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Active Control of Turbulence and Fluid- Structure Interactions Yu Zhou Institute for Turbulence-Noise-Vibration Interaction and Control Shenzhen Graduate School, Harbin Institute of Technology Shenzhen, China Acknowledgements This work was supported by grants from the RGC of the HKSAR (PolyU 5334/06E, PolyU 5350/10E, PolyU 5329/11E, PolyU 5319/12E) and from National Science Foundation of China through grant 11172085, with contributions from colleagues: Profs W J Li, J Mi, F Anselmet and X J Jing, my ex- and present PhD students/research Associates: Drs M M Zhang, S J Xu, L H Jin and H L Bai, and Messrs W G Zhang, P Zhang, B F Zhang and Y Wang.
Outline Introduction Turbulent 2D bluff body wake & fluid-structure interaction Vehicle aerodynamics Turbulent jet and flame Turbulent boundary layer 3/21
Outline Introduction Turbulent 2D bluff body wake & fluid-structure interaction Vehicle aerodynamics Turbulent jet and flame Turbulent boundary layer 4/21
Turbulent flow control attracted a great attention in the literature because of its engineering significance Flow-induced vibration: Off-shore structures, heat exchangers, high-rise buildings and bridges, etc. Aerodynamic noise: Blade-vortex interaction (BVI) noises in helicopters, turbomachines and fans, jet noise, etc. Drag reduction: Bluff bodies in cross flow, boundary layers, etc. Combustion Save fuels, cut down emissions, etc.
Control methods Passive Control Methods: requiring no external energy. Such as modifying structural geometry, adding longitudinal grooves or riblets to structural surface and using spoilers, etc. Active Control Methods: requiring external energy. Open-loop: using independent external disturbance. Such as acoustic excitation, bleeding technique, oscillating or rotating structures, etc. Closed-loop. Feedback Feedforward Combination of feedback and feedforward
Outline Introduction Turbulent 2D bluff body wake & fluid-structure interaction Vehicle aerodynamics Turbulent jet and flame Turbulent boundary layer 7/21
Perturbation Technique for fluid-structure/noise interaction control Initial wake instability Coherent structure Interaction /coupling Structure surface Noise Small local perturbation The local perturbation is imposed on the structural surface using piezoceramic actuators.
Generation of Perturbation Laser Vibrometer Hotwire Sensor Controller Closed-loop Actuator Signal Generator Open-loop
Actuator Characteristics Relatively Large displacement, high load capacity and lightweight. Installation Rigid supported end Movable supported end THUNDER
Open-loop control: Effect of Perturbation Frequency (f p* ) on Y and u (rigid Cylinder with Flexible Support) At f * p = 0.1 Y rms /h: 75% u rms /U : 68% At f * p = 0.13 Y rms /h } Double u rms /U Y rms / h, u rms / U 0.12 0.08 0.04 0 0 0.1 0.2 0.3 * f p = f p h / U Y rms / h u rms / U Re = 3500 h = 15 mm U = 3.6 m/s Resonance occurs Unperturbed: Y rms /h = 0.0546 u rms /U = 0.0362
Effect of f p * on Phase Shift φ Yu and Spectral Coherence Coh Yu at f s 3 2 1 φ Yu 0 Coh Yu -1-2 -3 0 0.1 0.2 0.3 1 0.8 0.6 0.4 f p h / U For bluff bodies with fixed separation points, synchronization range: 0.8f s ~ 2f s or f * p = 0.11 ~ 0.26. (Gowda 1975) 0.2 0 0 0.1 0.2 0.3 f p h / U
Closed-loop Control Drawbacks of Open-loop Control Narrow effective frequency range; High perturbation amplitude or large energy input needed. Objectives To improve control performance and find an optimum scheme; To shed light upon the underlying physics.
