Messages

151

Fluent / Re: What is DPM concentration in Fluent and how is it calculated?

« on: February 02, 2012, 05:46:35 PM »

The DPM concentration is defined as:

DPM concentration = (Avg. particle mass in cell * Part resi. Time * Strength of particle) / Cell volume ...1

Strength of particle = number in parcel / dt_flow ...2a
= total particle flow rate / mass of single particle in the stream ....2b

From equation 1 and equation 2b,

DPM conc. = (Avg. particle mass in cell * Part resi. Time * Total particle flow rate) / (mass of single particle in stream * cell volume) .....3

Example:

Consider the following example to make things more clear.

a. A simple channel flow with constant velocity of 1m/s is used.
b. The top and bottom surface are defined as symmetry to ensure the plug flow condition.
c. The particles with total mass of 0.1 Kg/s and velocity of 1m/s are injected as single injection at the center of inlet.
d. This ensures that DPM concentration is non-zero only at the center line and is zero everywhere else.

A single stream of particle is modelled, hence,

Average particle mass in cell = Average mass of particles in particle stream ...4

From equations 3 and 4,

DPM conc. = Part resi. Time * Total particle flow rate / Cell volume

For the test case, Part resi. Time = 0.002 s, Total particle flow rate = 0.1 Kg/s, Cell volume = 2.08e-6 m3

DPM conc. = 0.002 * 0.1 / 2.081 e-6 = 95 Kg / m3

152

Fluent / Re: DPM concentration in a cell using Fluent UDF

« on: February 02, 2012, 05:45:29 PM »

You can use

C_DPMS_CONCENTRATION(c,t)

to get the concentration of discrete phase in the cell.

153

Fluent / Re: How do I calculate concentration of water in a cell?

« on: February 02, 2012, 04:33:04 PM »

Thanks I will try that.

154

Fluent / How do I calculate concentration of water in a cell?

« on: February 02, 2012, 04:14:59 PM »

I have mass transfer from the spheres (lagrangian tracking) to the air (continuous phase). The mass transfer depends on the difference in concentration between particle surface vapours and water vapours concentration in the gas phase. For this I suppose I will need the water vapour concentration of the cell in which the particle is located.

If that is the case, then how do I calculate the concentration water in the cell?

Thanks.

155

Meshing / User guide for meshing in Ansys ICEM CFD

« on: January 27, 2012, 11:09:51 AM »

Hello,

Attached ICEM CFD Meshing tutorial guide is very useful for beginners who want to learn how to mesh using Ansys ICEM CFD 11.0.

156

Fluent / General guide lines for solving problems in converging a solution

« on: January 25, 2012, 06:19:27 PM »

Many people face problems in getting a stable and converged solution in CFD. Here are some elementary guidelines that I am sharing can help you get a better convergence.

1) Always start with the first order upwind discretization scheme, as this is the most stable scheme.

2) Once the first order upwind scheme solution is converged or the residuals (specially the continuity) are significantly lower <1 x10-3, then change the discretization scheme to second order upwind scheme and use the solution of first order scheme as the initial guessed values.

3) If large fluctuations in the residuals are observed, then change the under relaxation factors. Lowering the under relaxation factors help in converging the solution.

4) Sometimes residuals do not change with the iteration or fluctuate with the iteration about a mean value, in that case, it is better to plot cross-sectional averaged properties of any important variable like velocity, pressure etc to see how that is changing with iterations. If it is not changing appreciably, you can take that as your converged solution.

5) Sometimes the nature of the flow in the geometry is so complex that convergence using steady state approximation is not possible. In that case, transient simulation should be used.

157

Meshing / Re: Help in meshing using GAMBIT!

« on: January 24, 2012, 03:43:37 PM »

You have to merge the overlapping faces together using "connect faces" option in gambit. Fluent automatically makes a wall boundary condition for faces which are overlapping.

158

Fluent / Re: Stochastic vs Cloud tracking?

« on: January 20, 2012, 07:06:31 PM »

Stochastic tracking accounts for local variations in the flow properties. It adds random turbulent dispersion to each particle track. Tracks that start from the same point can be all different at the final location.
But it requires a large number of stochastic tries to achieve statistically significant sampling. It depends on the density of the mesh.
Insufficient number of tries can result in convergence issues.
It is recommended for use in complex geometries.

Cloud tracking uses statistical methods to trace the turbulent dispersion of particles about a mean trajectory. The mean trajectory is calculated from the ensemble average of the equation of motion for the particles represented in cloud. The distribution of particles inside a cloud is represented by a
Gaussian probability density function.
In cloud tracking, the local variations in the flow get averaged inside the particle cloud. It has a smooth distribution of particle coupling source terms. Each particle diameter requires its own cloud trajectory calculation.

159

Fluent / Re: How to calculate geometry diameter using UDF

« on: January 15, 2012, 10:29:24 AM »

Fluent offers geometry macros to get the coordinates of the node. Once you know the node coordinates, use the equation of the circle to get the radius. The code below shows how you implement it.


face_t f;
Thread *tf = DT_THREAD(dt);
real xnode, ynode, r;
int n;

begin_f_loop(f,tf) /* Loops over all faces in the thread passed in the DEFINE macro argument */

{
f_node_loop(f,tf,n) /* Loops over all nodes in the face */
{
Node *node;
xnode = NODE_X(node); /* calculate x-coordinate of a grid point on wall */
ynode = NODE_Y(node); /* calculate y-coordinate of a grid point on wall */
r = sqrt(pow(xnode,2)+pow(ynode,2)); /* calculate radial coordinate of the grid point on wall */
}

//Enter your equation here which you want to calculate as a function of radius


}
end_f_loop(f,tf);