How the flow is affected by the location of an obstruction

[amrbekhit]

Hello all,

I'm working on a project where I need to enclose some measuring instruments in a waterproof housing and mount that inside an operational straight water pipe, while trying to minimise any obstruction to the flow of water. Obviously, the frontal surface area and the shape of the housing will affect the forces exerted on the housing and the amount of obstruction, but I was wondering if the location of the housing inside the pipe could affect how much it obstructs the flow of water by? For example, if I mounted the housing on the pipe wall, would that cause less obstruction than mounting it so that it is in the centre of the pipe? I have not studied fluid mechanics, but I would be grateful for any pointers towards particular areas in the subject which could help answer my question.

Thanks

--Amr

 

[BigInch]

If its not in the center, look forward to creating lots of turbulence.

I would think the location would depend on the type of instruments you want to mount and what you're trying to measure, the fluid and the fluid velocity in the pipe.

If you have laminar flow, you might be able to mount anywhere with little effect to turbulence, and something like a pitot tube needs to go in the center to measure maximum velocity for example. A temperture probe might be able to go on the pipe wall, if the flow was well mixed.

In other words, with the little data you've supplied, nobody can tell you more than you already know yourself.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[amrbekhit]

Hi BigInch,

Thanks for your reply. Sorry for not providing much information - it's a bit sensitive and I'm not too sure how much I can disclose.

The location of my device in the pipe will have minimal, if any, impact on its operation so it is primarily a turbulence thing I am worried about. You mentioned at the start that unless my device is in the centre, I should expect a lot of turbulence. However, you later on mentioned that the turbulence would not change much with position during laminar flow. So is this only a concern if I know the flow in the pipe is turbulent?

Also, could you point me towards any reference material that mentions this? Not that I doubt your word - just need it for reference purposes.

Thanks

--Amr

 

[BigInch]

No its not only the location. Its size, shape, velocity, viscosity, symetry ...

Man ...its like everything.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[racookpe1978]

To mount any solid part in a flowing fluid (gas, water, steam, laminar or turbulent liquid) and create as little resistance to the fluid stream as possible, you MUST mount your sensors and parts inside a streamlined hollow shell, with absolutely smooth surfaces and radiused connectins to the pipe wall, and with as small a "frontal face" to the liquid flow as possible.

A round cylinder sticking into the fluid has a relative resistance of 1.0

A streamlined shape exactly matching the fluid characteristics *(remember the fluid viscosity, fluid speed, fluid temperature, shape symmetry, cavitation properties of the fluid, etc just mentioned above?) They matter a LOT. A ship design or airplane design or turbine blade or fan blade or pump immpellor shape treats all of those factors very very carefully to optimize their effecieny and effectiveness. You can be a little bit more coarse than somebody who trying to design a more efficeint pump, but you better check evry assumption yuo have made.

(Hint: If your design doesn't look like a rain drop when you get done with your preliminary design, you have failed. Just how long of a tail and how wide across the tear drop will be, and how tapered the tail needs to be I can't tell. You have not given us any information.)

To reduce ADDITIONAL turbulence build-up from the obstruction, you need to keep its length as short as possible. But you must go to the middle if you need to measure something there. As stated, it should be as skinny as possible to minimize frontal area.

You need to consider (Bernoulli's Law!) what the effect of your sensor enclosure will have on your reading: a pressure reading will be dead wrong if you put a probe right on the wall of the sensor. Exact temperature readings may change a little by the obstruction. (You're probably not going to be measuring that accurately.)

Supersonic flow possible?

Everything above changes.

 

[ione]

There are specific devices (i.e. flow conditioners) which minimize possibility of swirl and distortion of the flow profile. This stuff, fitted upstream your device, could help you to get a certain degree of repeatability in your measures.

 

[amrbekhit]

Hello all,

Thanks for taking the time to reply - once again, sorry for not providing enough information. I've been doing some more reading around the subject and want to confirm my understanding of the following:

Assuming that the location of my device inside the pipe has no effect on its operation, and that I am familiar with how to design the shell of my device to reduce drag, it is better to mount the device in the centre of the pipe. This is because there will be a boundary layer near the pipe wall where the flow of fluid will be reduced due to frictional resistance. Placing my enclosure in that area would further reduce the flow of fluid relative to the main flow and increase turbulence. There will, of course, be turbulence generated by my device in the centre of the pipe, but this would be less than if the device is mounted near the pipe wall.

Is that correct?

--Amr

 

[btrueblood]

"Assuming that the location of my device inside the pipe has no effect on its operation"

On the sensor's operation or the pipe's operation? Assuming the sensor...

"Placing my enclosure in that area would further reduce the flow of fluid relative to the main flow and increase turbulence. There will, of course, be turbulence generated by my device in the centre of the pipe, but this would be less than if the device is mounted near the pipe wall."

So, your device is sensitive to turbulence?

If not, and the first statement is true, then mount your sensor flush to the i.d. of the pipe - no added effect on turbulence then, right?

The turbulence, or more aptly, disturbance to the flow, of your sensor will be minimized if it is near the wall, yes - because there will not need to be any support struts or other structure bits impeding the flow.

 

[eadwine]

amr..., Sticking things in pipelines is like sticking resistors in a circuit. The larger the object in the pipe is like a installing a larger resistor. The object in your case would be to miniaturize the package in reference to the ID of the pipe or you could increase the ID of the pipe, within limits of course. And streamlining always helps.

 

[amrbekhit]

Thanks everyone for the useful replies. Streamlining has been mentioned several times for obvious reasons. Is there an engineering method for streamlining the body of an object to minimise turbulence and drag? Or is it a bit of a trial and error thing, where you look at the fluid flow around the object and try to optimise the worst parts?

--Amr

 

[eadwine]

If I remember correctly, the most efficient shape for minimizing turbulence and drag would be the "teardrop" shape. If you wish to find the most effective design for anything, study nature, it's only been at it for millions of years.

 

[whammett]

BigInch is right. However, simply stated, in a laminar flow situation fluids move slower towards the pipe walls.

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