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The performance benefits of deploying tidal turbines in close side-by-side proximity to exploit constructive interference effects are demonstrated experimentally using two 1.2 m diameter turbines. The turbines are arrayed side-by-side at 1/4 diameter tip-to-tip spacing, and their performance compared with that of a single rotor. Tests were complete...
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... are lower increases than for the uncorrected data but still represent an appreciable increase in the ratios of power-thrust and power-speed. Figure 10 shows a fast Fourier transform (FFT) of the blade RBMs for the single/North turbine when operating at 89 rpm (λ ≈ 7). Three cases are provided, the single turbine, twin with North and South at the same speed and twin with the South rotor operating at 1.57 times the speed of the North rotor. ...
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... each turbine at a different control point, i.e. rotational speed, affects the variation of loads experienced by the other rotor. Figure 12(c) shows how the thrust coefficient of each turbine varies with the TSR of the variable speed turbine. As is normal for a tidal turbine, thrust increases with TSR and this is clear for the variable turbine's operation. ...
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... assess the performance benefits between the single and twin configurations, the ratio of their thrust and power coefficient curves are shown against TSR in figure 13(a). For this comparison we use the BI-single data so as to isolate interference effects from overall global blockage effects. ...
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... rise in the power and thrust coefficient ratios between the single and twin configurations with increasing TSR are due to changes in the mass flow rate through the turbines. This effect may be captured by considering the power-to-thrust ratio, referred to as the basin efficiency η = C P /C T , shown in figure 13(b). The basin efficiency quantifies the ratio of the power generated by the turbine to the total power removed from the flow (power generated plus power lost to overcome friction and power lost in wake mixing). ...
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... is observed in the higher basin efficiency, and therefore mass flow rate, at peak C P observed in the twin rotor case. Conversely, the performance of the single rotor reduces significantly for λ 6.5 as too high a thrust is presented, reducing the mass flow rate through the turbine, which corresponds to the rapid reduction in basin efficiency observed in figure 13(b). ...
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... A38-21 Figure 14. Unblocked (for global blockage) performance coefficients for the BI-single and twin cases, computed using the global blockage velocity correction obtained from the BI-single data: (a) thrust coefficient; (b) power coefficient. ...
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... global blockage-corrected performance curves are shown in figure 14, with the same assumed channel widths of 6, 12 and 24 m considered. At high TSRs there are issues with the corrections collapsing and giving non-physical results, this is particularly true of the narrower channel and data has been curtailed to prevent the curves inverting. ...
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... data indicate that after correcting for global blockage, the twin performance is still significantly greater than that of the single, although the size of the shaded regions demonstrates a large amount of uncertainty on the final values. The performance coefficient ratios for the globally unblocked data are also included in figure 13(a). Up until a TSR of 7, the ratio of performance coefficients matches closely those from the uncorrected data with global blockage of 9.4 %. ...
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... before, the power coefficient is observed at the lower end of the presented TSR range owing to the physically measured values. Although these lower overall power coefficients are expected for larger channels, similar levels of basin efficiency are observed at peak performance ( figure 13b). Although a fully unblocked channel is not physical, this demonstrates that at the limiting case the potential for a more efficient system and lower cost of energy is possible through designing for constructive interference. ...
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... quantify whether 300 s of recorded data were sufficient to remove variability in the data, data were broken into N signals of length 300/N. Figure 15(a) shows the variation between the mean of each sample and the full 300 s signal. Torque is the most sensitive to the averaging period, with 2 % difference between the 150-and 300-second averaging periods, with other variables demonstrating less than 1 % variation. ...
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... the turbine and tank conditions should be the same, with only the position of the ADV being altered. Mean thrust and torque are presented as their variation from the overall mean in figure 15(b). The data are generally within 2 % of the mean, with some (likely with ADV near the rotor) showing 6 % difference. ...