Control Scheme and Controller Design Three feedback control schemes: PID-Y, PID-u and PID-Yu. I 1/s Laser vibrometer Y u Hotwire Integral Gain P Proportional Gain Actuation Signal D Derivative Gain s
Typical Time Histories of Y, u and V p Y / h 0.1 0.05 0-0.05 control on Y / h 0.1 0.05 0-0.05 control on Y / h 0.1 0.05 0-0.05 control on -0.1-0.1-0.1 u / U 0.8 0.4 0-0.4-0.8 u / U 0.8 0.4 0-0.4-0.8 u / U 0.8 0.4 0-0.4-0.8 V p (volts) 90 60 30 0-30 -60-90 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 V p (volts) 90 60 30 0-30 -60-90 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 V p (volts) 90 60 30 0-30 -60-90 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 t (secs) t (secs) t (secs) PID-Y PID-u PID-Yu
Typical Flow Visualization Photos, Re = 3500 (Zhang et al. 2004, POF) Unperturbed Open-loop f* p = 0.1 PID-Y PID-u PID-Yu
Performance Comparisons
Phase Shift φ Yu Unperturbed Open-loop f p = 0.1 * PID-Y PID-u PID-Yu
Effective Damping Ratio ζ e (Zhou et al. 2001, JFM) (ζ e = structural damping ratio ζ s fluid damping ratio ζ f ) ζ s
Outline Introduction Turbulent 2D bluff body wake & fluid-structure interaction Vehicle aerodynamics Turbulent jet and flame Turbulent boundary layer 20/21
Very limited success so far in active control. WHY? State of the art on turbulent flow around the Ahmed model (1) Our understanding of unsteady coherent structures is inadequate. Schematic of the flow structure behind a 3D Ahmed vehicle model (Ahmed et al. 1984) Schematic of flow structure models: α = 25 (Wang et al. 2013, EIF) The Ahmed vehicle model: most extensively studied in the literature due to its relative simplicity and 21/21 representativeness of the generic aerodynamic features associated with vehicles.
State of the art on turbulent flow around the Ahmed model (2) Rather scattered reports of Strouhal number St A : 0.18 ~ 0.50 over the rear window, 0.36 ~ 0.53 behind the base (α = 25 ). Subscript A denotes normalization by A ; A is frontal area of the Ahmed model. Over the rear window Behind the vertical base St A Re A Re A The Ahmed vehicle model: most extensively studied in the literature due to its relative simplicity 22/21 and representativeness of the generic aerodynamic features associated with vehicles.
Objectives of Stage 1 work Obtain an overall picture of coherent structures around an Ahmed body. Determine corresponding St. A Investigate possible Reynolds number effects on the coherent structures. 23/21
Experimental details Hotwire measurement arrangement U 5 Free-stream velocity ( ) : 8.33-32.0 m/s, Re = U A/ υ 0.62 2.6 10 A = ; Hotwire measurements were conducted over the roof, rear window, lateral side and behind the vertical base.
Experimental details Flow visualization measurement arrangement Flow visualization was conducted over the rear window and behind the vertical 5 base in the (x, z) planes at Re = 0.62 10. A Smoke release points in flow visualization experiments 25/21
Coherent structures over the rear window Dominated vortex frequencies St = 0.196 is captured over the rear window, which is attributed to the spanwise vortices. A St = 0.265 is measured along the side edge of the rear window, coinciding with the location of the C-pillar vortex. A y*=0 (x*, z*) (-0.47, 0.86) (-0.34, 0.82) (x*, z*) (-0.47, 0.86) (-0.34, 0.82) St A = 0.530 (x*, z*) (-0.47, 0.86) (-0.34, 0.82) 10-2 (-0.17, 0.76) (-0.17, 0.76) (-0.17, 0.76) E u 10-3 St A = 0.196 A 4 (0, 0.70) St A = 0.196 B 4 (0, 0.70) St A = 0.265 C 4 (0, 0.70) (a ) Merging vortices (b ) Spanwise vortices 10-4 10-2 10-1 10 0 A 1 - A 4 (y*= 0) φ = 25 o 10-2 10-1 10 0 f B 1 - B 4 (y*= 0.36) B C 1 1 C 4 A 1 B A 4 4 o z y 10-2 10-1 10 0 x C 1 - C 4 (y*= 0.57) Typical photographs of the flow structure in the (x, z) plane: (a) y* = 0, (b) y*=0.36. Power spectral density function E u 26/21
