In the extreme case, suppose one wants to model an instantaneous change in properties, such as: I use comsol multiphysics 5.2a and . 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 makes every reasonable effort to verify the information you view on this page. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. Hi Jason, Segregated approach and Direct vs. Again, introduce a Global Parameter that gets ramped from exactly zero to one. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. 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. 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. This guide applies solely to nonlinear stationary models. What sort of strategies would a medieval military use against a fantasy giant? There will always already be either a Segregated or Fully Coupled feature beneath this. Singular matrix., Make sure you defined your ports correctly:Boundary selectionType of port: e.g. [COMSOL_Users] output of stationary solver as input to time-dependent The coupling terms between the different groups are thus neglected. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. They worked with me. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. 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. That is, they are tuned to achieve convergence in as many cases as possible. You can unsubscribe from these emails at any time. About the Stationary Solver The following background information about the Stationary Solver discusses these topics: Damped Newton Methods, Termination Criterion for the Fully Coupled and Segregated Attribute Nodes, Linear Solvers versus Nonlinear Solvers, and Pseudo Time Stepping. 0 Replies, Please login with a confirmed email address before reporting spam. 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. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. 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. Improving Convergence of Nonlinear Stationary Models - COMSOL Version 5.3 They are usually called comp1.u, comp1.v, and comp1.w though. 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. To switch between these solver types, go to the Stationary Solver node within the Study sequence. This can arise as a consequence of extreme variations in the material properties, or high aspect ratio geometry. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. $140,000.00, $120,000.00 Screenshot showing a Solver Configuration that has been altered. The Fully Coupled solution approach, with the Plot While Solving enabled. As a rough rule of thumb, once the aspect ratio between the largest characteristic dimension to the smallest approaches 100:1, you might start to run into issues and should look to alternative ways of posing the problem, especially in a 3D model. Discussion Closed This discussion was created more than 6 months ago and has been closed. Improving Convergence of Nonlinear Stationary Models - Informazioni Multiphysics problems are often nonlinear. Connect and share knowledge within a single location that is structured and easy to search. My comment is perhaps a bit nave but it seems to me that you could simply deactivate the \frac{\partial \cdot}{\partial t} term of the background field equation but keep its connexion to the solid to get what you want. What did people search for similar to stationary stores in Brea, CA? Different physics have different default solvers. Trying to understand how to get this basic Fourier Series. 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. listed if standards is not an option). The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. This information is presented in the context of a very simple 1D finite element problem, and builds upon our previous entry on Solving Linear Static Finite Element Models. 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. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. Direct PARDISO Solver , PARDISO . Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. Asking for help, clarification, or responding to other answers. November 11, 2013. 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. Does ZnSO4 + H2 at high pressure reverses to Zn + H2SO4? I am solving a linear stationary finite element model but the software is not solving. Could you expand a little bit more why the coupling is impossible? Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. If it does so, use a finer increment in that range. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. 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. However, it is usually not possible to know this ahead of time. Here we introduce a more robust approach to solving nonlinear problems. The following are possible reasons why a linear stationary model will fail to solve, along with resolutions: The combination of the constraints and boundary conditions must be sufficient to define a unique solution to the problem, in terms of the field variables being solved. 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. Understanding the Fully Coupled vs. Stationary Solver Use the Stationary Solver () to find the solution to linear and nonlinear stationary problems (also called static or steady-state problems). 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. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. Full text of the 'Sri Mahalakshmi Dhyanam & Stotram'. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) 140K views 8 years ago COMSOL Multiphysics Tutorial for Beginners Please note that an updated version of the content in this video can be found in the Modeling Workflow video in the COMSOL. The segregated approach, on the other hand, solves sets of unknowns separately. Use a manually defined mesh to avoid elements with extreme aspect ratios and perform a mesh refinement study, as described here: Performing a Mesh Refinement Study, For problems that are ill-conditioned, using a direct solver is often called for. The objective here is to simplify the model to a state where the model will solve, with linear approximations. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). Stationary Solver Iterative Direct . If your matrix is singular than no solver in the world can solve Ax=B. Using the first order optimality. listed if standards is not an option). Not assigning proper boundary conditions: Especially if you have ports. $130,000.00. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) This algorithm was also useful for understanding what happens near a failure load. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. This approach is known as a Continuation Method with a Constant predictor. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. They deal with COMSOL package and train users. The coupling terms between the different groups are thus neglected. The technique of load ramping is not always reasonable for all problems. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. Leverage your professional network, and get hired. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. When you use an iterative solver, COMSOL Multiphysics estimates the error of the solution while solving. - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). "After the incident", I started to be more careful not to trip over things. These can be used alone, or in combination with other interfaces. replace it with the expression: Use this parameter to modify the nonlinearity expressions in the model. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. Most multiphysics problems are nonlinear. See Knowledge Base 1240: Manually Setting the Scaling of Variables. 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. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. Your Discussion has gone 30 days without a reply. Check the solver log to see if the continuation method is backtracking. See Knowledge Base 1240: Manually Setting the Scaling of Variables. To learn more, see our tips on writing great answers. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. The coupling terms between the different groups are thus neglected. Then use this file to provide the initial conditions in time dependent model. L'objectif de notre prsent travail se repose sur l'tude par simulation numrique du comportement de bton au jeune ge sous des conditions svres de temprature pendant les premires 24h aprs. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. rev2023.3.3.43278. Can someone help me fix the COMSOL error below? | ResearchGate Posted 26 set 2019, 11:57 GMT-4 Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. First, it is physically intuitive, often matching how one would perform an experiment. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. About the Stationary Solver - COMSOL Multiphysics As we saw previously in the blog entry on Solving Nonlinear Static Finite Element Problems, not all nonlinear problems will be solvable via the damped Newton-Raphson method. 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. COMSOL does not assume any legal liability for the accuracy of the data disclosed. It is also possible to manually refine the mesh. That is: Even if the forces on a part are opposite and equal, this is not sufficient information to say where the part is, so you must add some other condition, such as as Fixed Constraint to constrain displacement. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. It may also reveal that the model itself is ill-posed in some way. The algorithm is, generally speaking, a Newton's method approach. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Near the top of the Stationary Solver log, the software will report if a linear or nonlinear solver is being used. These are some highly rated cards & stationery in Brea, CA: What are some cards & stationery with a large number of reviews in Brea, CA? If you are unsure if your problem is linear and stationary, look at the Log. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Understanding stationary and time dependent solver COMSOL Multiphysics First, it is physically intuitive, often matching how one would perform an experiment. Why do many companies reject expired SSL certificates as bugs in bug bounties? - Feature: Stationary Solver 1 (sol1/s1) This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. 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.