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The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. Could you expand a little bit more why the coupling is impossible? Different physics have different default solvers. The objective here is to simplify the model to a state where the model will solve, with linear approximations. Singular matrix., Make sure you defined your ports correctly:Boundary selectionType of port: e.g. Troubleshooting for 3D trusses, if anyone's out there. : r/COMSOL - reddit -Detail: NaN or Inf found when solving linear system using SOR. 0 Replies, Please login with a confirmed email address before reporting spam. listed if standards is not an option). there is no defined multiphysics for it as I know, I have a standing accoustic wave and a flow in the background but I don't see their connection. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). 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. Minimising the environmental effects of my dyson brain. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. If you try to solve such a model, there will be an error at runtime that will provide information about the variable name and where it is being called. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. 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. 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. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. These are some highly rated cards & stationery in Brea, CA: What are some cards & stationery with a large number of reviews in Brea, CA? Consult your product manuals for complete trademark details. If it does so, use a finer increment in that range. This is relatively expensive to do, but will lead to the most robust convergence. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version This is relatively expensive to do, but will lead to the most robust convergence. This is relatively expensive to do, but will lead to the most robust convergence. The algorithm is, generally speaking, a Newton's method approach. We use COMSOL Multiphysics for solving distributed optimal control of un-steady Burgers equation without constraints and with pointwise control constraints. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. The advantages of the continuation method are two-fold. 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. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. "I chose this print shop based off yelp reviews and was very happy with the outcome! Numerically ill-conditioned means that the system matrix is nearly singular and that it will be difficult to solve on a finite-precision computer. 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. 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. Repeat this for every nonlinearity of the model. solving stationary solver after each time step - COMSOL Multiphysics - 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. This involves a systematic reduction in the model complexity. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. For example, if there is a temperature-dependent material property such as: Not meshing all the domains. Any trademarks referenced in this document are the property of their respective owners. Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. Note that while COMSOL employees may participate in the discussion forum, COMSOL software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team. The coupling terms between the different groups are thus neglected. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. [COMSOL_Users] output of stationary solver as input to time-dependent SGP handled 7 different prints for me at once and they all came out perfectly, in a timely manner. Set initial conditions in the physics to the appropriate dependent model variable names rather than the default 0. In the COMSOL Multiphysics software, this step of the modeling workflow is made. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: The fully coupled and segregated approaches are discussed below. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. P&S Comsol Team: Arif Gngr , Yannik Horst , Stefano Valente. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. The default Initial Values for the unknowns in most physics interfaces are zero. To switch between these solver types, go to the Stationary Solver node within the Study sequence. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. With sufficient simplification, a model can be reduced to a linear problem, and if this simplified model does not converge, see: What to do when a linear stationary model is not solving. This guide applies solely to nonlinear stationary models. Few days back i was also facing this problem in . Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. My comment is perhaps a bit nave but it seems to me that you could simply deactivate the term of the background field equation but keep its connexion to the solid to get what you want. If the material properties entered are incorrect for the governing equation, the model will generate an error at runtime, usually a Singular Matrix error. The other low-level default settings within the Stationary Solver are chosen for robustness. Wrong ordering of study steps. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. Not entering required material parameters. 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. 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. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. Save my name, email, and website in this browser for the next time I comment. Your Discussion has gone 30 days without a reply. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. This involves a systematic reduction in the model complexity. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. "After the incident", I started to be more careful not to trip over things. It is sometimes necessary to manually scale the dependent variables. Hi ! Is it possible to rotate a window 90 degrees if it has the same length and width? Solving for laminar flow using Comsol - YouTube Comsol help video number 2: Solving a laminar flow problem in a slit. Perhaps this approach could be adapted to represent your model. Thanks for contributing an answer to Stack Overflow! It may also reveal that the model itself is ill-posed in some way. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. Perhaps this approach could be adapted to represent your model. $131,100.00, Simplified Vehicle Operations Project Engineer, $115,000.00 If all of the above approaches have been tried and you are certain that the problem itself is well-posed, consider that the nonlinear problem may not, in fact, have a stationary (time-invariant) solution. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Solver . Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. Near the top of the Stationary Solver log, the software will report if a linear or nonlinear solver is being used. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. Find centralized, trusted content and collaborate around the technologies you use most. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: Hi Jason, Here we introduce a more robust approach to solving nonlinear problems. - 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. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. Can someone help me fix the COMSOL error below? | ResearchGate In a previous blog entry, we introduced meshing considerations for linear static problems. The problem is that when I run my model this message appear: Undefined value found. Posted 26 set 2019, 11:57 GMT-4 The advantages of the continuation method are two-fold. 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. The other low-level default settings within the Stationary Solver are chosen for robustness. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. COMSOL Blog Series on COMSOL Multiphysics Solvers Making statements based on opinion; back them up with references or personal experience. The Fully Coupled solution approach, with the Plot While Solving enabled. Such a large difference in the materials properties can be challenging. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. Acidity of alcohols and basicity of amines. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. 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. As we saw in Load Ramping of Nonlinear Problems, we can use the continuation method to ramp the loads on a problem up from an unloaded case where we know the solution. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. Consult your product manuals for complete trademark details. Your internet explorer is in compatibility mode and may not be displaying the website correctly. The technique of load ramping is not always reasonable for all problems. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. If so, see: Knowledgebase 1030: Error: "Out of memory". . Name: actdep_int1, Your email address will not be published. Dr.S.Ravindran Cite 1 Recommendation Popular answers (1). P&S: COMSOL Design Tool for Photonic Devices. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. Top 10 Best Stationary in Brea, CA - October 2022 - Yelp If your matrix is singular than no solver in the world can solve Ax=B. 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. listed if standards is not an option). 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. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. Common Mistakes: Not assigning materials to all the domains. If you are unsure if your problem is linear and stationary, look at the Log. Could you expand a little bit more why the coupling is impossible? If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. Not meshing all the domains. 3. You can unsubscribe from these emails at any time. 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. In many physics areas there exist alternative physics formulations specifically meant for solving cases where the geometry has an extreme aspect ratio. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. Why is there a voltage on my HDMI and coaxial cables? . For example, in an Electric Currents problem, you may want to consider a system of materials including a good conductor such as copper (with an electric conductivity of ~6e7 S/m) and an insulative material such as glass (which can have electric conductivity of ~1e-14 S/m.) An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: It's brand new in the hmart plaza and I wish it was open back when I would hangout in the plaza after school (although they would have taken all my allowance money! This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. 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. - the incident has nothing to do with me; can I use this this way? The stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. Why doesn't iterative solver converge in COMSOL? - ResearchGate Posted Sep 9, 2020, 1:44 p.m. EDT The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. I am solving a linear stationary finite element model but the software is not solving. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. What are people saying about cards & stationery in Brea, CA? One of the key concepts there was the idea of mesh convergence as you refine the mesh, the solution will become more accurate. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one.