Effect Of Tip Speed Ratio Research Articles

Effect of tip speed ratio on coherent dynamics in the near wake of a model wind turbine

The near wake of a small-scale wind turbine is investigated using particle image velocimetry experiments at different tip speed ratios ( $\lambda$ ). The wind turbine model had a nacelle and a tower mimicking real-scale wind turbines. The near wake is found to be dominated by multiple coherent structures, including the tip vortices, distinct vortex sheddings from the nacelle and tower, and wake meandering. The merging of the tip vortices is found to be strongly dependent on $\lambda$ . A convective length scale ( $L_c$ ) related to the pitch of the tip vortices is defined that is shown to be a better length scale than turbine diameter ( $D$ ) to demarcate the near wake from the far wake. The tower induced strong vertical asymmetry in the flow by destabilising the tip vortices and promoting mixing in the lower (below the nacelle) plane. The nacelle's shedding is found to be important in ‘seeding’ wake meandering, which, although not potent, exists close to the nacelle, and it becomes important only after a certain distance downstream ( $x>3L_c$ ). A link between the ‘effective porosity’ of the turbine and $\lambda$ is established, and the strength and frequency of wake meandering are found to be dependent on $\lambda$ . In fact, a decreasing trend of wake meandering frequency with $\lambda$ is observed, similar to vortex shedding from a porous plate at varying porosity. Such similarity upholds the notion of wake meandering being a global instability of the turbine, which can be considered as a ‘porous’ bluff body of diameter $D$ .

Effect of tip speed ratio on the aerodynamic noise of a small wind turbine: An optimization study

A dual-objective optimization study was carried out to illuminate how the tip speed ratio of a small wind turbine affects the aerodynamic noise as well as the blade geometry. A 0.75 kW three-bladed small horizontal axis wind turbine was selected as the case study. Two important goals were considered in the study: maximization of the output power and the minimization of the aerodynamic noise. The former was calculated by the well-known blade-element momentum theory while a validated semi-empirical model was adopted for the computation of the latter. A combination of these goals defined the objective function and the weighted-sum approach was employed to find the optimal values for the design variables of the optimization including the distributions of the chord and the twist angle along the blade together with the tip speed ratio. The extracted power was calculated at the rated wind speed of 10 m/s while the noise was computed at lower wind speeds of 5 and 7.5 m/s. The genetic algorithm technique was adopted for the muti-objective optimization. The results revealed the importance of the blade tip region in producing the emitted aerodynamic noise. Specifically, an increase in the chord and twist values near the tip results in the reduction in the emitted noise. Results also show that a good compromise between the two goals is achievable such that a noticeable reduction in the emitted aerodynamic noise is attainable in exchange for a very small drop in the power coefficient. The optimization results also indicated that the noise could be adequately reduced at the smaller value of the tip speed ratio without a large reduction in output power which is due to the decrease in the rotational speed of the turbine.

Effect of Tip Speed Ratio on the Performance of H-Darrieus Wind Turbine with NACA 0018 Airfoil

The effect of Tip Speed Ratio (TSR) to the performance of an H-Darrieus Vertical Axis Wind Turbine was studied using numerical method. The blade of the turbine was formed by using NACA 0018 mode. Three-dimensional governing equations are developed. The turbulent flow was modelled using k – epsilon equation. The commercial code Computational Fluid Dynamic is employed to solve the problem. The focus in here is to explore effect of the TSR. The tip speed ratio is varied from 1.65 to 1.8. As a results, the contour velocity was generated and discussed. The average velocity of the wind leaving from the turbine is employed to estimate the power coefficient. The power coefficient for tip speed ratio 1.65 and 1.8 are discussed. The conclusion here is that tip speed ratio and the power coefficient strongly affected each other. Increasing the tip speed ratio will increase the coefficient power.

Numerical modeling of aeroacoustic characteristics of different savonius blade profiles

Purpose This study aims to carry out numerical modeling to predict aerodynamic noise radiation from four different Savonius rotor blade profile. Design/methodology/approach Incompressible unsteady reynolds-averaged navier-stokes (URANS) approach using gamma–theta turbulence model is conducted to obtain the time accurate turbulent flow field. The Ffowcs Williams and Hawkings (FW-H) acoustic analogy formulation is used for noise predictions at optimal tip speed ratio (TSR). Findings The mean torque and power coefficients are compared with the experimental data and acceptable agreement is observed. The total and Mono+Dipole noise graphs are presented. A discrete tonal component at low frequencies in all graphs is attributed to the blade passing frequency at the given TSR. According to the noise prediction results, Bach type rotor has the lowest level of noise emission. The effect of TSR on the noise level from the Bach rotor is investigated. A direct relation between angular velocity and the noise emission is found. Practical implications The savonius rotor is a type of vertical axis wind turbines suited for mounting in the vicinity of residential areas. Also, wind turbines wherein operation are efficient sources of tonal and broadband noises and affect the inhabitable environment adversely. Therefore, the acoustic pollution assessment is essential for the installation of wind turbines in residential areas. Originality/value This study aims to investigate the radiated noise level of four common Savonius rotor blade profiles, namely, Bach type, Benesh type, semi-elliptic and conventional. As stated above, numbers of studies exploit the URANS method coupled with the FW-H analogy to predict the aeroacoustics behavior of wind turbines. Therefore, this approach is chosen in this research to deal with the aeroacoustics and aerodynamic calculation of the flow field around the aforementioned Savonius blade profiles. The effect of optimal TSR on the emitted noise and the contribution of thickness, loading and quadrupole sources are of interest in this study.

