Hydraulic hysteresis phenomenon

[horstborsch]

I have the following unexplained phenomenon:

(please excuse incomprehensible expressions I´m not a native speaker)

Experimental setup:

A 6 meter long measurment pipe is filled with a hydraulic fluid and has a square cross section (30mm²). The fluid in the pipe is pressurerised to 40 bar and has a temperature of 20°C. At the end of the pipe a device is installed to bring in the same fluid at arbitrary pressures (up to 2500 bar). Note the higher the pressure is the higher the temperature of the inserted fluid is (at 1500 bar ~100°C). The insertion can be timed within 0,5 and 4 ms. In this time depending on the actual pressure and time span the inserted mass is about 180mg.

The sense in that is to measure the progression of the timed mass insertion at high resolutions. For these purposes a pressure sensor is installed short after the insertion device. The additional inserted mass generates an excess pressure in the pipe which propagates with the speed of sound of the fluid (c(T)) (~1000m/s). At the other end of the pipe the additional inserted mass discharges through a restrictor.

The progression (gradient, characteristics) of the insertion can be seen as an idealised rectangular devolution. So the measured excess pressure of the passing pressure wave is expected to have the same shape. This is not the case. At the End of the insertion the pressure signal doesn´t fall back to zero but dies away by a 1/x behaviour. This decay develops even stronger if the insertion is repeated frequently so that the temperature is rising in the pipe. By choosing longer insertions times this phenomenon develops even stronger. So it seems to be at least partially dependend on the temperature gradient and it has some cumulative characteristics. The speed of sound in the fluid should be sufficiently high enough to carry off the excess pressure so that this hysteresis phenomenon shouldn´t appear.

A sample can be viewed here: http://www.bilder-hosting.de/img/WGOUG.jpg

Considerations so far:

1) calculation of cumulative reflections due to the developing temperature gradient
-> has some similarities to this characterisitcs but is not strong enough and doesn´t explain this hysteresis even with a "cold" pipe.
2) calculation of dissipative energy which heats up the local fluid volume and effects a pressure rise
-> the effect is too small and only visible at the maximun pressure and not at the decay
3) the reflected signal at the end of the pipe doesn´t reach the sensor in time so that it can´t overlay the source signal
4) cavitation should show dropping pressures through the collapsing bubbles

I would appreciate any suggestions for the source of this pheanomenon

 

[gtoguy97]

.One possible explanation for this phenomenon is that the pressure gradient created by the inserted fluid propagates through the pipe and interacts with its own pressure wave. This interaction can create a resonant frequency in the pipe, causing the pressure wave to amplify and persist in the pipe. This could explain why the pressure does not return to zero after the insertion of the fluid. Additionally, the resonance could be amplified even further if the frequency of the pressure wave is close to the natural frequency of the pipe, leading to an even greater amplification of the pressure wave.

 

[sir-pinski]

.

The phenomenon you are describing is known as hydraulic hysteresis. It is a common occurrence in hydraulic systems and can have various causes. One possible explanation for the behavior you are observing is the presence of air bubbles in the fluid. These bubbles can act as small compressible volumes that can absorb pressure waves, causing the pressure signal to decay in a non-linear manner. This effect can become more pronounced at higher pressures and temperatures, as the bubbles become more compressible and can absorb more energy.

Another possible explanation is the presence of turbulence in the fluid. Turbulence can cause fluctuations in pressure, which can also lead to a non-linear decay of the pressure signal. This turbulence can be caused by various factors such as flow rate, pipe roughness, and fluid viscosity.

In addition, the shape and size of the insertion device can also affect the pressure signal. If the device is not perfectly aligned with the pipe, it can create turbulence and cause the pressure signal to behave in a non-linear manner.

It is also possible that the material of the pipe itself could be contributing to the hysteresis phenomenon. The material may have certain properties that affect the propagation of pressure waves, causing them to decay in a non-linear manner.

Further investigation and experimentation may be needed to determine the exact cause of this phenomenon. It may also be helpful to compare your results with other similar experiments and to consult with experts in the field of hydraulics.