Simple CFD question for those in the know
 

Hi all,

I have a question that hopefully can be easily answered by someone who knows more about fluid dynamics than me.

As I understand, a velocity stack can be used to increase the velocity of fluid that passes through a tube when it is located at the entry of the tube and fluid is flowing towards and through the stack before it enters the tube.

But what about when fluid is flowing the other way?

Imagine a fluid moving along a small tube towards the centre of a face of a box. On the opposite face of the box is the fluid exit point.

How should the tube enter the box to allow fluids passing through it to enter at the greatest possible velocity?

My thoughts on the possibilities are:

a) a simple perpendicular butt joint where the tube is flush with the face of the box
b) the tube passes through the face of the box and protrudes internally by some small distance
c) the tube passes through the face of the box and is terminated with a velocity stack

Finally, if the exit point of the box was adjacent to the entry (rather than opposite), would this affect the answer?

Your thoughts are warmly welcomed :)

Do you want the fluid to go through at a maximum velocity or mass flow rate?

A flipped velocity stack is a diffuser. To minimize losses you would have a diffuser when entering the box and a velocity stack (contraction section) when exiting the box. This will help maximizing the flow rate. However, a diffuser will not maximize the velocity entering the box.

Yes thanks for correcting me - flow rate is the goal. I'm still getting my head around the terms!

So if I've understood correctly, this is what I should aim for:

http://i926.photobucket.com/albums/a...edorn/flow.jpg

Which direction of flow would produce the highest flow rate?

Or is there an even more efficient way like a more gradual tapered inlet/outlet?

Sort of. But the right and left side should look similar. In the case above, and this is a total guess, the flow going right to left may be more efficient. If the flow were going left to right there may be a stagnation point on the half circle and the flow would have to split, one streamline up, the other down. The up stream line would then eventually hit another stagnation point, which will become a separation point. Of course this is also true if the flow is going the other direction, except the stagnation point on the half circle becomes a separation point. I'm also guessing that the exit would be more sensitive to geometry than the entrance. So effort should be focused there. The overall objective for the entire system is to try minimize the creation of unwanted vortices. You'll also want to miminze the amount of turbulence created since some of it will go out the exit. For example, an ellipsoid "box" may be better than a rectangular box.

Edit: When I mention the stagnation point on the half circle, I mean the top half circle. The top stream line will go up the side of the box wall and the bottom stream line will go out the exit.

Oh, one more thing. I wrote that the entrance and exit should look similar. Not really true. If you are trying to optimize this, the two will look different. For the entrance to the box, the flow will want to separate somewhere on the nozzle/diffuser. On the exit of the box, the flow will want to separate in the throat.

Since a picture is worth a thousand words, I've attached a panel solution (Aero Troll + PANAIR) for a stretched donut.

When the flow is entering the donut it sees the minimum pressure at the throat and then a slight increase. This slight increase could cause flow separation and a blockage of the throat. The idea is to create a geometry that insures this does not occur. When the flow exists the donut it sees the minimum pressure at the throat and then an increasing pressure in the diffuser section. This will cause separation at some point. The idea is to minimize the effects of this. There may not be that much you can do about it. However, sometimes it is important to fix the separation point so unsteady flow does not occur.

Many thanks for your very helpful advice Martin.

Further to this I've since found some interesting data that is referenced in wiki.

http://i926.photobucket.com/albums/a...tionalflow.jpg

Sure enough, velocity stacks do perform differently when the flow is reversed. Assuming 'spitback' gives us a good idea of flow from right to left, I think it's fair to say an elipsed radiused velocity stack (bottom row) will provide the maximum flow rate in both directions.

This study also showed that differences in the radius of a velocity stack resulted in only a 3.5% difference in the measured mass flow rate on intake (top left & bottom left). Outlet readings were not measured, but looking at the right column, is it fair to assume the differences would probably be more significant?

Hard to say if it is significant. I'm guessing that the spitback results are transitory? Maybe steady state would be different. This is where analysis/CFD and experimentation would come in...

Good Luck.

Yes. It's the best I've got to go on at the moment so I'll have to press ahead with a prototype.

No prizes for guessing what it is... :)

http://i926.photobucket.com/albums/a...oxDesign2D.jpg