Range Of Tip Speed Ratios Research Articles

URANSE simulation of an active variable-pitch cross-flow Darrieus tidal turbine: Sinusoidal pitch function investigation

This article describes a 2D CFD simulation implementation of a crossflow tidal turbine, the blades of which have their pitch modified during revolution. Unsteady flow around the turbine is computed with an URANSE method, using the solver ANSYS-CFX. Spatial and temporal discretizations have been studied. The pitch motion of the blades is obtained through mesh deformation, and the main rotation is implemented through sliding boundaries, with general grid interface model. The turbulence model used is kω SST. Langtry Menter transition model was tried but showed high discrepancies with experimental results. Five experimental cases were used to assess the accuracy of the simulation. It provided accurate computed forces for a wide range of tip speed ratios, and proved to be suitable for exploratory simulations. Harmonic pitch control was thus implemented for a tip speed ratio of 5, close to an operational value for a crossflow turbine. First, second and third harmonics pitch function were tested. It was shown that an improvement of more than 50% could be achieved with the second harmonics, with a large reduction in thrust. The flow inside the turbine and close to the blade was examined so that the case of performance improvement due to pitch control could be clearly understood. It was observed that turbine efficiency improvement requires a very slight recirculation and an angle of attack decrease on the upstream part of the turbine, and an angle of attack increase on the downstream part. The flow deceleration through the turbine was found to be a primary factor in pitch function as well. Moreover the hydrodynamic torque and thus the energy required to control the pitch were found to be insignificant.

Computational and experimental investigations of an omni-flow wind turbine

Both numerical and experimental studies were conducted to evaluate the performance of an omni-flow wind turbine designed to provide renewable electricity on the top of urban buildings like skyscrapers. The numerical approach was based on Finite Volume Method (FVM) and the turbulence flow was studied with several commonly used Reynolds-averaged Navier–Stokes turbulence models. The results of the study were evaluated with the wind tunnel test results over a range of tip speed ratios. The numerical results showed the effect of blade number on both power output and starting capability. Although both the power and torque coefficient were improved significantly by the optimisation of the blade number, there was only a slight change when the blade number was greater than twenty. The results from wind tunnel testing also showed excellent starting capability with a starting wind velocity as low as 1.6 m/s. A numerical simulation was also conducted for the wind turbine working under non-uniform flow conditions. The numerical results have shown that the peak power coefficient of such a wind turbine under non-uniform flow, was lower than that under the uniform flow. Additionally, the applied thrust on a blade was subject to frequent and periodical changes. However, the effect of the change of thrust in magnitude and frequency was not significant. Therefore the omni-flow wind turbine has the potential to meet the challenge of unpredictable wind velocity and direction as a consequence of the urban environment.

Harvesting wind energy for improving performance of Savonius rotor

In this work three methods of controlling the wind direction were aiming at improving the performance of Savonius rotor. The idea behind the new designs was to harvest the incoming wind to generate a wind jet to the concave side of the advanced blade and prevent the convex side of return blade from coming upwind stream. The prevented wind was guided in different designs to impinge the concave side of the return blade and hence to eliminate the negative torque and increase the exerted positive torque. The study could be numerically introduced using commercial Fluent-software. The SST k-ω turbulence model was used to simulate the turbulence behavior. The results showed that the suggested designs improve the performance of Savonius rotor in view of the power coefficient and the operation range. One of them enhanced the performance to reach a power-coefficient peak of 0.52 with operation range of tip speed ratio λ≤2.2. However, the new designs generated large wakes behind the rotor that must be considered in the turbines farm arrangement.

Characteristic Analysis of Three-Bladed Darrieus Wind Turbine Based on the Multiple Streamtube Model

The paper analyzes the effect of airfoil thickness, camber and blade pitch angle on the performance of the three-bladed Darrieus wind turbines. The research results show that the increase of airfoil thickness, camber and pitch angle of blade, can improve power coefficient when the wind turbine tip speed ratio between zero and four. The increase of thickness and camber of the airfoil leads to running tip speed ratio range of wind turbine get narrowed, and reduces the power coefficient when wind turbine runs in high tip speed ratio range. When the pitch angle of blade is 1˚, power coefficient reaches the maximum value. Negative pitch angle has a bad impact on power coefficient and even creates negative power coefficients.

