COMSOL does not assume any legal liability for the accuracy of the data disclosed. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. Linear solvers. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. With respect to any nonlinearities, replace them by a reasonable linearized term. There will always already be either a Segregated or Fully Coupled feature beneath this. With respect to any nonlinearities, replace them by a reasonable linearized term. When you use an iterative solver, COMSOL Multiphysics estimates the error of the solution while solving. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) 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. 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. An example would be a linear static structural model of an aluminum beverage can. Therefore, it is recommended to use Adaptive Mesh Refinement which will automatically refine the mesh only in regions where it is needed, and coarsen the mesh elsewhere. The conditions on the geometric aspect ratio are relatively more strict. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. 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. 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. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. What are people saying about cards & stationery in Brea, CA? 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. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) 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. If it does so, use a finer increment in that range. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Alle Rechte vorbehalten. 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 Auxiliary Sweep can be used to implement ramping of any Global Parameter. The other low-level default settings within the Stationary Solver are chosen for robustness. 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. replace it with the expression: Wrong ordering of study steps. The technique of load ramping is not always reasonable for all problems. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. Set initial conditions in the physics to the appropriate dependent model variable names rather than the default 0. The segregated approach, on the other hand, solves sets of unknowns separately. The advantages of the continuation method are two-fold. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. In this page, we will be sharing some common errors that might occur in Comsol and their solutions. I want to conduct a simulation to find a solution (u) and its first derivative (ux) using a 3D stationary model. By default, the COMSOL Multiphysics software determines these values heuristically depending on the physics as, for example, the specified initial values or a solution from an earlier study step. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Multiphysics problems are often nonlinear. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. Any trademarks referenced in this document are the property of their respective owners. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. Your internet explorer is in compatibility mode and may not be displaying the website correctly. It is sometimes necessary to manually scale the dependent variables. The objective here is to simplify the model to a state where the model will solve, with linear approximations. Does ZnSO4 + H2 at high pressure reverses to Zn + H2SO4? 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. Posted 26 set 2019, 11:57 GMT-4 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. - Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. 3. 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. This involves a systematic reduction in the model complexity. Segregated approach and Direct vs. Iterative linear solvers, About the time step setting of the solver, Introducing Goal Seeking into the Segregated Solver. If you are unsure if your problem is linear and stationary, look at the Log. listed if standards is not an option). So far, weve learned how to mesh and solve linear and nonlinear single-physics finite element problems, but have not yet considered what happens when there are multiple different interdependent physics being solved within the same domain. Not assigning proper boundary conditions: Especially if you have ports. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. Review the model setup to resolve these. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Minimising the environmental effects of my dyson brain. 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. 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. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. The default Initial Values for the unknowns in most physics interfaces are zero. P&S Comsol Team: Yannik Horst, Manuel Kohli, Xinzhi Zhang. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. See Knowledge Base 1240: Manually Setting the Scaling of Variables. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Check the solver log to see if the continuation method is backtracking. Near the top of the Stationary Solver log, the software will report if a linear or nonlinear solver is being used. Stationary Solver Use the Stationary Solver () to find the solution to linear and nonlinear stationary problems (also called static or steady-state problems). The other low-level default settings within the Stationary Solver are chosen for robustness. By creating this job alert, you agree to the LinkedIn User Agreement and Privacy Policy. Create the time-dependent step or study. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). New Stationary Engineer jobs added daily. The coupling terms between the different groups are thus neglected. Contact COMSOL at Bangalore on their telephone: +9180 25323003. Note: there is no way to couple this . k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) This doesn't seem to me the most elegant of methods, since I am essentially solving a stationary solution using a time dependent They deal with COMSOL package and train users. ", "Absolutely the best place to shop for flowers and greenery, I spent an hour just looking around for fun prior to purchasing a bouquet, they have the best customer service people all over the place and it's a huge place", "Recently came in here to pick up 45 chocolate Seegars (cigars) for a dinner banquet in honor of my baby nephew. Alternatively, delete and re-create the study. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. 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 "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). Here, we will examine techniques for accelerating the convergence of these two methods. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: Click the link in the email we sent to to verify your email address and activate your job alert. Hello, Get notified about new Stationary Engineer jobs in Brea, California, United States. We are planning to continuously update this page throughout the semester and hopefully, this will become a reference during your projects as well. - the incident has nothing to do with me; can I use this this way? P&S: COMSOL Design Tool for Photonic Devices. What sort of strategies would a medieval military use against a fantasy giant? . - Feature: Stationary Solver 1 (sol1/s1)Failed to find a solution. That is, start by first solving a model with a small, but non-zero, load. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. 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. 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. That is, they are tuned to achieve convergence in as many cases as possible. This case is generally difficult, or impossible, to solve since this material property is non-smooth. 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. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. The solver settings are stored at Study > Solver Configurations > Solution. listed if standards is not an option). First, it is physically intuitive, often matching how one would perform an experiment. Function: / Failed to evaluate expression. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Such problems must solved in the time domain. Such a large difference in the materials properties can be challenging. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. 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. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. Check the solver log to see if the continuation method is backtracking. The segregated approach, on the other hand, solves sets of unknowns separately. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Note: there is no way to couple this field with the time dependent nature of this physics. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. Most multiphysics problems are nonlinear. They worked with me. - Feature: Stationary Solver 1 (sol1/s1) Learn how your comment data is processed. This guide applies solely to nonlinear stationary models. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. My comment is perhaps a bit nave but it seems to me that you could simply deactivate the term of the background field equation but keep its connexion to the solid to get what you want. In such cases, see if one material (or the other) can be omitted from the analysis completely. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. November 11, 2013. To switch between these solver types, go to the Stationary Solver node within the Study sequence. If you still need help with COMSOL and have an on-subscription license, please visit our Support Center for help. Required fields are marked *. 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. Cooling and Solidification of Metal. How can I use it? k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric 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. 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. 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. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. 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. 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. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. Wrong ordering of study steps. 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. Hi ! 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. Segregated approach and Direct vs. - 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. 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 involves a systematic reduction in the model complexity. 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. You can unsubscribe from these emails at any time. Stationary Solver Iterative Direct . Feature: Stationary Solver 1 (sol1/s1) Attempt to evaluate nonintegral power of negative number. 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. 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. That is, they are tuned to achieve convergence in as many cases as possible. Hello guys. The settings controlling the predictor type. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. There are also cases when an extremely poor quality mesh leads to an ill-conditioned problem, This issue often arises in combination with, and as a consequence of, geometries that have extreme aspect ratios. 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. Stationary (time-invariant) models with nonlinearities may converge very slowly. Solving for laminar flow using Comsol - YouTube Comsol help video number 2: Solving a laminar flow problem in a slit. Not meshing all the domains. $131,100.00, Simplified Vehicle Operations Project Engineer, $115,000.00 Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. The segregated approach, on the other hand, solves sets of unknowns separately. How do/should administrators estimate the cost of producing an online introductory mathematics class? Once the error estimate is small enough, as determined by the convergence criterion (20-16) the software terminates the computations and returns a 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. Sign in to create your job alert for Stationary Engineer jobs in Brea, California, United States. See Knowledge Base 1240: Manually Setting the Scaling of Variables. SGP handled 7 different prints for me at once and they all came out perfectly, in a timely manner. Is it possible to rotate a window 90 degrees if it has the same length and width? COMSOL 22.9K subscribers Adding a study to your simulation is necessary in order to solve your problem. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. The settings controlling the predictor type. See if these can be used instead of explicitly modeling parts with high-aspect ratio geometries. This involves a systematic reduction in the model complexity. 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. 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.
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