Damping (heave) plates having improved characteristics

This invention relates to improvements in damping plates (also known as heave plates) and, while not so limited, relates particularly to damping plates used in apparatus for converting energy contained in waves on the surface of bodies of water to useful energy.

The use of damping plates, e.g. a horizontally oriented flat plate attached to a submerged vertical spar of a floating oil platform, is generally known. The plates have the effect of increasing the mass and the viscous drag of the members to which they are attached, thus making the members less likely to be driven into destructive movements by excessively high surface waves and winds.

Such damping plates are also useful in certain types of apparatus, known as wave energy converters (WECs), for converting energy in surface waves on bodies of water to useful energy. One type of such WEC comprises a pair of floats which bob up and down in response to passing surface waves. Relative movements between the floats are used for driving an energy converter, for example, an electrical generator connected between the floats for generating electrical or any useful energy.

The amount of energy produced by a given WEC is a function of, among other things, the mass of the floats driven into motion by the passing waves. For increasing the effective mass of at least one of the floats, a practice is to suspend a submerged damping or heave plate from the float. The effective mass of the float from which the damping plate is suspended is increased due to the fact that the acceleration and deceleration of the plate through the water requires movements of some volume of water around the plate. The greater the amount of water moved by the plate, the greater is the increase of the float mass.

The amount of water moved by a given damping plate is a function of the size of the plate. The addition of a vertically extending structure, e.g. a lip along the edges of the heave plate, significantly increases the amount of water moved by the plate, hence the amount of mass added to the float.

While useful for adding effective mass to a float, a problem associated with the use of damping plates, at least in connection with WECs, is that the viscous drag forces associated with the plates tends to impede relative movements, thus tending to negatively affect power generation. The present invention provides means for minimizing the drag forces typically associated with the use of damping plates of the type described.

<patcit id="pcit0001" dnum="WO2004065785A1"><text>WO2004/065785A1</text></patcit> and <patcit id="pcit0002" dnum="US4232230A"><text>US4232230A</text></patcit> disclose structures in which damping plate comprises a first plate for being attached to the submerged end of a spar-like float in a body of liquid and subject to up and down movement in the vertical directions, said first plate being of generally flat configuration and having a pair of oppositely facing major surfaces extending transversely of the up and down movement of the spar-like float, and vertically extending structures mounted on said major surfaces.

SUMMARY OF THE INVENTION

The damping plate of the invention is characterised in that the vertically extending structures form a surrounding structure for said plate enclosing, with said plate, a volume of liquid, thereby increasing the volume of liquid pushed up and down during movements of the plate through the liquid, said vertically extending structures having streamlined configurations, thereby minimizing the amount of drag created by the damping plate.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are schematic and not to scale. <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1</figref> is a side view of a wave energy converter of known type including an elongated "spar" float having a damping plate attached to the lower end thereof;</li><li><figref idref="f0001">Figure 2</figref> shows the lower end of the spar float shown in <figref idref="f0001">Figure 1</figref> but with a known modification of the damping plate; and</li><li><figref idref="f0001">Figures 3 through 9</figref> are similar to <figref idref="f0001">Figure 2</figref> but show damping plates according to the instant invention.</li></ul>

DETAILED DESCRIPTION OF THE INVENTION

As shown schematically in <figref idref="f0001">Figure 1</figref>, a preferred WEC comprises a first, annular float 10 of a generally flat configuration, and a second, elongated float 20, referred to as a "spar" float, extending through an opening 22 through the annular float 10. In accordance with known technology, the two floats are configured to bob up and down in generally out of phase relationship in response to passing surface waves. A power take off device (PTO) 100 is connected between the float and spar to convert their relative motion into useful energy (e.g. electrical).

A damping plate 30 is disposed at the lower end of the spar float 20 for increasing its effective mass. As illustrated, the damping plate comprises an essentially flat plate having major surfaces 32 and 34 extending transversely to the directions of motion of the plate 30 through the water.

The effect of the damping plate 30 is to add mass significantly greater than that of the plate itself to the spar float 20. Such added mass is due to the fact that the acceleration or deceleration of the plate requires movement of some volume of fluid around the plate as it moves. It has been established that the volume of fluid that the plate will move, by itself and without the added structure herein disclosed, is proportional to the equivalent volume of the plate times some experimentally determined factor [see, <nplcit id="ncit0001" npl-type="b"><text>Chakrabarti, S. K. (1987), Hydrodynamics of Offshore Structures", WIT Press and Computational Mechanics Publications</text></nplcit>.] The volume of fluid moved is also a function of the frequency with which the plate is moving. This parameter is not discussed herein.

