CERES :: Browsing by Author "Nishino, Takafumi"

Browsing by Author "Nishino, Takafumi"

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Open Access
Analysing momentum balance over a large wind farm using a numerical weather prediction model
(IOP Publishing, 2020-09-21) Lun, Ma; Dunstan, Thomas D.; Nishino, Takafumi
This study attempts to better understand the mechanisms of wind farm blockage effect by analysing momentum balance in realistic atmospheric flow over an idealised large offshore wind farm. The analysis is performed following the two-scale momentum theory, which predicts the importance of three different terms in the farm-scale momentum balance, namely the streamwise pressure gradient, Coriolis force and acceleration/deceleration terms. A numerical weather prediction (NWP) model is used as a realistic farm-scale flow model in this study to investigate how these three terms tend to change in time. Initial results suggest that the streamwise pressure gradient may be enhanced substantially by the resistance caused by the wind farm, whereas its influence on the other two terms appears to be relatively minor. These results suggest the importance of modelling the farm-induced pressure gradient accurately for various weather conditions in future studies of wind farm blockage
Open Access
Blade-resolved CFD simulations of a periodic array of NREL 5 MW rotors with and without towers
(MDPI, 2022-01-14) Ma, Lun; Delafin, Pierre-Luc; Tsoutsanis, Panagiotis; Antoniadis, Antonis F.; Nishino, Takafumi
A fully resolved (FR) NREL 5 MW turbine model is employed in two unsteady Reynolds-averaged Navier–Stokes (URANS) simulations (one with and one without the turbine tower) of a periodic atmospheric boundary layer (ABL) to study the performance of an infinitely large wind farm. The results show that the power reduction due to the tower drag is about 5% under the assumption that the driving force of the ABL is unchanged. Two additional simulations using an actuator disc (AD) model are also conducted. The AD and FR results show nearly identical tower-induced reductions of the wind speed above the wind farm, supporting the argument that the AD model is sufficient to predict the wind farm blockage effect. We also investigate the feasibility of performing delayed-detached-eddy simulations (DDES) using the same FR turbine model and periodic domain setup. The results show complex turbulent flow characteristics within the farm, such as the interaction of large-scale hairpin-like vortices with smaller-scale blade-tip vortices. The computational cost of the DDES required for a given number of rotor revolutions is found to be similar to the corresponding URANS simulation, but the sampling period required to obtain meaningful time-averaged results seems much longer due to the existence of long-timescale fluctuations.
Open Access
CFD study of a bidirectional synthetic jet as an aerodynamic flow control device.
(2018-03) Fisher Fernandez, Jaime Rodrigo; Nishino, Takafumi
Control of the separation and reattachment of the boundary layer has been the focus of research for a wide range of engineering applications, and it is known that a synthetic jet can be used as an active flow control device for such issues. This work presents a numerical analysis of the potential benefits of changing the inflow and outflow directions of a synthetic jet separately. The basic concept of this new flow control approach, named Bi-Directional synthetic jet (BDSJ), is to induce the Coanda effect due to the jet outflow directed downstream, and change accordingly the inflow direction for an efficient and directed suction of the boundary layer. A widely-used reference case (so-called NASA 2-D Hump) is used to validate the numerical model and to compare results. Three well known Reynolds-averaged Navier-Stokes (RANS) turbulence models are employed and they all show similar trends. Eleven different angles are tested for the inflow and outflow directions of the synthetic jet; the most effective configuration is then compared with the classical synthetic jet and Directed synthetic jet. It is shown that the best BDSJ configuration results in a shorter separation bubble length over the hump, suggesting the potential of BDSJ as a future active flow control device. This work also attempts to answer the question; "How a Bi-directional synthetic jet, influences the aerodynamic coefficients on a aerofoil". Simulations are carried out on an NACA 23012 at a Reynolds number of Re =2.19 • 10⁶, where a two-dimensional structured mesh is used to evaluate the impact of the amplitude and frequency of the Bidirectional synthetic jet on the aerofoil performance over a wide range of angles of attack. Three different jet oscillating frequencies are considered as well as three blowing ratios. The phase difference between the inflow and outflow jets, the position and the configuration of the jet exits are also evaluated. The results suggest that the amplitude of the jet is a key parameter to increase the lift coefficient and a gap between the inflow and outflow jet exits could positively influence the lift coefficient if the central location of the jets coincides with the separation point. On the other hand, the jet frequencies and the phase at which the jets operate do not seem to influence the aerofoil performance significantly. Finally, some guidelines and recommendations are provided to develop further the actual Bi-directional synthetic jet which may lead to its optimal design and manufacture.
