Drag Coefficents

Laminar Flow over a Flat Plate:

Where the flow is Laminar the Drag Coefficient (CD) can be calculated with the equation.

ReL is the Reynolds Number of the unrestricted flow over the plate.

Once the drag cofficient is found, the following equation can rearranged to find the total drag force (D) acting on the plate:

• ρ is the density of the fluid.

• U is the speed of the flow.

• L is the length of the plate (parallel to the direction of the flow).

• b is the width of the plate (perpendicular to the direction of the flow).

Since the only drag force acting on the plate is in the case is friction the equation can be written as:

Coefficient of pressure drag:

The equation for calculating the coefficient of pressure drag is given by:

• • In this equation A is the frontal projected area perpendicular to the direction of the flow.

Combining drag coefficients:

Coefficients of pressure friction drag can be combinied to find an overall drag coefficient and and overall drag force.

Drag coefficients for common geometeries:

For more complex shapes it is difficult to find the pressure and friction drag separately and then added together. Instead an overall drag force is found empirically (but experimentation) for then a value for the drag can be found and recorded for that particular shape. The below tables show some of the Cd values for common geometeries: