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. 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. 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. By providing your email address, you consent to receive emails from COMSOL AB and its affiliates about the COMSOL Blog, and agree that COMSOL may process your information according to its Privacy Policy. 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 segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. Cooling and Solidification of Metal. Not meshing all the domains. 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. Does ZnSO4 + H2 at high pressure reverses to Zn + H2SO4? Full text of the 'Sri Mahalakshmi Dhyanam & Stotram'. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) Any trademarks referenced in this document are the property of their respective owners. The conditions on the geometric aspect ratio are relatively more strict. Minimising the environmental effects of my dyson brain. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. The fully coupled and segregated approaches are discussed below. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. To switch between these solver types, go to the Stationary Solver node within the Study sequence. Any trademarks referenced in this document are the property of their respective owners. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. If so, see: Knowledgebase 1030: Error: "Out of memory". 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. Wish you all the best. replace it with the expression: A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. 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. 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. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. What are people saying about cards & stationery in Brea, CA? I highly recommend this flower shop. The objective here is to simplify the model to a state where the model will solve, with linear approximations. As part of our solver blog series we have discussed solving nonlinear static finite element problems, load ramping for improving convergence of nonlinear problems, and nonlinearity ramping for improving convergence of nonlinear problems. 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. rev2023.3.3.43278. 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. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) Discussion Closed This discussion was created more than 6 months ago and has been closed. Consult your product manuals for complete trademark details. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. 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. Few days back i was also facing this problem in . 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. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. The objective here is to simplify the model to a state where the model will solve, with linear approximations. Dr.S.Ravindran Cite 1 Recommendation Popular answers (1). document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. If the model is nonlinear, see: Improving Convergence of Nonlinear Stationary Models. For example, if there is a temperature-dependent material property such as: Numerically ill-conditioned means that the system matrix is nearly singular and that it will be difficult to solve on a finite-precision computer. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Do you also know how to solve this problem: using stationary solution as the initial conditions in time dependent model, How Intuit democratizes AI development across teams through reusability. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Most multiphysics problems are nonlinear. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. Not the answer you're looking for? Hi Alexis, Create the time-dependent step or study. Iterative , Direct . 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. 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. The advantages of the continuation method are two-fold. Feature: Stationary Solver 1 (sol1/s1) Adaptation gives the same mesh: adaptmesh1. Different physics have different default solvers. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. That is: It is also possible to compute the derivative of the solution with respect to the continuation parameter and use that derivative (evaluated at the iteration) to compute a new initial value: where is the stepsize of the continuation parameter. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. 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. P&S Comsol Team: Manuel Kohli, Raphael Schwanninger, Feature: Stationary Solver 1 (sol1/s1) 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. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. Thanks, Andres. Hello guys. Here we introduce a more robust approach to solving nonlinear problems. 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. To switch between these solver types, go to the Stationary Solver node within the Study sequence. ", "Absolutely the best place to shop for flowers and greenery, I spent an hour just looking around for fun prior to purchasing a bouquet, they have the best customer service people all over the place and it's a huge place", "Recently came in here to pick up 45 chocolate Seegars (cigars) for a dinner banquet in honor of my baby nephew. 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. The Fully Coupled solution approach, with the Plot While Solving enabled. The advantages of the continuation method are two-fold. The technique of load ramping is not always reasonable for all problems. 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. The segregated approach, on the other hand, solves sets of unknowns separately. There will also be a red cross over the Materials branch icon. However, load ramping will not work in all cases, or may be inefficient. Name: actdep_int1, Your email address will not be published. The settings controlling the predictor type. In our previous blog entry, we introduced the Fully Coupled and the Segregated algorithms used for solving steady-state multiphysics problems in COMSOL. listed if standards is not an option). Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Understanding the Fully Coupled vs. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. Note: there is no way to couple this . 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. That is, start by first solving a model with a small, but non-zero, load. Your internet explorer is in compatibility mode and may not be displaying the website correctly. See if these can be used instead of explicitly modeling parts with high-aspect ratio geometries. Connect and share knowledge within a single location that is structured and easy to search. 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. The fully coupled and segregated approaches are discussed below. 