scholarly journals Blind test comparison of the performance and wake flow between two in-line wind turbines exposed to different turbulent inflow conditions

2017 ◽  
Vol 2 (1) ◽  
pp. 55-76 ◽  
Author(s):  
Jan Bartl ◽  
Lars Sætran
Keyword(s):  
Wind Tunnel ◽  
Wind Turbines ◽  
Turbulence Intensity ◽  
Wake Flow ◽  
Turbulent Shear ◽  
Mean Velocity ◽  
Blind Test ◽  
Turbulent Inflow ◽  
The Mean ◽  
Inflow Conditions

Abstract. This is a summary of the results of the fourth blind test workshop that was held in Trondheim in October 2015. Herein, computational predictions on the performance of two in-line model wind turbines as well as the mean and turbulent wake flow are compared to experimental data measured at the wind tunnel of the Norwegian University of Science and Technology (NTNU). A detailed description of the model geometry, the wind tunnel boundary conditions and the test case specifications was published before the workshop. Expert groups within computational fluid dynamics (CFD) were invited to submit predictions on wind turbine performance and wake flow without knowing the experimental results at the outset. The focus of this blind test comparison is to examine the model turbines' performance and wake development with nine rotor diameters downstream at three different turbulent inflow conditions. Aside from a spatially uniform inflow field of very low-turbulence intensity (TI = 0.23 %) and high-turbulence intensity (TI = 10.0 %), the turbines are exposed to a grid-generated highly turbulent shear flow (TI = 10.1 %).Five different research groups contributed their predictions using a variety of simulation models, ranging from fully resolved Reynolds-averaged Navier–Stokes (RANS) models to large eddy simulations (LESs). For the three inlet conditions, the power and the thrust force of the upstream turbine is predicted fairly well by most models, while the predictions of the downstream turbine's performance show a significantly higher scatter. Comparing the mean velocity profiles in the wake, most models approximate the mean velocity deficit level sufficiently well. However, larger variations between the models for higher downstream positions are observed. Prediction of the turbulence kinetic energy in the wake is observed to be very challenging. Both the LES model and the IDDES (improved delayed detached eddy simulation) model, however, consistently manage to provide fairly accurate predictions of the wake turbulence.

scholarly journals Blind test comparison of the performance and wake flow between two in-line wind turbines exposed to different atmospheric inflow conditions

2016 ◽  
Author(s):  
Jan Bartl ◽  
Lars Sætran
Keyword(s):  
Wind Tunnel ◽  
Wind Turbines ◽  
Turbulence Intensity ◽  
Wake Flow ◽  
Detached Eddy Simulation ◽  
Mean Velocity ◽  
Blind Test ◽  
The Mean ◽  
Inlet Conditions ◽  
Inflow Conditions

Abstract. This is a summary of the results of the fourth Blind test workshop which was held in Trondheim in October 2015. Herein, computational predictions on the performance of two in-line model wind turbines as well as the mean and turbulent wake flow are compared to experimental data measured at NTNU's wind tunnel. A detailed description of the model geometry, the wind tunnel boundary conditions and the test case specifications was published before the workshop. Expert groups within Computational Fluid Dynamics (CFD) were invited to submit predictions on wind turbine performance and wake flow without knowing the experimental results at the outset. The focus of this blind test comparison is to examine the model turbines' performance and wake development up until 9 rotor diameters downstream at three different atmospheric inflow conditions. Besides a spatially uniform inflow field of very low turbulence intensity (TI = 0.23 %) as well as high turbulence intensity (TI = 10.0 %), the turbines are exposed to a grid-generated atmospheric shear flow (TI = 10.1 %). Five different research groups contributed with their predictions using a variety of simulation models, ranging from fully resolved Reynolds Averaged Navier Stokes (RANS) models to Large Eddy Simulations (LES). For the three inlet conditions the power and the thrust force of the upstream turbine is predicted fairly well by most models, while the predictions of the downstream turbine's performance show a significantly higher scatter. Comparing the mean velocity profiles in the wake, most models approximate the mean velocity deficit level sufficiently well. However, larger variations between the models for higher downstream positions are observed. The prediction of the turbulence kinetic energy in the wake is observed to be very challenging. Both the LES model and the IDDES (Improved Delayed Detached Eddy Simulation) model, however, are consistently managing to provide fairly accurate predictions of the wake turbulence.

scholarly journals Wind tunnel experiments on wind turbine wakes in yaw: effects of inflow turbulence and shear

2018 ◽  
Vol 3 (1) ◽  
pp. 329-343 ◽  
Author(s):  
Jan Bartl ◽  
Franz Mühle ◽  
Jannik Schottler ◽  
Lars Sætran ◽  
Joachim Peinke ◽  
...  
Keyword(s):  
Wind Turbine ◽  
Laser Doppler Anemometry ◽  
Vortex Pair ◽  
Wake Flow ◽  
Turbulent Shear ◽  
Mean Velocity ◽  
Inflow Turbulence ◽  
The Mean ◽  
Inflow Conditions ◽  
Wake Characteristics

Abstract. The wake characteristics behind a yawed model wind turbine exposed to different customized inflow conditions are investigated. Laser Doppler anemometry is used to measure the wake flow in two planes at x∕D = 3 and x∕D = 6, while the turbine yaw angle is varied from γ=-30∘ to 0∘ to +30∘. The objective is to assess the influence of grid-generated inflow turbulence and shear on the mean and turbulent flow components. The wake flow is observed to be asymmetric with respect to negative and positive yaw angles. A counter-rotating vortex pair is detected creating a kidney-shaped velocity deficit for all inflow conditions. Exposing the rotor to non-uniform highly turbulent shear inflow changes the mean and turbulent wake characteristics only insignificantly. At low inflow turbulence the curled wake shape and wake center deflection are more pronounced than at high inflow turbulence. For a yawed turbine the rotor-generated turbulence profiles peak in regions of strong mean velocity gradients, while the levels of peak turbulence decrease at approximately the same rate as the rotor thrust.

Wakes behind two-dimensional surface obstacles in turbulent boundary layers

1974 ◽  
Vol 64 (3) ◽  
pp. 529-564 ◽  
Author(s):  
J. Counihan ◽  
J. C. R. Hunt ◽  
P. S. Jackson
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Turbulent Boundary Layer ◽  
The Body ◽  
Wake Flow ◽  
Two Dimensional ◽  
Mean Velocity ◽  
Velocity Deficit ◽  
Wind Tunnel Measurements ◽  
The Mean

By making simple assumptions, an analytical theory is deduced for the mean velocity behind a two-dimensional obstacle (of heighth) placed on a rigid plane over which flows a turbulent boundary layer (of thickness δ). It is assumed thath[Gt ] δ, and that the wake can be divided into three regions. The velocity deficit −uis greatest in the two regions in which the change in shear stress is important, a wall region (W) close to the wall and a mixing region (M) spreading from the top of the obstacle. Above these is the external region (E) in which the velocity field is an inviscid perturbation on the incident boundary-layer velocity, which is taken to have a power-law profileU(y) =U∞(y−y1)n/δn, wheren[Gt ] 1. In (M), assuming that an eddy viscosity (=KhU(h)) can be defined for the perturbed flow in terms of the incident boundary-layer flow and that the velocity is self-preserving, it is found thatu(x,y) has the form$\frac{u}{U(h)} = \frac{ C }{Kh^2U^2(h)} \frac{f(n)}{x/h},\;\;\;\; {\rm where}\;\;\;\; \eta = (y/h)/[Kx/h]^{1/(n+2)}$, and the constant which defines the strength of the wake is$C = \int^\infty_0 y^U(y)(u-u_E)dy$, whereu=uE(x, y) asy→ 0 in region (E).In region (W),u(y) is proportional to Iny.By considering a large control surface enclosing the obstacle it is shown that the constant of the wake flow is not simply related to the drag of the obstacle, but is equal to the sum of the couple on the obstacle and an integral of the pressure field on the surface near the body.New wind-tunnel measurements of mean and turbulent velocities and Reynolds stresses in the wake behind a two-dimensional rectangular block on a roughened surface are presented. The turbulent boundary layer is artificially developed by well-established methods (Counihan 1969) in such a way that δ = 8h. These measurements are compared with the theory, with other wind-tunnel measurements and also with full-scale measurements of the wind behind windbreaks.It is found that the theory describes the distribution of mean velocity reasonably well, in particular the (x/h)−1decay law is well confirmed. The theory gives the correct self-preserving form for the distribution of Reynolds stress and the maximum increase of the mean-square turbulent velocity is found to decay downstream approximately as$ (\frac{x}{h})^{- \frac{3}{2}} $in accordance with the theory. The theory also suggests that the velocity deficit is affected by the roughness of the terrain (as measured by the roughness lengthy0) in proportion to In (h/y0), and there seems to be some experimental support for this hypothesis.

