P Balakumar Flow Physics and Control Branch NASA Langley Research Center Symposium on Advances in Turbulence Modeling July 13 2017 Ann Arbor MI Objectives Perform DNSLES to compute turbulent separated flows at high Reynolds numbers ID: 634184
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Slide1
DNS/LES Simulations of Separated Flows
P. Balakumar Flow Physics and Control BranchNASA Langley Research CenterSymposium on Advances in Turbulence ModelingJuly 13, 2017Ann Arbor. MISlide2
Objectives
Perform DNS/LES to compute turbulent separated flows at high Reynolds numbers.Investigate the turbulent dynamics of separated flows by analyzing the mean flow profiles, Reynolds stresses, and the turbulent kinetic energy budget.Simulations are performed for flowsthrough a periodic channel with a constriction, over an NACA airfoil at high angle of attack.
2Slide3
Numerical Method
3Higher-order compact schemes are used for spatial discretizations.6th order in the homogeneous directions x and z.6th order schemes are used in the normal directions with third order scheme near the wall.8th
order filtering is used in every 10 time steps.3rd Order TVD explicit RK scheme is used for time-discretization.Slide4
4
Breuer et al. (2009), Frohlich et al. (2005), Ziefle et al. (2008)ModelFlow is periodic in x and z directions.Slide5
Instantaneous V
elocity Contours Reb=105955In the cross sectional plane x=1.7In the plan view , J=5 grid line (y+ ~ 5)In the longitudinal planeSlide6
Mean Quantities Re
b= 105956DNSRANS-SST (Rumsey NASA/TM 212412) Separation point 0.20Reattachment point 4.68Separation point 0.25Reattachment point 7.5Slide7
Mean Quantities Re
b= 105957Slide8
Reynolds Stress
Profiles Reb=105958
At x = 2
The ratios are
1 : 1/1.6 : 1/1.3
compared to
1 : 1/9 : 1/6.2 in plane channel flows.Slide9
Balance of Different Terms in the
TKE Equation Reb=28009
At x =
1
= (0.11) –(0.06) –(0.03) – (0.02)
Plane channel flowSlide10
TKE
Balance Re = 10595 10Slide11
Turbulent Statistical
Quantities Reb=1059511
Production is maximum near the start of the separated shear layer.
Dissipation is maximum near the wall. Outside the wall region it is confined to the separated shear layer.
Production/Dissipation is about 5
near the separation point region. It is about 1.8 for the channel flow.
TKE peaks in the middle of the separated shear layer.
Shear stress is concentrated near the start of the separated shear layer.Slide12
Reynolds Stress
Reb=1059512Slide13
Reynolds Stress
Reb=1059513Slide14
DNS of NACA-0012 Airfoil (Re
c= 1*106, α = 15 deg)14
N
x
N
y
N
z
3001
501
513
Grid sizes
Domain Size
Outer boundary
Span
15 c
0.2cSlide15
Isosurfaces and Velocity Contours (Re
c= 1*106, α=15 deg)15
Q=500Slide16
Contours of Mean U-Velocity (Re
c= 1*106, α=15 deg)16
Separation point 0.005Reattachment point
0.024
Separation point
0.80Slide17
Instantaneous and Mean Pressure Distribution (Rec= 1*10
6, α=15 deg)17
Mean
CpSlide18
18
C
D
C
L
α (deg)
Pressure
Friction
Total
Pressure
Friction
Total
15
0.0526
0.0042
0.0568
1.295
0.0005
1.295
15
0.0287
0.0055
0.0342
1.454
0
1.454
Mean cf (Re
c
= 1*10
6
, α = 15 deg)Slide19
Mean U-Velocity Profiles (Re
c= 1*106, α=15 deg )19Slide20
Reynolds Stresses (Re
c= 1*106, α=15 deg )20Slide21
Contours of Turbulent Quantities (Re
c= 1*106, α=15 deg )21Slide22
Conclusions
DNS for the hillThe Reynolds stresses peak near the separation point and the ratios of the three normal stresses are 1:1/1.6:1/1.3. For plane channel flows they are 1:1/9:1/6.2.TKE balance showed that near the separation point, production is balanced by the diffusion and dissipation. The diffusion is two times the dissipation. Production/Dissipation is about 5.DNS for the flow over an airfoil at high angle of attackA small laminar separation bubble on the order of 2% of the chord at the airfoil leading edge (x/c ~ 0.005 to 0.024) Two-layer structure develops downstream of the reattachment pointThe Reynolds stresses near the wall decrease along the chord and the Reynolds stresses gradually increase in the outer part of the boundary layer
22Slide23
END
.23Slide24
Instantaneous V
elocity Contours Reb=280024In the cross sectional plane x=2In the plan view , J=5 grid line (y+ ~ 2)In the longitudinal planeSlide25
Contours of the Streamwise
Velocity and Cf from RANS Computations Re = 280025
SST
RSMSlide26
Introduction
RANS Computations predict mean and turbulent quantities well in attached flows where turbulence is in equilibrium.Their performance is not very satisfactory in separated non-equilibrium flows.The predicted turbulent shear stresses are two to three times smaller than measured quantities.The reason may be the production/dissipation ratio in the shear layer region of the flow is very high ~ 6Large-Eddy Simulation (LES) is considered a viable option due to its accuracy and computational efficiency compared to Direct Numerical Simulation (DNS).
26Slide27
Mean Quantities Re
b=280027Skin frictionPressure coefficient Separation point 0.23Reattachment point 5.45Slide28
Comparison of Mean Quantities
Reb=280028U VelocitySlide29
Reynolds Stress
Profiles Reb=280029
At x = 1
The ratios are
1 : 1/2.25 : 1/1.8
compared to
1 : 1/9 : 1/6.2 in plane channel flows.Slide30
DNS Reb
=280030NxNyNzΔx+Δy+Δz+DNS5132572893.00.20
3.0Grid spacing
Grid
spacing respect to Kolmogorov scale
Grid
sizeSlide31
DNS Reb=10595
31NxNyNzΔx+Δy+Δz+DNS8015015134.70.503.6
Grid spacingGrid spacing respect to Kolmogorov scale
Grid
sizeSlide32
Turbulent Statistical
Quantities Reb=280032
Production is maximum near the start of the separated shear layer.
Dissipation is maximum near the wall. Outside the wall region it is confined to the separated shear layer.
Production/Dissipation is about
6
near the separation point region. It is about 1.8 for the channel flow.
TKE peaks in the middle of the separated shear layer.
Shear stress is concentrated near the start of the separated shear layer.Slide33
33
Mean Profiles (Re
c= 50K, α = 5 deg)Slide34
Objectives
Perform DNS/LES to compute turbulent separated flows at high Reynolds numbers.Investigate the turbulent dynamics of separated flows by analyzing the mean flow profiles, Reynolds stresses and the turbulent kinetic energy budget.Simulations are performed for flowsthrough a periodic channel with a constriction, over an NACA airfoil at high angle of attack.
34Slide35
Grid Distribution (Rec= 1*10
6, α=15 deg)35
Ratio of grid spacing to Kolmogorov scale at different locations
𝝙x/𝝶
𝝙y/𝝶
𝝙z/𝝶
15
6
5Slide36
Velocity and Reynolds Stress
Reb=1059536U Velocity