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Water jet exiting to atmosphere

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I am a 2nd semester UG Mechanical Engineering student and have no background in CFD and COMSOL. I was given a project by my fluid mechanics professor and was sort of stuck

I need to simulate a water jet exiting a nozzle and striking a plate. I need to find the pressure at the points that it strikes the plate and also the volume flow rate of the water exiting the nozzle.

If someone could provide a tutorial similar to my problem, I would really appreciate it.

How to plot time-dependent temperature profile at a point

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Hi all,

I have 1 question over here, i have been using the SAR head tutorial (Provided by comsol tutorial) to do research for my school project and i have 1 problem over here which is, i cant seems to plot temperature VS time graph. i had searched though the internet for solution and i did follow their steps but still i cant manage to get it. whatever i try to plot, the time will varies but the temperature remains the same throughout which makes my curve looks like one horizontal straight line.

I will really appreciate if someone can take a look at the attached picture (i can't seems to upload my model file here) and suggest to me what should i do next.

Thanks,
Boon

PDE General for Navier-Stokes equations

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Hi I am trying to use PDE General module in COMSOL 4.2 to simulate Euler equations. I know that we have a Laminar Flow module than I can put viscosity equal to zero to have Euler equations but I would like to solve it in PDE forms.

I have a question regarding defining different variables. So this is the case if I want to solve PDE general module using conservative forms of Euler equation I will have
U=[rho, rho*u,rho*v]
gamma=[rho*u rho*v
rho*u^2+p rho*u*v
rho*u*v rho*v^2+p]

as you can see I have a pressure term that I am not sure what should I do with it since I cannot express it as my conservative variables unless I define another set of equations for energy to solve Temperature and connect "rho", "T" to "P". Even in this case I need to have a relationship like the one that we have for ideal gas to solve this. Does anyone have any suggestions for my problem?

Bottom line how should I define my variables to solve something like Euler equation or to some extent Navier-Stokes equations with the General PDE module of COMSOL?

Cheers

mixer simulation, CFD

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Dear community,

Can you help me with the following:

Is there Comsol manual which has all the information about the rotating machinery which is an add-on of CFD ?
I have followed the tutorials available but in tutorials they are uploading the geometry and assigning the boundary conditions directly by selecting the predefined options in Comsol which is not possible when we create model from scratch.
Could you please tell me difference between rotating domain, rotating wall and rotating interior wall.
Why there is difference in selecting rotating domain in 2D ( by creating circle around impeller) and 3D ( directly selecting the impeller geometry).

I kindly request you to please help me in this

Centrifugal Force

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Hi,

I would like to include centrifugal force in my COMSOL (version 4.3a) model of a curved channel with laminar flow containing small particles.

Is it necessary for COMSOL to find the first and second time derivatives of the particles moving in x and y directions (to find the velocity and acceleration of these particles) at each point on the particle trajectory as the particles flow round the curve, and plug these values into the equation for the radius of curvature;

R= (x^'2+y^'2 )^(3/2)/(x^' y^''-y^' x^'' )

Since the radius of curvature is changing continuously for each particle?

Would COMSOL then use this R value to calculate the centrifugal force

F = mR(ω^2)

at each position the particle occupies?

Thanks in advance for any help that can be offered on this!

Laminar and turbulent flow in the same interface

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Hello everyone,
I have a non-isothermal flow ( a melt and argon) and I would like to know if it is possible to simulate a lamnar flow (melt) and a turbulent flow (argon) on the same fluid flow interface.
Thank you.

Two phase fluid flow adding material color

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Hello all,

I am attempting to model the combination of three fluids in an angled T-mixer (two of them are being modeled as water, and the other is a non-newtonian fluid). I think I have the physics set up correctly, but I am having a problem in postprocessing. I would like to be able to plot each fluid as a separate color in a stationary study. Any help would be greatly appreciated!

Best,
-Will

Fluid flow in porous media

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I'm modelling water flow in a cylindrical rock core under transient conditions. There, constant 'P' pressure is maintained at upstream and pressure built-up at downstream has to be investigated. Can anybody help me with this? How can I apply newly developed equation to simulate the flow into the software?

How to add inertial lift force and Dean force

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Hi everybody,

I m doing simulation for inertial focusing. I need to see the effect of inertial lift force and dean force on particle. I m just wanted to know where these forces can be defined. I tried to add them in particle tracing-> forces. still no use.

Appreciate help from someone.

Thanks in Advance.

Modelling of phase change

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Dear Comsol user,

I have one conceptual question...
I would like to model phase change of solid due to heat transfer from fluid above. Intuitively i should model the melting of solid using " heat transfer with phase change". But in different examples it is modeled differently. I found three examples till now "Cooling and Solidification of Metal" , "Tin Melting Front" and one with phase change of ice.
Use of either deformed geometry or phase change should give me the same result.

What is the difference between both of the approaches?

Coupling Pressure with EOF

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I am modeling electroosmotic flow (EOF) in a microfluidic chip. The chip has 2 inlets and 1 outlet. The EOF part seems to be working well. I am using Electric Currents (ec) and Laminar Flow (spf) physics.

