detached eddy simulations of an airfoil in turbulent inflow
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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
lg@civil.aau.dk
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
4
Inlet
Periodicity
Symmetry
Introduction – Computational Setup – Numerical Methods – Inflow Boundary Condition – Results and Discussion – Conclusions
O-mesh Close to the Airfoil
5
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
10
Spatial decay is studied• Test numerical
method • Test synthetic
turbulence
Spatial Decay of Isotropic Turbulence
11
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
12
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
13
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
14
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
15
• 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
16
• 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
17
• 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
19
• 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
20
• 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
21
• 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
lg@civil.aau.dk
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