1. Technical Field
This invention describes a simple apparatus and method for producing vortex ring bubbles of a gas in a host liquid. Once provided with a source of compressed gas, the basic geometry of the device establishes the conditions such that it will repetitively and endlessly produce gas-filled, vortex ring bubbles in a host liquid, at a rate determined only by the pressure and in-flow rate of the gas source. The device requires no high-tolerance components and is low-cost to manufacture. It has no moving parts, and will not wear out. It requires no periodic maintenance servicing, no human intervention, and no fine adjustments to sustain its operation.
2. Description of the Prior Art
Some forty years ago it was first reported that it is possible to generate rising toroidal ring-shaped bubbles, or ring bubbles as they are sometimes called, of gas within liquids. These are in fact vortex rings in the liquid, in which the gas collects in the ring-shaped core of the vortex and is thereby made visible as a circular tube of gas. In recent years it has become appreciated that these rings are a natural phenomenon that are even produced by whales and dolphins, evidently simply for amusement. Those creatures have sometimes been observed to create a vortex ring from their flippers, into which they exhale a bubble of air that is then drawn into the core of the ring to create the ring bubble. More often however, they create rings by rapidly exhaling a short upwards pulse of air which then evolves into the ring. Skilled professional divers have also been known to produce them by the analogous means of carefully exhaling a short pulse of air upwards into the surrounding water medium. Some experience on the part of the diver is necessary, but with practice quite impressive rings can be created, and these can travel upwards for large distances before breaking up. The skill required lies in being able to properly control the characteristics of the exhaled pulse of air so that rings will form, as opposed to the more familiar chaotic plumes of bubbles. If the right conditions are established, smooth circular rings will evolve. The ease with which this can be done follows from a mechanism of self organization or self stabilization, in which the swirl, or fluid dynamic circulation about the core of the gas ring stabilizes the entire ring so that it quickly develops into a smooth symmetric shape. Self stabilization and self organization of vortex rings is a common natural phenomenon that can be seen in smoke rings in which the self-induced motion quickly organizes even a distorted shape into a smooth circular ring. For the case of the ring bubble, the process leads to a ring that defies intuition by not collapsing into a chaotic plume of bubbles.
In fact it has sometimes been argued that these gas-filled toroidal bubbles are analogous to the familiar smoke rings in air. However, they are more complex as two distinct fluid phases are involved, namely the liquid medium, and the tubular core of gas. It has been known for over a hundred years that a tube of gas in a liquid should spontaneously collapse and break up through the effect of surface tension instability. That this does not happen for the toroidal tube of the ring bubble can be attributed to the stabilizing influence of the fluid dynamic circulation around the tubular core. In physical terms, the centrifugal force of the liquid spinning around the core opposes and balances the collapsing force of the surface tension (the same mechanism stabilizes the more familiar bath tub vortex).
In fluid dynamic terms, the surface tension pressure directed inwards on the gas at the gas/liquid interface of the core, is given by ΔP=σ/R, where σ is the coefficient of surface tension of the liquid and R is the radius of the core of the vortex. The magnitude of the outward pressure arising from the centrifugal force can be determined from an analysis of the forces on a small element of liquid at the interface and can be shown to be 2ρR2ω2, where ρ is the liquid density and ω is the angular spin velocity of the gas/liquid interface. For a stable ring bubble, these two components of force should be equal.
This condition will exist on the inside surface of the core of the ring bubble vortex and shows that a bubble ring is only possible if the right volume of gas is issued and if the right circulation is imparted to it so that the conditions are maintained. In addition, it can be seen that a small thin core will rotate relatively quickly to preserve stability, while a thicker core must turn more slowly.
For the rising vortex ring bubble, there is also an upward buoyancy force present, but that is balanced by a downward cross-flow force arising from the lateral spread of the spinning core of the ring, analogous to the lateral force on a spinning ball. Thus, the ring, once formed, will steadily rise and spread out and thin. If the ring rises a large distance, then the local static pressure falls in relation to the internal pressure within the ring, so that there will be a countering tendency that slows down the thinning of the ring. However, eventually a point is reached where viscosity dampens the energy of the circulation so that surface tension then dominates leading to breakup of the ring. Despite this, very long lived rings can be created before breakup occurs.
Various U.S. patents document methods of producing vortex rings of different co-mingled liquids and gasses. U.S. Pat. No. 3,589,603 by Fohl allows two different fluids to come together in a co-annular nozzle and mix to form a vortex ring. The fluid motions are generated by two moving pistons, but the device does not consider the case of one fluid being a liquid and the other being a gas as would be needed for forming a gas-filled ring bubble. The inventor gives no evidence that the device could produce toroidal ring bubbles.
U.S. Pat. No. 5,100,242 by Latto uses a technique in which a moving orifice plate generates a ring vortex that can be used to enhance fluid mixing. The inventor claims it can be used in water to produce aerated rings through seeding of the vortex flow with bubbles, but this is not the same as producing ring bubbles which are single, coherent self-organized structures. These coherent structures require very specialized conditions of pulse flow and pulse duration if they are to form.
