Explanation:

Boundary layer separation is a phenomenon in fluid mechanics where the boundary layer, which is the thin layer of fluid that is closest to the solid surface, separates from the surface due to adverse pressure gradients. This can occur on a variety of surfaces, including aircraft wings, turbine blades, and ship hulls.

The pressure gradient is a measure of how quickly the pressure changes in the fluid. If the pressure gradient is positive, this means that the pressure is increasing in the direction of flow, while a negative pressure gradient means that the pressure is decreasing in the direction of flow.

In the case of boundary layer separation, a positive pressure gradient is required to prevent the separation of the boundary layer. This is because a positive pressure gradient helps to maintain the velocity profile of the fluid, which in turn helps to keep the boundary layer attached to the surface.

On the other hand, a negative pressure gradient can cause the boundary layer to separate from the surface, which can lead to a loss of lift or an increase in drag. This is why it is important to design surfaces that have a positive pressure gradient in order to prevent boundary layer separation.

Conclusion:

In conclusion, the separation of the boundary layer takes place when the pressure gradient is positive. This is because a positive pressure gradient helps to maintain the velocity profile of the fluid and prevent the boundary layer from separating from the surface.