Weakly sheared turbulent flows generated by multiscale inhomogeneous grids

2018 ◽  
Vol 848 ◽  
pp. 788-820 ◽  
Author(s):  
Shaokai Zheng ◽  
P. J. K. Bruce ◽  
J. M. R. Graham ◽  
J. C. Vassilicos
Keyword(s):  
Velocity Profile ◽  
Turbulence Intensity ◽  
Shear Flows ◽  
Vertical Direction ◽  
Turbulent Shear ◽  
Incoming Flow ◽  
Mean Velocity ◽  
Free Stream Turbulence ◽  
The Mean ◽  
Vertical Bars

A group of three multiscale inhomogeneous grids have been tested to generate different types of turbulent shear flows with different mean shear rate and turbulence intensity profiles. Cross hot-wire measurements were taken in a wind tunnel with Reynolds number$Re_{D}$of 6000–20 000, based on the width of the vertical bars of the grid and the incoming flow velocity. The effect of local drag coefficient$C_{D}$on the mean velocity profile is discussed first, and then by modifying the vertical bars to obtain a uniform aspect ratio the mean velocity profile is shown to be predictable using the local blockage ratio profile. It is also shown that, at a streamwise location$x=x_{m}$, the turbulence intensity profile along the vertical direction$u^{\prime }(y)$scales with the wake interaction length$x_{\ast ,n}^{peak}=0.21g_{n}^{2}/(\unicode[STIX]{x1D6FC}C_{D}w_{n})$($\unicode[STIX]{x1D6FC}$is a constant characterizing the incoming flow condition, and$g_{n}$,$w_{n}$are the gap and width of the vertical bars, respectively, at layer$n$) such that$(u^{\prime }/U_{n})^{2}\unicode[STIX]{x1D6FD}^{2}(C_{D}w_{n}/x_{\ast ,n}^{peak})^{-1}\sim (x_{m}/x_{\ast ,n}^{peak})^{b}$, where$\unicode[STIX]{x1D6FD}$is a constant determined by the free-stream turbulence level,$U_{n}$is the local mean velocity and$b$is a dimensionless power law constant. A general framework of grid design method based on these scalings is proposed and discussed. From the evolution of the shear stress coefficient$\unicode[STIX]{x1D70C}(x)$, integral length scale$L(x)$and the dissipation coefficient$C_{\unicode[STIX]{x1D716}}(x)$, a simple turbulent kinetic energy model is proposed that describes the evolution of our grid generated turbulence field using one centreline measurement and one vertical profile of$u^{\prime }(y)$at the beginning of the evolution. The results calculated from our model agree well with our measurements in the streamwise extent up to$x/H\approx 2.5$, where$H$is the height of the grid, suggesting that it might be possible to design some shear flows with desired mean velocity and turbulence intensity profiles by designing the geometry of a passive grid.

scholarly journals Wind tunnel experiments on wind turbine wakes in yaw: Effects of inflow turbulence and shear

2018 ◽  
Author(s):  
Jan Bartl ◽  
Franz Mühle ◽  
Jannik Schottler ◽  
Lars Sætran ◽  
Joachim Peinke ◽  
...  
Keyword(s):  
Wind Turbine ◽  
Laser Doppler Anemometry ◽  
Vortex Pair ◽  
Wake Flow ◽  
Mean Velocity ◽  
Inflow Turbulence ◽  
The Mean ◽  
Center Deflection ◽  
Inflow Conditions ◽  
Wake Characteristics

Abstract. The wake characteristics behind a yawed model wind turbine exposed to different customized inflow conditions are investigated. Laser Doppler Anemometry is used to measure the wake flow in two planes at x/D = 3 and x/D = 6 while the turbine yaw angle is varied from −30° and 0° to +30°. The objective is to assess the influence of grid-generated inflow turbulence and shear on the mean and turbulent flow components. The wake flow is observed to be asymmetric with respect to negative and positive yaw angles. A counter-rotating vortex pair is detected creating a kidney-shaped velocity deficit for all inflow conditions. Exposing the rotor to non-uniform shear inflow changes the mean and turbulent wake characteristics only insignificantly. At low inflow turbulence the curled wake shape and wake center deflection are more pronounced than at high inflow turbulence. For a yawed turbine the rotor-generated turbulence profiles peak in regions of strong mean velocity gradients, while the levels of peak turbulence decrease at approximately the same rate as the rotor thrust.