Open Access
Comparison of low-order aerodynamic models and RANS CFD for full scale 3D vertical axis wind turbines
(Elsevier, 2017-03-21) Delafin, Pierre-Luc; Nishino, Takafumi; Athanasios, Kolios; Lin, Wang
A Double Multiple Streamtube model, a free-wake vortex model (both widely used for vertical axis wind turbine design) and RANS CFD simulations are used in this work to predict the performance of the 17 m Vertical Axis Wind Turbine, field tested by Sandia National Laboratories. The three-dimensional, full scale calculations are compared with the experiments in terms of power coefficient, power and instantaneous turbine torque to assess the validity of each model. Additionally, the two aerodynamic models and RANS CFD are compared to each other in terms of thrust and lateral force. The two models and CFD agree well with the experiments at the turbine optimal tip speed ratio. However, away from the optimal tip speed ratio, the streamtube model significantly deviates from the experimental data and from the other numerical models. RANS CFD gives a good agreement with the experiments, slightly underestimating the power coefficient at every tip speed ratio tested. The vortex model proves to be a useful tool with a better accuracy than the streamtube model and a much lower computational cost compared to RANS CFD.
Open Access
Development of a stochastic computational fluid dynamics approach for offshore wind farms
(IOP, 2018-06-19) Richmond, Mark; Kolios, Athanasios; Pillai, V. S.; Nishino, Takafumi; Wang, L.
In this paper, a method for stochastic analysis of an offshore wind farm using computational fluid dynamics (CFD) is proposed. An existing offshore wind farm is modelled using a steady-state CFD solver at several deterministic input ranges and an approximation model is trained on the CFD results. The approximation model is then used in a Monte-Carlo analysis to build joint probability distributions for values of interest within the wind farm. The results are compared with real measurements obtained from the existing wind farm to quantify the accuracy of the predictions. It is shown that this method works well for the relatively simple problem considered in this study and has potential to be used in more complex situations where an existing analytical method is either insufficient or unable to make a good prediction.
Open Access
Effect of the number of blades and solidity on the performance of a vertical axis wind turbine
(IOP Publishing: Conference Series / Institute of Physics (IoP), 2016-10-03) Delafin, Pierre-Luc; Nishino, Takafumi; Wang, Lin; Kolios, Athanasios
Two, three and four bladed phgr-shape Vertical Axis Wind Turbines are simulated using a free-wake vortex model. Two versions of the three and four bladed turbines are considered, one having the same chord length as the two-bladed turbine and the other having the same solidity as the two-bladed turbine. Results of the two-bladed turbine are validated against published experimental data of power coefficient and instantaneous torque. The effect of solidity on the power coefficient is presented and the instantaneous torque, thrust and lateral force of the two-, three- and four-bladed turbines are compared for the same solidity. It is found that increasing the number of blades from two to three significantly reduces the torque, thrust and lateral force ripples. Adding a fourth blade further reduces the ripples except for the torque at low tip speed ratio. This work aims to help choosing the number of blades during the design phase of a vertical axis wind turbine.
