GB2131489A - Obtaining useful energy from a flowing stream - Google Patents

Abstract

Parallel cylindrical bars 7 are mounted with their axes horizontal in a frame 4, 5 which is itself then mounted within a stream of flowing liquid so that the cylinder axes lie transverse to the flow. The movement of the stream over the cylinders causes the frame to oscillate, under elastic restraint, in a third direction which lies normal both to the flow and to the cylinder axes. The oscillations drive an energy converter such as an electrical generator 17. <IMAGE>

Description

SPECIFICATION Improvements in or relating to vibrators This invention relates to vibrators, especially vibrating devices that can be combined with energy converters by which the vibrations may be converted into more useful forms of energy. In particular it relates to a vibrator comprising a cylindrical or other elongated structure which can be mounted in a stream of moving fluid, for instance a river, so that the structure is set into transverse vibration by the reaction of the fluid against it, and thus becomes a source of useful power. Many methods have of course been tried for damping or otherwise diminishing the vibrations that elongaged structures like tall chimneys or underwater foundations tend to develop when fluid flows transversely past them.

This invention however arises from realising that the energy from a suitable body, deliberately arranged so as to be set into vibration within a flowing mass of fluid, may be put to useful effect.

According to the invention a vibrator comprises a cage member carrying at least one elongated structure, a framework in which the cage member is mounted for reciprocation in a direction perpendicular to the length dimension of the elongated structure, means for attaching the framework to the energy converter whereby the reciprocation may be converted to useful energy, and means for mounting the framework in a stream of fluid with the direction of flow at right angles both to the length of the structure and to the axis of reciprocation, whereby the interaction of fluid and structure sets the framework into reciprocation.

Preferably there are a plurality of elongated structures arranged with their length dimensions parallel one to another.

The framework may include or have attached to it an elastic restraint providing a restoring force which increases with departure of the cage member from the mid-point of its oscillation. The cage member may comprise frame members to hold the elongated structures in their desired arrangement. These frame members are preferably disposed so that when the cage as a whole is vibrating, they sweep out a minimum volume. They may for instance be plate-like lying edge-on to the directions of flow and parallel to the axis of reciprocation. The upstream and downstream faces of the cage should be as open as possible to allow free passage to the fluid through the cage. The elongated structures, which may be plain cylinders of circular crosssection, may be arranged in nested fashion in parallel rows, each row lying parallel to the axis of vibration.The spacing between the centres of adjacent bars of any one row may be in the range of about 2.5 D to 1.5 D, preferably about 2 D, where D is the diameter of a bar. The centres of the bars in one row may be staggered relative to those in an adjacent row, and the spacing between adjacent rows may be of the order of about 3.0 D to 1.0 D, preferably about 2 D.

The invention also includes a power generator comprising a vibrator, as already described, when fitted in a conduit for fluid, for instance a river or other watercourse. The axis of vibration may then be vertical, and the elongated structures may lie with their length dimensions transverse to the flow. In cross-section, the shape of the cage may closely follow the shape of the stream in which it is operating and is preferably symmetrical, thus calling for contouring of the banks of the stream if they doe not naturally show such symmetry. The generator may also be sufficiently symmetrical in an upstream-downstream direction to allow for reversal of the direction of the stream, e.g. in tidal conditions.

The mounting of the cage member may comprise bearings and a suspension, including for example a cantilever or other spring, above the surface of the fluid, and guides beneath the surface to constrain the cage to substantially linear vibration. Such guides may include anchors securing the bottom of the cage to the bed of the stream.

The invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic section on the line I-I in Figure 2; Figure 2 is a diagrammatic elevation of a vibrator set up in a river, taken in the direction of the arrow Il in Figure 1; Figure 3 is a partial and diagrammatic section on the line Ill-Ill in Figure 2, and Figure 4 is a graph.

The vibrator is set up in a river flowing in a bed having sloping banks 1 and a base 2. The vibrator comprises a cage member 3 having plate-like frame members 4 and top 5. The sides of the cage facing respectively upstream and downstream are unobstructed so that the water of the river flows through the cage as freely as possible. Spanning the cage, and supported by members 4 is a nest of cylindrical bars 7 arranged, as Figure 3 shows most plainly, in a matrix of vertical rows 8 and horizontal rows 9. Figure 3 is a partial view onlyin a typical case there will be more rows 8 and 9 than are shown in Figure 3. Two anchors 10, one upstream of the vibrator and the other downstream, are attached to the frame members and secure the cage in the direction of flow.A shaft 1 fixed to the top face 5 of the cage is supported by cantilever springs 12 mounted on a gantry 1 3, itself mounted on the bank 14 to either side of the river. Shaft 11 passes into a housing 1 5 mounted on gantry 13, and the upper end of shaft 11 carries the primary member 1 6 of a linear generator shown diagrammatically at 17: the secondary member 18 of this generator is fixed on gantry 13.Oscillation of shaft 11 in an axial direction, that is to say a direction at right angles both to the axes of cylinders 7 and to the direction of flow of the river, thus causes oscillation of primary member 1 6 within secondary 1 8 and generates useful electrical power to be conducted to a point of use by way of leads 1 9. The lines of anchors 10 lie nearly parallel to the river bed, and thus the anchors exert little restraint on the vertical movement of the cage.

