Wind turbine partial wake merging description and quantification

Individual turbine location within a wind plant defines the flow characterisitcs experienced by a given turbine. Irregular turbine arrays and inflow misalignment can reduce plant efficiency by producing highly asymmetric wakes with enhanced downstream longevity. Changes in wake dynamics as a result of turbine position were quantified in a wind tunnel experiment. Scale model turbines with a rotor diameter of 20 cm and a hub height of 24 cm were placed in symmetric, asymmetric, and rotated configurations. Simultaneous hub height velocity measurements were recorded at 11 spanwise locations for three distances downstream of the turbine array under two inflow conditions. Wake interactions are described in terms of the time‐average streamwise velocity and turbulence intensity as well as the displacement, momentum, and energy thicknesses. The effects of wake merging on power generation are quantified, and the two‐point correlation is used to examine symmetry in the mean velocity between wakes. The results indicate that both asymmetric and rotated wind plant arrangements can produce long‐lasting wakes. At shallow angles, rotated configurations compound the effects of asymmetric arrangements and greatly increase downstream wake persistence.     

Numerical investigation of wind turbine wake development in directionally sheared inflow

Turbines in wind farms are subject to complex mutual aerodynamic interactions, which in detail depend upon the characteristics of the atmospheric boundary layer. Our two objectives with this paper were to investigate the impact of directionally sheared inflow on the wake development behind a single wind turbine and to analyse the impact of the wakes on the energy yield and loading of a downstream turbine, which is exposed to partial and full wake conditions. We performed simulations with a framework based on a coupled approach of large‐eddy simulation and an actuator line representation of an aeroelastic turbine model. Our results show that directionally sheared inflow leads to a non‐symmetrical wake development, which transfers to distinct differences in the energy yield and loading of downstream turbines of equal lateral offsets in opposite direction. Therefore, the assumption of wakes being axisymmetrical could lead to notable deviations in the prediction of wake behaviour and their impact on downstream turbines for atmospheric inflow conditions, which include directional shear. Copyright © 2016 John Wiley & Sons, Ltd.     

An Experimental Study of Three-Dimensional Characteristics of Turbulent Wakes of Axial Compressor Rotors

This paper presents an experimental investigation of the characteristics of three-dimensional turbulent wakes of an isolated axial compressor rotor and a single-stage axial compressor rotor. The wakes were measured from hub to tip using a single-slant hot-wire and a four-hole conical high frequency pressure probe. The experiments were made at both design and near stall conditions. Variations of mean velocities, total pressure, static pressure and turbulence stresses in the wakes are shown and interpreted. The experimental data from the isolated compressor rotor wake are compared with that from the single-stage compressor rotor.     

A novel split-dimple interrupted fin configuration for heat transfer augmentation

The use of an interrupted plate fin with surface roughness in the form of split-dimples is investigated. High-fidelity time-dependent calculations are performed for a wide range of Reynolds number ranging from Re_H = 240 to 4000, covering the laminar to fully turbulent flow regimes. The split-dimples provide an additional mechanism for augmenting heat transfer by perturbing continuous boundary layer formation on the fin surface and generating energetic shear layers. High heat transfer regions are observed at the fin and split-dimple leading edges as a result of boundary layer restarts, in regions of flow acceleration between protrusions, and flow impingement on the protrusion surface. While the protruding geometry of the split-dimple also aids in augmenting heat transfer from the fin surface by generating unsteady or turbulent wakes, it also increases pressure losses. The split-dimple fin results in a heat conductance that is 60–175% higher than a plain interrupted plate fin, but at a cost of 4–8 times the frictional losses.     

A study of the near wake structure of a wind turbine comparing measurements from laboratory and full-scale experiments

Wake flow measurements have been performed using the technique of particle image velocimetry (PIV) at stations downstream from a model wind turbine rotor, and evaluated against experimental data from two full-scale machines. Comparisons include both mean velocity and turbulent intensity cross-wake profiles at a range of tip speed ratios. The application of PIV to the study of wind turbine wakes is described in detail, including the steps required to ensure appropriate and accurate simulation of the flow field conditions. The results suggest that the PIV method is a potentially useful tool in the investigation of detailed wake flow, though significant differences are observed between wake velocity deficits at full- and model scale. These are discussed with regard to scale effect, the influence of terrain, model similarity, and the phenomenon of wake meandering and effective cross-wake smoothing.     