Open Access
Energetic and exergetic study for cross-corrugated membrane-based total recovery exchanger for ventilation
(2017-10) Abduljabbar, Ahmed A.; Sher, Ilai; Nishino, Takafumi
Indoor air quality is an important component of the air conditioning of buildings due to its major effect on the health of the occupants, thus the air supplied to these buildings by the ventilation system should be sufficient, clean and healthy. A most promising development was the heat recovery system which offers better thermal energy efficiency and comfort with adequate fresh air. An energetic and exergetic analysis has been conducted on a cross-corrugated membrane based total heat exchanger core for ventilation of single dwellings. In order to enhance the sensible and latent effectiveness of the heat and mass transfer intensification was achieved by selecting Polyethersulfone for the membrane material, and a cross-corrugation arrangement of different dimensions for the primary surface exchanger. The design was tested against a ventilation air volume flow rate for an individual household; from 85 to 100 m³/hr. The dimensions of the exchanger were based on the polymer core being developed by Redring-Xpelair, Peterborough UK, with core dimensions of width and length both 250 mm, and a range of heights 100 – 500 mm. The cross-corrugated design of the test core had triangular openings with pitch lengths of 5, 10 and 25 mm. The ambient conditions were for a cold and humid winter in the UK. The ambient temperature test values were 2, 4, 6, 8 and 10 °C, and the inlet air velocities in the core were 0.5, 1.0, 1.5 and 2 m/s, with Reynolds numbers not exceeding 2200. CFD studies were conducted to investigate the thermal-fluid performance of the core, the Transition-SST model was used in the simulations within ANSYS Fluent 17.1 software and was validated using experimental data in the literature. The proposed model performed successfully in this study and proved that it was compatible with the test conditions. The exergetic analysis was conducted using the IPSEpro modelling software, by creating a system consisting of membrane core, a domestic dwelling, fresh air and exhaust fans. The energetic analysis results were the basis of the IPSEpro modelling to determine the exergy, the exergetic efficiency and exergy destruction in the system. The study concluded from both the energetic and the exergetic analysis that the membrane based exchanger core showed promising performance as a total heat and moisture recovery application with sensible and latent effectiveness values varying from 65% to 82%; and exergetic efficiency values varying from 30% to 60%, depending on core geometry and ambient conditions. The chemical exergy was the dominant factor in the performance in all cases, and the membrane core had the highest exergy destruction percentage comparing to the other system components. Decreasing the pitch length of the exchanger core intensified its performance, the 5 mm case showed the best performance, but there are likely to be difficulties in manufacturing such a compact core. But, and more directly, its use would mean unpleasant compromises due to the extremely higher pressure drop across such a core even at low Reynolds numbers. The 10 mm case gave a better performance than the 25 mm, but not substantially different, therefore, the optimum choice lies between the better heat and mass transfer performance of the 10 mm case and the lower pressure drop and relative ease of manufacture of the 25 mm.
Open Access
Flow through a very porous obstacle in a shallow channel
(The Royal Society, 2017-04-26) Creed, M. J.; Draper, S.; Nishino, Takafumi; Borthwick, A. G. L.
A theoretical model, informed by numerical simulations based on the shallow water equations, is developed to predict the flow passing through and around a uniform porous obstacle in a shallow channel, where background friction is important. This problem is relevant to a number of practical situations, including flow through aquatic vegetation, the performance of arrays of turbines in tidal channels and hydrodynamic forces on offshore structures. To demonstrate this relevance, the theoretical model is used to (i) reinterpret core flow velocities in existing laboratory-based data for an array of emergent cylinders in shallow water emulating aquatic vegetation and (ii) reassess the optimum arrangement of tidal turbines to generate power in a tidal channel. Comparison with laboratory-based data indicates a maximum obstacle resistance (or minimum porosity) for which the present theoretical model is valid. When the obstacle resistance is above this threshold the shallow water equations do not provide an adequate representation of the flow, and the theoretical model over-predicts the core flow passing through the obstacle. The second application of the model confirms that natural bed resistance increases the power extraction potential for a partial tidal fence in a shallow channel and alters the optimum arrangement of turbines within the fence.
