Explain drag of a Cylinder.

Consider a real fluid flowing past a cylinder of length L and diameter D. the drag coefficient is,

`F_D=C_D*(rho*U^2)/2*A`

Where A is the projected area, which is equal to `LD` for a cylinder.

At very small velocities, `R_e<0.5` the fluid sticks to the cylinder all the way round and never separates from the cylinder. This produces a streamline pattern similar to that of an ideal fluid as shown in figure. The dynamic pressure is achieved at the front stagnation point and vacuum equal to the dynamic pressure exists at the top and bottom where the velocity is at its greatest. 

As the velocity increases, the boundary layer breaks away and eddies are formed behind as shown in figure. The drag pressure increases at the front due to the pressure behind and drag pressure drops at the back. 

A further increase in velocity cause eddies to elongate and the drag coefficient becomes nearly constant as shown in figure. The pressure distributi....Show More

Consider a real fluid flowing past a cylinder of length L and diameter D. the drag coefficient is,

`F_D=C_D*(rho*U^2)/2*A`

Where A is the projected area, which is equal to `LD` for a cylinder.

At very small velocities, `R_e<0.5` the fluid sticks to the cylinder all the way round and never separates from the cylinder. This produces a streamline pattern similar to that of an ideal fluid as shown in figure. The dynamic pressure is achieved at the front stagnation point and vacuum equal to the dynamic pressure exists at the top and bottom where the velocity is at its greatest. 

As the velocity increases, the boundary layer breaks away and eddies are formed behind as shown in figure. The drag pressure increases at the front due to the pressure behind and drag pressure drops at the back. 

A further increase in velocity cause eddies to elongate and the drag coefficient becomes nearly constant as shown in figure. The pressure distribution shows that ambient pressure exists at the rear of the cylinder.

At Reynolds number of around 90, the vortices break away alternatively from the top and bottom of the cylinder producing a vortex street in the wake as shown in figure. The pressure distribution shows the vacuum at the rear.

Drag Coefficient as a function of Reynolds Number:

• For `R_e<1`the flow pattern will be symmetrical.

• For `R_e =1 1 to 2000, `C_D` decreases and attains a minimum value of 0.95 at `R_e`=2000.

• For `R_e=2000` to `3*10^4`, `C_D` increases and become 1.2 at `R_e=3*10^4`

• For `R_e=3*10^4` to `3*10^5`, `C_D` decreases and its value becomes 0.3 at a value of `R_e=3*10^5`

• For `R_e=3*10^6`, `C_D` increases and attains a value of o.7 in the end.