Patent Application
20110283569
The present invention relates to improvements in or relating to fluid jets and their effects and apparatus that can be developed to take advantage of specific forms of jets and methods of performing functions using such jets.
It is well known that when sea-going vessels operate in shallow water, the wash from the propeller can cause erosion of the bed. Propeller scour, as it is called, is the result of shear stresses, and to a lesser extent hydro-dynamic pressures, applied to the bed by the flow of water set in motion by the propeller. These forces cause surface particles to be dislodged, which then become carried along with the flow; the rate of erosion increasing as a higher power of the overlying flow velocity in excess of a certain threshold that depends on the bed material. Whilst propeller scour can be detrimental in ports, harbours and navigation channels, leading to undermining of structures and embankments, as well as unwanted siltation elsewhere, it can equally be beneficial if the process can be harnessed and applied in a controlled fashion.
U.S. Pat. No. 6,125,560 discloses a means for controlled application of the wash from a ducted propeller, for the purposes of seabed excavation and dispersal of the excavated material. No specific mention is made, however, about the nature of the flow or the excavation process; although it is envisaged that the main agency for dispersal of the material will be tidal currents. WO2004/065700 notes that since the propeller is located close to the duct outlet, the wash possesses certain flow features that are peculiar to propeller-generated flows. These include the fact that the flow is swirling (i.e. it has a component of rotation about the flow axis) and it is imbued with a number of concentrated vortical structures: including an axial hub vortex and a helical pattern of peripheral tip vortices.
The presence of swirl, together with the associated vortical structures, can enhance the excavation of seabed sediments by ducted propellers due, in part, to the unsteady nature of the flow. It can also enable the ducted propeller flow to be manipulated to enhance particular flow characteristics. WO2005/002735 describes a means for flow manipulation, which involves expansion of the flow by a diffusing, or flared, nozzle attached coaxially to the exit of the propeller duct.
It has been noticed, however, that if the flow from a ducted propeller is forced to converge, in a particular way, excavation will takes place: i) over a much larger area, ii) at a greatly increased rate, and iii) the excavated material will self-transport over long-distances before finally re-depositing. Each of these three attributes will be described in more detail below. The present invention is based upon the recognition that when all three attributes are operating in unison the ducted propeller has greatly increased utility for such applications as dredging, seabed levelling and underwater sediment management.
In order to appreciate the functional significance of the particular modification to a ducted propeller that produces the aforementioned desired attributes and which forms the subject matter of the present invention, it is necessary to have a basic understanding of propeller flows, particularly those from ducted propellers operating under high load. By high load is meant a ducted propeller operating in an essentially static mode and at maximum design propeller revs. In marine propulsion parlance this is often referred to as the bollard-pull (or maximum static thrust) condition. Close similarities thus exist between the ducted propeller of the present invention and such marine propulsion devices as: tunnel-thrusters of the type used on ferries and large vessels for slow-speed transverse manoeuvring. Similarly, an alternative usage for the present ducted propeller is as an axial flow propeller pump: for pumping large quantities of water at relatively low pressures.
The present invention provides an apparatus comprising a body having a fluid flow path defined between a fluid inlet and a fluid outlet and thrust means mounted within the fluid flow path to direct, in use, a flow of fluid, along the fluid flow path; wherein at least a portion of the fluid flow path comprises a duct and wherein the thrust means comprises a propeller mounted within the duct; characterised in that the apparatus further comprises a plate spaced from the fluid inlet defining a space therebetween; wherein a plurality of elongate pivotable vanes are positioned in a circular orientation in the space, about the axis of the flow path and with their pivoting axes aligned with the axis of the flow path; wherein the thrust means is adapted to rotate in a direction opposite to the direction of flow of fluid through the vanes into the space.
Preferably, the vanes are arranged in a circle so that their pivotal points are coincident with the lip of the fluid inlet and they have a height equal to the space between the fluid inlet and the plate; wherein the height to diameter ratio of the vanes is between 0.4 and 0.6, more preferably about 0.5.
Preferably, the vanes are collectively angled at an angle of between 45° and 75° to the radius of the circle that defines each pivot point, preferably about 60°.
