Create the time-dependent step or study. Check the solver log to see if the continuation method is backtracking. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. About the Stationary Solver - COMSOL Multiphysics How to solve "Undefined material property 'sigma - ResearchGate 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: 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. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Wrong ordering of study steps. 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. What version of COMSOL are you using? 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. Then use this file to provide the initial conditions in time dependent 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 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. Any trademarks referenced in this document are the property of their respective owners. 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. Discussion Closed This discussion was created more than 6 months ago and has been closed. Hi Alexis, Could you expand a little bit more why the coupling is impossible? I am solving a linear stationary finite element model but the software is not solving. This doesn't seem to me the most elegant of methods, since I am essentially solving a stationary solution using a time dependent 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. Common Mistakes: Not assigning materials to all the domains. Use a manually defined mesh to avoid elements with extreme aspect ratios and perform a mesh refinement study, as described here: Performing a Mesh Refinement Study, For problems that are ill-conditioned, using a direct solver is often called for. Could you expand a little bit more why the coupling is impossible? 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. 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. 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. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. Required fields are marked *. 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. The nature of simulating nature: A Q&A with IBM Quantum researcher Dr. Jamie We've added a "Necessary cookies only" option to the cookie consent popup. Studysteps might be listed in wrong order: Not assigning materials to all the domains. 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. The default Initial Values for the unknowns in most physics interfaces are zero. 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. It may also reveal that the model itself is ill-posed in some way. 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. 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. 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. Not meshing all the domains. Minimising the environmental effects of my dyson brain. What is the purpose of non-series Shimano components? This is relatively expensive to do, but will lead to the most robust convergence. 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. 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. Such problems must solved in the time domain. 3 Replies, Please login with a confirmed email address before reporting spam. P&S Comsol Team: Manuel Kohli, Raphael Schwanninger, Feature: Stationary Solver 1 (sol1/s1) Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) 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. Set "Values for dependent variables" in study step settings to User ControlledSolutionYour Stationary Study. replace it with the expression: Understanding the Fully Coupled vs. If so, see: Knowledgebase 1030: Error: "Out of memory". Again, introduce a Global Parameter that gets ramped from exactly zero to one. Not the answer you're looking for? The fully coupled and segregated approaches are discussed below. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. What did people search for similar to stationary stores in Brea, CA? Use this parameter to modify the nonlinearity expressions in the model. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. Hello, COMSOL Blog Series on COMSOL Multiphysics Solvers Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Save my name, email, and website in this browser for the next time I comment. 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, Galleria dei Modelli e delle App di Simulazione, 2023 da COMSOL. Name: actdep_int1, Your email address will not be published. If you are unsure if your problem is linear and stationary, look at the Log. There will always already be either a Segregated or Fully Coupled feature beneath this. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. The technique of load ramping is not always reasonable for all problems. The settings controlling the predictor type. It is also possible to manually refine the mesh. 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. This is relatively expensive to do, but will lead to the most robust convergence. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. The prices are good, and when I needed something delivered, same day, they were able to do it, and it looked beautiful!! The algorithm is, generally speaking, a Newton's method approach. Making statements based on opinion; back them up with references or personal experience. Find centralized, trusted content and collaborate around the technologies you use most. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. (Frequency Domain should be the last step) If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. 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. Version 5.3 This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. 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. The advantages of the continuation method are two-fold. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) Mesh 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. Check the solver log to see if the continuation method is backtracking. This guide applies solely to nonlinear stationary models. 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. 351 Stationary Engineer jobs in Brea, California, United States (15 new) How can I check before my flight that the cloud separation requirements in VFR flight rules are met? Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings.
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