Separation of flow refers to the phenomenon where the flow of a fluid separates from the surface it is flowing over. This can occur in a variety of situations, such as in pipes, channels, or around objects.


There are several factors that can cause flow separation, including the pressure gradient, velocity, and surface roughness. However, the primary cause of flow separation is the pressure gradient.


The pressure gradient refers to the change in pressure along the flow direction. In the context of flow separation, the pressure gradient plays a crucial role. In general, flow separation occurs when the pressure gradient exceeds a certain critical value.


Let's analyze the given options one by one:

a) The option states that separation of flow occurs when the pressure gradient tends to approach zero. However, this statement is incorrect. Flow separation occurs when the pressure gradient exceeds the critical value, not when it approaches zero.

b) The option states that separation of flow occurs when the pressure gradient becomes negative. However, this statement is also incorrect. Flow separation can occur for positive as well as negative pressure gradients, depending on the specific conditions.

c) The option states that separation of flow occurs when the pressure gradient reduces to a value when vapor formation starts. While vapor formation can contribute to flow separation in certain cases, it is not the only factor involved. Flow separation can occur even without vapor formation.

d) The correct answer is option 'D', which states that none of the given options is correct. This is the most accurate answer because none of the options correctly describe the conditions for flow separation.

In conclusion, flow separation occurs when the pressure gradient exceeds a critical value, and none of the given options accurately describe the conditions for flow separation.