The equivalent volume of the plate depends upon the geometry of the plate, and the general rule is that the equivalent volume is equal to the area of the plate multiplied by a height dimension equal to a lateral dimension of the plate; for example, the width of a rectangular plate, the side of a square plate, or the diameter of a circular plate.

The accelerated flow inertia force caused by the damping plate is the added mass of the system (the density of water times the equivalent volume times the experimentally determined factor) times the acceleration of the system, or: <maths id="math0001" num="Equation 1"><math display="block"><msub><mi>F</mi><mi mathvariant="italic">Interial</mi></msub><mo>=</mo><mfenced><mrow><msub><mi>C</mi><mi>m</mi></msub><msub><mi>V</mi><mi mathvariant="italic">equivalent</mi></msub><mi>ρ</mi></mrow></mfenced><mi>a</mi><mo>=</mo><mi mathvariant="italic">AddedMass</mi><mo>*</mo><mi>a</mi><mo>,</mo></math><img file="EP2016228B1_D0001.tif" /></maths> where C<i>_m</i> is the experimentally determined factor V<i>_equivalent</i> is the equivalent volume defined above , ρ is the density of water, and <i>a</i> is the acceleration. Note that the added mass term, with units of mass, is the combination of the variables inside of the parenthesis.

For purposes of analysis, the viscous drag can be separated into two components known as skin friction drag and form drag. Skin friction drag arises from the friction of a fluid against the surface ("skin") of the object that is moving through the fluid. Form (or profile or pressure) drag is mainly due to boundary layer separation of the fluid due to the form, i.e. shape, of the object. Form drag varies (non-linearly) as the square of velocity; thus the viscous drag of the system is non-zero only when there is a non-zero velocity.

A boundary layer of fluid is created around an object due to the friction of the fluid against the surface of the object. Flow separation, or boundary layer separation, occurs when this boundary layer of fluid detaches from the surface of the object. When separation occurs the fluid forms eddies and vortices (spinning and often turbulent flow). Flow separation can (and in the case of damping plates does) result in increased drag as the object moves through the fluid. An experimentally determined drag coefficient relates the strength of the viscous drag force to relative flow velocity. When there is no flow separation the drag coefficient will be lower than when vortices are formed. The thinness of the plate and porosity of the plate are important factors for the drag coefficient since they create a series of sharp angles from which the flowing water can separate [see, <nplcit id="ncit0002" npl-type="s"><text>Chua, K.H., Clelland, D., Huang, S., and Sworn, (2005) "Model Experiments of Hydrodynamic Forces on Heave Plates", Proceedings of 24th International Conference on Offshore Mechanics and Artie Engineering, Halkidiki, Greece</text></nplcit>]. Thus, when the spar damping plate unit is moving in heave, the flow of water past these sharp edges has a tendency to create vortices, creating a turbulent boundary layer, hence increasing the damping of the system. In contrast to sharp edges, the flow around blunt shaped structures without sharp corners, such as cylinders or spheres, can exhibit no flow separation, separation with no vortex shedding, and separation with vortex shedding coinciding with an increase in heave amplitude (often evaluated normalized by the diameter of the object in heave; see, <nplcit id="ncit0003" npl-type="b"><text>Faltinsen, O. M. 1990; "Sea Loads on Ships and Offshore Structures"; Cambridge University Press, Cambridge</text></nplcit>). Thus if a blunt shaped object is moved only a fraction of its diameter no flow separation occurs.

Returning to the matter of adding mass to the system, a vertical structure, such as shown in <figref idref="f0001">Figs. 2-9</figref>, is added to the damping plates for increasing the volume of fluid moved by the plates. Each figure shows the lower end of a spar float 20 to which a known type of damping plate 30 is attached. The figures are only a schematic and it is possible to have a "spar" that is comprised of multiple components, a truss spar. Because the spar float oscillates in the vertical direction, the added structure encompasses both the upper and lower surfaces of the damping plates.

As previously described, the damping plate 30, by itself, moves a volume of fluid proportional to the "equivalent volume" of the plate, determined primarily by the area of the plate. With the added vertical structure, an additional amount of fluid is moved as a function of the volume enclosed by the plate and the added surrounding structure, i.e. the height of the added structure multiplied by the area of the plate. Also, in some instances, the added structure functions to increase the effective area of the plate thereby further increasing the amount of fluid moved by the plate.