Coherent structures behind the base (y*=0) π St A = 0.442 y*=0 Φ 0 St A = 0.442 (a ) (b φ = 25 o z (a) Structure popping up from the lower recirculation bubble o St = 0.196 ) (b) Structure emanated Quasi-periodical toward the ground structures of St from the upper in the wake recirculation bubble -π z* 0.53 A 0.39 0.53 E u 0.32 y 10-1 0.39 0.25 10-2 0.13 0.32 10-3 St = 0.442 St =0.884 A A K 1 : 0 0.25 10-2 10-1 10 K 0 1 - K 6 f 0.13 (x*= 0.20, y*= 0) St = 0.442 A L 1 : 0 L 10-2 10-1 10 0 1 - L 6 (x*= 0.43, y*= 0) A 0 (a) = 0.442 0.5 1.0 1.5 2.0 f z* - St St = 0.442 M 10-2 10-1 10 0 1 M 6 (x*= 0.77, y*= 0) Power spectral density function E u A (b) 0.5 1.0 1.5 2.0 Spectral phase between hotwire signals at: (a) L 1 and L 3, (b) N 1 and N 3. z* 0.53 0.39 0.32 0.25 =0.884 A 0.13 M 1 : 0 z* 0.53 0.39 0.32 0.25 0.13 St = 0.442 A N 1 : 0 N 10-2 10-1 10 0 1 - N 6 (x*= 1.49, y*= 0) x 27/21
Coherent structures behind the base (y*=0.43) St A = 0.265 z* 0.53 φ = 25 o z y o x E u 10-1 10-2 10-3 10-2 10-1 10 P 0 1 - P 6 (x*= 0.20, y*= 0.43) C-pillar vortex 0.39 0.32 0.25 0.13 P 1 : 0 z* 0.53 0.39 0.32 0.25 z* 0.53 0.39 f 0.13 0.32 0.53 St = 0.442 A Q 1 : 0 0.25 0.39 Q 10-2 10-1 10 0 1 - Q 6 0.13 0.32 (x*= 0.43, y*= 0.43) St = 0.442 A R 1 : 0 0.25 R 10-2 10-1 10 0 1 - R 6 0.13 (x*= 0.77, y*= 0.43) St = 0.442 A S 1 : 0-10 -2 10-1 10 S 0 1 - S 6 (x*= 1.49, y*= 0.43) z* Power spectral density function E u Typical photographs of the flow structure in the x-z plane (y* = 0.43) in the wake. 28/21
Coherent structures near the lower edge of the base Two more predominant vortex frequencies were captured near the gap Power spectral density function E u 29/21
Outline Introduction Turbulent 2D bluff body wake & fluid-structure interaction Vehicle aerodynamics Turbulent jet and flame Turbulent boundary layer 30/21
Experimental details Main-jet assembly Microjet assembly 3-15
Two control parameters of microjet actuators Forcing frequency of microjets 0 < f * ( f e / f 0 ) < 1.4 where f 0 = 143 Hz is the preferred mode frequency in the uncontrolled jet. Mass flow ratio of microjets to main jet 0 < C m < 15.4% Control performance/mixing assessment Jet centreline velocity decay rate K, following Zhou et al (2012, AIAA J), is calculated by
Open-loop control Dependence of the decay rate K on C m for given forcing frequency f * = 1.02. Re D = 8000. I II III Uncontrolled 5-15
Perturbed jet 6-15
Flow structure Photographs of typical flow patterns from flow visualization Turbulent at x* = 0 uncontrolled jet Early roll-up Pairing 8-15
Dependence of the decay rate K on forcing frequency for C m = 0.8%. Optimal Forcing Freq f * = 0.89 (1) Control enhances mixing by 8 times; (2) Steady microjet control needs a mass ratio of C m = 4.0%, as much as 5 times, to achieve a similar decay rate to unsteady microjet control at C m = 0.8%. two steady microjets at C m = 4% (Zhou et al. 2013) 10-15
Possible formation of Side Jets Monkewitz et al. (1990, J. Fluid Mech.) : f * = 0.89 & C m = 0.8% in non-injection plane side jets emanating from the jet column (b) Side-jet-like structure hot-air jet Condition for Side Jets: ρ e ρ < 0.7 where ρ e is jet exit density; ρ is ambient fluid density. cold-air jet Side Jets occur in the controlled cold-air jet. 11-15
Closed-loop Control Objective: automatically find the optimal forcing frequency f e * using gradientbased extremum-seeking control (ESC) 1-4
Closed-loop Control ESC algorithm Block Diagram Implementation Two microjet manipulation is investigated for illustration: U e Microjet s 1 x/d = 0.1 2 x/d = 3 Time Average 3 x/d = 5 x 1: Monitoring wire for Re D (t) 2: Feedback wire for <K 3d (t)> 3: Monitoring wire for <K 5d (t)> Servo Motor ESC controller 2-4
Closed-loop Control ESC algorithm Block Diagram Implementation Result 1. Re D = 8000 f e f e (Hz) ESC Control on ---------------------------129 Hz ESC performance agrees well with the open-loop control Re D = 8000 K 3d K 3d K 5d 5d f e * =129 Hz f f f e (Hz) K 5d K 5d t, sec 3-4
