Pressure sensors for measuring pressure field on the bottom of the planing hull?

Discussion in 'Hydrodynamics and Aerodynamics' started by Binze, Apr 14, 2014.

  1. Binze
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    Binze Junior Member

    Hallo, I'm quite new to this forum but I was already going through topics related to this problem. I was also trying to find some good papers on this topic but nothing relevant...
    I would like to measure a pressure field on bottom of the hull. My idea is to use pressure sensors with stainless steel flush diaphragm. For example:
    Are there any other better ways how to messure pressure distribution on the bottom of the hull? I would like to try to verify a numerical model.
    I am aware of a problem to distinguish static / dynamic component of the pressure...
  2. tom28571
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    tom28571 Senior Member

    The main problem I see is, how do you measure he pressure at a given point in a way does not modify the true pressure? Any part of the sensor that affects the fluid flow will change the indicated pressure either positively or negatively.

    The point of maximum pressure is at the stagnation point but that will wander about with any relative vertical motion of the test hull. Therefore, the true pressure near the stagnation point cannot be measured unless there is zero heave in the model and the water surface is perfectly flat. These conditions can never be achieved in the real world outside a test tank.
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    There have been a number of studies done on measuring the bottom pressure of planing hulls, and you can do a search through the papers at SNAME for a detailed listing. The problems with measuring pressures are: They are highly localized--there are extremely high pressures that occur on very small points of area and then dissipate radially outward very quickly. Also, the time interval of pressure impacts is measured in milliseconds, extremely short periods of time. The time interval is so short that the structure does not necessarily have time to respond to the impact by the time the impact disappears.

    You can find a general description of this phenomenon in this paper:

    "A Simplified Method for Determining Structural Design-Limit Pressures on High Performance Vehicles" by R.G. Allen and R.R. Jones of the David W. Taylor Naval Ship Research and Development Cener, published as paper 78-754 by the AIAA/SNAME Advanced Marine Vehicles Conference in April, 1978.

    Known as "The Allen and Jones Paper" this paper forms the basis of the science in classification society high-speed craft scantling rules such as ABS, DNV, and the ISO RCD standards. As you can see, this science goes back over 30 years.

    I hope that helps.

  4. BMcF
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    BMcF Senior Member

    For measuring the quasi-steady static pressure field on a submerged body (or hull bottom), we've used arrays of cheap solid state sensors similar to the ones shown in the link I provided. (Similar...I can't find a link right off to the specific ones we have used..). Drill a small hole (~ 1/4") in the hull and glue the sensor in it. We carefully trim the sensor "nipple" to be flush with the hull surface if it protrudes at all after installation.

    That type of cheap sensors typically survive a couple of weeks of use.

    But to be clear, we use that method to obtain pressure distribution data under constant speed steady state running conditions; not to try and measure slam pressures or other transient responses.
  5. Remmlinger
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    Remmlinger engineer

  6. DCockey
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    DCockey Senior Member

  7. gonzo
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    gonzo Senior Member

    If you were to take the acceleration data and record the portion of the boat out of the water (which will give you total lift), it should be possible to compare the total pressures recorded versus lift + acceleration. At least it would give an idea if the measurements make sense or not.
  8. Binze
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    Binze Junior Member

    Hi, thanks everyone for attention and such a quick reply.

    We are aware of averaging the pressure field across the sensor area. The bigger the sensor the smaller the pressure... Our aim is to get "pressure map" across the hull (probably 2 longitudinal lines of sensors). We don't want to know absolute pressure peak - it is not possible. We want to know approximate pressure distribution and position of a spray root and with these data we want to validate our numerical models. We don't know how difficult it is going to be and how "naive" we are. But we assume that we will be able to deal with it somehow... This is one of our question. We will appreciate any comments for this topic.
    Fortunately we have quite good weather and geographic conditions for testing. We have flat/glassy water for most days a year. And we would like to perform series of steady velocity tests. We don't want to measure any dynamic instabilities ... yet :)

    We would like to perform other measurements as well. Such as measuring the force from the outboard motor, engine revs, velocity, accelerometer, gyro...