In <figref idref="f0001">Fig. 2</figref>, the added structure comprises a number of thin plates 40 secured to and extending completely around the edge of the damping plate 30 and perpendicular thereto. According to the invention, the plates are edge-joined to one another, thus comprising a cylinder enclosing the plate 30. The additional added mass achieved by the vertical plates is, for movements in either the up or down direction, proportional to the vertical height of the plates 40 above or below the plate 30, respectively, multiplied by the area of the plate 30. The structure shown in <figref idref="f0001">Fig. 2</figref> is disclosed in the previously cited provisional application SN <patcit id="pcit0003" dnum="WO60796388A"><text>60/796, 388</text></patcit>. A problem associated with the <figref idref="f0001">Fig. 2</figref> damping plate 30, however, is that the plates 40 terminate in sharp edges which tend to generate undesirably high drag forces. Conversely, in the structures shown in <figref idref="f0001">Figs. 3- 9</figref>, sharp edges are avoided and lower drag forces are present.

In <figref idref="f0001">Fig. 3</figref> the added structure comprises a tube 50 of circular cross-section and outer diameter D disposed along the edge of the plate 30. This results, similarly as in <figref idref="f0001">Fig. 2</figref>, in an added mass proportional to the height of the tube 50 above and below the plate 30 multiplied by the area of the plate 30, but with the added effect that the area of the plate is increased by the area of the circular tube 50. An advantage of this embodiment is that the smooth circular shape of the vertical portion of the tube wall keeps the fluid flow attached to the structure thus decreasing the drag coefficient.

In <figref idref="f0001">Fig. 4</figref>, the added structure comprises an elliptic cylinder 60 (a 3-dimensional object with an oblong cross-section) surrounding the plate 30. This results in a larger addition to the mass added to the plate 30 then that provided by the structure shown in <figref idref="f0001">Fig. 2</figref> provided the longer side is oriented perpendicular to the plate 30. The smooth shape of the elliptic cylinder tends to keep the fluid flow attached to the structure thus decreasing the drag coefficient.

In <figref idref="f0001">Fig. 5</figref>, the addition comprises semi-circular beads or rims 66 facing away from the plate 30 at the top and bottom ends of a vertical cylinder 68 (as in <figref idref="f0001">Fig. 2</figref>). The rims 66 add to the area of the damping plate thus increasing the equivalent volume thereof, hence the amount of fluid moved thereby. The additional added mass achieved by the vertical cylinder 68 is proportional to the height of the cylinder multiplied by the area of the damping plate. The curved shape of the rims keeps the fluid flow attached to the structure thus decreasing the drag coefficient.

In <figref idref="f0001">Fig. 6</figref>, the additional structure is similar to that shown in <figref idref="f0001">Fig. 5</figref> except that the rims 70 face inwardly of the plate edge. An advantage of this is that the fluid inside the rims is isolated from the fluid around the plate structure thus decreasing the strength of the vortex shedding and hence the drag coefficient.

In <figref idref="f0001">Fig. 7</figref>, the added member comprises a cylinder 72 having a wall of V-shaped cross section thus including both vertical and horizontal components. As in the foregoing examples, the horizontal components result in added mass by adding to the equivalent volume of the plate 30, while the added mass achieved by the vertical components is the height of such components multiplied by the area of the plate. <figref idref="f0001">Figure 7</figref> also shows the placement of smooth (blunt-shaped) objects, here, full circular tubes 74 at the ends of the cylinder 72 for keeping the fluid flow attached to the structure for decreasing the drag coefficient. Other objects, such as the semi-circular rims 66 and 70 shown respectively in <figref idref="f0001">Figs. 5 and 6</figref> can be used.

In <figref idref="f0001">Figs. 8 and 9</figref>, the addition of thick circular members 80 or 84 to the plate 30 with vertical dimensions curving towards or away from the spar 20, respectively, increase the added mass as a function, as in previous examples, of the product of the vertical dimension and the plate area. The members 80 and 84 are shown as being relatively thick and hollow. Alternatively, the circular members can comprise curved plates. The smooth shapes of the circular members 80 and 84 help to keep fluid flow attached to the structure.

In general, the elimination of sharp corners on the edges of the plates or on vertically extending members added to the damping plates will decrease the drag coefficient. This is advantageous for WECs since decreasing the drag coefficient will result in larger relative motions and hence allowing for a more efficient capture of energy.

Another use of the invention is in industrial mixing processes (where an actuator causes a plate to oscillate in a container of non-mixed liquids) in which it is desired to suppress (or enhance) formation of small scale vortices. This can be accomplished by the use of vertically extending lip structures as above-described with more or less streamlining of the lip structures to obtain the desired results.