scholarly journals Non-Equilibrium Scaling Applied to the Wake Evolution of a Model Scale Wind Turbine

Energies ◽  
2019 ◽  
Vol 12 (14) ◽  
pp. 2763 ◽  
Author(s):  
Victor P. Stein ◽  
Hans-Jakob Kaltenbach
Keyword(s):  
Wind Turbine ◽  
Reynolds Stress ◽  
Wake Region ◽  
Reynolds Stresses ◽  
Self Similarity ◽  
Horizontal Axis Wind Turbine ◽  
Velocity Deficit ◽  
Stress Components ◽  
Far Wake ◽  
Non Equilibrium

The present paper addresses the evolution of turbulence characteristics in wind turbine wakes immersed in a turbulent boundary layer. The study thereby focuses on finding physically consistent scaling laws for the wake width, the velocity deficit, and the Reynolds stresses in the far wake region. For this purpose, the concept of an added wake is derived which allows to analyse the self-similarity of the added flow quantities and the applicability of the non-equilibrium dissipation theory. The investigation is based on wind tunnel measurements in the wake of a three-bladed horizontal axis wind turbine model (HAWT) immersed in two neutrally-stratified turbulent boundary layers of different aerodynamic roughness length. The dataset also includes wake measurements for various yaw angles. A high degree of self-similarity is found in the lateral profiles of the velocity deficit and of the added Reynolds stress components. It is shown that these can be described by combined Gaussian shape functions. In the vertical, self-similarity can just be shown in the upper part of the wake. Moreover, it is observed that the degree of self-similarity is affected by the ground roughness. Results suggest an approximately constant anisotropy of the added turbulent stresses in the far wake, and the axial scaling of the added Reynolds stress components is found to be in accordance with non-equilibrium dissipation theory. It predicts a x − 1 decay of the added turbulent intensity I + , and a x − 2 evolution of the added Reynolds shear stresses Δ u i ′ u j ′ ¯ and the velocity deficit Δ u . Based on these findingsa semi-empirical model is proposed for predicting the Reynolds stresses in the far wake region which can easily be coupled with existing analytical wake models. The proposed model is found to be in good agreement with the measurement results.

Characterizing the Transitional Behavior of Wind Turbine Wake From Near to Far Wake Regimes

2021 ◽  
Author(s):  
Ravi Kumar ◽  
Ojing Siram ◽  
Niranjan Sahoo ◽  
Ujjwal K. Saha
Keyword(s):  
Wind Turbine ◽  
Energy Demand ◽  
Wind Farm ◽  
Flow Characteristics ◽  
Wind Farms ◽  
Horizontal Axis Wind Turbine ◽  
Velocity Deficit ◽  
Turbine Wake ◽  
Far Wake ◽  
Wake Characteristics

Abstract Knowledge of wind energy harvesting is an ever-growing process, and to meet the enormous energy demand, wind farms shall have a significant role. An efficient wind farm is required to have an in-depth knowledge of turbine wake characteristics. This article presents an experimental investigation of the wake expansion process defined by the transition of wake from near to far wake regimes. The study has been performed on models horizontal axis wind turbine (HAWT) composed of NACA 0012 profile, keeping the ratio of root chord to tip chord length is 5:2. A constant temperature hot-wire anemometer (HWA) has been used to examine the rotor’s fluctuating flow field. The subsequent time-averaged normalizes velocity deficit, and vortex shedding frequency are used for the flow characteristics. Time-averaged velocity deficit measurement suggests a drop in upstream velocity by 20–30% within the vicinity of rotor tip downstream of the rotor plane. The study shows that flow recovery is initiating from the near wake regime around 1.08R. Further, the spectral findings indicates the low frequency dominance within 4R (R being the rotor radius), and the Strouhal number falls close to 0.23. The present wind tunnel study on wake characteristics throws significant insight into further enhancing the WT wake modeling.

