Flow Control

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Flow Control

• By Dana Elam

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What is flow control?

• Any mechanism or process through which the
  boundary layer of a fluid flow is caused to
  behave differently than it normally would were
  the flow developing naturally along a smooth
  straight surface Flatt, 1961

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Turbulent Boundary layers

• Low-speed streaks
• Ejections of low-speed fluid outward from the
  wall
• Sweeps of high-speed fluid toward the fall
• Vortical structures of several proposed forms
• Strong internal shear layers in the wall zone
• Near wall pockets, observed as areas clear of
  marked fluid
• Backs surfaces across which the streamwise
  velocity changes abruptly
• Large scale motions in the outer layers
  (superlayers and deep valleys of free stream
  fluid) Kline and Robinson, 1990

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What is the benefit?

• Skin Friction drag accounts for
• 80 of total drag for ships
• 25-40 of total drag for commercial aircraft
• Also has implication in other industries due to
  skin friction in pipe and channel flows
• Increasing requirement with legislation,
  diminishing resources and oil now at 100 a
  barrel
• Ships and aircraft pose completely different
  problems
• Aircraft high velocity flow, temperature
  implications re 105
• Ships much lower speeds

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Passive v active control

• Passive flow control is where no energy is
  expended - added advantage of reduced maintenance
  requirements
• Active control is where energy is expended in
  controlling the flow - greater reductions in skin
  friction are possible

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Passive control

• Riblets - stabilising of vortices
• LEBU - break up of vortices
• Compliant surfaces
• Seen in nature, shark skin!

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Active methods- Open loop

• Suction
• Removal of flow through holes or slots
• Heating/cooling of boundary layer
• Cooling lowers viscosity of air, heating for
  water
• Surface additives
• polymers, bubbles, ultrasound

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Active Methods-Closed loop

• Lorentz Force
• Electromagnetic field not viable in air,
  inefficient
• Active blowing/sucking
• -25 reduction possible Choi,1994
• Wall deformation
• - MEMs acutation and sensors

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Issues

• Common issues for closed loop systems
• Sensor placement
• Computational requirement - linear system
• Actuator density, size
• Time delay
• System survival / ease of implementation
• Lack of research at higher Reynolds,
  effectiveness of control schemes may be
  different.