WO2014104978A1

WO2014104978A1 - A turbine

Info

Publication number: WO2014104978A1
Application number: PCT/SG2013/000556
Authority: WO
Inventors: Jing Liu; Andrew Thomas NORMAN; Mohan Dass Ettanoor Thuppale
Original assignee: Nanyang Technological University
Priority date: 2012-12-28
Filing date: 2013-12-27
Publication date: 2014-07-03

Abstract

A turbine (100) comprising a nacelle (104); and at least one projection (108) provided on the nacelle, wherein the at least one projection comprises at least one edge configured to generate complementary vortices that collectively increase velocity of a near-field wake generated by rotor blades (102) of the turbine.

Description

A Turbine

Field

The present invention relates to a turbine. More particularly, but not exclusively, it relates to a tidal current turbine configured to increase velocity of a near-field wake generated by rotor blades of the turbine.

Background

A turbine is a machine which converts kinetic energy of a fluid flow (free) stream (e.g. wind or tidal current) into mechanical or to mechanical/electrical energy. During the process, a wake is consequently generated behind the turbine as flow leaving the rotor of the turbine has lower kinetic energy, and the wake field is dominated by turbulent and low velocity flows. As the wake extends further downstream, neighbouring higher momentum flow will mix with the wake field flows, thereby re-energising the wake. But when unaided, the wake field can extend downstream up to a distance of 30 times the diameter (i.e. "D") of the rotor of the turbine until the velocity of the wake field recovers to approximately that of the free stream [1]. Any downstream turbines operating in the wake field may thus experience power loss and higher fatigue loading [2]. That is one crucial reason for arranging turbine arrays based on optimised power density per unit flow area of the available sea-bed real estate. For this reason, in practice, for wind turbines, the space between the turbines is arranged to be around 6D to 10D in the longitudinal direction, and about 1.5D to 3D in the lateral direction [2, 3]. On the other hand, for tidal current turbines, the longitudinal spacing is typically set at about or more than ZD, and the lateral space is around 2D [4, 5].

Therefore, it is a challenging task to arrange the turbines closer together to increase the power density derivable from the turbine arrays. Specifically, this requires recovering the low speed flow in the wake field to as close to that of the free stream and within as short a distance as possible. It is necessary to find effective ways to re-energise the wake field using adjacent high momentum flows. As known in the art, the wake field may be divided into a near-field wake and a far-field wake. The near-field wake is defined as a region extending to a maximum distance of 5D (i.e. ≤ 5D) behind the turbine rotor, and the far-field wake is a region extending beyond the distance of 5D (i.e. > 5D) [2, 6, 7]. Particularly, the flow characteristics of the near-field wake are determined b an associated geometry of the turbine and the turbine performance or its efficiency, whereas the flow characteristics of the far-field wake are mainly determined by the flow convection and turbulent mixing. Higher turbulence level can energise a wake field faster through efficient mixing of high and low velocity streams. Another known method to re-energise any wake field is to introduce, direct, or inject higher momentum stream into the wake field as illustrated in [8].

Indeed, the overall shape and velocity profile of the wake field affect arrangement of turbine arrays in a flow stream. The kinetic energy is lower in the near-field ake behind the turbine rotor compared to upstream of the turbine. As a result, it undesirably causes lower energy extraction potential for downstream turbines, even if those downstream turbines are configured with similar power efficiency as the upstream turbines. It is thus necessary to quickly recover the velocity/energy of the wake.

One object of the present invention is therefore to address at least one of the problems of the prior art and/or to provide a choice that is useful in the art.

Summary

According to a first aspect of the invention, there is provided a turbine comprising a nacelle; and at least one projection provided on the nacelle, wherein the at least one projection comprises at least one edge configured to generate complementary vortices that collectively increase velocity of a near- field wake generated by rotor blades of the turbine.

The at least one edge may comprise a plurality of edges that are arranged to be contiguous. Preferably, the plurality of edges may comprise a leading edge, a straight edge, a radial extension of the straight edge, and a trailing edge. The nacelle may have a shape of an airfoil profile configured to reduce hydraulic losses, and the airfoil profile may be a symmetrical profile. The airfoil profile may be a NACA 0038 profile.

Preferably, the leading edge may be arranged to begin from a section of the nacelle having a maximum diameter. The leading edge may be further arranged to be curved in one portion relative to the longitudinal axis of the nacelle. The maximurn diameter may be arranged to be 0.456D, D being the diameter of a circle circumscribed by the rotor blades. The maximum diameter of 0.456D may be approximately 38% of the chord length of the nacelle.

The at least one projection may be configured with a length of 0.45C, C being the chord length of the nacelle. The leading edge may be configured with a length of 0.285C, C being the chord length of the nacelle. C may have a value of 1.2D, and D is the diameter a circle circumscribed by the rotor blades.