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... water temperature during the tests varied between 16 • C and 17 • C which would have provided around 2.5 % variation in Reynolds number (for the flow mapping without turbines in May 2021, the temperature range was similar but reached a maximum of 18 • C). It is plotted in figure 16(a) to demonstrate how it is affected by upstream flow speed and TSR. Figure 16(b) shows the single rotor power coefficient for each of the tank flow speeds, again normalised by u x,2d rather than U N as in the main analysis. (This change in reference velocity is due to the full undisturbed upstream flow mapping described in § 3.1 only being conducted at 0.8 m s −1 and, hence, it is not possible to calculate the rotor equivalent speed as described by (3.2).) ...
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... is plotted in figure 16(a) to demonstrate how it is affected by upstream flow speed and TSR. Figure 16(b) shows the single rotor power coefficient for each of the tank flow speeds, again normalised by u x,2d rather than U N as in the main analysis. (This change in reference velocity is due to the full undisturbed upstream flow mapping described in § 3.1 only being conducted at 0.8 m s −1 and, hence, it is not possible to calculate the rotor equivalent speed as described by (3.2).) ...
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... is aligned with the main flow direction and so only affects the streamwise component. As the offset was consistent across the measured velocity range a correction was made to the instantaneous streamwise velocity component of ADV 2. Figure 17(b) shows the velocity magnitude at points measured by ADVs when the turbines were both out of and in the tank. Consistent with discussion in § 3, the flow speed is reduced on the 2d upstream rotor plane, whereas at 3d upstream there is less difference in the measurements, for both single and twin cases. ...
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... data (colours = subsets) Translated data (colours = subsets) Slope-adjusted data Data subset slope (used in average) Data subset slope (not used) Average subset slope Ideal slope Figure 18. Adjustment of blade RBM data using rotor thrust from (a) the implied values to (b) zeroed and scaled values. ...
Citations
... A turbine located within an array will be subject to the effects of blockage generated between adjacent turbines, and to the wakes from upstream devices. Some of these effects have been investigated both experimentally and computationally in Masters et al. (2013), Stallard et al. (2013), Stansby and Stallard (2016), Ouro and Nishino (2021), Ouro et al. (2023) and McNaughton et al. (2022). ...
... One reason for the increase in the fluctuation magnitude for the Tidal Bladed loading results from the inclusion of the dynamic inflow in the wake model, which would account for a greater change in the loads experienced as the turbine rotates. Overall the similarities in the mean flapwise bending moments is consistent with experimental findings in McNaughton et al. (2022), where the blockage effect of a short fence has more impact on thrust than root bending moments. ...
To maximise the availability of power extraction from a tidal stream site, tidal turbines need to be able to operate reliably when located within arrays. This requires a thorough understanding of the operating conditions, which include turbulence, velocity shear due to bed proximity and roughness, ocean waves and due to upstream turbine wakes, over the range of flow speeds that contribute to the loading experienced by the devices. High-fidelity models such as Large Eddy Simulation (LES) can be used to represent these complex flow conditions and turbine device models can be embedded to predict loading. However, to inform micro-siting of multiple turbines with an array, the computational cost of performing multiple simulations of this type is impractical. Unsteady onset conditions can be generated from the LES to be used in an offline coupling fashion as input to lower-fidelity load prediction models to enable computationally efficient array design. In this study, an in-house Blade Element Momentum (BEM) method is assessed for prediction of the unsteady loads on the turbines of a floating tidal device with unsteady inflow developed with the in-house LES solver DOFAS. Load predictions are compared to those obtained using the same unsteady inflow to the commercial tool Tidal Bladed and from an Actuator Line Model (ALM) embedded in the LES solver. Estimates of fatigue loads differ by up to 3% for mean thrust and 11% for blade root bending moment for a turbine subject to a turbulent channel flow. When subjected to more complex flows typical of a turbine wake, the predictions of rotor thrust fatigue differ by up to 10%, with loads reduced by the inclusion of a pitch controller.
... Large cant angles result in increased disturbance, which increases wave drag and reduces wing bending moment. A longer winglet achieves higher induced drag reduction at the expense of increased frictional drag and increased bending moment [46]. The choice of these two parameters is the result of a trade-off between different drag components and structural weight. ...
