Cooling and Solidification of Metal. About the Stationary Solver - COMSOL Multiphysics The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. listed if standards is not an option). Please dont hesitate to post comments below or send emails to us if you experience any other problems. That is, they are tuned to achieve convergence in as many cases as possible. When you use an iterative solver, COMSOL Multiphysics estimates the error of the solution while solving. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. In this blog post we introduce the two classes of algorithms that are used in COMSOL to solve systems of linear equations that arise when solving any finite element problem. If the default iterative solver is not converging, try switching to a direct solver, as described here: Understanding the Fully Coupled vs. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. As a second example, when solving for Electric Currents, do not model perfect electrical insulators as materials with zero conductivity, instead, omit the domain from the model and use the Electric Insulation boundary condition. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. This case is generally difficult, or impossible, to solve since this material property is non-smooth. To learn more, see our tips on writing great answers. With respect to multiphysics couplings, rather than solving the problem using a fully coupled approach (the default) solve the problem sequentially, with one physics being solved after another. Note: there is no way to couple this . There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. The algorithm is, generally speaking, a Newton's method approach. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. Save my name, email, and website in this browser for the next time I comment. The default Initial Values for the unknowns in most physics interfaces are zero. (I am begginer in comsol) Thank you. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. Near the top of the Stationary Solver log, the software will report if a linear or nonlinear solver is being used. I am trying to solve the coupling between a waveguide and a microring resonator. The segregated approach, on the other hand, solves sets of unknowns separately. Hence Comsol solved for the stationary solution at different points of time. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version There are also cases when an extremely poor quality mesh leads to an ill-conditioned problem, This issue often arises in combination with, and as a consequence of, geometries that have extreme aspect ratios. $140,000.00, $120,000.00 Multiphysics problems are often nonlinear. If it is not clear that any of the above strategies are working, it is useful to take a more general approach to verifying the general validity of the model. It is also possible to manually refine the mesh. Your Discussion has gone 30 days without a reply. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. One of the key concepts there was the idea of mesh convergence as you refine the mesh, the solution will become more accurate. In this page, we will be sharing some common errors that might occur in Comsol and their solutions. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. Posted 26 set 2019, 11:57 GMT-4 COMSOL makes every reasonable effort to verify the information you view on this page. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. COMSOL makes every reasonable effort to verify the information you view on this page. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. This is relatively expensive to do, but will lead to the most robust convergence. The software then computes an initial solution and from there it iteratively re-computes the solution, taking into account how these intermediate solutions affect the nonlinearities. P&S Comsol Team: Arif Gngr , Yannik Horst , Stefano Valente. For more details, see: Performing a Mesh Refinement Study, Mesh refinement may often need to be combined with load or nonlinearity ramping and may require a set of studies, first starting with a relatively coarse mesh for nonlinearity ramping, refining the mesh, and the ramping further on the refined mesh. Check the solver log to see if the continuation method is backtracking. Knowledgebase 1260: What to do when a linear stationary model is not solving, Knowledge Base 1240: Manually Setting the Scaling of Variables, What to do when a linear stationary model is not solving, Knowledge Base 1254: Controlling the Time Dependent solver timesteps, Galleria dei Modelli e delle App di Simulazione, 2023 da COMSOL. It may also reveal that the model itself is ill-posed in some way. Use this parameter to modify the nonlinearity expressions in the model. The software then computes an initial solution and from there it iteratively re-computes the solution, taking into account how these intermediate solutions affect the nonlinearities. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. The settings controlling the predictor type. 0 Replies, Please login with a confirmed email address before reporting spam. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Sometimes, reducing the model complexity can be quite challenging and it can be better to start from as simple a case as possible and gradually increase the complexity. Stationary (time-invariant) models with nonlinearities may converge very slowly. Does anyone know what should cause this problem? The idea behind the GCRO-DR method is to retain the subspace determined while solving previous systems and use it to reduce the cost of solving the next system. The coupling terms between the different groups are thus neglected. In this post, we will delve deeper into how to choose an appropriate mesh to start your mesh convergence studies for linear static finite element problems. Solver . Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. Does ZnSO4 + H2 at high pressure reverses to Zn + H2SO4? The coupling terms between the different groups are thus neglected. Can someone help me fix the COMSOL error below? | ResearchGate k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) Dr.S.Ravindran Cite 1 Recommendation Popular answers (1). If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. rev2023.3.3.43278. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. A linear finite element model is one in which all of the material properties, loads, boundary conditions, etc are constant with respect to the solution, and the governing partial differential equations are themselves linear. Tutti i diritti sono riservati. In this posting, we introduce the idea of ramping the nonlinearities in the problem to improve convergence. We are planning to continuously update this page throughout the semester and hopefully, this will become a reference during your projects as well. Unknown function or operator. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. That is, start by first solving a model with a small, but non-zero, load. A nonlinearity can be introduced into the model either in the governing equation, or by making any of the material properties, loads, or boundary conditions dependent upon the solution. Is it possible to rotate a window 90 degrees if it has the same length and width? That is, start by first solving a model with a small, but non-zero, load. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Solving for laminar flow using Comsol - YouTube In the COMSOL Multiphysics software, this step of the modeling workflow is made. The other low-level default settings within the Stationary Solver are chosen for robustness. Popular answers (1) This problem generally occurs when there is some mistake in the physics or study section or wrong selection of the mesh size. Some models are numerically ill-conditioned due to the setup. Once the error estimate is small enough, as determined by the convergence criterion (20-16) the software terminates the computations and returns a solution. Using the first order optimality. -Detail: NaN or Inf found when solving linear system using SOR. They deal with COMSOL package and train users. At low flow speeds the flow solution will be time invariant, but at higher flow rates there will be vortex shedding, a time-varying change in the flow field behind the cylinder. Such problems must solved in the time domain. For example, if ramping P over values of: 0.2,0.4,0.6,0.8,1.0 the nonlinear solver may fail to converge for a value of 0.8. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. The default Initial Values for the unknowns in most physics interfaces are zero. COMSOL does not assume any legal liability for the accuracy of the data disclosed. In that case, the continuation method will automatically backtrack and try to solve for intermediate values in the range of 0.6 through 0.8. Hi Alexis, Understanding stationary and time dependent solver COMSOL Multiphysics Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Stationary Solver Use the Stationary Solver () to find the solution to linear and nonlinear stationary problems (also called static or steady-state problems). The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. (COMSOL Multiphysics) : I use comsol multiphysics 5.2a and . This involves a systematic reduction in the model complexity. Here, we will examine techniques for accelerating the convergence of these two methods. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. For example, if there is a temperature-dependent material property such as: You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Hi ! Review the model setup to resolve these. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. Staging Ground Beta 1 Recap, and Reviewers needed for Beta 2, Simulation of effect of heated resistance on temperature distribution in laminar flow, COMSOL: Diffusion in Transport of Diluted Species Produces Unphysical Results. If it does so, use a finer increment in that range. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. Extending this logic, if one wants to solve for any arbitrary load on a nonlinear system, it makes sense to solve a sequence of intermediate problems with gradually increasing load values and using the solutions from each previous step as the initial condition for the next step. Within either of these features, it can also be helpful to enable the Results While Solving option, as shown in the screenshot below, to visualize the iterations being taken during the solution. . This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. One can say that, in general, if the loads on a nonlinear system are zero, the system will be at rest; that is, the solution will be zero. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. I am following the same model as Comsol provide us on the web. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. - Top 10 Best Stationary in Brea, CA - October 2022 - Yelp The objective here is to simplify the model to a state where the model will solve, with linear approximations. The advantages of the continuation method are two-fold. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) Load ramping and nonlinearity ramping can be used in combination, but start with only one or a few of the loads or nonlinearities being ramped. Consult your product manuals for complete trademark details. The Automatic predictor setting will use the constant predictor when a segregated solution approach is being used, and use the linear predictor when the fully coupled approach is used. P&S: COMSOL Design Tool for Photonic Devices - ETH Z The objective here is to simplify the model to a state where the model will solve, with linear approximations. Alternatively, delete and re-create the study. Within either of these features, it can also be helpful to enable the Results While Solving option, as shown in the screenshot below, to visualize the iterations being taken during the solution. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. With respect to any nonlinearities, replace them by a reasonable linearized term. Making statements based on opinion; back them up with references or personal experience. Reviews on Stationary Stores in Brea, CA - Typo, Mievic, Posh Paper, Morning Glory, Select Graphics & Printing, Daiso Japan, Urban Outfitters, Office Depot, Barnes & Noble Connect and share knowledge within a single location that is structured and easy to search. Then I stored the resulting solution, and used it to to solve the stationary solver again by using the solver manager. replace it with the expression: Assuming a well-posed problem, the solver may converge slowly (or not at all) if the initial values are poor, if the nonlinear solver is not able to approach the solution via repeated iterations, or if the mesh is not fine enough to resolve the spatial variations in the solution.