Novel energy coefficient used to predict efflux velocity of tidal current turbine

The efflux velocity is the basis for the prediction of turbine wake. A novel energy coefficient is defined to propose a new theoretical equation to predict the efflux velocity of tidal current turbine in this paper. Several CFD cases with different tip speed ratio and solidity is conducted using the DES-SA model. In order to overcome the limitations of the axial momentum theory, the effects of tip speed ratio and solidity on the efflux velocity are studied and the energy coefficients with different tip speed ratio and solidity are determined using the proposed equation based on the CFD results. Several semi-empirical efflux velocity equations are finally proposed by fitting the equation of the energy coefficient with tip speed ratio. The application of these equations in the prediction of wake flow and the power calculation of tidal turbine are also introduced in this paper.

Unsteady analysis on the instantaneous forces and moment arms acting on a novel Savonius-style wind turbine

This paper aims to present a transient analysis on the forces acting on a novel two-bladed Savonius-style wind turbine. Two-dimensional unsteady Reynolds Averaged Navier Stokes equations are solved using shear stress transport k–ω turbulence model at a Reynolds number of 1.23×105. The instantaneous longitudinal drag and lateral lift forces acting on each of the blades and their acting points are calculated. The corresponding moment arms responsible for the torque generation are obtained. Further, the effect of tip speed ratio on the force coefficients, moment arms and overall turbine performances are observed. Throughout the paper, the obtained results for the new design are discussed with reference to conventional semi-circular design of Savonius turbines. A significant performance improvement is achieved with the new design due to its increased lift and moment arm contribution as compared to the conventional design. More interestingly, the present study sets a platform for future aerodynamic research and improvements for Savonius-style wind turbines.

The effect of tip speed ratio on a vertical axis wind turbine at high Reynolds numbers

This work visualizes the flow surrounding a scaled model vertical axis wind turbine at realistic operating conditions. The model closely matches geometric and dynamic properties—tip speed ratio and Reynolds number—of a full-size turbine. The flow is visualized using particle imaging velocimetry (PIV) in the midplane upstream, around, and after (up to 4 turbine diameters downstream) the turbine, as well as a vertical plane behind the turbine. Time-averaged results show an asymmetric wake behind the turbine, regardless of tip speed ratio, with a larger velocity deficit for a higher tip speed ratio. For the higher tip speed ratio, an area of averaged flow reversal is present with a maximum reverse flow of $$-0.04U_\infty$$ . Phase-averaged vorticity fields—achieved by syncing the PIV system with the rotation of the turbine—show distinct structures form from each turbine blade. There were distinct differences in results by tip speed ratios of 0.9, 1.3, and 2.2 of when in the cycle structures are shed into the wake—switching from two pairs to a single pair of vortices being shed—and how they convect into the wake—the middle tip speed ratio vortices convect downstream inside the wake, while the high tip speed ratio pair is shed into the shear layer of the wake. Finally, results show that the wake structure is much more sensitive to changes in tip speed ratio than to changes in Reynolds number.

Mutual inductance of two helical vortices

The pairing of helical tip vortices in the wake of a two-bladed rotor is investigated experimentally. Time-resolved particle image velocimetry measurements provide a clear temporal and spatial evolution of the vortical structures, highlighting the transition to instability and the effect of tip speed ratio and helical spacing. The temporal growth rate of the vortex system instabilities were measured and are shown to be dependent on helical spacing. The evolution of filaments and their growth rates support the argument that the mutual inductance mode is the driving mechanism behind the transition to an unstable wake. The measurements are in agreement with maximum growth rates predicted by linear stability analyses of single- and double-helix arrangements. In addition, the wake topology due to varying rotor load through tip speed ratio variation is shown to play an important role in the initial symmetry breaking that drives the wake transition.

The laminar boundary layer on a rotating wind turbine blade

The present paper describes a method to calculate velocity profiles in the boundary layer of rotating blade. A differential approach is used to solve the laminar boundary layer equations. The effects of tip speed ratio, dimensionless radial position r/R and angle of attack have been analyzed. The test airfoils used in the simulations are NACA 63-215 and S809. The resulting velocity profiles in the chordwise and spanwise directions are mapped and stored in a Database according to the boundary layer parameters.

3D boundary layer study on a rotating wind turbine blade

The present paper describes a method to calculate velocity profiles in the boundary layer of a rotating blade. A differential approach is used to solve the laminar boundary layer equations. The effects of tip speed ratio, dimensionless radial position r/R and angle of attack have been analyzed. The test airfoils used in the simulations are NACA 63-215 and S809. The resulting velocity profiles in the chordwise and spanwise directions are mapped and stored in a Database according to the boundary layer parameters.

Effect of Tip Speed Ratio on the Longitudinal and Lateral Response Characteristics of a Lifting Rotor

Effect of Tip Speed Ratio on the Longitudinal and Lateral Response Characteristics of a Lifting Rotor

Effect of Tip Speed Ratio on the Longitudinal and Lateral Response Characteristics of a Lifting Rotor

Effect of Tip Speed Ratio on the Longitudinal and Lateral Response Characteristics of a Lifting Rotor