listed if standards is not an option). Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. 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 Auxiliary Sweep can be used to implement ramping of any Global Parameter. 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. In such cases, see if one material (or the other) can be omitted from the analysis completely. Communication over the phone, in person, and through email was very easy. If you still need help with COMSOL and have an on-subscription license, please visit our Support Center for help. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. Near the top of the Stationary Solver log, the software will report if a linear or nonlinear solver is being used. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. listed if standards is not an option). Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. 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 a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. - Segregated approach and Direct vs. 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. listed if standards is not an option). With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Singular matrix., Make sure you defined your ports correctly:Boundary selectionType of port: e.g. 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. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. Direct PARDISO Solver , PARDISO . The coupling terms between the different groups are thus neglected. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. 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. thanks for reply Discussion Closed This discussion was created more than 6 months ago and has been closed. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. 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. I want to conduct a simulation to find a solution (u) and its first derivative (ux) using a 3D stationary model. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version What did people search for similar to stationary stores in Brea, CA? Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) 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. Required fields are marked *. Linear solvers. Set "Values for dependent variables" in study step settings to User ControlledSolutionYour 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. Not entering required material parameters. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. 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. It may also reveal that the model itself is ill-posed in some way. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. 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. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. 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. The settings controlling the predictor type. See Knowledge Base 1240: Manually Setting the Scaling of Variables. 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. 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. 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. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. Dr.S.Ravindran Cite 1 Recommendation Popular answers (1). Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. With respect to any nonlinearities, replace them by a reasonable linearized term. It is also possible to manually refine the mesh. Your internet explorer is in compatibility mode and may not be displaying the website correctly. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. If these settings have been manually changed, you will see a small star symbol on the Solution feature, as shown in the screenshot below. First, it is physically intuitive, often matching how one would perform an experiment. 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. - Function: ^ Failed to evaluate variable. 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. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. Thanks for contributing an answer to Stack Overflow! 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. This involves a systematic reduction in the model complexity. Discussion Closed This discussion was created more than 6 months ago and has been closed. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. Not meshing all the domains. To switch between these solver types, go to the Stationary Solver node within the Study sequence. Full text of the 'Sri Mahalakshmi Dhyanam & Stotram'. - Feature: Stationary Solver 1 (sol1/s1)Failed to find a solution. The settings controlling the predictor type. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. 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. 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. 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. Learn how your comment data is processed. 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. That is: Even if the forces on a part are opposite and equal, this is not sufficient information to say where the part is, so you must add some other condition, such as as Fixed Constraint to constrain displacement. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. Wish you all the best. Name: actdep_int1, Your email address will not be published. Alternatively, delete and re-create the study. 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. Have you taken a look at this [blog post](https://www.comsol.com/blogs/modeling-fluid-structure-interaction-in-multibody-mechanisms/)? That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Not assigning proper boundary conditions: Especially if you have ports. Once the error estimate is small enough, as determined by the convergence criterion (20-16) the software terminates the computations and returns a solution. Stationary in the COMSOL Multiphysics Programming Reference Manual Damped Newton Methods The nonlinear solver uses an affine invariant form of the damped Newton method as described in Ref. If it does so, use a finer increment in that range. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Any trademarks referenced in this document are the property of their respective owners. Improving Convergence of Nonlinear Stationary Models, Knowledgebase 1030: Error: "Out of memory", Knowledgebase 1030: Performing a Mesh Refinement Study, 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. 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. 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. We use COMSOL Multiphysics for solving distributed optimal control of un-steady Burgers equation without constraints and with pointwise control constraints. The segregated approach, on the other hand, solves sets of unknowns separately. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) In this posting, we introduce the idea of ramping the nonlinearities in the problem to improve convergence. 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. It is sometimes necessary to manually scale the dependent variables. These are some cards & stationery with a large number of reviews in Brea, CA. 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. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. Repeat this for every nonlinearity of the model. (I am begginer in comsol) Thank you. Not meshing all the domains. The difference between the phonemes /p/ and /b/ in Japanese. 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. With respect to any nonlinearities, replace them by a reasonable linearized term. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Solver . Does ZnSO4 + H2 at high pressure reverses to Zn + H2SO4? Note: there is no way to couple this . Leverage your professional network, and get hired. See Knowledge Base 1240: Manually Setting the Scaling of Variables. The advantages of the continuation method are two-fold. 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 Auxiliary Sweep can be used to implement ramping of any Global Parameter. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. Why do many companies reject expired SSL certificates as bugs in bug bounties? "After the incident", I started to be more careful not to trip over things. 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! With respect to any nonlinearities, replace them by a reasonable linearized term.
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