listed if standards is not an option). Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. The Fully Coupled solution approach, with the Plot While Solving enabled. Any trademarks referenced in this document are the property of their respective owners. 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. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) The settings controlling the predictor type. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. 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. In this page, we will be sharing some common errors that might occur in Comsol and their solutions. Repeat this for every nonlinearity of the model. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. To switch between these solver types, go to the Stationary Solver node within the Study sequence. Multiphysics problems are often nonlinear. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: 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. 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. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. 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. Multiphysics problems are often nonlinear. listed if standards is not an option). 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. Review the model setup to resolve these. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. Sign in to create your job alert for Stationary Engineer jobs in Brea, California, United States. GCRO-DR is a method for Krylov subspace recycling. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. The advantages of the continuation method are two-fold. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. Such a case would be better to address instead with the Shell physics interface, which is specially formulated for handling thin-walled structural parts. Use this parameter to modify the nonlinearity expressions in the model. How can I use it? If you do not hold an on-subscription license, you may find an answer in another Discussion or in the Knowledge Base. The default Initial Values for the unknowns in most physics interfaces are zero. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. Tutti i diritti sono riservati. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. Linear solvers. -Detail: NaN or Inf found when solving linear system using SOR. If the model is nonlinear, see: Improving Convergence of Nonlinear Stationary Models. If the default iterative solver is not converging, try switching to a direct solver, as described here: Understanding the Fully Coupled vs. Cooling and Solidification of Metal. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: Set initial conditions in the physics to the appropriate dependent model variable names rather than the default 0. 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. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. I personally liked emailing them the file, ", "This flower shop is the best! Alle Rechte vorbehalten. What are people saying about cards & stationery in Brea, CA? Save my name, email, and website in this browser for the next time I comment. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. Note the star symbol on the Solution feature. Studysteps might be listed in wrong order: Not assigning materials to all the domains. This involves a systematic reduction in the model complexity. Direct PARDISO Solver , PARDISO . Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. Posted Sep 9, 2020, 1:44 p.m. EDT The other low-level default settings within the Stationary Solver are chosen for robustness. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. Near the top of the Stationary Solver log, the software will report if a linear or nonlinear solver is being used. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. Building on these topics, we will now address how to prepare your mesh for efficiently solving nonlinear finite element problems. Multiscale Modeling in High-Frequency Electromagnetics. Your email address will not be published. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. The prices are good, and when I needed something delivered, same day, they were able to do it, and it looked beautiful!! Numerically ill-conditioned means that the system matrix is nearly singular and that it will be difficult to solve on a finite-precision computer. COMSOL does not assume any legal liability for the accuracy of the data disclosed. If you are unsure if your problem is linear and stationary, look at the Log. 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. Comsol help video number 2: Solving a laminar flow problem in a slit.. In that case, the continuation method will automatically backtrack and try to solve for intermediate values in the range of 0.6 through 0.8. The stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. P&S Comsol Team: Manuel Kohli, Raphael Schwanninger, Feature: Stationary Solver 1 (sol1/s1) The fully coupled and segregated approaches are discussed below. Alternatively, delete and re-create the study. Your Discussion has gone 30 days without a reply. With respect to any nonlinearities, replace them by a reasonable linearized term. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Hello, Segregated approach and Direct vs. First, it is physically intuitive, often matching how one would perform an experiment. 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. The technique of load ramping is not always reasonable for all problems. Such problems must solved in the time domain. 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. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: Stationary (time-invariant) models with nonlinearities may converge very slowly. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. 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. 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. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. 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. November 11, 2013. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. 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. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). With respect to any nonlinearities, replace them by a reasonable linearized term. Tutti i diritti sono riservati. Again, introduce a Global Parameter that gets ramped from exactly zero to one. rev2023.3.3.43278. listed if standards is not an option). Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. 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. Solve the stationary study then the time dependent study. Required fields are marked *. Segregated approach and Direct vs. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. Screenshot showing a Solver Configuration that has been altered. What is the purpose of this D-shaped ring at the base of the tongue on my hiking boots? That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Again, introduce a Global Parameter that gets ramped from exactly zero to one. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. 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. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. Save this information to a file. 