The radial extension may have a length approximately 16% of the radius of a rotor formed by the rotor blades. A portion of the at least one projection comprising the straight edge may have a radial height of approximately 70% of a rotor formed by the rotor blades, and the radial height may be defined relative to the longitudinal axis of the nacelle.

The at least one projection may comprise a plurality of projections equal to a number of the rotor blades of the turbine. The number of rotor blades may be at least three. The at least one projection may comprise a rib. The turbine may be a tidal current turbine. The nacelle may have a cylindrical shape.

According to a second aspect of the invention, there is provided a turbine comprising a nacelle having an airfoil profile configured to reduce hydraulic losses for increasing velocity of a near-field wake generated by rotor blades of the turbine.

Preferably, the airfoil profile may be a symmetrical profile, and the airfoil profile may be a NACA 0038 profile. The nacelle may have a section configured with a maximum diameter. The maximum diameter may be arranged to be 0.456D, D being the diameter of a rotor formed by the rotor blades. The maximum diameter of 0.456D may be approximately 38% of the chord length of the nacelle.

It should be apparent that features relating to one aspect of the invention may also be applicable to the other aspects of the invention. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

Brief Description of the Drawings

Embodiments of the invention are disclosed hereinafter with reference to the accompanying drawings, in which:

FIG. 1 is a side-elevation view of a turbine arranged to increase velocity of a near-field wake generated by rotor blades of the turbine, according to a first embodiment of the invention;

FIG. 2 includes FIG. 2a and FIG. 2b, which are respectively a top view and a side-elevation view of a rib attached to the nacelle of the turbine of FIG. 1 ;

FIG. 3 is a perspective view of a generic turbine used in Computational Fluid

Dynamics (CFD) simulations to derive the turbine of FIG. 1 ;

FIG. 4 is a Table depicting relevant dimensions of the turbine of FIG. 3, and other physical parameters adopted for the CFD simulations;

FIG. 5 shows the relevant dimensions of a computational domain used for the

CFD simulations;

FIG. 6 includes FIG. 6a and FIG. 6b, which respectively depict two turbines each having a different nacelle shape investigated in the CFD simulations;

FIG. 7 includes FIG. 7a to FIG. 7d, which depict false-colour graphical results of the CFD simulations performed for the different turbines of FIG. 6;

FIG. 8 is a Table depicting relevant numerical results of the CFD simulations performed for the different turbines of FIG. 6;

FIG. 9 includes FIG. 9a and FIG. 9b, which respectively illustrate visualisation of tip vortices generated by the turbine of FIG. 6b, and path-lines of vortices around the near-field wake of the same turbine;

FIG. 10 illustrates path-lines of vortices along surfaces of the nacelle and the attached three ribs of the turbine of FIG. 1 ;

FIG. 1 1 shows a comparison plot of the normalized velocity distribution of a wake field for the turbine of FIG. 6b, and the turbine of FIG. 1 ;

FIG. 12 shows a plot of velocity magnitude along the flow direction from the domain inlet of the computational domain of FIG. 5 to a distance of 7D downstream of the computational domain; FIG. 13 shows a plot of percentage increase of velocity magnitude and kinetic energy at different locations downstream of the turbine of FIG. 6b, and the turbine of FIG. 1 ; and

FIG. 14 includes FIG. 14a and FIG. 14b, which respectively depict false-colour graphical results of velocity contours of the turbine of FIG. 6b, and the turbine of FIG. 1.

Detailed Description of Preferred Embodiments

A turbine 100 configured to increase velocity of a near-field wake generated by rotor blades 102 of the turbine 00 is disclosed, according to a first embodiment as shown in FIG. 1. The number of rotor blades 102 in this case is configured to be at least three and based on the National Advisory Committee for Aeronautics (NACA) profile 9618. It is to be appreciated that the blades' dimensions may be configured to suit the requirements of the application and are preferably optimized for maximising power extraction. The configuration of the rotor blade 102 may be optimized for maximising the power extraction. Specifically, to improve the near-field wake energy distribution/level, a novel technique of increasing the level of turbulence generated by the turbine 100 is disclosed herein. It will be appreciated that the turbine 100 may be a wind turbine or a tidal current turbine; in this instance, reference will be made (in a non-limiting manner) in the context of the tidal current turbine. For convenience, the three- dimensional Cartesian coordinate axes (hereinafter 3-D reference axes) 103 are shown on the right- bottom corner of FIG. 1 for reference to aid understanding of how the turbine 100 is orientated. Specifically, the 3-D reference axes 103 is arranged such that the x-axis points out of the paper, the z-axis is on the left of the x-axis, while the y-axis is above the x-axis. The x-axis, y-axis and z-axis directions as mentioned hereinafter are made with reference to this 3-D reference axes 103, which is also shown in other drawings, where necessary, for the same purpose. In FIGS. S and 10, the 3-D reference axes 103 is depicted as a perspective view to be in line with the 3D context of those drawings as shown.

