# Modelling & Simulations

**URL:** https://discourse.julialang.org/c/domain/models/21.md

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## [About the Modelling & Simulations category](https://discourse.julialang.org/t/about-the-modelling-simulations-category/284)

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**Author:** [@vchuravy](https://discourse.julialang.org/u/vchuravy)\
**Replies:** 1\
**Last updated:** [November 26, 2016, 12:51pm UTC](https://discourse.julialang.org/t/about-the-modelling-simulations-category/284 "2016-11-26T12:51:51Z")

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(Replace this first paragraph with a brief description of your new category. This guidance will appear in the category selection area, so try to keep it below 200 characters. Until you edit this description or create top…

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## [How to compose discrete-time static and dynamic models for systems simulation similar to Simulink?](https://discourse.julialang.org/t/how-to-compose-discrete-time-static-and-dynamic-models-for-systems-simulation-similar-to-simulink/74858)

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**Author:** [@JanKap](https://discourse.julialang.org/u/JanKap)\
**Replies:** 9\
**Last updated:** [September 11, 2026, 6:19pm UTC](https://discourse.julialang.org/t/how-to-compose-discrete-time-static-and-dynamic-models-for-systems-simulation-similar-to-simulink/74858 "2026-09-11T18:19:36Z")

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Hi all, I’m searching for a Simulink replacement. The main purpose is to have a simplified system simulation for quick prototyping. The domain is mechanical/electrical Usually, I have 4 kinds of models/subsystems Sen…

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## [Mixing time-discrete and time-continuous systems in a simulation](https://discourse.julialang.org/t/mixing-time-discrete-and-time-continuous-systems-in-a-simulation/74277)

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**Author:** [@tqml](https://discourse.julialang.org/u/tqml)\
**Replies:** 22\
**Last updated:** [September 11, 2026, 5:23pm UTC](https://discourse.julialang.org/t/mixing-time-discrete-and-time-continuous-systems-in-a-simulation/74277 "2026-09-11T17:23:15Z")

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Hello all! Does anyone what’s currently the best way in the Julia ecosystem to mix time-continuous and time-discrete systems in one simulation? (e.g. I want to simulate a continuous system regulated by time-discrete con…

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## [Pvlib for Julia?](https://discourse.julialang.org/t/pvlib-for-julia/118952)

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**Author:** [@langestefan](https://discourse.julialang.org/u/langestefan)\
**Replies:** 19\
**Last updated:** [September 10, 2026, 2:47pm UTC](https://discourse.julialang.org/t/pvlib-for-julia/118952 "2026-09-10T14:47:10Z")

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Hi, I have been happily using Julia for a year or so now after being a python user for many more years. The only thing I am really missing is a substitute for pvlib. pvlib is a popular toolbox for photovoltaic systems …

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## [How to use a convolution integral with NeuralPDE.discretize](https://discourse.julialang.org/t/how-to-use-a-convolution-integral-with-neuralpde-discretize/137997)

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**Author:** [@baez-rafael](https://discourse.julialang.org/u/baez-rafael)\
**Replies:** 8\
**Last updated:** [September 9, 2026, 9:53am UTC](https://discourse.julialang.org/t/how-to-use-a-convolution-integral-with-neuralpde-discretize/137997 "2026-09-09T09:53:13Z")

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Hello everyone! I am trying to include the convolution integral \\int\_0^tKr(t -\\tau)\\dot x(\\tau)d\\tau in my equation and I keep coming up with this error when trying to do NeuralPDE.discretize ERROR: KeyError: key :ta…

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## [Simulink alternative in Julia for control engineering](https://discourse.julialang.org/t/simulink-alternative-in-julia-for-control-engineering/131961)

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**Author:** [@ohmsweetohm1](https://discourse.julialang.org/u/ohmsweetohm1)\
**Replies:** 32\
**Last updated:** [September 8, 2026, 7:05pm UTC](https://discourse.julialang.org/t/simulink-alternative-in-julia-for-control-engineering/131961 "2026-09-08T19:05:27Z")

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In my group, many use Matlab and Simulink for control engineering. It is very easy for them since this his how you learn control engineering, by block diagrams. Regardless of the user interface, what options would I have…

