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. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. 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 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. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. By creating this job alert, you agree to the LinkedIn User Agreement and Privacy Policy. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. COMSOL makes every reasonable effort to verify the information you view on this page. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. 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. 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. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: Stationary (time-invariant) models with nonlinearities may converge very slowly. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version If these settings have been manually changed, you will see a small star symbol on the Solution feature, as shown in the screenshot below. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. Find centralized, trusted content and collaborate around the technologies you use most. 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. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) 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. I am solving a linear stationary finite element model but the software is not solving. The Fully Coupled solution approach, with the Plot While Solving enabled. 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. The settings controlling the predictor type. (Frequency Domain should be the last step) Such a large difference in the materials properties can be challenging. Hi ! GCRO-DR is a method for Krylov subspace recycling. Again, introduce a Global Parameter that gets ramped from exactly zero to one. 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. Studysteps might be listed in wrong order: Not assigning materials to all the domains. See Knowledge Base 1240: Manually Setting the Scaling of Variables. Function: / Failed to evaluate expression. 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. Segregated approach and Direct vs. Consult your product manuals for complete trademark details. Your internet explorer is in compatibility mode and may not be displaying the website correctly. The problem is that when I run my model this message appear: Undefined value found. 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. Posted 26 set 2019, 17:57 CEST Mesh Version 5.3 0 Replies. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. What are some highly rated cards & stationery in Brea, CA? Hi Jason, The fully coupled and segregated approaches are discussed below. Most multiphysics problems are nonlinear. The segregated approach, on the other hand, solves sets of unknowns separately. That is, they are tuned to achieve convergence in as many cases as possible. How can I use it? See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. The fully coupled and segregated approaches are discussed below. 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. This is relatively expensive to do, but will lead to the most robust convergence. Solver . 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. - Feature: Stationary Solver 1 (sol1/s1) That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. Your internet explorer is in compatibility mode and may not be displaying the website correctly. 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. listed if standards is not an option). 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. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. thanks for reply COMSOL does not assume any legal liability for the accuracy of the data disclosed. The conditions on the geometric aspect ratio are relatively more strict. How can I check before my flight that the cloud separation requirements in VFR flight rules are met? Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. thanks for reply 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. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. - Check the solver log to see if the continuation method is backtracking. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. I use comsol multiphysics 5.2a and . With respect to any nonlinearities, replace them by a reasonable linearized term. 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. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Here, we begin an overview of the algorithms used for solving nonlinear static finite element problems. 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. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: Once the error estimate is small enough, as determined by the convergence criterion (20-16) the software terminates the computations and returns a solution. 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. The stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. Contact COMSOL at Bangalore on their telephone: +9180 25323003. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. -Detail: NaN or Inf found when solving linear system using SOR. 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. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: In such cases, see if one material (or the other) can be omitted from the analysis completely. Improving Convergence of Nonlinear Stationary Models, Knowledgebase 1030: Error: "Out of memory", Knowledgebase 1030: Performing a Mesh Refinement Study, Understanding the Fully Coupled vs. Not assigning proper boundary conditions: Especially if you have ports. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. The stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. Trying to understand how to get this basic Fourier Series. One of the key concepts there was the idea of mesh convergence as you refine the mesh, the solution will become more accurate. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. 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. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. 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. Save my name, email, and website in this browser for the next time I comment. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. Not the answer you're looking for? In the COMSOL Multiphysics software, this step of the modeling workflow is made. This approach is known as a Continuation Method with a Constant predictor. See if these can be used instead of explicitly modeling parts with high-aspect ratio geometries. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Stationary Solver Use the Stationary Solver () to find the solution to linear and nonlinear stationary problems (also called static or steady-state 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. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. If it does so, use a finer increment in that range. Note: there is no way to couple this . Leverage your professional network, and get hired. In many physics areas there exist alternative physics formulations specifically meant for solving cases where the geometry has an extreme aspect ratio. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: Use this parameter to modify the nonlinearity expressions in the model. 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. If it does so, use a finer increment in that range. Use this parameter to modify the nonlinearity expressions 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. The advantages of the continuation method are two-fold. The prices are good, and when I needed something delivered, same day, they were able to do it, and it looked beautiful!! This can arise as a consequence of extreme variations in the material properties, or high aspect ratio geometry. What are people saying about cards & stationery in Brea, CA? Repeat this for every nonlinearity of the model. Linear solvers. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. Multiphysics problems are often nonlinear. 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. Version 5.3 0 Replies, Please login with a confirmed email address before reporting spam. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Your internet explorer is in compatibility mode and may not be displaying the website correctly. The segregated approach, on the other hand, solves sets of unknowns separately. These are some highly rated cards & stationery in Brea, CA: What are some cards & stationery with a large number of reviews in Brea, CA? Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. 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. 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. You can write the discrete form of the equations as f(U) = 0, where f(U) is the residual vector and U is the solution vector. For example, if there is a temperature-dependent material property such as: The other low-level default settings within the Stationary Solver are chosen for robustness. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. To start a new discussion with a link back to this one, click here. However, if a lower-level change has been manually made to the solver settings, the software will not automatically use the correct default solver. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. 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 objective here is to simplify the model to a state where the model will solve, with linear approximations. First, it is physically intuitive, often matching how one would perform an experiment.