The turbine 100 comprises a nacelle 04, a rotor comprising a rotor hub 106 and rotor blades 102 attached thereto, and a plurality of projections 108 provided on the nacelle 104. The nacelle 104 is formed of metal, although other suitable materials such as composite materials may also be used depending on the application of the turbine 100. The nacelle 104 is preferably configured to have a symmetrical airfoil profile. The . airfoil profile is preferably selected from the NACA 00XX series to enable downstream flows behind the rotor blades 102 to have identical flow paths over 360 degrees of sweep. Such a shape chosen for the nacelle 104 minimises hydraulic losses, to be elaborated below in subsequent paragraphs. The nacelle 104 is slightly truncated at its head in order to allow the rotor hub 106 to be fitted to the nacelle 104. An amount of truncation of the head of the nacelle 104 is approximately equal to a diameter of the rotor hub 106.

·

For this embodiment, a NACA 0038 profiled shape is chosen for the nacelle 104 to appropriately house a size and shape of an off-the-shelf 1 kW electrical generator driven by the rotor hub 106. The selected NACA 0038 profiled shape for the nacelle 104 beneficially results in increased radial velocity component of turbulent flows formed behind the rotor blades 102. The maximum lateral cross section "D_n" of the nacelle 104 as shown in FIG. 6b is configured based on a diameter of the chosen 1 kW electrical generator and its associated mechanical mounting requirements and the metal skin of the nacelle 104. In this case, "D_n" is defined with a value of 0.456D, which is about 38% of the (non-truncated) chord length, "C" of the nacelle 104 (being configured to be around 1.2D). To reiterate, "D" represents the diameter of the rotor of the turbine 100. A view of just the shape of the nacelle 104, without adding the plurality of projections 108, can be seen from FIG. 6b. The plurality of projections 108 provided on the nacelle 104 may be configured to be any suitable shape and size, but in this case, the projections 108 comprise fins or ribs, each of which is arranged with a plurality of edges (see FIGS. 2a and 2b) that are contiguously formed and designed to generate complementary vortices that collectively increase the velocity of the near-field wake. The plurality of projections 108 are all similarly configured in this instance. FIGS. 2a and 2b respectively show a top view and a side elevation view of one of the ribs 108. The projections 108 serve to re-energise the near-field wake by creating highly turbulent flows that consequently enhance mixing between high and low momentum turbulent flows behind the turbine 100, while not causing any loss in power of the associated turbine 100. The projections 108 may thus be referred to as flow modifiers. Preferably, the projections 108 are provided substantially close to the rear of the nacelle 104 as can be clearly seen from FIG. 1 , and are equally spaced to the neighbouring adjacent projections 108. Preferably, a number of the projections 108 mounted on the nacelle 104 are equal to a number of rotor blades 102 for optimum performance, but this is not to be construed as being limiting in any way, since other suitable configurations may be adopted depending on requirements of an intended application. Hence, a minimum of at least one projection 1 08 is all that is required. Hereinafter, the projections 108 in this embodiment will be referred to as ribs 108 for convenience.

Each rib 108 has a plurality of edges which comprise a leading edge 202, a straight edge 204, a radial extension 206 of the straight edge 204, and a trailing edge 208. Specifically, the leading edge 202 is configured to start from a point located at the maximum lateral cross section "D_n" of the nacelle 104, where the diameter is the greatest on the nacelle 104. From a side-elevation view, the leading edge 202 has a convex shape until it gradually straightens to form the straight edge 204. That is_t the leading edge 202 is made blunt radially in the radial direction to avoid non-beneficial vortices from being induced. For the same reason, the portion of the rib 108 comprising the leading edge 202 is configured to be curved when viewed from the top as shown in FIG. 2a, so as to be approximately aligned with turbulent flows emerging from and in between the rotating rotor blades 102 of the turbine 100 and flowing over the nacelle 104, as shown in FIG. 1. In particular, the degree of curvature of the leading edge 202 is defined by an acute angle, " 3" 210, which is also the angle of the above mentioned emerging turbulent flows measured with respect to the longitudinal axis of the nacelle 104 when the turbine is operating at its maximum rated power. That is, the angle "β" 210 is also considered the arc angle of the curvature of the rib 108. The angle "β" depends on the turbine 100 design and can be approximated to the attack angle of the rotor blades 102 with respect to the fluid flow. Specifically, the angle "β" 210 is determined by a range of factors including a rotational speed of the turbine 100, a configured angle of attack of the rotor blades 102, an efficiency of the turbine 100, velocity of free streams encountered by the turbine 00, and geometry of the nacelle 104. Similarly, this portion of the rib 108, where the leading edge 202 is formed, gradually straightens out over the angle "β" 210, at where it merges with another adjacent section of the rib 108, on which the straight edge 204 is formed, to be in line with the longitudinal axis of the nacelle 104. As will be appreciated, the preceding statement is described in the context from a top view of the rib 108 as shown in FIG. 2a.

