The present invention relates to a fluidic oscillator, and particularly to one used in a display fountain application.

Display fountains are essentially of two types. Static fountains are either on or off and when on send jets or sprays of water in a constant and unvarying pattern. Dynamic fountains vary the sprays and jets, increasing and decreasing their powers, and switching them on and off in an aesthetic display. This gives the fountain more interest, but requires expensive and potentially unreliable control mechanisms. Particularly favoured are displays involving bullets, that is to say, cohesive slugs of water issuing from a jet and arcing through the air with both a cleanly cut-off front and tail . Such a mechanism requires precise pump mechanisms and is difficult to sustain continuously for indefinite periods .

Fluidic devices do not involve moving mechanical components and are therefore generally more reliable. WO-A-2004/047997 discloses various fountain arrangements, some of which use a fluidic diverter with closed loop control that switches the diverter with a frequency dependent essentially on the length of the control loop. However, the frequency is generally too high for convenience. Various uses of the diverter are suggested.

The present invention is concerned with an oscillator whose frequency can be arranged more suited to fountain applications .

In accordance with the present invention, there is provided a fluidic device having an input and at least

two alternative outputs, wherein flow to at least a first of said outputs is to a pressure accumulator resulting in increasing pressure in said first output until a first high pressure threshold is reached at which point the device switches flow to a second of said outputs, the flow therefrom including said supply flow and at least a proportion of said original flow into said accumulator and until a second low pressure threshold is reached in said first output, whereupon the cycle is repeated.

Preferably, said first and second thresholds are substantially different pressures, resulting in considerable hysteresis and therefore low frequency of oscillation between flows from said first and second outputs.

Preferably, said fluidic device comprises a Coanda-effect diverter, the first output of which supplies a vertically-arranged column which is gradually filled until the pressure in said first outflow reaches said first threshold and flow switches to the second output.

In one arrangement, said diverter is bistable, said second output being provided with a feedback conduit to a control port which, when said conduit receives flow, switches said flow from said second to said first output, said conduit being arranged to receive flow only when the second output flow has reduced by exhaustion of the proportion of said original flow in said second output flow such that the pressure in said feedback conduit increases. In this event, it is not the reduction in pressure in said first output that directly switches flow from the second to the first outputs; rather, such switching is merely a consequence thereof.

In another arrangement, said diverter is mono-stable having an asymmetric nozzle and consequently preferring flow into said first output, at least until such first threshold pressure is reached.

In an alternative to said vertically-arranged column, said accumulator may comprise a chamber in which a compressible fluid is trapped and compressed as flow increases through said first output.

Alternatively, said first output may comprise a first, conical diffuser whose axis is coincident with the direction of said supply, and said second output may comprise a second diffuser whose axis is transverse said supply, the mouth of each diffuser and the supply being coincident.

Preferably, said second diffuser is a radial diffuser whose mouth surrounds the junction between the supply and the mouth of said first diffuser.

Said radial diffuser may deflect into an annular diffuser axially coincident with said first diffuser.

Preferably, said device comprises part of a fountain arrangement, wherein at least one of said accumulator and second output are provided with a jet. Of course, in the event that said accumulator is provided with a jet, the jet must not permit greater flow than said supply flow. Likewise, in the event that said second output is provided with a jet, the jet must permit greater flow than said supply flow..

Indeed, this is, of course, a general condition of the device to maintain oscillation that loads from the first

and second outputs must be controlled so that pressure accumulates in the accumulator when flow is to the first output, and is dissipated when flow is to the second output .

^• The invention is further described hereinafter, by way ^' of example only, with reference to the accompanying drawings which are schematic illustrations of fluidic arrangements, and in which: -

Figure 1 is an unvented bistable fluidic amplifier;

Figure 2 is the amplifier of Figure 1 with feedback control;

Figure 3 is a load-switched fluidic feedback oscillator in accordance with the present invention;

Figure 4 is as Figure 3, configured as a water display fountain;

Figure 5 is as Figure 4, but with a modified accumulator able to trap air and means for adjusting the trapped air volume;

Figure 6 is a fluidic diverter having internal feedback;

Figure 7 is a reverse flow fluidic diverter;

Figure 8 is an incorporation of the diverter of Figure 8 in an alternative arrangement of load-switched fluidic feedback oscillator in accordance with the present invention, in a water display application;

Figure 9 is a variation of the arrangement of Figure 8; and

Figure 10 are two arrangements of planar devices suitable for use in devices in accordance with the present invention.

The objective of these applications of fluidics is to produce interesting and artistic water displays in which

water jets fluctuate or jump rather than remain constant as in usual simple fountains . Although fluctuation and jumping can be achieved by "high technology" devices, these typically use moving parts and electric and/or pneumatic controls which incur high costs and may demand expensive maintenance. The innovative systems described here use no-moving-part fluidic devices, thereby reducing costs and potentially giving high reliability.

All of the proposed systems use a form of fluidic diverter. The first six figures relate to the so-called "wall-attachment" or "Coanda-effect" fluidic diverters (or "fluid amplifiers") . These are derived from the well established type of device shown in Fig 1. These are "planar" elements represented by profiles of constant- depth channels 2 cut into a plate 3 of material. Despite the symmetrical design, . supply flow 8 entering at the left is diverted to one of two outputs 14,15. The flow can be switched from one output to the other by brief control flows through control ports 19,20, but the control flow does not need to be maintained to maintain the state of switching. In a logical sense the device "remembers" the result of past control inputs, so it can also be regarded as a logic element. Fluidic logic systems can be based on such devices. The essential characteristic is a hysteretic or switching response to inputs. The hysteresis comes from the interaction of the turbulent supply jet with the side walls just down stream of the supply nozzle. This interaction • whereby a jet tends to stick to a wall is called the "Coanda effect". Signal amplification can be achieved because usually a small control flow can switch a much larger supply flow. By applying feedback from the outputs to the control ports, as shown in Fig 2, the diverter can be made to oscillate continuously. This is well known technology.

However new forms of oscillator are more versatile and better suited for fountain applications .

Other forms of diverter depend on the flow properties in diffusers. In fluid mechanics diffusers are diverging channels (round, square annular, radial or perhaps other configurations) in which flow is gradually decelerated. The purpose is to convert dynamic pressure (and kinetic energy) in the inlet flow into ^' static pressure (and potential energy) in the much slower outflow. Devices in which a jet is projected into the junction between two diffusers can also function as flow diverters because they can have multivalued flow-pressure characteristics, so that when connected to the correct type of output load, the overall system exhibits hysteresis and can divert flow. Another description of these effects is "negative output impedance" ^' (or resistance) . Certain types of load can also cause continuous oscillation resulting in large flow changes. These oscillatory effects are the basis for the fountain applications.

The standard feedback oscillator shown in Fig 2 is rather inconvenient for fountain applications. Drawbacks are that unless capacitive (compressible volume) -elements are inserted into the feedback paths 18, inertia alone is the main cause of signal delay. Unless very long pipes are used, this inertial delay is too small. The frequency of oscillation is then too high. Alternatively if capacitive volumes (eg vessels containing trapped air, or tall open-topped water containers), are inserted into the feedback paths 18, typically two are needed and the overall system is complicated and unwieldy for fountain applications .

A load switched feedback oscillator in accordance -with

the present invention is shown in. Fig 3. This relies on the fact that the switching point of a wall attachment diverter depends both on the control flows and on the output flows. In general, as the output flow is reduced for example by a resistive "load" (ie a restriction or back pressure) the control flow needed to switch the diverter decreases . Eventually if the outflow is very greatly restricted, the diverter may switch with zero control flow. This load sensitivity effect is used to cause oscillation as follows:

1) Supply flow of water is initially considered to be switched into first output 14 feeding a tall vertical container ^• 12 serving as a pressure accumulator. The container is empty at first so there is a very low output pressure at 14. The flow is held firmly to the upper sidewall 16 because there is no control flow in feedback channel 18. Upper control port 20 is, in fact, permanently blocked.

2) After a delay, the container fills with water so the pressure at output 14 of the diverter 10 (meaning the output of the diverter) becomes high. At a specific maximum threshold level, the output pressure is so high that the outflow is reduced and the diverter switches. The exact switching point depends on the design of the diverter and on the supply pressure in relation to the height of the water in the container.

