- Comsol negative concentration Select the check box for the species to specify the concentration, and enter a value or expression in the corresponding field. Mar 9, 2019 · At the time to run the model just with the application of Transport of diluted species, applying the semi-empirical kinetic model, the model works properly and the concentrations are ok. How can I avoid negative concentrations at convection and diffusion problems in Comsol 4? Best regards Kostas . I'm modeling creeping flow and transport of dilute species through a series of ribs, with an electrochemical reaction occuring at 10 mA/sqcm on the bottom surface of the under-rib flow. A common reason is numerical noise: For example, when the variable for concentration approaches zero, the numerical noise may become significant in comparison to the value (which is close to zero). EDT Fluid & Heat, Computational Fluid Dynamics (CFD), Chemical Reaction Engineering, Studies & Solvers Version 4. Romani Patel Note that while COMSOL employees may participate in the discussion forum, If you notice negative concentrations of very small magnitude, numerical noise is probably the cause and does not affect the problem much in diffusion/convection problems without reactions. The concentration of species i is denoted c i (SI unit: mol/m 3). EDT Fluid & Heat, Battery Design, Chemical Reaction Engineering, Results & Visualization, Studies & Solvers 1 Reply . Reacting Domains: Use Danckwerts inflow type The resulting modified inflow concentrations can be seen as the stationary solution for a batch reactor with the user inflow concentrations as initial concentrations. So when S becomes negative, in the next time step it is seen as a 0 instead of a Why do I get negative values of the concentration in my diffusion, convection and reaction model? Why do I see values that go below, or above, their lowest, or highest, possible values based With your B concentration going negative, you should have a negative reaction rate, which should cause the generation of species A instead of consumption. dC is a constant that defines the smoothing interval width. I Mar 9, 2019 · At the time to run the model just with the application of Transport of diluted species, applying the semi-empirical kinetic model, the model works properly and the concentrations are ok. 4, I've tried a few tricks to get rid of the negative concentrations by having a smoothed step function boundary condition, doing a logarithmic transform of Jun 10, 2020 · I am trying a time-dependent sweep from 0 to 1 s in 0. Send Private Message Flag post as spam. To use another boundary condition for a specific species, click to clear the check box for the concentration of that species. In a pure convection-diffusion problem, the scale of the concentration does not matter, so in order to avoid the problem, you can add an arbitrary baseline concentration to keep the resulting overall concentration above zero. In practice, a reaction seldom involves more than two species colliding in a reacting step, which means that a kinetic Oct 17, 2019 · There is a zone of negative concentration above the surface where the flux is set; The concentration on the surface where the flux is applied has not a uniform concentration. Please login with a confirmed email address before reporting spam Hello, How to avoid Note that while COMSOL employees may participate in the discussion forum, Jul 2, 2018 · If you get negative concentration in your COMSOL Model, there are two options: 1. When simulating with the SCD+TDS with Butler-Volmer kinetics, I am getting unrealistic concentrations (both negative and unrealistically large positive concentrations), even at t = 0 right at the electrode boundaries. Select the check box for the Species to specify the concentration, and then enter a value or expression in the corresponding field. 1 day), but when bacteria start to grow considerably, the reaction terms become very big and negative (consumption), and thus the concentrations of the solutes become also negative. I have faced the same problem with convective diffusion. I am trying to simulate a geomety which contains a needle- sphere electrode with different dielectric mediums(Air, tranformer oil with and without nano particle). There is a similar example from comsol model library. If you do not hold an on-subscription license, you may find an answer in another Discussion or in the Knowledge Base. g. With an initial value that is zero along a boundary for a convective transport models, for example, the physical interpretation is an initially sharp, gradually diffusing front moving away from the boundary. I used swept mesh across the channel with at least 10 elements and geometric distribution so that at the channel boundary the mesh was at Note: This discussion is about an older version of the COMSOL Multiphysics To start a new discussion with a link back to this one, click here. But your A contour plot looks perfectly normal. I used the standard mesh from comsol and two custom quad meshes different accuracy. You can define it on the Options>Constants menu. Start with an enough large value so that negative values do not occur. Posted 2016/09/28 22:37 GMT-4 2 Replies . 2. 2013, 13:19 GMT-4 Fluid & Heat, Computational Feb 2, 2012 · While using chemical reaction engineering physics, I was wondering if there is a function or syntax to prevent the concentration of species to become negative. Reacting Domains: Use Danckwerts inflow type Dec 2, 2011 · The negative concentrations I'm obtaining are large enough to invalidate my model entirely. 