Such problems must solved in the time domain. If it does so, use a finer increment in that range. Hi Alexis, First, it is physically intuitive, often matching how one would perform an experiment. By providing your email address, you consent to receive emails from COMSOL AB and its affiliates about the COMSOL Blog, and agree that COMSOL may process your information according to its Privacy Policy. Therefore, it is recommended to use Adaptive Mesh Refinement which will automatically refine the mesh only in regions where it is needed, and coarsen the mesh elsewhere. The coupling terms between the different groups are thus neglected. 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. It is also possible to manually refine the mesh. - Hence Comsol solved for the stationary solution at different points of time. Version 5.3 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. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. listed if standards is not an option). This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. I am following the same model as Comsol provide us on the web. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Cooling and Solidification of Metal. Hi ! 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. What did people search for similar to stationary stores in Brea, CA? 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. 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. 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. Solver . Different physics have different default solvers. Singular matrix., Make sure you defined your ports correctly:Boundary selectionType of port: e.g. Your Discussion has gone 30 days without a reply. 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. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. Most multiphysics problems are nonlinear. 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]) Multiphysics problems are often nonlinear. 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. 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. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. 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. 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. The settings controlling the predictor type. That is, they are tuned to achieve convergence in as many cases as possible. Repeat this for every nonlinearity of the model. "I chose this print shop based off yelp reviews and was very happy with the outcome! There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Again, introduce a Global Parameter that gets ramped from exactly zero to one. Consult your product manuals for complete trademark details. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. 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. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. 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. If it does so, use a finer increment in that range. I personally liked emailing them the file, ", "This flower shop is the best! This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. 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. 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. We are planning to continuously update this page throughout the semester and hopefully, this will become a reference during your projects as well. The technique of load ramping is not always reasonable for all problems. The solver settings are stored at Study > Solver Configurations > Solution. Linear solvers. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. In this page, we will be sharing some common errors that might occur in Comsol and their solutions. listed if standards is not an option). They worked with me. It may also reveal that the model itself is ill-posed in some way. The technique of load ramping is not always reasonable for all problems. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. These are some highly rated cards & stationery in Brea, CA: What are some cards & stationery with a large number of reviews in Brea, CA? Posted Sep 9, 2020, 1:44 p.m. EDT To learn more, see our tips on writing great answers. Why is there a voltage on my HDMI and coaxial cables? In the extreme case, suppose one wants to model an instantaneous change in properties, such as: Therefore, it is recommended to use Adaptive Mesh Refinement which will automatically refine the mesh only in regions where it is needed, and coarsen the mesh elsewhere. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. The default Initial Values for the unknowns in most physics interfaces are zero. 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. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Improving Convergence of Nonlinear Stationary Models, Knowledgebase 1030: Error: "Out of memory", Knowledgebase 1030: Performing a Mesh Refinement Study, Understanding the Fully Coupled vs. so many cute little stationary items , hair". 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. P&S Comsol Team: Yannik Horst, Manuel Kohli, Xinzhi Zhang. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. 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. Discussion Closed This discussion was created more than 6 months ago and has been closed. 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. It is sometimes necessary to manually scale the dependent variables. Could you expand a little bit more why the coupling is impossible? Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. 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. Mesh The issue here has do with the iterative algorithm used to solve nonlinear stationary models. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) Communication over the phone, in person, and through email was very easy. The other low-level default settings within the Stationary Solver are chosen for robustness. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Segregated approach and Direct vs. The objective here is to simplify the model to a state where the model will solve, with linear approximations. 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. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. 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. Save this information to a file. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. So far, weve learned how to mesh and solve linear and nonlinear single-physics finite element problems, but have not yet considered what happens when there are multiple different interdependent physics being solved within the same domain. Required fields are marked *. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. 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. 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. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. 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. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) The Fully Coupled solution approach, with the Plot While Solving enabled. Again, introduce a Global Parameter that gets ramped from exactly zero to one. listed if standards is not an option). 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. Note: there is no way to couple this field with the time dependent nature of this physics. The Fully Coupled solution approach, with the Plot While Solving enabled. Wrong ordering of study steps. Understanding the Fully Coupled vs. This solver is automatically used when a Stationary or Frequency Domain study is added to the model. To start a new discussion with a link back to this one, click here. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Not assigning proper boundary conditions: Especially if you have ports. 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. Note: there is no way to couple this . The objective here is to simplify the model to a state where the model will solve, with linear approximations. Dr.S.Ravindran Cite 1 Recommendation Popular answers (1). Therefore, it is recommended to use Adaptive Mesh Refinement which will automatically refine the mesh only in regions where it is needed, and coarsen the mesh elsewhere. 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. Connect and share knowledge within a single location that is structured and easy to search. In the COMSOL Multiphysics software, this step of the modeling workflow is made. The stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. Your internet explorer is in compatibility mode and may not be displaying the website correctly. 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. I want to conduct a simulation to find a solution (u) and its first derivative (ux) using a 3D stationary model. 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. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. 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. How can I check before my flight that the cloud separation requirements in VFR flight rules are met? This involves a systematic reduction in the model complexity. The default Initial Values for the unknowns in most physics interfaces are zero. 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. This algorithm was also useful for understanding what happens near a failure load. Check the solver log to see if the continuation method is backtracking. Get notified about new Stationary Engineer jobs in Brea, California, United States. 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. Direct Iterative , Direct . Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. However, it is usually not possible to know this ahead of time. 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. Changes to these low-level settings from the defaults will usually be quite model- and case-specific.
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