Autodesk Simulation CFD - Calculator Volume Flow Rate Need Help. Tks

[Anonymous]

Dear Mr Jon Wilde,

I had designed a part, like picture below. And I want to reduce the Volume Flow Rate from 10-20l/h to 1-2l/h by a narrow way for water. But in Simulation I still got outface water have a same Volume Flow Rate with inface water. Can you help me to explain why and how to fix it? 

Here is my CFD file: it 13Mb, I can't upload to this forums...http://www.mediafire.com/download/mk7tu100ttgds97/TEST+PART+CFD.cfz

AND the narrow way for water

I have attached my DWG file design and my CFD Simulation. If possible, pls help me to understand why the Volume Flow Rate isn't change . Many thanks and Best Regards.

Hoang Su

[Jon.Wilde]

Hi,

This is conservation of mass right? So unless your density changes, the flow rates must be identical?

Kind regards,

Jon

[Anonymous]

Thanks for the prompt reply . 

I just wonder why the Volume Flow Rate aren't change,I mean the Volume Flow Rate alway same value in place water in, water out ( red ellipse) whether we have a narrow way for water in my part (white ellipse). You can imagine this part purpose like a valve, I want to reduce flow...but I dont know why it isn't. Can you pls download my Simulation File and help me to find out the problem...

Thank you

[Jon.Wilde]

Because whatever enters the chamber must also leave it again. Either that or we are creating/destroying matter right?

Unless there is another way for the fluid to escape?

I would think that you are increasing the pressure drop through the model, so at a given inlet pressure, reducing the outlet diameter is going to reduce the flow rate? But at a fixed flow rate, the outlet must equal the inlet.

Does that help?

Kind regards,

Jon

[Anonymous]

i think i understand what you mean. Because I set a fix inlet flow, so whatever narrow space inside my part, I always have a same value outlet flow, equal with inlet flow right?

Thanks but my question now is : if my part is a valve to control flow of water, I need to know how it work by compare value of flow inlet and outlet of part. In Autodesk Simulation CFD, how I simulate it? Can you teach me to set inlet value?

Suppose I have a constant value of Velocity, or Pressure inlet water, how I set it in Autodesk Simulation CFD to compare the value flow outlet?

Thanks so much

[Jon.Wilde]

Hi,

If you have an incompressible fluid (you do) and a fixed flow rate, the inlet and outlet are going to be the same. Changing the geometry will change the pressure drop though.

If instead you set a pressure drop to the model higher at inlet and a P=0 at outlet, then CFD will give you the flow rate this would produce. This would change depending on the geometry.

I would use Advection Scheme 5 for all of these. Be sure that you have inlet and outlet extensions also (5x diamater on inlet and 10x on the outlet). This is to avoid recirculation over the boundary, which can negatively effect results.

Thanks,

Jon

[Anonymous]

Dear Mr Jon Wilde,

Thanks for your reply very details. I have a picture of my case:

Suppose A, h = constant and the water is incompressible fluid, that mean h isn't change all time. So,we have a constant pressure of water at (a), and my part is a flow control valve, it control flow by control section for water inside. In real life, the flow of (b) allways smaller than (a).

My question need your help is:

1. If I known pressure value (a), how to set it in boundary condition, because when I set, it become fixed flow and the result of flow rate allways same beetwen (a) and (b). I think this is my problem, but I dont know how to fix it, how to set right my intent.

2. And your answer "If instead you set a pressure drop to the model higher at inlet", I don't understand this case, because I just know value or (a): pressure ,rate flow, or velocity of water WITHOUT VALVE

3. And your answer "Be sure that you have inlet and outlet extensions also (5x diamater on inlet and 10x on the outlet)". Is that extension pipe like the picture below of ball valve right? (red circle)

Sorry but I just beginning using CFD. Thanks for your help so much.

Thanks and Best Regards.

[Jon.Wilde]

No problem.

I will answer your questions one at a time but firstly, I cannot understand how you can have less flow at point (b) than point (a), where does the rest of the fluid go?

I suggest that they are always equal but that the speed of the fluid is controlled by the amount that the valve is opened, which is controlling the pressure drop.

