If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. What is \newluafunction? It is also possible to manually refine the mesh. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. 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. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Wrong ordering of study steps. Thanks, Andres. Segregated approach and Direct vs. 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. Your internet explorer is in compatibility mode and may not be displaying the website correctly. If the default iterative solver is not converging, try switching to a direct solver, as described here: Understanding the Fully Coupled vs. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. See Knowledge Base 1240: Manually Setting the Scaling of Variables. 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. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Your internet explorer is in compatibility mode and may not be displaying the website correctly. However, it is usually not possible to know this ahead of time. Such problems must solved in the time domain. 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. Hello, "After the incident", I started to be more careful not to trip over things. Most multiphysics problems are nonlinear. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. If the material properties entered are incorrect for the governing equation, the model will generate an error at runtime, usually a Singular Matrix error. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) Stationary (time-invariant) models with nonlinearities may converge very slowly. COMSOL makes every reasonable effort to verify the information you view on this page. 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. 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. 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. Repeat this for every nonlinearity of the model. - Function: ^ Failed to evaluate variable. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. Here, we will examine techniques for accelerating the convergence of these two methods. 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. Tutti i diritti sono riservati. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. $140,000.00, $120,000.00 Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. 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. Multiphysics problems are often nonlinear. Singular matrix., Make sure you defined your ports correctly:Boundary selectionType of port: e.g. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. The conditions on the geometric aspect ratio are relatively more strict. 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. 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. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. November 11, 2013. Perhaps this approach could be adapted to represent your model. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. They worked with me. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. 0 Replies, Please login with a confirmed email address before reporting spam. This case is generally difficult, or impossible, to solve since this material property is non-smooth. Posted 26 set 2019, 17:57 CEST Mesh Version 5.3 0 Replies. 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. Use this parameter to modify the nonlinearity expressions in the model. 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. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. With respect to any nonlinearities, replace them by a reasonable linearized term. The Fully Coupled solution approach, with the Plot While Solving enabled. What is the purpose of non-series Shimano components? I use comsol multiphysics 5.2a and . Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: Check the solver log to see if the continuation method is backtracking. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. Communication over the phone, in person, and through email was very easy. Save my name, email, and website in this browser for the next time I comment. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: Repeat this for every nonlinearity of the model. Thanks for contributing an answer to Stack Overflow! Click the link in the email we sent to to verify your email address and activate your job alert. Linear solvers. The default solver for most 3D models is an iterative solver, which is more sensitive to ill-conditioned problems. Not meshing all the domains. COMSOL does not assume any legal liability for the accuracy of the data disclosed. For example, in an Electric Currents problem, you may want to consider a system of materials including a good conductor such as copper (with an electric conductivity of ~6e7 S/m) and an insulative material such as glass (which can have electric conductivity of ~1e-14 S/m.) The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). The stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. However, it is usually not possible to know this ahead of time. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. What sort of strategies would a medieval military use against a fantasy giant? Note: there is no way to couple this field with the time dependent nature of this physics. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. That is, start by first solving a model with a small, but non-zero, load. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. Consult your product manuals for complete trademark details. What did people search for similar to stationary stores in Brea, CA? P&S Comsol Team: Manuel Kohli, Raphael Schwanninger, Feature: Stationary Solver 1 (sol1/s1) This is a review for cards & stationery in Brea, CA: "Love this store!!! Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. It is also possible to manually refine the mesh. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. Such a case would be better to address instead with the Shell physics interface, which is specially formulated for handling thin-walled structural parts. 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. 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. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. 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. What are people saying about cards & stationery in Brea, CA? If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. In our previous blog entry, we introduced the Fully Coupled and the Segregated algorithms used for solving steady-state multiphysics problems in COMSOL. Reviews on Stationary in Brea, CA - Mievic, Typo, Tokyo Japanese Lifestyle, Fancy, Posh Paper, Morning Glory, Velvet Fox Designs, Barnes & Noble, Michaels, Select Graphics & Printing Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Feature: Stationary Solver 1 (sol1/s1) Attempt to evaluate nonintegral power of negative number. I want to conduct a simulation to find a solution (u) and its first derivative (ux) using a 3D stationary model. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. Not entering required material parameters. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Function: / Failed to evaluate expression. P&S Comsol Team: Arif Gngr , Yannik Horst , Stefano Valente. 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. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. Multiphysics problems are often nonlinear. Get notified about new Stationary Engineer jobs in Brea, California, United States. 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. 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. 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. That is, they are tuned to achieve convergence in as many cases as possible. Again, introduce a Global Parameter that gets ramped from exactly zero to one. Reply . I am solving a linear stationary finite element model but the software is not solving. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. 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. listed if standards is not an option). Save this information to a file. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. 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. Cooling and Solidification of Metal. GCRO-DR is a method for Krylov subspace recycling. listed if standards is not an option). 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. The problem is that when I run my model this message appear: Undefined value found. 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. 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. 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. This case is generally difficult, or impossible, to solve since this material property is non-smooth. 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. Multiscale Modeling in High-Frequency Electromagnetics. 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. For example, if there is a temperature-dependent material property such as: Check the solver log to see if the continuation method is backtracking. 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 algorithm is, generally speaking, a Newton's method approach. 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. To switch between these solver types, go to the Stationary Solver node within the Study sequence. 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. Building on these topics, we will now address how to prepare your mesh for efficiently solving nonlinear finite element problems. Linear solvers. - The segregated approach, on the other hand, solves sets of unknowns separately. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. It may also reveal that the model itself is ill-posed in some way. Again, introduce a Global Parameter that gets ramped from exactly zero to one. 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. This approach is known as a Continuation Method with a Constant predictor.
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