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## [Does MTK nominal metadata for variables set abstol when solving ODE](https://discourse.julialang.org/t/does-mtk-nominal-metadata-for-variables-set-abstol-when-solving-ode/139123)

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**Author:** [@mark.garnett](https://discourse.julialang.org/u/mark.garnett)\
**Replies:** 4\
**Last updated:** [September 4, 2026, 1:33pm UTC](https://discourse.julialang.org/t/does-mtk-nominal-metadata-for-variables-set-abstol-when-solving-ode/139123 "2026-09-04T13:33:06Z")

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when you define a variables @variables begin x(t), \[nominal=1e6\] y(t), \[nominal=1e-3\] end will it essentially do the equivalent of setting abstol to a vector of absolute tolerances rather than a scalar, by multiplying …

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## [Manually update operator in a SplitODEProblem](https://discourse.julialang.org/t/manually-update-operator-in-a-splitodeproblem/139138)

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**Author:** [@homocarbonis](https://discourse.julialang.org/u/homocarbonis)\
**Replies:** 1\
**Last updated:** [September 1, 2026, 3:00pm UTC](https://discourse.julialang.org/t/manually-update-operator-in-a-splitodeproblem/139138 "2026-09-01T15:00:46Z")

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Hello, I have some library code which constructs a SplitODEProblem with a DiagonalOperator as f1, and I’m trying to write a function to remake the problem with problem.f.f1.A updated according to new parameter values. I…

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## [What's all the current development of MTK currently going on related to?](https://discourse.julialang.org/t/whats-all-the-current-development-of-mtk-currently-going-on-related-to/138286)

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**Author:** [@mark.garnett](https://discourse.julialang.org/u/mark.garnett)\
**Replies:** 30\
**Last updated:** [August 20, 2026, 3:30pm UTC](https://discourse.julialang.org/t/whats-all-the-current-development-of-mtk-currently-going-on-related-to/138286 "2026-08-20T15:30:40Z")

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I’m seeing a whole lot of commits and minor version increases the past week or two. It seems like a lot of it is internal reworking / refactoring. Just curious to know what it’s all about

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## [Modelling fiber lasers in Julia as a BVDAE problem](https://discourse.julialang.org/t/modelling-fiber-lasers-in-julia-as-a-bvdae-problem/138925)

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**Author:** [@mich](https://discourse.julialang.org/u/mich)\
**Replies:** 0\
**Last updated:** [August 19, 2026, 10:16am UTC](https://discourse.julialang.org/t/modelling-fiber-lasers-in-julia-as-a-bvdae-problem/138925 "2026-08-19T10:16:50Z")

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Hello, I am trying to model a fiber laser using Julia. I’ve managed to simulate an amplifier which is an initial value DAE problem, now i need to extend the code to a BVDAE problem to handle different laser resonator ar…

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## [Understanding jump model preparation cost](https://discourse.julialang.org/t/understanding-jump-model-preparation-cost/138472)

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**Author:** [@lmshk](https://discourse.julialang.org/u/lmshk)\
**Replies:** 18\
**Last updated:** [August 18, 2026, 3:57pm UTC](https://discourse.julialang.org/t/understanding-jump-model-preparation-cost/138472 "2026-08-18T15:57:19Z")

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I am using Catalyst.jl to make JumpProblems of around 160k species and a fairly sparse interaction structure that includes what JumpProcesses.jl calls “constant-rate jump” (CRJ) reactions. This works, but JumpProblem con…

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## [Likelihood model in Bayesian NODE](https://discourse.julialang.org/t/likelihood-model-in-bayesian-node/138664)

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**Author:** [@slwu89](https://discourse.julialang.org/u/slwu89)\
**Replies:** 2\
**Last updated:** [August 10, 2026, 10:39pm UTC](https://discourse.julialang.org/t/likelihood-model-in-bayesian-node/138664 "2026-08-10T22:39:21Z")

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Hi all, I’m reading Uncertainty Quantified Deep Bayesian Model Discovery · Overview of Julia's SciML. to play around with Bayesian NODEs. I noticed the loss function which actually gets passed to AdvancedHMC’s samplers i…

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## [Online hackathon (Aug 2026): composing differentiable simulation pipelines](https://discourse.julialang.org/t/online-hackathon-aug-2026-composing-differentiable-simulation-pipelines/138669)

