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. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. 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. - Feature: Stationary Solver 1 (sol1/s1) The latter method is known as the Continuation Method with a Linear predictor, and is controlled within the Study Configurations as shown in the screenshot below. This is relatively expensive to do, but will lead to the most robust convergence. What version of COMSOL are you using? Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. A linear finite element model is one in which all of the material properties, loads, boundary conditions, etc are constant with respect to the solution, and the governing partial differential equations are themselves linear. 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. 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. Repeat this for every nonlinearity of the model. Such a large difference in the materials properties can be challenging. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. First, it is physically intuitive, often matching how one would perform an experiment. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) The Auxiliary Sweep can be used to implement ramping of any Global Parameter. That is, they are tuned to achieve convergence in as many cases as possible. COMSOL does not assume any legal liability for the accuracy of the data disclosed. 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. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. 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 guide applies solely to nonlinear stationary models. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Today's top 351 Stationary Engineer jobs in Brea, California, United States. In many physics areas there exist alternative physics formulations specifically meant for solving cases where the geometry has an extreme aspect ratio. Then I stored the resulting solution, and used it to to solve the stationary solver again by using the solver manager. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. The fully coupled and segregated approaches are discussed below. 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. Your internet explorer is in compatibility mode and may not be displaying the website correctly. 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. For example, in an Electric Currents problem, you may want to consider a system of materials including a good conductor such as copper (with an electric conductivity of ~6e7 S/m) and an insulative material such as glass (which can have electric conductivity of ~1e-14 S/m.) It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. 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. What is \newluafunction? Posted Sep 9, 2020, 1:44 p.m. EDT This solver is automatically used when a Stationary or Frequency Domain study is added to the model. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. 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. listed if standards is not an option). 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. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. Save this information to a file. Wrong ordering of study steps. How can I use it? Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. Computational Fluid Dynamics (CFD), API This case is generally difficult, or impossible, to solve since this material property is non-smooth. Why do many companies reject expired SSL certificates as bugs in bug bounties? See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. 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. Minimising the environmental effects of my dyson brain. 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. Near the top of the Stationary Solver log, the software will report if a linear or nonlinear solver is being used. There will always already be either a Segregated or Fully Coupled feature beneath this. Connect and share knowledge within a single location that is structured and easy to search. The solver settings are stored at Study > Solver Configurations > Solution. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. Here, we begin an overview of the algorithms used for solving nonlinear static finite element problems. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). listed if standards is not an option). Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) Repeat this for every nonlinearity of the model. This is relatively expensive to do, but will lead to the most robust convergence. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. 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. Your internet explorer is in compatibility mode and may not be displaying the website correctly. The default Initial Values for the unknowns in most physics interfaces are zero. 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. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. COMSOL does not assume any legal liability for the accuracy of the data disclosed. With respect to any nonlinearities, replace them by a reasonable linearized term. The algorithm is, generally speaking, a Newton's method approach. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. There will always already be either a Segregated or Fully Coupled feature beneath this. 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. An example would be a linear static structural model of an aluminum beverage can. 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. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. 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. 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. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Multiphysics problems are often nonlinear. Such problems must solved in the time domain. - Not meshing all the domains. Not entering required material parameters. COMSOL makes every reasonable effort to verify the information you view on this page. Feature: Stationary Solver 1 (sol1/s1) Attempt to evaluate nonintegral power of negative number. They worked with me. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version It can be useful while solving sequences of linear systems arising from, for example, nonlinear problems. Discussion Closed This discussion was created more than 6 months ago and has been closed. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. Asking for help, clarification, or responding to other answers. 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. 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. 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. Click the link in the email we sent to to verify your email address and activate your job alert. Could you expand a little bit more why the coupling is impossible? This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. 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. This is relatively expensive to do, but will lead to the most robust convergence. If it does so, use a finer increment in that range. Stationary Solver Use the Stationary Solver () to find the solution to linear and nonlinear stationary problems (also called static or steady-state problems). This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Stationary Solver Iterative Direct . They deal with COMSOL package and train users. 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. Thanks for contributing an answer to Stack Overflow! She opened prepackaged 6-packs of Seegars to make ", All "stationary stores" results in Brea, CA, Search stationary stores in popular locations. Not assigning proper boundary conditions: Especially if you have ports. 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 stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. there is no defined multiphysics for it as I know, I have a standing accoustic wave and a flow in the background but I don't see their connection. 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. listed if standards is not an option). Any trademarks referenced in this document are the property of their respective owners. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Hello, It is also possible to manually refine the mesh. (Frequency Domain should be the last step) It is also possible to manually refine the mesh. See if these can be used instead of explicitly modeling parts with high-aspect ratio geometries. Cooling and Solidification of Metal. Most multiphysics problems are nonlinear. Common Mistakes: Not assigning materials to all the domains. numeric (each ports needs their ownboundary mode analysis in the study if they are numerically defined)Wave excitation: on/off(input/output), - Feature: Stationary Solver 1 (sol1/s1) Division by zero. Not meshing all the domains. For example, if there is a temperature-dependent material property such as: However, it is usually not possible to know this ahead of time. For example, if there is a temperature-dependent material property such as: You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. Get notified about new Stationary Engineer jobs in Brea, California, United States. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) 3. The objective here is to simplify the model to a state where the model will solve, with linear approximations. 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 example, if there is a temperature-dependent material property such as: The algorithm is, generally speaking, a Newton's method approach. Such problems must solved in the time domain. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. 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. To learn more, see our tips on writing great answers. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) 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. 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.
The conditions on the geometric aspect ratio are relatively more strict. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. Use this parameter to modify the nonlinearity expressions in the model. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. It is sometimes necessary to manually scale the dependent variables.