The spectral signature of wind turbine wake meandering: A wind tunnel and field‐scale study

Field‐scale and wind tunnel experiments were conducted in the 2D to 6D turbine wake region to investigate the effect of geometric and Reynolds number scaling on wake meandering. Five field deployments took place: 4 in the wake of a single 2.5‐MW wind turbine and 1 at a wind farm with numerous 2‐MW turbines. The experiments occurred under near‐neutral thermal conditions. Ground‐based lidar was used to measure wake velocities, and a vertical array of met‐mounted sonic anemometers were used to characterize inflow conditions. Laboratory tests were conducted in an atmospheric boundary layer wind tunnel for comparison with the field results. Treatment of the low‐resolution lidar measurements is discussed, including an empirical correction to velocity spectra using colocated lidar and sonic anemometer. Spectral analysis on the laboratory‐ and utility‐scale measurements confirms a meandering frequency that scales with the Strouhal number St = fD/U based on the turbine rotor diameter D. The scaling indicates the importance of the rotor‐scaled annular shear layer to the dynamics of meandering at the field scale, which is consistent with findings of previous wind tunnel and computational studies. The field and tunnel spectra also reveal a deficit in large‐scale turbulent energy, signaling a sheltering effect of the turbine, which blocks or deflects the largest flow scales of the incoming flow. Two different mechanisms for wake meandering—large scales of the incoming flow and shear instabilities at relatively smaller scales—are discussed and inferred to be related to the turbulent kinetic energy excess and deficit observed in the wake velocity spectra.     

Direct Numerical Simulation of Air Heated Cylinder Wake in Transitional State. Part I: Cylinder Heating Effect on Strong Turbulence Anisotropy (STA), and Determination of STA Suppression

A variation of strong turbulence anisotropy (STA) with cylinder heating, and, as a result, STA suppression, are here first elucidated in the air heated wake in the transitional state at Reynolds number Re = 300 and Richardson number Ri = 0.3. Simultaneously, new facts for variations of velocities u, v, and w, i.e., the upward, horizontal, and spanwise directions, and wake motion by cylinder heating are first elucidated. Here STA is defined such that, in velocities u, v, and w, laminar velocities arise in at least one directions but turbulence arises in the remainder of directions. 1. STA is determined to arise in isothermal and heated wakes by employing the new method. With cylinder heating, STA is found to be suppressed because the turbulent region in w is decreased, the three-dimensionality is suppressed, but the two-dimensionality and laminarization are enhanced in the heated wake.

2. With cylinder heating, the spanwise velocity |w| is found to be suppressed strongly in the probability density function (PDF), whose peak value is decreased strongly near w ≈ 0, and becomes 20% of that in the isothermal wake.

3. The above decrease in |w| is found to be equivalent to the increase of laminar state in time records. With cylinder heating, the laminar region is increased, but the region with turbulence is decreased.

4. The wake stability is extremely different in thermal flows with or without turbulence in w. When w has turbulence, here u and v are laminar, STA arises, and the thermal flow field becomes the transitional state, has instability, and is destabilized.

5. When w does not have turbulence, and is laminar, velocities u, v , and w are all laminar, form only 3-D flow, and do not have turbulence anisotropy. The wake does not have instability, and is stabilized.

     

Reynolds number dependence of turbulence statistics in the wake of wind turbines

A wind tunnel experiment has been performed to quantify the Reynolds number dependence of turbulence statistics in the wake of a model wind turbine. A wind turbine was placed in a boundary layer flow developed over a smooth surface under thermally neutral conditions. Experiments considered Reynolds numbers on the basis of the turbine rotor diameter and the velocity at hub height, ranging from Re = 1.66 × 10⁴ to 1.73 × 10⁵. Results suggest that main flow statistics (mean velocity, turbulence intensity, kinematic shear stress and velocity skewness) become independent of Reynolds number starting from Re ≈ 9.3 × 10⁴. In general, stronger Reynolds number dependence was observed in the near wake region where the flow is strongly affected by the aerodynamics of the wind turbine blades. In contrast, in the far wake region, where the boundary layer flow starts to modulate the dynamics of the wake, main statistics showed weak Reynolds dependence. These results will allow us to extrapolate wind tunnel and computational fluid dynamic simulations, which often are conducted at lower Reynolds numbers, to full‐scale conditions. In particular, these findings motivates us to improve existing parameterizations for wind turbine wakes (e.g. velocity deficit, wake expansion, turbulence intensity) under neutral conditions and the predictive capabilities of atmospheric large eddy simulation models. Copyright © 2011 John Wiley & Sons, Ltd.     