Aerodynamic performance and characteristic of vortex structures for Darrieus wind turbine. I. Numerical method and aerodynamic performance

A recently developed in-house computational fluid dynamics (CFD) code is used to simulate an H-Darrieus wind turbine. Aerodynamic performance of the simulated Darrieus turbine having different number of blades and turbine solidity is analyzed and compared for different tip speed ratios. As expected, the power coefficient of the simulated Darrieus turbine increases with the increase of tip speed ratio until a maximum is reached. However, the power coefficient then decreases with further increases in the tip speed ratio. The calculated power curve is in good agreement with experimental results. The results obtained suggest that this developed CFD code can accurately predict the aerodynamic characteristics of an H-Darrieus turbine. In addition, it is found that the solidity has considerable influence on the power coefficient of the simulated turbine in the present work. The smaller the solidity, the higher will be the optimal tip speed ratio and the wider will be the range of tip speed ratios at which the H-Darrieus turbine remains high power coefficient. If solidity is very low, the performance of a 2-bladed Darrieus turbine is obviously better than that of turbines with 3 and 4 blades. For moderate to high solidity, the power coefficients of the 2-bladed Darrieus wind turbine are similar to those of the 3-bladed turbine and are higher than those of the 4-bladed turbine. Moreover, the power coefficient increases with increasing solidity at low tip speed ratios. When the tip speed ratio is close to the optimum value, the power coefficient initially increases and then decreases with the increase of solidity. At high tip speed ratio, the power coefficient decreases with increasing solidity. An in-depth investigation is also conducted on the findings observed in this study and presented in Part 2 of this work, in which the mechanism of the effect of solidity on power coefficient has been explored based on the vortex structure of the flow field with the aid of this self-developed CFD code.

Assessment of low-order theories for analysis and design of shrouded wind turbines using CFD

The use of a shroud around the rotor of a wind turbine has been known to augment the airflow through the rotor plane and hence result in improved performance. This work uses Computational Fluid Dynamics (CFD) to assess the validity of several simple theories which attempt to extend Betz theory to shrouded turbines. Two CFD models are employed and compared to predictions of previously published models. The first makes use of a fixed pressure-drop actuator disk, while the second incorporates the twist and chord distribution of the turbine blade as well as an airfoil polar using a technique much like the classical blade element momentum (BEM) method. Calculations are performed for a sweep of turbine loadings using the fixed pressure-drop model and a sweep of tip speed ratios using the BEM model for both an open and shrouded turbine. Power is computed using a control volume approach for the fixed pressure-drop model and by integrating tangential forces for the BEM model. Information including mass flow ratio, power coefficient ratio, axial induction, and shroud force is extracted from the solution fields and compared against the predictions of low-order theories. Finally, the blade element model is used to redesign the turbine twist distribution to achieve greater performance across a range of tip speed ratios.

Turbulence modelling of low Reynolds number flow effects around a vertical axis turbine at a range of tip-speed ratios

This paper presents a two-dimensional numerical investigation of a vertical axis turbine with focus on the prediction from two different turbulence modelling schemes. The three bladed turbine configuration tested is that of Roa et al. (2010) which has a high solidity of 1.1 and is tested at a chord-based Reynolds number of 1.5 × 105. Results are provided at three different tip speed ratios: the optimal value, and values 25% higher and lower. A detailed investigation of the flow ensues and results are presented as obtained from the original SST model and a modified version for low Reynolds number effects, recently tested by Wang et al. (2010) for a similar flow. Post-processing includes the evolution of the skin-friction coefficient over the blade throughout a cycle which enables a better understanding of the flow structure and the dynamic stall effects, as well as blade–vortex interaction. The low Reynolds number version of the model acts to reduce turbulence viscosity, leading to laminar flow at various parts of the cycle, which is seen to correctly enable the formation of leading edge vortices observed in the literature.

Conceptual design and numerical simulations of a vertical axis water turbine used for underwater mooring platforms

Energy is a direct restriction to the working life of an underwater mooring platform (UMP). In this paper, a vertical axis water turbine (VAWT) is designed to supply energy for UMPs. The VAWT has several controlled blades, which can be opened or closed by inside plunger pumps. Two-dimensional transient numerical studies are presented to determine the operating performance and power output of the turbine under low ocean current velocity. A standard k-e turbulence model is used to perform the transient simulations. The influence of structural parameters, including foil section profile, foil chord length and rotor diameter, on the turbine performance are investigated over a range of tipspeed-ratios ( TSRs ). It was found that turbine with three unit length NACA0015 foils generated a maximum averaged coefficient of power, 0.1, at TSR = 2.

