- The other low-level default settings within the Stationary Solver are chosen for robustness. Reviews on Stationary Stores in Brea, CA - Typo, Mievic, Posh Paper, Morning Glory, Select Graphics & Printing, Daiso Japan, Urban Outfitters, Office Depot, Barnes & Noble - Function: ^ Failed to evaluate variable. To start a new discussion with a link back to this one, click here. I want to conduct a simulation to find a solution (u) and its first derivative (ux) using a 3D stationary 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. Solve the stationary study then the time dependent study. 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. (Frequency Domain should be the last step). With respect to any nonlinearities, replace them by a reasonable linearized term. 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. I am following the same model as Comsol provide us on the web. If you try to enter a variable that is undefined, the automatic syntax highlighting will identify this as you enter it, and it is best practice to resolve this immediately. Studysteps might be listed in wrong order: Not assigning materials to all the domains. The default Initial Values for the unknowns in most physics interfaces are zero. If the model is very large, and if you do not have very much memory in your computer, you may get an error message regarding memory. This consent may be withdrawn. 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. Segregated approach and Direct vs. 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. The settings controlling the predictor type. listed if standards is not an option). The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. If the default iterative solver is not converging, try switching to a direct solver, as described here: Understanding the Fully Coupled vs. This approach is known as a Continuation Method with a Constant predictor. 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. Is there a way to use the stationary solution obtained in Comsol 4.2 as the initial conditions in a time dependent model? The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. 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. P&S Comsol Team: Yannik Horst, Manuel Kohli, Xinzhi Zhang. 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. (Frequency Domain should be the last step) Again, introduce a Global Parameter that gets ramped from exactly zero to one. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. 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. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. Once the error estimate is small enough, as determined by the convergence criterion (20-16) the software terminates the computations and returns a solution. 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. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. Why do many companies reject expired SSL certificates as bugs in bug bounties? - the incident has nothing to do with me; can I use this this way? 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. Such a case would be better to address instead with the Shell physics interface, which is specially formulated for handling thin-walled structural parts. I am trying to solve the coupling between a waveguide and a microring resonator. The nature of simulating nature: A Q&A with IBM Quantum researcher Dr. Jamie We've added a "Necessary cookies only" option to the cookie consent popup. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Create the time-dependent step or study. It is sometimes necessary to manually scale the dependent variables. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. The algorithm is, generally speaking, a Newton's method approach. Not the answer you're looking for? That is, start by first solving a model with a small, but non-zero, load. 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. Using the first order optimality. 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. GCRO-DR is a method for Krylov subspace recycling. COMSOL makes every reasonable effort to verify the information you view on this page. My comment is perhaps a bit nave but it seems to me that you could simply deactivate the \frac{\partial \cdot}{\partial t} term of the background field equation but keep its connexion to the solid to get what you want. I use comsol multiphysics 5.2a and . An example would be a linear static structural model of an aluminum beverage can. 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 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. If it does so, use a finer increment in that range. If you have both as steps in the same study, then solve that study. 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 default solver for most 3D models is an iterative solver, which is more sensitive to ill-conditioned problems. Not assigning proper boundary conditions: Especially if you have ports. P&S Comsol Team: Manuel Kohli, Raphael Schwanninger, Feature: Stationary Solver 1 (sol1/s1) The segregated approach, on the other hand, solves sets of unknowns separately. 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. The objective here is to simplify the model to a state where the model will solve, with linear approximations. 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. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. Trying to understand how to get this basic Fourier Series. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. 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! Reply . Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. Numerically ill-conditioned means that the system matrix is nearly singular and that it will be difficult to solve on a finite-precision computer. $140,000.00, $120,000.00 Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. Understanding the Fully Coupled vs. It is sometimes necessary to manually scale the dependent variables. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. thanks for reply This doesn't seem to me the most elegant of methods, since I am essentially solving a stationary solution using a time dependent Multiphysics problems are often nonlinear. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. 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. However, it is usually not possible to know this ahead of time. 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. Use this parameter to modify the nonlinearity expressions in the model. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. 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. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. Again, introduce a Global Parameter that gets ramped from exactly zero to one. - It is sometimes necessary to manually scale the dependent variables. In a previous blog entry, we introduced meshing considerations for linear static problems. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. It can be useful while solving sequences of linear systems arising from, for example, nonlinear problems. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Cooling and Solidification of Metal. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. 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. listed if standards is not an option). What sort of strategies would a medieval military use against a fantasy giant? The solver settings are stored at Study > Solver Configurations > Solution. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. For example, if there is a temperature-dependent material property such as: What are people saying about cards & stationery in Brea, CA? "I chose this print shop based off yelp reviews and was very happy with the outcome! $131,100.00, Simplified Vehicle Operations Project Engineer, $115,000.00 A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. Dr.S.Ravindran Cite 1 Recommendation Popular answers (1). Stationary (time-invariant) models with nonlinearities may converge very slowly. Cooling and Solidification of Metal. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. The segregated approach, on the other hand, solves sets of unknowns separately. See Knowledge Base 1240: Manually Setting the Scaling of Variables. This approach is known as a Continuation Method with a Constant predictor. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. Could you expand a little bit more why the coupling is impossible? This guide applies solely to nonlinear stationary models. 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. COMSOL 22.9K subscribers Adding a study to your simulation is necessary in order to solve your problem. replace it with the expression: Reviews on Stationary in Brea, CA - Mievic, Typo, Tokyo Japanese Lifestyle, Fancy, Posh Paper, Morning Glory, Velvet Fox Designs, Barnes & Noble, Michaels, Select Graphics & Printing Any trademarks referenced in this document are the property of their respective owners. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) Not meshing all the domains. This is relatively expensive to do, but will lead to the most robust convergence. Connect and share knowledge within a single location that is structured and easy to search. Segregated approach and Direct vs. Iterative linear solvers, Time dependent function and stationary study, Combining Adaptive Mesh Refinement with Data Filtering, What to do when a linear stationary model is not solving, Galleria dei Modelli e delle App di Simulazione, 2023 da COMSOL. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. If it does so, use a finer increment in that range. Perhaps this approach could be adapted to represent your model. 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. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. The problem is that when I run my model this message appear: Undefined value found. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. Common Mistakes: Not assigning materials to all the domains. That is, they are tuned to achieve convergence in as many cases as possible. Making statements based on opinion; back them up with references or personal experience. The Fully Coupled solution approach, with the Plot While Solving enabled. 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. Asking for help, clarification, or responding to other answers. If you do not hold an on-subscription license, you may find an answer in another Discussion or in the Knowledge Base. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. Such a large difference in the materials properties can be challenging. (Frequency Domain should be the last step) COMSOL makes every reasonable effort to verify the information you view on this page. Repeat this for every nonlinearity of the model. Not assigning proper boundary conditions: Especially if you have ports. For example, if there is a temperature-dependent material property such as: That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. P&S Comsol Team: Arif Gngr , Yannik Horst , Stefano Valente. 3. 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. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. 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. Please dont hesitate to post comments below or send emails to us if you experience any other problems. 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. First, it is physically intuitive, often matching how one would perform an experiment. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. In this page, we will be sharing some common errors that might occur in Comsol and their solutions. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. Here we introduce a more robust approach to solving nonlinear problems. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. She opened prepackaged 6-packs of Seegars to make ", All "stationary stores" results in Brea, CA, Search stationary stores in popular locations. "After the incident", I started to be more careful not to trip over things. The technique of load ramping is not always reasonable for all problems. 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. 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. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Posted 26 set 2019, 11:57 GMT-4 Also, keep in mind that a linear stationary model should solve regardless of how coarse the mesh is (albeit to a low accuracy) so you can always start with as coarse a mesh as possible, and refine the mesh (See also: Knowledgebase 1030: Performing a Mesh Refinement Study. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. Consult your product manuals for complete trademark details. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. Segregated approach and Direct vs. Click the link in the email we sent to to verify your email address and activate your job alert. The issue here has do with the iterative algorithm used to solve nonlinear stationary models.
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