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10.3 Flow Separation and Wakes

2 min readLast Updated on July 19, 2024

Flow separation occurs when fluid detaches from a surface due to adverse pressure gradients. This phenomenon leads to wake formation, increased drag, and reduced lift. Understanding flow separation is crucial for designing efficient aerodynamic and hydrodynamic systems.

Engineers employ various techniques to control flow separation and minimize its effects. These methods include streamlining body shapes, active flow control like boundary layer suction, and passive control such as vortex generators. Mastering these techniques is essential for optimizing fluid flow in real-world applications.

Flow Separation

Flow separation causes

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  • Boundary layer detaches from a solid surface when fluid lacks sufficient momentum to overcome adverse pressure gradient (increasing pressure in flow direction)
    • Adverse pressure gradient opposes fluid motion causing deceleration
    • Viscous effects near surface reduce fluid velocity to zero
  • Separation point occurs where boundary layer detaches from surface
    • Characterized by reversal of flow direction near surface
  • Influenced by surface geometry (sharp corners, curved surfaces), flow velocity, and fluid properties (viscosity, density)

Wake formation behind bodies

  • Region of disturbed flow downstream of an immersed body formed due to flow separation and interaction of shear layers
  • Characterized by recirculation zones with reversed flow immediately behind body
  • Vortex shedding involves periodic release of vortices from alternating sides of body
    • von Kármán vortex street displays regular pattern of vortices in wake
  • Increased turbulence and mixing in wake region
  • Reduced velocity and increased pressure compared to freestream flow
  • Wake size and shape depend on body geometry (streamlined vs. bluff bodies) and flow conditions (Reynolds number)

Effects and Control of Flow Separation

Effects on drag and lift

  • Increased drag forces due to flow separation
    • Pressure drag (form drag) caused by pressure difference between upstream and downstream sides of body
      • Separation creates larger wake increasing pressure drag
    • Skin friction drag caused by shear stresses acting on body surface
      • Separation reduces skin friction drag due to detached boundary layer
  • Affects lift forces
    • Separation on upper surface of airfoil can lead to stall (sudden decrease in lift force at high angles of attack)
    • Separated flow disrupts pressure distribution around airfoil reducing lift
  • Controlling flow separation is important for aerodynamic and hydrodynamic efficiency

Control of separation and wakes

  • Streamlining involves designing body shapes to minimize flow separation
    • Gradual changes in surface contours prevent sharp pressure gradients
  • Active flow control methods
    • Boundary layer suction removes low-momentum fluid near surface
    • Boundary layer blowing injects high-momentum fluid near surface
    • Vortex generators create vortices to energize boundary layer
  • Passive flow control methods
    • Vortex strakes generate streamwise vortices as fin-like protrusions
    • Riblets reduce turbulent skin friction with micro-grooved surfaces
    • Dimples create localized vortices delaying separation as surface indentations
  • Oscillatory control uses unsteady actuation to control separation
    • Synthetic jets create oscillatory flow as zero-net-mass-flux devices
    • Plasma actuators induce flow oscillations using ionized air
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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