Performance Enhancement Analysis of a Horizontal Axis Wind Turbine by Vortex Trapping Cavity

2021 ◽  
pp. 1-13
Author(s):  
Khaoula Qaissi ◽  
Omer A Elsayed ◽  
Mustapha Faqir ◽  
Elhachmi Essadiqi
Keyword(s):  
Wind Turbine ◽  
Performance Enhancement ◽  
Lift Coefficient ◽  
Separated Flow ◽  
National Renewable Energy Laboratory ◽  
Wake Region ◽  
High Twist ◽  
Horizontal Axis Wind Turbine ◽  
Wind Speeds ◽  
Nrel Phase Vi

Abstract A wind turbine blade has the particularity of containing twisted and tapered thick airfoils. The challenge with this configuration is the highly separated flow in the region of high twist. This research presents a numerical investigation of the effectiveness of a Vortex Trapping Cavity (VTC) on the aerodynamics of the National renewable Energy laboratory (NREL) Phase VI wind turbine. First, simulations are conducted on the S809 profile to study the fluid flow compared to the airfoil with the redesigned VTC. Secondly, the blade is simulated with and without VTC to assess its effect on the torque and the flow patterns. The results show that for high angles of incidence at Rec=106, the lift coefficient increases by 10% and the wake region appears smaller for the case with VTC. For wind speeds larger than 10 m/s, the VTC improves the torque by 3.9%. This is due to the separation that takes place in the vicinity of the VTC and leads to trapping early separation eddies inside the cell. These eddies roll up forming a coherent laminar vortex structure, which in turn sheds periodically out of the cell. This phenomenon favourably reshapes excessive flow separation, reenergizes the boundary layer and globally improves blade torque.

Flow characterization in the near-wake region of a horizontal axis wind turbine

Wind Energy ◽  
2015 ◽  
Vol 19 (7) ◽  
pp. 1249-1267 ◽  
Author(s):  
Pooyan Hashemi Tari ◽  
Kamran Siddiqui ◽  
Horia Hangan
Keyword(s):  
Wind Turbine ◽  
Horizontal Axis ◽  
Wake Region ◽  
Horizontal Axis Wind Turbine ◽  
Near Wake ◽  
Flow Characterization

scholarly journals Experimental Study and Simulation of a Small-Scale Horizontal-Axis Wind Turbine

2017 ◽  
Vol 139 (5) ◽  
Author(s):  
Randall S. Jackson ◽  
Ryoichi Amano
Keyword(s):  
Wind Turbine ◽  
High Speed ◽  
Wind Farm ◽  
Reynolds Stress Model ◽  
Turbulence Closure ◽  
Small Scale ◽  
Stress Model ◽  
Horizontal Axis Wind Turbine ◽  
Cfd Simulations ◽  
Velocity Deficit

The advancement of wind energy as an alternative source to hydrocarbons depends heavily on research activities in turbulence modeling and experimentation. The velocity deficit behind wind turbines affects the power output and efficiency of a wind farm. Being able to simulate the wake dynamics of a wind turbine effectively can result in optimum spacing, longer wind turbine life, and shorter payback on the wind farm investment. Two-equation turbulence closure models, such as k–ε and k–ω, are used extensively to predict wind turbine performance and velocity deficit profiles. The application of the Reynolds stress model (RSM) turbulence closure method has been limited to few studies where the rotor is modeled as an actuator disk (AD). The computational cost associated with RSM has made it challenging for simulations where the rotor is discretized directly; however, with advances in computer speed and power coupled with parallel computing architecture, RSM may be a better turbulence closure option. In this research, wind tunnel experiments were conducted, using hot-wire anemometry, to measure the velocity deficit profiles at different wake locations behind a small-scale, three-bladed, horizontal-axis wind turbine (HAWT). Experiments were also performed with two and three HAWTs in series to evaluate the change in velocity deficit and turbulence intensity (TI). High-speed imaging with an oil-based mist captured the vortices produced at the blade tips and showed the vortices dissipated approximately three rotor diameters downstream. Computational fluid dynamics (CFD) simulations were performed to predict the velocity deficit at wake locations matching the experiments. The Reynolds stress model was applied to a fully discretized rotor with a tower and nacelle included in the simulation. A steady-state moving reference frame (MRF) model was created with the computational domain subdivided into rotating and stationary domains. The MRF results were used as an initial condition for time-accurate rigid body motion (RBM) simulations. The RBM CFD simulations showed excellent agreement with experimental measurements for velocity deficit after properly accounting for experimental boundary effects. Isosurfaces of the Q-criterion highlighted the vortices produced at the blade tips and were consistent with high-speed images.

