Could you expand a little bit more why the coupling is impossible? If the model is nonlinear, see: Improving Convergence of Nonlinear Stationary Models. Required fields are marked *. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. 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. Does anyone know what should cause this problem? 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. To switch between these solver types, go to the Stationary Solver node within the Study sequence. Check the solver log to see if the continuation method is backtracking. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. there is no defined multiphysics for it as I know, I have a standing accoustic wave and a flow in the background but I don't see their connection. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: 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. Name: actdep_int1, Your email address will not be published. There will also be a red cross over the Materials branch icon. Minimising the environmental effects of my dyson brain. Your email address will not be published. For example, if there is a temperature-dependent material property such as: Have you taken a look at this [blog post](https://www.comsol.com/blogs/modeling-fluid-structure-interaction-in-multibody-mechanisms/)? Hi Jason, The other low-level default settings within the Stationary Solver are chosen for robustness. Thanks, Andres. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. Posted Sep 9, 2020, 1:44 p.m. EDT Set "Values for dependent variables" in study step settings to User ControlledSolutionYour Stationary Study. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. 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. There will always already be either a Segregated or Fully Coupled feature beneath this. Improving Convergence of Nonlinear Stationary Models, Knowledgebase 1030: Error: "Out of memory", Knowledgebase 1030: Performing a Mesh Refinement Study, Understanding the Fully Coupled vs. 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. The coupling terms between the different groups are thus neglected. 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. The Fully Coupled solution approach, with the Plot While Solving enabled. What are some highly rated cards & stationery in Brea, CA? Alternatively, delete and re-create the study. listed if standards is not an option). Review the model setup to resolve these. Your internet explorer is in compatibility mode and may not be displaying the website correctly. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. By default, the COMSOL Multiphysics software determines these values heuristically depending on the physics as, for example, the specified initial values or a solution from an earlier study step. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Dr.S.Ravindran Cite 1 Recommendation Popular answers (1). Your internet explorer is in compatibility mode and may not be displaying the website correctly. 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. The problem is that when I run my model this message appear: Undefined value found. The fully coupled and segregated approaches are discussed below. 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. How can I check before my flight that the cloud separation requirements in VFR flight rules are met? This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. Feature: Stationary Solver 1 (sol1/s1) Attempt to evaluate nonintegral power of negative number. If it does so, use a finer increment in that range. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. As part of our solver blog series we have discussed solving nonlinear static finite element problems, load ramping for improving convergence of nonlinear problems, and nonlinearity ramping for improving convergence of nonlinear problems. Not assigning proper boundary conditions: Especially if you have ports. replace it with the expression: I am following the same model as Comsol provide us on the web. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. You can unsubscribe from these emails at any time. 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. Stationary Engineer Jobs in Brea, California, United States, $87,400.00 Again, introduce a Global Parameter that gets ramped from exactly zero to one. Reply . 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. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. It may also reveal that the model itself is ill-posed in some way. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. 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. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. Some models are numerically ill-conditioned due to the setup. The settings controlling the predictor type. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). 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. With respect to any nonlinearities, replace them by a reasonable linearized term. The stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. Multiphysics problems are often nonlinear. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. thanks for reply 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. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). listed if standards is not an option). Hello, However, it is usually not possible to know this ahead of time. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. Learn how your comment data is processed. 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. The coupling terms between the different groups are thus neglected. November 11, 2013. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Cecilia M. came from the backroom to help me and went out of her way to help me even though I had not called before going into the store. 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, 2023 by COMSOL. Here we introduce a more robust approach to solving nonlinear problems. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. 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. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. $130,000.00. This guide applies solely to nonlinear stationary models. Have you taken a look at this blog post? $125,000.00, Project Engineer (In-person/Hybrid/Remote), $100,000.00 Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. 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. What sort of strategies would a medieval military use against a fantasy giant? She opened prepackaged 6-packs of Seegars to make ", All "stationary stores" results in Brea, CA, Search stationary stores in popular locations. 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. Any trademarks referenced in this document are the property of their respective owners. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. In the COMSOL Multiphysics software, this step of the modeling workflow is made. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. Wrong ordering of study steps. Leverage your professional network, and get hired. - Any trademarks referenced in this document are the property of their respective owners. However, load ramping will not work in all cases, or may be inefficient. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) Do you also know how to solve this problem: using stationary solution as the initial conditions in time dependent model, How Intuit democratizes AI development across teams through reusability. (Frequency Domain should be the last step). Iterative , Direct . The algorithm is, generally speaking, a Newton's method approach. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. 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. Please dont hesitate to post comments below or send emails to us if you experience any other problems. Consult your product manuals for complete trademark details. Alle Rechte vorbehalten. If it does so, use a finer increment in that range. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. If it does so, use a finer increment in that range. listed if standards is not an option). Segregated approach and Direct vs. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. Making statements based on opinion; back them up with references or personal experience. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. Find centralized, trusted content and collaborate around the technologies you use most. Sign in to create your job alert for Stationary Engineer jobs in Brea, California, United States. If you see this, right-click on the Solution feature and select Reset Solver to Default. 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. The default Initial Values for the unknowns in most physics interfaces are zero. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. 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. 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. The objective here is to simplify the model to a state where the model will solve, with linear approximations. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. This approach is known as a Continuation Method with a Constant predictor. P&S Comsol Team: Manuel Kohli, Raphael Schwanninger, Feature: Stationary Solver 1 (sol1/s1) (Frequency Domain should be the last step) Solver . This case is generally difficult, or impossible, to solve since this material property is non-smooth. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. The fully coupled and segregated approaches are discussed below. 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. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version This algorithm was also useful for understanding what happens near a failure load. Full text of the 'Sri Mahalakshmi Dhyanam & Stotram'. To learn more, see our tips on writing great answers. Screenshot showing a Solver Configuration that has been altered. Here, we will examine techniques for accelerating the convergence of these two methods. Why? In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. Again, introduce a Global Parameter that gets ramped from exactly zero to one. Not meshing all the domains. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. Stationary (time-invariant) models with nonlinearities may converge very slowly. Posted 26 set 2019, 11:57 GMT-4 These can be used alone, or in combination with other interfaces. A linear finite element model is one in which all of the material properties, loads, boundary conditions, etc are constant with respect to the solution, and the governing partial differential equations are themselves linear. Common Mistakes: Not assigning materials to all the domains. Note: there is no way to couple this field with the time dependent nature of this physics. 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 segregated approach, on the other hand, solves sets of unknowns separately. 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. Direct PARDISO Solver , PARDISO . Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. 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. Your Discussion has gone 30 days without a reply. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. Perhaps this approach could be adapted to represent your model. It can be useful while solving sequences of linear systems arising from, for example, nonlinear problems. Such a large difference in the materials properties can be challenging. Version 5.3 We are planning to continuously update this page throughout the semester and hopefully, this will become a reference during your projects as well. Your internet explorer is in compatibility mode and may not be displaying the website correctly. COMSOL does not assume any legal liability for the accuracy of the data disclosed. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. Not assigning proper boundary conditions: Especially if you have ports. 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. In this post, we will delve deeper into how to choose an appropriate mesh to start your mesh convergence studies for linear static finite element problems. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model.