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1
##### Fluent / Setting up a partially-premixed combustion simulation using CHEMKIN database
« on: November 13, 2016, 02:57:49 PM »
Procedure for setting up a partially-premixed combustion simulation using materials from CHEMKIN database:

See below:

2
##### CFX / Choosing a suitable physical timescale in CFX calculation
« on: November 13, 2016, 02:27:21 PM »
For a single phase turbulent flow with or without heat transfer, the 'Auto Timescale' should be used. If you have more complicated physics, such as multiphase flow, flow resistance, or flow with reactions, then you should choose a physical timescale that is appropriate for your physics.

Most physical models require additional source terms and the timescale can often be estimated from them. For example, if we consider a quadratic flow resistance on a filter of length scale, d, then the momentum equation would be:

density*dv/dt = ... + 0.5*Cd*density*(v^2)/d

The timescale, dt, implied by this model is estimated from

v/dt = 0.5*Cd*(v^2)/d or dt = 2*d/(Cd*v)

So if d = 1 mm, Cd = 0.6 and v = about 10 m/s, dt = 3 x 10^-4 s.

The form of the source term can be obtained from the ANSYS CFX theory manual.

Once you have run a first case then look at the last number on the right in the residual table for each iteration in the out file. For example:

----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
| U-Mom-Bulk | 0.84 | 6.3E-05 | 3.4E-03 | 1.6E-03 OK|
| V-Mom-Bulk | 0.87 | 1.4E-04 | 6.5E-03 | 4.4E-03 OK|
| W-Mom-Bulk | 0.84 | 6.3E-05 | 3.8E-03 | 1.5E-03 OK|
| P-Vol | 0.77 | 2.4E-06 | 1.0E-04 | 9.0 2.9E-03 OK|
+----------------------+------+---------+---------+------------------+
| Mass-Fluid1 | 0.91 | 2.3E-05 | 1.2E-03 | 10.7 3.3E-08 OK|

The numbers in the 'Linear Solution' column (1.6E-3, 4.4E-3, etc) tell you the residual from the linear solver but they can also be considered to be the fraction of non-linearity in the equation. Ideally, you want to choose the timescale so that the non-linearity is about 1 or 2% (i.e. 1e-2). If you increase the timescale, the amount of non-linearity also increases in proportion. In the above example, the timescale for u, v, w and p could be increased by a factor of 3 to get faster convergence. However, the mass fraction called 'Mass-Fluid1' has a non-linearity of only 3.3e-8, which is very small. The timescale for this equation (in CFX Pre Solver Control > Equation Class Settings) could be increased by almost 1,000 times, and the calculation would still be stable.

3
##### Fluent / Re: How can I calculate time-average velocity in transient simulation?
« on: November 25, 2012, 01:11:34 PM »
Does it work?
CAN anyone help me?

You can follow the instructions given. If you have any issues then you can post it here.

4
##### Fluent / Re: How can I get new particles after a particle impacting a solid wall?
« on: November 09, 2012, 03:00:47 PM »
This feature is available built-in in Fluent. Use the splash option in wall boundary condition for DPM.

5
##### Fluent / Re: moving wall problem
« on: November 09, 2012, 02:53:56 PM »
hello guys
I am new to fluent so please be gentle with me

I am also trying to simulate moving objects in a chamber. It is hard for me to understand UDFs (was looking around the internet for a while without finding REAL tutorial).
Can i define a moving wall without UDF? a practical (even very simple one ) example might be very helpful

thanks a lot

Doron

Please see the tutorial below.
http://www.engr.uconn.edu/~ewebhk/buttons/flfem/fluent5/tutorials/tutor1618.pdf

Please next time start your own thread if you need any help rather than replying to existing thread.

6
##### Fluent / Re: error in udf interpretation
« on: October 26, 2012, 11:48:28 AM »
Please compile the UDF. Do not use interpreter, it does not work for UDFs in which you have defined your own variables.

7
##### CFX / Re: Getting heat flux
« on: October 22, 2012, 04:03:46 PM »
You can select your coordinates of line that correspond to the surface of the wall. For details of how to plot see the CFX post manual.

8
##### CFX / Re: Getting heat flux
« on: October 22, 2012, 01:52:42 PM »

9
##### Fluent / Re: Body Force UDF problem
« on: June 20, 2012, 05:32:31 PM »
To loop over all the injections, you first need to get a list of all the injections you have defined. This can be done using the following command

Injection *Ilist = Get_dpm_injections();

You will also have to define a generic injection pointer

Injection *I;

Now, looping over all the injections can be done using the following command

loop(I, Ilist)
{
}
You can loop over all the particles from a given injection. To do that, you first need to define a particle pointer

Particle *p;

Looping over all the particles can be done using

loop(p, I->p)
{
}
In unsteady DPM calculations, if you want to initialize the transient particles, you will require the following loop
loop(p, I->p_init)
{
}

You can use these loops to fetch particle data using particle specific macros like

P_DIAM(p) for particle diameter
P_T(p) for particle temperature etc.

10
##### Fluent / Re: Unsteady flow and DPM
« on: March 21, 2012, 04:15:13 PM »
You dont need to solve the flow field before you switch to DPM. You can turn the DPM along with the flow field. When you see constant cross-sectional velocities or constant discrete phase propeties at the outlet, it means that the simulation is converged.

11
##### Fluent / Re: Unsteady flow and DPM
« on: March 19, 2012, 06:33:42 PM »
You can solve the DPM as a steady state case within the transient simulation. In such a case the flow of fluid will be solved for each time step but after each time step, particles trajectories will be solved. But you can have the discrete phase in transient manner as well. In which the flow of DPM is also solved for each time step. The option is available in Fluent.

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