Download - CFD LES Synthetic Jet.pdf
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M.A. Leschziner*Imperial College London
Simulation of Synthetic Jets for Separation ControlSimulation of Synthetic Jets for Separation Control
*Contributions from: Geoff Fishpool, Alexandros Avdis, Don Wu, Anne Dejoan
Sponsored by: EPSRC, Airbus, BAE Systems
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Engineering contextEngineering context
On-demand control of separation
Local elimination of separation e.g. wing-bodyjunction
Removal of heavy, mechanical high-lift components
Improve overall efficiency
Control of aero-acoustics in separated flows
Alternatives in separation controlAlternatives in separation control
Passive Control Active Control
Suction/blowing
Active flaps
Acoustic excitation
Active dimples
Jets
Round, square, slot
continuous, pulsed, synthetic
Vortex generators
Fences
Chevrons
Wedges
Dimples
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Synthetic jetsSynthetic jets
Zero net mass flux no mass supply!
Finite momentum and vorticity flux
Pulsing by piston or diaphragm in cavityGenerally high injection velocity
Jet causes unsteady streamwise vortices, turbulence, mixing
Jet parametersJet parameters
Maximum aperture-averaged velocity,
Aperture diameter / width,
Injection Reynolds number,
Period of blowing, (half of cycle)
Dimensionless stroke length
slug equivalent,
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Jet propertiesJet properties
Circulation
Vorticity advected across orifice
Flux of vorticity
= rate of change of circulation
Time-integrated flux
total circulation
Jet propertiesJet properties
Circulation in primary vortex capped at high stroke length
Additional circulation in trailing structures
Zhong et al, FTaC (2007)
Total circulation
Primary vortex
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Comparison with mean-flow experiments by Cater & Soria (2002)
=5000,=0.003
Square, body-fitted and
IBM representation of orifice
Synthetic jet in stagnant environmentSynthetic jet in stagnant environment
Synthetic jet in stagnant environmentSynthetic jet in stagnant environment
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FluidFluid--mechanic issuesmechanic issues
Fundamentals of vortex formation, propagation and breakdown
Cavity-orifice-jet interaction
Interaction with cross-flow
Control effectiveness (practical)
intensity
longevity, persistence
region of influence
reduction of separation
Mechanisms underpinning enhanced mixing (fundamental)
Resonance with instability modes
Interaction with turbulence scales
Computational challengesComputational challenges
Scale-disparity effects jet size = O(0.001) x controlled-flow length
Complex geometry / topology cavity, orifice, outside flow
Cavity small, but influential
Jet-injection period >> turbulence time scales:
very long simulations
difficulty of obtaining phase-averaged data
Turbulent upstream conditions: full spectral description of inlet flow
Wide, multi-D parameter space
Strong unsteadiness due to high-frequency injection O(200-1000 Hz)
RANS unpromising; models unsuitable for unsteady conditions
High: near-wall resolution is a serious problem
Compressibility and resonance in cavity
Membrane (and jet) response to voltage actuation unknown
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Importance of cavityImportance of cavity
2d phase-averaged flow
Importance ofImportance of motionmotion aroundaround orificeorifice
Phase-averaged flow in turbulent boundary layer
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Resolution of orificeResolution of orifice
Examined in double back-to-back cavities
16 cells
32 cells
Orifice
Influence of inlet conditionsInfluence of inlet conditions
Generated by recycling needs very long domains
Persistent, long-lived structures
Spanwise inhomogeneity despite very long precursor domain
and long integration times
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Principal configurationPrincipal configuration
Ramp designed by reference to experiments of Song & Eaton
(Experiments in Fluids, 2004)
Modified / optimised with RANS computations (without jets)
= 5-20
Inter-orifice spacing = 10
= 1100; 13700; =2170;
Earlier configurationsEarlier configurations
Rescal
Slot injection into a back-step flow
Slot injection into a separated flow
behind a dune-shaped body
Round/square-jet injection into
turbulent boundary layer
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BackstepBackstep configurationconfiguration
Reynolds number:
Expts. by S.Yoshioka, S. Obi and S. Masuda (2001))
Strouhal number:
Optimum frequency
Shedding-mode instability;
flapping shear layerShear-layer-mode
instability Unforced-flow
Spectral analysisSpectral analysis
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Reduction in recirculation length: Expt - 30%; LES - 26%
Skin frictionSkin friction,, velocity profilesvelocity profiles, shear stress, shear stress
Distance normalised by unforcedreattachment distance
WallWall-- mounted NASA 2d humpmounted NASA 2d hump
Targeted at separation control
With / without synthetic jet
Experiment:=0.1, =935892
Jet frequency =0.216 (based on
bump height)
Jet velocity 0.66
= 5900 and 6770
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Geometry and overall view of f lowGeometry and overall view of flow
Expt. reattachment: 1.1c
Predicted reattachment: 1.07c
Expt. reattachment: 0.99c
Predicted reattachment: 0.97c
25% reduction in recirculation length
No jet
With jet
Effects of actuationEffects of actuation
Phase-averaged field
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PhasePhase-- averaged fieldsaveraged fields
Quantitative comparisons and POD analysis: Avdis et al, FTaC (2009)
Expt.
