That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. 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. This algorithm was also useful for understanding what happens near a failure load. 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. 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. Click the link in the email we sent to to verify your email address and activate your job alert. What are people saying about cards & stationery in Brea, CA? 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. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: See Knowledge Base 1240: Manually Setting the Scaling of Variables. (Frequency Domain should be the last step) Please dont hesitate to post comments below or send emails to us if you experience any other problems. 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. Stationary in the COMSOL Multiphysics Programming Reference Manual Damped Newton Methods The nonlinear solver uses an affine invariant form of the damped Newton method as described in Ref. Hello guys. Common Mistakes: Not assigning materials to all the domains. COMSOL makes every reasonable effort to verify the information you view on this page. 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. Unknown function or operator. The fully coupled and segregated approaches are discussed below. 0 Replies, Please login with a confirmed email address before reporting spam. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. 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. 3 Replies, Please login with a confirmed email address before reporting spam. Thanks for contributing an answer to Stack Overflow! The segregated approach, on the other hand, solves sets of unknowns separately. Iterative , Direct . Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. I am following the same model as Comsol provide us on the web. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. That is, start by first solving a model with a small, but non-zero, load. Sign in to create your job alert for Stationary Engineer jobs in Brea, California, United States. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. However, if a lower-level change has been manually made to the solver settings, the software will not automatically use the correct default solver. However, it is usually not possible to know this ahead of time. The fully coupled and segregated approaches are discussed below. 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: 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. - Feature: Stationary Solver 1 (sol1/s1) Why? Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. Posted 26 set 2019, 11:57 GMT-4 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. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. To start a new discussion with a link back to this one, click here. For example, if there is a temperature-dependent material property such as: 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. Leverage your professional network, and get hired. Get notified about new Stationary Engineer jobs in Brea, California, United States. With respect to any nonlinearities, replace them by a reasonable linearized term. In such cases, see if one material (or the other) can be omitted from the analysis completely. How can I use it? The algorithm is, generally speaking, a Newton's method approach. The settings controlling the predictor type. If the model is very large, and if you do not have very much memory in your computer, you may get an error message regarding memory. Comsol help video number 2: Solving a laminar flow problem in a slit.. Such problems must solved in the time domain. 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. 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. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. The technique of load ramping is not always reasonable for all problems. What are some highly rated cards & stationery in Brea, CA? This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. 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. It is sometimes necessary to manually scale the dependent variables. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. Find centralized, trusted content and collaborate around the technologies you use most. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Wrong ordering of study steps. If it does so, use a finer increment in that range. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. 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. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) These are some highly rated cards & stationery in Brea, CA: What are some cards & stationery with a large number of reviews in Brea, CA? Repeat this for every nonlinearity of the model. 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. 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. This consent may be withdrawn. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. 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. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. 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. The solver settings are stored at Study > Solver Configurations > Solution. Understanding the Fully Coupled vs. In our previous blog entry, we introduced the Fully Coupled and the Segregated algorithms used for solving steady-state multiphysics problems in COMSOL. Any trademarks referenced in this document are the property of their respective owners. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. 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. 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. 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. Your internet explorer is in compatibility mode and may not be displaying the website correctly. This guide applies solely to nonlinear stationary models. 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. 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. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. Does anyone know what should cause this problem? -Detail: NaN or Inf found when solving linear system using SOR. Perhaps this approach could be adapted to represent your model. 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. See if these can be used instead of explicitly modeling parts with high-aspect ratio geometries. ", "Absolutely the best place to shop for flowers and greenery, I spent an hour just looking around for fun prior to purchasing a bouquet, they have the best customer service people all over the place and it's a huge place", "Recently came in here to pick up 45 chocolate Seegars (cigars) for a dinner banquet in honor of my baby nephew. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. For example, if there is a temperature-dependent material property such as: The Fully Coupled solution approach, with the Plot While Solving enabled. The objective here is to simplify the model to a state where the model will solve, with linear approximations. - the incident has nothing to do with me; can I use this this way? This approach is known as a Continuation Method with a Constant predictor. The segregated approach, on the other hand, solves sets of unknowns separately. Such a case would be better to address instead with the Shell physics interface, which is specially formulated for handling thin-walled structural parts. Understanding the Fully Coupled vs. rev2023.3.3.43278. 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. Use this parameter to modify the nonlinearity expressions in the model. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. (I am begginer in comsol) Thank you. Near the top of the Stationary Solver log, the software will report if a linear or nonlinear solver is being used. 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. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. 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.