Flow separation occurs when the fluid travels away from the adverse pressure gradient.

Flow separation is a phenomenon that occurs when the flow of a fluid, such as air or water, separates from a solid surface or an object, causing a change in the flow pattern. It is often characterized by the formation of vortices, eddies, or turbulent regions in the flow.

Adverse pressure gradient:
An adverse pressure gradient refers to a situation where the pressure of a fluid decreases in the direction of flow. This means that the fluid experiences a higher pressure at one point and a lower pressure at another point along its path. In the case of flow separation, the adverse pressure gradient causes the fluid to separate from the surface or object it is flowing over.

Causes of flow separation:
1. Boundary layer separation: The boundary layer is a thin layer of fluid that forms on the surface of an object. As the fluid flows over the surface, it adheres to the surface due to viscous forces. However, as the adverse pressure gradient increases, the boundary layer may not be able to follow the surface contours, leading to separation.

2. Flow separation due to curvature: When the fluid flows over a curved surface, the adverse pressure gradient can cause the flow to separate. This is because the fluid on the outer side of the curve experiences lower pressure compared to the fluid on the inner side, leading to separation.

3. Flow separation due to flow obstruction: Flow separation can also occur when there is an obstruction or an object in the flow path. The adverse pressure gradient caused by the obstruction can cause the flow to separate.

Effects of flow separation:
Flow separation can have several effects on the performance and behavior of a fluid flow:

1. Increased drag: Flow separation increases the drag force on the object or surface. This can result in higher energy consumption for vehicles or decreased efficiency for fluid systems.

2. Loss of lift: In aerodynamics, flow separation can cause a loss of lift on airfoils, wings, or other lifting surfaces. This can affect the stability and control of aircraft or other flying objects.

3. Increased turbulence: Flow separation often leads to the formation of turbulent regions in the flow. This can result in increased mixing and fluctuations in the flow, which may have adverse effects on the performance of systems.

In conclusion, flow separation occurs when the fluid travels away from the adverse pressure gradient. It is a complex phenomenon that can have significant effects on the behavior and performance of fluid flows.