Detached Eddy Simulations of an Airfoil in Turbulent Inflow
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Transcript of Detached Eddy Simulations of an Airfoil in Turbulent Inflow
Detached Eddy Simulations of an Airfoil in Turbulent Inflow
Lasse Gilling, Aalborg University, DenmarkNiels N. Sørensen, Nat. Lab. Sustainable Energy, Risø/DTU, DenmarkLars Davidson, Chalmers University of Technology, Sweden
Agenda
• Introduction• Computational Setup• Numerical Methods• Inflow Boundary Condition• Results and Discussion• Conclusions
2Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Introduction
• The most common approach to DES of airfoils is to use a mesh like this
• Coarse grid far from the airfoil• Fine grid close the airfoil• Laminar inflow with low eddy
viscosity
• Wind turbines operate close to the ground and are subjected to high levels of turbulence
• This work investigates the importance of resolving the inflow turbulence
3Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Computational Setup
• Geometry like the wind tunnel
• NACA 0015 airfoil• Re=1.6×106
• 21 million cells
• Extruded 2D mesh• O-mesh close to
the airfoil• Cartesian cells
everwhere else• The cells are
stretched prior to the outlet
• Here every 8th cell is shown
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Inlet
Periodicity
Symmetry
Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
O-mesh Close to the Airfoil
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384×64 cells in O-mesh - 128 cells in spanwise direction
Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Cell Sizes
Close to the wall• Cell size in wall units is
shown in the figure• Non-constant friction
velocity
In the Cartesian part• Δx ≈ 1.4×10-2 c• Δy ≈ 1.6×10-2 c• Δz ≈ 1.2×10-2 c
6Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Numerical Methods
EllipSys3D• Developed by J. Michelsen and N. Sørensen from DTU and Risø• Incompressible Navier-Stokes equations• Finite volume (cell-centered)• Structured, multi-block grid• Rhie/Chow interpolation• PISO algorithm• Detached eddy simulations with the k-ω SST subgrid turbulence
model• Momentum equations are solved with 4th order central difference
scheme• 2nd order accurate dual time stepping algorithm
7Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Inflow Boundary Condition
• Fluctuating velocity field is used for inflow boundary condition
• Synthetic inflow turbulence is created by the method of Mann• All three
velocity components
• Components are correlated
• Velocity field is divergence free
8Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Precursor Simulation
• Random phases and incorrect statistical moments of third and higher order
• The synthetic turbulence is run through a precursor simulation to• Let the flow solver correct random phases and incorrect higher
order moments• Let the turbulence adopt to the grid and the numerical method
9Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Spatial Decay of Homogenous Turbulence
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Spatial decay is studied• Test numerical
method • Test synthetic
turbulence
Spatial Decay of Isotropic Turbulence
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The three curves should have the same slope as the emperical line
Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Results and Discussion: Lift and Drag
• Flow is sensitive to turbulence• DES with no inflow turbulence predicts stall too late• DES with 0.5% turbulence intensity (TI) gives good agreement before stall• DES with 2.0% TI gives poor results for low AOA but better after stall• 2D RANS is good for low AOA, but fails to predict stall• Experiment: ~0.1% turbulence intensity
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0 2 4 6 8 10 12 14 16 18 200
0.5
1
1.5
Angle of attack [deg]
CL
2D RANS
DES, TI=0.0%
DES, TI=0.5%
DES, TI=2.0%
Measurements
0 2 4 6 8 10 12 14 16 18 200
0.05
0.1
0.15
0.2
0.25
0.3
Angle of attack [deg]
CD
2D RANS
DES, TI=0.0%
DES, TI=0.5%
DES, TI=2.0%
Measurements
Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Surface Pressure
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AOA=16° AOA=18°
AOA=14°
• Good agreement• Low TI best for low AOA• High TI best for high AOA• Flow very sensitive at 16° AOA
Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Skin Friction
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For low AOA:• Increased TI moves separation
point upstreamFor high AOA:• Increased TI moves separation
point downstream
AOA=16° AOA=18°
AOA=14°
Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Force History
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• AOA is 16° – close to stall• Required simulation time
depends on the TI
Low TI • Long flow development time• Shows large, slow oscillationsHigh TI • Short flow development time• Only small, fast oscilations
Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Flow Visualization – Low Turbulence
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• TI is 0.1% and AOA is 16°• Surface limited streamlines
and iso-vorticity • Large separation gives low lift
and vice versa• Very unsteady, large
spanwise variations• Modeling full width of tunnel
is requiredIntroduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion –
Conclusions
Flow Visualization – High Turbulence
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• TI is 2.0% and AOA is 16°• Surface limited streamlines
and iso-vorticity • Much smaller variations in
time and spanwise direction• More steady lift
Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Averaged Turbulence Intensity
• AOA is 12° and TI is 0.5%• Leading edge is located at x/c=0• Only little decay upstream of the airfoil• Turbulence is generated in the separation bubble and the first part of the wake• Larger decay in stretched part of the grid (for x/c>6)
18Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Eddy Viscosity
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• Eddy viscosity normalized by the molecular viscosity• AOA is 12° and TI is 0.5%• High eddy viscosity in the wake and separated region• Eddy viscosity far from the airfoil is constant
Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Subgrid Kinetic Energy
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• Subgrid kinetic energy normalized by the mean velocity squared• AOA is 12° and TI is 0.5%• High subgrid kinetic energy close to the wall• Far from the airfoil is constant and low• Intermediate values in the wake
Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Resolved Kinetic Energy
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• Resolved kinetic energy normalized by the mean velocity squared• AOA is 12° and TI is 0.5%• High resolved kinetic energy in the wake• Far from the airfoil is is constant with a value corresponding to the
intensity of the resolved turbulence
Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Conclusions
• Computed lift and drag depends on the resolved turbulence intensity
• Stall is predicted best with TI similar to the one in the experiment• Low AOA: Increased turbulence moves separation point
upstream• High AOA: Increased turbulence moves separation point
downstream
• Best agreement with measurements is obtained• Low AOA: Low turbulence intensity• High AOA: High turbulence intensity
22Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Future Plans
• Implement an actuator disc approach of imposing the turbulence• Turbulence can be imposed immediately upstream of the airfoil• Save mesh points
• Investigate the influence of the turbulence length scale
23Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
Detached Eddy Simulations of an Airfoil in Turbulent Inflow
Lasse Gilling, Aalborg University, DenmarkNiels N. Sørensen, Nat. Lab. Sustainable Energy, Risø/DTU, DenmarkLars Davidson, Chalmers University of Technology, Sweden