Understanding the Fully Coupled vs. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. 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. Check the solver log to see if the continuation method is backtracking. 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. With respect to any nonlinearities, replace them by a reasonable linearized term. 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. so many cute little stationary items , hair". Solve the stationary study then the time dependent study. In many physics areas there exist alternative physics formulations specifically meant for solving cases where the geometry has an extreme aspect ratio. 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. Any trademarks referenced in this document are the property of their respective owners. To learn more, see our tips on writing great answers. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. That is, start by first solving a model with a small, but non-zero, load. Stationary (time-invariant) models with nonlinearities may converge very slowly. Such problems must solved in the time domain. Studysteps might be listed in wrong order: Not assigning materials to all the domains. 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. Wrong ordering of study steps. 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. Function: / Failed to evaluate expression. 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. 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. Could you expand a little bit more why the coupling is impossible? This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). listed if standards is not an option). Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. 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!!! Click the link in the email we sent to to verify your email address and activate your job alert. First, it is physically intuitive, often matching how one would perform an experiment. P&S: COMSOL Design Tool for Photonic Devices. "I chose this print shop based off yelp reviews and was very happy with the outcome! Some models are numerically ill-conditioned due to the setup. Your Discussion has gone 30 days without a reply. Making statements based on opinion; back them up with references or personal experience. 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. 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. The algorithm is, generally speaking, a Newton's method approach. - The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. . Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. For example, if there is a temperature-dependent material property such as: Multiphysics problems are often nonlinear. In our previous blog entry, we introduced the Fully Coupled and the Segregated algorithms used for solving steady-state multiphysics problems in COMSOL. COMSOL does not assume any legal liability for the accuracy of the data disclosed. Such problems must solved in the time domain. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. This is relatively expensive to do, but will lead to the most robust convergence. The Fully Coupled solution approach, with the Plot While Solving enabled. An example would be a linear static structural model of an aluminum beverage can. Common Mistakes: Not assigning materials to all the domains. For example, if there is a temperature-dependent material property such as: Reply . In this page, we will be sharing some common errors that might occur in Comsol and their solutions. Trying to understand how to get this basic Fourier Series. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: 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. 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. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. replace it with the expression: Posted 26 set 2019, 17:57 CEST Mesh Version 5.3 0 Replies. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. (Frequency Domain should be the last step). If you see this, right-click on the Solution feature and select Reset Solver to Default. ", "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. . - Function: ^ Failed to evaluate variable. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Cooling and Solidification of Metal. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. This guide applies solely to nonlinear stationary 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. Minimising the environmental effects of my dyson brain. Stationary (time-invariant) models with nonlinearities may converge very slowly. The other low-level default settings within the Stationary Solver are chosen for robustness. They are usually called comp1.u, comp1.v, and comp1.w though. The fully coupled and segregated approaches are discussed below. 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. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. 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. This algorithm was also useful for understanding what happens near a failure load. The Fully Coupled solution approach, with the Plot While Solving enabled. comp1.u2, comp1.v2, and comp1.w2 are usually variables associated with the x,y, and z component of deformation in COMSOL. Check the solver log to see if the continuation method is backtracking. Create the time-dependent step or study. 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. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. Comsol help video number 2: Solving a laminar flow problem in a slit.. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Not assigning proper boundary conditions: Especially if you have ports. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. - Feature: Stationary Solver 1 (sol1/s1) 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. Feature: Stationary Solver 1 (sol1/s1) Attempt to evaluate nonintegral power of negative number. It is also possible to manually refine the mesh. $131,100.00, Simplified Vehicle Operations Project Engineer, $115,000.00 Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. Why is there a voltage on my HDMI and coaxial cables? There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Repeat this for every nonlinearity of the model. 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. 3 Replies, Please login with a confirmed email address before reporting spam. Review the model setup to resolve these. listed if standards is not an option). This is relatively expensive to do, but will lead to the most robust convergence. Here, we begin an overview of the algorithms used for solving nonlinear static finite element problems. What are some highly rated cards & stationery in Brea, CA? Unknown function or operator. The default Initial Values for the unknowns in most physics interfaces are zero. This approach is known as a Continuation Method with a Constant predictor. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. 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. 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. (Frequency Domain should be the last step) The idea behind the GCRO-DR method is to retain the subspace determined while solving previous systems and use it to reduce the cost of solving the next system. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. Multiphysics problems are often nonlinear. If the default iterative solver is not converging, try switching to a direct solver, as described here: Understanding the Fully Coupled vs. These are some cards & stationery with a large number of reviews in Brea, CA. Any trademarks referenced in this document are the property of their respective owners. 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. COMSOL makes every reasonable effort to verify the information you view on this page. If your matrix is singular than no solver in the world can solve Ax=B. They worked with me. Sign in to create your job alert for Stationary Engineer jobs in Brea, California, United States. 