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Back and forth, cycling flow

Question asked by Chris Morrow on Jul 30, 2014
Latest reply on Aug 4, 2014 by Jared Conway

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Hey,

I'm new to flow simulation and I have gone through a lot of examples and I'm getting a grasp on how effective this tool can be.

I have a few questions that maybe some of you could help answer.

I have some images here of my project that I'm working on. I am creating a flow study that has time dependancies. The inlet flow rate is what I will be changing over time.

I have found where I can specify either an equation or a table of values for flow rate as a function of time. The main problem I'm running into is that I need to specify a back and forth cyclical flow from +30 ul/s to -30 ul/sec. Using the table, it will not accept negative values for flow rate (which I guessed it wouldn't but I tried anyway).

How can I create boundry conditions that will allow back and forth flow? As you can see from the image above (or not due to resolution) I have a flow cycle period of 3 cycles / 5 seconds.

Thanks in advance for any help

Chris Morrow Aug 4, 2014 7:20 PM

Correct Answer

Okay, I figured out a good way to do this last week.

Because I have a single inlet that will switch between +30uL/s to -30uL/s and the outlet is static pressure at atmospheric, I decided to create a Y shaped inlet that leads into my single inlet. This is created only for the sake of simulation.

So now you can see I have an inlet where I specify a volume flow rate boundary condition. I used the mode where I can specify values in a table essentially cycling from 0 - 30 - 0 ul/s. The other branch I specify an OUTLET volume flow rate in the same manner as the inlet just with a phase shift so that it creates a cycling flow. In essence, while the INLET has flow > 0, the OUTLET Volume flow = 0.

I made sure to keep the feature long enough for flow to develop properly before it enters the lower portion of the assembly where I'm interested in.

So far the results have turned out great.

If anyone is attempting to do something similar and needs more info, let me know I'd be glad to help.

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Chris Ma Jul 30, 2014 4:29 PM

Chris,

You are correct in that you cannot input negative flow rates in the software. One way you could approach this would be to use pressure boundary conditions instead of flow conditions. For example, at t=1s you can set the inlet pressure to be higher than the outlet pressure, and then at t=2s set the outlet pressure to be higher than the inlet, causing the flow direction to reverse.

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Chris Morrow @ Chris Ma on Jul 30, 2014 4:35 PM

Interesting approach, thanks for your help. I'll try that. Is it possible to specify in the goals to dynamically change the pressures based on calculated volume flow rates at the inlet and outlet?

I don't have information on the pressures. And I'd rather drive the simulation using what I know rather than itterating using pressures until I can get the volume flow rate to what I want.

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Chris Ma @ Chris Morrow on Jul 30, 2014 5:53 PM

Unfortunately, you cannot drive the boundary conditions based on goals. You would need to determine the relationship between pressure drop and the flow rate inputs you have, and "convert" your flow rate boundary condition into a set of pressure boundary conditions.

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Jared Conway @ Chris Morrow on Jul 31, 2014 4:20 PM

what do you want to learn in this analysis?

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Chris Morrow @ Jared Conway on Jul 31, 2014 6:10 PM

There are several things I want to learn from this study.
First off, from the image, you see there is an orange part. It is a gasket material bonded to the cylindrical part below it. The cylindrical part is glass. When it is pressed against the thin square piece it closes off the system creating a flow path with the orange piece directing the flow. The flow path become very shallow and wide. The width of the flow path within the orage part is only ~ 2.5mm. The thickness of the gasket after sealing on the surface ~ .125mm deep. The inlets and outlets lead in perpendicular from the glass piece into the flow path directed by the orange piece.

The things I need to understand are the following:

1. The shear stress at the surface of the thin square piece (first picture)

I want this shear stress to be as even as possible across the width of the flow path and across the length of the 3 longer flow areas.

2. After I grasp how to properly specify back and forth flow, I will then impliment a second fluid. And I need to understand the mixing of the two fluids as they pass through the flow path.

The system this runs on uses syringe pumps so they dispense at a very defined volume flow rate. The volume flow rates go 30 microlitres / sec in on direction for ~0.88seconds, then it switches direction and flows at 30 microlitres / sec in the oposite direction. There is obviously a very small time durring which the speed ramps down and switched direction but I'm neglecting this for the time being.

Any suggestions on setting this up using volume flow rate?

I tried doing the differential pressures like Chris Ma suggested however its not a great approach because the pressures change a lot to be able to keep a defined volume flow rate.

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Jared Conway @ Chris Morrow on Aug 1, 2014 12:57 AM

I think it is a limitation. Have you checked the swx kb or with your var?

Is there any reason you can't start a first order approximation using one direction?

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Chris Morrow @ Jared Conway on Aug 4, 2014 7:20 PM

Okay, I figured out a good way to do this last week.

I made sure to keep the feature long enough for flow to develop properly before it enters the lower portion of the assembly where I'm interested in.

So far the results have turned out great.

If anyone is attempting to do something similar and needs more info, let me know I'd be glad to help.

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Jared Conway @ Chris Morrow on Aug 4, 2014 11:27 PM

tricky, I like it

only thing i'd suggest is moving your bc away from your area of interest

remember that an outlet volume flow rate creates a uniform outlet flow profile

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8 Replies

Chris Ma Jul 30, 2014 4:29 PM
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Correct Answer
Chris,

You are correct in that you cannot input negative flow rates in the software. One way you could approach this would be to use pressure boundary conditions instead of flow conditions. For example, at t=1s you can set the inlet pressure to be higher than the outlet pressure, and then at t=2s set the outlet pressure to be higher than the inlet, causing the flow direction to reverse.
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- Chris Morrow @ Chris Ma on Jul 30, 2014 4:35 PM
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  Interesting approach, thanks for your help. I'll try that. Is it possible to specify in the goals to dynamically change the pressures based on calculated volume flow rates at the inlet and outlet?
  
  I don't have information on the pressures. And I'd rather drive the simulation using what I know rather than itterating using pressures until I can get the volume flow rate to what I want.
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  - Chris Ma @ Chris Morrow on Jul 30, 2014 5:53 PM
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    Unfortunately, you cannot drive the boundary conditions based on goals. You would need to determine the relationship between pressure drop and the flow rate inputs you have, and "convert" your flow rate boundary condition into a set of pressure boundary conditions.
    Like • Show 1 Like1
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  - Jared Conway @ Chris Morrow on Jul 31, 2014 4:20 PM
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    what do you want to learn in this analysis?
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    - Chris Morrow @ Jared Conway on Jul 31, 2014 6:10 PM
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      Correct Answer
      There are several things I want to learn from this study.
      First off, from the image, you see there is an orange part. It is a gasket material bonded to the cylindrical part below it. The cylindrical part is glass. When it is pressed against the thin square piece it closes off the system creating a flow path with the orange piece directing the flow. The flow path become very shallow and wide. The width of the flow path within the orage part is only ~ 2.5mm. The thickness of the gasket after sealing on the surface ~ .125mm deep. The inlets and outlets lead in perpendicular from the glass piece into the flow path directed by the orange piece.
      
      The things I need to understand are the following:
      1. The shear stress at the surface of the thin square piece (first picture)
      I want this shear stress to be as even as possible across the width of the flow path and across the length of the 3 longer flow areas.
      
      2. After I grasp how to properly specify back and forth flow, I will then impliment a second fluid. And I need to understand the mixing of the two fluids as they pass through the flow path.
      
      The system this runs on uses syringe pumps so they dispense at a very defined volume flow rate. The volume flow rates go 30 microlitres / sec in on direction for ~0.88seconds, then it switches direction and flows at 30 microlitres / sec in the oposite direction. There is obviously a very small time durring which the speed ramps down and switched direction but I'm neglecting this for the time being.
      
      Any suggestions on setting this up using volume flow rate?
      I tried doing the differential pressures like Chris Ma suggested however its not a great approach because the pressures change a lot to be able to keep a defined volume flow rate.
      Like • Show 0 Likes0
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      - Jared Conway @ Chris Morrow on Aug 1, 2014 12:57 AM
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        I think it is a limitation. Have you checked the swx kb or with your var?
        
        Is there any reason you can't start a first order approximation using one direction?
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        Chris Morrow @ Jared Conway on Aug 4, 2014 7:20 PM
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        Correct Answer
        Okay, I figured out a good way to do this last week.
        Because I have a single inlet that will switch between +30uL/s to -30uL/s and the outlet is static pressure at atmospheric, I decided to create a Y shaped inlet that leads into my single inlet. This is created only for the sake of simulation.
        So now you can see I have an inlet where I specify a volume flow rate boundary condition. I used the mode where I can specify values in a table essentially cycling from 0 - 30 - 0 ul/s. The other branch I specify an OUTLET volume flow rate in the same manner as the inlet just with a phase shift so that it creates a cycling flow. In essence, while the INLET has flow > 0, the OUTLET Volume flow = 0.
        I made sure to keep the feature long enough for flow to develop properly before it enters the lower portion of the assembly where I'm interested in.
        
        So far the results have turned out great.
        If anyone is attempting to do something similar and needs more info, let me know I'd be glad to help.
        Like • Show 1 Like1
        Actions
        Jared Conway @ Chris Morrow on Aug 4, 2014 11:27 PM
        Mark Correct
        Correct Answer
        tricky, I like it
        only thing i'd suggest is moving your bc away from your area of interest
        remember that an outlet volume flow rate creates a uniform outlet flow profile
        Like • Show 0 Likes0
        Actions

Back and forth, cycling flow

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