Open Access
Fluid dynamic mechanisms of enhanced power generation by closely spaced vertical axis wind turbines
(Elsevier, 2016-08-18) Zanforlin, Stefania; Nishino, Takafumi
We present a comprehensive set of two-dimensional (2D) unsteady Reynolds-averaged Navier-Stokes (URANS) simulations of flow around a pair of counter-rotating vertical-axis wind turbines (VAWTs). The simulations are performed for two possible configurations of the counter-rotating VAWT pair, with various gaps between the two turbines, tip-speed-ratios and wind directions, in order to identify key flow mechanisms contributing to the enhanced performance of a pair of turbines compared to an isolated turbine. One of the key mechanisms identified, for the case of two turbines arrayed side-by-side with respect to the incoming wind, is the change of lateral velocity in the upwind path of each turbine due to the presence of the neighbouring turbine, making the direction of local flow approaching the turbine blade more favourable to generate lift and torque. The results also show that the total power of a staggered pair of turbines cannot surpass that of a side-by-side pair of turbines. Some implications of the present results for the prediction of the performance of single and multiple rows (or a farm) of VAWTs are also discussed. The local flow mechanisms identified in the present study are expected to be of great importance when the size of the farm is relatively small.
Open Access
Local blockage effect for wind turbines
(IOP Publishing, 2015-06-11) Nishino, Takafumi; Draper, Scott
This paper presents a combined theoretical and CFD study on the fluid-mechanical limit of power extraction by a closely-spaced lateral array of wind turbines. The idea of this study originates in recent studies on the array optimisation of tidal/marine turbines, for which the power coefficient of each turbine is known to increase significantly if the lateral spacing between turbines, or the local blockage, is optimised. The present study, using 3D Reynolds- averaged Navier-Stokes (RANS) simulations of a boundary-layer flow over a closely-spaced lateral array of up to 9 actuator discs, suggests that a similar—albeit less significant—power increase due to the effect of local blockage can be achieved even for wind turbines. A possible theoretical approach to estimating this power increase is also discussed.
Open Access
Momentum balance in a fully developed boundary layer over a staggered array of NREL 5MW rotors
(IOP Publishing, 2017-06-13) Delafin, Pierre-Luc; Nishino, Takafumi
3D Reynolds-averaged Navier-Stokes (RANS) simulations of a fully developed wind farm boundary layer over a staggered array of NREL 5MW turbines are presented. The turbine is modeled as an actuator disk and as a fully resolved rotor to compare the effect of the turbine model on the wind farm aerodynamics, in particular the streamwise momentum balance across the farm. Results show that the difference in the turbine model affects the average wind speed through the farm as well as the local flow pattern around each turbine; both contributing to the difference in the prediction of farm performance. Results are also compared with a simple theoretical model of very large wind farms proposed recently. The actuator disk simulations agree very well with the theoretical model, whereas the fully resolved rotor simulations show some consistent and expected differences from the model. Paper presented at the Wake Conference 2017 30 May to 1 June 2017, Visby, Sweden.
Open Access
Numerical Analysis of a Bidirectional Synthetic Jet for Active Flow Control
(American Institute of Aeronautics and Astronautics, 2017-01-11) Nishino, Takafumi; Savill, Mark A.
Open Access
Numerical study of airfoil stall cells using a very wide computational domain
(Elsevier, 2016-10-05) Manni, Luca; Nishino, Takafumi; Delafin, Pierre-Luc
The formation of stall cells over a NACA 0012 airfoil at a Reynolds number of one million has been investigated numerically, using unsteady Reynolds-averaged Navier–Stokes (URANS) and delayed detached-eddy simulation (DDES) approaches. The simulations are performed with a very wide computational domain (10 chord length) to minimize the influence of spanwise periodic boundary conditions. For the URANS simulations, four different spanwise mesh resolutions are tested to determine the minimum resolution required to capture the formation of stall cells. Both URANS and DDES results show a sudden decrease in lift and increase in drag between 16° and 17° angle of attack, accompanied by a significant change of separated flow patterns. Stall cell structures are observed clearly in the URANS solutions between 17° and 19° with a spanwise spacing of about 1.4 to 1.8 chord length, which agrees well with a theoretical prediction based on the slope of the lift curve in this angle-of-attack range. The DDES results show much more complex flow patterns over the airfoil at these high angles of attack, although the spectral analysis of wall shear stress suggests the existence of flow structures having a similar spanwise length scale to the stall cells.