The influence of surface gravity waves on marine current turbine performance

Surface gravity waves can significantly impact operating conditions for a marine current turbine, imparting unsteady velocities several orders of magnitude larger than the ambient turbulence. The influence of surface waves on the performance characteristics of a two-bladed horizontal axis marine current turbine was investigated experimentally in a large towing tank facility at the United States Naval Academy. The turbine model had a 0.8m diameter (D) rotor with a NACA 63-618 cross section, which is Reynolds number independent with respect to lift coefficient in the operating range of Rec≈4×105. The torque, thrust and rotational speed were measured at a range of tip speed ratios (TSR) from 5<TSR<11. Tests were performed at two rotor depths (1.3D and 2.25D) with and without waves. The average turbine performance characteristics were largely unchanged by depth or the presence of waves. However, tests with waves indicate large variations in thrust, rotational speed, and torque occurred with the passage of the wave. These results demonstrate the impact of surface gravity waves on power production and structural loading and suggest that turbines should be positioned vertically within the water column at a depth which maximizes power output while minimizing material fatigue.

Turbulent flow and loading on a tidal stream turbine by LES and RANS

This paper presents results from numerical simulations of a 3-bladed horizontal axis tidal stream turbine. Initially, Reynolds Averaged Navier Stokes (RANS) k–ω Shear Stress Transport eddy-viscosity and Launder–Reece–Rodi models were used for code validation and testing of a newly implemented sliding mesh technique for an unstructured finite volume code. Wall- and blade-resolved large-eddy simulations (LES) were then performed to study the complete geometry at various tip speed ratios (TSR). Thrust and power coefficients were compared to published experimental measurements obtained from a towing tank for a range of TSR (4, 5, 6, 7, 8, 9 and 10) at a fixed hub pitch angle. A strong meandering is observed downstream of the supporting tower due to interaction between the detached tip vortices and vortex shedding from the support structure. The wake profiles and rate of recovery of velocity deficit show high sensitivity to the upstream turbulence intensities. However, the mean thrust and power coefficients were found to be less sensitive to the upstream turbulence. Comparisons between RANS and LES are also presented for the mean sectional blade pressures and mean wake velocity profiles. The paper also presents an overview of modelling and numerical issues relating to simulations for such rotating geometries.

Experimental measurements of the hydrodynamic performance and structural loading of the transverse horizontal axis water turbine: Part 2

This paper is the second of three, which outline the procedures and results for a set of experiments on various configurations of the Transverse Horizontal Axis Water Turbine (THAWT), which is a horizontally orientated variant of the Darrieus cross-flow turbine. Tests were conducted in the combined wind, wave and current tank at Newcastle University on a 0.5 m diameter rotor, while the flow depth and velocity were varied over a range of realistic Froude numbers for tidal streams. Various configurations of the device were tested to assess the merits of varied blade pitch, rotor solidity, blockage ratio and truss oriented blades. Experiments were carried out using a speed controlled motor/generator, allowing quasi-steady results to be taken over a range of tip speed ratios. Measurements of power, thrust, blade loading and free surface deformation provide extensive data for future validation of numerical codes and demonstrate the ability of the device to exceed the Lanchester–Betz limit for kinetic efficiency, by exploiting high blockage. This second paper covers the instrumentation and analysis for the structural loading for the parallel bladed variant of the THAWT device. The first paper covers the experimental setup and hydrodynamic performance of the parallel bladed rotor, and the third paper covers both performance and loading of the truss configured THAWT device.