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. The coupling terms between the different groups are thus neglected. 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. Check the solver log to see if the continuation method is backtracking. However, if a lower-level change has been manually made to the solver settings, the software will not automatically use the correct default solver. Function: / Failed to evaluate expression. 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. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. Common Mistakes: Not assigning materials to all the domains. If the default iterative solver is not converging, try switching to a direct solver, as described here: Understanding the Fully Coupled vs. Tutti i diritti sono riservati. Segregated approach and Direct vs. It is sometimes necessary to manually scale the dependent variables. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Then use this file to provide the initial conditions in time dependent model. It is also possible to manually refine the mesh. This guide applies solely to nonlinear stationary models. COMSOL does not assume any legal liability for the accuracy of the data disclosed. - Feature: Stationary Solver 1 (sol1/s1) This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. Sign in to create your job alert for Stationary Engineer jobs in Brea, California, United States. Again, introduce a Global Parameter that gets ramped from exactly zero to one. When you use an iterative solver, COMSOL Multiphysics estimates the error of the solution while solving. Today's top 351 Stationary Engineer jobs in Brea, California, United States. What is the purpose of this D-shaped ring at the base of the tongue on my hiking boots? That is, they are tuned to achieve convergence in as many cases as possible. That is, start by first solving a model with a small, but non-zero, load. Studysteps might be listed in wrong order: Not assigning materials to all the domains. Save this information to a file. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. The algorithm is, generally speaking, a Newton's method approach. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. $125,000.00, Project Engineer (In-person/Hybrid/Remote), $100,000.00 Segregated approach and Direct vs. In many physics areas there exist alternative physics formulations specifically meant for solving cases where the geometry has an extreme aspect ratio. 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. Such a large difference in the materials properties can be challenging. This involves a systematic reduction in the model complexity. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. 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. To switch between these solver types, go to the Stationary Solver node within the Study sequence. Not assigning proper boundary conditions: Especially if you have ports. New Stationary Engineer jobs added daily. so many cute little stationary items , hair". Dun & Bradstreet gathers Office Supplies, Stationery, and Gift Retailers business information from trusted . -Detail: NaN or Inf found when solving linear system using SOR. Linear solvers. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. We use COMSOL Multiphysics for solving distributed optimal control of un-steady Burgers equation without constraints and with pointwise control constraints. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. This is relatively expensive to do, but will lead to the most robust convergence. With respect to any nonlinearities, replace them by a reasonable linearized term. (Frequency Domain should be the last step) Mesh It is also possible to manually refine the mesh. For the Nozomi from Shinagawa to Osaka, say on a Saturday afternoon, would tickets/seats typically be available - or would you need to book? Get email updates for new Stationary Engineer jobs in Brea, California, United States. thanks for reply 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. Unknown function or operator. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). The algorithm is, generally speaking, a Newton's method approach. Contact COMSOL at Bangalore on their telephone: +9180 25323003. Some models are numerically ill-conditioned due to the setup. 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. "I chose this print shop based off yelp reviews and was very happy with the outcome! Therefore, an initial value of zero is almost always reasonable if a very small load is applied. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. The problem is that when I run my model this message appear: Undefined value found. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. 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. GCRO-DR is a method for Krylov subspace recycling. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. If you do not hold an on-subscription license, you may find an answer in another Discussion or in the Knowledge Base. Repeat this for every nonlinearity of the model. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: Thanks for contributing an answer to Stack Overflow! The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. Again, introduce a Global Parameter that gets ramped from exactly zero to one. - If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. listed if standards is not an option). A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. Multiphysics problems are often nonlinear. If you are unsure if your problem is linear and stationary, look at the Log. It can be useful while solving sequences of linear systems arising from, for example, nonlinear problems. 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. Once the error estimate is small enough, as determined by the convergence criterion (20-16) the software terminates the computations and returns a solution. $140,000.00, $120,000.00 There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. To learn more, see our tips on writing great answers. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. The objective here is to simplify the model to a state where the model will solve, with linear approximations. Repeat this for every nonlinearity of the 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. Linear solvers. What is \newluafunction? Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. listed if standards is not an option). Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. 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 difference between the phonemes /p/ and /b/ in Japanese. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version If the model is very large, and if you do not have very much memory in your computer, you may get an error message regarding memory.
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