The Auxiliary Sweep can be used to implement ramping of any Global Parameter. Discussion Closed This discussion was created more than 6 months ago and has been closed. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Any trademarks referenced in this document are the property of their respective owners. Not meshing all the domains. 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. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. 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. Today's top 351 Stationary Engineer jobs in Brea, California, United States. Mesh 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. The other low-level default settings within the Stationary Solver are chosen for robustness. For example, if there is a temperature-dependent material property such as: 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. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. 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. 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. 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 Fully Coupled solution approach, with the Plot While Solving enabled. . Hi Alexis, Unknown function or operator. The objective here is to simplify the model to a state where the model will solve, with linear approximations. P&S Comsol Team: Arif Gngr , Yannik Horst , Stefano Valente. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. listed if standards is not an option). If you still need help with COMSOL and have an on-subscription license, please visit our Support Center for help. If you are unsure if your problem is linear and stationary, look at the Log. There will always already be either a Segregated or Fully Coupled feature beneath this. 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. 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. The fully coupled and segregated approaches are discussed below. Perhaps this approach could be adapted to represent your model. Please dont hesitate to post comments below or send emails to us if you experience any other problems. 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. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. Set initial conditions in the physics to the appropriate dependent model variable names rather than the default 0. That is, they are tuned to achieve convergence in as many cases as possible. Not meshing all the domains. 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. - As a second example, when solving for Electric Currents, do not model perfect electrical insulators as materials with zero conductivity, instead, omit the domain from the model and use the Electric Insulation boundary condition. I want to conduct a simulation to find a solution (u) and its first derivative (ux) using a 3D stationary model. 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. Comsol help video number 2: Solving a laminar flow problem in a slit.. Such a large difference in the materials properties can be challenging. Common Mistakes: Not assigning materials to all the domains. Learn how your comment data is processed. Also, keep in mind that a linear stationary model should solve regardless of how coarse the mesh is (albeit to a low accuracy) so you can always start with as coarse a mesh as possible, and refine the mesh (See also: Knowledgebase 1030: Performing a Mesh Refinement Study. Most multiphysics problems are nonlinear. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. An example would be a linear static structural model of an aluminum beverage can. Wrong ordering of study steps. - Feature: Stationary Solver 1 (sol1/s1) Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. If it does so, use a finer increment in that range. Any trademarks referenced in this document are the property of their respective owners. I personally liked emailing them the file, ", "This flower shop is the best! It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. In our previous blog entry, we introduced the Fully Coupled and the Segregated algorithms used for solving steady-state multiphysics problems in COMSOL. 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. COMSOL does not assume any legal liability for the accuracy of the data disclosed. 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. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. The settings controlling the predictor type. 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. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. The problem is that when I run my model this message appear: Undefined value found. Any trademarks referenced in this document are the property of their respective owners. COMSOL 22.9K subscribers Adding a study to your simulation is necessary in order to solve your problem. Solve the stationary study then the time dependent study. 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. 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. Repeat this for every nonlinearity of the model. This case is generally difficult, or impossible, to solve since this material property is non-smooth. 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. The settings controlling the predictor type. 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. Most multiphysics problems are nonlinear. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. Minimising the environmental effects of my dyson brain. In such cases, use the same continuation method, but instead ramp the nonlinearities in the model. These are some highly rated cards & stationery in Brea, CA: What are some cards & stationery with a large number of reviews in Brea, CA? Hi Jason, If some, or all, of the needed materials properties needed by the physics interfaces are not defined, the model will generate an error at runtime. Using the first order optimality. 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. Use this parameter to modify the nonlinearity expressions in the model. listed if standards is not an option). Wish you all the best. COMSOL does not assume any legal liability for the accuracy of the data disclosed. Discussion Closed This discussion was created more than 6 months ago and has been closed. 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. COMSOL makes every reasonable effort to verify the information you view on this page. Studysteps might be listed in wrong order: Not assigning materials to all the domains. There will always already be either a Segregated or Fully Coupled feature beneath this. View the Settings window for the Materials branch to get a list of all domains with undefined materials and add a material to those domains. Building on these topics, we will now address how to prepare your mesh for efficiently solving nonlinear finite element problems. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. Does ZnSO4 + H2 at high pressure reverses to Zn + H2SO4? Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. 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. Communication over the phone, in person, and through email was very easy. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. However, it is usually not possible to know this ahead of time. 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. Multiphysics problems are often nonlinear. The segregated approach, on the other hand, solves sets of unknowns separately. Not assigning proper boundary conditions: Especially if you have ports. Near the top of the Stationary Solver log, the software will report if a linear or nonlinear solver is being used. 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. 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. Repeat this for every nonlinearity of the model. Is it possible to rotate a window 90 degrees if it has the same length and width? Stationary Solver Iterative Direct . This approach is known as a Continuation Method with a Constant predictor. If it does so, use a finer increment in that range. Segregated approach and Direct vs. Iterative linear solvers, About the time step setting of the solver, Introducing Goal Seeking into the Segregated Solver. This involves a systematic reduction in the model complexity. Use this parameter to modify the nonlinearity expressions in the model. Hi Jason, Your internet explorer is in compatibility mode and may not be displaying the website correctly. The algorithm is, generally speaking, a Newton's method approach. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: - 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. Repeat this for every nonlinearity of the model. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) In such cases, see if one material (or the other) can be omitted from the analysis completely. SGP handled 7 different prints for me at once and they all came out perfectly, in a timely manner. Set "Values for dependent variables" in study step settings to User ControlledSolutionYour Stationary Study. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. Segregated approach and Direct vs. Dun & Bradstreet gathers Office Supplies, Stationery, and Gift Retailers business information from trusted . With respect to any nonlinearities, replace them by a reasonable linearized term. 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. 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 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. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. Asking for help, clarification, or responding to other answers. 0 Replies, Please login with a confirmed email address before reporting spam. To start a new discussion with a link back to this one, click here. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. Linear solvers. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. Ramping the nonlinearities over time is not as strongly motivated, but step changes in nonlinearities should be smoothed out throughout the simulation. listed if standards is not an option). Singular matrix., Make sure you defined your ports correctly:Boundary selectionType of port: e.g. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. 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. 3 Replies, Please login with a confirmed email address before reporting spam. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. Create the time-dependent step or study. Does anyone know what should cause this problem? Such problems must solved in the time domain. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. November 11, 2013. 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.
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