Breakdown of Pulsating Water Jets
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A pulsating water jet, shortly after leaving the nozzle, is disintegrating into single drops, which is obvious through strobe illumination. Two mechanisms are effective. First the so called Rayleigh instability; mainly induced by surface tension. Second a kinematic effect at the nozzle, caused by a variation of velocities with time. These two mechanisms have either similar or have contradictory effects, depending on the character of the modulation. This modulation is applied in a plenum chamber just in front of the nozzle, with an outlet of 5 mm in diameter.
For closer inspection we have partially demounted the apparatus. Under the plenum chamber you see the inlet tube. Here the water radially and laminarily enters the plenum chamber by
way of a cotton wad filter.
The vibrator, left, is transferring its motion by a long rod on a semipermeable piston of sinter material to the stream of water. We can follow this motion well, in a demonstration at a low frequency. In the real experiment the displacement of the piston is far smaller with frequencies between 20 and 200Hz. The modulation amplitude of the velocity is overlapping the main flux,
which is 2 m/s. This modulation is indicated by a probe, which is calibrated in meters per second. The flexible wire leads to the oscilloscope.
For calibration the momentary velocity of the water jet is determined by a hot wire anemometer probe.
First the modulation starts with a frequency of 100Hz and the amplitude is 5% of the main flux. An oscilloscope shows the momentary characteristic of the modulation.
With small amplitudes a sinusoidal modulation of the velocity is generated.
On a first sight the well defined motion of the piston generates an apparently highly turbulent water jet.
This soon proves to be an illusion, for we at once get a fundamentally different impression as soon as we use stroboscopic illumination as in the initial scene. Flash and displacement of the piston are controlled by the same sine generator. Therefore we observe a stationary phenomenon, which is comparable to a freeze frame. During film exposure, the operation of the camera shutter was also synchronised. The frequencies of flash and picture were identical. The motion of the jet seems to be frozen, in accordance with the constant period of the nonstationary phenomenon. As soon as the modulation frequency is slightly differing from the flash frequency, we get the impression of an extremely slow motion exposure of the water jet. By that means details of the breakdown become clearly visible. What influence has the frequency of the modulation on the breakdown behaviour of water jets? At a constant relative modulation amplitude of 5% and a frequency of 50Hz, we can observe the beginning breakdown of the jet. Here in the lower right of the picture. Whereas at 25Hz the jet shows only a slight undulation. What effect has the amplitude of the modulation? This becomes obvious, if we slowly increase the modulation amplitude from 0 to 5%. The frequency is kept constant at 80Hz. Even at lower frequencies the jet soon starts breaking down, and single droplets appear. At a frequency of 80Hz obviously both mechanisms show optimal mutual enhancement. A sequence of various velocities in the nozzle causes periodically changing cross-sections of the jet. This enhances the tendency to drop formation. Under the influence of the surface tension the jet is interrupted at the thin regions. What effect is introduced with high modulation frequencies? If the jet is modulated at a frequency as high as 200Hz, a new phenomenon appears. Please watch the increasing instabilities shortly after ejection from the nozzle. Farther outside a damping phenomenon has a stabilising effect. This damping prevents the breakdown. First a velocity variation across the nozzle starts the formation of drops. In the preceding experiments at low modulation frequencies the surface tension enhanced this tendency until the jet broke down. Now, at the high frequency of 200Hz the surface tension acts in the opposite direction. In so far as it is damping the surface deformation, it is smoothing the jet. Kinematic effects caused by different velocities within the cross section at the nozzle and surface tension have opposite effects. This becomes obvious, when we increase the modulation amplitude. At a high modulation amplitude kinematic effects overcome the effect of surface tension. We observe a strong tendency to interrupt.
Let us now watch details of the breakdown of water jets. The modulation frequency is now 60 Hz. The amplitude is slowly being increased to 5% and then kept constant at this level. The camera is now slowly following the water jet into a region far off from the nozzle. Here the disintegration is starting a vibration, which produces droplets of different shapes. Now in a close-up view. An even closer look reveals clearly the disintegration of the jet with formation and subsequent vibrations of single droplets. Here the modulation amplitude is less than 5%. What happens when we increase the modulation amplitude to far more than 5% to within the range of the basic velocity? Qualitatively new variations of the cross section are observed. At higher amplitudes of modulation even closely behind the nozzle, parts of the liquid at different velocities first accumulate and then start flowing sideways, which at low amplitudes is prevented by surface tension. At a vibration frequency of only 20Hz gravity can pull the bouncing area downwards. This leads to total disintegration of the jet. Therefore bizarre formations in the shape of strings of pearls are being formed. Liquid particles, started in front of others, are overrun by those, started later. This becomes very obvious in a blown up section of 5 cm width.
Gravity spots the pulsating jet into several parallel strands. When contracted to a common axis, they can overlap each other and at certain places bouncing occurs. In a vertical jet, gravity acts in the direction of the axis of the jet. We now may understand what becomes obvious with the help of the slow motion effect already used before.
A thick high velocity part of the jet passes a thinner one. We observe bouncing and a sideways displaced liquid lamella. In the vertical jet we use a nozzle of 3.2 mm diameter and a flux of 4m/s. Some bizarrely shaped bubbles emerge symmetrically to the axis of the jet. Those sections, where lamellae are being formed, follow theoretical predictions. At first surface tension has only little influence on these shapes. Later on, downstream, it causes some metamorphoses.
Because of the different curvatures and the times of capture of the liquid volumina, a lamella symmetrically contracts towards the jet axis. At the free end of the lamella surface tension has an additional effect. Small marginal undulations produce little columns, which soon disintegrate into droplets. They are forming a crown and later on a flask appears. At last: bubbles. To the upper left of a bubble, two droplets stay remarkably stationary and reappear periodically, and accompany also the following bubbles. Obviously small marginal irregularities at the mouth of the nozzle influence the shape of the crown. Therefore slowly turning the nozzle makes the different peaks rotate in the same sense. Partial disturbance of the jet, here on the left of the axis, only slightly influences the stability of the right hand structure. This is a very striking evidence for the fact, that mainly initial local and temporal disturbances are responsible for the shape and metamorphosis of the jet. Instabilities, however, caused by friction and surface tension, are negligable. Here, as in many nonstationary phenomena, kinematic processes determine the local and temporal structure.
|Titel||Breakdown of Pulsating Water Jets|
|Alternativer Titel||Zerfall pulsierender Wasserstrahlen|
Meier, G. E. A.
Gotthard Glatzer (Redaktion)
Gerhard Matzdorf (Kamera und Schnitt)
Es gilt deutsches Urheberrecht. Der Film darf zum eigenen Gebrauch kostenfrei genutzt, aber nicht im Internet bereitgestellt oder an Außenstehende weitergegeben werden.
|IWF-Filmdaten||Film, 16 mm, LT, 174 m ; F, 16 min|
|Abstract||Interaction of running-time effects and surface tension (Rayleigh instability) in a fluid beam with velocity modulation. Basic velocities: 4 m/s and 2 m/s; jet diameter: 3.2 mm or 5 mm; disturbance amplitudes: 0 to 5 %. Pressure chamber with pulsator, pressure probe, and anemometer. Decay in drops. Slow motion photography (20-200 Hz) with synchronous stroboscopic illumination. Overlap effects. In a horizontal beam: bead chains and umbrellas. In a vertical beam: crowns, bottles, and bubbles.|
water jet / disintegration
run time effect