Attached flow simply means that the fluid (usually air or water) is flowing smoothly along a surface without separating from it.
In more detail:
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Definition: Attached flow is the flow regime where the fluid streamlines closely follow the contours of a body. This indicates that the fluid is "stuck" to the surface and there isn't a region of reversed or recirculating flow near the surface.
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Example (Airfoil): A classic example is airflow over an airfoil (like an airplane wing) at a low angle of attack. The air smoothly flows over the upper and lower surfaces of the wing, generating lift without detaching from the surface.
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Contrast with Separated Flow: The opposite of attached flow is separated flow, also known as flow separation. This occurs when the fluid's momentum is insufficient to overcome an adverse pressure gradient, causing the flow to detach from the surface. This can lead to phenomena like stall on an airfoil, dramatically reducing lift and increasing drag.
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Key Characteristics of Attached Flow:
- Streamlines follow the body's contour.
- Minimal turbulence near the surface.
- Efficient transfer of momentum and energy.
- Predictable aerodynamic performance.
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Conditions Promoting Attached Flow:
- Smooth surfaces
- Low angles of attack (for airfoils)
- Favorable pressure gradients (pressure decreasing in the direction of flow)
In summary, attached flow is a desirable condition in many aerodynamic and hydrodynamic applications as it leads to efficient and predictable performance.