3) The flow is now diverted to second output 15, which is at a low pressure. Simultaneously output 14 is no longer fed by the diverter so flow reverses in this output and the container 12 starts to empty. The supply flow and reverse flow from first output 14 then discharge from second output 15. Despite the feedback channel 18

from output 15 to the control port 19, the control flow needed to switch the diverter back to output 14 cannot yet be supplied by the feedback connection. This is because the very high outflow (low output "load" pressure) demands a very high control flow, and this cannot be provided by the low output pressure in 15.

4) Eventually the container empties sufficiently for the output 14 pressure, and reverse flow, to drop. Now, the outflow from second output 15 is low enough to permit switching by feedback control flow in loop 18. The cycle is then complete and it repeats forming a regular oscillation.

The advantages of this system for fountain use are that a single container is used to provide the time constants. A single feedback channel is necessary, but it is not a cause of a delay time-constant, so it can be a short pipe or channel integrated into the diverter module. The control port 20, and any control connection on one side of the diverter, can be eliminated. Although the overall system is asymmetric, this is not a disadvantage. For example, the outflow from second output 15 can undergo very large fluctuations because its maximum value is the sum of the supply flow and the reverse flow from the container. Its minimum value can be zero or, in fact slightly negative. These large fluctuations can produce spectacular water display effects.

Hitherto, attention would have concentrated on the output available from what is referred to above as the first output 14, ie that output attached to the container or accumulator. Although this output is a useful signal source, particular emphasis here is on the output available from the second output 15. This output can

carry a fluctuating flow whose maximum value is significantly bigger than the average supply flow. This is because the contents of the container or accumulator discharge predominantly through the- second output 15, thereby adding to the supply flow which must also discharge through this outlet.

Fig 4 shows how the load switched feedback oscillator can be applied as a fountain 100. Two fountain nozzles 21,22 are supplied from outputs .1 and 2. Water jets therefore issue alternately from them. In the case of nozzle 21, the pressure tends to fluctuate in a fairly smooth manner, following the varying height of liquid in the container 12. The flow from nozzle 22 can be more sudden. This is most evident when the diverter switches from output 1 to output 2. The flow may change from zero to its maximum ¹ value at this instant.

The nozzles 21,22 may take various forms, for example nozzle 21 may be one of many, all fed from the first' output-side 14 of the diverter. Nozzle 22 on the second output side 15 of the fountain 100 can be configured as an annular gap producing a bell-like water flow. The fountain then alternates between a spray and a smooth bell-shaped flow. It is evident, of course, that output from nozzle 21 must be less than the supply flow 8, otherwise the container 12 will never fill. Likewise, the flow from nozzle 22 must be more than the supply flow, or the container 12 will never empty.

The maximum pressure at nozzle 21 is limited by the height of the container (and the water column within it) when its top is open as shown in Fig 4. In this case, the supply pressure must be kept below the value which would cause the container to overflow because this would

prevent switching.

To avoid this limitation, the top 30 of the container may be sealed, as in Fig.5, so that air is trapped in its upper part 32. In this case the pressure at the bottom 34 of the container (in the vicinity of nozzle 23) is not determined solely by hydrostatic pressure due to the height of the liquid column, but mainly by the compressed state of the trapped air 32. This can be relatively high, within limits set by the strength of the container or accumulator and on the supply pressure. Therefore with a sealed container or accumulator, very powerful fluctuating jets can be produced.

Indeed, nozzle 23 could be omitted. Fountain effects would then come entirely from the right-hand, second output 15 and its nozzle (s) 24. Multiple nozzles could be used having a wide range of designs, however the common characteristic of the jets would be that a large fluctuation could be provided by the oscillator.

Thus, a container accumulator with a sealed top enables high-pressure oscillations to be generated, whereas a container accumulator which is open topped leads to lower frequencies and more gentle effects.

Moreover, by adding or subtracting (by bleeding-off) air to or from the otherwise sealed container 30, the frequency can be adjusted to achieve the optimum effect. A wide variety of effects can be got from a single basic system by modulating the air volume in the container. Two methods of adjusting the air volume are shown in Fig 5. One method simply allows air to be bled from the upper part of the container via a tube 35 and conventional valve 37. This valve could be remote from

the fountain and could be manually or automatically controlled. Another method uses a water driven jet pump 70 to entrain atmospheric air and inject it via tube 36 into the container. High pressure water supplied to a jet 39 entrains air at inlet 40 and discharges it along with some water into the receiver 38. Alternatively a conventional source of compressed air could be used to supply the air, again via tube 36. Control of the inflow or outflow of air from the container can be effected by various conventional means including the use of solenoid valves, non-return valves or other standard methods of pneumatics .

