ME 58:143 Computational Fluid & Thermal Engineering
Computer Lab #5
PARTICLE TRAJECTORIES IN A CURVED DUCT
Introduction:
In this lab you’ll learn how to inject some fluid particles into a curved duct shown below
and observe the trajectories of the particles along the flow field in the duct. You’ll model
flow into a 90-degree circular bend in a two-dimensional duct. The boundaries of the
computational domain will be formed by two circular arcs and two straight line segments
as sketched below:
Outlet
(outflow)
Inner wall
Y
Outer wall
Inlet
(velocity
inlet)
X
In the sketch, the origin is the center of curvature of the duct. The radius of the inner wall
is 0.4 m, and that of the outer wall is 0.5 m.
Generation of 2D grid with Gambit:
By now you shall be able to generate the geometry of this curved duct without any
difficulty. Use Fluent 5/6 as the solver. When meshing the duct face, put 50 grid points
on both inner wall and outer wall, and 20 grid points for both inlet and outlet boundaries.
To bunch grids near walls, a ratio of 1.1 is recommended for both ends of the edges of
inlet and outlet. The boundary types should be specified same as those shown in the
sketch.
Hint: How to create circular arcs? Look under Geometry (Operation)-Edge (Geometry)Create Edge (Edge) - mouse/right click-Arc.
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Fluent - Particle trajectories in a curved duct:
The procedures to run this case are same as before, you can check the previous lab
handouts if you don’t remember them. Some brief descriptions are as follows:
1. The default fluid is air, so we don't have to change it.
2. Set the inlet velocity as 2.0 as the velocity magnitude(in m/s) normal to the inlet.
3. Try laminar flow at first, you can also try turbulent flow if you have interests in
this problem.
4. The operating pressure now needs to be specified. In the main Fluent window,
Define-Operating Conditions. The default Operating Pressure is fine (standard
atmospheric pressure), but the Pressure Reference Location is outside the
computational domain. Change the Reference Pressure Location to X = 0.0 and Y
= 0.45, so that the reference pressure is defined at the center of the duct outlet.
5. The gravity term must be included in the momentum equations. Define-Operating
Conditions, and turn on Gravity. The x and y components of the gravitational
acceleration can be changed in the area of the window called Gravitational
Acceleration. For the first part of this problem, gravity will be neglected, so
leave both the x and y components at 0.
6. At the end of 100 iterations, check to see how the solution is progressing. Check
the velocity vectors at first. It may help to change Scale to around 0.4 so that the
velocity vector arrows do not overlap. Select Vector Options. In the Vector
Options window, change Style to arrow and change Scale Head to 0.3. Apply.
This makes the arrows nicer looking. Then check the contours of the stream
function (streamlines).
Inject some particles at the inlet, and plot their trajectories:
1. Now that the carrier fluid flow field has been established, particles will be
injected at the centerline of the duct inlet to see how they move in this flow.
2. In the main Fluent window, Define-Injections-Create. A window called Set
Injection Properties will open up. In this window, at the top, specify the Injection
Name as "micron-10".
3. From the drop-down list under Material, select (highlight) water-liquid - we will
examine the trajectories of water droplets of several different sizes (spherical
particles are assumed).
4. Under Point Properties, enter X-Position as 0.45 and Y-Position as 0. (This
location is in the middle of the inlet - all particles will be injected from this spatial
location.)
5. In like manner, specify the X-Velocity component as 0 and the Y-Velocity
component as 2.0, so that the particles are initially moving exactly with the fluid.
As the fluid turns in the duct bend, however, the particles may follow a different
path, due to their inertia.
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6. Specify this particle diameter as 10.E-06 meters, i.e. 10 microns. OK.
7. Create three other water droplet particles in the same fashion, with diameters of
50, 100, and 500 microns, and with Injection Names "micron-50", "micron-100",
and "micron-500" respectively. Note: the copy function in the Injections window
is handy for this type of operation. To use the copy utility, highlight the injection
name to be copied, and Copy. The only things you will need to modify are the
Injection Name and the particle Diameter.
8. After all four injection particles have been properly defined as labeled, Close the
Injections window.
9. Now the particle trajectories will be plotted. In the main Fluent window, DisplayParticle Tracks. A window called Particle Tracks will open up.
10. Under Color By, the default selection Particle Variables is fine, but under that,
choose Particle Diameter. With this selection, Fluent will assign a different color
to each particle trajectory.
11. Under Release From Injections, select (i.e. highlight) all of the particles. Display.
One particle trajectory should appear on the plot for each of the different diameter
particles defined above. If everything is working properly, very small particles
nearly follow the streamlines of the flow, but the larger (more massive) particles
get flung to the outer wall.
12. By default in Fluent, particles are assumed to reflect off of solid walls. If instead
the particles actually "stick" to walls, this boundary condition can easily be
changed. In the main Fluent window, Define-Boundary Conditions. Select the
Zone called inner wall, and Set.
13. Change Boundary Cond. Type (under Discrete Phase Model Conditions) from
reflect to trap. OK.
14. Repeat for the outer wall.
15. Plot the particle trajectories again for this case using Display-Particle TracksDisplay.
16. When the plot is to your satisfaction, a hardcopy file of this plot will be saved.
Add the phrase "no gravity" to the bottom of the plot.
Turn on gravity and re-create the particle trajectories:
1. In the main Fluent window, Define-Operating Conditions. Under Gravitational
Acceleration, change the Y-component to -9.81. OK.
2. The addition of gravity to the flow should have no effect on the carrier fluid.
Verify this by running 10 or 20 iterations. There should be no significant change
in the residuals.
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3. Plot the particle trajectories for this case using Display-Particle Tracks-Display.
Have any of the trajectories changed from the previous case?
4. Edit the label on your plot to indicate that gravity is acting downward. (Instead of
"no gravity", type "gravity down".)
5. Repeat for the case of gravity up, i.e. change the Y-component of Gravitational
Acceleration in the Operating Conditions window to 9.81, and re-plot the particle
trajectories.
Assignments (20 pts):
1. Print out the velocity vector plot and the contour of stream-functions plot
2. Print out the particle trajectories in the three gravity cases.
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