Question

Why is flow separation in flow over cylinders delayed in turbulent flow?

Short answer

Answer: The primary reason for the delay in flow separation in turbulent flow is the increased momentum transfer in the boundary layer due to turbulence. This enhanced mixing, caused by rapid fluctuations and eddies, enables the fluid to maintain its momentum for a longer distance around the cylinder, pushing the separation point further downstream and resulting in reduced drag.

Step-by-step solution

Introduction to Laminar and Turbulent Flow

In fluid dynamics, there are two different types of flow regimes: laminar and turbulent flow. Laminar flow occurs when a fluid flows smoothly, with uniform or no fluctuations in velocity. In contrast, turbulent flow has chaotic and rapid variations in velocity and pressure. Turbulent flow has many scales of motion, ranging from large-scale eddies to small-scale turbulence.

Flow Separation in Flow over Cylinders

Flow separation is a phenomenon that occurs when fluid flowing over a surface, like a cylinder, experiences an abrupt change in pressure, causing the fluid to separate from the surface and create a recirculation region behind the body. This separation can result in a significant increase in drag and loss of lift forces, affecting the overall performance of the system.

Flow Separation in Laminar Flow

In laminar flow, the fluid particles flow smoothly in parallel layers, with the boundary layer (a thin layer of fluid close to the body's surface) remaining attached to the surface, as seen at low Reynolds numbers. However, as the flow goes around the cylinder, an adverse pressure gradient develops because of the change in pressure distribution, causing the fluid momentum to decrease. This eventually leads to flow separation, with the formation of a low-pressure recirculation region behind the cylinder.

Flow Separation in Turbulent Flow

Turbulent flow, on the other hand, is characterized by fluctuations and eddies. These fluctuations transport momentum throughout the flow, allowing for the boundary layer to have more momentum or kinetic energy. This increased momentum helps the fluid overcome the adverse pressure gradient, delaying the flow separation as the flow moves around the cylinder. Consequently, turbulent flow separates at a later point on the cylinder's surface compared to laminar flow.

Delay in Flow Separation in Turbulent Flow

The primary reason for the delay in flow separation in the turbulent flow is the increased momentum transfer in the boundary layer due to turbulence. The rapid fluctuations and eddies in the turbulent flow mix the fluid more effectively compared to the laminar flow, allowing the fluid to maintain its momentum for a longer distance around the cylinder. This enhanced mixing delays flow separation by pushing the separation point further downstream, resulting in a narrower wake and reduced drag.