Fluid dynamics simulations are powerful tools for studying complex flows and phenomena, such as turbulence, multiphase flows, and heat transfer. However, how do you know if your simulations are accurate and reliable? Here are some tips and methods to check the quality and validity of your fluid dynamics simulations.
Before you start any simulation, you need to define the problem you want to solve and the objectives you want to achieve. This will help you choose the appropriate physical models, boundary conditions, numerical schemes, and solver settings for your simulation. You also need to identify the relevant parameters and variables that you want to measure and compare with experimental or analytical data.
The mesh is the discretization of the domain into small elements or cells, where the governing equations are solved. The mesh quality and resolution can affect the accuracy and convergence of your simulation. You need to choose a mesh that is suitable for the geometry, flow features, and physical models of your problem. You also need to perform a mesh independence study, where you compare the results of different mesh sizes and refine the mesh until the results do not change significantly.
Meshing is important step during preprocessing. Different forms of meshing algorithms are presently available these days and most the commercial software packages tend to give you reasonably good mesh with the default setting itself. But, CFD engineers should take care about mesh quality and its applicability for the fluid mechanics problem that is being solved. There are mathematical quality parameters which must be checked after meshing before proceeding to the solver. Parameters such as orthogonal quality, skewness and aspect ration are some of the most important ones. Mesh elements can also impact the results based on the physics being solved. A tetrahedral mesh or hexahedral mesh can have different behavior while solving.
The residuals are the errors or imbalances in the governing equations at each iteration of the solver. They indicate how well the solution is converging to a steady or unsteady state. You need to monitor the residuals during your simulation and check if they are decreasing and reaching a desired level of tolerance. If the residuals are oscillating, diverging, or stagnating, you may need to adjust your solver settings, such as the relaxation factors, time step, or convergence criteria.
The results of your simulation are the outputs of the solver, such as the velocity, pressure, temperature, and other quantities of interest. You need to compare the results with experimental or analytical data, if available, or with other sources of information, such as literature, empirical correlations, or physical intuition. You need to evaluate the agreement and discrepancy between your simulation and the reference data, and analyze the possible sources of error and uncertainty.
A sensitivity analysis is a method to assess how the results of your simulation depend on the input parameters or assumptions. You need to perform a sensitivity analysis to identify the most influential factors that affect your simulation and quantify their impact on the output. You can vary one or more parameters, such as the physical models, boundary conditions, initial conditions, or numerical schemes, and observe how the results change accordingly.
The final step of checking your fluid dynamics simulation is to document and report your simulation process and results. You need to provide a clear and detailed description of your problem definition, mesh generation, solver settings, results comparison, and sensitivity analysis. You also need to present your results in an appropriate and consistent format, such as tables, graphs, or images, and highlight the main findings and conclusions of your simulation.