Wake Region Research Articles

Fatigue degradation of wind turbines considering dynamic wake meandering effects

This paper systematically investigates the effects of six factors, namely wind shear, ambient turbulence, wake meandering, wake-added turbulence, turbulence intensities, and wind speed, on the dynamic responses and fatigue degradation of wind turbines in the wake region. An improved mid-fidelity code, FAST.Farm, is used, which considers wake-added turbulence by adopting an additional wind domain with the Mann model. Different distributions of short-term damage equivalent stresses are found in two selected components, namely figure-8 and elliptical distributions, respectively, for nonturbulent and turbulent inflows at the blade root and a consistent ∞-distribution at the tower section for different inflows. An apparent lifetime reduction is observed at the blade root and tower base after the introduction of ambient turbulence, wake meandering, and wake-added turbulence. This lifetime reduction is more critical at the tower bases of downstream wind turbines, where the fatigue lifetime can be shorter than the designed service lifetime (20 years) owing to the wake effect. Considering a Rayleigh distribution with an average wind speed of 8.5 m/s, the fatigue lifetimes of turbines located 4D and 8D downstream of another turbine are approximately 45.86 % and 13.12 %, respectively, lower than the fatigue lifetime of the upstream turbine, whereas the reductions are 38.80 % and 5.03 %, respectively, for an average wind speed of 10.0 m/s. Given that the wake effect on the fatigue lifetime has already been highlighted in recent design standards, the findings of the present study are expected to provide guidance for fatigue evaluation in practice, which will help improve fatigue estimations, extend turbine lifetimes, and reduce the construction costs.

Understanding the Effect of Three-Dimensional Design in Tandem Blade

Abstract In order to maximize the pressure ratio and efficiency, compressor designers have tried several unconventional design approaches. Tandem blading is one such unconventional design that promises a higher pressure ratio per stage through a higher diffusion factor. The nozzle shape created between the forward and aft blades of a tandem configuration acts as a passive boundary layer control mechanism. The boundary layer growth over the aft rotor is therefore effectively controlled with the help of this gap-nozzle flow. The flow complexity is likely to increase in the case of a tandem rotor due to the twin leakage vortices, twin wake regions, and their interaction with the hub and casing boundary layers. Modern compressor blades are often designed with three-dimensional blade techniques such as sweep, lean, dihedral, end bent, etc., to reduce the various losses and achieve optimum performance. However, to the best of the author’s knowledge, the effect of 3D blade designs on the performance of tandem rotors has not been fully explored so far. A comprehensive numerical investigation is undertaken to understand the effect of 3D designs on the performance of tandem blades. Axial sweep and dihedral failed to improve the performance of the tandem rotor. Significant improvement in the stall margin is observed for the forward chordwise-swept and negative lean tandem rotors and is largely attributed to lower tip incidence. The performance penalty of the forward-swept and negatively leaned cases can be reduced by integrating compound or variable lean and sweep into the design.

Experimental and numerical validation of a hybrid method for modelling the wake flow of two in-line wind turbines

To forecast the wake flow and power reduction affecting downstream turbines reliably and accurately, a hybrid wake model CFD(ALM)-IDWM was improved, in which the forecasting of Vθmax is improved from linear to cubic. Then, a partially overlapping computational domain is added behind the far-wake domain of upstream wind turbine to calculate the aerodynamic performance of a downstream wind turbine, and the numerical model of full CFD(ALM) and CFD(ALM)-IDWM for two in-line wind turbines are developed. Subsequently, a wind tunnel model test on two in-line wind turbines was carried out to validate the full CFD(ALM) model firstly. Subsequently, the hybrid model of CFD(ALM)-IDWM is numerically validated by the full CFD(ALM) simulations. The results show that CFD(ALM)-IDWM cannot only predict the wake characteristics such as vortices and wakes in the wake region more accurately, but can also accurately simulate the average values of the downstream turbine thrust and torque. By ignoring certain flow field details including acceleration, turbulent viscosity, and Reynolds stress during the simulation process, the computational time is reduced. Base on the same CFD(ALM), the computation time of CFD(ALM)-IDWM was approximately 60% of that of full CFD(ALM). The longer the computational domain of IDWM, the greater reduction in computational time.