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. Cooling and Solidification of Metal. Asking for help, clarification, or responding to other answers. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: Again, introduce a Global Parameter that gets ramped from exactly zero to one. For example, if there is a temperature-dependent material property such as: 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. In such cases, see if one material (or the other) can be omitted from the analysis completely. Does anyone know what should cause this problem? Stationary (time-invariant) models with nonlinearities may converge very slowly. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. We are planning to continuously update this page throughout the semester and hopefully, this will become a reference during your projects as well. To start a new discussion with a link back to this one, click here. This algorithm was also useful for understanding what happens near a failure load. I highly recommend this flower shop. That is, they are tuned to achieve convergence in as many cases as possible. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). Is there a way to use the stationary solution obtained in Comsol 4.2 as the initial conditions in a time dependent model? What is the purpose of non-series Shimano components? - We use COMSOL Multiphysics for solving distributed optimal control of un-steady Burgers equation without constraints and with pointwise control constraints. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. In that case, the continuation method will automatically backtrack and try to solve for intermediate values in the range of 0.6 through 0.8. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. 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. This case is generally difficult, or impossible, to solve since this material property is non-smooth. 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 did people search for similar to stationary stores in Brea, CA? It may also reveal that the model itself is ill-posed in some way. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. 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. Is it possible to rotate a window 90 degrees if it has the same length and width? 3 Replies, Please login with a confirmed email address before reporting spam. See Knowledge Base 1240: Manually Setting the Scaling of Variables. This guide applies solely to nonlinear stationary models. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). Iterative , Direct . Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. 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. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. It is also possible to manually refine the mesh. 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 technique of load ramping is not always reasonable for all problems. Direct Iterative , Direct . 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. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. Stationary (time-invariant) models with nonlinearities may converge very slowly. If the material properties entered are incorrect for the governing equation, the model will generate an error at runtime, usually a Singular Matrix error. See if these can be used instead of explicitly modeling parts with high-aspect ratio geometries. Name: actdep_int1, Your email address will not be published. P&S: COMSOL Design Tool for Photonic Devices. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. They deal with COMSOL package and train users. I am solving a linear stationary finite element model but the software is not solving. 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. Feature: Stationary Solver 1 (sol1/s1) Adaptation gives the same mesh: adaptmesh1. 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. 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. If it does so, use a finer increment in that range. Solver . 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. There will always already be either a Segregated or Fully Coupled feature beneath this. Acidity of alcohols and basicity of amines. This approach is known as a Continuation Method with a Constant predictor. 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. Stationary Engineer Jobs in Brea, California, United States, $87,400.00 What are some highly rated cards & stationery in Brea, CA? - Trying to understand how to get this basic Fourier Series. 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. Not entering required material parameters. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. Feature: Stationary Solver 1 (sol1/s1) Attempt to evaluate nonintegral power of negative number. 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. 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. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. replace it with the expression: This is relatively expensive to do, but will lead to the most robust convergence. comp1.u2, comp1.v2, and comp1.w2 are usually variables associated with the x,y, and z component of deformation in COMSOL. In the COMSOL Multiphysics software, this step of the modeling workflow is made. The coupling terms between the different groups are thus neglected. Find detailed information on Office Supplies, Stationery, and Gift Retailers companies in Brea, California, United States of America, including financial statements, sales and marketing contacts, top competitors, and firmographic insights. The other low-level default settings within the Stationary Solver are chosen for robustness. 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. Discussion Closed This discussion was created more than 6 months ago and has been closed. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) Consult your product manuals for complete trademark details. 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. 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. Have you taken a look at this [blog post](https://www.comsol.com/blogs/modeling-fluid-structure-interaction-in-multibody-mechanisms/)? 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. 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. 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. 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. The advantages of the continuation method are two-fold. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Not assigning proper boundary conditions: Especially if you have ports. What version of COMSOL are you using? 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. This case is generally difficult, or impossible, to solve since this material property is non-smooth. 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. Perhaps this approach could be adapted to represent your model. One of the key concepts there was the idea of mesh convergence as you refine the mesh, the solution will become more accurate. Posted 26 set 2019, 11:57 GMT-4 In many physics areas there exist alternative physics formulations specifically meant for solving cases where the geometry has an extreme aspect ratio.
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