As shown in FIG. 1 , an optimum radial height, h, of the section of the rib 108 comprising the straight edge 204 to the longitudinal axis, L_c, of the nacelle 104 is preferably about 70% of the radius of the rotor of the turbine 100, so as to be able to induce formation of a continuous beneficial vortex flow (extending from the leading edge 202 to the trailing edge 208) that complements spiral flow emanating from the rotational plane of the rotor blades 102, as shown in FIG. 10.

An optimal length of the rib 108 is determined by its flow straightening characteristics, which begins from the maximum lateral cross section "D_n" of the nacelle 104, i.e., at where the leading edge 202 starts. In this instance, the optimal total length of the entire rib 108 is arranged to be 0.45C, while the length of the straight edge 204 is arranged to be about 0.285C. "C" represents the (non-truncated) chord length of the nacelle 104 and preferably has a configured value of 1.2D. The afore stated values are all related to the dimensions of the nacelle 104, which are in turn related to the diameter of the turbine rotor. These relationships are derived for optimum beneficial vortex flow generation and turbulent mixing of spiral flows emanating from the rotational plane of the rotor blades 102 of the turbine 100. The straight edge 204 is further extended radially upwards, away from the longitudinal axis of the nacelle 104, by about another 16% of the radius of the rotor of the turbine 100 to form the radial extension 206. That is, the radial extension 206 has a partial concave shape. The radial extension 206 is concluded to meet the trailing edge 208, as shown in FIG. 2b, which has an overall effect of forcing turbulent flows over the rib 108 to climb higher and thus create useful vortices (see FIG. 10). The trailing edge 208 facilitates generation of useful vortices that combine with vortices generated by the leading edge 202 of the rib 108, as shown in FIG. 10. The results of FIG. 10 will be explained in greater detail below. FIG. 1 also depicts the configured rotational direction, ω, of the rotor of the turbine 100 (which is in a counter-clockwise direction as indicated by an arc arrow 110), the turbulent flows over the surface of the nacelle 104, and the effect of the ribs 108 (i.e. the flow modification features). Particularly, turbulent flows just upstream of the ribs 108 meet one lateral side of the associated ribs 108 and are forced to climb up and over the leading, and straight edges 202, 204, plus the radial extension 206, which are collectively denoted by the symbol "( )" in FIG. 1 for simplicity. Subsequently, the turbulent flows downstream of the ribs 108 passing at the opposing lateral side of the associated ribs 108 attach to the wall of the ribs 108 and exit from the trailing edge 208, which is denoted by the symbol "©" in FIG. 1. These two (up and down) streams of turbulent flows consequently twist together to form a complimentary flow vortex that extend into the near-field wake of the turbine 100, which in the process transfer the momentum of the complimentary flow vortex to the near-field wake. The related velocity path-lines of the turbulent flows induced by the nacelle 104 and ribs 108 of the turbine 100 are shown in FIG. 10. By comparing with FIG. 9b, it can clearly be appreciated that a diameter of the turbulent domain is much larger than that of a similar turbine that is arranged without any ribs 108. In this way, by utilising the nacelle 104 (with a NACA profiled shape) and ribs 108 for the turbine 100, mixing between high velocity and low velocity turbulent flows is thus improved and enhanced in the near-field wake, which advantageously re- energises the near-field wake effectively. This re-energising of the near-field wake manifests as increased velocity of the near-field wake.

In summary, the ribs 108 serve to deflect high kinetic energy turbulent streams into the centre of the near-field wake, and also to create beneficial vortices at the leading edge 202, at the straight edge 204 and at the trailing edge 208, in order to increase the energy level of the near-field wake. More specifically, the ribs 108 mounted substantially at the rear of the nacelle 104 (with NACA profiled shape) create a pair of complimentary spiral turbulent flows per rib 108 that additively form a complimentary flow vortex to re-energise the near-field wake. Compared to a conventional cylindrically shaped nacelle, the present proposed nacelle 104, having an airfoil profile shape based on the NACA 0038 profile, beneficially reduces hydraulic losses in the near-field wake. In this way, the nacelle 104 and the ribs 1 8 synergistically function to increase the energy level of the near-field wake. It is to be appreciated that the proposed turbine 100 may be utilised to further increase the energy extractable from a given sea-bed real estate.

Next, a process showing how the design for the turbine 100 of FIG. 1 is obtained is described. Discussion of the process will include explanations of the numeral simulation methods used (i.e. Computational Fluid Dynamics), and relevant performance results of CFD simulation studies ("CFD simulations") to verify effectiveness of the design arrived for the turbine 100.

