Question

5) (1 point) Drag on cylinders and spheres decreases when the Reynolds number is in the region of 2 x 105 since (a) Flow separation occurs due to transition to turbulence (b) Flow separation is delayed due to onset of turbulence (C) Flow separation is advanced due to transition to turbulence (d) Flow reattachment occurs

Answer 1

Ans. Drag on cylinder and spheres decreases when reynolds number in the region is 2*10^5. Since flow separation is delayed due to onset of turbulence.

Reynolds no 2*10^5 represents the region of the transition flow. In which there will be transition from laminar flow to turbulent flow. Turbulent begin when critical reynolds number is reaches a value 5*10^5.

Boundary layer change from laminar flow to turbulent flow..

Changeover from an early separation to a delayed separation is brought by the following methods also.

1. By increasing reynolds number of flow which may be achieved by increasing velocity of free stream.

2. increasing initial level of turbulence in the free stream.

3. Roughening a part of leading face of cylinder or spheres.

4. Providing a trip-wire ring in the laminar boundary layer region.

Hence Option b is correct.

Option a is incorrect because there will be delay in flow separation.

Option c is incorrect because turbulence delay formation of wakes and doesn't promote advance separation of flow.

Option d is incorrect because flow after transition will detach from the surfaces of cylinder and spheres.