Learning Forum

[Chris Roberts]

Hello,

I am having a problem trying to find the total resistance of a tube. I have a tube that has 30 holes in all along the sides of the tube. This tube is placed in a large box that is 600 mL. There is an inlet into the box which fills the box with water and then the water enters the tube holes and goes into the tube and flows out of the sealed box from the inner outlet of the tube. I have 3 simulations. In simulation 1 the tube is inserted into the box by 5 cm. In the second simulation by 10 cm and the third by 20 cm. I would like to calculate the resistance of each of the simulations to see how it changes for the tube. I took the resistance (pressure drop) of each hole with the mass flow rate in each and the overall resistance seems to be the same in all tubes. But should the resistance not be different ? I think I am making a mistake in how I calculate this if someone can please give me some advice. Thank you very much!

[NickFL]

I am having a little trouble understanding exactly what you are simulating, sometimes pictures are worth more than 1000 words ? But based upon my understanding, here are some thoughts. You seem to have a pipe with holes in it (like the elementary school instrument the recorder). Water fills a box so slowly water filles these holes and then moves down the pipe towards an outlet. The resistance coefficient for each one of the holes will be the same, but the resistance down the pipe from each of the holes will be different. Think of the hole furthest from the outlet, in the pipe there is only the water from the one hole going through it. But at the hole nearest the outlet, you will have the contribution from the other 29 holes.

You mention multiple simulations. Are these at different depths where there is a different hydrostatic pressure driving the fluid into the pipe? What quantities are you trying to obtain from simulations?

And go Tigers! (Unfortunately it is not their year, again)

[Chris Roberts]

Hello NickFL,

Thanks for your reply! Yes it is basically like a recorder. The tip of the tube is sealed but the sides have multiple holes in it. So water is injected into the box at a constant laminar flow rate and the fluid enters the sides of all of the holes and exits the box from the inner tube leading outside the box. When I mean multiple simulations I mean the tubes have different lengths. For example in simulation 1 a box that has 800 mL of fluid had a tube inserted 8 cm into it. Simulation 2 has the same 800 mL of fluid but now the tube is advanced to a depth of 14 cm, and simulation 3 35 cm. I am trying to calculate what is the total resistance that each piple based on the different depths is providing to the system. So is the pipe that is 14 cm vs 35 cm more resistant to flow ? Is the resistance of the total holes ? the drag outside the tube ? which settings do I used in the post solver to help me calculate the resistance as a measure of pressure drop vs flowrate in the holes and the drag from the surface. This is what I am looking to do and not sure how to do that in fluent post results. Thank you so very much!

 

And yes totally not their year but can't give up on them!

[NickFL]

It sounds like an interesting problem. There is not a one button solution to get what you want, but maybe I can describe how I would attack this problem—and Rob’s comments below are always excellent recommendations.

Usually in fluid dynamics we talk about the pressure drop. We use the pressure at various points or cross-sections of the flow to determine what is the drop between them. Since this is a laminar solution, I will assume there is a linear relationship between velocity and pressure drop. This is where the idea of flow resistance comes into play as it is analogous to an electrical circuit.

You could then find what the resistance for each component (a hole, or a section of pipe) and then make an equivalent electrical circuit. Then you could make approximations for the other simulations and then conduction the simulation to see how close your prediction was.

In the electrical circuit analogy, the volumetric flow rate (Q) is the current (I) and the pressure drop (dP) is the voltage drop (dV). Thus, to determine the resistance, we need the pressure on either side of the component and the flow rate (R = dP /Q). This is where Rob’s idea with the named locations comes into play.  You may also be able to create such surfaces in post-processing.  

I am old enough to remember ’84. 35-5 was quite remarkable and will likely never be topped.

[Rob]

If you take the inlet pressure and (I assume) set outlet pressure you get a bulk dP value. If you set interior surfaces on the holes at the geometry stage, and label each with a unique value you can also get those values. It depends on what you need. 

You may want to look up "effective area", it was a method I used for my PhD to help figure out a dP against flow for a gas turbine combustor where I was effectively altering the port size of the primary inlet using a fluidic device. As my thesis is public domain I can mention the approach. 

[Chris Roberts]

Hi Rob,

Thank you for the reply. What I need is to basically find out how resistant is each tube to the given volume. Is the tube that is 14 cm more resistant compared to the other tubes ? I am able to get the inlet and outlet pressure difference. Would this is delta P be the resistance of the system to the tube ? I assumed taking the flow rate through each hole against the pressure drop in each hole would give me the slope which would be resistance ? Also if there a way to find the pressure drop in each hole surface from fluent solver ? For example I see that I can select each hole face and choose total pressure, dynamic and such. Is there a choice that would give me the total pressure drop across the hole ? Thank you Rob.

[Rob]

You need to decide on what you're defining. Are you looking at inlet to holes on the tube (each at a different pressure) or to a set distance into the surrounding? 

[Chris Roberts]

Hi Rob,

That is a good question.  So my goal is to find the total resistance of the tube in relation to the volume. So my plan was to first get the resistance of the holes. I was going to see if I can get the pressure drop through the holes to start with. So measurement 1. measure the flowrate through each hole depth ( from the outer to inner diameter of the hole) with respect to the total pressure drop across the hole. That is measurement 1. Next measurement 2 what is the total pressure drop inside the tube itself from the tip to the exit with respect to the flow rate at the very tip ( little low) to the end of the tube near the exit (fully developed flow). Measurement 3 will be the drag across the outer surface of the tube. I would like to check each of these to add it as a sum total resistance. However i am not sure which settings to "click on" in fluent to get these values if you can suggest some help on that it would be very helpful. Thank you Rob.

[Rob]

Probably you just need pressure and flow at the various positions. Area weighted average on the walls and mass weighted on the interior zones. 

I suggest you sketch out what you're doing (and what data you need) then review the options in Fluent to see what you have to set to get what you need. Exactly as you would an experiment.