As Figure 1 shows, the vibrator will act as a weir across the river causing a difference between the upstream and downstream depths. If the upstream and downstream flow velocities and depths are respectively v1 and d1, and the v2 and d2, as shown in Figure 1, then it may be shown that the energy change across the weir for each Newton of fluid flowing is: (d1 +v12)-(d2+v22) 29 2g The frame members 4 are flat and lie edge-on to the direction of flow and parallel to shaft 11.Thus the flowing water does minimum work on these members, but it does considerable work in passing through the array of cylindrical bars, and promotes vigorous vibration of the whole elastically-supported cage in the vertical direction, that is to say up and down the axis of shaft 1 This vibration is caused by alternate but simultaneous pressure changes on individual bars of the cage. The change of energy across the cage is partly dissipated within the cage and partly in the turbulent wake, and the invention depends upon the recovery of such energy from the cage by the application of damping which is additional to that already inherent in the motion of the spring and the cage bars through the fluid.In the apparatus described the additional damping is provided by the electricity generator 17, but of course the invention includes apparatus for generating other forms of useful energy.

Additional damping applied to the vibrating system will, inevitably, reduce the amplitude of vibration.

For the bars 7, and thus for the cage 3 that holds them, to vibrate at all when the river flows through the cage, it may be shown that the value of the function V/ND, known as the "period pzarameter", should lie between certain limits. In this formula V=the flow velocity of the river, N=the frequency of the vibration and D=the diameter of each bar. Roughly, this value must lie between 2.5 and 8. It may be shown that the period parameter has typical values for each of the three forms of excitation which a cylinder placed in flowing fluid may undergo. Its value is determined by secondary boundary layer behaviour in the region of separated flow around the bar. Where there is cross-flow vibration, as in the cases already described by way of example, the dominant value of the parameter is normally about 6.5.Whereas my invention includes vibrators with only a single bar within the cage, I have found that by arranging a plurality of bars within the cage in certain ways the magnitude of V/ND may be diminished to values between say 2.7 and 3.2 for a given flow velocity V, leading to a significant and desirable increase in the frequency of vibration N which enhances the efficiency of the conversion of the vibratory electrical energy. Another and equally important consequence of such improved arrangement of the bars within the cage is to produce a stable response curve, that is to say a stable relationship between the functions V/ND and a/D, where a is the the amplitude of vibration of the cage taken from the mean position.Figure 4 shows two curves (a) and (b): curve (b) is an example of a typical unstable relationship that is liable to exist when the cage contains only a single bar, or a plurality of bars inappropriately arranged, while curve (a) exemplifies the more stable relationship that I have found to be possible. The advantage of the stable relationship is that the reduced amplitude a/D, which is representative of the energy available, remains relatively unchanged for quite large changes in stream velocity V.

Amplitude of vibration of the cage is related to the value of function Am/pD2 in which 3 is the logarithmic decrement of successive amplitudes of the decay curve of the system in still fluid, high values indicating that much damping is present in the system, m is the effective mass per unit length of the bars, and p the density of the flowing fluid. The function bm/pD2 appears to represent a ratio between a typical damping force and a typical exciting force, high values leading to the cessation of vibration and low values to high amplitudes of vibration. If energy is to be extracted from the system the value of Sm must be as low as possible before additional damping is applied.This means that the construction of the cage and other vibrating parts must be as light as possible to minimise m and that a must be as small as practicable.

Figure 3 illustrates a form of arrangement of bars within the cage which I have found tends to lead to the advantageous decrease in the value V/ND that I have already referred to. The bars are arranged in vertical rows or ladders 8, and in horizontal rows 9. The centres of the bars or rungs of each ladder are spaced from each other by a vertical interval lying in the range of about 2.5 D to 1.5 D, preferably 2.0 D, and the spacing between adjacent ladders is in the range of about 3.0 D to 1.0 D, preferably 2.0 D. The bars in one ladder are horizontally aligned with those in the next ladder but one, but displaced by half the vertical interval from those in the adjacent ladder.

The cage should be located so that the top bars 7 in the cage do not break surface and so that the top member 5 is clear of the water when it is operating, and so that the bottom bars do not strike the bed of the river. As Figure 2 shows most plainly, the outlines of the cage and of the bed and banks of the river may be closely similar, clearance between them being sufficient only for movement and adjustment of the order of say 2D.