An extremely simple but effective mode of control is by using a single bleed tube 35 and to have a fixed restriction in place of valve 37. When suitably chosen such a restrictor allows air to gradually leak from the accumulator causing a gradually increasing frequency of the fountain display. Eventually when the accumulator is filled with water to the maximum level, air can be replenished simply by turning off the water supply to the fountain. Water then drains from the accumulator and air enters it from atmosphere via tube 35.

A further use for the sealed container-type of accumulator is to facilitate the operation of the oscillator when supplied by a purely gaseous medium, such as compressed air. Oscillatory pneumatic power would then be available from the nozzles previously described • as fountain nozzles.

Fig 6 shows the general shape of a further simplified design for the basic fluidic diverter. Here both control ports have been eliminated. What was the feedback channel in the designs of Figs 3-5 has been replaced by

an asymmetric configuration in the sidewalls 17,18 downstream of the supply 8. The same load-switching can be achieved as in the devices of Figs 3-5 provided the device is essentially monostable, or at least preferentially one-sided being that side which is provided with the pressure accumulator. Hence this special purpose diverter can be used for all the foregoing applications .

Similar fountain effects can be generated by oscillators using so-called negative-resistance phenomena. Like the diverter-based oscillators described with reference to Figs 1 to 6, the negative resistance oscillator depends on a fluidic device with three pipe connections (or three "terminals") . The foremost example is the A-type reverse flow diverter (or RFD) 150 shown in Fig 7. This is a section through an axisymmetric device which consists of a nozzle 128 of a supply 8 discharging axially into a conical diffuser 121 (having output 122, separated from the nozzle by a radial diffuser 123. The three terminals are therefore the supply nozzle 128, output from the conical diffuser 121, and output 124 formed by the annular gap between the periphery of the two discs 130,132 constituting the radial diffuser 123.

The important characteristics of the RFD are that, if the supply is fed at a constant flow, the output pressure and flow from output 122 exhibits a negative output resistance phenomenon when the output is restricted beyond a specific degree. Thus, as the output flow diminishes, the output pressure also diminishes. It only increases when the outflow is negative however. For a ^■ significant- range of reverse ^' flow the pressure is still low. Hence it can be said that the device favours reverse flow, and that it acts as a reverse flow diverter

for certain ranges of output pressure. [1]

The operating characteristics of the RFD allow it to be used in an oscillatory water display system as shown in Fig 8. Here again the system is mainly axisymmetric. The conical diffuser 121 feeds a container 112 and fountain nozzles 140. The radial diffuser 123 discharges into a plenum 142 which feeds fountain nozzles 144. The overall operation is similar to that of the planar diverter-type fountain oscillators, however all feedback effects causing the oscillation are intrinsic to the RFD.

The oscillatory cycle starts with the container 112 at a low pressure being fed with flow and pressure in the positive output resistance region of the RFD' s characteristics. Eventually at a certain maximum pressure, the RFD jumps across the negative resistance region into one of reverse flow, and the container empties causing a large outflow from nozzles 144.

As with the planar devices the same variety of options can be used concerning . the trapping of air in the container and the existence or otherwise of the associated nozzles 140.

The radial diffuser may make it difficult to accommodate the RFD into practical systems. A more compact design is shown in Fig 9. Here the radial diffuser is replaced by an annular diffuser 146 surrounding the central conical diffuser 121. The annular output from this annular diffuser is directly analogous to that from the radial diffuser in the basic RFD of Fig 7.

The RFD can be made in planar forms as shown in Fig 10. These profiles represent constant-depth channels cut into

a plate. Like the axisymmetric RFDs, the main design features are a supply nozzle 158 aimed at the mouth 162 of a receiving diffuser 160, a second diffuser 166 originating in the gap between the nozzle 128 and mouth 162. The alternative configurations shown in Fig 10 are to suit different arrangements of nozzles and containers connected to the outputs. All the same nozzle and container variants can be used with these planar devices.

Reference

[1] Anderson, NM and Tippetts, JR, Analysis of a Fluidic Oscillator with Power Output. 84-WA/DSC-8 Am. Soc Mech Engineers, New York 1984