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Gas mixture flow simulation

May 2, 2016, 10:20 pm
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Hi everybody,

Gas 1 is flowing in my geometry (a round pipe) and I want to inject gas 2 into the flow.

My idea was to assign both gases to the same geometry (using pipe flow module twice) and then use the transport of concentrated species module to adjust the molar fraction of gases in a way to tell COMSOL that initially there was just gas 1 and then an x molar fraction of gas 2 is injected into the flow.

My question is: Is this a right way to deal with the problem and how to tell COMSOL to assign different materials (gas 1 or gas 2) to the same domain dependent on the physics?

Thanks.
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Matrix has zero on diagonal

May 3, 2016, 5:29 am
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I have to solve a lid driven cavity (laminar flow) + melting problem (heat transfer in fluids and solids with moving mesh). I was able to successfully implement this in 2D.

I have now shifted to a 3D model, still solving the same physics. I get an distinct ( i haven't encountered this previously) error message which I’m unable to understand :
Failed to find a solution for the initial parameter.
Matrix has zero on diagonal for the SSOR update submatrix in the Vanka algorithm.
Returned solution is not converged.

In the Comsol reference manual, explanation for matrix equation for vanka algorithm is attached in the picture

Is the diagonal element (Sj) = 0? This S corresponds to the average values of the velocity vectors?
My initial condition is set to zero.

I have also attached my model file.

Kindly help
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Thermal resistance in non-isothermal pipe flow (nipfl)

May 3, 2016, 5:30 am
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Gooday,

I'm using non-isothermal pipe flow model and heat transfer in solids physics in order to simulate a bore hole heat exchanger (BHE). the problem consist of a tube modeling by nipfl and a ground around the tube modeling as a solid heat transfer. The program seems to simulate the problem in a reasonable way but when the thermal resistance (R) is calculated, the obtained R by Comsol is higher almost 30 % compared with R obtained by formula or 2D model. I guess since this module simulate the pipe flow as a 1D problem and there is a gap between the pipe and solid which in graphics shows as a solid part, it adds the value of thermal resistance. Can anyone help or has a suggestion?

In attachment you can find the file.

kind regards,
Amin
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Parachute CFD simulation

May 3, 2016, 10:51 pm
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Hello,

I am trying to perform a CFD flow field simulation of a 3D parachute to find its lift and drag. The geometry model is imported from a CAD software as surfaces only (no thickness). I attached one picture of how it looks like. In order to make the simulation, I created the computational domain (box) big enough to neglect the boundary effects (I saw in a post from an airfoil SST model that a recommended value of 100 chord-lengths toward all sides is enough). For the physics, I am using the SST turbulence model with inlet (velocity input), outlet (pressure output) and symmetry (box lateral sides) boundary conditions. I used an interior wall boundary condition on the surfaces of the parachute with no slip condition. For the mesh, I have tetahedrals for the domains and prisms for the boundary layer over the parachute surfaces. After running the simulation, I visualized clearly the vortices created in the interior of the parachute. However, when calculating the surface integrals to determine lift and drag (I saw in a comsol blog to use spf.T_stress to calculate lift and drag at the correspondent axis direction), I am getting nonsense values (my drag is on the opposite direction resulting in a propulsive force). For sanity checks, I was also trying to see how the pressure differs on the parachute surfaces from the inside and outside but I only see the same values on both sides.
My questions would be:
1. Is this approach valid for this type of scenario?
2. How would it be possible to find the pressure difference on the inside and outside surfaces of the parachute? (I am not sure if this has something to do on how the interior wall boundary condition works). From my understanding, I thought this internal wall boundary condition would act as a baffle (very thin surface) for the parachute surfaces (which will therefore have both inside and outside surfaces). Please correct me if I am wrong on this.

I would really appreciate some help with this issues. Thanks.
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Problem in 1D EHD simulation

May 4, 2016, 3:01 am
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Dear all,

I have built a 1D EHD in COMSOL. But it does not work. There always exists an error. Can anybody give me any suggestion? Please help me! My *.mph file is in the attachment. The error is as follows

The following feature has encountered a problem:
Failed to find a solution.
Singular matrix.
There are 249511 void equations (empty rows in matrix) for the variable comp1.P.
at coordinates: (2.5949,0), (2.5948,0), (2.59482,0), (2.59484,0), (2.59486,0), ...
There are 1 degrees of freedom that do not occur in any equation (empty columns in matrix) for the variable comp1.H0.
at coordinates: (0,0), ...
Returned solution is not converged.
- Feature: Stationary Solver 1 (sol1/s1)

Thanks a lot!
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Inlet pressure boundary condition not allowing COMSOL to converge

May 5, 2016, 7:19 pm
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Hi,

In my simulation (file attached here), I'm modeling a slight variation of the 2D axi-symmetric 'stirrer in a cylinder' situation, which is on COMSOL model library.
In my variation, there is applied pressure on the top of the cylinder in which the stirrer rotates. Below the cylinder-stirrer setup is a highly impermeable layer (modeled by Brinkmann equation) through which the fluid has to pass.
This is a simulation of a lab setup, so I know that the applied pressure at the top is usually 4 bar. The outlet below the membrane is open to atmosphere (so the fluid has to pass though the cylinder, through the, membrane and then the open outlet) .

