Changes to these low-level settings from the defaults will usually be quite model- and case-specific. 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 the star symbol on the Solution feature. As we saw in Load Ramping of Nonlinear Problems, we can use the continuation method to ramp the loads on a problem up from an unloaded case where we know the solution. The technique of load ramping is not always reasonable for all problems. 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. In this blog post we introduce the two classes of algorithms that are used in COMSOL to solve systems of linear equations that arise when solving any finite element problem. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. This solver is automatically used when a Stationary or Frequency Domain study is added to the model. 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. thanks for reply 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. 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. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. 0 Replies, Please login with a confirmed email address before reporting spam. replace it with the expression: With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) Hi Jason, If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. If you are unsure if your problem is linear and stationary, look at the Log. It is sometimes necessary to manually scale the dependent variables. It can be useful while solving sequences of linear systems arising from, for example, nonlinear problems. Check the solver log to see if the continuation method is backtracking. 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. COMSOL makes every reasonable effort to verify the information you view on this page. Solver . listed if standards is not an option). Connect and share knowledge within a single location that is structured and easy to search. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. 140K views 8 years ago COMSOL Multiphysics Tutorial for Beginners Please note that an updated version of the content in this video can be found in the Modeling Workflow video in the COMSOL. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. 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. Leverage your professional network, and get hired. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. 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. 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. Not assigning proper boundary conditions: Especially if you have ports. The default solver for most 3D models is an iterative solver, which is more sensitive to ill-conditioned problems. Minimising the environmental effects of my dyson brain. As a second example, when solving for Electric Currents, do not model perfect electrical insulators as materials with zero conductivity, instead, omit the domain from the model and use the Electric Insulation boundary condition. Is there a way to use the stationary solution obtained in Comsol 4.2 as the initial conditions in a time dependent model? 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. 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. Reviews on Stationary Stores in Brea, CA - Typo, Mievic, Posh Paper, Morning Glory, Select Graphics & Printing, Daiso Japan, Urban Outfitters, Office Depot, Barnes & Noble so many cute little stationary items , hair". The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. Get email updates for new Stationary Engineer jobs in Brea, California, United States. See if these can be used instead of explicitly modeling parts with high-aspect ratio geometries. If the model is nonlinear, see: Improving Convergence of Nonlinear Stationary Models. However, load ramping will not work in all cases, or may be inefficient. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: "I chose this print shop based off yelp reviews and was very happy with the outcome! This doesn't seem to me the most elegant of methods, since I am essentially solving a stationary solution using a time dependent Improving Convergence of Nonlinear Stationary Models, Knowledgebase 1030: Error: "Out of memory", Knowledgebase 1030: Performing a Mesh Refinement Study, Understanding the Fully Coupled vs. This is relatively expensive to do, but will lead to the most robust convergence. COMSOL does not assume any legal liability for the accuracy of the data disclosed. 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. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. This approach is known as a Continuation Method with a Constant predictor. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. This case is generally difficult, or impossible, to solve since this material property is non-smooth. 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. 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. Get notified about new Stationary Engineer jobs in Brea, California, United States. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. These can be used alone, or in combination with other interfaces. 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. listed if standards is not an option). The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. The other low-level default settings within the Stationary Solver are chosen for robustness. Not assigning proper boundary conditions: Especially if you have ports. Any trademarks referenced in this document are the property of their respective owners. If the default iterative solver is not converging, try switching to a direct solver, as described here: Understanding the Fully Coupled vs. In many physics areas there exist alternative physics formulations specifically meant for solving cases where the geometry has an extreme aspect ratio. COMSOL 22.9K subscribers Adding a study to your simulation is necessary in order to solve your problem. The algorithm is, generally speaking, a Newton's method approach. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). Not the answer you're looking for? It is sometimes necessary to manually scale the dependent variables. Not entering required material parameters. I personally liked emailing them the file, ", "This flower shop is the best! 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. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. 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. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. The objective here is to simplify the model to a state where the model will solve, with linear approximations. 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. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. 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. 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. 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. 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 settings controlling the predictor type. Your email address will not be published. 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. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. The objective here is to simplify the model to a state where the model will solve, with linear approximations. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. One of the key concepts there was the idea of mesh convergence as you refine the mesh, the solution will become more accurate. COMSOL does not assume any legal liability for the accuracy of the data disclosed. - Feature: Stationary Solver 1 (sol1/s1)Failed to find a solution. They deal with COMSOL package and train users. It is also possible to manually refine the mesh. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. This case is generally difficult, or impossible, to solve since this material property is non-smooth. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) Your internet explorer is in compatibility mode and may not be displaying the website correctly. The coupling terms between the different groups are thus neglected. COMSOL makes every reasonable effort to verify the information you view on this page. Such problems must solved in the time domain. How can I use it? Alle Rechte vorbehalten. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. listed if standards is not an option). 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. New Stationary Engineer jobs added daily. Any trademarks referenced in this document are the property of their respective owners. The Fully Coupled solution approach, with the Plot While Solving enabled. This involves a systematic reduction in the model complexity. Iterative , Direct . Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. How can I check before my flight that the cloud separation requirements in VFR flight rules are met? $130,000.00. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. 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. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Stationary Engineer Jobs in Brea, California, United States, $87,400.00 Solve the stationary study then the time dependent study. - Function: ^ Failed to evaluate variable. 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. Segregated approach and Direct vs. 3 Replies, Please login with a confirmed email address before reporting spam. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. If you see this, right-click on the Solution feature and select Reset Solver to Default. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. What are people saying about cards & stationery in Brea, CA? 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. Linear solvers. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. 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. 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. Again, introduce a Global Parameter that gets ramped from exactly zero to one. There will always already be either a Segregated or Fully Coupled feature beneath this. Check the solver log to see if the continuation method is backtracking. 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. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). For example, if there is a temperature-dependent material property such as: Why? Some models are numerically ill-conditioned due to the setup. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. The problem is that when I run my model this message appear: Undefined value found. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. Singular matrix., Make sure you defined your ports correctly:Boundary selectionType of port: e.g. Segregated approach and Direct vs. Iterative linear solvers, About the time step setting of the solver, Introducing Goal Seeking into the Segregated Solver. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. Asking for help, clarification, or responding to other answers. 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. 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. 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. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. P&S Comsol Team: Arif Gngr , Yannik Horst , Stefano Valente. That is, they are tuned to achieve convergence in as many cases as possible. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. Any trademarks referenced in this document are the property of their respective owners. listed if standards is not an option). For the Nozomi from Shinagawa to Osaka, say on a Saturday afternoon, would tickets/seats typically be available - or would you need to book? In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. This guide applies solely to nonlinear stationary models. In our previous blog entry, we introduced the Fully Coupled and the Segregated algorithms used for solving steady-state multiphysics problems in COMSOL. 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. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. Such problems must solved in the time domain. Repeat this for every nonlinearity of the model. Then use this file to provide the initial conditions in time dependent model. $125,000.00, Project Engineer (In-person/Hybrid/Remote), $100,000.00 It's brand new in the hmart plaza and I wish it was open back when I would hangout in the plaza after school (although they would have taken all my allowance money! The technique of load ramping is not always reasonable for all problems. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. 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 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. Segregated approach and Direct vs. Iterative linear solvers, Time dependent function and stationary study, Combining Adaptive Mesh Refinement with Data Filtering, What to do when a linear stationary model is not solving, Galleria dei Modelli e delle App di Simulazione, 2023 da COMSOL. Comsol help video number 2: Solving a laminar flow problem in a slit.. Thanks for contributing an answer to Stack Overflow! Any trademarks referenced in this document are the property of their respective owners. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. Using the first order optimality. GCRO-DR is a method for Krylov subspace recycling. It is sometimes necessary to manually scale the dependent variables. 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. Use this parameter to modify the nonlinearity expressions in the model. COMSOL makes every reasonable effort to verify the information you view on this page. My comment is perhaps a bit nave but it seems to me that you could simply deactivate the \frac{\partial \cdot}{\partial t} term of the background field equation but keep its connexion to the solid to get what you want. Feature: Stationary Solver 1 (sol1/s1) Attempt to evaluate nonintegral power of negative number. Wrong ordering of study steps. Not entering required material parameters. Few days back i was also facing this problem in . A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Stationary (time-invariant) models with nonlinearities may converge very slowly. Dun & Bradstreet gathers Office Supplies, Stationery, and Gift Retailers business information from trusted . Studysteps might be listed in wrong order: Not assigning materials to all the domains. Stationary (time-invariant) models with nonlinearities may converge very slowly. 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. See Knowledge Base 1240: Manually Setting the Scaling of Variables. 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. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Such problems must solved in the time domain. With respect to any nonlinearities, replace them by a reasonable linearized term. 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. 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. 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. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version If the material properties entered are incorrect for the governing equation, the model will generate an error at runtime, usually a Singular Matrix error.
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stationary solver comsol