• TURBINE GEOMETRY USED FOR S TUDYING WAKE FIELD

FIG. 3 shows a perspective view of the structure of a generic turbine 300, used as a base design for deriving the design of the turbine 100 of FIG. 1. Thus, the generic turbine 300 can also be termed as a "Bare Turbine". The generic turbine 300 is a tidal current turbine comprising three rotor blades 302 secured with a rotor nose 308 to a rotor hub 304, and a cylindrical nacelle 306 configured to house an associated generator (not shown) driven by the rotor hub 304. The diameter of the cylindrical nacelle 306 is arranged to be the same as the rotor hub 304. For the CFD simulations performed, the generic turbine 300 was placed in a simulated environment configured with 2 m/s flow streams. Specific dimensions of the generic turbine 300 and other physical parameters used for the CFD simulations are set out in the Table 400 shown in FIG. 4. The rotor dimension is the same as the turbine 600 of FIG. 6b.

• NUMERICAL METHODS USED

As can be seen from FIG. . 5, relevant dimensions of a computational (rectangular-like) domain 500 used for the afore mentioned CFD simulations comprise a 4D upstream length immediately preceding the rotating plane of the turbine 300, a 15D downstream length immediately subsequent to the rotating plane of the turbine 300 (i.e. representing a total length of 19D), a 10D height, and a 10D width. "D" represents the diameter of the rotor of the turbine 300 of FIG. 3. . For the CFD simulations, width-wise, the same turbine 300 was positioned in the centre of the computational domain 500, and thus the turbine 300 was spaced at a length of 5D to each opposing lateral boundary walls of the computational domain 500. The boundary conditions employed for the CFD simulations include velocity inlet, pressure outlet, and no-slip boundary walls (i.e. for the turbine solid walls and computational lateral boundaries).

An unstructured tetrahedral mesh was generated in the whole computational domain 500 using the ICEM CFD™ software from ANSYS. In particular, a fine tetrahedral mesh was created for the wake field behind the rotor of the turbine 300 (to be used as a mesh motion method) to capture details of turbulent flows in the wake field. A total number of around 15 million mesh cells were configured. In the CFD simulations, one time step in seconds was defined to be equal to one degree of the rotor rotation of the turbine 300. A fluid dynamic solver software, Fluent™ from ANSYS, was adopted for related analysis. The computational domain 500 is further divided into a rotational zone (which encompasses the rotor hub 304 and three rotor blades 302) and a stationary zone (which encompasses the rotor nose 308 and cylindrical nacelle 306). The mesh motion method is then used for a moving zone, which encompasses the flow streams. All the boundaries of the computational domain 500 and mesh nodes are rotated together with the rigid bodies - the rotor hub 304 and rotor blades 302, but the mesh cells are however not allowed to deform. A sliding mesh interface is employed on overlapping surfaces between an internal rotating fluid zone boundary and an external stationary fluid zone boundary. The flow properties communication is carried out via linear interpolation based on the locations of the mesh cells. The k-ε RNG model is then used to solve for the turbulent flows. Additionally, the turbulence intensity is set to be at about 5% at the inlet of the computational domain 500, and the turbulence length is configured to extend for about 0.5 m. To reduce the computational time needed for the CFD simulations, a standard wall function is selected to solve for the near wall region. Therefore, it is then not necessary to build finer computational nodes to resolve the inner wall boundary layer.

• WAKE FIELD STUDY RESULTS

To improve an energy level of the near-field wake of an associated turbine, two different nacelle shapes are studied. FIGS. 6a and 6b show the associated geometries of the nacelle shapes studied, being respectively labelled as "Nacelle 1" 306 in FIG. 6a and "Nacelle 2" 104 in FIG. 6b. "Nacelle 1 " 306 is the same nacelle 306 of the generic turbine 300 of FIG. 3 (i.e. the turbine 300 in FIGS. 3 and 6a are the same). "Nacelle 1 " 306 is of a conventional cylindrical shape, similar to existing ones typicaliy employed in the market for most tidal current turbines. The diameter of "Nacelle 1 " 306 is arranged to be the same as the diameter of hub 304 of the turbine 300 to avoid introducing any flow restriction downstream. The diameter of the rotor of the turbine 300 in FIG. 6a is the same as the diameter of the rotor of the turbine 600 in FIG. 6b, and is denoted as "D". In this instance, the diameter of "Nacelle 1 " 306 is configured to be 0.22D and an overall length of "Nacelle 1 " 306 is configured to be 0.4D. It is found that the overall length of "Nacelle 1 " 306 has no significant effect on the turbine performance.

By comparison, "Nacelle 2" 104 of a turbine 600 in FIG. 6b is the same nacelle 104 of the turbine 100 of FIG. 1 but without any ribs 108 as shown in FIG. 6b. Thus, "Nacelle 2" 104 has a NACA 0038 profiled shape, which is chosen to accommodate a size and a shape of an off-the-shelf 1 kW electrical generator, as explained before. Due to the NACA 0038 profiled shape, "Nacelle 2" 104 has a maximum cross section "D„", configured to be 0.456D, which is about 38% of the non-truncated chord length, C, of "Nacelle 2" 04. Correspondingly, the non- truncated chord length, C, of "Nacelle 2" 104 is thus 1.2D. The above proposed value for the maximum cross section "D„" of "Nacelle 2" 104 is determined based on the diameter of the chosen 1 kW electrical generator in view of its mechanical mounting requirements and also due to the metal skin of "Nacelle 2" 104. "Nacelle 2" 104 is slightly truncated at its head in order to allow the rotor hub 106 to be fitted thereto, and the amount of truncation of the head of "Nacelle 2" 104 is approximately equal to a diameter of the rotor hub 106.

