Flow Control

Inflow pulsation on a turbulent coaxial jet

-      To investigate dynamic and mixing properties in a confined coaxial jet flow with pulsating turbulent inflow

·        Dynamic SGS model (Germano et al. 1991)

·        Fully-implicit fractional step method (Kim et al. 2002)

·        Fully conservative finite difference scheme (Morinish et al. 2004)

·        Pressure Poisson equation solver: V-shape multigrid method

·        Bounded QUICK scheme for scalar advection (Herrmann et al. 2006)

-      Inflow pulsation

-      St effects

·        0.1<St<0.9

·        Optimal pulsation frequency for the reduction of reattachment length: St=0.327

·        Optimal pulsation frequency for the mixing enhancement: St=0.180

( Jang & Sung, International Journal of Heat and Fluid Flow, Vol.31, pp.351-367, 2010)

Drag reduction

Ultrasonic forcing for skin friction reduction

-      Objective

·        Reducing skin friction in a turbulent boundary layer

-      Schematic diagram of Experiment

 

-      Ultrasonic cleaner mechanism

-      Forcing effect (mean velocity)

-      Forcing effect (turbulent intensities)

·        Streamwise direction turbulent intensity is decreased (reduced skin friction)

·        Wall normal direction turbulent intensity is increased (forcing effect)

Turbulent boundary layer control using periodic suction/blowing

-      Schematic diagram of Experiment

 

 

·        Periodic suction and blowing through a spanwise slot

·        Effect of forcing frequency and forcing angle α

-      Control mechanism

-      Vortex spatial fraction

·        A spanwise vortex which is generated frequency is more effective in reducing skin friction.

-      Experimental results

 

 

 

·        Periodic suction/blowing reduces the wall region velocity so the velocity profile is upward shifted.

·        At that time, effect of higher frequency forcing and reverse forcing angle persists farther downstream.

-      Wall region ω_z and velocity behavior with forcing frequencies

 

 

-      Wall region ω_z and velocity behavior with forcing angles

 

 

-      Experimental results

·        High frequency and α=120° are more effective to reduce skin friction

·        Spanwise vortex retards wall region velocity

DNS of turbulent boundary layer with local forcing

-      Objective

·        Effects of local forcing on skin friction in a turbulent boundary layer

-      Flow configuration

·        Spatially developing turbulent boundary layer

 

 

 

-      Numerical method

·        DNS by fully-implicit decoupling method

-      Vorticity field after the slot

 

 

-      Convectional forcing parameter σ=vW/U_∞θ

DNS of TBL with periodic blowing through a spanwise slot

-      Objective

·        To investigate the effects of unsteady blowing on turbulent boundary layer

 

 

 

-      Insensitivity of long time mean velocity for unsteady forcing

-      Convection of lower wall-shear stress region

 

 

 

-      Unsteady forcing enhances energy redistribution

-      Spanwise vortical structure inducing a reverse flow near the wall reducing wall shear stress periodically

-      Pressure strain correlation tensor

 

 

 

Experiment with local forcing in turbulent boundary layer

-      Understanding the flow structure behind the local blowing/suction in a flat plate boundary

-      Skin friction was reduced up to 40% compared with the undisturbed boundary layer

 

 

 

Control of TBL by local spanwise oscillating Lorentz force

-      Objective

·        Drag reduction by Lorentz force

·        Effects of local forcing on turbulent near wall coherent structures

-      Numerical method

·        DNS by fully-implicit decoupling method

 

 

 

 

 

 

Spanwise wall oscillation

-      Objective

·        Drag reduction

-      Flow configuration

·        Periodic channel / pipe flows

-      Spanwise wall oscillation

 

 

-      Scale similarity on drag reduction

-      Conditionally-averaged flow fields (Streamwise Vorticity ω_x and streaks u’)

 

 

