💥💥💥 How to speed up convergence for steady state analysis in Ansys Fluent - 5 tips

There are several factors that can affect the convergence speed for steady state analysis in ANSYS Fluent, such as the mesh quality, the solver settings, the initial conditions, the boundary conditions, and the physical models. Here are some general tips that may help you to speed up your convergence¹²³:

• Check your mesh quality and refine it if necessary. A good mesh should have a high element quality, a low aspect ratio, and a smooth transition between different sizes. You can use the mesh adaption feature to refine the mesh in regions of high gradients or curvature.
• Choose the appropriate solver settings for your problem. For example, if you are simulating natural or mixed convection, you may need to use a lower convergence criteria for the energy equation (e.g., 1e-12) and turn on gravity and radiation models. You may also need to use a small velocity in the direction opposite to gravity as an initial condition for steady state problems. You can also adjust the under-relaxation factors (URF) for different equations to improve stability and convergence. A common choice is to use 0.7 for pressure and 0.3 for momentum.
• Use a good initial guess for your solution. You can use the patching feature to specify different values for different regions of your domain. You can also use a previous solution from a similar case or a coarser mesh as an initial guess. A good initial guess can reduce the number of iterations needed to reach convergence.
• Check your boundary conditions and make sure they are consistent and realistic. For example, if you are using a pressure outlet boundary condition, you may need to specify a backflow temperature or use an outflow boundary condition instead. You can also use a mass flow inlet boundary condition instead of a velocity inlet boundary condition if you have compressible flow or variable density flow.
• Choose the appropriate physical models for your problem. For example, if you are simulating turbulent flow, you may need to use a turbulence model that can capture the effects of buoyancy and wall functions. You can also use a coupled pressure-velocity solver instead of a segregated solver if you have high-speed flow or high-pressure gradients.

I hope these tips can help you to speed up your convergence for steady state analysis in ANSYS Fluent. 😊

What are the mechanics of how relaxation factors work in Ansys Fluent?

The relaxation factors in Ansys Fluent are parameters that control the update of the computed variables at each iteration. They are used to improve the stability and convergence of the solution process. The relaxation factors are based on the following formula:

xk+1 = w.xcal + (1-w).xk

where xk is the value of the variable at iteration k, xcal is the value calculated from the equation, and w is the relaxation factor. The relaxation factor can range from 0 to 2, but usually it is between 0 and 1. A relaxation factor of 1 means that the variable is fully updated with the calculated value, while a relaxation factor of 0 means that the variable is not updated at all. A relaxation factor between 0 and 1 means that the variable is partially updated with a weighted average of the previous and calculated values.

The relaxation factors can affect the speed and accuracy of the solution. A higher relaxation factor can increase the convergence rate, but it can also cause instability or divergence if it is too high. A lower relaxation factor can increase the stability, but it can also slow down the convergence or cause oscillations if it is too low. Therefore, choosing appropriate relaxation factors for different equations and problems is important for obtaining a good solution.

Ansys Fluent provides default values for the relaxation factors that are suitable for most cases. However, some problems may require adjusting the relaxation factors to achieve better convergence or stability. For example, some turbulent flows or high-Rayleigh-number natural-convection problems may need lower relaxation factors for pressure, momentum, energy, and turbulence equations. Conversely, some flows with constant density or weak coupling between temperature and momentum may allow higher relaxation factors for temperature equation.

You can set or change the relaxation factors for each equation in the Solution Controls task page under Under-Relaxation Factors. You can also click the Default button to restore the default values. For more details about how to specify solution controls in Ansys Fluent, you can refer to this course or this user’s guide. You can also watch some video tutorials (https://www.youtube.com/watch?v=gZc7eS1xcFU) (https://www.youtube.com/watch?v=PrSpOf-TXiE) on how to model different types of flows in Ansys Fluent. I hope this helps you understand how relaxation factors work in Ansys Fluent. 😊