... The phenomenon of rotation, which typically relies on the torque produced by various types of driving forces, including electromagnetic [1], gas-dynamic [2], hydrodynamic [3] and mechanical contacted [4] forces, is ubiquitous, as illustrated in Fig. 1. Recently, a novel approach utilizing acoustic vortex waves, carrying orbital angular momentum, was proposed for transferring mechanical torque to small objects and rotational manipulation [5,6] without any physical touch. ...
... It is noteworthy that the trapped objects undergo rapid rotation in the vortex potential well that simultaneously ensures the spin stability of objects [40]. Inspired by the rotational phenomenon of an object in the twisted wave [7,8] and utilizing the higher intensity of focusing waves, this work studies the radiation torque of converged [1][2][3][4] that convert certain types of forces into mechanical torques and transfer them to objects being manipulated. Sound can also be used to generate and transfer mechanical torque by phase modulation. ...
The use of acoustic vortex waves with angular momentum is a promising means of transferring mechanical torque to an object without making any physical contact. However, the existing passive-conversion vortex lenses are unable to produce strong acoustic radiation torque. To address this issue, we propose a flat reflector sculpted by spiral grooves. It reflects and converts incident waves into vortex-focusing waves that generate radiation torque. Compared with a transmission-type lens, a reflection-type vortex focusing plate exhibits superior functionality in converging sound waves and creating a stronger circular intensity flow. In this work, we analyze the vortex-focusing sound by Rayleigh-Sommerfeld integral theory and boundary element simulation, confirmed by experiments. The number, depth and direction of spiral grooves determine the topological charge, focusing intensity and torque direction, respectively. We evaluate the local and total acoustic radiation forces and torques, acting on small and large fixed spherical targets in the vortex-focusing beam. As a result, spinning a centimeter-size origami pinwheel several times larger than the wavelength at 40 kHz is realized without direct contact, by utilizing the enhanced acoustic radiation torque converged by the reflector. This technology enables efficient convergence of acoustic waves and simultaneous contactless transfer of mechanical torque to an object, thereby presenting potential applications in sound energy harvesting and wireless power supplies.
... For this two-scale problem of a long array of turbines partially spanning the width of a much wider channel (vanishing global blockage) the efficiency of energy extraction, normalised on the undisturbed kinetic energy flux, rises from the Lanchester-Betz limit of 0.593 to the partial fence limit of 0.798 [7]. Experiments on pairs of side-by-side turbines at large laboratory scale [8] have confirmed the important aspects of the underlying partial fence theory and that some of the performance benefits offered by constructive interference effects can be achieved in practice. ...
... Various other singleturbine corrections have been proposed [14]- [17]; see [9] for detailed numerical and [10] for detailed experimental comparisons of these blockage corrections. These single-scale blockage corrections can however only account for global blockage, and simplifications for turbine arrays must currently be made based on the assumption that global and local blockage effects can be linearly decoupled [8]. ...
It has been shown theoretically that tidal fences consisting of multiple turbines placed side-by-side can makeuse of constructive interference (local blockage) effects to raise the energy extraction efficiency of the fenceabove that of the Betz limit applicable to unblocked flow problems. For the two-scale problem of a longarray of turbines partially spanning the width of a much wider channel (vanishing global blockage) theefficiency of energy extraction, normalised on the undisturbed kinetic energy flux, rises from the Betz limitof 0.593 to the partial fence limit of 0.798 [1]. Experiments on pairs of side-by-side turbines at largelaboratory scale [2] have confirmed the important aspects of the underlying partial fence theory and thatsome of the performance benefits offered by constructive interference effects can be achieved in practice. Experimental validation in wind tunnels, towing tanks and other laboratory facilities are however prone toglobal blockage effects not seen in full-scale open flows due to the close proximity of flow boundaries to thebody. These global blockage effects modify the thrust and power performance of the turbines, such thatcorrections to experimental curves are necessary to either translate laboratory-scale experimental results tofull-scale conditions, or to calculate the expected loads and power on tidal turbines deployed in blocked-flowconditions [3][4]. The difficulty applying blockage corrections to turbine arrays is the non-linear interactionbetween local and global blockage. These two effects cannot be simply decoupled as for various turbine tip-to-tip spacings (affecting local blockage), changes in the global blockage have a different impact on turbineperformance. A number of blockage corrections have been developed for single turbines operating in blocked flowconditions. These corrections typically seek to describe an equivalent free-stream velocity which, in theabsence of global blockage, would result in the same thrust and velocity through the turbine as in the blockedcase. Thrust and power curves are then scaled non-linearly with the ratio of the experimental tank velocityand the equivalent free-stream velocity [5]. These single turbine blockage corrections can however onlyaccount for global blockage, and simplifications must currently be made based on the assumption that globaland local blockage effects can be linearly decoupled [2]. This work therefore presents an analytical blockage correction for co-planar arrays of tidal turbines based ontwo-scale momentum theory [1]. This correction is then compared to other models, particularly for turbinearray experimental test data. Finally, RANS computations for a turbine array at various global blockageratios is compared to the analytical model, demonstrating its validity. A particularly useful aspect of thetheoretical model is to allow for experimental quantification of the local-blockage effect for finite lengthfences. For instance, doubling the fence length doubles the global blockage, but increases in fence thrust andpower cannot be attributed only to the change in global blockage due to non-linear coupling. This correctionallows for a decoupling of these two effects, such that the local blockage effect can be isolated andquantified.
... 6 To lower the cost of electricity generation, 7 large-scale tidal energy development is mostly accomplished through a tidal farm consisting of many tidal turbines. 8 However, the energy capture of a single turbine will be affected by the upstream turbine due to the interaction between turbines in a tidal farm, 9 especially the wake effect. Therefore, it is necessary to fully understand the turbine wake, reducing the mutual interference through a more rational arrangement scheme. ...
The wake development of a tidal turbine should be fully considered in the array arrangement. There are many studies on wake characteristics, mainly focusing on a conventional horizontal-axis turbine, while a ducted turbine has attracted little attention. This paper investigates the wake characteristic of a ducted turbine using flume experiments and large eddy simulations. An analytical wake model of the ducted turbine is proposed and verified by the wake profile under different inflow velocities and the downstream turbine performance under different tandem arrangements. The results show that a ducted turbine wake still maintains a high self-similarity, and the wake profile is approximately the double-Gaussian curve. Compared with a conventional tidal turbine, a ducted turbine has a faster wake recovery speed, but a larger radial influence range. Therefore, ducted turbine arrays should be configured with wider radial distances and shorter axial distances.
... Additionally, the model has been applied to multiple rows of tidal turbines (Draper & Nishino 2014), where it was found that a single row of turbines outperforms a staggered arrangement, and to shallow channels with non-negligible bed friction, which was found to change both the power extraction potential and the optimum fence arrangement (Creed et al. 2017). The scale-separation effect has been demonstrated experimentally using porous discs (Cooke et al. 2015), and power uplift through the local blockage effect has been shown in large laboratory turbine experiments (McNaughton et al. 2022). ...
This paper presents an analytic model for the analysis of co-planar turbine fences that partially span the width of a channel in which the flow is driven by a sinusoidally oscillating driving head. The thrust presented by the turbines reduces the flow rate through the channel leading to a solution for overall power that is dependent upon turbine resistance and flow blockage as well as on channel characteristics. We introduce a return parameter, in terms of power per turbine area, to assess optimum turbine fence deployment for a given channel. We find that the optimal deployment rests on a universal curve independent of the channel characteristics, and that these characteristics – namely the integrated channel bed friction and a modified channel Froude number – move the optimum along this curve. We find that blockage considerations play a large role in the performance of a tidal farm – its achievable power, optimal return, channel flow rate reduction and device thrust – and that the scales of blockage must be considered even when designing relatively unblocked farms. The impact of the channel characteristics on the optimal arrangement, alongside environmental constraints that may limit permissible flow blockage, are quantified and discussed.
