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Different physics have different default solvers. If you try to enter a variable that is undefined, the automatic syntax highlighting will identify this as you enter it, and it is best practice to resolve this immediately. The algorithm is, generally speaking, a Newton's method approach. Not assigning proper boundary conditions: Especially if you have ports. However, load ramping will not work in all cases, or may be inefficient. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. The default solver for most 3D models is an iterative solver, which is more sensitive to ill-conditioned problems. 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. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. 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. It is quite rare that changing these settings is superior to using a combination of the other techniques in this Knowledgebase, although it is possible to tune these settings to reduce solution time and memory requirements, once a model is already converging. Comsol help video number 2: Solving a laminar flow problem in a slit.. 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. Save my name, email, and website in this browser for the next time I comment. Note the star symbol on the Solution feature. Feature: Stationary Solver 1 (sol1/s1)" . Consult your product manuals for complete trademark details. Hi ! Leverage your professional network, and get hired. Set "Values for dependent variables" in study step settings to User ControlledSolutionYour Stationary Study. This involves a systematic reduction in the model complexity. Posted Sep 9, 2020, 1:44 p.m. EDT 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. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. The latter method is known as the Continuation Method with a Linear predictor, and is controlled within the Study Configurations as shown in the screenshot below. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. To start a new discussion with a link back to this one, click here. If you define this nonlinearity ramping such that the first case (P=0) is a purely linear problem, then you are guaranteed to get a solution for this first step in the ramping. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. This involves a systematic reduction in the model complexity. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) If some, or all, of the needed materials properties needed by the physics interfaces are not defined, the model will generate an error at runtime. However, it is usually not possible to know this ahead of time. if I want to do an adaptive mesh refinement, I get this error. If the material properties entered are incorrect for the governing equation, the model will generate an error at runtime, usually a Singular Matrix error. This case is generally difficult, or impossible, to solve since this material property is non-smooth. We are planning to continuously update this page throughout the semester and hopefully, this will become a reference during your projects as well. . Version 5.3 Iterative , Direct . Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. Get notified about new Stationary Engineer jobs in Brea, California, United States. 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. Create the time-dependent step or study. Wrong ordering of study steps. Such problems must solved in the time domain. It may also reveal that the model itself is ill-posed in some way. Alle Rechte vorbehalten. Any trademarks referenced in this document are the property of their respective owners. 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. 0 Replies, Please login with a confirmed email address before reporting spam. The continuation method will again backtrack and try intermediate values of the ramping parameter, thus giving you the nearest approximation to the abrupt transition that is solvable. Common Mistakes: Not assigning materials to all the domains. Most multiphysics problems are nonlinear. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. 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. Then I stored the resulting solution, and used it to to solve the stationary solver again by using the solver manager. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. For example, if there is a temperature-dependent material property such as: I want to conduct a simulation to find a solution (u) and its first derivative (ux) using a 3D stationary model. If it does so, use a finer increment in that range. In a previous blog entry, we introduced meshing considerations for linear static problems. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. 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. Therefore, it is recommended to use Adaptive Mesh Refinement which will automatically refine the mesh only in regions where it is needed, and coarsen the mesh elsewhere. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. Please dont hesitate to post comments below or send emails to us if you experience any other problems. At a value of P=0 the above expression is linear, and at a value of P=1 the expression is equal to the original nonlinear expression. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. In such cases, see if one material (or the other) can be omitted from the analysis completely. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. These are some cards & stationery with a large number of reviews in Brea, CA. Stationary (time-invariant) models with nonlinearities may converge very slowly. 3 Replies, Please login with a confirmed email address before reporting spam. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: Hi Jason, This case is generally difficult, or impossible, to solve since this material property is non-smooth. 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. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. This algorithm was also useful for understanding what happens near a failure load. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Solver . See Knowledge Base 1240: Manually Setting the Scaling of Variables. Not assigning proper boundary conditions: Especially if you have ports. In many physics areas there exist alternative physics formulations specifically meant for solving cases where the geometry has an extreme aspect ratio. 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. Cooling and Solidification of Metal. That is, start by first solving a model with a small, but non-zero, load. When the difference in the computed solutions between successive iterations is sufficiently small, or when the residual is sufficiently small, the problem is considered converged to within the specified tolerance. In this case, it would likely be reasonable to treat the insulative material as a perfect insulator, omit it from the analysis, and use the Electric Insulation boundary condition instead of modeling those domains. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. 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. 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. Your internet explorer is in compatibility mode and may not be displaying the website correctly. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. 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. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. How can I check before my flight that the cloud separation requirements in VFR flight rules are met? There will also be a red cross over the Materials branch icon. Check the solver log to see if the continuation method is backtracking. What is \newluafunction? Here, we will examine techniques for accelerating the convergence of these two methods. P&S Comsol Team: Manuel Kohli, Raphael Schwanninger, Feature: Stationary Solver 1 (sol1/s1) Posted 26 set 2019, 17:57 CEST Mesh Version 5.3 0 Replies. Also, keep in mind that a linear stationary model should solve regardless of how coarse the mesh is (albeit to a low accuracy) so you can always start with as coarse a mesh as possible, and refine the mesh (See also: Knowledgebase 1030: Performing a Mesh Refinement Study. If these settings have been manually changed, you will see a small star symbol on the Solution feature, as shown in the screenshot below. 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. 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, 2023 by COMSOL. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. They are usually called comp1.u, comp1.v, and comp1.w though. Posted 26 set 2019, 11:57 GMT-4 With respect to any nonlinearities, replace them by a reasonable linearized term. If you do not hold an on-subscription license, you may find an answer in another Discussion or in the Knowledge Base. 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. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). The technique of load ramping is not always reasonable for all problems. The nature of simulating nature: A Q&A with IBM Quantum researcher Dr. Jamie We've added a "Necessary cookies only" option to the cookie consent popup. 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. - Changes to these low-level settings from the defaults will usually be quite model- and case-specific. There will always already be either a Segregated or Fully Coupled feature beneath this. The default Initial Values for the unknowns in most physics interfaces are zero. Not meshing all the domains. However, if a lower-level change has been manually made to the solver settings, the software will not automatically use the correct default solver. What did people search for similar to stationary stores in Brea, CA? 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. Numerically ill-conditioned means that the system matrix is nearly singular and that it will be difficult to solve on a finite-precision computer. I use comsol multiphysics 5.2a and . This guide applies solely to nonlinear stationary models. The solver settings are stored at Study > Solver Configurations > Solution. 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. 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.