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

        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

 
 

-      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