On top of the chip are fluid reservoirs. As fluid flows from inlet to outlet the fluid height in the inlet reservoirs decreases and the fluid height at the outlet reservoir increases. This should cause a change in pressure that I would like to model. I have attached a simplified sketch (not to scale) with one inlet and outlet that should clarify my problem.

I am new to fluid dynamics so I may be approaching this problem in the wrong way but here is what I have so far.

The initial pressure (P0) at each reservoir is the same. Once the problem starts (t=0) an electric field is applied between one of the inlets and the outlet. The second inlet is unbiased for now. At any time t>0 there will now be a pressure difference between the inlets and outlet.

I have derived an equation for the pressure in the reservoirs based on the equation P=rho*g*h, where P is pressure, rho is the fluid density, g is gravity, and h is the fluid height in a reservoir.

At a reservoir the pressure will be P = P0 + Pt, where Pt is the pressure change as a function of time. Which is to say the pressure change due to fluid emptying from the inlet or collecting at the outlet.

Therefore, Pt = rho*g*ht, where ht is the change in fluid height in a reservoir. Now volume (V) in the cylindrical reservoirs is given as V=pi*r^2*h, therefore, h = V/(pi*r^2). The change in volume in the reservoirs is equal to the volume flow rate Q multiplied by time. Q = A*v where A is the cross sectional area of the microfluidic channel and v is the flow velocity.

So Pt = rho*g*ht = rho*g*V/(pi*r^2) = rho*g*(Q*t)/(pi*r^2) = rho*g*(A*v*t)/(pi*r^2)

Therefore, at the inlet P = P0 - rho*g*(A*v*t)/(pi*r^2),
and at the outlet P = P0 + rho*g*(A*v*t)/(pi*r^2)

In Comsol for inlet 1, I am using the pressure boundary condition, suppress backflow is unchecked, and flow direction is normal flow. Under pressure condition the units are Pa and I have written the following:

101325-rho*g*((A*spf.U)*t)/(pi*rR^2)

where rR is the reservoir radius, and I have already identified the other variables.

I am leaving the second inlet electrically floating for the moment. The conditions are otherwise the same, so under the pressure condition I have simply left it as

101325

For the outlet, I am using the pressure boundary condition, suppress backflow and normal flow are unchecked. Under pressure condition the units are Pa and I have written the following:

101325+rho*g*((A*spf.U)*t)/(pi*rR^2)

I have tried solving the problem as segregated and as fully coupled, but either way I get the following error:

Attempt to evaluate negative power of zero.
- Function: ^
Failed to evaluate temporary symbolic derivative variable.
- Variable: comp1.spf.U@VDN$comp1.u
- Defined as: (0.5*(((comp1.w^2)+((comp1.v^2)+(comp1.u^2)))^(-0.5)))*(2*comp1.u)
Failed to evaluate expression.
- Expression: (0.5*(((comp1.w^2)+((comp1.v^2)+(comp1.u^2)))^(-0.5)))*(2*comp1.u)
Failed to evaluate Jacobian of expression.
- Expression: comp1.spf.U
Failed to evaluate Jacobian of operator.
- Operator: mean
- Geometry: geom1
- Boundary: 6
Failed to evaluate Jacobian of variable.
- Variable: comp1.spf.U
- Geometry: geom1
- Boundary: 6
Failed to evaluate Jacobian of expression.
- Expression: ((-comp1.spf.f0)*comp1.spf.nzmesh)*dvol
Failed to evaluate Jacobian of expression.
- Expression: (-comp1.spf.f0*(test(comp1.u)*comp1.spf.nxmesh+test(comp1.v)*comp1.spf.nymesh+test(comp1.w)*comp1.spf.nzmesh))*(dvol)
- Feature: Time-Dependent Solver 1 (sol1/t1)


I get no errors and the model seems to be working as expected when I set the pressure condition for the inlets and outlet to 101325 Pa. Therefore the problem seems to be with my equation/implementation.

Any help with how I can address this error or approach the problem differently is welcome.

Thanks.

Wall condition (no slip or sliding), fluid outlet condition

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Hi all,

I am modeling a sphere moving in a moving laminar fluid domain (2D). The sphere is moved by the motion of the fluid, through boundary load on the sphere surface. I am wondering what kind of wall condition should I apply to the sphere surface, 'no slip' or 'sliding'.

Also, what kind of outlet condition should I apply to the outlet? Currently I am using 'pressure, no viscous stress', with pressure '0'. However, in the pressure plot, the pressure close to the outlet is larger than the pressure far away, which seems strange. How can the sphere move if the pressure ahead is smaller than the pressure behind?

I have attached the model and the pressure plot.

Thanks in advance for your help!

Frequency dependent pressure waves

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Hi,

I am trying to apply pressure waves to a mass contained in a closed volume in order to make it vibrate. So far I got it to vibrate, but I can't seem to find a way to set the frequency of the waves. Is there any way to set the frequency of the applied waves?

