Fluid Mechanics Laboratory

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Low-Re Separation Control by Periodic Suction
Surface Motion
The 54th Annual Meeting of the Division of Fluid
Dynamics American Physical Society

• David Munday, George Huang and Jamey Jacob
• Department of Mechanical Engineering
• University of Kentucky
• 19 November 2001

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Motivation

• µAVs
• Re 104 - 105
• UAVs
• Re 105 - 106
• High Altitude
• Other
• atmospheres (Mars)

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Laminar Separation Bubble

• Adverse Pressure gradient on a laminar flow
  causes separation
• Transition occurs. Fluid is entrained and
  turbulent flow re-attaches

Figure from Lissaman
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Active Flow Control

• Constant sucking or blowing
• Intermittent sucking and blowing (synthetic jets)
• Wygnanski, Glezer
• Suggests existence of sweet spots in frequency
  range
• Mechanical momentum transfer
• Modi, V. J.
• Change of the shape of the wing (Adaptive
  Airfoils)

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Adaptive Airfoils

• Can change shape to adapt to flow
• Simple examples Flaps, Slats, Droops
• Move slowly, quasi-static
• Change shape parameter (usually camber) to adapt
  to differing flight regimes
• Rapid Actuation
• Can adapt to rapid changes in flow condition
• May produce the same sort of sweet spot
  frequency response as synthetic jets

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Piezoelectric Actuation

• Rapid actuation requires either large forces or
  light actuators
• Piezo-actuators are small and light
• They are a natural choice for µAV designs

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Adaptive Wing Construction

• NACA 4415
• well measured, room for internal actuator
  placement
• Modular (allows variation in aspect ratio)

• Multiple independent actuators
• Flexible insulating layer and skin

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Foil shape (mode shapes grossly exaggerated)
Actual amplitude 0.002c
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Dynamic Model Flow Visualization

• Flow Visualization is by the smoke wire technique
• As described in Batill and Mueller (1981)
• A wire doped with oil is stretched across the
  test section
• The wire is heated by Joule heating and the oil
  evaporates making smoke trails
• Limited to low Re
• Limit due to requirement for laminar flow over
  wire
• Limited to a wire diameter based Red lt 50

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Dynamic Model Flow Visualization
a 0
Actuator Fixed
Actuation on
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Dynamic Model Flow Visualization
a 9
Actuator Fixed
Actuation on
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Separation as a function of Reduced Frequency
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Separation with and without actuation
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Numerical Simulation

• 2nd order backward temporal scheme
• 3rd order QUICK spatial scheme
• Chimera grid with moving overlapping grid
  capability, can handle moving boundaries
• Parallel MPI

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Conclusions

• Oscillation of the actuator has a pronounced
  effect on the size of the separated flow
• Oscillation holds the separation at 70 to 4-6
  of chord while un-actuated flow separates as much
  as 15 of chord
• Separation can be reduced by from 30 to 60
  relative to un-actuated flow-field

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Further Work

• Expand the range of Re
• Force measurements of Dynamic Mode
• effect on L/D
• PIV measurements of Dynamic Mode
• flow control
• Phase average PIV data
• Comparisons with Numerical Simulation
• Examine behavior with artificial turbulation
• Compare gains in performance with power required

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Questions?