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**Author:** [@dionhaefner](https://discourse.julialang.org/u/dionhaefner)\
**Replies:** 2\
**Last updated:** [August 8, 2026, 9:08am UTC](https://discourse.julialang.org/t/online-hackathon-aug-2026-composing-differentiable-simulation-pipelines/138669 "2026-08-08T09:08:11Z")

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Hi there! I work at Pasteur Labs on Tesseract, an open-source tool for wrapping simulation components so they compose into end-to-end differentiable pipelines (w/ some existing Julia examples via juliacall). We’re runni…

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## [DynamicalSystems.jl - news, updates, and announcements](https://discourse.julialang.org/t/dynamicalsystems-jl-news-updates-and-announcements/122079)

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**Author:** [@Datseris](https://discourse.julialang.org/u/Datseris)\
**Replies:** 11\
**Last updated:** [August 5, 2026, 7:25am UTC](https://discourse.julialang.org/t/dynamicalsystems-jl-news-updates-and-announcements/122079 "2026-08-05T07:25:36Z")

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Hello everyone, this Topic will serve as a “mega-thread” sorts of post where all updates regarding the DynamicalSystems.jl library are posted. The updates here will range from noteworthy new features to potentially brea…

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## [Warning in SciMLBase about ContinousClock](https://discourse.julialang.org/t/warning-in-scimlbase-about-continousclock/138534)

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**Author:** [@c-nicomar](https://discourse.julialang.org/u/c-nicomar)\
**Replies:** 5\
**Last updated:** [July 31, 2026, 5:57pm UTC](https://discourse.julialang.org/t/warning-in-scimlbase-about-continousclock/138534 "2026-07-31T17:57:42Z")

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During precompilation I get this warning. How could I avoid this? I have this version v2.155.1 SciMLBase │ ┌ Warning: the bare singleton variant syntax \`ContinuousClock\` is deprecated, write \`ContinuousClock()\` instead…

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## [How good is the support for hybrid system modelling in MTK?](https://discourse.julialang.org/t/how-good-is-the-support-for-hybrid-system-modelling-in-mtk/138499)

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**Author:** [@ufechner7](https://discourse.julialang.org/u/ufechner7)\
**Replies:** 4\
**Last updated:** [July 28, 2026, 12:42pm UTC](https://discourse.julialang.org/t/how-good-is-the-support-for-hybrid-system-modelling-in-mtk/138499 "2026-07-28T12:42:46Z")

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How good is the support of modeling hybrid systems with MTK in the summer of 2026? It used to be incomplete/ buggy for a long time. Did this change? I am looking at the control of electrical machines. The controller ca…

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## [Can Symbolics/MTK split the generated RHS into multiple smaller functions?](https://discourse.julialang.org/t/can-symbolics-mtk-split-the-generated-rhs-into-multiple-smaller-functions/138469)

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**Author:** [@Bart\_van\_de\_Lint](https://discourse.julialang.org/u/Bart_van_de_Lint)\
**Replies:** 1\
**Last updated:** [July 26, 2026, 7:39pm UTC](https://discourse.julialang.org/t/can-symbolics-mtk-split-the-generated-rhs-into-multiple-smaller-functions/138469 "2026-07-26T19:39:39Z")

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I have a large ModelingToolkit model. mtkcompile inlines ~32k observed equations into a single big RHS function in around 60 seconds, and first-call compilation of that one function takes \>1000 s (dominated by type infer…

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## [Spiking Neural Networks](https://discourse.julialang.org/t/spiking-neural-networks/63270)

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**Author:** [@mefob](https://discourse.julialang.org/u/mefob)\
**Replies:** 33\
**Last updated:** [July 9, 2026, 4:32pm UTC](https://discourse.julialang.org/t/spiking-neural-networks/63270 "2026-07-09T16:32:19Z")

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What is the state of the art for modeling spiking neural networks in Julia? I haven’t found much recent work: Spiking Neural Network (Discussion from 2019 on implementing a SNN as an ODE) GitHub - AStupidBear/Spikin…