Experimental measurements of the hydrodynamic performance and structural loading of the Transverse Horizontal Axis Water Turbine: Part 1

This paper is the first of three, which outline the procedures and results for a set of experiments carried out on various configurations of the Transverse Horizontal Axis Water Turbine (THAWT), which is a horizontally orientated variant of the Darrieus cross-flow turbine. Tests were conducted in the combined wind, wave and current tank at Newcastle University on a 0.5 m diameter rotor, while the flow depth and velocity were varied over a range of realistic Froude numbers for tidal streams. Various configurations of the device were tested to assess the merits of varied blade pitch, rotor solidity, blockage ratio and truss oriented blades. Experiments were carried out using a speed controlled motor/generator, allowing quasi-steady results to be taken over a range of tip speed ratios. Measurements of power, thrust, blade loading and free surface deformation provide extensive data for future validation of numerical codes and demonstrate the ability of the device to exceed the Lanchester-Betz limit for kinetic efficiency by using high blockage. This paper covers the experimental procedures and results for the hydrodynamic performance for the parallel bladed variant of the THAWT device. The second paper covers the hydrodynamic loading of the parallel bladed rotor and the third covers both hydrodynamic performance and loading of the truss configured THAWT device.

Experimental measurements of the hydrodynamic performance and structural loading of the Transverse Horizontal Axis Water Turbine: Part 3

This paper is the third of three, which describe the procedures and results for a set of experiments on various configurations of the Transverse Horizontal Axis Water Turbine (THAWT), which is a horizontally orientated variant of the Darrieus cross-flow turbine. Tests were conducted in the combined wind, wave and current tank at Newcastle University on a 0.5 m diameter rotor, while the flow depth and velocity were varied over a range of realistic Froude numbers for tidal streams. Various configurations of the device were tested to assess the merits of varied blade pitch, rotor solidity, blockage ratio and truss oriented blades. Experiments were carried out using a speed-controlled motor/generator, allowing quasi-steady results to be taken over a range of tip speed ratios. Measurements of power, thrust, blade loading and free surface deformation provide extensive data for future validation of numerical codes and demonstrate the ability of the device to exceed the Lanchester–Betz limit for kinetic efficiency by using high blockage. This paper covers the instrumentation, hydrodynamic performance and loading of the truss bladed variant of the THAWT device. The first paper covers the experimental setup and hydrodynamic performance of the parallel bladed rotor and the second paper covers the instrumentation and hydrodynamic loading of the parallel bladed rotor.

The effect of surface waves on the performance characteristics of a model tidal turbine

Scale model tests were conducted on a three bladed horizontal axis tidal turbine in a large tow tank facility at the United States Naval Academy. Performance characteristics are presented for a turbine towed at a constant carriage speed for cases with and without surface waves. Intermediate waves were modeled that are representative of swells typically found on the continental shelf of the United States eastern seaboard. The oscillatory wave velocity present in the water column results in significant variations in measured turbine torque and rotational speed as a function of wave phase. This in turn produces cyclic variations in tip speed ratio and power coefficient. The power coefficient over the entire wave phase did not show a difference from the experiments without waves for a range of tip speed ratios, as reported in previous studies. The waves limited the lower range of tip speed ratios at which the turbine could operate. These results highlight the impact of surface waves on turbine design and performance, and the importance of understanding the site-specific wave conditions.

Modeling of the flow in a Darrieus water turbine: Wall grid refinement analysis and comparison with experiments

This paper presents some aspects concerning the 2D RANS numerical modeling of a Darrieus cross flow marine turbine. Two main features of the modeling are studied. The first deals with the influence of the near wall grid density on the numerical results. Most of the available literature concerning the occurrence of stalling foils emphasizes the need for a fine grid mesh at wall fitting y+ around the unity or less at the first near wall cell center. Nevertheless, in the case of a Darrieus turbine, the influence of this parameter has not yet been studied precisely. In particular, the exact y+ specification is not known, and its influence either on the global turbine performance or on the local flow field, has not been outlined. The present work provides insight into the y+ influence in a 2D Darrieus turbine and deals with its maximum acceptable value. The second feature concerns the ability of a 2D modeling to represent, the actual 3D flow in the turbine. The power coefficients CP are compared to those obtained in the hydrodynamic LEGI tunnel on a small scale model. The experimental power coefficients are presented with their associated precision. The comparisons show a medium tip speed ratio range around the nominal point for which the instantaneous ratio of the experimental and numerical power coefficients is a constant significantly lower than 1 regardless of the azimuthal position of the blades. This constant ratio is thought to be representative of the tip and arm-blade junctions losses.