There are also a number of U.S. patents that describe different methods of creating gas-filled rings by generating the required pulsed flow of gas in some way. For example, U.S. Pat. No. 4,534,914 to Takahashi et al. describes a device that uses an accumulator with a diaphragm in one wall that unseats a spring loaded valve when under pressure allowing gas to flow out into a nozzle. The nozzle has a second elastic valve at its exit which is driven open by the pressure it is exposed to following the opening of the spring valve. As the flow exits through the two valves, the pressure in the accumulator falls, both valves close, creating a short duration pulse of gas. If the mechanical parameters of the device are chosen properly, a gas-filled vortex ring forms at the tip of the elastic valve. In a further embodiment, they replace the spring loaded valve with a pressure sensitive switch on the diaphragm to open the flow from the accumulator to the elastic valve, once a predefined pressure is reached. In a third embodiment, they use a timed pulse to a solenoid-actuated valve to feed the accumulator so that the rising pressure in the accumulator opens the second elastic valve creating the flow. Thus while operator skill or human intervention is not required to produce ring bubbles, proper tuning and setting of the valve parameters is required. If the valves leak, or jam, of fail in some other way, the operation of the device will be compromised.
In another example, in U.S. Pat. No. 5,947,784 to Cullen, a very similar device is described. In one embodiment it uses a small spring loaded annular nozzle at the end of a tube into which an operator blows to unseat the valve momentarily and create the ring. This device attempts to minimize the operator skill that is needed to generate rings. However, the operator effectively acts as a second valve that determines the strength and duration of the pulse that creates the vortex ring, so that some skill and human intervention is needed.
In a second embodiment, the pulse is created by an electrically driven pump actuated by a timed circuit. This is very similar to the third embodiment of Takahashi et al. As before, the pressure at which the vortex forms is a consequence of the resilience of the valves, and the duration of the pulse is also determined by this pressure and the volume of the tubing feeding the valve. Failure or jamming of the valve will compromise the operation of the device.
The method described by Whiteis, U.S. Pat. No. 6,488,270, is somewhat different and allows gas to flow from a pressurized source and to build up in a contained pocket under a plate. This plate tilts around a pivot in response to the buoyancy of the gas buildup. This directs the gas to a nozzle and allows it to momentarily escape into the surrounding liquid. The weight of the plate terminates the flow once a certain volume of gas has been expended. Therefore, although the device does not have a valve in the usual sense, the tilting plate clearly acts as a valve to create the required momentary flow of gas. If the mechanism fails or jams, the device will no longer generate rings. In a second, but different device by the same inventor, Whiteis, U.S. Pat. No. 6,736,375, the gas is captured within an inverted bell-like container and is released by an operator momentarily depressing a lever. This opens a valve at the top of the bell thereby creating a flow out of the container. The duration of the flow is determined by the skill of the operator so that some human intervention is required for the device to work.
Finally, an alternative device, developed by the present inventor, U.S. Pat. No. 6,824,125, uses an electric solenoid valve and timing circuit to open and close the flow from a pressure accumulator through a specially configured nozzle. Because of the short time that the valve is open, and because of unique features of the design of the nozzle exit that control capillary effects, very controlled exit flow can be established. Additionally, the sudden acceleration of the flow through the nozzle generates the fluid dynamic vorticity that is known to be essential to the formation of vortex rings. These two features allow very repeatable vortex ring bubbles to be formed on demand and without operator skill. The unique feature of the invention is that it allows one apparatus to develop different size and shaped rings.
This, and the other inventions that have been described, all use specially configured valves, for the creation of a momentary exit of gas flow through a nozzle, in an attempt to establish the favorable conditions that are necessary to give rise to rings. It can be inferred from these inventions, and as is well known from the science of fluid mechanics, that there are two important characteristics that need to be controlled in order that the exiting flow will self evolve into a vortex ring:                1. The first is the strength of the expended pulse of gas, namely its source pressure. A reasonably high source pressure gives rise to a sudden acceleration of the gas when the valves are opened. This creates an exiting flow that is rich in fluid dynamic vorticity which is well known to be important to developing the fluid dynamic circulation needed for forming vortex rings.        2. The second is the time duration of the pulse. The flow into the ring must be sustained for just the right amount of time so that the evolving flow field will self organize into a single ring. If too short, a ring will not fill out. If too long, the ring will be broken up by the subsequent flow.        
It is apparent that the various inventions that have been described strive to properly achieve these two conditions by various means. However, from the preceding discussion, a number of observations can be made and which can be summarized as follows:                1. The various configurations generate a pulse flow but generally require a specially sized mechanical valve, or a resilient valve, or a spring loaded valve to control the pulse flow. Some of them even require a second additional, properly-sized valve in order to operate. Indeed, the prior art clearly suggests that a complexity of valves is the only possible way that the right flow conditions can be established for vortex ring bubbles formation.        2. If any of these valves fail mechanically, or leak, the devices will cease to work correctly. For the devices to operate continuously, periodic maintenance is needed to prevent this.        3. Even with proper maintenance, valves such as these will eventually wear out, so that long-term continuous operation of the devices can not be expected.        4. Some of these devices require properly tuned electronic circuits to operate properly. Failure of any electrical component, or loss of electrical power will cause the devices to cease top operate.        5. Some of these devices will not operate independently of human intervention. Indeed, the skill of the operator may even be essential to the successful generation of ring bubbles.        6. Some of the devices, with their multiplicity of valves and moving parts are quite complex and consequently, would not be low-cost to manufacture.        