Drag

Drag Characteristics

Drag is a force that opposes the motion of a body through a fluid (such as air or water). It is a critical factor in the design and performance of vehicles, aircraft, and other objects moving through fluids. Here are the key characteristics and concepts related to drag:

Types of Drag

• Pressure Drag (Form Drag): Caused by the pressure differential between the front and rear of the body. It is influenced by the shape of the body.
• Friction Drag (Skin Friction): Caused by the frictional forces between the fluid and the surface of the body. It is influenced by the surface roughness and the viscosity of the fluid.
• Induced Drag: Occurs in lifting bodies (such as wings) due to the creation of lift. It is associated with the vortices generated at the wingtips.
• Wave Drag: Occurs in high-speed flows, especially near the speed of sound, due to the formation of shock waves.

Drag Coefficient (C_D)

Drag Coefficient (C_D): A dimensionless number that quantifies the drag force relative to the dynamic pressure and the reference area. It is defined as:

$$C_D = \frac{D}{0.5 \rho U^2 A}$$

Where:

• D: Drag force
• ρ: Fluid density
• U: Flow velocity
• A: Reference area

Factors Affecting Drag

• Shape of the Body: Streamlined shapes reduce pressure drag, while blunt shapes increase it.
• Surface Roughness: Smoother surfaces reduce friction drag.
• Flow Velocity: Higher velocities increase both pressure and friction drag.
• Reynolds Number (Re): Indicates the flow regime (laminar or turbulent) and affects the drag characteristics.

Flow Separation

• Flow Separation: Occurs when the boundary layer detaches from the surface of the body, leading to an increase in pressure drag. It is influenced by the shape of the body and the Reynolds number.

Drag Reduction Techniques

• Streamlining: Designing bodies with smooth, tapered shapes to minimize flow separation and reduce pressure drag.
• Surface Treatments: Using smooth coatings or riblets to reduce friction drag.
• Active Flow Control: Techniques such as boundary layer suction or blowing to delay flow separation.

Example: Drag on a Sphere

When fluid flows over a sphere, the drag characteristics can be analyzed using the following steps:

1. Determine the Reynolds Number: Calculate the Reynolds number based on the flow conditions.
2. Identify the Flow Regime: Determine whether the flow is laminar, turbulent, or in transition.
3. Calculate Drag Coefficient: Use empirical correlations or experimental data to estimate the drag coefficient.
4. Calculate Drag Force: Use the drag coefficient to calculate the drag force acting on the sphere.