A wind tunnel simulation of the mean velocity fields behind upright porous fences

2007 ◽  
Vol 146 (1-2) ◽  
pp. 82-93 ◽  
Author(s):  
Zhibao Dong ◽  
Wanyin Luo ◽  
Guangqiang Qian ◽  
Hongtao Wang
Keyword(s):  
Wind Tunnel ◽  
Velocity Fields ◽  
Mean Velocity ◽  
Wind Tunnel Simulation ◽  
The Mean

Investigation of Boundary Layer Flows Over a Flat Plate With Different-Size Transverse Square Grooves at s/w = 10

2007 ◽  
Author(s):  
Redha Wahidi ◽  
Walid Chakroun ◽  
Sami Al-Fahad
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Skin Friction ◽  
Turbulence Intensity ◽  
Viscous Sublayer ◽  
Velocity Profiles ◽  
Boundary Layer Flows ◽  
Mean Velocity ◽  
Uncertainty Range ◽  
The Mean

Turbulent boundary layer flows over a flat plate with multiple transverse square grooves spaced 10 element widths apart were investigated. Mean velocity profiles, turbulence intensity profiles, and the distributions of the skin-friction coefficients (Cf) and the integral parameters are presented for two grooved walls. The two transverse square groove sizes investigated are 5mm and 2.5mm. Laser-Doppler Anemometer (LDA) was used for the mean velocity and turbulence intensity measurements. The skin-friction coefficient was determined from the gradient of the mean velocity profiles in the viscous sublayer. Distribution of Cf in the first grooved-wall case (5mm) shows that Cf overshoots downstream of the groove and then oscillates within the uncertainty range and never shows the expected undershoot in Cf. The same overshoot is seen in the second grooved-wall case (2.5mm), however, Cf continues to oscillate above the uncertainty range and never returns to the smooth-wall value. The mean velocity profiles clearly represent the behavior of Cf where a downward shift is seen in the Cf overshoot region and no upward shift is seen in these profiles. The results show that the smaller grooves exhibit larger effects on Cf, however, the boundary layer responses to these effects in a slower rate than to those of the larger grooves.

Control of trailing-edge separation by tangential blowing inside the bubble

2004 ◽  
Vol 108 (1086) ◽  
pp. 419-425 ◽  
Author(s):  
P. R. Viswanath ◽  
K. T. Madhavan
Keyword(s):  
Separation Bubble ◽  
Effective Means ◽  
Separated Flow ◽  
Wake Flow ◽  
Turbulent Shear ◽  
Trailing Edge ◽  
Mean Velocity ◽  
Turbulent Shear Stress ◽  
Edge Separation ◽  
Tangential Blowing

Abstract Experiments have been performed investigating the effectiveness of steady tangential blowing, with the blowing slot located downstream of separation (but inside the separation bubble) to control a trailing-edge separated flow at low speeds. Trailing-edge separation was induced on a two-dimensional aerofoil-like body and the shear layer closure occurred in the near-wake. Measurements made consisted of model surface pressures and mean velocity, turbulent shear stress and kinetic energy profiles in the separated zone using a two-component LDV system. It is explicitly demonstrated that the novel concept of tangential blowing inside the bubble can be an effective means of control for trailing-edge separated flows as well. Blowing mass and momentum requirements for the suppression of wall and wake flow reversals have been estimated.

Combined effects of flow curvature and rotation on uniformly sheared turbulence

2009 ◽  
Vol 628 ◽  
pp. 371-394 ◽  
Author(s):  
D. C. ROACH ◽  
A. G. L. HOLLOWAY
Keyword(s):  
Shear Stress ◽  
Wind Tunnel ◽  
Model Development ◽  
Three Dimensional ◽  
Simple Shear Flow ◽  
Test Case ◽  
Mean Velocity ◽  
Tunnel Section ◽  
Flow Curvature ◽  
The Mean

This paper describes an experiment in which a uniformly sheared turbulence was subjected to simultaneous streamwise flow curvature and rotation about the streamwise axis. The distortion of the turbulence is complex but well defined and may serve as a test case for turbulence model development. The uniformly sheared turbulence was developed in a straight wind tunnel and then passed into a curved tunnel section. At the start of the curved section the plane of the mean shear was normal to the plane of curvature so as to create a three-dimensional or ‘out of plane’ curvature configuration. On entering the curved tunnel, the flow developed a streamwise mean vorticity that rotated the mean shear about the tunnel centreline through approximately 70°, so that the shear was nearly in the plane of curvature and oriented so as to have a stabilizing effect on the turbulence. Hot wire measurements of the mean velocity, mean vorticity, mean rate of strain and Reynolds stress anisotropy development along the wind tunnel centreline are reported. The observed effect of the mean shear rotation on the turbulence was to diminish the shear stress in the plane normal to the plane of curvature while generating non-zero values of the shear stress in the plane of curvature. A rotating frame was identified for which the measured mean velocity field took the form of a simple shear flow. The turbulence anisotropy was transformed to this frame to estimate the effects of frame rotation on the structure of sheared turbulence.

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