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## [ModelingToolkit.jl / Multibody.jl with interpolated joint properties](https://discourse.julialang.org/t/modelingtoolkit-jl-multibody-jl-with-interpolated-joint-properties/133754)

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**Author:** [@Jskacines](https://discourse.julialang.org/u/Jskacines)\
**Replies:** 11\
**Last updated:** [July 8, 2026, 2:52pm UTC](https://discourse.julialang.org/t/modelingtoolkit-jl-multibody-jl-with-interpolated-joint-properties/133754 "2026-07-08T14:52:43Z")

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Hello Julia community, I am working on a multibody dynamics simulation, and was looking to use ModelingToolkit.jl along with Multibody.jl The specific problem I am working on involves an externally solved mapping of jo…

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## [Initialization in ModelingToolkit](https://discourse.julialang.org/t/initialization-in-modelingtoolkit/137983)

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**Author:** [@BLI](https://discourse.julialang.org/u/BLI)\
**Replies:** 0\
**Last updated:** [July 7, 2026, 12:17pm UTC](https://discourse.julialang.org/t/initialization-in-modelingtoolkit/137983 "2026-07-07T12:17:13Z")

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Questions, etc. If any of the functionality discussed below already exists, please let me know! Does anyone else find the ideas useful? Or am I the only one that struggles with initialization? If anyone with better prog…

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## [How to use Multibody.jl?](https://discourse.julialang.org/t/how-to-use-multibody-jl/134354)

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**Author:** [@SietseAchterop](https://discourse.julialang.org/u/SietseAchterop)\
**Replies:** 8\
**Last updated:** [July 3, 2026, 12:16pm UTC](https://discourse.julialang.org/t/how-to-use-multibody-jl/134354 "2026-07-03T12:16:15Z")

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Hello all, I am new to Julia, now on version 1.12.2 on Linux/Debian version 13.2. Played around with it and with Dyad. Now trying to use the Pendulum example from Multibody.jl I can do add and then using ModelingTool…

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## [Boundary Condition Error in MethodOfLines](https://discourse.julialang.org/t/boundary-condition-error-in-methodoflines/137819)

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**Author:** [@ducksoverip](https://discourse.julialang.org/u/ducksoverip)\
**Replies:** 2\
**Last updated:** [June 29, 2026, 12:53pm UTC](https://discourse.julialang.org/t/boundary-condition-error-in-methodoflines/137819 "2026-06-29T12:53:59Z")

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I’m trying to use MethodOfLines to solve the time-independent Schrodinger equation for hydrogen, as a prelude to more complicated systems. The equation is as follows: \\nabla^2 \\psi(\\vec{r}) - \\frac{1}{r}\\psi(\\vec{r}) =…

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## [Sensitivity Analysis on ODE system](https://discourse.julialang.org/t/sensitivity-analysis-on-ode-system/137757)

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**Author:** [@violabini](https://discourse.julialang.org/u/violabini)\
**Replies:** 4\
**Last updated:** [June 25, 2026, 9:40am UTC](https://discourse.julialang.org/t/sensitivity-analysis-on-ode-system/137757 "2026-06-25T09:40:40Z")

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Hi everyone, I am trying to use automatic differentiation in julia to compute the Jacobian of my outputs wrt my design variables for local sensitivity, but my ODEs are nearly coupled as for example du\[3\]=A\*du\[2\]+B u\[1\].…

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## [How to Create Global Parameters in ModelingToolkit.jl](https://discourse.julialang.org/t/how-to-create-global-parameters-in-modelingtoolkit-jl/60065)

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**Author:** [@Brad\_Carman](https://discourse.julialang.org/u/Brad_Carman)\
**Replies:** 6\
**Last updated:** [June 25, 2026, 9:37am UTC](https://discourse.julialang.org/t/how-to-create-global-parameters-in-modelingtoolkit-jl/60065 "2026-06-25T09:37:00Z")

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Is it possible to create global parameters? For example, when modeling a gas or fluid, it would be convenient to define them globally, similar to how is done in SimScape with a domain definition. I attempted to create …

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## [Solver differences between sim!(::SimModel) and sim!(::PredictiveController) in ModelPredictiveControl.jl](https://discourse.julialang.org/t/solver-differences-between-sim-simmodel-and-sim-predictivecontroller-in-modelpredictivecontrol-jl/120819)