A numerical study of blade thickness and camber effects on vertical axis wind turbines

This article presents the results of a two-dimensional computational study into the effects of rotor blade thickness and camber on the performance of a 5 kW scale vertical axis wind turbine. Validation is provided by reference to experimental data for a pitching aerofoil with dynamic stall phenomenon. The performance of the turbine is mapped out for a variety of different tip speed ratios and detailed investigations are presented to determine how and, most importantly, why the turbine performance varies with thickness and camber as it does. The rotor blades chosen were the NACA0012, NACA0022, NACA5522 and LS0421. The turbine rotor has a diameter of d = 3.1 m with a blade chord length of c = 0.18 m. Over the range of tip speed ratios examined, the NACA0012 profile performed with the highest overall performance of 50% at λ = 3.5. Slightly cambered aerofoils (such as the LS0421) can improve the overall performance of the vertical axis wind turbine, whereas a camber of 5% (NACA5522) results in unfavourable performance. Of the cambered blades tested, the LS0421 performs the best with maximum CP of 0.40 at λ = 3.5. A camber along the blade path causes the blades to generate higher values of torque in both the upwind and the downwind regions. It was also determined that inverted cambered profiles produce power mostly in the upwind region.

Numerical Research on the Characteristics of Lift-Drag Type Vertical Axis Wind Turbine

Numerical studies are conducted to research the performance of a kind of lift-drag type vertical axis wind turbine (VAWT) affected by solidity with the CFD method. Moving mesh technique is used to construct the model. The Spalart-Allmaras one equation turbulent model and the implicit coupled algorithm based on pressure are selected to solve the transient equations. In this research, how the tip speed ratio and the solidity of blade affect the power coefficient (Cp) of the small H-VAWT is analyzed. The results indicate that Cp curves exhibit approximate parabolic form with its maximum in the middle range of tip speed ratio. The two-blade wind turbine has the lowest Cp while the three-blade one is more powerful and the four-blade one brings the highest power. With the certain number of blades, there is a best chord length, and too long or too short chord length may reduce the Cp.

1406 滝用水車の開発 : 衝動型ランナと貫流型ランナの性能比較(OS14-2 再生可能流体エネルギー,オーガナイズドセッション)

The aim of this study is to develop the open type runner utilizing waterfall for environmentally friendly nano-hydraulic turbine. Impulse type runner and cross-flow type one was selected and compared with each runner performance. As a result, the cross-flow runner is superior to the impulse runner. Power coefficient of cross-flow runner is reached at 0.74 in wide range of tip speed ratio.

Experimental testing of the transverse horizontal axis water turbine

This study outlines the procedures and results for a set of experiments on the transverse horizontal axis water turbine (THAWT), which is a variant of a Darrieus turbine. Testing of a 1/20th scale device was conducted in the combined wind, wave and current tank at Newcastle University. Flow depth and velocity were varied over a range of realistic Froude numbers for tidal streams. Various configurations of the device were tested to assess the merits of the THAWT design. Variants included a parallel-bladed device, a ‘truss’ device and a parallel-bladed device configured with blades pitched relative to the pitch circle tangent. Experiments were carried out using a speed controlled motor, allowing quasi-steady results to be taken over a range of tip speed ratios. The results demonstrate that, over a range of flow conditions, the device is capable of exceeding the Lanchester–Betz limit for kinetic efficiency. This is principally because of the relatively high blockage ratio that can be achieved with such a device.

Blade Offset and Pitch Effects on a High Solidity Vertical Axis Wind Turbine

A high solidity, small scale, 2.5m diameter by 3m high Vertical Axis Wind Turbine (VAWT) consisting of three NACA 0015 profile blades, each with a span of 3m and a chord length of 0.4m, was tested in an open-air wind tunnel facility to investigate the effects of preset toe-in and toe-out turbine blade pitch. The effect of blade mount-point offset was also investigated. The results from these tests are presented for a range of tip speed ratios, and compared with an extensive base data set obtained for a nominal wind speed of 10m/s. Results show measured performance decreases of up to 47% for toe-in, and increases of up to 29% for toe-out blade pitch angles, relative to the zero preset pitch case. Also, blade mount-point offset tests indicate decreases in performance as the mount location is moved from mid-chord towards the leading edge, as a result of an inherent toe-in condition. Observations indicate that these performance decreases may be minimized by compensating for the blade mount offset with a toe-out preset pitch. The trends of the preset blade pitch tests agree with those found in literature for much lower solidity turbines.