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A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. The default Initial Values for the unknowns in most physics interfaces are zero. 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. The default solver for most 3D models is an iterative solver, which is more sensitive to ill-conditioned problems. In a previous blog entry, we introduced meshing considerations for linear static problems. It is sometimes necessary to manually scale the dependent variables. What is the purpose of non-series Shimano components? Note: there is no way to couple this . I am solving a linear stationary finite element model but the software is not solving. 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. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. The coupling terms between the different groups are thus neglected. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. 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. Minimising the environmental effects of my dyson brain. Not assigning proper boundary conditions: Especially if you have ports. Check the solver log to see if the continuation method is backtracking. Your internet explorer is in compatibility mode and may not be displaying the website correctly. If the default iterative solver is not converging, try switching to a direct solver, as described here: Understanding the Fully Coupled vs. They are usually called comp1.u, comp1.v, and comp1.w though. For example, if there is a temperature-dependent material property such as: Check the solver log to see if the continuation method is backtracking. Any trademarks referenced in this document are the property of their respective owners. The algorithm is, generally speaking, a Newton's method approach. Stationary Solver Iterative Direct . The fully coupled and segregated approaches are discussed below. - Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. Not entering required material parameters. If these settings have been manually changed, you will see a small star symbol on the Solution feature, as shown in the screenshot below. Asking for help, clarification, or responding to other answers. Create the time-dependent step or study. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. 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. 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. However, it is usually not possible to know this ahead of time. 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. 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. Few days back i was also facing this problem in . 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. 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. listed if standards is not an option). This is relatively expensive to do, but will lead to the most robust convergence. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. 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. Feature: Stationary Solver 1 (sol1/s1) Adaptation gives the same mesh: adaptmesh1. Not assigning proper boundary conditions: Especially if you have ports. 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. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. She opened prepackaged 6-packs of Seegars to make ", All "stationary stores" results in Brea, CA, Search stationary stores in popular locations. 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. I am following the same model as Comsol provide us on the web. This is relatively expensive to do, but will lead to the most robust convergence. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) Posted 26 set 2019, 17:57 CEST Mesh Version 5.3 0 Replies. "After the incident", I started to be more careful not to trip over things. Solving for laminar flow using Comsol - YouTube Comsol help video number 2: Solving a laminar flow problem in a slit. Some models are numerically ill-conditioned due to the setup. Multiscale Modeling in High-Frequency Electromagnetics. It is also possible to manually refine the mesh. Please dont hesitate to post comments below or send emails to us if you experience any other problems. 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. Stationary Solver Use the Stationary Solver () to find the solution to linear and nonlinear stationary problems (also called static or steady-state problems). 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. Any trademarks referenced in this document are the property of their respective owners. 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. 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! This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Computational Fluid Dynamics (CFD), API The prices are good, and when I needed something delivered, same day, they were able to do it, and it looked beautiful!! The algorithm is, generally speaking, a Newton's method approach. The problem is that when I run my model this message appear: Undefined value found. 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 exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. 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. How can I check before my flight that the cloud separation requirements in VFR flight rules are met? An example would be a linear static structural model of an aluminum beverage can. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). 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. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. Your email address will not be published. Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. To switch between these solver types, go to the Stationary Solver node within the Study sequence. The advantages of the continuation method are two-fold. Consult your product manuals for complete trademark details. The coupling terms between the different groups are thus neglected. Consult your product manuals for complete trademark details. This solver is automatically used when a Stationary or Frequency Domain study is added to the model. COMSOL does not assume any legal liability for the accuracy of the data disclosed. 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. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. 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. Such problems must solved in the time domain. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. 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. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. Different physics have different default solvers. 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 advantages of the continuation method are two-fold. 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. Your internet explorer is in compatibility mode and may not be displaying the website correctly. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: Second, the continuation method will automatically take smaller load increments if a solution cannot be found. Version 5.3 Most multiphysics problems are nonlinear. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. Not meshing all the domains. 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. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. The other low-level default settings within the Stationary Solver are chosen for robustness. 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. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. There will always already be either a Segregated or Fully Coupled feature beneath this. This involves a systematic reduction in the model complexity. November 11, 2013. The other low-level default settings within the Stationary Solver are chosen for robustness. For example, if there is a temperature-dependent material property such as: Changes to these low-level settings from the defaults will usually be quite model- and case-specific. 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. This is relatively expensive to do, but will lead to the most robust convergence. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. See Knowledge Base 1240: Manually Setting the Scaling of Variables. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. They deal with COMSOL package and train users. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. 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. COMSOL 22.9K subscribers Adding a study to your simulation is necessary in order to solve your problem. thanks for reply To subscribe to this RSS feed, copy and paste this URL into your RSS reader. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. How can I use it? It is also possible to manually refine the mesh. It is sometimes necessary to manually scale the dependent variables. Perhaps this approach could be adapted to represent your model. 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. For example, if there is a temperature-dependent material property such as: 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. Sign in to create your job alert for Stationary Engineer jobs in Brea, California, United States. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. 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. This case is generally difficult, or impossible, to solve since this material property is non-smooth. 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. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. The objective here is to simplify the model to a state where the model will solve, with linear approximations. The technique of load ramping is not always reasonable for all problems. Reply . Changes to these low-level settings from the defaults will usually be quite model- and case-specific. 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. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. Reviews on Stationary Stores in Brea, CA - Typo, Mievic, Posh Paper, Morning Glory, Select Graphics & Printing, Daiso Japan, Urban Outfitters, Office Depot, Barnes & Noble - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. 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. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Here, we will examine techniques for accelerating the convergence of these two methods. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. Dr.S.Ravindran Cite 1 Recommendation Popular answers (1). 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. Cooling and Solidification of Metal. It can be useful while solving sequences of linear systems arising from, for example, nonlinear problems. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. Then use this file to provide the initial conditions in time dependent model. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. - the incident has nothing to do with me; can I use this this way? The technique of load ramping is not always reasonable for all problems. If you see this, right-click on the Solution feature and select Reset Solver to Default. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). . That is, they are tuned to achieve convergence in as many cases as possible. P&S Comsol Team: Manuel Kohli, Raphael Schwanninger, Feature: Stationary Solver 1 (sol1/s1) That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. Does ZnSO4 + H2 at high pressure reverses to Zn + H2SO4? 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. Posted 26 set 2019, 11:57 GMT-4 Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. I highly recommend this flower shop.