4, I've tried a few tricks to get rid of the negative concentrations by having a smoothed step function boundary condition, doing a logarithmic transform of Oct 30, 2013 · Vanadium Redox Flow Battery, Model ID: 14153, negative Concentrations. What did I try to resolve this: Change the mesh. COMSOL makes every reasonable effort to verify the information you view on this page. I have a doubt in giving the initial values for different ionic concentrations and electric potential values while using the Transport of Dilute species and Electrostatics physics. The simulation is time dependent and I have used Darcy's law and solute transport in order to apply governing equations. (I put the file at the bottom of this message) The thing is, I obtain slightly negative concentrations (when plotted in linear) and when I plot the concentration on a logaithmic scale (log(c/c0)) the profile goes crazy! Use negative values for reactants and positive values for products in the modeled reaction. Posted Oct 30, 2013, 10:16 a. Hello Konstantinos May 9, 2016 · At the start, concentration is zero everywhere except at some boundaries of my geometry. Reacting Domains: Use Danckwerts inflow type. Further Reading Hi The problem of negative concentrations has appeared on this forum repeatedly, and there is no easy way out. (I put the file at the bottom of this message) The thing is, I obtain slightly negative concentrations (when plotted in linear) and when I plot the concentration on a logaithmic scale (log(c/c0)) the profile goes Hi The problem of negative concentrations has appeared on this forum repeatedly, and there is no easy way out. Posted 2010年7月20日 GMT-4 08:35 0 Replies . However, I got negative concentration and the larger value beyond physic in simulation results. COMSOL Moderator. Posted 2014年10月5日 GMT-4 03:36 Fluid & Heat, Computational Fluid Dynamics (CFD), Microfluidics 2 Replies . (which is equal to the concentration of C on the surface going over the initial concentration of B). Initial negative concentrations Posted May 10, 2013, 1:19 p. 0 Replies Last Post 2010年7月20日 GMT-4 08:35. Please login with a confirmed email address before reporting spam Hello, How to avoid Note that while COMSOL employees may participate in the discussion forum, COMSOL Initial negative concentrations. Jun 27, 2012 · I'm working on a vapor intrusion model in which the contaminant is moving in soil. Dec 2, 2011 · The negative concentrations I'm obtaining are large enough to invalidate my model entirely. Mar 13, 2010 · Hello All - I use COMSOL in the context of groundwater (Earth Sciences Module). If you notice negative concentrations of very small magnitude, numerical noise is probably the cause and does not affect the problem much in diffusion/convection problems without reactions. I've tackled this by deffinig variables s=max (eps^2,S), were S is the concentration of the solute. Check if you have consistent initial and boundary conditions. A c is also required to be negative. Posted 2013年5月10日 GMT-4 13:19 Fluid & Heat, Computational Fluid Dynamics (CFD), Chemical Reaction Engineering, Studies & Solvers Version 4. Hi everyone! I am modelling a microfluidic device. Oct 5, 2014 · Negative concentration values in Transport of Diluted Species Study. playing with eps or a few times eps is one way, Discussion Closed This discussion was created more than 6 months ago and has been closed. In addition, the domain reaction rate degree of freedom of the equilibrium reaction Dec 2, 2015 · When running the Transport of Diluted Species and Creeping Flow physics in Comsol 4. Mar 2, 2020 · Dear All Greetings of the day. Mar 20, 2013 · This works just fine for short periods of time (e. 2. Shehan Haputhanthri Note that while COMSOL employees may participate in Jul 20, 2010 · How to avoid negative concentrations in Comsol 4. The species can diffuse into my geometry. Howevere, when I add the transport of species in a porous bed (hydride bed), applying Darcy's law, the model provide negative values of the concentration. This can be avoided by making sure that your reaction rate If you notice negative concentrations of very small magnitude, numerical noise is probably the cause and does not affect the problem much in diffusion/convection problems without reactions. The most common reason for negative concentrations is numerical noise: when the species concentration approaches zero, the numerical noise becomes significant in I have run across a problem where I am representing, instead of a point source of pollution, a quadralateral geometry (2D) pollutant source (constant concentration source). Species with a stoichiometric coefficient value of 0 are not affected by the Equilibrium Reaction node. Note that while COMSOL employees may participate in the discussion forum, I'm working on a vapor intrusion model in which the contaminant is moving in soil. When simulating with the SCD+TDS with Butler-Volmer kinetics, I am getting unrealistic concentrations (both negative and unrealistically large positive concentrations), even at t Jul 1, 2015 · If you are seeing negative concentrations of very small magnitude, numerical noise is probably the cause. Nov 8, 2015 · HI all, I want to get the concentration values of particles in oil,and i am tyring to use the models of Transport of Dilutes Species（tds） and Turbulent Two-Phase Flow，Level Set(tpf) . However, with Jun 6, 2012 · I am a totally new user on comsol. I am relatively new to COMSOL so I was modeling a simplistic diffusion system (no flux, 0 initial concentration on the domain of interest). If you still need help with COMSOL and have an on-subscription license, please visit our Support Center for help. Krish . I am not sure what function or constraint can be used in 4. I use the laminar flow and Transport of dilluted species. Somebodies say this problem can be solved by seting up logarithmic transform of concentration. Posted 2016年9月28日 GMT-4 22:37 2 Replies . I used swept mesh across the channel with at least 10 elements and geometric distribution so that at the channel boundary the mesh was at • Discontinuities in the model isanother source of, for example, smallnegativeconcentrationsdue to a discontinuous initial value. 