The turbine 300 in FIG. 6a and the turbine 600 in FIG. 6b are configured with the same set of three rotor blades 302, rotor hub 304, and rotor nose 308 as shown in FIG. 3. Hence, the turbine 300 in FIG. 6a, and the turbine 600 in FIG. 6b only differ in the shape of the nacelle arranged thereto. The CFD simulations are executed for 12 revolutions of the rotor of the associated turbine 300, 600 to reach computational convergence, and the corresponding results obtained are presented in FIG. 7. From the false-colour images shown in FIG. 7, it is evident that wake meandering occurs behind "Nacelle 1 " 306 for even of the smallest possible diameter (see FIG. 7a). In particular, wake meandering can lead to extra fatigue loads and yaw loads on a turbine and also on a subsequent turbine downstream of the turbine array. A large vortex is also generated around "Nacelle 1 " 306, as seen in FIG. 7b, which is caused by the low fluid pressure encountered at that region. The diameter of the large vortex was determined to be almost half the diameter of the rotor of the turbine 300 of FIG. 6a. This large vortex was seen to considerably resist the flow over "Nacelle 1 " 306 from the root region of the rotor blade 302.

The above phenomena observed for "Nacelle 1 " 306 were however not observed for "Nacelle 2" 104. In particular, from FIG. 7c, the wake field behind "Nacelle 2" 104 appears to be totally straight, and the flow vectors display a smooth flow pattern over "Nacelle 2" 104, as seen from FIG. 7d. Hence, this helps to increase the mass of flow across the plane of the rotor of the turbine 600 of FIG. 6b, and consequently leads to an increase in the energy that can be extracted. This increase in energy extracted from the flow stream is verified from the Table 800 in FIG. 8, where it is determined from results that a higher power (of 820W) is produced by the turbine 600 with "Nacelle 2" 104 compared to the turbine 300 with "Nacelle 1 " 306 (which generates a lower power of 792W) and also it is confirmed by the higher thrust generated across the rotor blades as experienced by the turbine 600 of FIG. 6b as compared to turbine 300 of FIG. 6a (1 196 N vs. 1 131 N). Also, the thrust exerted on the whole structure (excluding the rotor blades 102 but including the rotor nose 308, rotor hub 304, and nacelle 104) of the turbine 600 of FIG. 6b (configured with "Nacelle 2" 104) is found to be just slightly lower than the thrust exerted on the whole structure (excluding the rotor blades 302 but including the rotor nose 308, rotor hub 304, and nacelle 306) of the turbine 300 of FIG. 6a (i.e., 1 186 N vs 1208 N, as seen from FIG. 8). This may be explained by the low intensity turbulent backflow observed at the rear of "Nacelle 2" 104, as seen in FIG. 7d.

Accordingly, as seen from the above results, due to the advantages displayed by "Nacelle 2" 104, it was adopted as the nacelle to be used in the next stage of the study aimed at deriving the design of the turbine 100 of FIG. 1 . FIG. 9 includes FIGS. 9a and 9b, which respectively illustrate visualisation of tip vortices generated by the turbine 600 of FIG. 6b, and path-lines of vortices around the near-field wake of the same turbine 600. When viewed from the front of the turbine 600 of FIG. 6b, the associated rotor rotates in the counter-clockwise direction, whilst spiral flows caused to be generated by the tips of the rotor blades 302 in the downstream direction are formed in the clockwise direction (refer to FIG. 9a). From FIG. 9b, it can be observed that the turbulence is fairly strong in the centre of the wake, from the overall density of the path-lines depicted.

With the adoption of the NACA 0038 profiled shape for "Nacelle 2" 104, the design of the turbine 100 of FIG. 1 was derived by further adding the ribs 108 to "Nacelle 2" , 104, the structural details of which are already explained above, and hence not repeated for brevity. FIG. 10 illustrates path-lines of vortices along surfaces of the nacelle 104 (i.e. "Nacelle 2") and the attached three ribs 108 of the turbine 100 of FIG. 1. It can be seen that the edges 202-208 of the ribs 108 collectively induce the formation of useful complimentary vortices that extend into the near-field wake and transferring their momentum to the near-field wake in the process, thus achieving the objective of re-energising the near-field wake.

