It can be useful while solving sequences of linear systems arising from, for example, nonlinear problems. Tutti i diritti sono riservati. The default solver for most 3D models is an iterative solver, which is more sensitive to ill-conditioned problems. The Fully Coupled solution approach, with the Plot While Solving enabled. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. COMSOL makes every reasonable effort to verify the information you view on this page. First, it is physically intuitive, often matching how one would perform an experiment. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. Most multiphysics problems are nonlinear. Not assigning proper boundary conditions: Especially if you have ports. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. In a previous blog entry, we introduced meshing considerations for linear static problems. Stationary (time-invariant) models with nonlinearities may converge very slowly. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. It may also reveal that the model itself is ill-posed in some way. 3 Replies, Please login with a confirmed email address before reporting spam. If so, see: Knowledgebase 1030: Error: "Out of memory". As we saw in Load Ramping of Nonlinear Problems, we can use the continuation method to ramp the loads on a problem up from an unloaded case where we know the solution. Wrong ordering of study steps. 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. New Stationary Engineer jobs added daily. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. "After the incident", I started to be more careful not to trip over things. Again, introduce a Global Parameter that gets ramped from exactly zero to one. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. COMSOL makes every reasonable effort to verify the information you view on this page. What sort of strategies would a medieval military use against a fantasy giant? Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. 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. 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. 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. replace it with the expression: k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) Under Initial values of variables solved for, the default value of the Settingslist is Physics controlled. 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. Solving for laminar flow using Comsol - YouTube Comsol help video number 2: Solving a laminar flow problem in a slit. if I want to do an adaptive mesh refinement, I get this error. 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. replace it with the expression: Could you expand a little bit more why the coupling is impossible? 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. 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. The technique of load ramping is not always reasonable for all problems. We use COMSOL Multiphysics for solving distributed optimal control of un-steady Burgers equation without constraints and with pointwise control constraints. Repeat this for every nonlinearity of the model. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Note the star symbol on the Solution feature. This involves a systematic reduction in the model complexity. Common Mistakes: Not assigning materials to all the domains. 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. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. What is the purpose of non-series Shimano components? The objective here is to simplify the model to a state where the model will solve, with linear approximations. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. When you use an iterative solver, COMSOL Multiphysics estimates the error of the solution while solving. 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. The segregated approach, on the other hand, solves sets of unknowns separately. P&S: COMSOL Design Tool for Photonic Devices. We are planning to continuously update this page throughout the semester and hopefully, this will become a reference during your projects as well. thanks for reply 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. Your email address will not be published. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. It is sometimes necessary to manually scale the dependent variables. This case is generally difficult, or impossible, to solve since this material property is non-smooth. Tutti i diritti sono riservati. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. They are usually called comp1.u, comp1.v, and comp1.w though. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Also, keep in mind that a linear stationary model should solve regardless of how coarse the mesh is (albeit to a low accuracy) so you can always start with as coarse a mesh as possible, and refine the mesh (See also: Knowledgebase 1030: Performing a Mesh Refinement Study. It may also reveal that the model itself is ill-posed in some way. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. Here we introduce a more robust approach to solving nonlinear problems. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. An example would be a linear static structural model of an aluminum beverage can. To switch between these solver types, go to the Stationary Solver node within the Study sequence. Version 5.3 First, it is physically intuitive, often matching how one would perform an experiment. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. SGP handled 7 different prints for me at once and they all came out perfectly, in a timely manner. This guide applies solely to nonlinear stationary models. Does ZnSO4 + H2 at high pressure reverses to Zn + H2SO4? Hi Alexis, (Frequency Domain should be the last step) Have you taken a look at this blog post? Use this parameter to modify the nonlinearity expressions in the model. How can I use it? listed if standards is not an option). For example, if there is a temperature-dependent material property such as: See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Hi ! Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version In this posting, we introduce the idea of ramping the nonlinearities in the problem to improve convergence. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. The difference between the phonemes /p/ and /b/ in Japanese. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. listed if standards is not an option). I personally liked emailing them the file, ", "This flower shop is the best! If you have both as steps in the same study, then solve that study. The Fully Coupled solution approach, with the Plot While Solving enabled. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) With respect to any nonlinearities, replace them by a reasonable linearized term. 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. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. That is, they are tuned to achieve convergence in as many cases as possible. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. 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. The default Initial Values for the unknowns in most physics interfaces are zero. Learn how your comment data is processed. P&S Comsol Team: Arif Gngr , Yannik Horst , Stefano Valente. Screenshot showing a Solver Configuration that has been altered. Not assigning proper boundary conditions: Especially if you have ports. At low flow speeds the flow solution will be time invariant, but at higher flow rates there will be vortex shedding, a time-varying change in the flow field behind the cylinder. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. Here, we begin an overview of the algorithms used for solving nonlinear static finite element problems. so many cute little stationary items , hair". Discussion Closed This discussion was created more than 6 months ago and has been closed. 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. 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