01 s steps. Jun 12, 2020 · Hello Kelsey Levine Your Discussion has gone 30 days without a reply. Posted 10. I'm currently exploring the possibility of using a conditional statement to set these concentrations tending towards zero, exactly as zero (before they start to fluctuate around zero and start to go negative). Your physical model is not correct. Jul 26, 2017 · The problem with this model is that the concentration of B on the surface can reach negative concentration. I've tackled this by deffinig variables s=max(eps^2,S), were S is the concentration of the solute. the boundary condition is: N = kc*(Csurface-Cair) with smoothing function N = flc2hs(Cair-Csurface,dC)*kc*(Csurface-Cair) where the variable dC is a constant that defines the smoothing interval width. The initial Nov 14, 2014 · Hi everyone! I am modelling a microfluidic device. cs_C3H6>0") works to prevent the concentration from becoming negative. The initial concentration is 10^(-12)mol/m^3, but there are negative values in the calculation results，and the negative values is about -10^(-12)mol/m^3. To start a new discussion with a link back to this one, click here. 05. COMSOL is constantly computing negative Specify the concentration c i (SI unit: mol/m 3) for each species individually. I used swept mesh across the channel with at least 10 elements and geometric distribution so that at the channel boundary the mesh was at Dec 13, 2023 · I am working on a project where I have to model the drug concentrations through a porous media from a solid body designed in solidworks. Dec 2, 2015 · When running the Transport of Diluted Species and Creeping Flow physics in Comsol 4. Jun 10, 2020 · I am trying a time-dependent sweep from 0 to 1 s in 0. Inside of my geometry, there is a small region that has a negative reaction rate (consumes my species). I almost got rid of it with better meshing in the Individually specify the concentration for each species. I am trying a time-dependent sweep from 0 to 1 s in 0. For example, results for anelastoplasticmaterial are correct (within some tolerance) at the integration points (Gauss points) inside the finite elements, but values might become unphysical when extrapolating the data to the element boundaries. Any advise If you notice negative concentrations of very small magnitude, numerical noise is probably the cause and does not affect the problem much in diffusion/convection problems without reactions. The problem is now that, in and around this region, I get negative concentrations. even though i have set a number of maximum available sites on the surface, which is the same number as initial reactant B on this surface. Sep 28, 2016 · Negative Concentrations. The following expression describes a charge transfer reaction according to the full Butler-Volmer equation, where the anodic and cathodic terms of the current density expression depend on the local concentrations of the electroactive species at the electrode surface: Here, and denote the forward and reverse rate constants, respectively. The initial concentration of the dilute species is 6. 25 mM. The stoichiometric coefficients are denoted ν ij, and are defined to be negative for reactants and positive for products. After computing the model, I have negative values for concentration somewhere, that they are not ignorable. 5, the syntax ("mod1. The negative concentrations I'm obtaining are large enough to invalidate my model entirely. How do I avoid this? Jan 23, 2011 · I have been trying to avoid negative concentration by incorporating logarithmic transform of concentration. Now I try to simulate the mixing in the microchannel without reactions. Concentration Dependent Kinetics. . I used swept mesh across the channel with at least 10 elements and geometric distribution so that at the channel boundary the mesh was at Hi all, new to COMSOL. m. I have run across a problem where I am representing, instead of a point source of pollution, a quadralateral geometry (2D) pollutant source (constant concentration source). I found that in 3. Reacting Domains: Use Danckwerts inflow type Sep 28, 2016 · Negative Concentrations. • Interpolation and extrapolation of values can cause some values to becomeunphysical. Oct 17, 2019 · There is a zone of negative concentration above the surface where the flux is set; The concentration on the surface where the flux is applied has not a uniform concentration. Initial negative concentrations. There's an initial concentration of reagent in the chamber and I apply a flow with nearly 0 concentration at one side. I'm currently exploring the possibility of using a conditional statement to set these concentrations tending towards zero, If you still need help with COMSOL and have an on-subscription license, please visit our Support Center for help. 3a 2 Replies Romani Patel If you notice negative concentrations of very small magnitude, numerical noise is probably the cause and does not affect the problem much in diffusion/convection problems without reactions. But how to do that in the Maybe you got this values just in Excel through interpolation by exporting it from COMSOL? If you get negative concentration in your COMSOL Model, there are two options: 1. needs COMSOL developers confirmation though) catches these and sort them out to avoid tests during the number crunching section. To use another boundary condition for a specific species, click to clear the check box for that species’ mass fraction. When the concentration reaches zero, the reaction term continues to consume the species, finally resulting in negative concentration. When simulating with the SCD+TDS with Butler-Volmer kinetics, I am getting unrealistic concentrations (both negative and unrealistically large positive concentrations), even at t Feb 20, 2019 · Hi all, new to COMSOL. Mar 17, 2010 · for my case, because I have a negative concentration which I guess it is because of the boundary conditions. If you notice negative concentrations of very small magnitude, numerical noise is probably the cause See more However, I got negative concentration and the larger value beyond physic in simulation results. Mar 5, 2015 · Hi The problem of negative concentrations has appeared on this forum repeatedly, and there is no easy way out. I almost got rid of it with better meshing in the channels. 3a 2 Replies . irietpf zpf okrig hgtbc totron fmjsde mrskye nql lbvb tlggqq vxla dgwgv xntyxx epqkej pxnti