However the pressure boundary condition at inlet does not allow comsol to converge.

I've seen this happen even in a simple pipe flow situation where beyond a certain max pressure, COMSOL can't converge.

Please provide me suggestions!
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inertial focusing in microfluidic channel

May 11, 2016, 5:34 pm
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Firstly，I want to modeling inertial focusing in straight and curved microfluidic channels（may the channel geometry seems like more complicated ）,just like
cn.comsol.com/story/download/1...85/Veryst_Mass_Gen_CN2013.pdf;
here is my model scheme:
1、sovle a CFD probelm(steady) without the particle;
2、calculate the force act on the particle induced by the “stress tensor and pressure ”,just like
www.chem.mtu.edu/~fmorriso/cm3...esonSurfacesinComsol5_2015.pdf
(I know there have “drag force” “lift force”etc.,but I CAN NOT find an explict formula of lift force suited complicated channel)
Lift Force(x)=sphint(r,spf.T_stressx); Lift Force(y)=sphint(r,spf.T_stressy);
The sphint(r, expr) operator computes the surface integral of the expression expr on the surface with radius r around the point in which it is evaluated.;r is the radius of the particle.
3、model particle tracing for fluid flow ,and "Force_x" =lift force(x),"Force_y"=lift force(y)

But the force seems like have influence to the particle, as "the particle inserted at a specific location in the straight channel remian at that location"

So， can anyone give some advises？
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Convective heat flux

May 12, 2016, 12:25 am
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Hi,
I'm trying to see the heat conduction from the electric power conduit pipe to the soil surrounding the pipe.
The square part which I attached as model.png is the pipe and inside.
I built two point source inside.
Inside the pipe is just air, so I applied the heat transfer in fluid.
Wanted to apply convective heat flux to the pipe walls, but there was only inward heat flux in my version.
So I just applied the inward heat fluxes to the walls.
However, the result came out like result.png. Even though I applied heat transfer in solid and heat transfer fluid for the analysis, it doesn't read the heat conduction or convection inside the pipe box at all.
Anyone knows the solution? using inward heat flux to realize the convective heat flux or finding the convective heat flux.

Is it just the problem of this version?
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A closed pipe

May 15, 2016, 6:37 pm
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I want to model a pipe has only an outlet, In fact, I changed inlet to wall. How can I define boundary condition for this case?

I attached an image,

TX
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Stopping the flow in a pipe after a certain time

May 16, 2016, 4:22 am
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Hi, I am new to COMSOL and I am simulating the expansion of steam in a a condenser from a cylinder. They are connected by a pipe. The expansion is due to pressure difference. I need to run the simulation for about 1 s.

Is there any way I can block the flow of steam from the cylinder to the condenser after a certain time say about 0.5 s and then simulate just the condenser for the remaining 0.5 s?
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Flow Separation in fluid flow around a sphere

May 17, 2016, 9:37 pm
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I am new to COMSOL MULTIPHYSICS. I want to find out at which Reynolds number the flow separation starts in laminar flow.How can I exactly find the point of separation ?
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Problems with STEP FUNCTION

May 18, 2016, 1:06 pm
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Hi everybody,

I have an inlet in a water reactor working in the following way: 10 s on, 80 s off, 10 s on, 80 s off....

-Fist of all, I have built a PIECEWISE (pw1) function (0<t<10 = 1, 10<t<90 = 0)
-Then, using this piecewise function I have built an ANALYTIC function (an1) with a periodic extension 0-90, and plot parameters 0-900. When I plot it, everything is OK.
- Finally I have created a variable for the inlet velocity: 0.265[m/s]*an1(t[1/s])

When I compute my model, the inlet works only in the first 10 seconds, but after 90 seconds it doesn´t start working agains as an inlet, there is no velocity.

Why????

Thank you so much

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Failed to find consistent initial values

May 20, 2016, 2:54 am
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Dear COMSOL users!
I am trying to model wormhole formation during the acid injection in the reservoir. It is two scale model: Darcy scale and pore scale. I am using 3 physics: Darcy law, transport of diluted species in porous media and Domain ODEs and DAEs. Initial porosity is distributed random: por = por0 + rn(x,y); But when I am doing simulation I have error message "Failed to find consistent initial values.
Last time step is not converged.
- Feature: Time-Dependent Solver 1 (sol1/t1)"

So, I couldn't find where is the problem. Actually problem is associated with porosity.

