What is the purpose of this D-shaped ring at the base of the tongue on my hiking boots? An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: This is relatively expensive to do, but will lead to the most robust convergence. Again, introduce a Global Parameter that gets ramped from exactly zero to one. listed if standards is not an option). The segregated approach, on the other hand, solves sets of unknowns separately. You can write the discrete form of the equations as f(U) = 0, where f(U) is the residual vector and U is the solution vector. 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. COMSOL makes every reasonable effort to verify the information you view on this page. The solver settings are stored at Study > Solver Configurations > Solution. Acidity of alcohols and basicity of amines. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. 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. "After the incident", I started to be more careful not to trip over things. The coupling terms between the different groups are thus neglected. Required fields are marked *. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. 0 Replies, Please login with a confirmed email address before reporting spam. For the Nozomi from Shinagawa to Osaka, say on a Saturday afternoon, would tickets/seats typically be available - or would you need to book? Your internet explorer is in compatibility mode and may not be displaying the website correctly. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. 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. That is, start by first solving a model with a small, but non-zero, load. How to solve "Undefined material property 'sigma - ResearchGate 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. They are usually called comp1.u, comp1.v, and comp1.w though. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. COMSOL 22.9K subscribers Adding a study to your simulation is necessary in order to solve your problem. 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 there is a temperature-dependent material property such as: Use this parameter to modify the nonlinearity expressions in the model. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. 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. The Fully Coupled solution approach, with the Plot While Solving enabled. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. The objective here is to simplify the model to a state where the model will solve, with linear approximations. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. Tutti i diritti sono riservati. The conditions on the geometric aspect ratio are relatively more strict. 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. 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. Common Errors in Comsol - P&S: COMSOL Design Tool for Photonic Devices Why do many companies reject expired SSL certificates as bugs in bug bounties? Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. The technique of load ramping is not always reasonable for all problems. 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. If it does so, use a finer increment in that range. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. This solver is automatically used when a Stationary or Frequency Domain study is added to the model. Why doesn't iterative solver converge in COMSOL? - ResearchGate COMSOL makes every reasonable effort to verify the information you view on this page. Singular matrix., Make sure you defined your ports correctly:Boundary selectionType of port: e.g. 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. COMSOL Blog Series on COMSOL Multiphysics Solvers There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. Save my name, email, and website in this browser for the next time I comment. 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. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Improving Convergence of Nonlinear Stationary Models - COMSOL In the extreme case, suppose one wants to model an instantaneous change in properties, such as: Connect and share knowledge within a single location that is structured and easy to search. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) If the model is nonlinear, see: Improving Convergence of Nonlinear Stationary Models. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. I have searched all over comsol forum to fix this stationary solver configuration and still doesn't work because I don't know the logic behind the solver system. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. What is \newluafunction? See Knowledge Base 1240: Manually Setting the Scaling of Variables. Not the answer you're looking for? Stationary Engineer Jobs in Brea, California, United States, $87,400.00 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. It is sometimes necessary to manually scale the dependent variables. 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. 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. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. 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. With respect to any nonlinearities, replace them by a reasonable linearized term. [COMSOL_Users] output of stationary solver as input to time-dependent It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. 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. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. The problem is that when I run my model this message appear: Undefined value found. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. 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. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. 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. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. Linear solvers. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. comsol - using stationary solution as the initial conditions in time Not assigning proper boundary conditions: Especially if you have ports. Under Initial values of variables solved for, the default value of the Settingslist is Physics controlled. Your internet explorer is in compatibility mode and may not be displaying the website correctly. - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. This guide applies solely to nonlinear stationary models. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Therefore, an initial value of zero is almost always reasonable if a very small load is applied. Office Supplies, Stationery, and Gift Retailers Companies in Brea 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. Such problems must solved in the time domain. Your Discussion has gone 30 days without a reply. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) 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. When you use an iterative solver, COMSOL Multiphysics estimates the error of the solution while solving. Have you taken a look at this blog post? The objective here is to simplify the model to a state where the model will solve, with linear approximations. This case is generally difficult, or impossible, to solve since this material property is non-smooth. Hello guys. 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 parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. if I want to do an adaptive mesh refinement, I get this error. That is, they are tuned to achieve convergence in as many cases as possible. - The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). The coupling terms between the different groups are thus neglected. The settings controlling the predictor type. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. (I am begginer in comsol) Thank you. 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. Specify Material Properties in COMSOL Multiphysics (3/8) Is it possible to rotate a window 90 degrees if it has the same length and width? You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Click the link in the email we sent to to verify your email address and activate your job alert. Wish you all the best. listed if standards is not an option). However, it is usually not possible to know this ahead of time. Reply . 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. These are some highly rated cards & stationery in Brea, CA: What are some cards & stationery with a large number of reviews in Brea, CA? Name: actdep_int1, Your email address will not be published. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. P&S Comsol Team: Yannik Horst, Manuel Kohli, Xinzhi Zhang. Hi Jason, 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. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. Different physics have different default solvers. View the Settings window for the Materials branch to get a list of all domains with undefined materials and add a material to those domains. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: Get notified about new Stationary Engineer jobs in Brea, California, United States. If it does so, use a finer increment in that range. The segregated approach, on the other hand, solves sets of unknowns separately. The technique of load ramping is not always reasonable for all problems. Function: / Failed to evaluate expression. 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. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Wrong ordering of study steps. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Improving Convergence of Nonlinear Stationary Models - Informazioni As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. The default solver for most 3D models is an iterative solver, which is more sensitive to ill-conditioned problems. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. The advantages of the continuation method are two-fold. 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. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. It may also reveal that the model itself is ill-posed in some way. Any trademarks referenced in this document are the property of their respective owners. For example, if there is a temperature-dependent material property such as: The settings controlling the predictor type. In this page, we will be sharing some common errors that might occur in Comsol and their solutions. The algorithm is, generally speaking, a Newton's method approach. This approach is known as a Continuation Method with a Constant predictor. About the Stationary Solver - COMSOL Multiphysics Solve the stationary study then the time dependent study. The other low-level default settings within the Stationary Solver are chosen for robustness. This guide applies solely to nonlinear stationary models. Check the solver log to see if the continuation method is backtracking. 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. Direct PARDISO Solver , PARDISO . Can someone help me fix the COMSOL error below? | ResearchGate 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. 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. 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 unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. 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 algorithm is, generally speaking, a Newton's method approach. 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. Understanding the Fully Coupled vs. To start a new discussion with a link back to this one, click here. Iterative , Direct . The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). Linear solvers. 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. 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. 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. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Discussion Closed This discussion was created more than 6 months ago and has been closed. Iterative - COMSOL Multiphysics Full text of the 'Sri Mahalakshmi Dhyanam & Stotram'. That is, start by first solving a model with a small, but non-zero, load. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). 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. That is, start by first solving a model with a small, but non-zero, load. Not entering required material parameters. This guide applies solely to nonlinear stationary models. It is also possible to manually refine the mesh. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. Common Study Step Settings - COMSOL Multiphysics Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. Use this parameter to modify the nonlinearity expressions in the model. The idea behind the GCRO-DR method is to retain the subspace determined while solving previous systems and use it to reduce the cost of solving the next system. 3 Replies, Please login with a confirmed email address before reporting spam. However, if a lower-level change has been manually made to the solver settings, the software will not automatically use the correct default solver. That is, they are tuned to achieve convergence in as many cases as possible. We use COMSOL Multiphysics for solving distributed optimal control of un-steady Burgers equation without constraints and with pointwise control constraints. listed if standards is not an option). thanks for reply This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge.