Wave absorbers in water tunels

[Benrazar]

A water tunnel in the lab I work in was designed poorly, and will require the implementation of some sort of wave absorber to eliminate waves disturbing the test section. The tunnel has a very short diverging section (downstream of test section) and afterwards redirects the flow 90 degrees downwards with a vertical wall. The tunnel is used for flow visualization and Particle Image Velocimetry.

Has anyone dealt with a similar or relevant issue? Does anyone know of some resources I can look at? Where would you look to find literature or examples on this issue?

Thank you

-Ben

[Sensei]

Nowadays people start simulation in AutoCAD and similar CAD programs before they build anything physically..

When the simulation goes OK, they build a small model, if everything is OK, they build a larger size..

Waves can interfere and "attack" more than one would expect in some areas. To fight with it, you can try the same techniques as with sound waves e.g. an anechoic room. https://en.wikipedia.org/wiki/Anechoic_chamber

 

 

[sethoflagos]

9 hours ago, Benrazar said:

Does anyone know of some resources I can look at? Where would you look to find literature or examples on this issue?

You could start by doing a literature search for current work referencing these citations:

Milgram, J. H. 1965 Compliant water-wave absorbers. M.I.T. Department of Naval Architecture and Marine Engineering Report no. 65–13.

Ursell, F., Dean, R. & Yu, Y. 1960 Forced small amplitude water waves; a comparison of theory and experiment. J. Fluid Mech. 7, 33–52.

[studiot]

10 hours ago, Benrazar said:

A water tunnel in the lab I work in was designed poorly, and will require the implementation of some sort of wave absorber to eliminate waves disturbing the test section. The tunnel has a very short diverging section (downstream of test section) and afterwards redirects the flow 90 degrees downwards with a vertical wall. The tunnel is used for flow visualization and Particle Image Velocimetry.

Has anyone dealt with a similar or relevant issue? Does anyone know of some resources I can look at? Where would you look to find literature or examples on this issue?

Thank you

-Ben

We need more information.

A sketch would be useful.

You say a water tunnel. Is it running full ? If so what happens at the divergent section ?

[Benrazar]

7 hours ago, sethoflagos said:

You could start by doing a literature search for current work referencing these citations:

Milgram, J. H. 1965 Compliant water-wave absorbers. M.I.T. Department of Naval Architecture and Marine Engineering Report no. 65–13.

Ursell, F., Dean, R. & Yu, Y. 1960 Forced small amplitude water waves; a comparison of theory and experiment. J. Fluid Mech. 7, 33–52.

Thanks for the leads on those papers. I haven't thought of doing a reference search, thanks for the tip.

17 hours ago, Sensei said:

Nowadays people start simulation in AutoCAD and similar CAD programs before they build anything physically..

When the simulation goes OK, they build a small model, if everything is OK, they build a larger size..

Waves can interfere and "attack" more than one would expect in some areas. To fight with it, you can try the same techniques as with sound waves e.g. an anechoic room. https://en.wikipedia.org/wiki/Anechoic_chamber

 

 

My knowledge of cfd or similar software is very limited, but if given the opportunity I will try to have a simulation conducted. Thanks for your input.

8 hours ago, studiot said:

We need more information.

A sketch would be useful.

You say a water tunnel. Is it running full ? If so what happens at the divergent section ?

Sorry, I tried to find some photos but didn't have anything good. here's a sketch.

[studiot]

14 minutes ago, Benrazar said:

Thanks for the leads on those papers. I haven't thought of doing a reference search, thanks for the tip.

My knowledge of cfd or similar software is very limited, but if given the opportunity I will try to have a simulation conducted. Thanks for your input.

Sorry, I tried to find some photos but didn't have anything good. here's a sketch.

Thank you that will give us something to work on.

[Benrazar]

As an update, pending data collection of the characteristics of the disturbing waves, we will most likely be going with a porous plate design. Anybody have experience with using them as wave absorbers?

[studiot]

OK a few more questions.

20 hours ago, Benrazar said:

The tunnel is used for flow visualization and Particle Image Velocimetry.

 

Are you viewing from the top or the sides (or both)  ?

You plate idea (as a top plate on the surface) is a likely fix is you don't need top viewing, make it removable for acees though.

The test length seems exceedingly short, almost certainly too short for the convergent and divergent orifices effects to be smoothed out.

Depending upon the type of pump you are in danger of introducing vertical axis vortices in the outlet, with the diagrammed arrangement.

By 'particle' do you mean fluid element or suspended matter.

Are you interested in sedimentation rates or differential sedimentation rates .

[Benrazar]

6 hours ago, studiot said:

OK a few more questions.

Are you viewing from the top or the sides (or both)  ?

You plate idea (as a top plate on the surface) is a likely fix is you don't need top viewing, make it removable for acees though.

The test length seems exceedingly short, almost certainly too short for the convergent and divergent orifices effects to be smoothed out.

Depending upon the type of pump you are in danger of introducing vertical axis vortices in the outlet, with the diagrammed arrangement.

By 'particle' do you mean fluid element or suspended matter.

Are you interested in sedimentation rates or differential sedimentation rates .

We are viewing from one side with cameras, and a laser plane with a traverse is projected from underneath the test section for PIV. The water is "seeded" with particles which are then illuminated by the laser plane. Two cameras take pictures at different times and angles which provides the data to build a velocity field of the volume containing the test model.

On the form of the tunnel, yeah it is quite short and squashed compared to other ones. I believe we had someone design it specifically for the space we had, but obviously designed it poorly.

On the plates, they will be vertical and normal to the flow direction or slightly inclined, all based on articles investigating wave absorbers.

In the diagram I drew, I did not include some flow straighteners and plumbing for simplicity, but vortices in the vertical axis could be a source of disturbance. Thanks for that observation.

[studiot]

4 hours ago, Benrazar said:

On the plates, they will be vertical and normal to the flow direction or slightly inclined, all based on articles investigating wave absorbers.

I presume the waves are surface waves.

In general these do not disturb water very deeply.

So I would suggest trying a horizontal perforated or cellular plate at or just below, the water surface.
Suitable sized and spaced holes would act as resonant absorbers of the waves without causing changes to the flow regime or pressures beneath as the water would 'pump' up and down through the holes. The plate thickness would need to be similar in size to the observed wave heights.

Edited January 30, 2023 by studiot

[Benrazar]

14 hours ago, studiot said:

I presume the waves are surface waves.

In general these do not disturb water very deeply.

So I would suggest trying a horizontal perforated or cellular plate at or just below, the water surface.
Suitable sized and spaced holes would act as resonant absorbers of the waves without causing changes to the flow regime or pressures beneath as the water would 'pump' up and down through the holes. The plate thickness would need to be similar in size to the observed wave heights.

It all depends on results from data collection, but people who have worked on the tunnel far longer than me do not believe they are surface waves. The flow is slow, 15 mm/s, so no possibly prevalent surface waves are observable. Should have said this before, but the fluctuations we are seeing are ~.2 cycles per second. I believe this means that we have relatively short wave height and wavelength, long period waves. According to articles on wave absorbers, wavelength and period seem to be the most important characteristics to design around. 

Thanks again for your time and input

[studiot]

11 hours ago, Benrazar said:

but the fluctuations we are seeing are ~.2 cycles per second.

Sounds like pressure pulses from the pump reflecting of the confining walls.

Not quite as bad as water hammer, but not what you need either.