Transient CFD Analysis of Unsteady Fluids
In engineering, there are many unsteady fluid problems that cannot be simply solved using steady-state methods. Unsteady flows are primarily caused by two factors. One is due to internal fluid instability or a non-equilibrium initial state of the fluid, such as turbulence of various scales, shockwaves, convection, etc. The other is due to changing boundary conditions or source terms, like pulsating flows or the rotation of rotor blades in machinery. For these unsteady flows, it is necessary to use transient analysis methods in order to understand the state of the fluid and its solid contact surfaces.
Transient analysis involves calculating the flow field for multiple time steps. The computational workload also increases linearly with the duration of the simulation. Therefore it is a common approach to compute a steady state within a short time interval at each time step, numerically. Then, the results of this steady state are used as the initial conditions for the next time step’s calculation. Depending on the time solver used, the choice of time step may vary slightly, with implicit solvers typically allowing for larger time steps than explicit solvers. The well-known open-source multi-physics solver SU2 has been proven to work well for unsteady CFD problems. WELSIM also added support for SU2 in its 2023R3 release; for more details, refer to the article “Generate SU2 solver scripts using WELSIM.”
Steps for Unsteady CFD Analysis
Below is an example demonstrating how to perform transient CFD analysis:
(1) Using a 2D model as an example, open WELSIM and create a new project, setting the model as a 2D transient fluid model.
(2) Import the geometry model.
(3) Create the mesh, setting the maximum element size to 0.03 meter.
(4) Set the time step for the solver to 0.0005 second, with a total run time of 0.6 second.
(5) Use the SU2 solver.
(6) The governing equations use RANS for compressible fluids, and the turbulence model is Spalart-Allmaras.
(7) Configure relevant solver parameters.
(8) Set the free-stream field conditions, including a Mach number of 0.3, an angle of attack of 17 degrees, standard temperature and pressure, and a Reynolds number of 1000.
(9) Define the far-field boundary conditions.
(10) Specify the thermal boundary conditions with a value of zero for heat flux and no heat convection.
Click the solve button. Since it is a transient computation, it will require a significant amount of computational time based on the mesh density and duration. After the calculation is complete, add a Mach number result object and display contour plots. The images below show the Mach number in the flow field at 0.027 seconds and 0.597 seconds, respectively.
The calculation result video for this case is as follows.
Additionally, this case has been integrated into WELSIM’s automated regression testing library, which can be beneficial for the long-term maintenance of the solver and front-end software. The test files have been open-sourced and shared on GitHub at the following address:
https://github.com/WelSimLLC/WelSimAutoTests
WELSIM’s Support for SU2 Commands
The config file of SU2 is the main solver input file, and currently, WELSIM as a preprocessor supports a significant number of core commands. SU2 version 7.5.1 includes a total of 671 commands, and WELSIM already supports 134 commands, accounting for 20% of the total. The supported commands are listed below.