Thanks in advance

Modelling seperation of ice and salt

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Hello,

I am a begginer at CFD modelling, with only basic experience in MATLAB with Euler Methods.

My objective is to model the seperation of ice and salt from seawater. This occurs when a liquid is brought to the eutectic point ( a temperature ranging from -10 to -30 celsius). I am aware of how to model the heat exchange of the problem, my issue is with the phase change that occurs.

If anyone could suggest a general pathway on solving this I would be very grateful.

Kind Regards,

Argyris

(Two phase)Bubble shrink in a two phase flow

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I want to simulate a bubble dynamics in the water.
I initialize a bubble in the water, and use phase-field model.
I give the water a velocity in a channel, the bubble moves with the water.
After a few time steps, the bubble become smaller and smaller, than disappear!!!
It is very odd, as I set incompressible flow and a uniform mesh.
Other parameters are default.
What is the reason for this phenomenon.

Thanks.

Undefined value found in the equation residual vector.

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I'm solving for lid driven cavity with stratified flow.

My error message looks:
Undefined value found.
- Detail: Undefined value found in the equation residual vector.
There are 3 degrees of freedom giving NaN/Inf in the vector for the variable comp1.T.
at coordinates: (0.080597,0.2), (0.0835821,0.2), (0.0820896,0.202422), .

Thus the error would belong to the heat transfer domain. As I understand, residual vector is the vector that is formed to solve my equation, which in this case would be the temperature equation. It would consist of my thermal properties, Temperature and velocity as a variable along with boundary conditions. But I’m unable to figure what is actually missing.

According to manual I followed “Check for undefined numerical values after each operation” in the assembly settings. After that it display this error :

Function evaluation returns undefined numerical value.
- Function: ^2
- Expression: (comp1.spf.xi_m*comp1.spf.res_v)^2
Failed to evaluate variable.
- Variable: comp1.spf.vhNS_lin
Failed to evaluate expression.
- Expression: nojac((1.4142135623730951^(1-shapeorder(comp1.u)))*max(comp1.spf.vhNS_lin-comp1.spf.vhNS_quad,0))
Failed to evaluate variable.
- Variable: comp1.spf.vhNS
Failed to evaluate temporary symbolic derivative variable.
- Variable: comp1.spf.crosswindns@VDN${test@85}
Failed to evaluate expression.
- Expression: d((comp1.spf.crosswindns)*(dvol),{test@85})
Failed to evaluate Jacobian of expression.
- Expression: (comp1.spf.crosswindns)*(dvol)

But here it seems that it is referring to components of velocities. My guess would be that it is unable to resolve velocities at the boundary or getting division by zero there.. I’m lost on how to fix this.

I don't think it is due boundary conditions or material properties are the culprits, due to fact that there are 200 iterations. Although the error plot looks really bad i.e no convergence is achieved. My initial guess is good enough since it gives an error of magnitude ~ 10^0 .

Kindly help!!


Regards
Parth Swaroop

Creeping flow through a hydrogel

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Hi everyone,

I am trying to model the diffusion (creeping flow) of molecules through a collagen type I hydrogel. Does anyone have any idea on how to model a portion of a microfluidic device that contains this hydrogel?

Best,
Kevin

Error in Two Phase Flow

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I am using the Level Set Method in Comsol. When I am using the Time dependent solvers I get the following error-
Failed to find consistent initial values.
Last time step is not converged.

The same problem using Stationary Solver I am getting the following error-\

Wrong number of dofs in initial value.


Can anyone please tell the error in the model

Calculate Drag Coefficient

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Background: simulate the drag coefficient for a truck model

I have read the following post
www.comsol.de/blogs/how-do-i-compute-lift-and-drag/
and looked at the flow over an Ahmed body example
which shows that the total drag force in the Y direction is: spf.T_stressy

To calculate c_d i need to calculate
c_d = \frac{ 2F_d}{ \rho u^2 A} (en.wikipedia.org/wiki/Drag_coefficient)

where (please correct if wrong):
F_d = spf.T_stressy
/rho = spf.rho
u = spf.u

Is the above correct? How do I calculate Area? The area should only be the surface exposed in the Y direction (spf.nymesh?)?

thanks in advance!
Jess

A Vortex Pair Impinging on a Solid Boundary

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I am trying to model the Problem of 9.13 from Plawsky Transport Phenomena 3rd ed (below). Any ideas for how to set this up better? Thanks

The method of images uses precisely oriented collections of sources, sinks, and vortices
to establish artificial walls and so simulate more complicated flow fields. One such flow
field is formed by positioning two counter-rotating vortices a distance, d, from the
location of the hypothetical wall. The stream function for this flow is given by:

ψ = qo / 4π{ −ln [(x − d)^2 + y^2] + ln[ (x + d)^2 + y^2]}

a) Demonstrate that there is no flow through the wall.
b) How does the velocity vary along the length of the wall?
c) What is the pressure distribution along the wall?
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