Turbulent kinetic energy estimates from profiling wind LiDAR measurements and their potential for wind energy applications

This study shows that turbulent kinetic energy (TKE) estimates, derived from static LiDARs in Doppler Beam Swing (DBS) mode, permit a qualitative and quantitative characterization and analysis of turbulent structures as wind turbine wakes, and convective or shear generated eddies in the lower atmospheric boundary layer. The analysed data, collected by a WINDCUBE™ v1 in a wind park in Austria, is compared to WINDCUBE™ v1 and sonic data from the WINd Turbine Wake EXperiment Wieringermeer (WINTWEX-W). Although turbulence measurements with a WINDCUBE™ v1 are limited to a specific length scale, processed measurements above this threshold are in a good agreement with sonic anemometer data. In contrast to the commonly used turbulence intensity, the calculation of TKE not only provides an appropriate measure of turbulence intensities but also gives an insight into its origin. The processed data show typical wake characteristics, as flow decelerations, turbulence enhancement and wake rotation. By comparing these turbulence characteristics to other turbulent structures in the atmospheric boundary layer, we found that convection driven eddies in the surface layer have similar turbulence characteristics as turbine wakes, which makes convective weather situations relevant for wind turbine fatigue considerations.     

Vapor pressure distribution of a flat plate heat pipe

The vapor pressure distributions of an asymmetric vapor flow of a flat plate heat pipe were calculated using the concept of suction and injection principle. Both symmetrical and asymmetrical flow conditions were considered. Comparisons were made with existing results. The agreement was reasonably good.     

Comparison of CFD Simulation to Experiment for Convection Heat Transfer in an Enhanced Channel

The numerical and experimental study of heat transfer characteristics in an enhanced channel with turbulent flow is presented. Numerical computations have been done for a periodic element of the channel with periodically fully developed flow using a commercial finite element code. The main objective of this study was to use computational fluid dynamics to obtain convection heat transfer coefficients with air as the fluid. Numerical predictions were compared with experimental results, and a reasonably good agreement was found between the two. It is shown that the channel investigated in this study improves the convection heat transfer coefficient. For high Reynolds number flow conditions, Nusselt numbers in this channel exceeded those in the parallel plate channel by approximately 220%.     

A stochastic wind turbine wake model based on new metrics for wake characterization

Understanding the detailed dynamics of wind turbine wakes is critical to predicting the performance and maximizing the efficiency of wind farms. This knowledge requires atmospheric data at a high spatial and temporal resolution, which are not easily obtained from direct measurements. Therefore, research is often based on numerical models, which vary in fidelity and computational cost. The simplest models produce axisymmetric wakes and are only valid beyond the near wake. Higher‐fidelity results can be obtained by solving the filtered Navier–Stokes equations at a resolution that is sufficient to resolve the relevant turbulence scales. This work addresses the gap between these two extremes by proposing a stochastic model that produces an unsteady asymmetric wake. The model is developed based on a large‐eddy simulation (LES) of an offshore wind farm. Because there are several ways of characterizing wakes, the first part of this work explores different approaches to defining global wake characteristics. From these, a model is developed that captures essential features of a LES‐generated wake at a small fraction of the cost. The synthetic wake successfully reproduces the mean characteristics of the original LES wake, including its area and stretching patterns, and statistics of the mean azimuthal radius. The mean and standard deviation of the wake width and height are also reproduced. This preliminary study focuses on reproducing the wake shape, while future work will incorporate velocity deficit and meandering, as well as different stability scenarios. Copyright © 2016 John Wiley & Sons, Ltd.     

Evaluation of layout and atmospheric stability effects in wind farms using large‐eddy simulation

Large‐eddy simulation (LES) has been used previously to study the effect of either configuration or atmospheric stability on the power generated by large wind farms. This is the first study to consider both stability and wind farm configuration simultaneously and methodically with LES. Two prevailing wind directions, two layouts (turbines aligned versus staggered with respect to the wind) and three stabilities (neutral and moderately unstable and stable) were evaluated. Compared with neutral conditions, unstable conditions led to reduced wake losses in one configuration, to enhanced wake losses in two and to unchanged wake losses in one configuration. Conversely, stable conditions led to increased wake losses in one, decreased wake losses in two and unchanged wake losses in one configuration. Three competing effects, namely, rates of wake recovery due to vertical mixing, horizontal spread of wakes and localized regions of acceleration caused by multiple upstream wakes, were identified as being responsible for the observed trends in wake losses. The detailed flow features responsible for these non‐linear interactions could only be resolved by the LES. Existing analytical models ignore stability and non‐linear configuration effects, which therefore need to be incorporated. Copyright © 2017 John Wiley & Sons, Ltd.     

