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. 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. The coupling terms between the different groups are thus neglected. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. What did people search for similar to stationary stores in Brea, CA? Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). By providing your email address, you consent to receive emails from COMSOL AB and its affiliates about the COMSOL Blog, and agree that COMSOL may process your information according to its Privacy Policy. Your internet explorer is in compatibility mode and may not be displaying the website correctly. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. 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. They are usually called comp1.u, comp1.v, and comp1.w though. This doesn't seem to me the most elegant of methods, since I am essentially solving a stationary solution using a time dependent Could you expand a little bit more why the coupling is impossible? 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. 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. 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. 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. 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. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: Numerically ill-conditioned means that the system matrix is nearly singular and that it will be difficult to solve on a finite-precision computer. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. The advantages of the continuation method are two-fold. 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. 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. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. - This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. listed if standards is not an option). Set initial conditions in the physics to the appropriate dependent model variable names rather than the default 0. If you do not hold an on-subscription license, you may find an answer in another Discussion or in the Knowledge Base. - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. listed if standards is not an option). if I want to do an adaptive mesh refinement, I get this error. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: If it does so, use a finer increment in that range. 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. To switch between these solver types, go to the Stationary Solver node within the Study sequence. It is sometimes necessary to manually scale the dependent variables. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. . There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. 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. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. We use COMSOL Multiphysics for solving distributed optimal control of un-steady Burgers equation without constraints and with pointwise control constraints. - Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. Check the solver log to see if the continuation method is backtracking. Set "Values for dependent variables" in study step settings to User ControlledSolutionYour Stationary Study. What is the purpose of non-series Shimano components? Therefore, an initial value of zero is almost always reasonable if a very small load is applied. 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. The objective here is to simplify the model to a state where the model will solve, with linear approximations. The coupling terms between the different groups are thus neglected. 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. Multiscale Modeling in High-Frequency Electromagnetics. Linear solvers. Singular matrix., Make sure you defined your ports correctly:Boundary selectionType of port: e.g. The Fully Coupled solution approach, with the Plot While Solving enabled. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. Repeat this for every nonlinearity of the model. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. 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. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. Using the first order optimality. For example, if there is a temperature-dependent material property such as: 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. 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. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. I personally liked emailing them the file, ", "This flower shop is the best! Hi ! However, it is usually not possible to know this ahead of time. Not assigning proper boundary conditions: Especially if you have ports. If so, see: Knowledgebase 1030: Error: "Out of memory". To start a new discussion with a link back to this one, click here. 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. 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. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: This approach is known as a Continuation Method with a Constant predictor. 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. 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. Perhaps this approach could be adapted to represent your model. - the incident has nothing to do with me; can I use this this way? 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. For the Nozomi from Shinagawa to Osaka, say on a Saturday afternoon, would tickets/seats typically be available - or would you need to book? The coupling terms between the different groups are thus neglected. I am solving a linear stationary finite element model but the software is not solving. 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. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Dr.S.Ravindran Cite 1 Recommendation Popular answers (1). It is also possible to manually refine the mesh. There will always already be either a Segregated or Fully Coupled feature beneath this. Does ZnSO4 + H2 at high pressure reverses to Zn + H2SO4? There will always already be either a Segregated or Fully Coupled feature beneath this. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. 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. I am following the same model as Comsol provide us on the web. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. 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. 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. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. That is, start by first solving a model with a small, but non-zero, load. Review the model setup to resolve these. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) Use this parameter to modify the nonlinearity expressions in the model. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. 0 Replies, Please login with a confirmed email address before reporting spam.