1. So, if (a) and (b) are not the same, then we are creating or destroying matter - I propose this is not happening 🙂
2. You can can calculate the pressure at (a) from the height of fluid right? I would think that this can be assumed to be a constant. With or without the valve, this would be the same. We would then use a P=0 far away, down the end of the outlet tube. Try this with varying inlet pressures and note the change in flow rate that CFD predicts
3. Yes, that is correct. Especially so on the outlet here, to keep the P=0 far from the region in question and also to prevent any recirculation occuring over the P=0

Hope this makes sense and helps,

Jon

[Anonymous]

Dear Mr Jon Wilde,

Thanks for your help. But I think you aren't understand my intent . I already know the point of my problem is setup inlet flow rate of water. If it allways the same because no way for the rest of the fluid go.

"I cannot understand how you can have less flow at point (b) than point (a), where does the rest of the fluid go?

I suggest that they are always equal but that the speed of the fluid is controlled by the amount that the valve is opened, which is controlling the pressure drop.

1. So, if (a) and (b) are not the same, then we are creating or destroying matter - I propose this is not happening"

BUT, I really would like you to help me how to fix my problem ( I need the result flow at point (b) lower than flow at point (a). You know in the real life, the valve in toilet is the same purpose with my part (flow rate controller). That mean whatever flow rate at (a), we allways have the flow rate at (b) lower than  (a), the pressure of inlet water will substract with pressure inside the valve, and the result is flow at outlet lower than flow at inlet.

And, I want to simulation the valve above ( caculate the flow rate beetwen inlet and outlet the valve). If the flow allways the same, why do we need to use the flow controll valve like this. So, I think we can simulation that valve by change option for input of inlet water.

Thanks and Best Regards.

[apolo_vanderberg · Accepted solution]

Hoang,

     Let me take a large step back as I don't believe you are taking what Jon is stating in to consideration along with the practicality of the examples you are referencing.

As Jon is stating, there are basic prinicples that we cannot create or destroy mass, so for any valve, pipe, etc if we are using constant properties (fixed density) then we will get the same flowrate in and out of the system. This is good and to be expected.

What you are referencing as far as how a valve works (in a toilet or for your garden), is there isn't a prescribed flowrate at the inlet.

The way all of those systems work is that there is a fixed Pressure upstream. (P1)

By opening your valve you are changing the pressure drop of the system to the outlet P(2)

So as the Pressure drop changes (due to opening or closing the valve) you will get more or less flow.

But to circle back to the comments made - at any point in this example when you open the valve there is exactly the same flow going in to that valve and coming out to conserve mass.

For your analyses there are two approaches most people will take:

1) Set it up using Prescribed pressure drop (High Pressure on Inlet - Low Pressure on Outlet)

    As you change your valve position you will see the resulting flow rate that corresponds to the valve position

2) Set up as you have done with a Flowrate at the inlet

     Here, as you set the flowrate and alter the valve position the Pressure response will change. At the end of the runs you would then compare the pressure response to whatever the known source pressure you have for the design you are working with.

[Anonymous]

Many thanks to Mr Jon and Mr Apolo, thanks for your help 

Mr Apolo is right about my problem, that is a situation valve in real life. Now , I will try to fix my problem base on your solution.

Thanks and Best Regards

Su

[Anonymous]

Sorry Mr Apolo but We still have the problem. " Set it up using Prescribed pressure drop (High Pressure on Inlet - Low Pressure on Outlet)", and I found:

1. If you setup Higher Pressure on Inlet and P=0 on outlet, you will have exponentially of pressure, velocity of water, we can't have a fix...inlet pressure, velocity and outlet pressure, velocity ...You can see a basic model I tested below, if we increase the Iteration/step, we also increase the velocity, and it will become infinite..that mean we have an exponentially of pressure inlet.

How we fix it? Did I setup wrong any step? Can you help me to know please.

You can see I found 2 video on youtube, my intent worked with other software like solidwork

https://www.youtube.com/watch?v=xBtWaeBZ5x4

https://www.youtube.com/watch?v=OhiYamCDPe8

Thanks and Best Regard. 

Hoang Su

[apolo_vanderberg]

Hoang,

 I'm not sure I'm following the current issue.

For an assigned pressure drop there will be a given flowrate that develops within the model (for example, flow through a pipe - an assigned pressure drop will generate a given flowrate).

I'm not sure I understand the increase of iterations means the increase of velocity. What do you mean by exponentially prsesure?

From the questions you are asking and the difficulties you are having, I would highly recommend going through some of our Tutorials (accessible via the Ribbon).

Both videos are things that are capable within Simulation CFD

Apolo

[Anonymous]

Dear Mr Apolo,

With your advice about pressure drop, I tested by setup a higher pressure on inlet, and P=0 on outlet and make a simulate. And I have a conclude:

1. with the same inlet section, and same Iteration/step number, same pressure inlet, we will have a difference velocity or pressure outlet. This things prove pressure drop work ,it will dependent of pressure inside the part (Pressure drop changes (due to opening or closing the valve))

2. It work but I find out another problem, the volocity isn't fix with fixed part. That mean if you increase number of Iteration/step, the velocity will increase , and nolimit. First time I wait for it established, but I'm wrong, it will going to infinity if we increase number of Iteration/step to infinity.

BUT we just have a fixed pressure inlet?, So, how to fix it...You can open Simulation CFD Software and try it.

Thanks and Best Regards

[Jon.Wilde]

Hi Hoang,

We run models like this every day, no problem 🙂

Set a model up like this:

Inlet = 0.5bar

Outlet = 0bar

Use Advection Scheme 5

Run this to convergence - this is enough iterations for CFD to automatically stop when it has a solution.

Place a cut-plane through the cross section and measure the flow rate. Of course - if your pipe is not long enough, the flow might still be developing before it reaches the outlet even, another thing to consider.

Once you have done this, right click and clone the scenario.

Change the inlet pressure in the new scenario to 1 bar and run again until convergence from 0 iterations. Measure the flow rate.

This would demonstrate a difference in flow rates. Provided of course we are not causing some compressible flow 🙂

Check your mesh quality too - we need a minimum of 4-5 elements across the pipe diameter (not including the boundary layer). I would say that it might be better to aim for 10 even.

Apolo's advice of running through some of the tutorials is absolutely the best place to start here too!

Kind regards,

Jon

[Anonymous]

Dear Mr Jon,

1. Firstly you advice :"Set a model up like this: Inlet = 0.5bar & Outlet = 0bar". My question is : with my case, I just need to setup pressure inlet only, and pressure zero outlet, and using Advection Scheme 5 right?. Because I found we can setup many inlet input at the same time.

2. And I think I have a trouble with ITERATION number, if I set 0, it nothing happen at outlet, but I set it to 200, it run too long and increases follow the iteration and I dont know how many ITERATION is enough? Can you advice me for this?

Hope you have a nice weekend.

Thanks and Best Regards

Hoang Su

[Anonymous]

And Another point is:

3. When I have a Velocity Magtitus outlet, how can I have the volume flow? Do I use A*V ( with V is Velocity Magtitus {m/s) and A is area of outlet section {m^2} ? is it right?

Thanks and Best Regards

Hoang Su

[Anonymous]

Dear Mr Jon,

Thank you for your help. I think it worked, but I'm still have 3 point need to know, pls advice me 😉

1. Firstly you advice :"Set a model up like this: Inlet = 0.5bar & Outlet = 0bar". My question is : with my case, I just need to setup pressure inlet only, and pressure zero outlet, and using Advection Scheme 5 right?. Because I found we can setup many inlet input at the same time.

2. And I think I have a trouble with ITERATION number, if I set 0, it nothing happen at outlet, but I set it to 200, it run too long and increases follow the iteration and I dont know how many ITERATION is enough? Can you advice me for this?

3. When I have a Velocity Magnitude outlet, how can I have the volume flow? 

Do I use Volume flow = 3600*1000*A*V           (litre/h)

( with V is Velocity Magnitude {m/s) 

and A is area of outlet section {m^2} ? 

and 1000 because 1m^3 we have 1000 litre water

and 3600 because 1 hour we have 3600 second   )

Am I caculate the outlet flow right? Btw, Pls help me to delete 2 my reply above. I see no option to delete it. And can't edit it too

Hope you have a nice weekend.

Thanks and Best Regards

Hoang Su

[Anonymous]

Dear Mr Jon,

Btw, I dont know why the Velocity Magnitude change not much, just drop from 1000mm/s to 850 mm/s (after 220 iterations step), can you help me caculate for the flow?

Thanks and Best Regards

Hoang Su

[Jon.Wilde · Accepted solution]

Hi,

1) Yes, just set a pressure drop or you will be overconstraining the model

2) Just run to convergence, start with about 500 iterations (it should auto stop in less)

3) Use the Bulk Calculator

Kind regards,

Jon