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Numerical Results

We can use either FLUENT or CFD-Post as post-processing tool. CFD-Post is preferable as it is more user friendly and gives you more freedom. The first check, however, will be made in FLUENT

Check mass flow

It's always good to check the mass flow rate after CFD simulation. The solver tries to keep it satisfied, but sometimes a representative imbalance is obtain, showing that something has to be done in order to get more accurate results.

To do this, we will use FLUENT.

Highlight "Reports" in the left box. Then select "Fluxes" and click "Set Up...".

FIGURE 1

Note that "Mass flow rate" is already selected.

Here you can play around to see the mass balance through each boundaries. Since most of our boundaries are single circular element, it is expected that the imbalance at each boundary alone be zero. We will check the mass flow rate though two boundaries.

First, the external boundaries. In the "Flux Reports" window, locate and highlight "farfield1" and "farfield2" and hit "Compute". The mass flow rate though that boundary is now printed in the command window.

FIGURE 2

Note that 73.5kg/s get into out domain and virtually everything leaves. The imbalance is 7.4e-9kg/s which is negligible considering the total amount of flux in the system. Nice!

Now, let's check the imbalance inside the hub. For that we only have one boundary completely circling the zone, hub_inner (note that hub_outer is essentially the same boundary).

Proceed similarly as before and check the mass flow through the hub_inner boundary. You could also select hub_outer and the result would be almost identical.

FIGURE 3

Remember to deselect farfield1 and farfield2 before hitting compute! You should get an imbalance of 3.42e-10kg/s which is essentially zero. Cool!

 

Velocity Contours

To plot the velocity contours we will use CFD-Post. You can now close FLUENT.

In Workbench, under Project Schematic, double click Results. This will launch CFD-Post

FIGURE 4

CFD-Post usually opens with an isometric view of the part. Since we're in a 2D analysis, this is not very useful for us. So, the first thing to do is click on the Z axis arrow (bottom-right corner of graphics window) to change the view.

Now insert a Contour. Click on the Contour icon (or Insert > Contour)

FIGURE 5

Name it "Velocity contour". A new box will appear on the left side of the screen. Summary of what to do:

Domains: leave default ("All Domains")

Locations: click on the three dots "..." on the side. select all names with "symmetry 1" in it. Hold the Ctrl key for that. You will select 5 zones in total, see figure.

FIGURE 6

Variable: change is to "Velocity".

Range: leave as "Global".

# of Contours: change to around 51.

Click Apply.

You can zoom into the bug, drawing a box with the right mouse button.

FIGURE 7

Note that it's very clear the effect of when the blade is perpendicular to the flow: a huge recirculation bubble is made. This will negatively affect other turbines placed downstream of this one. This is a very important thing to consider when designing an array of VAWTs.

This is a single snapshot of the spinning of the turbine, or a particular position. A transient analysis with a complete animation will be created in the future.

You can also zoom out and see that far downstream of the turbine, the flow has not yet recover its freestream condition. If you probe the velocity inside that slightly brighter area you will see that the velocity is around 9m/s (instead of 10m/s of the freestream).

To probe: Click on "Probe" icon. Then change the variable to "Velocity" and click on the screen where you wanna probe.

FIGURE 8

 

 

Check mass flow

Velocity contours

(save image)

Blade velocity (TSR)?

Pressure contours

Torque

Vorticity?

 

 

 

 

 

When extracting the Torque, explain that the Moving Frame of Reference is used to calculate (FOR FEA) omega squared times the radius times the mas of each element to compute the force exerted by the fluid on the blades. (check Wind Blade 2 tutorial, under Physics Setup, second video, around 3:30. It is for FEA, we should get the analogy to fluid before).

Under Construction



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