Use this parameter to modify the nonlinearity expressions in the model. 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. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) In this case, it would likely be reasonable to treat the insulative material as a perfect insulator, omit it from the analysis, and use the Electric Insulation boundary condition instead of modeling those domains. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. 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 is relatively expensive to do, but will lead to the most robust convergence. Comsol help video number 2: Solving a laminar flow problem in a slit.. Such problems must solved in the time domain. Unknown function or operator. Most multiphysics problems are nonlinear. 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. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. 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. Your Discussion has gone 30 days without a reply. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Any trademarks referenced in this document are the property of their respective owners. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Screenshot showing a Solver Configuration that has been altered. Please dont hesitate to post comments below or send emails to us if you experience any other problems. 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. 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. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. This approach is known as a Continuation Method with a Constant predictor. What are people saying about cards & stationery in Brea, CA? 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. COMSOL makes every reasonable effort to verify the information you view on this page. 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. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Hi Alexis,
Common Mistakes: Not assigning materials to all the domains. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. 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. numeric (each ports needs their ownboundary mode analysis in the study if they are numerically defined)Wave excitation: on/off(input/output), - Feature: Stationary Solver 1 (sol1/s1) Division by zero. (Frequency Domain should be the last step). By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. Multiphysics problems are often nonlinear. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. One of the key concepts there was the idea of mesh convergence as you refine the mesh, the solution will become more accurate. This involves a systematic reduction in the model complexity. 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. 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 coupling terms between the different groups are thus neglected. The Fully Coupled solution approach, with the Plot While Solving enabled. 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. First, it is physically intuitive, often matching how one would perform an experiment. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. 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. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Again, introduce a Global Parameter that gets ramped from exactly zero to one. Wrong ordering of study steps. In our previous blog entry, we introduced the Fully Coupled and the Segregated algorithms used for solving steady-state multiphysics problems in COMSOL. See Knowledge Base 1240: Manually Setting the Scaling of Variables. Alternatively, delete and re-create the study. 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. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: Get notified about new Stationary Engineer jobs in Brea, California, United States. Singular matrix., Make sure you defined your ports correctly:Boundary selectionType of port: e.g. Not assigning proper boundary conditions: Especially if you have ports. Could you expand a little bit more why the coupling is impossible? Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. The Fully Coupled solution approach, with the Plot While Solving enabled. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Near the top of the Stationary Solver log, the software will report if a linear or nonlinear solver is being used. 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. 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. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. Consult your product manuals for complete trademark details. Create the time-dependent step or study. listed if standards is not an option). An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: Could you expand a little bit more why the coupling is impossible? Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. Solver . What is the purpose of this D-shaped ring at the base of the tongue on my hiking boots? If the default iterative solver is not converging, try switching to a direct solver, as described here: Understanding the Fully Coupled vs. - the incident has nothing to do with me; can I use this this way? The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. - Feature: Stationary Solver 1 (sol1/s1) The objective here is to simplify the model to a state where the model will solve, with linear approximations. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. Cooling and Solidification of Metal. 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. With respect to any nonlinearities, replace them by a reasonable linearized term. 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. See Knowledge Base 1240: Manually Setting the Scaling of Variables. P&S: COMSOL Design Tool for Photonic Devices. 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. Cooling and Solidification of Metal. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. 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. 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. 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. This algorithm was also useful for understanding what happens near a failure load. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. 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. Not meshing all the domains. There will always already be either a Segregated or Fully Coupled feature beneath this. The Fully Coupled solution approach, with the Plot While Solving enabled. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: In many physics areas there exist alternative physics formulations specifically meant for solving cases where the geometry has an extreme aspect ratio. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. 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. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. As a rough rule of thumb, once the aspect ratio between the largest characteristic dimension to the smallest approaches 100:1, you might start to run into issues and should look to alternative ways of posing the problem, especially in a 3D model. Here we introduce a more robust approach to solving nonlinear problems. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. listed if standards is not an option). This doesn't seem to me the most elegant of methods, since I am essentially solving a stationary solution using a time dependent 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. Stationary (time-invariant) models with nonlinearities may converge very slowly. Any trademarks referenced in this document are the property of their respective owners. Minimising the environmental effects of my dyson brain. Does anyone know what should cause this problem? Asking for help, clarification, or responding to other answers. $140,000.00, $120,000.00 The settings controlling the predictor type. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Not entering required material parameters. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Such a large difference in the materials properties can be challenging. I personally liked emailing them the file, ", "This flower shop is the best! Such problems must solved in the time domain. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. Why is there a voltage on my HDMI and coaxial cables? The Auxiliary Sweep can be used to implement ramping of any Global Parameter. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. 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. 