Closed-loop Control ESC algorithm Block Diagram Implementation Result 2. ESC s adaptivity as Re D varies Re D = 9333 Re D Re D = 8000 155 Hz K 5d K 5d Re D 8000 9333 129 Hz 155 Hz f e (Hz) 129 Hz f e * t, sec f e (Hz) 4-4
Outline Introduction Turbulent 2D bluff body wake & fluid-structure interaction Vehicle aerodynamics Turbulent jet and flame Turbulent boundary layer 42/21
Objectives: investigate the boundary layer control through a local wall-normal surface oscillation (Bai et al. 2013, under the consideration of JFM)
Experimental Details: piezo-ceramic actuator array A total of 16 elements 44
Characteristic parameters of the uncontrolled turbulent boundary layer U (m/s) δ 99 (mm) θ (mm) Re θ H 12 u τ (m/s) 2.4 60 6.5 1,000 1.4 0.111 4.0 54 5.8 1,540 1.4 0.176 Control parameters (normalization based on u τ at U = 2.4 m/s ) A o (oscillation amplitude) 0.83 ~ 2.77 f o (oscillation frequency) 0.13 ~ 0.65 ϕ i,i1 (i = 1,...,15) (phase-shift) 0 o ~ 180 o λ z (wavelength) 41.6 ~ 45
Perturbation of One actuator y U u rms Parameters: A o = 3.2 f o = 0.17 Normalization based on u τ at U = 4.0 m/s without perturbation. y z x Actuator cross-section 46
S u K u δ τ Distributions of (a) mean streamwise velocity, (b) rootmean-square value u rms ; (c) skewness S u ; and (d) kurtosis K u. Natural flow: present data,, Re θ = 1,000; Degraaff & Eaton (2000, Re θ = 1,430): ; Murlis et al. (1982, Re θ = 791): ; Purtell et al. (1981, Re θ = 1,340):. Presently controlled flow [A o = 1.94, f o = 0.39, and λ z = 416 (or ϕ i, i1 = 18 o ), δ τw = -35% at x = 35]:. w ( τ w) on ( τ w) = ( τ w) off off 100% 47
Dependence of drag reduction on control parameters (%) Dependence of δ τw on (a) f o, (b) A o, and (c) both A o and f o (x = 35). The wavelength formed by the actuators is λ z = 416 (or ϕ i, i1 = 18 o ). U = 2.4 m/s. δ τ w = ( τ w ) on ( τ ( τ w ) off w ) off 100% 48
Dependence of drag reduction on the wavelength Maximum effective wavelength (%) Dependence of δ τw on (a) λ z and (b) ϕ i, i1 at (A o, f o ) = (1.66, 0.39), (1.94, 0.39), and (1.94, 0.65). U = 2.4 m/s. 49
Downstream drag recovery (%) Dependence of δ τw on the location x at z = 0 downstream of actuators [A o = 1.94, f o = 0.39, and λ z = 416 (or ϕ i, i1 = 18 o )]. U = 2.4 m/s. 50
Flow structure in xz-plane (LIF flow visualization) Flow uncontrolled controlled Click for movie Typical photographs of instantaneous flow structure in the xz-plane at y = 10 from smoke-wire (placed at y = 8) flow visualization: (a) uncontrolled, (b) controlled [A o = 2.22, f o = 0.65, and λ z = 416 (or ϕ i, i1 = 18 o )]. Flow at U = 1.5 m/s is left to right. 51
Flow structure in xz-plane (LIF flow visualization) uncontrolled Flow controlled Playback speed 0.5x Typical photographs of instantaneous flow structure in the xz-plane at y = 10 from smoke-wire flow visualization: (a) uncontrolled, (b) controlled [A o = 2.22, f o = 0.65, and λ z = 416 (or ϕ i, i1 = 18 o )]. Flow at U = 1.5 m/s is left to right. 52
Scaling of drag-reduction λ z 250 (or ϕ i, i1 30 o ) ( A o f o Π δτ w = δτ w / e 2 ) : energy input of the travelling wave o ( A 2) = 36.1ξ 2 : connected to the penetration depth of perturbation Π : drag reduction per unit penetration depth Scaling factor: ξ ae 36.1 Ao fo b1 2 Ao fo b2 2 1 ( ) ( ) c1 c2 = Π ae 1 2
Proposed mechanism of drag reduction The oscillating-actuator-generated transverse travelling wave produces a layer of highly regularized external streamwise vortices. The vortices naturally interact with large-scale coherent structures and act to weaken or even completely break the connection between these structures and the wall, thus interfering the turbulence production cycle, which consists largely of vortex formation, streak formation and breakdown (Kim 2011), and reducing friction drag.