SOLVER
MATH_PROBLEM
KIND_TURB_MODEL
KIND_TRANS_MODEL
BODY_FORCE
BODY_FORCE_VECTOR
RESTART_SOL
FLUID_MODEL
SPECIFIC_HEAT_CP
VISCOSITY_MODEL
MU_CONSTANT
CONDUCTIVITY_MODEL
THERMAL_CONDUCTIVITY_CONSTANT
REYNOLDS_NUMBER
REYNOLDS_LENGTH
PRANDTL_LAM
PRANDTL_TURB
MACH_NUMBER
INIT_OPTION
FREESTREAM_OPTION
FREESTREAM_PRESSURE
FREESTREAM_DENSITY
FREESTREAM_TEMPERATURE
FREESTREAM_TEMPERATURE_VE
INC_DENSITY_MODEL
INC_ENERGY_EQUATION
INC_DENSITY_INIT
INC_VELOCITY_INIT
INC_TEMPERATURE_INIT
FREESTREAM_VELOCITY
FREESTREAM_VISCOSITY
FREESTREAM_INTERMITTENCY
FREESTREAM_TURBULENCEINTENSITY
FREESTREAM_NU_FACTOR
SIDESLIP_ANGLE
AOA
REF_ORIGIN_MOMENT_X
REF_ORIGIN_MOMENT_Y
REF_ORIGIN_MOMENT_Z
REF_AREA
REF_LENGTH
REF_DIMENSIONALIZATION
MARKER_PLOTTING
MARKER_MONITORING
MARKER_ANALYZE
MARKER_DESIGNING
MARKER_EULER
MARKER_FAR
MARKER_SYM
MARKER_NEARFIELD
INLET_TYPE
INC_INLET_TYPE
MARKER_INLET
MARKER_INLET_SPECIES
MARKER_INLET_TURBULENT
MARKER_SUPERSONIC_INLET
MARKER_SUPERSONIC_OUTLET
MARKER_OUTLET
INC_OUTLET_TYPE
MARKER_ISOTHERMAL
MARKER_HEATFLUX
MARKER_HEATTRANSFER
MARKER_PRESSURE
MARKER_DAMPER
TIME_MARCHING
CFL_NUMBER
CFL_ADAPT
CFL_ADAPT_PARAM
RK_ALPHA_COEFF
TIME_DISCRE_FLOW
TIME_DISCRE_FEM_FLOW
TIME_DISCRE_ADJFLOW
TIME_DISCRE_TURB
LINEAR_SOLVER
LINEAR_SOLVER_PREC
LINEAR_SOLVER_ERROR
LINEAR_SOLVER_ITER
CONV_RESIDUAL_MINVAL
CONV_STARTITER
CONV_CAUCHY_ELEMS
CONV_CAUCHY_EPS
CONV_FIELD
MGLEVEL
MGCYCLE
MG_PRE_SMOOTH
MG_POST_SMOOTH
MG_CORRECTION_SMOOTH
MG_DAMP_RESTRICTION
MG_DAMP_PROLONGATION
NUM_METHOD_GRAD
NUM_METHOD_GRAD_RECON
VENKAT_LIMITER_COEFF
ADJ_SHARP_LIMITER_COEFF
CONV_NUM_METHOD_FLOW
MUSCL_FLOW
SLOPE_LIMITER_FLOW
JST_SENSOR_COEFF
LAX_SENSOR_COEFF
CONV_NUM_METHOD_ADJFLOW
MUSCL_ADJFLOW
SLOPE_LIMITER_ADJFLOW
MESH_FORMAT
MESH_FILENAME
MESH_OUT_FILENAME
CONV_FILENAME
SOLUTION_FILENAME
SOLUTION_ADJ_FILENAME
RESTART_FILENAME
RESTART_ADJ_FILENAME
VOLUME_FILENAME
VOLUME_ADJ_FILENAME
GRAD_OBJFUNC_FILENAME
VALUE_OBJFUNC_FILENAME
SURFACE_FILENAME
SURFACE_ADJ_FILENAME
SURFACE_SENS_FILENAME
VOLUME_SENS_FILENAME
TIME_DOMAIN
TIME_ITER
ITER
RESTART_ITER
TIME_STEP
SCREEN_OUTPUT
HISTORY_OUTPUT
VOLUME_OUTPUT
OUTPUT_WRT_FREQ
OUTPUT_FILES
Conclusion
SU2 is an excellent open-source CFD solver known for its performance and accessible licensing. It is capable of rapidly solving unsteady fluid problems involving turbulence. With its user-friendly graphical interface, WELSIM makes working with SU2 easier. WELSIM seamlessly integrates with SU2 for solving and displaying results, as well as generating SU2 computation input scripts as needed. Currently, WELSIM is recognized as the leading pre- and post-processing software worldwide for supporting SU2.
WELSIM is the #1 engineering simulation CAE software for the open-source community.