The above and other aspects of the present invention will now be described in further detail, by way of example only, with reference to the accompanying drawings, in which:
The general form of the ducted propeller utilised in the present invention is shown in sectional view in
It should be noted that: i) the annular flow through the duct is forced to converge before it passes through the plane of the propeller by the combined shape of the duct and the motor housing, ii) the hub of the propeller has a diameter, which is approximately 0.3 times the diameter of the propeller and iii) the propeller has a slightly unusual pitch distribution (the blades are over-pitched in the hub region and under-pitched towards the tips). The latter is a subtle propeller design feature, intended to enhance static thrust that is not evident from either
For the purposes of seabed excavation or other applications involving impingement of the propeller duct flow against a surface, the duct arrangement shown in
The streamtube represents a free shear surface, across which there is a jump in axial as well as swirl velocity. Vorticity is associated with shearing between two fluid bodies and in the present context it can be thought of as the fluid-equivalent of roller bearings—allowing the duct flow to move relative to the still ambient without significant friction or exchange of momentum.
In order to appreciate the significance of voracity, and to better understand the features of this invention, the reader is invited to perform a simple demonstration. Take a pencil, and place it on the base of the palm of the left hand. Hold it in place with the finger tips of the right hand, and then move the right hand forward (while keeping the left hand still) so that the base of the palm of the right hand comes to coincide with the finger tips of the left hand. In carrying out this action it will be noticed that: i) the pencil rotates with a sense of rotation that is anti-clockwise for a forward movement of the right hand, and ii) the pencil moves a distance of one hand length, while the relative distance of movement of the hands is two hand lengths. This demonstration serves to highlight that whatever the relative speed of movement of the hands (provided that one hand is kept still), the pencil will always move at half this speed. Thus within a free shear layer, vorticity (the pencil) will always be transported at approximately half the relative speed of the adjacent fluid bodies (provided that one is static) and the sense of rotation of the vorticity will be determined by the relative direction of shear (relative movement of the hands). Note that if both hands (fluid bodies) move in opposite directions, the pencil (vorticity) may remain static and only rotate.
Since the flow from a ducted propeller possess both axial and swirl momentum the vorticity residing within the streamtube will be helical in character. Helical vorticity (or helicity) can be thought of as a combination of axial vorticity (which is associated with tangential or swirl fluid movement) and azimuthal or ring vorticity (which is associated with axial fluid movement). The familiar smoke ring vortex is an example of pure azimuthal vorticity, being always associated with axial flow. If the axial flow that sustains the smoke ring were also to rotate, the smoke ring would take the form of a helix.
The propeller blades, being moving boundary surfaces, are where most of the vorticity originates, as indicated diagrammatically in
Vorticity has no capacity for self-transport—just as the pencil only moves by virtue of the hand moving. Vorticity is, therefore, transported (advected) by the flow and for this reason it is often described as being ‘frozen’ within a flow. However, in real fluids with strong vorticity, the vorticity can equally be considered as driving the flow, through the concept of vortex singularities acting as momentum sources. This is tantamount to saying that the pencil causes the hands to move!
In normal ducted propeller jets the outer stream tube vorticity can be considered as driving the whole of the axial flow as well as a component of the azimuthal (swirl) flow; specifically the outer part of the swirl flow. The hub vortex vorticity can be considered as driving the remainder of the azimuthal (swirl) flow and a counter component of the axial flow. What the latter means is that in normal ducted propeller jets the centreline part of the jet actually has near zero axial velocity. Near zero axial velocity equates to elevated stagnation pressure and it is this hydrodynamic pressure force which accounts for static thrust in ducted propellers used for slow-speed propulsion.
A feature of normal propeller-generated flows is that at a certain distance downstream the vortical structures start to exhibit increasing instability. This is shown in
The present invention, which is shown diagrammatically in
The vanes (22) shown in
The effect of the vanes and the resulting inlet pre-swirl is to change the vorticity produced by the propeller dramatically; essentially removing the hub vortex and the axial vorticity component of the tip vortices. The resulting jet has little or no swirl, while the axial flow has uniformly high velocity across the width of the jet. Static thrust is thus sacrificed for the sake of increase axial flow production. Importantly, the streamtube (tip vortex) vorticity is retained so that the jet remains columnar and does not interact with the ambient fluid.
The change in inlet flow characteristics associated with the inlet vanes results, in effect, in a reduction in the angle of attack of the incident flow relative to the propeller blades. As a result, the propeller is obliged to operate in a less heavily-loaded condition (the propeller absorbs less torque for the same rotation speed), and so produces significantly less static thrust. The consequent reduction in outlet swirl is the flow manifestation of this change in propeller operating characteristics. These effects are particularly evident when a single ducted propeller, with inlet flow vanes, is operated in a suspended mode with the jet pointing vertically downwards. Under these circumstances the equipment exhibits a higher apparent submerged weight (due to the reduced thrust) and a decreased tendency to rotate about the point of suspension (decreased torque reaction from the propeller).
