What sort of strategies would a medieval military use against a fantasy giant? It is sometimes necessary to manually scale the dependent variables. - Feature: Stationary Solver 1 (sol1/s1)Failed to find a solution. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. SGP handled 7 different prints for me at once and they all came out perfectly, in a timely manner. 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. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. 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. That is, start by first solving a model with a small, but non-zero, load. Multiphysics problems are often nonlinear. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. If so, see: Knowledgebase 1030: Error: "Out of memory". If you try to solve such a model, there will be an error at runtime that will provide information about the variable name and where it is being called. Hence Comsol solved for the stationary solution at different points of time. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. 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. Could you expand a little bit more why the coupling is impossible? 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. 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. - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Common Mistakes: Not assigning materials to all the domains. 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. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. - the incident has nothing to do with me; can I use this this way? Repeat this for every nonlinearity of the model. What are people saying about cards & stationery in Brea, CA? Perhaps this approach could be adapted to represent your model. 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. Reviews on Stationary Stores in Brea, CA - Typo, Mievic, Posh Paper, Morning Glory, Select Graphics & Printing, Daiso Japan, Urban Outfitters, Office Depot, Barnes & Noble Alternatively, delete and re-create the study. Your internet explorer is in compatibility mode and may not be displaying the website correctly. 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. The default solver for most 3D models is an iterative solver, which is more sensitive to ill-conditioned problems. The segregated approach, on the other hand, solves sets of unknowns separately. Multiscale Modeling in High-Frequency Electromagnetics. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. The prices are good, and when I needed something delivered, same day, they were able to do it, and it looked beautiful!! See Knowledge Base 1240: Manually Setting the Scaling of Variables. If the model is nonlinear, see: Improving Convergence of Nonlinear Stationary Models. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. I use comsol multiphysics 5.2a and . There are also cases when an extremely poor quality mesh leads to an ill-conditioned problem, This issue often arises in combination with, and as a consequence of, geometries that have extreme aspect ratios. Near the top of the Stationary Solver log, the software will report if a linear or nonlinear solver is being used. Use this parameter to modify the nonlinearity expressions in the model. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) See if these can be used instead of explicitly modeling parts with high-aspect ratio geometries. 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. With respect to any nonlinearities, replace them by a reasonable linearized term. 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 parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. 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. 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. 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. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. COMSOL does not assume any legal liability for the accuracy of the data disclosed. I am solving a linear stationary finite element model but the software is not solving. Your Discussion has gone 30 days without a reply. In this page, we will be sharing some common errors that might occur in Comsol and their solutions. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. Can I tell police to wait and call a lawyer when served with a search warrant? Why is there a voltage on my HDMI and coaxial cables? Direct PARDISO Solver , PARDISO . They worked with me. 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. Trying to understand how to get this basic Fourier Series. 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. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. 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. This case is generally difficult, or impossible, to solve since this material property is non-smooth. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Here we introduce a more robust approach to solving nonlinear 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. 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. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. 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. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. For example, if there is a temperature-dependent material property such as: This guide applies solely to nonlinear stationary models. - Again, introduce a Global Parameter that gets ramped from exactly zero to one. If it does so, use a finer increment in that range. 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. November 11, 2013. An example would be a linear static structural model of an aluminum beverage can. Solving such models in a stationary sense should simply require solving a single (large) system of linear equations and should always be solvable, but there are cases when the software will fail to find a solution. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. Reply . Such a case would be better to address instead with the Shell physics interface, which is specially formulated for handling thin-walled structural parts. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. How do/should administrators estimate the cost of producing an online introductory mathematics class? 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. Repeat this for every nonlinearity of the model. 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. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. Popular answers (1) This problem generally occurs when there is some mistake in the physics or study section or wrong selection of the mesh size. For example, in a Solid Mechanics (wherein the software is solving for the displacement field within the solid) applying two opposite and equal Boundary Load conditions on a part is not sufficient to define the displacement. New Stationary Engineer jobs added daily. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. Any trademarks referenced in this document are the property of their respective owners. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. Most multiphysics problems are nonlinear. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Segregated approach and Direct vs. Discussion Closed This discussion was created more than 6 months ago and has been closed. It can be useful while solving sequences of linear systems arising from, for example, nonlinear problems. Making statements based on opinion; back them up with references or personal experience. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. This is relatively expensive to do, but will lead to the most robust convergence. Hi Alexis, In the COMSOL Multiphysics software, this step of the modeling workflow is made. With respect to any nonlinearities, replace them by a reasonable linearized term. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. However, load ramping will not work in all cases, or may be inefficient. The coupling terms between the different groups are thus neglected. Again, introduce a Global Parameter that gets ramped from exactly zero to one. Use this parameter to modify the nonlinearity expressions in the model. However, if a lower-level change has been manually made to the solver settings, the software will not automatically use the correct default solver. 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.
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