That is, they are tuned to achieve convergence in as many cases as possible. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. 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. Some models are numerically ill-conditioned due to the setup. Any trademarks referenced in this document are the property of their respective owners. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. 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. COMSOL does not assume any legal liability for the accuracy of the data disclosed. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. 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. Is it possible to rotate a window 90 degrees if it has the same length and width? Use this parameter to modify the nonlinearity expressions in the model. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Alle Rechte vorbehalten. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) This case is generally difficult, or impossible, to solve since this material property is non-smooth. See Knowledge Base 1240: Manually Setting the Scaling of Variables. It is sometimes necessary to manually scale the dependent variables. 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. To start a new discussion with a link back to this one, click here. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version These are some highly rated cards & stationery in Brea, CA: What are some cards & stationery with a large number of reviews in Brea, CA? 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. It is also possible to manually refine the mesh. 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. By creating this job alert, you agree to the LinkedIn User Agreement and Privacy Policy. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. View the Settings window for the Materials branch to get a list of all domains with undefined materials and add a material to those domains. Check the solver log to see if the continuation method is backtracking. Direct Iterative , Direct . 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. 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. Could you expand a little bit more why the coupling is impossible? From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) Have you taken a look at this [blog post](https://www.comsol.com/blogs/modeling-fluid-structure-interaction-in-multibody-mechanisms/)? Set "Values for dependent variables" in study step settings to User ControlledSolutionYour Stationary Study. To start a new discussion with a link back to this one, click here. If it does so, use a finer increment in that range. The other low-level default settings within the Stationary Solver are chosen for robustness. One of the key concepts there was the idea of mesh convergence as you refine the mesh, the solution will become more accurate. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Improving Convergence of Nonlinear Stationary Models, Knowledgebase 1030: Error: "Out of memory", Knowledgebase 1030: Performing a Mesh Refinement Study, Understanding the Fully Coupled vs. To switch between these solver types, go to the Stationary Solver node within the Study sequence. However, if a lower-level change has been manually made to the solver settings, the software will not automatically use the correct default solver. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. It may also reveal that the model itself is ill-posed in some way. Such a case would be better to address instead with the Shell physics interface, which is specially formulated for handling thin-walled structural parts. In this posting, we introduce the idea of ramping the nonlinearities in the problem to improve convergence. See if these can be used instead of explicitly modeling parts with high-aspect ratio geometries. Segregated approach and Direct vs. Iterative linear solvers, Time dependent function and stationary study, Combining Adaptive Mesh Refinement with Data Filtering, What to do when a linear stationary model is not solving, Galleria dei Modelli e delle App di Simulazione, 2023 da COMSOL. If one particular material is missing one property, that material will also be highlighted with a red cross over that material icon in the Model Builder. Thanks for contributing an answer to Stack Overflow! Stationary Solver Use the Stationary Solver () to find the solution to linear and nonlinear stationary problems (also called static or steady-state problems). Solver . I'm trying to model a solid that's moving through a steady background field in a background flow, I want to take into account the effect of movement of the solid after each time step so I have to use stationary solver after each time step in order to see how field has changed after solid moved. Solve the stationary study then the time dependent study. I want to conduct a simulation to find a solution (u) and its first derivative (ux) using a 3D stationary model. The segregated approach, on the other hand, solves sets of unknowns separately. Segregated approach and Direct vs. Iterative linear solvers, About the time step setting of the solver, Introducing Goal Seeking into the Segregated Solver. . As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. November 11, 2013. Required fields are marked *. Posted 26 set 2019, 17:57 CEST Mesh Version 5.3 0 Replies. 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. Leverage your professional network, and get hired. 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. Solving for laminar flow using Comsol - YouTube Comsol help video number 2: Solving a laminar flow problem in a slit. 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. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. Then use this file to provide the initial conditions in time dependent model. P&S Comsol Team: Yannik Horst, Manuel Kohli, Xinzhi Zhang. 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. They worked with me. Dr.S.Ravindran Cite 1 Recommendation Popular answers (1). The Fully Coupled solution approach, with the Plot While Solving enabled. 3 Replies, Please login with a confirmed email address before reporting spam. The conditions on the geometric aspect ratio are relatively more strict. The default solver for most 3D models is an iterative solver, which is more sensitive to ill-conditioned problems. Such problems must solved in the time domain. Again, introduce a Global Parameter that gets ramped from exactly zero to one. 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. 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. 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. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. 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. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Posted 26 set 2019, 11:57 GMT-4 Hence Comsol solved for the stationary solution at different points of time. The technique of load ramping is not always reasonable for all problems. 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. Cecilia M. came from the backroom to help me and went out of her way to help me even though I had not called before going into the store. To learn more, see our tips on writing great answers. This guide applies solely to nonlinear stationary models. Direct PARDISO Solver , PARDISO . 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. I am solving a linear stationary finite element model but the software is not solving. P&S Comsol Team: Arif Gngr , Yannik Horst , Stefano Valente. 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. Any trademarks referenced in this document are the property of their respective owners. Consult your product manuals for complete trademark details. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. 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. Hi Alexis, The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. comp1.u2, comp1.v2, and comp1.w2 are usually variables associated with the x,y, and z component of deformation in COMSOL. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Version 5.3 This guide applies solely to nonlinear stationary models. Not assigning proper boundary conditions: Especially if you have ports. If so, see: Knowledgebase 1030: Error: "Out of memory". Alternatively, delete and re-create the study. The other low-level default settings within the Stationary Solver are chosen for robustness. Contact COMSOL at Bangalore on their telephone: +9180 25323003. With respect to any nonlinearities, replace them by a reasonable linearized term. Is there a way to use the stationary solution obtained in Comsol 4.2 as the initial conditions in a time dependent model? If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. Posted Sep 9, 2020, 1:44 p.m. EDT Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. 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. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. "I chose this print shop based off yelp reviews and was very happy with the outcome! Can I tell police to wait and call a lawyer when served with a search warrant? This approach is known as a Continuation Method with a Constant predictor. Solving such models in a stationary sense should simply require solving a single (large) system of linear equations and should always be solvable, but there are cases when the software will fail to find a solution. However, it is usually not possible to know this ahead of time. An example would be a linear static structural model of an aluminum beverage can. The algorithm is, generally speaking, a Newton's method approach. Minimising the environmental effects of my dyson brain. Stationary Solver Iterative Direct . Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. We use COMSOL Multiphysics for solving distributed optimal control of un-steady Burgers equation without constraints and with pointwise control constraints. What version of COMSOL are you using? The issue here has do with the iterative algorithm used to solve nonlinear stationary models. Any trademarks referenced in this document are the property of their respective owners. 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. 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 approach is used by default for most 1D, 2D, and 2D-axisymmetric models. Stationary (time-invariant) models with nonlinearities may converge very slowly. 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. Perhaps this approach could be adapted to represent your model. 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. Reviews on Stationary Stores in Brea, CA - Typo, Mievic, Posh Paper, Morning Glory, Select Graphics & Printing, Daiso Japan, Urban Outfitters, Office Depot, Barnes & Noble 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. Few days back i was also facing this problem in . Your internet explorer is in compatibility mode and may not be displaying the website correctly. Not the answer you're looking for? 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. Note the star symbol on the Solution feature. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. - Feature: Stationary Solver 1 (sol1/s1) By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. The fully coupled and segregated approaches are discussed below. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. Asking for help, clarification, or responding to other answers. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. What are some highly rated cards & stationery in Brea, CA? This is relatively expensive to do, but will lead to the most robust convergence. 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. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. (Frequency Domain should be the last step) Does anyone know what should cause this problem? The default Initial Values for the unknowns in most physics interfaces are zero. This involves a systematic reduction in the model complexity. Not entering required material parameters. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. 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. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. If you have both as steps in the same study, then solve that study. 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. COMSOL does not assume any legal liability for the accuracy of the data disclosed. 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. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. 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. By default, the COMSOL Multiphysics software determines these values heuristically depending on the physics as, for example, the specified initial values or a solution from an earlier study step. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Screenshot showing a Solver Configuration that has been altered. 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. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Discussion Closed This discussion was created more than 6 months ago and has been closed. Once the error estimate is small enough, as determined by the convergence criterion (20-16) the software terminates the computations and returns a solution. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Segregated approach and Direct vs. With respect to any nonlinearities, replace them by a reasonable linearized term. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) However, it is usually not possible to know this ahead of time. 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. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. If these settings have been manually changed, you will see a small star symbol on the Solution feature, as shown in the screenshot below. The stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. 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. 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. GCRO-DR is a method for Krylov subspace recycling. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. rev2023.3.3.43278. -Detail: NaN or Inf found when solving linear system using SOR. 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. 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. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) 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. That is, start by first solving a model with a small, but non-zero, load. The advantages of the continuation method are two-fold. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. In particular, choosing an improper initial condition or setting up a problem without a solution will simply cause the nonlinear solver to continue iterating without converging. Near the top of the Stationary Solver log, the software will report if a linear or nonlinear solver is being used. Hi Jason, There will always already be either a Segregated or Fully Coupled feature beneath this. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. 3. Cooling and Solidification of Metal. Use this parameter to modify the nonlinearity expressions in the model. Set initial conditions in the physics to the appropriate dependent model variable names rather than the default 0. The advantages of the continuation method are two-fold. The other low-level default settings within the Stationary Solver are chosen for robustness. I have searched all over comsol forum to fix this stationary solver configuration and still doesn't work because I don't know the logic behind the solver system. Such problems must solved in the time domain. The technique of load ramping is not always reasonable for all problems. Find detailed information on Office Supplies, Stationery, and Gift Retailers companies in Brea, California, United States of America, including financial statements, sales and marketing contacts, top competitors, and firmographic insights. Wrong ordering of study steps. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. Check the solver log to see if the continuation method is backtracking. How do/should administrators estimate the cost of producing an online introductory mathematics class? k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Stationary (time-invariant) models with nonlinearities may converge very slowly. Check the solver log to see if the continuation method is backtracking. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. Why? 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. 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. 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. Comsol help video number 2: Solving a laminar flow problem in a slit.. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. 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. The advantages of the continuation method are two-fold. Your internet explorer is in compatibility mode and may not be displaying the website correctly. I am trying to solve the coupling between a waveguide and a microring resonator. replace it with the expression: 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. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. listed if standards is not an option). We have also introduced meshing considerations for linear static problems, as well as how to identify singularities and what to do about them when meshing. 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. P&S: COMSOL Design Tool for Photonic Devices. There will also be a red cross over the Materials branch icon. The problem is that when I run my model this message appear: Undefined value found. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. COMSOL makes every reasonable effort to verify the information you view on this page. I use comsol multiphysics 5.2a and . 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. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. Multiphysics problems are often nonlinear. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. It is sometimes necessary to manually scale the dependent variables. If the default iterative solver is not converging, try switching to a direct solver, as described here: Understanding the Fully Coupled vs. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt.
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