That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. For example, in an Electric Currents problem, you may want to consider a system of materials including a good conductor such as copper (with an electric conductivity of ~6e7 S/m) and an insulative material such as glass (which can have electric conductivity of ~1e-14 S/m.) Get email updates for new Stationary Engineer jobs in Brea, California, United States. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. Extending this logic, if one wants to solve for any arbitrary load on a nonlinear system, it makes sense to solve a sequence of intermediate problems with gradually increasing load values and using the solutions from each previous step as the initial condition for the next step. 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. COMSOL does not assume any legal liability for the accuracy of the data disclosed. 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. listed if standards is not an option). 3. The segregated approach, on the other hand, solves sets of unknowns separately. Note the star symbol on the Solution feature. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. Save this information to a file. If this was solved using the Solid Mechanics physics interface the thin walls of the container would need to be explicitly modeled, but the wall thickness is much smaller than the overall can dimensions. Here we introduce a more robust approach to solving nonlinear problems. - Feature: Stationary Solver 1 (sol1/s1) 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. The difference between the phonemes /p/ and /b/ in Japanese. Why is there a voltage on my HDMI and coaxial cables? Sign in to create your job alert for Stationary Engineer jobs in Brea, California, United States. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. Understanding the Fully Coupled vs. Your internet explorer is in compatibility mode and may not be displaying the website correctly. However, it is usually not possible to know this ahead of time. 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. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. That is: Even if the forces on a part are opposite and equal, this is not sufficient information to say where the part is, so you must add some other condition, such as as Fixed Constraint to constrain displacement. What are some highly rated cards & stationery in Brea, CA? 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. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. 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 trying to solve the coupling between a waveguide and a microring resonator. 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. P&S: COMSOL Design Tool for Photonic Devices. 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. One of the key concepts there was the idea of mesh convergence as you refine the mesh, the solution will become more accurate. It is also possible to manually refine the mesh. The algorithm is, generally speaking, a Newton's method approach. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. There will always already be either a Segregated or Fully Coupled feature beneath this. See Knowledge Base 1240: Manually Setting the Scaling of Variables. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Dun & Bradstreet gathers Office Supplies, Stationery, and Gift Retailers business information from trusted . There will also be a red cross over the Materials branch icon. 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. Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Thanks for contributing an answer to Stack Overflow! Connect and share knowledge within a single location that is structured and easy to search. For the Nozomi from Shinagawa to Osaka, say on a Saturday afternoon, would tickets/seats typically be available - or would you need to book? 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. In our previous blog entry, we introduced the Fully Coupled and the Segregated algorithms used for solving steady-state multiphysics problems in COMSOL. L'objectif de notre prsent travail se repose sur l'tude par simulation numrique du comportement de bton au jeune ge sous des conditions svres de temprature pendant les premires 24h aprs. 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. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: They are usually called comp1.u, comp1.v, and comp1.w though. Your internet explorer is in compatibility mode and may not be displaying the website correctly. There are also cases when an extremely poor quality mesh leads to an ill-conditioned problem, This issue often arises in combination with, and as a consequence of, geometries that have extreme aspect ratios. 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. What sort of strategies would a medieval military use against a fantasy giant? Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. We use COMSOL Multiphysics for solving distributed optimal control of un-steady Burgers equation without constraints and with pointwise control constraints. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. This case is generally difficult, or impossible, to solve since this material property is non-smooth. Such problems must solved in the time domain. 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. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. Alle Rechte vorbehalten. Perhaps this approach could be adapted to represent your model. Such problems must solved in the time domain. 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. 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. 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. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. The stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. Making statements based on opinion; back them up with references or personal experience. The advantages of the continuation method are two-fold. Repeat this for every nonlinearity of the model. Multiphysics problems are often nonlinear. 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. Minimising the environmental effects of my dyson brain. Solver . This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. 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. Name: actdep_int1, Your email address will not be published. Again, introduce a Global Parameter that gets ramped from exactly zero to one. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. The technique of load ramping is not always reasonable for all 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. Not entering required material parameters. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) In such cases, see if one material (or the other) can be omitted from the analysis completely. Again, introduce a Global Parameter that gets ramped from exactly zero to one. In the extreme case, suppose one wants to model an instantaneous change in properties, such as: Stationary Solver Iterative Direct . comp1.u2, comp1.v2, and comp1.w2 are usually variables associated with the x,y, and z component of deformation in COMSOL. numeric (each ports needs their ownboundary mode analysis in the study if they are numerically defined)Wave excitation: on/off(input/output), - Feature: Stationary Solver 1 (sol1/s1) Division by zero. 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. 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. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. Feature: Stationary Solver 1 (sol1/s1)" . Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. For example, if there is a temperature-dependent material property such as: If you have both as steps in the same study, then solve that study. Discussion Closed This discussion was created more than 6 months ago and has been closed. Check the solver log to see if the continuation method is backtracking. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. (Frequency Domain should be the last step) First, it is physically intuitive, often matching how one would perform an experiment. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. ", "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. -Detail: NaN or Inf found when solving linear system using SOR. If you still need help with COMSOL and have an on-subscription license, please visit our Support Center for help. The segregated approach, on the other hand, solves sets of unknowns separately. What is the purpose of non-series Shimano components? This case is generally difficult, or impossible, to solve since this material property is non-smooth. The fully coupled and segregated approaches are discussed below. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. What is \newluafunction? It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Consult your product manuals for complete trademark details. Multiscale Modeling in High-Frequency Electromagnetics. 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. This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. listed if standards is not an option). 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. Use a manually defined mesh to avoid elements with extreme aspect ratios and perform a mesh refinement study, as described here: Performing a Mesh Refinement Study, For problems that are ill-conditioned, using a direct solver is often called for. Acidity of alcohols and basicity of amines. 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. 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. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version This approach is known as a Continuation Method with a Constant predictor. The advantages of the continuation method are two-fold. Some models are numerically ill-conditioned due to the setup. Iterative , Direct . Stationary (time-invariant) models with nonlinearities may converge very slowly. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. 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. With respect to any nonlinearities, replace them by a reasonable linearized term. 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. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) 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. Screenshot showing a Solver Configuration that has been altered. Common Mistakes: Not assigning materials to all the domains. Cooling and Solidification of Metal. Can I tell police to wait and call a lawyer when served with a search warrant? This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. 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. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) Therefore, an initial value of zero is almost always reasonable if a very small load is applied. There will always already be either a Segregated or Fully Coupled feature beneath this. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. However, load ramping will not work in all cases, or may be inefficient. Posted 26 set 2019, 17:57 CEST Mesh Version 5.3 0 Replies. 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. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. Most multiphysics problems are nonlinear. Cooling and Solidification of Metal. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version This involves a systematic reduction in the model complexity. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. They deal with COMSOL package and train users. Does ZnSO4 + H2 at high pressure reverses to Zn + H2SO4? I highly recommend this flower shop. 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. 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. The algorithm is, generally speaking, a Newton's method approach. Note that while COMSOL employees may participate in the discussion forum, COMSOL software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. See also: Knowledge Base 1254: Controlling the Time Dependent solver timesteps. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. - An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: $131,100.00, Simplified Vehicle Operations Project Engineer, $115,000.00 replace it with the expression: I use comsol multiphysics 5.2a and . Thanks, Andres. Under Initial values of variables solved for, the default value of the Settingslist is Physics controlled. listed if standards is not an option). 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. 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. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version The settings controlling the predictor type. It is sometimes necessary to manually scale the dependent variables. 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. Wrong ordering of study steps. 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). Is it possible to rotate a window 90 degrees if it has the same length and width? Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. In this posting, we introduce the idea of ramping the nonlinearities in the problem to improve convergence. Required fields are marked *. This guide applies solely to nonlinear stationary models. If the material properties entered are incorrect for the governing equation, the model will generate an error at runtime, usually a Singular Matrix error. Tutti i diritti sono riservati. replace it with the expression: Second, the continuation method will automatically take smaller load increments if a solution cannot be found. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). Feature: Stationary Solver 1 (sol1/s1) Attempt to evaluate nonintegral power of negative number. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. Leverage your professional network, and get hired. The finite element mesh must be fine enough to resolve the spatial variations in the solution fields. Note: there is no way to couple this . The objective here is to simplify the model to a state where the model will solve, with linear approximations. 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 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. Not entering required material parameters. It's brand new in the hmart plaza and I wish it was open back when I would hangout in the plaza after school (although they would have taken all my allowance money! If these settings have been manually changed, you will see a small star symbol on the Solution feature, as shown in the screenshot below. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. For example, in a Solid Mechanics (wherein the software is solving for the displacement field within the solid) applying two opposite and equal Boundary Load conditions on a part is not sufficient to define the displacement. Posted 26 set 2019, 11:57 GMT-4 About the Stationary Solver The following background information about the Stationary Solver discusses these topics: Damped Newton Methods, Termination Criterion for the Fully Coupled and Segregated Attribute Nodes, Linear Solvers versus Nonlinear Solvers, and Pseudo Time Stepping. Stationary (time-invariant) models with nonlinearities may converge very slowly. 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. Find centralized, trusted content and collaborate around the technologies you use most. 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. These are some cards & stationery with a large number of reviews in Brea, CA. For example, if there is a temperature-dependent material property such as: Function: / Failed to evaluate expression. First, it is physically intuitive, often matching how one would perform an experiment. Computational Fluid Dynamics (CFD), API The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. This algorithm was also useful for understanding what happens near a failure load. That is, start by first solving a model with a small, but non-zero, load. This can arise as a consequence of extreme variations in the material properties, or high aspect ratio geometry. 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.
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