An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. There will always already be either a Segregated or Fully Coupled feature beneath this. 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. -Detail: NaN or Inf found when solving linear system using SOR. Posted 26 set 2019, 11:57 GMT-4 Multiscale Modeling in High-Frequency Electromagnetics. If so, see: Knowledgebase 1030: Error: "Out of memory". COMSOL makes every reasonable effort to verify the information you view on this page. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) Any trademarks referenced in this document are the property of their respective owners. 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 fully coupled and segregated approaches are discussed below. In such cases, see if one material (or the other) can be omitted from the analysis completely. Contact COMSOL at Bangalore on their telephone: +9180 25323003. 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. Is there a way to use the stationary solution obtained in Comsol 4.2 as the initial conditions in a time dependent model? The objective here is to simplify the model to a state where the model will solve, with linear approximations. Few days back i was also facing this problem in . comp1.u2, comp1.v2, and comp1.w2 are usually variables associated with the x,y, and z component of deformation in COMSOL. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. What is the purpose of this D-shaped ring at the base of the tongue on my hiking boots? 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. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version The settings controlling the predictor type. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. To start a new discussion with a link back to this one, click here. As a rough rule of thumb, once the aspect ratio between the largest characteristic dimension to the smallest approaches 100:1, you might start to run into issues and should look to alternative ways of posing the problem, especially in a 3D model. 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. Your internet explorer is in compatibility mode and may not be displaying the website correctly. This involves a systematic reduction in the model complexity. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. It is also possible to manually refine the mesh. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. Studysteps might be listed in wrong order: Not assigning materials to all the domains. Your internet explorer is in compatibility mode and may not be displaying the website correctly. L'objectif de notre prsent travail se repose sur l'tude par simulation numrique du comportement de bton au jeune ge sous des conditions svres de temprature pendant les premires 24h aprs. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. 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. It can be useful while solving sequences of linear systems arising from, for example, nonlinear problems. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. 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. listed if standards is not an option). This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. Sometimes, reducing the model complexity can be quite challenging and it can be better to start from as simple a case as possible and gradually increase the complexity. 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. See Knowledge Base 1240: Manually Setting the Scaling of Variables. However, it is usually not possible to know this ahead of time. It's brand new in the hmart plaza and I wish it was open back when I would hangout in the plaza after school (although they would have taken all my allowance money! A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. 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. 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. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: 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. Minimising the environmental effects of my dyson brain. Again, introduce a Global Parameter that gets ramped from exactly zero to one. P&S Comsol Team: Arif Gngr , Yannik Horst , Stefano Valente. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. It is sometimes necessary to manually scale the dependent variables. Sign in to create your job alert for Stationary Engineer jobs in Brea, California, United States. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. 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. When you use an iterative solver, COMSOL Multiphysics estimates the error of the solution while solving. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. 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. This guide applies solely to nonlinear stationary models. 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. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. What version of COMSOL are you using? That is, start by first solving a model with a small, but non-zero, load. 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. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. 3 Replies, Please login with a confirmed email address before reporting spam. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. However, it is usually not possible to know this ahead of time. 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. 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. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. For example, if there is a temperature-dependent material property such as: This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. 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. 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. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. 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. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. The segregated approach, on the other hand, solves sets of unknowns separately. Could you expand a little bit more why the coupling is impossible? A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. 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. The Fully Coupled solution approach, with the Plot While Solving enabled. Not assigning proper boundary conditions: Especially if you have ports. Solving such models in a stationary sense should simply require solving a single (large) system of linear equations and should always be solvable, but there are cases when the software will fail to find a solution. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. It is also possible to manually refine the mesh. Consult your product manuals for complete trademark details. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. 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 Fully Coupled solution approach, with the Plot While Solving enabled. Here, we begin an overview of the algorithms used for solving nonlinear static finite element problems. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. In the COMSOL Multiphysics software, this step of the modeling workflow is made. 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. If the material properties entered are incorrect for the governing equation, the model will generate an error at runtime, usually a Singular Matrix error. P&S Comsol Team: Manuel Kohli, Raphael Schwanninger, Feature: Stationary Solver 1 (sol1/s1) That is, start by first solving a model with a small, but non-zero, load. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. 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.
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