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. I am solving a linear stationary finite element model but the software is not solving. 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. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. First, it is physically intuitive, often matching how one would perform an experiment. It may also reveal that the model itself is ill-posed in some way. She opened prepackaged 6-packs of Seegars to make ", All "stationary stores" results in Brea, CA, Search stationary stores in popular locations. Iterative , Direct . 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. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. 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). The settings controlling the predictor type. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. 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 Auxiliary Sweep can be used to implement ramping of any Global Parameter. This involves a systematic reduction in the model complexity. Communication over the phone, in person, and through email was very easy. Use this parameter to modify the nonlinearity expressions in the model. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. In this post, we will delve deeper into how to choose an appropriate mesh to start your mesh convergence studies for linear static finite element problems. 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. In a previous blog entry, we introduced meshing considerations for linear static problems. Save my name, email, and website in this browser for the next time I comment. If one particular material is missing one property, that material will also be highlighted with a red cross over that material icon in the Model Builder. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. Note: there is no way to couple this field with the time dependent nature of this physics. Learn how your comment data is processed. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Under Initial values of variables solved for, the default value of the Settingslist is Physics controlled. $125,000.00, Project Engineer (In-person/Hybrid/Remote), $100,000.00 SGP handled 7 different prints for me at once and they all came out perfectly, in a timely manner. Such problems must solved in the time domain. Thanks for contributing an answer to Stack Overflow! - 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. 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 case is generally difficult, or impossible, to solve since this material property is non-smooth. To switch between these solver types, go to the Stationary Solver node within the Study sequence. Segregated approach and Direct vs. Iterative linear solvers, About the time step setting of the solver, Introducing Goal Seeking into the Segregated Solver. The other low-level default settings within the Stationary Solver are chosen for robustness. Why? 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. The default solver for most 3D models is an iterative solver, which is more sensitive to ill-conditioned problems. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. 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. Such a large difference in the materials properties can be challenging. Wrong ordering of study steps. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. Check the solver log to see if the continuation method is backtracking. Please dont hesitate to post comments below or send emails to us if you experience any other problems. Not the answer you're looking for? Stationary Solver Iterative Direct . It may also reveal that the model itself is ill-posed in some way. Tutti i diritti sono riservati. Have you taken a look at this blog post? 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. Using the first order optimality. Consult your product manuals for complete trademark details. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. 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. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. Version 5.3 Screenshot showing a Solver Configuration that has been altered. The default Initial Values for the unknowns in most physics interfaces are zero. If these settings have been manually changed, you will see a small star symbol on the Solution feature, as shown in the screenshot below. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Direct PARDISO Solver , PARDISO . 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. Dun & Bradstreet gathers Office Supplies, Stationery, and Gift Retailers business information from trusted . Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Can I tell police to wait and call a lawyer when served with a search warrant? That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. This case is generally difficult, or impossible, to solve since this material property is non-smooth. One of the key concepts there was the idea of mesh convergence as you refine the mesh, the solution will become more accurate. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. Use this parameter to modify the nonlinearity expressions in the model. 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. The technique of load ramping is not always reasonable for all problems. 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. Set "Values for dependent variables" in study step settings to User ControlledSolutionYour Stationary Study. listed if standards is not an option). This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. If the model is nonlinear, see: Improving Convergence of Nonlinear Stationary Models. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: How can I check before my flight that the cloud separation requirements in VFR flight rules are met? You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Not entering required material parameters. 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. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) These can be used alone, or in combination with other interfaces. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. Acidity of alcohols and basicity of amines. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. Leverage your professional network, and get hired. Again, introduce a Global Parameter that gets ramped from exactly zero to one. Note: there is no way to couple this . Your internet explorer is in compatibility mode and may not be displaying the website correctly. Most multiphysics problems are nonlinear. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: The advantages of the continuation method are two-fold. thanks for reply That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. 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. listed if standards is not an option). (I am begginer in comsol) Thank you. Set initial conditions in the physics to the appropriate dependent model variable names rather than the default 0. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. The segregated approach, on the other hand, solves sets of unknowns separately. Feature: Stationary Solver 1 (sol1/s1) Adaptation gives the same mesh: adaptmesh1. Perhaps this approach could be adapted to represent your model. replace it with the expression: - Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. COMSOL makes every reasonable effort to verify the information you view on this page. Does anyone know what should cause this problem? For example, in Solid Mechanics, if the Poisson Ratio set to 0.5, then the model will not solve, as this value in incompatible with the theory of linear elasticity. Consult your product manuals for complete trademark details. If the material properties entered are incorrect for the governing equation, the model will generate an error at runtime, usually a Singular Matrix error.