Numerical Investigation of Actuator Disc Thickness Effect on Predicting the Performance and Far Wake of the Horizontal Axis Wind Turbine

2015 ◽  
Author(s):  
Ali Behrouzifar ◽  
Masoud Darbandi ◽  
Gerry E. Schneider
Keyword(s):  
Wind Turbine ◽  
Thickness Distribution ◽  
The Other ◽  
Thickness Effect ◽  
Navier Stokes ◽  
Constant Thickness ◽  
Horizontal Axis Wind Turbine ◽  
Design Data ◽  
Actuator Disc ◽  
Far Wake

In this paper, the actuator disk (AD) method is used to simulate the wind turbine performance and far-wake behavior. In this work, we incorporate the AD method with an axisymmetric full Navier-Stokes solver. In other words, the calculated AD load is suitably distributed on the disc to impose the aerodynamic forces acting on the blade. One important factor among various different factors, which affect the AD modeling prediction considerably, is the utilized grid thickness. In this work, we first choose the grid thickness recommended by the other researches and study the actual thickness of NREL 5MW wind turbine. Next, many other configurations are considered in our AD thickness modeling including a constant thickness and the actual thickness configurations. The latter one has a linear thickness distribution from 3.54 m at the root to 0.7 m at tip. The wind speed is ranged from 3 to 11 m/s consistent with the practical tests performed on the NREL 5MW wind turbine. We calculate the generated power for all the simulated configurations and their percentages of differences. The results show that the lowest difference is about 4.5% for a constant AD thickness of 0.2 m. Therefore, we conclude that the current AD model predicts results very close to the NREL design data. Additionally, this accurate prediction is similarly observed in the other ranges of wind turbine operational speeds.

Effects of Axial Casing Grooves on the Structure of Turbulence in the Tip Region of an Axial Turbomachine Rotor

2021 ◽  
pp. 1-46
Author(s):  
Huang Chen ◽  
Yuanchao Li ◽  
Subhra Shankha Koley ◽  
Joseph Katz
Keyword(s):  
Reynolds Stress ◽  
Turbulence Modeling ◽  
Suction Side ◽  
Vortex Center ◽  
Reynolds Stresses ◽  
Tip Leakage ◽  
Stress Components ◽  
Anisotropy Tensor ◽  
Stress Models ◽  
Turbomachine Rotor

Abstract Challenges in turbulence modeling in the tip region of turbomachines include anisotropy, inhomogeneity, and non-equilibrium conditions, resulting in poor correlations between Reynolds stresses and the corresponding mean strain rate components. The geometric complexity introduced by casing grooves exacerbates this problem. Taking advantage of a large database collected in the refractive index-matched liquid facility at JHU, this paper examines the effect of axial casing grooves on the distributions of turbulent kinetic energy (TKE), Reynolds stresses, anisotropy tensor, and TKE production rate in the tip region of an axial turbomachine. Comparisons are performed at flow rates corresponding to prestall and best efficiency points of the untreated machine. Common features include high TKE near the tip leakage vortex center, and in shear layer connecting it to the blade suction side tip corner. The turbulence is highly anisotropic and inhomogeneous, with the anisotropy tensor shifting from one dimensional (1D) to 2D and to 3D structures over small distances. With the grooves, the flow structure, hence the distribution of Reynolds stresses, becomes more complex. Additional sites with elevated turbulence include the corner vortex that develops at the entrance to the grooves, and in the flow jetting out of the grooves into the passage. Consistent with trends of the production rates of normal Reynolds stress components, the grooves increase the axial but reduce the radial velocity fluctuations as the inflow and outflow from the groove interacts with the passage flow. These findings might assist the development of Reynolds stress models suitable for tip flows.