    Eric Sponberg:
    Thank you very much for your interest and pointing "The Allen and Jones Paper". There are interesting and important informations. We are aware of short time duration of pressure peaks so our intention is to have a measuring freq about 100 Hz. Do you think it should be enough or we should go for higher freq? But as I already wrote to Tom, mainly we would like to capture pressure distribution not pressure peaks as itself.

    We would like to measure pressure distribution under constant speed steady states on a flat water - same as you did.
    These sensors are nice and cheap! But we are AFRAID of having a "flute/whistle" effect due to the hydrodynamic paradox. (Sorry, I don't know how is it called in English.) I guess that there can become local negative pressure at the inlet of small sensor hole and therefore it can suck small amount of air and devalue measured pressure.
    BUT if you have positive experiences with these sensors we will be more than happy... :)
    Anyway we would like to measure pressure distribution around steps and interceptors in the future with areas of negative pressure so I guess that we will have to go for stainless steel flush diaphragm anyway. But for now we can go for your type of sensors. Did you have any problems with them?

    Thanks, I will go through it...

    Interesting, thanks for the tip. We have gone through published verified results but we have to do our own measurements to validate our own numerical models and to get better understanding on what is going on...

    We will record acceleration data anyway so we can play with it later...

    Thanks again to everyone and as I already said I will be grateful for any information on this topic.
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  9. BMcF
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    BMcF Senior Member

    They performed very well for what we wanted..the static pressure distribution over the surface they were installed in. As long as the sensor "tap" is installed flush with the surface, the local static pressure is unaffected by the presence of the small sensor hole; there is no flow in or out of that hole and the effective local velocity at the hull surface is zero (boundary layer effects) anyway.

    We used fairly high data acquisition rates and were able to determine, from analyzing the dynamic content in the pressure signals, where flow around a submerged body transitioned from laminar to turbulent.

    The only drawback was the relatively short lifespan of the cheap sensors. Most of them quit providing reliable measurements due to fouling of the pressure tap..others simply quit working. However, the duration of our test programs are such that we don't really care that the sensors are "throwaways".
  10. Binze
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    Binze Junior Member

    Hi, thank you for your interest.

    I'm glad to hear that. Did you use it on a whole surface of a hull? Was it planing or a displacement hull? Did you use it just aft the spray root?
    We would like to measure area around (also in front of) spray root. So I'm quite worried that it can suck some air in higher trim (5 deg) situations when we go over the hump...

    And what was your sampling rate? We intend to have a frequency 100 Hz...

    BTW: I think I didn't mention that we are going to measure 1:1 model of a 6m boat.
  11. BMcF
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    BMcF Senior Member

    All of the measurements that we have done were on fully-submerged bodies (and/or foils), or submerged hull surfaces. We would sample at 20 Hz to obtain data sets where we only cared about average values and at 100 Hz if we intended to analyze the signal for dynamic content. So we did not have to be concerned about the mixed-media zone that you are interested in; I have a feeling that the "cheap sensor" solution would still work, but the true surface mount solution would, it seems to me, eliminate the guesswork there.

    A lot of our work involves models too..although often some rather large ones, manned, and up to 18 meters or more in length.
  12. Binze
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    Binze Junior Member

    Thanks for reply. We are concerned about this mixed-media zone with this kind of sensors. So we will probably do some testing with both sensors - these "cheap" ones and one with steel flush diaphragm which should provide valuable results and compare it...
  13. tspeer
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    tspeer Senior Member

    You might want to consider piezoelectric film. It has a high frequency response, can be molded to the shape of the bottom of your boat, and you can etch it to form pressure sensors of various sizes for different purposes. Compatibility with water might be a big issue, however.
  14. Binze
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    Binze Junior Member

    Hi, it is very interesting idea. I was searching the web but couldn't find any similar application - measuring pressure under water - with piezoelectric film elements. Have you ever seen it used in a similar application e.g. wings etc?

  15. SukiSolo
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    SukiSolo Senior Member

    I believe piezo films are used in professional quality microphones, maybe that area might help. Effectively your requirements are reversing the way they work so it should be possible, water proofing etc considered.
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