Control of turbulent channel flow

-      Objective

·        Drag reduction

-      Flow configuration

·        Periodic channel flows

-      Suboptimal control algorithm

·        Spatial optimum

-      Actuators: suction / blowing (φ₂), spanwise sliding velocity (φ₃)

 

 

 

-      Control mechanism and instantaneous vorticity field

 

 

 

-      Assessment of suboptimal control

 

 

 

 

 

 

 

 

 

 

Micro flow control system

Particle Image Velocimetry

-      Velocity field measurement technique using particle images in fluid flow

-      Displacement of particle images between the Nd:Yag laser pulses is determined through spatial correlation

·        Cross-correlation method covers linear shifts only (interrogation window size should be sufficiently small)

Influence of unsteady wake on a backward-facing step flow

-      Control of separated flows over a backward-facing step

-      Flow configuration

·        Backward-facing step flow at Re_H=560

·        Wake disturbance within 10% of U_∞

-      Experimental results

·        Effective flow control at St_H=fH/U_∞=0.2

Influence of unsteady wake

-      Objective

·        Flow control

-      Flow configuration

·        Turbulent separation bubble

-      Phase averaging technique

·        Effective means of investigation

-      Velocity: Hot-wire anemometry

-      Pressure: Microphone array

-      x_R measurement

·        Further reduction of x_R on the ‘pressure’ side

 (‘wrapping’ regime, ‘cutting’ regime)

-      Spreading rate

·        Larger spreading rate at St_H=0.2

·        Further enhancement on the ‘pressure’ side (CCW)

-      Turbulence intensity

·        Larger turbulent intensity at St_H=0.2

·        Further increase for CCW

·        Active enhancement of rolled-up vortices

·        Reduction as St_H increases

·        Further reduction for CCW

-      Pressure spectrum for CCW

·        Significant increase at x/H=0.8

·        Fast decay after x_R/H=2.4

-      Coherence (St=0 and x₀/H=0.8)

·        Flapping motion at St=0.02 : 0<ξ/H<3

-      Coherence (St=0 and x₀/H=8.0)

·        Weakened flapping motion : -8<ξ/H<-3

·        Shedding motion St ξ/H=Const. after reattachment (white dotted curve)

-      Coherence (St_H=0.02 and x₀/H=0.8, CW)

·        Coherent motion of St=0.02 and its harmonics

·        Broad coherent region of St=0.37 observed

-      Coherence (St_H=0.02 and x₀/H=0.8, CCW)

·        Weakened coherent motion of St=0.02

·        Significant broad coherent region of St=0.3

·        Possible cause of different behavior between suction and pressure sides

·        Coherent motion of St=0.2 and first harmonic at St=0.4

·        Broad coherent region of St=0.14

-      Coherence (St_H=0.20 and x₀/H=0.8, CCW)

·        Weakened coherent motion of St=0.2

·        Significant repetitive motion (spatial version of unsteady wake)

·        High coherence means x_R reduction

Large-scale vertical structure of turbulent separation bubble

-      Objective

·        Quantitative visualization of large-scale vertical structure of turbulent separation bubble utilizing pressure-velocity correlation

-      Flow configuration

·        Flows over a thick blunt body with unsteady wake by a wake generator

·        Re_H=560 (comparison purpose with the results by Hwang et al..2001)

·        Glycerol St_H=0, 0.2 (clockwise and counter-clockwise rotation of a wake generator)

-      Method : Spatial box filtering to smooth out the 3-dimensional irregularities on spanwise large-scale vortices ( Lee and Sung, 2002)

-      Formation of large-scale vertical structure and its convection

-      Control effect of unsteady wake by reducing vertical structure

-      CCW more effective than CW for the more active interaction between turbulent separation bubble and unsteady wake

-      Dipole noise calculation with Curle’s extension formula

-      Noise source region reduced by interacting with wake

-      Noise source strengthened by active interaction between separation bubble and unsteady wake