Appraisal and calibration of the actuator line model for the prediction of turbulent separated wakes

The aim of this study is to further investigate the accuracy and the reliability of the actuator line model (ALM) predictions for turbulent separated wakes. Large eddy simulations (LES) of the flow around a NACA0009 airfoil are performed mimicking the geometry with the immersed boundary method. Results are validated against experiments and used to assess the accuracy of the ALM predictions for the same airfoil, with different values of the spreading parameter and of the reference velocity and for two values of the angle of attack. It is found that the ALM setup recently derived from linearized inviscid analysis leads to accurate results for the lower angle of attack, while at the higher one for which a significant separation of the boundary layer occurs, the ALM requires a different set of model parameters. This calls for a systematic investigation of the sensitivity to the ALM parameters for separated flows, which is carried out herein through a stochastic approach allowing continuous response surfaces to be obtained in the parameter space. The ALM parameters are calibrated against the results obtained with the immersed boundaries. With the calibrated model parameters, the ALM gives good predictions of the velocity and turbulent kinetic energy in the far wake. Finally, the proposed model parameters are used to predict the flow past a different geometry, a flat plate, at high angle of attack. The accuracy of the prediction of the far wake is again good, showing the robustness of the identified setup.     

Observations of wind turbine wakes and surface roughness effects on wind flow variability

Wind data collected at nine meteorological towers at the Goodnoe Hills MOD-2 wind turbine site were analyzed to characterize the wind flow over the site both in the absence and presence of wind turbine wakes. Free-flow characteristics examined were the variability of wind speed and turbulence intensity across the site as a function of wind direction and surface roughness. The nine towers' data revealed that scattered areas of trees upwind of the site caused pronounced variations in the wind flow over the site. At two towers that were frequently downwind of an extensive grove of trees, up to 30% reductions in wind speed and a factor of 2 to 3 increase in turbulence intensity were measured. A substantial increase in the magnitude of the wind gusts, as well as a considerable decrease in the mean wind speed, was observed when a tower was downwind of the trees.Wind turbine wake characteristics analyzed included the average velocity deficits, wake turbulence, wake width, wake trajectory, vertical profile of the wake, and the stratification of wake properties as a function of the ambient wind speed and turbulence intensity. The wind turbine rotor disk spanned a height of 15 m to 107 m. The nine towers' data permitted a detailed analysis of the wake behavior at a height of 32 m at various downwind distances from 2 to 10 rotor diameters (D). The relationship between velocity deficit and downwind distance was surprisingly linear, with average maximum deficits ranging from 34% at 2 D to 7% at 10 D. Largest deficits were at low wind speeds and low turbulence intensities. Average wake widths were 2.8 D at a downwind distance of 10 D. Implications for turbine spacing are that, for a wind farm with a 10-D row separation, array losses would be significantly greater for a 2-D than a 3-D spacing because of incremental effects caused by overlapping wakes. Other interesting wake properties observed were the wake turbulence (which was greatest along the flanks of the wake). the vertical variation of deficits (which were greater below hub height than above), and the trajectory of the wake (which was essentially straight).     

Large‐eddy simulations of wind‐farm wake characteristics associated with a low‐level jet

In this study, we performed a suite of flow simulations for a 12‐wind‐turbine array with varying inflow conditions and lateral spacings, and compared the impacts of the flow on velocity deficit and wake recovery. We imposed both laminar inflow and turbulent inflows, which contain turbulence for the Ekman layer and a low‐level jet (LLJ) in the stable boundary layer. To solve the flow through the wind turbines and their wakes, we used a large‐eddy simulation technique with an actuator‐line method. We compared the time series for the velocity deficit at the first and rear columns to observe the temporal change in velocity deficit for the entire wind farm. The velocity deficit at the first column for LLJ inflow was similar to that for laminar inflow. However, the magnitude of velocity deficit at the rear columns for the case with LLJ inflow was 11.9% greater because of strong wake recovery, which was enhanced by the vertical flux of kinetic energy associated with the LLJ. To observe the spatial transition and characteristics of wake recovery, we performed statistical analyses of the velocity at different locations for both the laminar and LLJ inflows. These studies indicated that strong wake recovery was present, and a kurtosis analysis showed that the probability density function for the streamwise velocity followed a Gaussian distribution. In a quadrant analysis of the Reynolds stress, we found that the ejection and sweep motions for the LLJ inflow case were greater than those for the laminar inflow case.     

