replace it with the expression: 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. They worked with me. First, it is physically intuitive, often matching how one would perform an experiment. I am following the same model as Comsol provide us on the web. 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. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. 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. What are some highly rated cards & stationery in Brea, CA? Screenshot showing a Solver Configuration that has been altered. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. Set initial conditions in the physics to the appropriate dependent model variable names rather than the default 0. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version 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. Solving for laminar flow using Comsol - YouTube Comsol help video number 2: Solving a laminar flow problem in a slit. 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. How can I use it? The advantages of the continuation method are two-fold. Again, introduce a Global Parameter that gets ramped from exactly zero to one. Posted 26 set 2019, 11:57 GMT-4 COMSOL does not assume any legal liability for the accuracy of the data disclosed. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. Common Mistakes: Not assigning materials to all the domains. 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. Check the solver log to see if the continuation method is backtracking. Leverage your professional network, and get hired. - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. The problem is that when I run my model this message appear: Undefined value found. By creating this job alert, you agree to the LinkedIn User Agreement and Privacy Policy. 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 Fully Coupled solution approach, with the Plot While Solving enabled. Please dont hesitate to post comments below or send emails to us if you experience any other problems. See if these can be used instead of explicitly modeling parts with high-aspect ratio geometries. Why? This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. 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. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. They are usually called comp1.u, comp1.v, and comp1.w though. See Knowledge Base 1240: Manually Setting the Scaling of Variables. 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. Contact COMSOL at Bangalore on their telephone: +9180 25323003. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: These can be used alone, or in combination with other interfaces. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. P&S Comsol Team: Arif Gngr , Yannik Horst , Stefano Valente. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. Numerically ill-conditioned means that the system matrix is nearly singular and that it will be difficult to solve on a finite-precision computer. listed if standards is not an option). This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. COMSOL makes every reasonable effort to verify the information you view on this page. 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. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. Consult your product manuals for complete trademark details. 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. Note: there is no way to couple this . If it does so, use a finer increment in that range. Reviews on Stationary in Brea, CA - Mievic, Typo, Tokyo Japanese Lifestyle, Fancy, Posh Paper, Morning Glory, Velvet Fox Designs, Barnes & Noble, Michaels, Select Graphics & Printing November 11, 2013. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) Could you expand a little bit more why the coupling is impossible? Changes to these low-level settings from the defaults will usually be quite model- and case-specific. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. The default Initial Values for the unknowns in most physics interfaces are zero. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. listed if standards is not an option). - This approach is known as a Continuation Method with a Constant predictor. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. The segregated approach, on the other hand, solves sets of unknowns separately. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) 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. -Detail: NaN or Inf found when solving linear system using SOR. Your Discussion has gone 30 days without a reply. I want to conduct a simulation to find a solution (u) and its first derivative (ux) using a 3D stationary model. Segregated approach and Direct vs. Iterative linear solvers, About the time step setting of the solver, Introducing Goal Seeking into the Segregated Solver. Is there a way to use the stationary solution obtained in Comsol 4.2 as the initial conditions in a time dependent model? Such a case would be better to address instead with the Shell physics interface, which is specially formulated for handling thin-walled structural parts. (Frequency Domain should be the last step) The technique of load ramping is not always reasonable for all problems. That is, they are tuned to achieve convergence in as many cases as possible. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) We are planning to continuously update this page throughout the semester and hopefully, this will become a reference during your projects as well. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. Discussion Closed This discussion was created more than 6 months ago and has been closed. We have also introduced meshing considerations for linear static problems, as well as how to identify singularities and what to do about them when meshing. The coupling terms between the different groups are thus neglected. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. 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. 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. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. Find centralized, trusted content and collaborate around the technologies you use most. Understanding the Fully Coupled vs. COMSOL 22.9K subscribers Adding a study to your simulation is necessary in order to solve your problem. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Singular matrix., Make sure you defined your ports correctly:Boundary selectionType of port: e.g. Use this parameter to modify the nonlinearity expressions in the model. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. Not assigning proper boundary conditions: Especially if you have ports. Required fields are marked *. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). If you see this, right-click on the Solution feature and select Reset Solver to Default. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. Stationary (time-invariant) models with nonlinearities may converge very slowly. replace it with the expression: The Auxiliary Sweep can be used to implement ramping of any Global Parameter. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. 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.