Local forcing over a backward-facing step

-      Objective

·        Separation control

-      Flow configuration

·        Turbulent backward-facing step

·        On the separation edge at 45°

·        Effective means of flow control

-      Hot-wire anemometry

·        Definition of local focing

·        Time-mean flow field

·        Velocity spectrum

-      Local forcing

·        Effective forcing frequency: St_H=0.27

·        Decrease of x_R with increasing A0

·        Two minima

St_H=0.27 : shedding type instability

St_H=0.40 : shear layer instability

-      Velocity spectrum

·        Effective forcing frequency: St_H=0.27

·        Vortex merging: St_H/2, St_H/4

·        Initial region (x/H=1)

Enhancement of vortices

Increased turbulence level

·        Reattachment region

Earlier reattachment

No forcing: x_R/H=7.8

Effective forcing: x_R/H=4.9

Control of turbulent separated and reattaching flow by spanwise local forcing

-      Objective

·        Mixing enhancement of turbulent separated flow

·        Control of coherent structure of turbulent separated flow

·        Control of noise radiation from wall pressure fluctuations

-      Control strategy

·        Local forcing by using acoustic excitation

·        Spanwise variation(d,t)

·        Swirling effect(θ)

Control of separated flows by local variations at separation edge

-      Objective

·        Mixing enhancement at separation edge (passive control)

-      Velocity measurement by PIV

-      Wall pressure fluctuations by microphones

DNS/LES of backward-facing step flow

-      Numerical method

·        Implicit velocity decoupling procedure of Kim et al.(2002)

·        Multi-grid method to solve pressure Poisson’s equation

·        Inlet boundary condition : Lund et al.(1998)

·        Parallelization using MPI (Message-Passing interface)

-      Physics

·        Flow control with local forcing

Unsteady  RANS and CFD

-      Turbulence heat transfer model

·        Low-Reynolds-number 4-equation model(IJHMT 1996, IJHFF 1997)

·        Algebraic heat transfer model (IJHMT 2000)

-      Computation Fluid Dynamics

·        Inflow condition for RANS closure (AIAA J.2000)

·        New decoupling algorithm for unsteady RANS and LES(on going)

-      Unsteady RANS computation

·        Locally forced turbulent boundary layer with heat transfer( on going)

·        Locally forced backward-facing step( FDR 2000)

·        Enhancement of heat transfer in backward-facing step ( FDR 2000)

·        Impinging Jet with periodic inflow oscillation (on going)

-      Locally-forced backstep flow

-      Enhancement of heat transfer

Control of vortex shedding from a cylinder

-      Phase relations & Control result

-      System identification in lock-on range

-      Feedback control system

Lock-on behavior of the cylinder wake

-      Alternate shedding of vortices in the near wake leads to large fluctuating pressure forces in a direction transverse to the flow

·        natural shedding frequency

-      Rotary oscillation of a cylinder is applied

-      Maximum value of C_L and time of negative maximum C_L

·        Phase change of the vortex shedding, relative to the body oscillation

Large-scale vertical structure of turbulent separation bubble

-      In the forced wake flows, vortex shedding is entrained by the cylinder motion.

-      Self-excited vortex shedding frequency synchronizes with the forcing frequency provided that the two frequencies are not too different

-      If the difference in the two frequencies is large enough, a quasi-periodic or chaotic oscillation may occur.

-      Power spectra on the cylinder wake & Frequency selection

Response to superharmonic excitation

-      The lock-on of frequency still occurs when the ratio between the natural frequency of the self-excited oscillation and forcing frequency is in the neighborhood of an integer (different from unity) or a fraction(Hayashi 1985). => superharmonic lock-on and subharmonic lock-on

-      Schematic diagram of the vortex formation patterns

Control of separated TBL by local Lorentz force

-      Objective

·        Separation control by local forcing

·        Understanding the characteristics of the separated turbulent boundary layer

-      Flow configuration

·        Spatially developing turbulent boundary layer with separation bubble

-      Numerical method

·        Direct Numerical Simulation