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. This involves a systematic reduction in the model complexity. Wrong ordering of study steps. (Frequency Domain should be the last step) I use comsol multiphysics 5.2a and . 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. In the COMSOL Multiphysics software, this step of the modeling workflow is made. Sign in to create your job alert for Stationary Engineer jobs in Brea, California, United States. 3. 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. 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. 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. Few days back i was also facing this problem in . Any trademarks referenced in this document are the property of their respective owners. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. "After the incident", I started to be more careful not to trip over things. 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. What version of COMSOL are you using? The issue here has do with the iterative algorithm used to solve nonlinear stationary models. Linear solvers. 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. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. This is a review for cards & stationery in Brea, CA: "Love this store!!! It is also possible to manually refine the mesh. Not assigning proper boundary conditions: Especially if you have ports. They are usually called comp1.u, comp1.v, and comp1.w though. Connect and share knowledge within a single location that is structured and easy to search. A linear finite element model is one in which all of the material properties, loads, boundary conditions, etc are constant with respect to the solution, and the governing partial differential equations are themselves linear. The technique of load ramping is not always reasonable for all problems. 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. To start a new discussion with a link back to this one, click here. New Stationary Engineer jobs added daily. However, it is usually not possible to know this ahead of time. However, it is usually not possible to know this ahead of time. 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. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Communication over the phone, in person, and through email was very easy. In a previous blog entry, we introduced meshing considerations for linear static problems. Again, introduce a Global Parameter that gets ramped from exactly zero to one. You can write the discrete form of the equations as f(U) = 0, where f(U) is the residual vector and U is the solution vector. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. Stationary Engineer Jobs in Brea, California, United States, $87,400.00 If it does so, use a finer increment in that range. 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. 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. Tutti i diritti sono riservati. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. The objective here is to simplify the model to a state where the model will solve, with linear approximations. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Here, we will examine techniques for accelerating the convergence of these two methods. rev2023.3.3.43278. 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.
It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. Note: there is no way to couple this field with the time dependent nature of this physics. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. 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. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. Perhaps this approach could be adapted to represent your model. 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. Feature: Stationary Solver 1 (sol1/s1)" . 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. By creating this job alert, you agree to the LinkedIn User Agreement and Privacy Policy. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. Your internet explorer is in compatibility mode and may not be displaying the website correctly. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. "I chose this print shop based off yelp reviews and was very happy with the outcome! It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. 0 Replies, Please login with a confirmed email address before reporting spam. Not entering required material parameters. Set initial conditions in the physics to the appropriate dependent model variable names rather than the default 0. 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. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. Segregated approach and Direct vs. 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, Galleria dei Modelli e delle App di Simulazione, 2023 da COMSOL. 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. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. The segregated approach, on the other hand, solves sets of unknowns separately. The default Initial Values for the unknowns in most physics interfaces are zero. Direct Iterative , Direct . Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. $131,100.00, Simplified Vehicle Operations Project Engineer, $115,000.00 Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues.
The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. This involves a systematic reduction in the model complexity. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). Leverage your professional network, and get hired. Solve the stationary study then the time dependent study. 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. For example, in an Electric Currents problem, you may want to consider a system of materials including a good conductor such as copper (with an electric conductivity of ~6e7 S/m) and an insulative material such as glass (which can have electric conductivity of ~1e-14 S/m.) Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Check the solver log to see if the continuation method is backtracking. This approach is known as a Continuation Method with a Constant predictor. Stationary Solver Iterative Direct . That is, they are tuned to achieve convergence in as many cases as possible. 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. 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. Consult your product manuals for complete trademark details. Making statements based on opinion; back them up with references or personal experience. . That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. Near the top of the Stationary Solver log, the software will report if a linear or nonlinear solver is being used. 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. 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. - Get email updates for new Stationary Engineer jobs in Brea, California, United States. (I am begginer in comsol) Thank you. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. 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. 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. Check the solver log to see if the continuation method is backtracking. Thanks for contributing an answer to Stack Overflow! You can unsubscribe from these emails at any time. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. Improving Convergence of Nonlinear Stationary Models, Knowledgebase 1030: Error: "Out of memory", Knowledgebase 1030: Performing a Mesh Refinement Study, Understanding the Fully Coupled vs. The technique of load ramping is not always reasonable for all problems. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. There will also be a red cross over the Materials branch icon. 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. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. The other low-level default settings within the Stationary Solver are chosen for robustness. . The fully coupled and segregated approaches are discussed below. Use this parameter to modify the nonlinearity expressions in the model. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. What is \newluafunction? That is, start by first solving a model with a small, but non-zero, load. Not the answer you're looking for? Cooling and Solidification of Metal. 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 the material properties entered are incorrect for the governing equation, the model will generate an error at runtime, usually a Singular Matrix error.