LDA Measurements on Rough Turbulent Boundary Layers Subject to Strong Favorable Pressure Gradients

2006 ◽  
Author(s):  
Rau´l Bayoa´n Cal ◽  
Brian Brzek ◽  
Gunnar Johansson ◽  
Luciano Castillo
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Rough Surface ◽  
Reynolds Stress ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Velocity Deficit ◽  
The Mean ◽  
Stress Profiles

Laser-Doppler anemometry (LDA) measurements of the mean velocity and Reynolds stresses are carried out on a rough surface favorable pressure gradient (FPG) turbulent boundary layer. These data is compared with smooth FPG turbulent boundary layer data possessing with the same strength of pressure gradient and also with rough zero pressure gradient (ZPG) data. The scales for the mean velocity deficit and Reynolds stresses are obtained through means of equilibrium similarity analysis of the RANS equations [1]. The mean velocity deficit profiles collapse, but to different curves when normalized using the free-stream velocity. The effects of the pressure gradient and roughness are clearly distinguished and separated. However, these effects are removed from the outer flow when the profiles are normalized using the Zagarola and Smits [2] scaling. It is also found that there is a clear effect of the roughness and pressure gradient on the Reynolds stresses. The Reynolds stress profiles augment due to the rough surface. Furthermore, the strength of the pressure gradient imposed of the flow changes the shape of the Reynolds stress profiles especially on the < v2 > and < uv > components. The rough surface influence is mostly noticed on the < u2 > component of the Reynolds stress, where the shape of the profiles change entirely. The boundary layer parameter δ*/δ shows the effects of the roughness and a dependence on the Reynolds number for the smooth FPG case. The pressure parameter, A, describes a development of the turbulent boundary layer and no influence of the roughness is linked with the parameter, k+. The boundary layers grow differently and depict the influence of the studied effects in their development. These measurements are the first of their nature due to the extensive number in downstream locations (12) and the combination of the studied external conditions (i.e., the strength of the pressure gradient and the surface roughness).

Application of the Reynolds Stress Model to Direct Modeling and Actuator Disk Simulations of a Small-Scale Horizontal-Axis Wind Turbine

2016 ◽  
Author(s):  
Randall Jackson ◽  
Ryoichi S. Amano
Keyword(s):  
Wind Turbine ◽  
Reynolds Stress ◽  
Reynolds Stress Model ◽  
Horizontal Axis ◽  
Tip Vortex ◽  
Small Scale ◽  
Stress Model ◽  
Closure Model ◽  
Horizontal Axis Wind Turbine ◽  
Turbulence Closure Model

Computational Fluid Dynamics (CFD) has become a staple in wind energy research and studies cover a broad range of topics including atmospheric wind profiles, airfoil design, wind turbine design, terrain effects, and wake dynamics. One of the most important aspects of applying CFD methods is the selection of a turbulence closure model when solving the Reynolds Averaged Navier-Stokes (RANS) equations. In this research, the Reynolds Stress Model (RSM) was applied to predict the wake turbulence and velocity profiles for a small scale, 3-bladed, horizontal-axis wind turbine (HAWT) using a commercial CFD software, Star CCM+. The wind turbine was modeled directly by discretizing the rotor and also using an actuator disc concept to simulate the rotor. Wind tunnel experiments were performed using hot-wire anemometry to measure the velocity deficit at various downstream locations. High speed images were also captured to examine qualitatively the wake and tip vortex dissipation created from an oil mist. The CFD results show the RSM turbulence closure model to be excellent in predicting the wake velocity and tip vortex structure when compared to experimental results.

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