Can anyone help me to decide this problem? Thank you everyone, in advance!
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Turbulent to Laminar

May 21, 2016, 1:20 am
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Hi,
I want to draw a model in which I wish to simulate whether a turbulent flow converts into laminar flow or not.!
I have made a cylindrical pipe and at a further distance I have divided the pipes into many pipes to lower the reynolds no. I have to provide turbulent flow at inlet and further to see whether the flow achieves laminar flow in the smaller cross-section pipes or not.
How to simulate this model? what physics should I take???
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Unable to evaluate an expression

February 4, 2016, 3:40 am
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Excuse me, I am getting an error as

Attempt to evaluate real square root of negative number.
Function: sqrt
Failed to evaluate expression.
Expression: nojac(min(1,(sqrt((comp1.fsi.h2GNS*((comp1.fsi.rho^2)*(((comp1.u fluid-d(r,TIME))^2)+((comp1.w fluid-d(z,TIME))^2)))))/(comp1.fsi.mu*24))))
Failed to evaluate variable.
Variable: comp1.fsi.xi m, Defined as: nojac(min(1,(sqrt((comp1.fsi.h2GNS*((comp1.fsi.rho^2)*(((comp1.u fluid-d(r,TIME))^2)+((comp1.w fluid-d(z,TIME))^2)))))/(comp1.fsi.mu*24))))
Failed to evaluate variable.
Variable: comp1.fsi.vhNS lin, Defined as: ((0.5*sqrt((max((comp1.fsi.tau m*(((comp1.fsi.res u*comp1.fsi.xi m)^2)+((comp1.fsi.res v*comp1.fsi.xi m)^2))),0)*comp1.fsi.B0YigijtauNorm2)))/(comp1.fsi.B0YigijtauNorm2+eps))
Failed to evaluate expression.
Expression: nojac(((1.4142135623730951^(1-shapeorder(comp1.u fluid)))*max((comp1.fsi.vhNS lin-comp1.fsi.vhNS quad),0)))
Failed to evaluate variable.
Variable: comp1.fsi.vhNS, Defined as: nojac(((1.4142135623730951^(1-shapeorder(comp1.u fluid)))*max((comp1.fsi.vhNS lin-comp1.fsi.vhNS quad),0)))
Failed to evaluate temporary symbolic derivative variable.
Variable: comp1.fsi.crosswindnsVDN${test8}, Defined as: ((comp1.fsi.rho*comp1.fsi.vhNS)*((comp1.u fluidr*(-comp1.fsi.gijrr))-(comp1.u fluidz*comp1.fsi.gijrz)))
Failed to evaluate expression.
Expression: d((2*comp1.fsi.crosswindns*pi*r)*(dvol spatial),{test8})
Last time step is not converged.

Can Any one answer this pls
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difference between Capacitively coupled plasma and AC-DBD

February 11, 2015, 7:38 am
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Hello Everyone !

Can anyone help me in understanding the difference between capacitively coupled plasma (CCP) and an AC-DBD?

I appreciate your effort and help.

Thanks a lot!!

Best regards
Zaid
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Simulation of crack propagation using mixed mode intensity

May 23, 2016, 11:00 am
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Hi all,
I have read 2 following documents that related to crack propagation.

www.comsol.com/model/mixed-mod...of-a-laminated-composite-19961
www.comsol.com/model/single-edge-crack-988

The examples talk about the first mode of crack propagation and they based on symmetric plane. I am thinking that if we can implement a procedure by using mixed mode intensity (K1, K2) a crack propagate in an arbitrary direction. On this condition, do you think that it is possible to simulate a crack propagation in an arbitrary direction with COMSOL? If so, do you have any idea on how to implement the problem? In fact I want to solve a poroelasticity problem which I have a porous media and the pressure is supplying by fluid pumping. But I have some problem in my example and it does not show a proper result.
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Solution does not converge when source term is added?

May 23, 2016, 8:27 pm
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I am trying to solve laminar flow in porous media and vapor domain. The vapor domain is attached to porous media. I use laminar flow in vapor domain and flow in porous matrix in the another domain. The porous media has a source term which is about 0.1 Firstly, I solved this problem without the source term and it is converged. When I add the source term, the solution does not converge.
This is also happened if I used free and porous media physics.

Appreciate your help
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how to use function defined in the global in inlet condition(laminar flow)

May 23, 2016, 10:57 pm
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Hey,
I m working on laminar flow in falling film. I got the laminar profile with some intial conditions. Now i want to use the profile as an inlet condition.Therefore i created one function V using interpolation. I dont how to you use the function in the inlets conditions.Please help me out.
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permeability

May 24, 2016, 10:55 am
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hi i am doing my final work of grade with comsol.
I am doing a reactor with porous membrane and a layer of paladium and i have a lot of doubts about to together the physics and get a flow across of membrane. I want to say, i put de velocity inlet in axial directions and to across of membrane the flow is radial directions and i don´t get it.
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