Computational fluid dynamics (CFD) is a powerful tool for engineering design, but it also comes with some challenges and limitations. To get the most out of your CFD simulations, you need to optimize them for better accuracy and reliability. Here are some tips on how to do that.
The first step to optimize your CFD simulations is to choose the right model for your problem. There are different types of models, such as laminar, turbulent, compressible, incompressible, multiphase, and so on. Each model has its own assumptions, equations, and parameters that affect the accuracy of the results. You should select the model that best represents the physics of your fluid flow, and avoid using models that are too simple or too complex for your case.
Optimizing the accuracy of CFD simulations significantly depends on selecting the appropriate models for the specific flow phenomena being studied: Turbulence Models: Choosing the right turbulence model (like RANS for less computational demand or LES/DES for complex, transient phenomena) is key for accurately capturing flow details. Multiphase Models: For simulations involving multiple phases, selecting the correct approach (Eulerian, Lagrangian, or Volume of Fluid) is crucial to accurately model the interactions between phases. Reaction and Combustion Models: Selecting the appropriate combustion model based on the type of combustion (premixed, non-premixed, or partially premixed) to predict flame characteristics and emissions correctly.
The second step to optimize your CFD simulations is to refine your mesh. The mesh is the grid that divides your domain into small elements, where the fluid variables are calculated. The quality and size of your mesh can have a significant impact on the accuracy and convergence of your simulation. You should use a mesh that is fine enough to capture the important features of your flow, such as boundary layers, wakes, shocks, and so on. You should also use a mesh that is smooth, uniform, and orthogonal, to avoid numerical errors and instabilities.
Mesh refinement is a critical aspect of CFD simulation that directly impacts accuracy. I have always compared mesh to the resolution in photographs. Photos with large pixels are not clear and the resolution is low, it is difficult to understand what the photo taken is. As we reduce the size of pixels, we increase the resolution of the photo and get a more accurate and clear image. With mesh independency study, implementing local mesh refinement, optimizing mesh quality, choosing the appropriate mesh type, you can significantly improve the accuracy of your CFD simulations. This approach allows for a more precise representation of the physical phenomena being modeled, enhancing the reliability of the simulation results.
Mesh independency tests are a crucial step for getting a high quality mesh. Take a critical parameter such as temperature in specific regions and see how its calculated value changes with the mesh refinement. If we reach a point where further refinements will not change or affect the temperature (or any other monitored parameter), or the change is very small (matching a pre-established acceptance criteria), then we have found a mesh we can work with.
The third step to optimize your CFD simulations is to validate your results. Validation is the process of comparing your simulation results with experimental data or analytical solutions, to check if they are in good agreement. Validation can help you identify and correct any errors or discrepancies in your model, mesh, or boundary conditions. You should use validation data that is relevant, reliable, and representative of your problem. You should also use validation metrics that are meaningful, such as error norms, correlation coefficients, and confidence intervals.
It is important to perform a comprehensive literature review before starting a CFD study. You can use the example closest to your problem as a basis. Then, depending on the similarity of the results, you can continue. The most accurate-most costly method to verify the results is experiment. A cheaper-complex method is to confirm the analysis analytically. What differentiates CFD from 'Colorful'FD is the measurability of the analyzed situation and the physical accuracy of the results. Check for the Newton's Laws (conservation of mass, conservation of energy, etc.). If the general condition looks correct both physically and 'visually', validation can be performed by experimental or analytical methods.
The fourth step to optimize your CFD simulations is to perform sensitivity analysis. Sensitivity analysis is the process of evaluating how your simulation results change with respect to variations in your input parameters, such as geometry, material properties, initial conditions, and so on. Sensitivity analysis can help you understand the effects of uncertainty and variability on your output variables, such as pressure, velocity, temperature, and so on. You should use sensitivity analysis methods that are appropriate, such as perturbation, design of experiments, or Monte Carlo.
Sensitivity analysis is another step that should not be overlooked. Sensitivity analysis allows us to answer questions like "What would happen with my output Y if my input X changes 5% (for example)?" That kind of questions are sometimes more complicated or costly when answered with experiments. If we want to understand how the different shapes of an airfoil affect the laminar-turbulent transition region using a rig, for example, we would need to build the different airfoils, which in turn would imply an increase in time and cost (due to the manufacturing of the airfoils). That apart from the time required to set up and run the tests with the different airfoils.
The fifth step to optimize your CFD simulations is to optimize your design. Optimization is the process of finding the optimal values of your design variables, such as shape, size, angle, and so on, that maximize or minimize your objective function, such as drag, lift, heat transfer, and so on. Optimization can help you improve the performance and efficiency of your engineering system, such as a wing, a nozzle, a heat exchanger, and so on. You should use optimization techniques that are efficient, such as gradient-based, evolutionary, or surrogate-based.
The sixth and final step to optimize your CFD simulations is to document your work. Documentation is the process of recording and reporting your simulation setup, results, and analysis, in a clear and concise manner. Documentation can help you communicate your findings and conclusions to your peers, clients, or reviewers. You should use documentation standards that are consistent, such as ISO, ASME, or NAFEMS. You should also use documentation tools that are effective, such as tables, graphs, images, or videos.