Effect of wall heating on turbulent boundary layers with temperature-dependent viscosity



Skin-friction reduction by viscosity stratification


-      To investigate the effects of wall heating on turbulent skin friction in turbulent boundary layer

        Direct numerical simulation (DNS)

        Fractional step algorithm with a local volume-flux formulation (Rosenfeld, Kwak & Vinokur 1991)

        The Arrhenius-type viscosity model for water (White 2006)

        Inflow simulation from the by-pass transition due to isotropic free-stream turbulence (Jacobs & Durbin 2001)


                                       Schematic diagram of computational domain


-      The effects of the wall temperature for high Prandtl-number fluid — 30, 70 and 99C

     Decreased skin friction due to the decreased viscosity near the heated wall,

        Weakened turbulent energy in the outer region

        Increased near-wall energy which leads to enhanced dissipation

        Decreased growth rate of the boundary layer


Skin-friction coefficient (Cf) and drag reduction rate (DR). T=30C.


Vortical structures with the lambda-2 criteria


(Lee et al., Journal of Fluid Mechanics, Vol. 726, pp.196-225, 2013)



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


        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


-      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 z and velocity behavior with forcing frequencies



-      Wall region z and velocity behavior with forcing angles


-      Experimental results


        High frequency and =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 =vW/U





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 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 (2), spanwise sliding velocity (3)



-      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


        Statistical processing using correlation between two interrogation windows

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


Separation control


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 ReH=560

        Wake disturbance within 10% of U


-      Experimental results

        Effective flow control at StH=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


-      xR measurement

        Further reduction of xR on the pressure side

 (wrapping regime, cutting regime)



-      Spreading rate

        Larger spreading rate at StH=0.2

        Further enhancement on the pressure side (CCW)


-      Turbulence intensity

        Larger turbulent intensity at StH=0.2

        Further increase for CCW

        Active enhancement of rolled-up vortices


-      Cp measurement

        Reduction as StH increases

        Further reduction for CCW


-      Pressure spectrum for CCW

        Significant increase at x/H=0.8

        Fast decay after xR/H=2.4



-      Coherence (St=0 and x0/H=0.8)

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


-      Coherence (St=0 and x0/H=8.0)

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

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



-      Coherence (StH=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 (StH=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


-      Coherence (StH=0.20 and x0/H=0.8, CW)

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

        Broad coherent region of St=0.14


-      Coherence (StH=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 xR 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

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

        Glycerol StH=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 Curles 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


-      Local forcing

        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: StH=0.27

        Decrease of xR with increasing A0

        Two minima

StH=0.27 : shedding type instability

StH=0.40 : shear layer instability



-      Velocity spectrum

        Effective forcing frequency: StH=0.27

        Vortex merging: StH/2, StH/4

        Initial region (x/H=1)

Enhancement of vortices

Increased turbulence level

        Reattachment region

Earlier reattachment

No forcing: xR/H=7.8

Effective forcing: xR/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()






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 Poissons 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 CL and time of negative maximum CL

        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