressure between its front and rear surfaces. It mainly occurs when the fluid flow separates from the surface of the body, creating a wake or low-pressure region behind it.

In simple words, pressure drag is caused by the uneven pressure distribution around an object. It is more dominant in bluff bodies, where the flow separation is large, and less in streamline bodies, where the fluid flow remains smooth and attached.

Detailed Explanation :

Pressure Drag

Pressure drag, also known as form drag, is a type of aerodynamic or hydrodynamic resistance that occurs when an object moves through a fluid such as air or water. It arises because of the pressure difference between the front and back sides of the object. The front of the object faces high pressure as it pushes through the fluid, while the rear side experiences low pressure due to flow separation and the formation of a wake. This pressure imbalance produces a net force opposite to the motion of the body, known as pressure drag.

When a body moves through a fluid, the fluid must move aside to allow the body to pass. The fluid in contact with the surface slows down due to viscosity, forming a boundary layer. For streamlined shapes, the boundary layer remains attached over most of the surface, resulting in a small wake and less pressure difference. However, in bluff bodies or bodies with abrupt shapes, the boundary layer separates early, forming a large wake region with low pressure. The large difference between the high pressure at the front and low pressure at the rear leads to significant pressure drag.

Mechanism of Pressure Drag Formation

1. Front Pressure Formation:
   As the fluid strikes the front portion of a moving body, it decelerates and builds up a high-pressure zone known as the stagnation point.
2. Flow Separation:
   The fluid moves around the surface of the body. Due to the body’s shape or surface roughness, the flow separates from the surface before reaching the rear end.
3. Wake Region:
   Behind the body, the separated flow forms vortices and turbulent eddies. This creates a low-pressure region called the wake.
4. Pressure Difference:
   The difference between the high pressure at the front and the low pressure in the wake generates a drag force that opposes motion.

Mathematically, pressure drag can be expressed as part of the total drag force:

Here,  is the total drag force,  is the drag coefficient,  is the fluid density,  is the frontal area, and  is the relative velocity of the object. The total drag () is a combination of pressure drag and friction drag. In most bluff bodies, pressure drag forms the major portion of total drag.

Factors Affecting Pressure Drag

1. Shape of the Body:
   The shape of an object is the main factor. Streamlined shapes delay flow separation and reduce pressure drag, while bluff shapes cause early separation and high drag.
2. Velocity of Flow:
   As the velocity increases, the drag increases rapidly since it varies with the square of velocity.
3. Surface Roughness:
   A smoother surface delays flow separation and reduces drag, while rough surfaces cause early separation.
4. Fluid Density and Viscosity:
   Higher fluid density increases drag because more particles exert pressure on the surface.
5. Reynolds Number:
   It determines whether the flow is laminar or turbulent, which affects how and where the separation occurs.

Examples of Pressure Drag

• Flat Plate Perpendicular to Flow: A flat plate facing the flow directly has a large pressure difference between front and rear, creating high pressure drag.
• Cylinder: A circular cylinder in airflow experiences significant pressure drag due to early flow separation and a large wake region.
• Vehicles: Trucks, buses, and trains with blunt shapes experience high pressure drag, reducing speed and fuel efficiency.
• Buildings and Chimneys: These tall structures face large pressure drag due to wind, leading to vibrations and structural stresses.
• Spheres and Balls: Sports balls experience pressure drag, which influences their trajectory and speed during motion.

Methods to Reduce Pressure Drag

1. Streamlining:
   Designing bodies with smooth, tapered shapes helps the fluid flow remain attached for longer, reducing wake size and pressure difference.
2. Flow Control Devices:
   Devices like vortex generators and fins help in controlling flow separation and minimizing turbulence behind the object.
3. Surface Polishing:
   A smoother surface helps the flow adhere longer to the body, reducing drag.
4. Boat Tails and Fairings:
   These are added to vehicles or projectiles to smoothen the rear flow and minimize the wake region.
5. Boundary Layer Control:
   Suction or blowing techniques are sometimes used to manage the boundary layer and delay separation.

Significance of Pressure Drag

Pressure drag plays a major role in designing vehicles, aircraft, ships, and structures exposed to moving fluids. In aerodynamics, reducing pressure drag improves efficiency, speed, and fuel economy. In civil engineering, understanding pressure drag helps in ensuring stability of buildings and bridges against wind forces. It also helps in designing efficient industrial components such as heat exchangers and turbines.

Conclusion:

Pressure drag is a resistive force caused by the pressure difference between the front and rear surfaces of a body moving through a fluid. It mainly results from flow separation and the formation of a low-pressure wake region. Pressure drag is dominant in bluff bodies and can be minimized by streamlining, smooth surface design, and proper flow control techniques. Understanding and reducing pressure drag is essential in achieving better performance, stability, and energy efficiency in various engineering applications.