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With respect to any nonlinearities, replace them by a reasonable linearized term. 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. If so, see: Knowledgebase 1030: Error: "Out of memory". In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. 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. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. Segregated approach and Direct vs. Iterative linear solvers, About the time step setting of the solver, Introducing Goal Seeking into the Segregated Solver. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. This information is presented in the context of a very simple 1D finite element problem, and builds upon our previous entry on Solving Linear Static Finite Element Models. Wrong ordering of study steps. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). Your internet explorer is in compatibility mode and may not be displaying the website correctly. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. Communication over the phone, in person, and through email was very easy. COMSOL does not assume any legal liability for the accuracy of the data disclosed. Today's top 351 Stationary Engineer jobs in Brea, California, United States. 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. Reply . This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. Is there a way to use the stationary solution obtained in Comsol 4.2 as the initial conditions in a time dependent model? Such a case would be better to address instead with the Shell physics interface, which is specially formulated for handling thin-walled structural parts. Required fields are marked *. 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. Posted 26 set 2019, 11:57 GMT-4 Your internet explorer is in compatibility mode and may not be displaying the website correctly. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. If you see this, right-click on the Solution feature and select Reset Solver to Default. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: See if these can be used instead of explicitly modeling parts with high-aspect ratio geometries. How can I use it? 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. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version COMSOL makes every reasonable effort to verify the information you view on this page. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. The objective here is to simplify the model to a state where the model will solve, with linear approximations. Set initial conditions in the physics to the appropriate dependent model variable names rather than the default 0. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Consult your product manuals for complete trademark details. This approach is known as a Continuation Method with a Constant predictor. They are usually called comp1.u, comp1.v, and comp1.w though. 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. 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. They worked with me. "After the incident", I started to be more careful not to trip over things. Please dont hesitate to post comments below or send emails to us if you experience any other problems. 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. 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. 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. 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. COMSOL makes every reasonable effort to verify the information you view on this page. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. 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. Then use this file to provide the initial conditions in time dependent model. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the 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. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. Not assigning proper boundary conditions: Especially if you have ports. The fully coupled and segregated approaches are discussed below. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. 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. If the material properties entered are incorrect for the governing equation, the model will generate an error at runtime, usually a Singular Matrix error. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. In a previous blog entry, we introduced meshing considerations for linear static problems. Use this parameter to modify the nonlinearity expressions in the model. 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. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. Common Mistakes: Not assigning materials to all the domains. 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. The segregated approach, on the other hand, solves sets of unknowns separately. 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 algorithm is, generally speaking, a Newton's method approach. $131,100.00, Simplified Vehicle Operations Project Engineer, $115,000.00 In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. Version 5.3 From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. Note: there is no way to couple this field with the time dependent nature of this physics. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. 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. The other low-level default settings within the Stationary Solver are chosen for robustness. 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. 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. Then I stored the resulting solution, and used it to to solve the stationary solver again by using the solver manager. Check the solver log to see if the continuation method is backtracking. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. The settings controlling the predictor type. 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. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. To start a new discussion with a link back to this one, click here. 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. The coupling terms between the different groups are thus neglected. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) In this posting, we introduce the idea of ramping the nonlinearities in the problem to improve convergence. 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. What version of COMSOL are you using? Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. There will also be a red cross over the Materials branch icon. Can I tell police to wait and call a lawyer when served with a search warrant? I am following the same model as Comsol provide us on the web. 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. This involves a systematic reduction in the model complexity. What sort of strategies would a medieval military use against a fantasy giant? Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. 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. The settings controlling the predictor type. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. Function: / Failed to evaluate expression. Use this parameter to modify the nonlinearity expressions in the model. That is, they are tuned to achieve convergence in as many cases as possible. 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. 3. Mesh COMSOL makes every reasonable effort to verify the information you view on this page. Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. It may also reveal that the model itself is ill-posed in some way. Does anyone know what should cause this problem? Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. For example, if there is a temperature-dependent material property such as: - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. What did people search for similar to stationary stores in Brea, CA? One of the key concepts there was the idea of mesh convergence as you refine the mesh, the solution will become more accurate. Some models are numerically ill-conditioned due to the setup. listed if standards is not an option). Segregated approach and Direct vs. We are planning to continuously update this page throughout the semester and hopefully, this will become a reference during your projects as well. 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. $130,000.00. Alternatively, delete and re-create the study. This consent may be withdrawn. This approach is known as a Continuation Method with a Constant predictor. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. 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. 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. (Frequency Domain should be the last step). 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. Here, we will examine techniques for accelerating the convergence of these two methods. 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. Most multiphysics problems are nonlinear. 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. . thanks for reply - Posted Sep 9, 2020, 1:44 p.m. EDT The algorithm is, generally speaking, a Newton's method approach. How can I check before my flight that the cloud separation requirements in VFR flight rules are met? Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. Name: actdep_int1, Your email address will not be published. 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. I personally liked emailing them the file, ", "This flower shop is the best! The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. Connect and share knowledge within a single location that is structured and easy to search.