The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. If this was solved using the Solid Mechanics physics interface the thin walls of the container would need to be explicitly modeled, but the wall thickness is much smaller than the overall can dimensions. With respect to any nonlinearities, replace them by a reasonable linearized term. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. Solving for laminar flow using Comsol - YouTube Comsol help video number 2: Solving a laminar flow problem in a slit. 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. If it does so, use a finer increment in that range. 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. Minimising the environmental effects of my dyson brain. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. The coupling terms between the different groups are thus neglected. 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 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. 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. 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. Your internet explorer is in compatibility mode and may not be displaying the website correctly. 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 exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. 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. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) Reviews on Stationary in Brea, CA - Mievic, Typo, Tokyo Japanese Lifestyle, Fancy, Posh Paper, Morning Glory, Velvet Fox Designs, Barnes & Noble, Michaels, Select Graphics & Printing You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Cooling and Solidification of Metal. This involves a systematic reduction in the model complexity. This is relatively expensive to do, but will lead to the most robust convergence. 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. 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 example, if there is a temperature-dependent material property such as: The Auxiliary Sweep can be used to implement ramping of any Global Parameter. It is also possible to manually refine the mesh. Here, we begin an overview of the algorithms used for solving nonlinear static finite element problems. 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. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. Hi Alexis, The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. 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. 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. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. That is, start by first solving a model with a small, but non-zero, load. - In this posting, we introduce the idea of ramping the nonlinearities in the problem to improve convergence. Segregated approach and Direct vs. Not assigning proper boundary conditions: Especially if you have ports. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. 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. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. 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. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. 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. Get email updates for new Stationary Engineer jobs in Brea, California, United States. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. If you still need help with COMSOL and have an on-subscription license, please visit our Support Center for help. The objective here is to simplify the model to a state where the model will solve, with linear approximations. -Detail: NaN or Inf found when solving linear system using SOR. COMSOL does not assume any legal liability for the accuracy of the data disclosed. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Note: there is no way to couple this . The segregated approach, on the other hand, solves sets of unknowns separately. Alle Rechte vorbehalten. Multiphysics problems are often nonlinear. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Segregated approach and Direct vs. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Posted Sep 9, 2020, 1:44 p.m. EDT . Understanding the Fully Coupled vs. 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. 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. Staging Ground Beta 1 Recap, and Reviewers needed for Beta 2, Simulation of effect of heated resistance on temperature distribution in laminar flow, COMSOL: Diffusion in Transport of Diluted Species Produces Unphysical Results. Comsol help video number 2: Solving a laminar flow problem in a slit.. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. 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. Linear solvers. Then I stored the resulting solution, and used it to to solve the stationary solver again by using the solver manager. Get notified about new Stationary Engineer jobs in Brea, California, United States. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. replace it with the expression: However, if a lower-level change has been manually made to the solver settings, the software will not automatically use the correct default solver. Stationary in the COMSOL Multiphysics Programming Reference Manual Damped Newton Methods The nonlinear solver uses an affine invariant form of the damped Newton method as described in Ref. Posted 26 set 2019, 17:57 CEST Mesh Version 5.3 0 Replies. COMSOL makes every reasonable effort to verify the information you view on this page. Note: there is no way to couple this field with the time dependent nature of this physics. 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. 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. That is, start by first solving a model with a small, but non-zero, load. The technique of load ramping is not always reasonable for all problems. Again, introduce a Global Parameter that gets ramped from exactly zero to one. 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. I am following the same model as Comsol provide us on the web. Most multiphysics problems are nonlinear. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. Mesh It is sometimes necessary to manually scale the dependent variables. 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. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. COMSOL makes every reasonable effort to verify the information you view on this page. This case is generally difficult, or impossible, to solve since this material property is non-smooth. This is relatively expensive to do, but will lead to the most robust convergence. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. $125,000.00, Project Engineer (In-person/Hybrid/Remote), $100,000.00 Discussion Closed This discussion was created more than 6 months ago and has been closed. 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. Feature: Stationary Solver 1 (sol1/s1)" . This case is generally difficult, or impossible, to solve since this material property is non-smooth. If your matrix is singular than no solver in the world can solve Ax=B. Have you taken a look at this [blog post](https://www.comsol.com/blogs/modeling-fluid-structure-interaction-in-multibody-mechanisms/)? The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. COMSOL does not assume any legal liability for the accuracy of the data disclosed. 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. 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. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: If the default iterative solver is not converging, try switching to a direct solver, as described here: Understanding the Fully Coupled vs. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Stationary (time-invariant) models with nonlinearities may converge very slowly. 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. First, it is physically intuitive, often matching how one would perform an experiment. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. Use this parameter to modify the nonlinearity expressions in the model. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. Learn how your comment data is processed. Multiphysics problems are often nonlinear. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. To switch between these solver types, go to the Stationary Solver node within the Study sequence. 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.

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