Explanation:

Flow separation is a phenomenon that occurs when the fluid flow over a surface separates away from it, creating turbulence and reducing the lift force. It can occur in both laminar and turbulent flows and is caused by several factors.

Pressure Gradient:

One of the leading causes of flow separation is a positive pressure gradient. When the pressure gradient is positive, the pressure on the surface increases in the direction of the flow, and the velocity of the fluid decreases. As a result, the boundary layer thickness increases, and the flow separates from the surface. This phenomenon is commonly observed in the flow over an airfoil, where the pressure on the surface is higher than that of the free stream.

Boundary Layer Thickness:

Another factor that affects flow separation is the thickness of the boundary layer. When the boundary layer becomes too thick, the flow separates from the surface, creating turbulence and reducing the lift force. This phenomenon is commonly observed in the flow over a blunt body, where the boundary layer thickness is large.

Negative Pressure Gradient:

Flow separation can also occur when there is a negative pressure gradient. In this case, the pressure on the surface decreases in the direction of the flow, and the velocity of the fluid increases. As a result, the boundary layer thickness decreases, and the flow separates from the surface. This phenomenon is commonly observed in the flow over a backward-facing step.

Reduction of Pressure:

Finally, flow separation can also occur when the pressure on the surface is reduced to the local vapor pressure, causing the fluid to boil and creating vapor cavities. This phenomenon is commonly observed in the flow over a hydrofoil, where the pressure on the surface can become very low.

Conclusion:

In conclusion, flow separation is a complex phenomenon that can be caused by several factors. Understanding these factors is essential in designing aerodynamic surfaces to reduce flow separation and improve lift force.