If you do not hold an on-subscription license, you may find an answer in another Discussion or in the Knowledge Base. $140,000.00, $120,000.00 listed if standards is not an option). 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. Posted 26 set 2019, 17:57 CEST Mesh Version 5.3 0 Replies. (Frequency Domain should be the last step) k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) 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. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. I want to conduct a simulation to find a solution (u) and its first derivative (ux) using a 3D stationary model. Does anyone know what should cause this problem? In the extreme case, suppose one wants to model an instantaneous change in properties, such as: 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, in Solid Mechanics, if the Poisson Ratio set to 0.5, then the model will not solve, as this value in incompatible with the theory of linear elasticity. It is also possible to manually refine the mesh. $131,100.00, Simplified Vehicle Operations Project Engineer, $115,000.00 Use this parameter to modify the nonlinearity expressions in the model. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. It is sometimes necessary to manually scale the dependent variables. The other low-level default settings within the Stationary Solver are chosen for robustness. Segregated approach and Direct vs. Iterative linear solvers, About the time step setting of the solver, Introducing Goal Seeking into the Segregated Solver. 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. COMSOL makes every reasonable effort to verify the information you view on this page. 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. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. I use comsol multiphysics 5.2a and . The problem is that when I run my model this message appear: Undefined value found. The Fully Coupled solution approach, with the Plot While Solving enabled. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Making statements based on opinion; back them up with references or personal experience. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Posted Sep 9, 2020, 1:44 p.m. EDT Such a case would be better to address instead with the Shell physics interface, which is specially formulated for handling thin-walled structural parts. 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. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. - Alternatively, delete and re-create the study. 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. Staging Ground Beta 1 Recap, and Reviewers needed for Beta 2, Simulation of effect of heated resistance on temperature distribution in laminar flow, COMSOL: Diffusion in Transport of Diluted Species Produces Unphysical Results. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. For example, if there is a temperature-dependent material property such as: Repeat this for every nonlinearity of the model. P&S Comsol Team: Arif Gngr , Yannik Horst , Stefano Valente. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). Understanding the Fully Coupled vs. 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. 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. 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. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. - Second, the continuation method will automatically take smaller load increments if a solution cannot be found. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. 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. 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. Save this information to a file. Then use this file to provide the initial conditions in time dependent model. Note: there is no way to couple this . There will always already be either a Segregated or Fully Coupled feature beneath this. Repeat this for every nonlinearity of the model. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. (I am begginer in comsol) Thank you. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. The advantages of the continuation method are two-fold. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: There will always already be either a Segregated or Fully Coupled feature beneath this. 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. The coupling terms between the different groups are thus neglected. Posted 26 set 2019, 11:57 GMT-4 Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. 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. November 11, 2013. About the Stationary Solver The following background information about the Stationary Solver discusses these topics: Damped Newton Methods, Termination Criterion for the Fully Coupled and Segregated Attribute Nodes, Linear Solvers versus Nonlinear Solvers, and Pseudo Time Stepping. These are some cards & stationery with a large number of reviews in Brea, CA. If you have both as steps in the same study, then solve that study. 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. How can I check before my flight that the cloud separation requirements in VFR flight rules are met? The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. 3. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. Your internet explorer is in compatibility mode and may not be displaying the website correctly. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. 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. In this page, we will be sharing some common errors that might occur in Comsol and their solutions. Wrong ordering of study steps. This case is generally difficult, or impossible, to solve since this material property is non-smooth. 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. It may also reveal that the model itself is ill-posed in some way. The Automatic predictor setting will use the constant predictor when a segregated solution approach is being used, and use the linear predictor when the fully coupled approach is used. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Alle Rechte vorbehalten. View the Settings window for the Materials branch to get a list of all domains with undefined materials and add a material to those domains. rev2023.3.3.43278. Here, we will examine techniques for accelerating the convergence of these two methods. That is, they are tuned to achieve convergence in as many cases as possible. 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. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. This is relatively expensive to do, but will lead to the most robust convergence. The solver settings are stored at Study > Solver Configurations > Solution. 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. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. It is also possible to manually refine the mesh. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. Your internet explorer is in compatibility mode and may not be displaying the website correctly. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. 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. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Stationary Solver Iterative Direct . Discussion Closed This discussion was created more than 6 months ago and has been closed. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. 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. 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. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. 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. Stationary (time-invariant) models with nonlinearities may converge very slowly. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. 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. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Set "Values for dependent variables" in study step settings to User ControlledSolutionYour Stationary Study. In this blog post we introduce the two classes of algorithms that are used in COMSOL to solve systems of linear equations that arise when solving any finite element problem. Check the solver log to see if the continuation method is backtracking. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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 Second, the continuation method will automatically take smaller load increments if a solution cannot be found.
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