FIG. 1 1 shows a comparison plot 1 100 of the normalized velocity distribution of a wake field for the turbine 600 of FIG. 6b (which is configured without the ribs 108 shown in FIG. 2), and the turbine 100 of FIG. 1. In particular, V_w represents the velocity of the wake field and V_f represents the velocity of the free stream. It can be seen from FIG. 1 1 that the ribs 108 enhance the energy recovery of the near-field wake even up to a distance of 6D downstream of the turbine 100 of FIG. 1 . The velocity distribution along the y-axis at a distance of 2D shows that the partial mixing of high and low velocity turbulent streams is asymmetrical for the turbine 100 of FIG. 1 .

FIG. 12 shows a plot 1200 of velocity magnitude along the flow direction from the domain inlet to a distance of 7D downstream of the computational domain 500 of FIG. 5. The averaged longitudinal velocity vector along the flow direction across a 1 D flow stream cross section in the wake field is much higher for the turbine with ribs 108 as compared to the turbine without ribs 108 in the near-field wake. A velocity magnitude increase of about 3% is observed to occur from the 5D point up to the 7D point, a point which is described in literature as an optimal spacing for a tidal turbine array (i.e. to achieve highest power density) [4, 5]. The beneficial consequence of this is that it results in a 9% increase in kinetic energy, or a 27% increase in the extractable power available for each of the downstream turbines in the turbine array.

FIG. 13 shows a plot of percentage increase of velocity magnitude and kinetic energy at different locations downstream of the turbine 600 of FIG. 6b (which is configured without the ribs 108 of FIG. 2), and of the turbine 100 of FIG. 1. FIG. 14 includes FIGS. 14a and 14b, which respectively depict false-colour graphical results of velocity contours of the turbine 600 of FIG. 6b (which is configured without the ribs 108 of FIG. 2), and the turbine 100 of FIG. 1. Higher velocities in the near-field wake of the turbine 100 of FIG. 1 (with the ribs 108), are observed compared to the turbine 600 of FIG. 6b.

Further embodiments of the invention will be described hereinafter. For the sake of brevity, description of like elements, functionalities and operations that are common between the embodiments are not repeated; reference will instead be made to similar parts of the relevant embodiment(s).

According to a second embodiment, there is proposed a first variant turbine (not shown) of the turbine 100 of FIG. 1 , in which the first variant turbine has a nacelle the same as the nacelle 104 of the turbine 600 shown in FIG. 6b, but without any projections 108 provided thereon. In other words, the first variant turbine differs only from the turbine 100 of FIG. 1 in that the former is not configured to have projections 108 on the nacelle 104. Other than the lack of projections 108, the rest of the structure of the first variant turbine is similar to the turbine 100 of FIG. 1, Thus, to increase the velocity of the near-field wake, the first variant turbine relies only on the NACA profiled shape of the nacelle 104 to reduce hydraulic losses. As before, the NACA profiled shape may be selected to be the NACA 0038 or alternatively any from the NACA 00XX series (depending on applications).

According to a third embodiment, there is proposed a second variant turbine (not shown) of the turbine -100 of FIG. 1 , in which, for this second variant turbine, the nacelle has a conventional cylindrical shape as illustrated in FIG. 6a. The projections 108 however are still provided on the cylindrically shaped nacelle. Hence, the second variant turbine relies primarily on the projections 108 to re- energise the near-field wake by collectively inducing formation of useful complimentary vortices that extend into the near-field wake and transferring their momentum to the near-field wake during the process, thus increasing the velocity of the near-field wake. It is noted that the projections 108 may be implemented in the form of the ribs disclosed in the first embodiment, but other suitable forms of projections may nonetheless also be used.

In conclusion, the proposed turbine(s) discussed in the first to third embodiments are advantageously able to increase the energy level of the turbine's near-field wake by: (1 ) reducing hydraulic losses, and/or (2) enabling effective mixing of high and low velocity flows downstream of the turbine through induced beneficial vortices. Objective (1 ) is attained by shaping the nacelle to a suitable NACA profiled shape, whereas objective (2) is achievable by providing carefully sized and shaped flow modifiers (i.e. the projections 108 such as ribs) onto the nacelle (which optionally may be a NACA profiled shape, or any other conventional shapes). As a result, the potential commercial benefits that can be reaped from the proposed turbines include ability to increase the overall energy output generated by any tidal turbine array, coupled with using an optimal turbine spacing of less than or equal to 7D. Specifically, the increase in velocity in the near-field wake results in associated increase of kinetic energy of subsequent free streams (re-assembled from the near-field wake) to be made available to the downstream turbines in the turbine array, and hence causes a corresponding increase in overall energy output that can be generated by the turbine array.

The described embodiments should not however be construed as limitative. For example, in the first embodiment, the number of rotor blades 102 may alternatively be dissimilar to the number of projections 108 required, and need not be three in total. Moreover, the concepts proposed in the first to third embodiments are similarly applicable to any types of turbines that are configured to generally be driven using fluid. Also, for the turbine 100 of FIG. 1 , the leading edge 202 of the ribs 108 may not necessarily need to start from the maximum lateral cross section "D„" of the nacelle 104; other suitable positions on the nacelle 104 for placing the ribs 108 may be determined and used as well. Further, the projections 108 may not be confined to the specific shape shown in FIG. 2b; other appropriate shapes are possible so long the intended functionality of increasing velocity of the near-field wake is achieved.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary, and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practising the claimed invention.