With the correct setting angle of the inlet vanes, static thrust (due to hydrodynamic pressure) and torque reaction can be all but eliminated. This is the condition, which in practice has been found to produce the maximum rate of seabed excavation. An approximately 60° negative vane setting angle has been found to be the optimum. This angle (28) is measured relative to the plane of each vane and a radial line (27) passing through the duct centreline and the vane pivot point, as indicated in the inset diagram in
The general features of the exit flow (i.e. submerged jet) from the propeller duct of the present invention are shown diagrammatically in
In a number of respects, including its impingement behaviour against a surface, the submerged jet from this device resembles a free-fall liquid-into-air jet. To illustrate this behaviour the reader is invited to carry out the following simple experiment. Turn on a kitchen tap slightly so that a thin steady laminar stream of fluid is produced. It will be noticed that the fluid stream remains circular and continues to contract (but progressively less rapidly) from the tap to the point where it impinges against the base of the sink. These characteristics of a free-falling jet result from the fact that the jet is driven by gravity rather than by fluid pressure. It will be further noticed that where the jet strikes the sink base it turns sharply to form a thin wall flow that runs out radially across the surface. The thin fluid wall flow has a radial velocity approximately equal to that of the free-falling jet. That is to say, there is no loss of momentum or turbulence-generation where the jet strikes the surface. At a certain distance from the point of impingement, which is large compared to the diameter of the free-falling jet, the thin-film wall flow suddenly increases in depth and its velocity decreases appreciably. This is referred to as a circular hydraulic jump, and it represents a transition from super-critical (laminar) flow to sub-critical (turbulent) flow.
Impingement of the jet from this invention against a surface is illustrated in
The combination of high-velocity jet impact, and high-velocity outward-deflected radial wall jet flow, is what makes this invention so effect for seabed excavation and other applications involving surface removal of material. Unlike the free-falling jet, however, which is constrained by gravity to flow downwards, the jet from the present device can be made to flow in any direction.
The present jet, like the free-falling jet, is able to extend out radially across the surface to many jet diameters before it becomes unstable. Instability in this case results in the vorticity finally rolling up to form a large roll vortex (30) as indicated in
Note also that the lateral distance at which the roll vortex forms relative to the diameter of the impinging jet is dependent on the impinging velocity of the jet. It is not particularly sensitive to the distance of the jet nozzle above the jetting surface.
During seabed excavation operations, it is believed that the roll vortex goes through repeated cycles of growth and collapse, for the following reasons. During the latter part of the growth stage, the roll vortex is so highly charged with suspended material that it becomes gravitationally unstable. This is where gravity acting on the dense fluid overcomes circulation, resulting in collapse and the spontaneous formation of a dense fluid outflow across the surface. Such a flow is known as a density- or gravity-current, and it provides a very effective means for transporting sediment over long distances, even across flat or very gently inclined slopes. For seabed excavation it provides the means for long-distance self transport of the excavated material into deeper water. Collapse to form a density-current effectively destroys the roll vortex, which then starts to reform—hence the cyclic process, which also results in sequential waves of density-current flow being produced.
Thus by the simple addition of a set of vanes, of the correct size and orientation, a ducted propeller of fairly standard design can be converted into an extremely effective and efficient means for seabed excavation and controlled dispersal of the material. The fact that the excavated material invariably gravitates into deeper water, in the direction of seabed slope, is particularly important for navigation dredging and bed levelling operations, where the object is generally to lower the bed to some specified minimum level. It is also important from an environmental standpoint since density-current transport occurs very close to the bed with very little lofting of sediment to higher levels in the water column.
While underwater excavation is the intended primary application of this invention, alternative applications include: underwater cleaning, such as bio foul removal from ships' hulls, and in a land context, sweeping of leaves and dust. The latter being an alternative to conventional brush sweeping or the use of air blowers. Note that because leaves and dust are gathered into a roll vortex it is possible to exercise a much greater degree of control over their onward transport. By tilting the jet slightly it is also possible to displace the material in a preferred direction. For the latter application it is envisaged that the simple ducted propeller, with inlet vanes, might be attached to the rotating shaft enclosure of a garden strimmer, providing an alternative ‘attachment tool’ to the strimmer head.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0724592.1 | Dec 2007 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2008/051191 | 12/16/2008 | WO | 00 | 6/3/2011 |