Since as already stated it is generally advantageous for the profiles of the cage and watercourse to match as nearly as possible, each vertical row or ladder 8 preferably extends from as near the water surface to the bed of the stream as possible. with the bars at the preferred spacing.

For a given rung diameter an increase in the number of ladders, that is to say in the horizontal dimension of the matrix of bars, will result in an increase in the energy removed by the cage from the flow of liquid. In general, maximum energy should be removed consistent with such operating restraints as the maximum premissible change in head between the upstream and the downstream sides of the cage. Small diameter rungs will require a relatively large number of ladders, and vice versa, the number of ladders being inversely proportional, approximately, to the rung diameter. For an energy change which is a fixed proportion of the upstream energy-say 1/5th-tests suggest that as alternative arrangements using different numbers and diameters of bars are tried, the total volume of the bars in the resulting cage varies little.It should also be observed that the system shown in Figure 1, having symmetry (especially of anchorage and of bar shape) in both directions relative to the fluid flow, is suitable for use in tidal or other conditions in which the flow may periodically reverse. The operation of the vibrator is essentially that of a damped, forced vibration at resonance.

Claims (Filed on 11.3.83) 1. A vibrator comprising a cage member carrying at least one elongated structure, a framework in which the cage member is mounted for reciprocation in a direction perpendicular to the length dimension of such elongated structures, means for attaching the framework to an energy converter whereby the reciprocation may be converted to useful energy, and means for mounting the framework in a stream of fuid with the direction of flow at right angles both to the said length dimension and to the axis of reciprocation, whereby the interaction of fluid and any such elongated structure sets the framework into reciprocation.

2. A vibrator according to Clairn 1 comprising a plurality of elongated structures, arranged with their length dimensions parallel one to another.

3. A vibrator according to Claim 1 including an elastic restraining member associated with the framework, and providing a restoring force which increases with departure of the cage member from the mid-point of its oscillation.

4. A vibrator according to Claim 2 in which the elongated structures are plain cylinders of circular cross-section, arranged in nested fashion in parallel rows with each row lying parallel to the axis of vibration, and with the centres of the cylinders in one row being staggered relative to those in an adjacent row.

5. Power generating means comprising a vibrator according to Claim 1 , fitted in a conduit for fluid such as a river or other watercourse and operatively connected to an electrical generator.

6. A vibrator according to Claim 1, substantially as described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. as possible. with the bars at the preferred spacing. For a given rung diameter an increase in the number of ladders, that is to say in the horizontal dimension of the matrix of bars, will result in an increase in the energy removed by the cage from the flow of liquid. In general, maximum energy should be removed consistent with such operating restraints as the maximum premissible change in head between the upstream and the downstream sides of the cage. Small diameter rungs will require a relatively large number of ladders, and vice versa, the number of ladders being inversely proportional, approximately, to the rung diameter. For an energy change which is a fixed proportion of the upstream energy-say 1/5th-tests suggest that as alternative arrangements using different numbers and diameters of bars are tried, the total volume of the bars in the resulting cage varies little.It should also be observed that the system shown in Figure 1, having symmetry (especially of anchorage and of bar shape) in both directions relative to the fluid flow, is suitable for use in tidal or other conditions in which the flow may periodically reverse. The operation of the vibrator is essentially that of a damped, forced vibration at resonance. Claims (Filed on 11.3.83)

1. A vibrator comprising a cage member carrying at least one elongated structure, a framework in which the cage member is mounted for reciprocation in a direction perpendicular to the length dimension of such elongated structures, means for attaching the framework to an energy converter whereby the reciprocation may be converted to useful energy, and means for mounting the framework in a stream of fuid with the direction of flow at right angles both to the said length dimension and to the axis of reciprocation, whereby the interaction of fluid and any such elongated structure sets the framework into reciprocation.

2. A vibrator according to Clairn 1 comprising a plurality of elongated structures, arranged with their length dimensions parallel one to another.

3. A vibrator according to Claim 1 including an elastic restraining member associated with the framework, and providing a restoring force which increases with departure of the cage member from the mid-point of its oscillation.

4. A vibrator according to Claim 2 in which the elongated structures are plain cylinders of circular cross-section, arranged in nested fashion in parallel rows with each row lying parallel to the axis of vibration, and with the centres of the cylinders in one row being staggered relative to those in an adjacent row.

5. Power generating means comprising a vibrator according to Claim 1 , fitted in a conduit for fluid such as a river or other watercourse and operatively connected to an electrical generator.

6. A vibrator according to Claim 1, substantially as described with reference to the accompanying drawings.