This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. Review the model setup to resolve these. It can be useful while solving sequences of linear systems arising from, for example, nonlinear problems. See Knowledge Base 1240: Manually Setting the Scaling of Variables. If you see this, right-click on the Solution feature and select Reset Solver to Default. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. 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. Check the solver log to see if the continuation method is backtracking. Segregated approach and Direct vs. 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. As a rough rule of thumb, once the aspect ratio between the largest characteristic dimension to the smallest approaches 100:1, you might start to run into issues and should look to alternative ways of posing the problem, especially in a 3D model. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Your internet explorer is in compatibility mode and may not be displaying the website correctly. For example, if there is a temperature-dependent material property such as: That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. 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. (Frequency Domain should be the last step) Linear solvers. 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. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. 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. Troubleshooting for 3D trusses, if anyone's out there. : r/COMSOL - reddit Asking for help, clarification, or responding to other answers. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. What did people search for similar to stationary stores in Brea, CA? Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Understanding the Fully Coupled vs. What is the purpose of this D-shaped ring at the base of the tongue on my hiking boots? 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. Stationary Solver Use the Stationary Solver () to find the solution to linear and nonlinear stationary problems (also called static or steady-state problems). 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. Function: / Failed to evaluate expression. The problem is that when I run my model this message appear: Undefined value found. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. In this posting, we introduce the idea of ramping the nonlinearities in the problem to improve convergence. Common Mistakes: Not assigning materials to all the domains. 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. The algorithm is, generally speaking, a Newton's method approach. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) See Knowledge Base 1240: Manually Setting the Scaling of Variables. 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. Stationary Engineer Jobs in Brea, California, United States, $87,400.00 Again, introduce a Global Parameter that gets ramped from exactly zero to one. In many physics areas there exist alternative physics formulations specifically meant for solving cases where the geometry has an extreme aspect ratio. The advantages of the continuation method are two-fold. To switch between these solver types, go to the Stationary Solver node within the Study sequence. 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. This guide applies solely to nonlinear stationary models. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Using the first order optimality. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. (Frequency Domain should be the last step). 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. When you use an iterative solver, COMSOL Multiphysics estimates the error of the solution while solving. Does anyone know what should cause this problem? An example would be a linear static structural model of an aluminum beverage can. This involves a systematic reduction in the model complexity. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. 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. Repeat this for every nonlinearity of the model. 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. However, load ramping will not work in all cases, or may be inefficient. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. 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. That is, start by first solving a model with a small, but non-zero, load. Multiphysics problems are often nonlinear. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. 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. replace it with the expression: Acidity of alcohols and basicity of amines. How to solve "Feature: Stationary Solver 1 (sol1/s1) Attempt - COMSOL How can I check before my flight that the cloud separation requirements in VFR flight rules are met? The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. Posted 26 set 2019, 11:57 GMT-4 This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. What are people saying about cards & stationery in Brea, CA? Computational Fluid Dynamics (CFD), API In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. 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. This involves a systematic reduction in the model complexity. How can I use it? If so, see: Knowledgebase 1030: Error: "Out of memory". This approach is known as a Continuation Method with a Constant predictor. Please dont hesitate to post comments below or send emails to us if you experience any other problems. (I am begginer in comsol) Thank you. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. The objective here is to simplify the model to a state where the model will solve, with linear approximations. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). Set initial conditions in the physics to the appropriate dependent model variable names rather than the default 0. Your internet explorer is in compatibility mode and may not be displaying the website correctly. - Feature: Stationary Solver 1 (sol1/s1) First, it is physically intuitive, often matching how one would perform an experiment. Feature: Stationary Solver 1 (sol1/s1) Attempt to evaluate nonintegral power of negative number. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) replace it with the expression: This guide applies solely to nonlinear stationary models. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. The algorithm is, generally speaking, a Newton's method approach. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. 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. If the default iterative solver is not converging, try switching to a direct solver, as described here: Understanding the Fully Coupled 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. As we saw previously in the blog entry on Solving Nonlinear Static Finite Element Problems, not all nonlinear problems will be solvable via the damped Newton-Raphson method. Tutti i diritti sono riservati. Not meshing all the domains. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. COMSOL does not assume any legal liability for the accuracy of the data disclosed. That is, they are tuned to achieve convergence in as many cases as possible. Set "Values for dependent variables" in study step settings to User ControlledSolutionYour Stationary Study. This is relatively expensive to do, but will lead to the most robust convergence. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. Dun & Bradstreet gathers Office Supplies, Stationery, and Gift Retailers business information from trusted . Is there a way to use the stationary solution obtained in Comsol 4.2 as the initial conditions in a time dependent model? 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. Get notified about new Stationary Engineer jobs in Brea, California, United States. - 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. Your Discussion has gone 30 days without a reply. Click the link in the email we sent to to verify your email address and activate your job alert. 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. The coupling terms between the different groups are thus neglected. In a previous blog entry, we introduced meshing considerations for linear static problems. It may also reveal that the model itself is ill-posed in some way. Have you taken a look at this [blog post](https://www.comsol.com/blogs/modeling-fluid-structure-interaction-in-multibody-mechanisms/)? - Function: ^ Failed to evaluate variable. The Fully Coupled solution approach, with the Plot While Solving enabled. 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. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Iterative - COMSOL Multiphysics There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. To switch between these solver types, go to the Stationary Solver node within the Study sequence. 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! If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. The fully coupled and segregated approaches are discussed below. 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. The algorithm is, generally speaking, a Newton's method approach. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) 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. 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. 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. Improving Convergence of Nonlinear Stationary Models - COMSOL Stationary (time-invariant) models with nonlinearities may converge very slowly. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. Can I tell police to wait and call a lawyer when served with a search warrant? 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. If you have both as steps in the same study, then solve that study. Consult your product manuals for complete trademark details. This approach is known as a Continuation Method with a Constant predictor. Hi ! 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. Version 5.3 Could you expand a little bit more why the coupling is impossible? To start a new discussion with a link back to this one, click here. - the incident has nothing to do with me; can I use this this way? Understanding the Fully Coupled vs. thanks for reply Hence Comsol solved for the stationary solution at different points of time. 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. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. 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. 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. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. 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. Once the error estimate is small enough, as determined by the convergence criterion (20-16) the software terminates the computations and returns a solution. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. 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. If it does so, use a finer increment in that range. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. Wrong ordering of study steps. GCRO-DR is a method for Krylov subspace recycling. Find detailed information on Office Supplies, Stationery, and Gift Retailers companies in Brea, California, United States of America, including financial statements, sales and marketing contacts, top competitors, and firmographic insights. 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. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. 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. Minimising the environmental effects of my dyson brain. 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. 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. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Why do many companies reject expired SSL certificates as bugs in bug bounties? Consult your product manuals for complete trademark details. COMSOL makes every reasonable effort to verify the information you view on this page. Cooling and Solidification of Metal. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Building on these topics, we will now address how to prepare your mesh for efficiently solving nonlinear finite element problems. Can someone help me fix the COMSOL error below? | ResearchGate You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Use this parameter to modify the nonlinearity expressions in the model. About the Stationary Solver The following background information about the Stationary Solver discusses these topics: Damped Newton Methods, Termination Criterion for the Fully Coupled and Segregated Attribute Nodes, Linear Solvers versus Nonlinear Solvers, and Pseudo Time Stepping. The segregated approach, on the other hand, solves sets of unknowns separately. This solver is automatically used when a Stationary or Frequency Domain study is added to the model. If you still need help with COMSOL and have an on-subscription license, please visit our Support Center for help. rev2023.3.3.43278. Some models are numerically ill-conditioned due to the setup. Not meshing all the domains. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. It may also reveal that the model itself is ill-posed in some way. 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. 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. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. 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. 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. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Any trademarks referenced in this document are the property of their respective owners. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. $131,100.00, Simplified Vehicle Operations Project Engineer, $115,000.00 November 11, 2013. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. Improving Convergence of Nonlinear Stationary Models - Informazioni 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. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Screenshot showing a Solver Configuration that has been altered. 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. 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. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Improving Convergence of Nonlinear Stationary Models, Knowledgebase 1030: Error: "Out of memory", Knowledgebase 1030: Performing a Mesh Refinement Study, Understanding the Fully Coupled vs. Tutti i diritti sono riservati. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model.