Open Access
Numerical study on the limit of power extraction by a dense cross-stream array of wind turbines
(Elsevier, 2017-06-27) Tartari, Filiberto; Nishino, Takafumi
A numerical study is presented on the upper limit of power extraction by a dense cross-stream array of wind turbines, using 3D Reynolds-averaged Navier–Stokes simulations of flow over porous discs. The main objectives are: (i) to investigate the effect of ‘local blockage’ due to neighbouring turbines on the limit of power extraction; and (ii) to clarify how this effect compares with the effect of ‘local flow acceleration’ obtained by staggering the array in the streamwise direction. Some unconventional array configurations with vertical turbine arrangements, following the so-called ‘multi-rotor’ concept, are also investigated. Results show that the limit of power extraction by a non-staggered array increases moderately with the number of turbines arrayed (about 5% increase in the power coefficient compared to the Betz limit when 9 turbines are arrayed side-by-side). This power increase due to the local blockage can be enhanced further, but only slightly for the case of 9 turbines, by arranging turbines vertically as well as horizontally. Staggering the array in the streamwise direction may increase the power of downstream turbines due to the effect of local flow acceleration but reduce the power of upstream turbines as the local blockage effect diminishes, resulting in a total power reduction.
Open Access
Numerical validation of the two-scale actuator disc theory for marine turbine arrays
(Unknown, 2015-09-11) Perez-Campos, Edgar; Nishino, Takafumi
The challenge in the hydrodynamic modelling of tidal and marine turbine farms is to take into account the interaction of flow events across a wide range of scales, such as the blade scale, turbine scale, array scale and regional scale. Whilst the interaction of the blade and turbine scales can be studied using the classical Blade-Element-Momentum (BEM) theory, no basic theory was available until recently to describe the interaction of the turbine and larger scales. The two-scale actuator disc theory (ADT), first proposed in 2012 by Nishino and Willden, explains the interaction of the turbine and array scales at a fundamental level; however, its validity or applicability to real problems has only partially been confirmed. Hence in this study we perform 3D RANS simulations of single and double rows of porous discs (8 discs for each row) in the middle of a shallow open channel with a vertically sheared flow. The simulation results are shown to agree qualitatively with the two-scale ADT and importantly, the optimal intra-disc spacing predicted by the simulations (to maximise the total power) agrees well with the theory, for both single- and double-row cases.
Open Access
Performance of an ideal turbine in an inviscid shear flow
(Journal of Fluid Mechanics, 2016-04-28) Draper, S.; Nishino, Takafumi; Adcock, T. A. A.; Taylor, P. H.
Although wind and tidal turbines operate in turbulent shear flow, most theoretical results concerning turbine performance, such as the well-known Betz limit, assume the upstream velocity profile is uniform. To improve on these existing results we extend the classical actuator disc model in this paper to investigate the performance of an ideal turbine in steady, inviscid shear flow. The model is developed on the assumption that there is negligible lateral interaction in the flow passing through the disc and that the actuator applies a uniform resistance across its area. With these assumptions, solution of the model leads to two key results. First, for laterally unbounded shear flow, it is shown that the normalised power extracted is the same as that for an ideal turbine in uniform flow, if the average of the cube of the upstream velocity of the fluid passing through the turbine is used in the normalisation. Second, for a laterally bounded shear flow, it is shown that the same normalisation can be applied, but allowance must also be made for the fact that non-uniform flow bypassing the turbine alters the background pressure gradient and, in turn, the turbines ‘effective blockage’ (so that it may be greater or less than the geometric blockage, defined as the ratio of turbine disc area to cross-sectional area of the flow). Predictions based on the extended model agree well with numerical simulations approximating the incompressible Euler equations. The model may be used to improve interpretation of model-scale results for wind and tidal turbines in tunnels/flumes, to investigate the variation in force across a turbine and to update existing theoretical models of arrays of tidal turbines.
