It is sometimes necessary to manually scale the dependent variables. The Fully Coupled solution approach, with the Plot While Solving enabled. The advantages of the continuation method are two-fold. Thanks for contributing an answer to Stack Overflow! Here we introduce a more robust approach to solving nonlinear problems. 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. . Asking for help, clarification, or responding to other answers. First, it is physically intuitive, often matching how one would perform an experiment. When you use an iterative solver, COMSOL Multiphysics estimates the error of the solution while solving. The software then computes an initial solution and from there it iteratively re-computes the solution, taking into account how these intermediate solutions affect the nonlinearities. Any trademarks referenced in this document are the property of their respective owners. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. 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. 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]*exp(-(T-293[K])/100[K]) What is the purpose of non-series Shimano components? Again, introduce a Global Parameter that gets ramped from exactly zero to one. Posted 26 set 2019, 11:57 GMT-4 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. Understanding the Fully Coupled vs. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. The coupling terms between the different groups are thus neglected. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. The objective here is to simplify the model to a state where the model will solve, with linear approximations. It may also reveal that the model itself is ill-posed in some way. To switch between these solver types, go to the Stationary Solver node within the Study sequence. 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. Alternatively, delete and re-create the study. I am trying to solve the coupling between a waveguide and a microring resonator. Save this information to a file. The following are possible reasons why a linear stationary model will fail to solve, along with resolutions: The combination of the constraints and boundary conditions must be sufficient to define a unique solution to the problem, in terms of the field variables being solved. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) Why is there a voltage on my HDMI and coaxial cables? Such a large difference in the materials properties can be challenging. 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. 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. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version The segregated approach, on the other hand, solves sets of unknowns separately. 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. Reviews on Stationary Stores in Brea, CA - Typo, Mievic, Posh Paper, Morning Glory, Select Graphics & Printing, Daiso Japan, Urban Outfitters, Office Depot, Barnes & Noble Such problems must solved in the time domain. Note the star symbol on the Solution feature. The software then computes an initial solution and from there it iteratively re-computes the solution, taking into account how these intermediate solutions affect the nonlinearities. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. Linear solvers. Hello guys. If so, see: Knowledgebase 1030: Error: "Out of memory". Cooling and Solidification of Metal. 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. Segregated approach and Direct vs. Tutti i diritti sono riservati. The other low-level default settings within the Stationary Solver are chosen for robustness. Discussion Closed This discussion was created more than 6 months ago and has been closed. 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. 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. Repeat this for every nonlinearity of the model. Your email address will not be published. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. 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. Hi ! 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. 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. 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. 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. - Function: ^ Failed to evaluate variable. Have you taken a look at this [blog post](https://www.comsol.com/blogs/modeling-fluid-structure-interaction-in-multibody-mechanisms/)? It is also possible to manually refine the mesh. Segregated approach and Direct vs. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. This solver is automatically used when a Stationary or Frequency Domain study is added to the model. However, it is usually not possible to know this ahead of time. (Frequency Domain should be the last step). COMSOL makes every reasonable effort to verify the information you view on this page. The stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. 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 former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. Name: actdep_int1, Your email address will not be published. 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. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. 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. 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. Wrong ordering of study steps. replace it with the expression: 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. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. 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. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown 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. These are some highly rated cards & stationery in Brea, CA: What are some cards & stationery with a large number of reviews in Brea, CA? - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. The conditions on the geometric aspect ratio are relatively more strict. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Find centralized, trusted content and collaborate around the technologies you use most. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. Function: / Failed to evaluate expression. For example, in a Solid Mechanics (wherein the software is solving for the displacement field within the solid) applying two opposite and equal Boundary Load conditions on a part is not sufficient to define the displacement. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Does ZnSO4 + H2 at high pressure reverses to Zn + H2SO4? Solver . Feature: Stationary Solver 1 (sol1/s1) Attempt to evaluate nonintegral power of negative number. The technique of load ramping is not always reasonable for all problems. 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. Reply . If you still need help with COMSOL and have an on-subscription license, please visit our Support Center for help. 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. The software then computes an initial solution and from there it iteratively re-computes the solution, taking into account how these intermediate solutions affect the nonlinearities. P&S: COMSOL Design Tool for Photonic Devices. Most multiphysics problems are nonlinear. 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. For example, if there is a temperature-dependent material property such as: Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. Alle Rechte vorbehalten. Cooling and Solidification of Metal. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. They worked with me. 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. Full text of the 'Sri Mahalakshmi Dhyanam & Stotram'. The other low-level default settings within the Stationary Solver are chosen for robustness. $140,000.00, $120,000.00 If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. What is \newluafunction? k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) Iterative , Direct . They are usually called comp1.u, comp1.v, and comp1.w though.
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