Active Flow Control Using Transverse Travelling Waves

Speaker: Aki Pakarinen25.04.2006 Imperial College

Design Objectives

• A net skin friction reduction of 20% would save the airlines roughly $40m. per day worldwide on direct fuel costs. (2004 fuel prices and consumption)

• Rugged, reliable control system• Surface mounted• Generous net energy savings• Open loop, always “on-design”• Commercial operation

25.04.2006

Control Schemes

• Large-scale longitudinal vortices• “Wall turbulence manipulation by

large-scale streamwise vortices,” Gaetano Iuso, Michele Onorato, Pier Giorgio Spazzini, Gaetano Maria Di Cicca, JFM (2002)

• Oscillating wall• “On the effects of lateral wall

oscillations on a turbulent boundary layer,” Pierre Ricco, Shengli Wu, Experimental Thermal and Fluid Science (2004)

25.04.2006

Du, Symeonidis & Karniadakis (2002)

• “Drag reduction in wall-bounded turbulence via a transverse travelling wave” by Du, Symeonidis and Karniadakis, JFM 2002

• DNS simulations, using a force resembling a transverse travelling wave, showed > 30% reduction in wall shear stress through the suppression of turbulence production.

• The forcing was confined to the viscous sublayer.

25.04.2006

Flow Visualisation

At y+ = 4. Blue indicates low-speed streaks and yellow-red high speed streaks.

• Plots of instantaneous streamwise velocity on both unactuated (top) and actuated (bottom) walls.

• Top plot shows characteristics 100 viscous unit spacing of low-speed/high-speed streak pairs.

• Controlled case shows distinct lack of streaks, which are replaced by a large region of low-speed fluid.

25.04.2006

Control Variables

• Frequency T+

• Strong dependence

• Force magnitude I• Smaller “interaction parameter” showed better

results

• Wavelength +

• The longer the better

• Penetration length • For Re = 150, they found I • T+ • 1 to produce

Cƒ of 30%.

25.04.2006

Non-Ideal Waveforms

(a) Cƒ of -30%(b) Cƒ increase(c) no change(d) Cƒ of ≈ -20%

• Initial simulations were done with an “ideal” sine wave.

• Further studies showed the feasibility of using piecewise approximations.

25.04.2006

Zhao et al. (2004)

• “Turbulent drag reduction by travelling wave of a flexible wall,” H. Zhao, J.-Z. Wu, J.-S. Luo, Fluid Dynamics Research (2004)

• DNS simulation showed 30% drag reduction, using actual wall motion.

• A pseudo-Stokes layer was seen, which has a large spanwise structure and is relatively homogenous in a streamwise direction and interferes with the turbulence regeneration cycle.

25.04.2006

Re Effects

• As Re increases, viscous sublayer becomes thinner.

• Power input decreases with decreasing actuation deflection.

• Therefore, as Re increases, control becomes “easier”, though the associated frequencies increase.

• Power required due to skin friction ~ Rex17/6

25.04.2006

Re Effects cont.

Net power savings per unit area Power saved Power spent

• Using simple boundary layer and structural mechanics approximations, an equation for the net power savings of monolith flow control device can be derived.

• Positive laboratory results should translate directly to higher Reynolds numbers, despite the requirement for higher frequencies.

25.04.2006

Into the Lab?

• Produce an accurate, transverse travelling wave (TTW) of < viscous sublayer height, with ability to control amplitude, wavelength and frequency…

• At lab conditions, these starting values are:• wavelength λ: 5 - 30mm• amplitude A: 0.4 - 0.8mm• frequency ƒ: 20 - 100 Hz• length and width of actuated flat plate: to suit tunnel

25.04.2006

Model Actuation

• Design parameters signify a definite challenge.• Many options were considered and rejected:

• Springs• Magnets• Hydraulics• Mechanical (piano keys, revolving disks, moving ridges)

• Not one could produce the frequency and the amplitude criteria.

• The search for a viable system continued until a very specific actuator was found.

25.04.2006

Model Actuation cont.

• Face® International Thunder® TH11R-3 pre-curved piezoceramic actuators

• Peak to peak response: 0.8mm• Frequencies up to 100Hz• ±500V operating voltage

Teflon rail

Perspex cylinder

Actuator

Perspex base plate

Simply Supported Mounting

Frequency Response 50Vpp

3mm

25.04.2006

• Apparatus is capable of the following:• Wavelength: 7mm – 105mm

• Frequency: up to 100hz• Amplitude: up to 0.8mm

• Net savings? Possibly, but worst case scenario would be 11.5W spent to save 0.9W.

Apparatus Capabilities

7mm Up to 105mm

25.04.2006

Future Work

• Initial system validation (ongoing)• Current set-up has frequency related issues.• System evolution ongoing, with two analogue back-up

designs ready. • Complete system manufacture• Testing

• Investigating effects of changing control variables (frequency, amplitude and wavelength)

• Including mass balance, hotwire and PIV measurements

• This work leads onto specification and design of the more practical surface, as presented by Kevin.

25.04.2006

Conclusion

• DNS has shown the possibility of significant drag reductions using transverse travelling waves confined to the viscous sublayer.

• An experimental set-up and further work has been suggested.

• Any questions?

25.04.2006