COMSOL 22.9K subscribers Adding a study to your simulation is necessary in order to solve your problem. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. Linear solvers. There will always already be either a Segregated or Fully Coupled feature beneath this. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Today's top 351 Stationary Engineer jobs in Brea, California, United States. Use this parameter to modify the nonlinearity expressions in the model. Set "Values for dependent variables" in study step settings to User ControlledSolutionYour Stationary Study. 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. Here, we will examine techniques for accelerating the convergence of these two methods. Consult your product manuals for complete trademark details. 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. This approach is known as a Continuation Method with a Constant predictor. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. November 11, 2013. The following are possible reasons why a linear stationary model will fail to solve, along with resolutions: The combination of the constraints and boundary conditions must be sufficient to define a unique solution to the problem, in terms of the field variables being solved. Cooling and Solidification of Metal. This is relatively expensive to do, but will lead to the most robust convergence. 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. Not the answer you're looking for? 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. That is, start by first solving a model with a small, but non-zero, load. I highly recommend this flower shop. Numerically ill-conditioned means that the system matrix is nearly singular and that it will be difficult to solve on a finite-precision computer. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. (Frequency Domain should be the last step). In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. 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. This involves a systematic reduction in the model complexity. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. The algorithm is, generally speaking, a Newton's method approach. The other low-level default settings within the Stationary Solver are chosen for robustness. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). The objective here is to simplify the model to a state where the model will solve, with linear approximations. 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. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. 3. Minimising the environmental effects of my dyson brain. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. 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. Name: actdep_int1, Your email address will not be published. Your email address will not be published. Solver . 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, see if one material (or the other) can be omitted from the analysis completely. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Your internet explorer is in compatibility mode and may not be displaying the website correctly. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. P&S: COMSOL Design Tool for Photonic Devices. Get notified about new Stationary Engineer jobs in Brea, California, United States. Your internet explorer is in compatibility mode and may not be displaying the website correctly. 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 is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. 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. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. Solving for laminar flow using Comsol - YouTube Comsol help video number 2: Solving a laminar flow problem in a slit. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. What version of COMSOL are you using? Such problems must solved in the time domain. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. Most multiphysics problems are nonlinear. Use this parameter to modify the nonlinearity expressions in the model. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. Linear solvers. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. comp1.u2, comp1.v2, and comp1.w2 are usually variables associated with the x,y, and z component of deformation in COMSOL. 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. Stationary (time-invariant) models with nonlinearities may converge very slowly. 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. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. listed if standards is not an option). Posted Sep 9, 2020, 1:44 p.m. EDT Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. COMSOL does not assume any legal liability for the accuracy of the data disclosed. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. 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. Your internet explorer is in compatibility mode and may not be displaying the website correctly. 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. 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: In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. The Fully Coupled solution approach, with the Plot While Solving enabled. Click the link in the email we sent to to verify your email address and activate your job alert. Dr.S.Ravindran Cite 1 Recommendation Popular answers (1). See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Solve the stationary study then the time dependent study. The other low-level default settings within the Stationary Solver are chosen for robustness. Trying to understand how to get this basic Fourier Series. Hi ! Any trademarks referenced in this document are the property of their respective owners. These can be used alone, or in combination with other interfaces. 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. Not assigning proper boundary conditions: Especially if you have ports. Function: / Failed to evaluate expression. Perhaps this approach could be adapted to represent your model. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Leverage your professional network, and get hired. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. That is, start by first solving a model with a small, but non-zero, load. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. 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. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. Cooling and Solidification of Metal. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. Not meshing all the domains. Different physics have different default solvers. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. 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. Stationary Engineer Jobs in Brea, California, United States, $87,400.00 The other low-level default settings within the Stationary Solver are chosen for robustness. Asking for help, clarification, or responding to other answers. 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. If you are unsure if your problem is linear and stationary, look at the Log. 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. 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. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version - COMSOL does not assume any legal liability for the accuracy of the data disclosed. (Frequency Domain should be the last step) Again, introduce a Global Parameter that gets ramped from exactly zero to one. Wrong ordering of study steps. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. The technique of load ramping is not always reasonable for all problems. $125,000.00, Project Engineer (In-person/Hybrid/Remote), $100,000.00 k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) Save my name, email, and website in this browser for the next time I comment. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Here, we begin an overview of the algorithms used for solving nonlinear static finite element problems. First, it is physically intuitive, often matching how one would perform an experiment. Reply . The default Initial Values for the unknowns in most physics interfaces are zero. P&S Comsol Team: Manuel Kohli, Raphael Schwanninger, Feature: Stationary Solver 1 (sol1/s1) An example would be a linear static structural model of an aluminum beverage can. Tutti i diritti sono riservati. New Stationary Engineer jobs added daily. Tutti i diritti sono riservati. Iterative , Direct . - I am following the same model as Comsol provide us on the web. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. 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. 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. thanks for reply 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. To switch between these solver types, go to the Stationary Solver node within the Study sequence. Is there a way to use the stationary solution obtained in Comsol 4.2 as the initial conditions in a time dependent model? 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. - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. Posted 26 set 2019, 11:57 GMT-4 In particular, choosing an improper initial condition or setting up a problem without a solution will simply cause the nonlinear solver to continue iterating without converging. 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 advantages of the continuation method are two-fold. 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. Reviews on Stationary Stores in Brea, CA - Typo, Mievic, Posh Paper, Morning Glory, Select Graphics & Printing, Daiso Japan, Urban Outfitters, Office Depot, Barnes & Noble If so, see: Knowledgebase 1030: Error: "Out of memory". - Function: ^ Failed to evaluate variable. The objective here is to simplify the model to a state where the model will solve, with linear approximations. Any trademarks referenced in this document are the property of their respective owners. The coupling terms between the different groups are thus neglected. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. 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. $131,100.00, Simplified Vehicle Operations Project Engineer, $115,000.00 The objective here is to simplify the model to a state where the model will solve, with linear approximations. However, it is usually not possible to know this ahead of time. SGP handled 7 different prints for me at once and they all came out perfectly, in a timely manner. That is, they are tuned to achieve convergence in as many cases as possible. The advantages of the continuation method are two-fold. Understanding the Fully Coupled 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. This algorithm was also useful for understanding what happens near a failure load. Wrong ordering of study steps. Thanks for contributing an answer to Stack Overflow! It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. The fully coupled and segregated approaches are discussed below. 140K views 8 years ago COMSOL Multiphysics Tutorial for Beginners Please note that an updated version of the content in this video can be found in the Modeling Workflow video in the COMSOL. Repeat this for every nonlinearity of the model. However, it is usually not possible to know this ahead of time. "After the incident", I started to be more careful not to trip over things. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. - 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. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. Hi Jason, Discussion Closed This discussion was created more than 6 months ago and has been closed. COMSOL does not assume any legal liability for the accuracy of the data disclosed. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. 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. Not meshing all the domains. 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. listed if standards is not an option). Any trademarks referenced in this document are the property of their respective owners. Discussion Closed This discussion was created more than 6 months ago and has been closed. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. Repeat this for every nonlinearity of the model. Cecilia M. came from the backroom to help me and went out of her way to help me even though I had not called before going into the store. This guide applies solely to nonlinear stationary models. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. 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. Feature: Stationary Solver 1 (sol1/s1) Adaptation gives the same mesh: adaptmesh1. 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. Wish you all the best. This doesn't seem to me the most elegant of methods, since I am essentially solving a stationary solution using a time dependent They are usually called comp1.u, comp1.v, and comp1.w though. Consult your product manuals for complete trademark details. Save this information to a file. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. Making statements based on opinion; back them up with references or personal experience. 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 incident has nothing to do with me; can I use this this way? Sign in to create your job alert for Stationary Engineer jobs in Brea, California, United States. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. By creating this job alert, you agree to the LinkedIn User Agreement and Privacy Policy. Feature: Stationary Solver 1 (sol1/s1) Attempt to evaluate nonintegral power of negative number. 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. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. Could you expand a little bit more why the coupling is impossible? COMSOL makes every reasonable effort to verify the information you view on this page. In our previous blog entry, we introduced the Fully Coupled and the Segregated algorithms used for solving steady-state multiphysics problems in COMSOL. Such a large difference in the materials properties can be challenging. 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. 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. It is sometimes necessary to manually scale the dependent variables. Not entering required material parameters. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. Hence Comsol solved for the stationary solution at different points of time.