Open Access
Preliminary estimate of the impact of support structures on the aerodynamic performance of very large wind farms
(IOP, 2018-06-19) Ma, Lun; Nishino, Takafumi
An extended theoretical model, which is based on a two-scale coupled momentum conservation argument, is proposed to estimate aerodynamic effects of support structures on the performance of ideal very large wind farms. A key implication of this extended model is that the parameter (As /A) bold dot ${C}_{D}^{* }$ where A and As are the rotor swept area and support-structure frontal projected area, respectively, and ${C}_{D}^{* }$ is an effective support-structure drag coefficient, plays an important role in the design of very large wind farms. In particular, the optimal farm density tends to decrease as the normalised support-structure drag increases. To validate this extended model, Wall-Modelled Large-Eddy Simulations (WMLES) of a periodic array of actuator discs with and without support structures are conducted; results agree qualitatively with the model.
Open Access
Structural optimisation of vertical-axis wind turbine composite blades based on finite element analysis and genetic algorithm
(Elsevier, 2016-06-02) Wang, Lin; Kolios, Athanasios; Nishino, Takafumi; Delafin, Pierre-Luc; Bird, Theodore
A wind turbine blade generally has complex structures including several layers of composite materials with shear webs, making its structure design very challenging. In this paper, a structural optimisation model for wind turbine composite blades has been developed based on a parametric FEA (finite element analysis) model and a GA (genetic algorithm) model. The optimisation model minimises the mass of composite blades with multi-criteria constraints. The number of unidirectional plies, the locations of the spar cap and the thicknesses of shear webs are taken as design variables. The optimisation model takes account of five constraints, i.e. stress constraint, deformation constraint, vibration constraint, buckling constraint, and manufacturing manoeuvrability and continuity of laminate layups constraint. The model has been applied to the blade structural optimisation of ELECTRA 30 kW wind turbine, which is a novel VAWT (vertical-axis wind turbine) combining sails and V-shape arm. The mass of the optimised blade is 228 kg, which is 17.4% lower than the initial design, indicating the blade mass can be significantly reduced by using the present optimisation model. It is demonstrated that the structural optimisation model presented in this paper is capable of effectively and accurately determining the optimal structural layups of composite blades.
Open Access
Two-scale dynamics of flow past a partial cross-stream array of tidal turbines
(Cambridge University Press, 2016-07-30) Nishino, Takafumi; Willden, R. H. J.
The characteristics of flow past a partial cross-stream array of (idealized) tidal turbines are investigated both analytically and computationally to understand the mechanisms that determine the limiting performance of partial tidal fences. A two-scale analytical partial tidal fence model reported earlier is further extended by better accounting for the effect of array-scale flow expansion on device-scale dynamics, so that the new model is applicable to short fences (consisting of a small number of devices) as well as to long fences. The new model explains theoretically general trends of the limiting performance of partial tidal fences. The new model is then compared to three-dimensional Reynolds-averaged Navier–Stokes (RANS) computations of flow past an array of various numbers (up to 40) of actuator disks. On the whole, the analytical model agrees well with the RANS computations, suggesting that the two-scale dynamics described in the analytical model predominantly determines the fence performance in the RANS computations as well. The comparison also suggests that the limiting performance of short partial fences depends on how much of device far-wake mixing takes place within the array near-wake region. This factor, however, depends on the structures of the wake and therefore on the type/design of devices to be arrayed.
Open Access
Two-scale momentum theory for very large wind farms
(IOP Publishing: Conference Series / Institute of Physics (IoP), 2016-10-03) Nishino, Takafumi
A new theoretical approach is proposed to predict a practical upper limit to the efficiency of a very large wind farm. The new theory suggests that the efficiency of ideal turbines in an ideal very large wind farm depends primarily on a non-dimensional parameter λ/Cf0, where λ is the ratio of the rotor swept area to the land area (for each turbine) and Cf0 is a natural friction coefficient observed before constructing the farm. When X/Cf approaches to zero, the new theory goes back to the classical actuator disc theory, yielding the well-known Betz limit. When λ/Cf0 increases to a large value, the maximum power coefficient of each turbine reduces whilst a normalised power density of the farm increases asymptotically to an upper limit. A CFD analysis of an infinitely large wind farm with 'aligned' and 'displaced' array configurations is also presented to validate a key assumption used in the new theory.