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Cf considerably higher when using the compressible solver |
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#1 |
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Hi everyone, this is my first post in this forum. I'm relatively new to SU2, but I have a background in CFD using in-house (academic) solvers. My background has been mostly in high-speed flows, but currently, I am interested in simulating low-speed flows.
__________________________________________________ Test case A NACA4412 finite wing with a chord of 1 m and a span of 6 m (AR=6). The Mach number is 0.09 and the Reynolds number is 1.52E6. The wing has a blunt trailing edge (1% of the chord length). __________________________________________________ Issue High skin-friction coefficients are predicted by the compressible solver on the same mesh compared to the incompressible solver. Does anyone observe similar discrepancies or am I missing something? The predicted pressure coefficient agrees well between the solvers and the experiment (see Results). __________________________________________________ What I've tried - I've tried adjusting the Venkatakrishnan limiter coefficient value (increased to 0.2). This resulted in a less overprediction of Cf, but the drag was still overpredicted compared to the incompressible solution. - I've tried the Van-Albada limiter for the flow. For turbulence, MUSCL was set to NO. This resulted in a Cf and drag prediction comparable to the default Venkatakrishnan limiter coefficient (0.05). Drag is still overpredicted. - Y+ in both solutions (compressible/incompressible) is < 1. __________________________________________________ Domain and boundary conditions - FARFIELD - SYMMETRY - HEATFLUX domain.png __________________________________________________ Setup Compressible (Roe, MUSCL=YES, Venkatakrishnan=0.05, Entropy Fix=1e-5) Code:
SOLVER= RANS KIND_TURB_MODEL= SST SST_OPTIONS= V2003m MATH_PROBLEM= DIRECT RESTART_SOL= NO %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% MACH_NUMBER= 0.09 AOA= 0 SIDESLIP_ANGLE= 0.0 INIT_OPTION= REYNOLDS FREESTREAM_OPTION= TEMPERATURE_FS FREESTREAM_TEMPERATURE= 297.222 REYNOLDS_NUMBER= 1.52E6 REYNOLDS_LENGTH= 1.0 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% FLUID_MODEL= STANDARD_AIR GAMMA_VALUE= 1.4 GAS_CONSTANT= 287.058 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% VISCOSITY_MODEL= SUTHERLAND MU_REF= 1.716E-5 MU_T_REF= 273.15 SUTHERLAND_CONSTANT= 110.4 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% CONDUCTIVITY_MODEL= CONSTANT_PRANDTL PRANDTL_LAM= 0.72 PRANDTL_TURB= 0.90 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% REF_ORIGIN_MOMENT_X = 0.25 REF_ORIGIN_MOMENT_Y = 0.00 REF_ORIGIN_MOMENT_Z = 0.00 REF_LENGTH= 1.0 REF_AREA= 3.0 REF_DIMENSIONALIZATION= FREESTREAM_VEL_EQ_ONE %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% MARKER_HEATFLUX= ( tri_WING, 0.0, quad_WING, 0.0 ) MARKER_FAR= ( FARFIELD) MARKER_PLOTTING= ( tri_WING, quad_WING) MARKER_MONITORING= ( tri_WING, quad_WING) MARKER_SYM= ( tri_SYMMETRY, quad_SYMMETRY) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% NUM_METHOD_GRAD= GREEN_GAUSS CFL_NUMBER= 5.0 CFL_ADAPT= NO CFL_ADAPT_PARAM= ( 0.5, 1.5, 5.0, 25.0 ) ITER=10000 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% LINEAR_SOLVER= FGMRES LINEAR_SOLVER_PREC= ILU LINEAR_SOLVER_ERROR= 1E-10 LINEAR_SOLVER_ITER= 10 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% CONV_NUM_METHOD_FLOW= ROE ENTROPY_FIX_COEFF=1E-5 MUSCL_FLOW= YES SLOPE_LIMITER_FLOW= VENKATAKRISHNAN VENKAT_LIMITER_COEFF= 0.05 TIME_DISCRE_FLOW= EULER_IMPLICIT %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% CONV_NUM_METHOD_TURB= SCALAR_UPWIND MUSCL_TURB= YES SLOPE_LIMITER_TURB= VENKATAKRISHNAN TIME_DISCRE_TURB= EULER_IMPLICIT %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% CONV_FIELD= LIFT CONV_STARTITER= 10 CONV_CAUCHY_ELEMS= 100 CONV_CAUCHY_EPS= 1E-6 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% MESH_FILENAME= naca4412_wing.cgns MESH_FORMAT= CGNS SOLUTION_FILENAME= restart_flow.dat TABULAR_FORMAT= CSV CONV_FILENAME= history RESTART_FILENAME= restart_flow.dat VOLUME_FILENAME= flow SURFACE_FILENAME= surface_flow OUTPUT_WRT_FREQ= 250 SCREEN_OUTPUT= (INNER_ITER, WALL_TIME, RMS_DENSITY, RMS_NU_TILDE, LIFT, DRAG) HISTORY_OUTPUT= (ITER, RMS_RES, AERO_COEFF) OUTPUT_FILES = (RESTART, PARAVIEW_ASCII, SURFACE_PARAVIEW_ASCII) Code:
SOLVER= INC_RANS KIND_TURB_MODEL= SST SST_OPTIONS= V2003m MATH_PROBLEM= DIRECT RESTART_SOL= NO %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% INC_DENSITY_INIT=0.8956 INC_VELOCITY_INIT=(31.1051, 0.0, 0.0) INC_NONDIM=INITIAL_VALUES INC_DENSITY_REF=1.0 INC_VELOCITY_REF=1.0 INC_TEMPERATURE_REF=1.0 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% VISCOSITY_MODEL= CONSTANT_VISCOSITY MU_CONSTANT=1.8327E-5 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% REF_ORIGIN_MOMENT_X = 0.25 REF_ORIGIN_MOMENT_Y = 0.00 REF_ORIGIN_MOMENT_Z = 0.00 REF_LENGTH= 1.0 REF_AREA= 3.0 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% MARKER_HEATFLUX= ( tri_WING, 0.0, quad_WING, 0.0 ) MARKER_FAR= ( FARFIELD) MARKER_PLOTTING= ( tri_WING, quad_WING) MARKER_MONITORING= ( tri_WING, quad_WING) MARKER_SYM= ( tri_SYMMETRY, quad_SYMMETRY) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% NUM_METHOD_GRAD= GREEN_GAUSS CFL_NUMBER= 5.0 CFL_ADAPT= NO CFL_ADAPT_PARAM= ( 0.5, 1.5, 5.0, 25.0 ) ITER=10000 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% LINEAR_SOLVER= FGMRES LINEAR_SOLVER_PREC= ILU LINEAR_SOLVER_ERROR= 1E-10 LINEAR_SOLVER_ITER= 10 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% CONV_NUM_METHOD_FLOW= FDS MUSCL_FLOW= YES SLOPE_LIMITER_FLOW= VENKATAKRISHNAN VENKAT_LIMITER_COEFF= 0.05 TIME_DISCRE_FLOW= EULER_IMPLICIT %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% CONV_NUM_METHOD_TURB= SCALAR_UPWIND MUSCL_TURB= YES SLOPE_LIMITER_TURB= VENKATAKRISHNAN TIME_DISCRE_TURB= EULER_IMPLICIT %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% CONV_FIELD= LIFT CONV_STARTITER= 10 CONV_CAUCHY_ELEMS= 100 CONV_CAUCHY_EPS= 1E-6 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% MESH_FILENAME= naca4412_wing.cgns MESH_FORMAT= CGNS SOLUTION_FILENAME= restart_flow.dat TABULAR_FORMAT= CSV CONV_FILENAME= history RESTART_FILENAME= restart_flow.dat VOLUME_FILENAME= flow SURFACE_FILENAME= surface_flow OUTPUT_WRT_FREQ= 250 SCREEN_OUTPUT= (INNER_ITER, WALL_TIME, RMS_DENSITY, RMS_NU_TILDE, LIFT, DRAG) HISTORY_OUTPUT= (ITER, RMS_RES, AERO_COEFF) OUTPUT_FILES = (RESTART, PARAVIEW_ASCII, SURFACE_PARAVIEW_ASCII) Results Cp_comparison.png CLCD_history.png comparison_Cf.jpg |
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#2 |
Senior Member
bigfoot
Join Date: Dec 2011
Location: Netherlands
Posts: 596
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What is the difference in CL and CD for both cases? It looks very similar. Do you have some values, and the corresponding value from literature?
Is this the case from the nasa larc website? https://turbmodels.larc.nasa.gov/naca4412sep_val.html The nasa validation was done for high angle of attack, and you simulate it for AoA=0, right? It would help here if the expected CL and CD are given. It would then be possible to say if all nondimensionalization was done correctly. There are also some pressure peaks at the trailing edge, but this might be due to the mesh quality there, the mesh needs some special attention in that region. |
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#3 |
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Join Date: Jul 2024
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Thank you for the prompt reply! The purpose of these simulations was for me to benchmark the compressible solver for low-Mach problems. More specifically - if the compressible solver can reproduce the solution from the incompressible solver. I decided to use the freestream conditions from the NACA 4412 TMR case. This case, as you noted, is 2D and is at an AOA of 10 deg. Using the freestream conditions from the NACA4412 TMR case was an arbitrary decision (I am interested in comparing the solvers). Before performing the 3D simulations, I have done the 2D simulations for the NACA4412 and NACA0012 2D TMR cases.
The 3D simulations of the NACA4412 wing result in the following CL and CD: CL: 0.3275 (compressible) CD: 0.0341 (compressible) CL: 0.3145 (incompressible) CD: 0.0206 (incompressible) The differences in the CL are reasonable, but the difference in CD is substantial. The peaks at the trailing edge are due to the "bluntness" of the trailing edge. Most SU2 cases use sharp trailing edges, but for complex 3D geometries sharp trailing edges usually result in poor mesh quality. Therefore the simulations above employ a blunt trailing edge. I have 3 cells across (coarse mesh). The location of the blunt trailing edge is at 99% of the chord. PS: I have noted that when the Reynolds number is lowered the results for CD from the incompressible and compressible solver begin to grow apart. I've done 2D simulations of the NACA0012 TMR case at 6e6, both solvers agree on the CD although the compressible solver predicts a slightly higher CD. I've lowered the Re to 6e5 and repeated the simulations. The differences in CD between the solvers increased to 35%. |
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