References

1. Malki, R., et al. "The variation in wake structure of a tidal stream turbine with flow velocity marine 2011".

2. Sanderse, B. "Aerodynamics of wind turbine wakes". 2009; Available from: http://www.ecn.nl/docs/library/report/2009/e09016.pdf.

3. .Milborrow, D.J., "The performance of arrays of wind turbines". Journal of Wind Engineering and Industrial Aerodynamics, 1980. 5(3-4): p. 403-430.

4. Turnock, S.R., et al., "Mode/ling tidal current turbine wakes using a coupled RANS-BEMT approach as a tool for analysing power capture of arrays of turbines". Ocean Engineering, 201 1 . 38(1 1 -12): p. 1300-1307. . Harrison, M.E., W.M.J. Batten, and A.S. Bahaj. "A blade element actuator disc approach applied to tidal stream turbines". 2010.

. Holland, R. "Wind turbines Wake Turbulence and Separation". Available from: http://www.arising.eom.au/aviation/windturbines/wind-turbine.html. . Bahaj, A.S., et al., "Characterising the wake of horizontal axis marine current turbines", in Proceeings of the 7th european wave and tidal energy conferece2007, Porto, Portugal.

. Werle, K.M., "Method and apparatus to improve wake flow and power productionof wind and water turbines" , 201 1 , United States.

Claims

1. A turbine comprising:

a nacelle; and

at least one projection provided on the nacelle, wherein the at least one projection comprises at least one edge configured to generate complementary vortices that collectively increase velocity of a near-field wake generated by rotor blades of the turbine.

2. The turbine of claim 1 , wherein the at least one edge comprises a plurality of edges that are arranged to be contiguous.

3. The turbine of claim 2, wherein the plurality of edges comprise a leading edge, a straight edge, a radial extension of the straight edge, and a trailing edge.

4. The turbine of any preceding claim, wherein the nacelle has a shape of an airfoil profile configured to reduce hydraulic losses.

5. The turbine of claim 4, wherein the airfoil profile is a symmetrical profile.

6. The turbine of claim 5, wherein the airfoil profile is a NACA 0038 profile.

7. The turbine of any one of claims 4 to 6 when dependent on claim 3, wherein the leading edge is arranged to begin from a section of the nacelle having a maximum diameter.

8. The turbine of claim 7, wherein the leading edge is further arranged to be curved in one portion relative to the longitudinal axis of the nacelle.

9. The turbine of claim 7 or 8, wherein the maximum diameter is arranged to be 0.456D, D being the diameter of a circle circumscribed by the rotor blades.

10. The turbine of claim 9, wherein the maximum diameter of 0.456D is approximately 38% of the chord length of the nacelle.

1 1. The turbine of any one of claims 7 to 10, wherein the at least one projection is configured with a length of 0.45C, C being the chord length of the nacelle.

12. The turbine of any one of claims 7 to 1 1 , wherein the leading edge is configured with a length of 0.285C, C being the chord length of the nacelle.

13. The turbine of any one of claims 1 1 or 19, wherein C has a value of 1.2D, D being the diameter a circle circumscribed by the rotor blades.

14. The turbine of any one of claims 7 to 13, wherein the radial extension has a length approximately 16% of the radius of a rotor formed by the rotor blades.

15. The turbine of any of claims 7 to 14, wherein a portion of the at least one projection comprising the straight edge has a radial height of approximately 70% of a rotor formed by the rotor blades, the radial height being defined relative to the longitudinal axis of the nacelle.

16. The turbine of any preceding claim, wherein the at least one projection comprises a plurality of projections equal to a number of the rotor blades of the turbine.

17. The turbine of claim 16, wherein the number of rotor blades is at least three.

18. The turbine of any preceding claim, wherein the at least one projection comprises a rib.

19. The turbine of any preceding claim, wherein the turbine is a tidal current turbine.

The turbine of claim 1 , wherein the nacelle has a cylindrical shape.

21. A turbine comprising:

a nacelle having an airfoil profile configured to reduce hydraulic losses for increasing velocity of a near-field wake generated by rotor blades of the turbine.

22. The turbine of claim 21 , wherein the airfoil profile is a symmetrical profile.

23. The turbine of claim 22, wherein the airfoil profile is a NACA 0038 profile.

24. The turbine of any one of claims 21 to 23, wherein the nacelle has a section configured with a maximum diameter.

25. The turbine of claim 24, wherein the maximum diameter is arranged to be 0.456D, D being the diameter of a rotor formed by the rotor blades.

26. The turbine of claim 25, wherein the maximum diameter of 0.456D is approximately 38% of the chord length of the nacelle.