Heat Transfer and Flow Characteristics of Al2O3/Water Nanofluid in Various Heat Exchangers: Experiments on Counter Flow

Abstract

On account of nanofluids influence on heat exchangers (HEs), a vigorous discussion can be made to concurrently contrast HEs to one another under the same conditions to detect maximum efficacy. Based on an extensive experimental study, this research is established to examine the effect of nanofluids on the performance of heterogeneous HEs with the same heat transfer surface area considering counter flow arrangement. A double pipe HE, a shell and tube HE and a plate HE are intended to accomplish the experiments. The experiments are executed under turbulent flow conditions using distilled water and Al₂O₃/water nanofluid with 0.2, 0.5, and 1% particle volume concentrations. From the results shown in the article, the double pipe HE revealed the best outcome for the heat transfer coefficient with a maximum enhancement of 60% while a maximum enhancement in the heat transfer coefficient of 11% was reported for the plate HE. Utilizing a nanofluid represented the lowest penalty in the pressure drop with a maximum enhancement of 27% for the plate HE while the highest penalty in the pressure drop with a maximum enhancement of 85% was observed in the double pipe and shell and tube HEs.     

The near wake of a model horizontal-axis wind turbine—II. General features of the three-dimensional flowfield

The three-dimensional near-wake of a model horizontal-axis wind turbine has been measured for three operating conditions: stalled flow over the blades, close to optimum performance, and approaching runaway. The measurements of the mean velocity and turbulence at six axial locations document the formation and development of the near-wake. For the two highest tip speed ratios, the tip vortices are clearly identifiable from the contours of axial velocity and vorticity, and turbulent kinetic energy. At the lowest tip speed ratio, the turbulence level is also high within the blade wakes and these wakes are larger, because of separation in the flow over the blades. The wake structure is simplest for the condition closest to the optimum where the bound vorticity is almost constant with radius. As the tip speed ratio increases, the pitch of the tip vortices decreases and the angular momentum within them increases. This angular momentum reduces the power available from the turbine. The implication is that the structure of the tip vortices must be included in computational models intended to cover the entire operating range of a turbine.     

Experimental Study on Wake Development in a Three-cylinder Model within a Tunnel in Transitional Flow

ABSTRACT

The characteristics of the flow and heat transfer in transitional flow are not clear and especially the lack of some reliable experimental data hinders the numerical study of the heat exchanger. Based on this, a two-dimensional time-resolved digital particle image velocimetry from the American-TSI-corporation was adopted. Experiments were carried out in an open-loop, recirculating water tunnel that operated at steady-state conditions. The experimental object is a three-cylinder with an equilateral triangle configuration having center-to-center distance ratio of 3. Reynolds number based on the tube diameter and the velocity of inlet is set at 70, 90, 120, 150, 200, 250 and 300. The flow parameters were quantified to research the effect of Reynolds number on flow field for supporting some facts to analyze the heat transfer. Time-mean flow field, turbulence statistics and velocity processed by proper orthogonal decomposition were investigated at different Reynolds numbers. The wake closure length, oscillation and Karman vortex in the wake of the three-cylinder model were discussed in detail.     

Evaluation of the effects of turbulence model enhancements on wind turbine wake predictions

The modelling of wind turbine wakes is investigated in this paper using a Navier–Stokes solver employing the k–ω turbulence model appropriately modified for atmospheric flows. It is common knowledge that even single‐wind turbine wake predictions with computational fluid dynamic methods underestimate the near wake deficit, directly contributing to the overestimation of the power of the downstream turbines. For a single‐wind turbine, alternative modelling enhancements under neutral and stable atmospheric conditions are tested in this paper to account for and eventually correct the turbulence overestimation that is responsible for the faster flow recovery that appears in the numerical predictions. Their effect on the power predictions is evaluated with comparison with existing wake measurements. A second issue addressed in this paper concerns multi‐wake predictions in wind farms, where the estimation of the reference wind speed that is required for the thrust calculation of a turbine located in the wake(s) of other turbines is not obvious. This is overcome by utilizing an induction factor‐based concept: According to it, the definition of the induction factor and its relationship with the thrust coefficient are employed to provide an average wind speed value across the rotor disk for the estimation of the axial force. Application is made on the case of five wind turbines in a row. Copyright © 2010 John Wiley & Sons, Ltd.