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**Author:** [@Bart\_van\_de\_Lint](https://discourse.julialang.org/u/Bart_van_de_Lint)\
**Replies:** 19\
**Last updated:** [June 24, 2026, 5:15pm UTC](https://discourse.julialang.org/t/solver-differences-between-sim-simmodel-and-sim-predictivecontroller-in-modelpredictivecontrol-jl/120819 "2026-06-24T17:15:12Z")

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I am trying to use NMPC in ModelPredictiveControl.jl, but while my model runs with sim!(model) it gets a boundserror with sim!(nmpc). To try to find out what is going on, I tried to run the two sim! functions on a simpl…

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## [Dyad 3.0 slower Transient Analysis](https://discourse.julialang.org/t/dyad-3-0-slower-transient-analysis/137148)

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**Author:** [@gruemelmonster](https://discourse.julialang.org/u/gruemelmonster)\
**Replies:** 12\
**Last updated:** [June 19, 2026, 5:19pm UTC](https://discourse.julialang.org/t/dyad-3-0-slower-transient-analysis/137148 "2026-06-19T17:19:26Z")

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Hi I just got greeted in VSCode with a message that made me update to Dyad 3.0… After using the AI to update my models I now realized that the TransientAnalysis is now much slower that before. (5min instead of 3min mayb…

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## [Update initialization guesses on an existing problem without rebuilding?](https://discourse.julialang.org/t/update-initialization-guesses-on-an-existing-problem-without-rebuilding/137694)

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**Author:** [@Bart\_van\_de\_Lint](https://discourse.julialang.org/u/Bart_van_de_Lint)\
**Replies:** 0\
**Last updated:** [June 19, 2026, 11:06am UTC](https://discourse.julialang.org/t/update-initialization-guesses-on-an-existing-problem-without-rebuilding/137694 "2026-06-19T11:06:55Z")

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I have a DAE built with ModelingToolkit. Between solves I re-initialize from a new operating point. For variables that have an initial constraint\*I can update them in place via the Initial parameter: prob.ps\[Initial(x)\]…

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## [Invalid Shapes error on creating ODEProblem from MTK system](https://discourse.julialang.org/t/invalid-shapes-error-on-creating-odeproblem-from-mtk-system/137671)

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**Author:** [@mark.garnett](https://discourse.julialang.org/u/mark.garnett)\
**Replies:** 2\
**Last updated:** [June 18, 2026, 8:00pm UTC](https://discourse.julialang.org/t/invalid-shapes-error-on-creating-odeproblem-from-mtk-system/137671 "2026-06-18T20:00:21Z")

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I have been running into this problem in many situations and am at a loss to figure it out. It always seems to occur with ifelse() statements, abs(), and the like. Unfortunately I have to have a lot of these types of sta…

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## [Gridap use for finite element modelling for electrochemical electrode modelling](https://discourse.julialang.org/t/gridap-use-for-finite-element-modelling-for-electrochemical-electrode-modelling/135377)

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**Author:** [@algoking](https://discourse.julialang.org/u/algoking)\
**Replies:** 1\
**Last updated:** [June 17, 2026, 12:35pm UTC](https://discourse.julialang.org/t/gridap-use-for-finite-element-modelling-for-electrochemical-electrode-modelling/135377 "2026-06-17T12:35:11Z")

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Hi everyone, I am currently transitioning our lab’s electrochemical modeling workflow from COMSOL to Gridap.jl. My goal is to move away from proprietary licenses and leverage the flexibility of the Julia ecosystem. The…

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## [Projection of Gradient of Tensor object](https://discourse.julialang.org/t/projection-of-gradient-of-tensor-object/135467)

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**Author:** [@Ankan\_Man](https://discourse.julialang.org/u/Ankan_Man)\
**Replies:** 3\
**Last updated:** [June 17, 2026, 12:31pm UTC](https://discourse.julialang.org/t/projection-of-gradient-of-tensor-object/135467 "2026-06-17T12:31:26Z")

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Hi all. I am having an issue with projecting the derivative of a tensor on a sphere. This is the term in the weak form. Let P be the projector ( P = I - nxn). How to properly do the projection. Please help term = ∇(q) ⊙…

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