2500 Samples collected over 250 000 time-steps
13 flow-through times
22 jet periods
Jet-on
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Circular jet in attached turbulent boundary layerCircular jet in attached turbulent boundary layer
Test cases by reference to PIV and hot-wire data from Garcillan
et al (2008)
Jet flow
IBM representation of orifice
Velocity ratio
Strouhal number based on orifice diameter and
Cross-flow
Boundary-layer thickness
Momentum-thickness Reynolds number
Injection without upstream turbulenceInjection without upstream turbulence
Non-turbulent cross-flow, only mean profile imposed at the inlet
Iso-surface of instantaneous, normalised vorticity magnitude
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Injection with upstream turbulenceInjection with upstream turbulence
Fully turbulent cross-flow
Iso-surface of instantaneous, normalised vorticity magnitude
(/)
PhasePhase--averageaveragedd fieldsfields
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TimeTime--averaged flow around orificeaveraged flow around orifice
TimeTime
--averaged modification of boundary layeraveraged modification of boundary layer
Streamwise velocity and vorticity
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BoundaryBoundary--layer propertieslayer properties
Streamwise evolution of
momentum thickness
and shape factorat centre-plane
Large excursion from
standard boundary-layer
profile not unexpected
Quantitative comparisons: Wu and Leschziner, IJHFF (2009)
Circular jet in attached laminar boundary layerCircular jet in attached laminar boundary layer
Investigate fundamental effects on cross-flow
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Mean transverse motionMean transverse motion control effectivenesscontrol effectiveness
Upward motion: red 5%; Downward motion: blue 5%
Separation control with circular jetsSeparation control with circular jets
Baseline flow no injection
Experiments: Zhang & Zhong(private communication, 2009)
Principal configuration
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Baseline flowBaseline flow
No injection
With injectionWith injection whole domainwhole domain
Phase- and spanwise-averaged over 6 cycles
Present conditions=12.6,=0.18 flapping instability
Alternative: shear-layer instability mode
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Cavity flowCavity flow
Zoom onto the separation zoneZoom onto the separation zone
=0.18
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Zoom onto the separation zoneZoom onto the separation zone
=0.18
Mean effect on separation (Mean effect on separation (unconvergedunconverged))
=0.18
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= ; ; = 40 Hz; = 19.5
Spanwise position [z/S]
y
/h
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.50
0.05
0.1
0.15
0.2
0.25
0.3
VR=0.2
VR=0.3
VR=0.4
VR=0.5
PIV experimentsPIV experiments
= ; ; = 40 Hz; = 19.5
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Mean effect on separation (Mean effect on separation (unconvergedunconverged))
=1
Concluding remarksConcluding remarks
Much of the control is derived from large-scale flapping of the
separated shear layer, due to jet perturbation.
For slot jets in separated flow, the =O(0.2) seems to havesome significance, associated with flapping instability.
The relationship to the shear-layer instability is rather unclear.
Control depends on high injection velocities and spanwise extent of
injection.Control effectiveness is limited to a small spanwise extent in which
streamwise vorticity is generated.
Widely-spaced round jets appear much less effective than slot jets.
Much remains to be studied: sensitivity to
jet-BL spectral interaction,
injection period