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Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. 3 Replies, Please login with a confirmed email address before reporting spam. The other low-level default settings within the Stationary Solver are chosen for robustness. Mesh Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. The advantages of the continuation method are two-fold. This guide applies solely to nonlinear stationary models. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Singular matrix., Make sure you defined your ports correctly:Boundary selectionType of port: e.g. Hi Alexis, Your internet explorer is in compatibility mode and may not be displaying the website correctly. If your matrix is singular than no solver in the world can solve Ax=B. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. 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. Building on these topics, we will now address how to prepare your mesh for efficiently solving nonlinear finite element problems. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Not assigning proper boundary conditions: Especially if you have ports. Connect and share knowledge within a single location that is structured and easy to search. 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. - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. How can I check before my flight that the cloud separation requirements in VFR flight rules are met? 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. However, load ramping will not work in all cases, or may be inefficient. 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. COMSOL Error for Initial Values(RF Module) Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. 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. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. COMSOL makes every reasonable effort to verify the information you view on this page. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. Not entering required material parameters. Create the time-dependent step or study. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. 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. The settings controlling the predictor type. Find centralized, trusted content and collaborate around the technologies you use most. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. 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. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: With respect to any nonlinearities, replace them by a reasonable linearized term. The segregated approach, on the other hand, solves sets of unknowns separately. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. 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. The technique of load ramping is not always reasonable for all problems. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. thanks for reply 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. 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. November 11, 2013. Thanks for contributing an answer to Stack Overflow! Posted 26 set 2019, 11:57 GMT-4 We use COMSOL Multiphysics for solving distributed optimal control of un-steady Burgers equation without constraints and with pointwise control constraints. Your internet explorer is in compatibility mode and may not be displaying the website correctly. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. See Knowledge Base 1240: Manually Setting the Scaling of Variables. 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. Under Initial values of variables solved for, the default value of the Settingslist is Physics controlled. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) What did people search for similar to stationary stores in Brea, CA? Perhaps this approach could be adapted to represent your model. Not assigning proper boundary conditions: Especially if you have ports. Save this information to a file. 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. 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. What to do when a linear stationary model is not solving - COMSOL listed if standards is not an option). There will always already be either a Segregated or Fully Coupled feature beneath this. Few days back i was also facing this problem in . If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. Stationary Solver Use the Stationary Solver () to find the solution to linear and nonlinear stationary problems (also called static or steady-state problems). Cooling and Solidification of Metal. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Can someone help me fix the COMSOL error below? | ResearchGate There will always already be either a Segregated or Fully Coupled feature beneath this. solving stationary solver after each time step - COMSOL Multiphysics Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. This algorithm was also useful for understanding what happens near a failure load. Wish you all the best. This involves a systematic reduction in the model complexity. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. 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. 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. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. 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. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. We are planning to continuously update this page throughout the semester and hopefully, this will become a reference during your projects as well. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) The coupling terms between the different groups are thus neglected. listed if standards is not an option). Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. 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.) Solving for laminar flow using Comsol - YouTube Comsol help video number 2: Solving a laminar flow problem in a slit. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. The objective here is to simplify the model to a state where the model will solve, with linear approximations. What is the purpose of non-series Shimano components? Different physics have different default solvers. The technique of load ramping is not always reasonable for all problems. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Dr.S.Ravindran Cite 1 Recommendation Popular answers (1). 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. $130,000.00. Consult your product manuals for complete trademark details. 3. Minimising the environmental effects of my dyson brain. The objective here is to simplify the model to a state where the model will solve, with linear approximations. How do/should administrators estimate the cost of producing an online introductory mathematics class? This solver is automatically used when a Stationary or Frequency Domain study is added to the model. In this posting, we introduce the idea of ramping the nonlinearities in the problem to improve convergence. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. You can unsubscribe from these emails at any time. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. 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. "After the incident", I started to be more careful not to trip over things. Repeat this for every nonlinearity of the model. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. 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. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. Set "Values for dependent variables" in study step settings to User ControlledSolutionYour Stationary Study.