CFD LES Synthetic Jet.pdf

8/10/2019 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


2/23

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


3/23

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,


4/23

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


5/23

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


6/23

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


7/23

Importance of cavityImportance of cavity

2d phase-averaged flow

Importance ofImportance of motionmotion aroundaround orificeorifice

Phase-averaged flow in turbulent boundary layer


8/23

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


9/23

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


10/23

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


11/23

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


12/23

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


13/23

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


14/23

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


16/23

TimeTime--averaged flow around orificeaveraged flow around orifice

TimeTime

--averaged modification of boundary layeraveraged modification of boundary layer

Streamwise velocity and vorticity


17/23

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


18/23

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


19/23

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


20/23

Cavity flowCavity flow

Zoom onto the separation zoneZoom onto the separation zone

=0.18


21/23

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


23/23

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

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CFD LES Synthetic Jet.pdf