Spatial distribution of void fraction in the liquid slug in vertical Gas-Liquid slug flow

Vertical slug flow is characterized by the intermittent passage of two structures, the Taylor bubble and the liquid slug. The Taylor bubble is an elongated gas bubble with a bullet shape that flows upward and fills most of the radial cross-section, leaving room for an annular downward flow of a liquid film. The liquid slug is composed of the liquid phase carrying small, scattered bubbles and flowing upwards. The liquid slug presents three main regions: the wake, the development and the fully developed one. The liquid slug has a high concentration of dispersed bubbles in the wake region because of the entrainment mechanism. The detachment point (Dt) is a stagnation point with almost no dispersed bubbles around it, occurring close to the wall in the wake region end. Experimental data for the gas fraction in three different pipe diameters (26, 40.8 and 50 mm) were gathered in the 14-meter-high vertical flow experimental facility of the NUEM/UTFPR MULTILAB. The gas fraction along the liquid slug was evaluated by using a capacitive wire-mesh sensor (WMS) for quantitative results and a high-speed camera was used to analyze qualitative results. A new correlation to predict the average volumetric gas fraction along the unit cell is proposed. Radial and axial gas fraction distribution were analyzed and the detachment point was identified. The Reynolds and Froude number influence on the Dt point was evaluated and a new correlation to predict the Dt location (LDt) is proposed.

Effects of integral length scale variations on the stall characteristics of a wing at high free-stream turbulence conditions

The effect of variations in the integral length scale of incoming free-stream turbulence on a NACA0012 wing is investigated with the use of force, moment and particle image velocimetry measurements. At a chord-based Reynolds number ( $Re = U_\infty c/\nu$ where c is the chord length, $U_\infty$ is the free-stream velocity and $\nu$ is the kinematic viscosity) of $2\times 10^5$ , an active grid generates turbulence intensities of 15 % at normalised integral length scales ranging from 0.5 $c$ to 1 $c$ . The introduction of turbulence improves the time-averaged performance characteristics of the wing by delaying stall and increasing the peak lift coefficient. It is found that for half-chord integral length scales, the magnitude of the fluctuations in forces and moments is larger than that of full-chord integral length scales, as the former amplifies the naturally occurring unsteadiness in the flow (when there is no free-stream turbulence). The increase in magnitude is ascribed to a larger density of smaller-scale vortices within the separated flow and wake region of the wing.

DRAG REDUCTION OF A SIMPLIFIED TRUCK MODEL USING CAB ROOF FAIRINGS

Adverse effects of climate change have prompt transition towards a low-carbon transportation sector. Electrification of the transportation sector is one of the ongoing efforts to reduce the Greenhouse Gas emissions. However, electrification of long haul and heavy-duty vehicles remain a huge challenge due to cost and limitation in the current battery technology. Improving the aerodynamic design of current truck-trailer is an alternative to electrification. This initiative involves modifying the current truck-trailer body design or attaching accessories for aerodynamic drag reduction that can lead to improved fuel economy. In this paper, the effectiveness of four Cab Roof Fairing (CRF) designs on the aerodynamic drag reduction of a simplified truck model is investigated. The results of a numerical simulation performed on a two-dimensional model show that the CRF can reduce aerodynamic drag by up to 45%. The CRF is found to reduce the vertical velocity component of the flow at fore part of the truck body. This leads to a relatively smaller wake region when compared against the baseline case. Wind tunnel results is performed to verify the results of the numerical simulation. At Reynolds number , the measured coefficient of drag reduction is about 6% when compared with the baseline case.

Entropy analysis and mixed convection of nanofluid flow in a pillow plate heat exchanger in the presence of porous medium

A pillow plate heat exchanger (PPHE) is one of the types of heat exchangers that haven't received the attention they deserve despite their high efficiency and operational capability. A unique feature of PPHEs is their pillow-shaped structure, which is achieved through hydroforming. In light of this, scholars have been interested in analyzing and optimizing the thermo-hydraulic properties of PPHEs. In this study, the interior of the PPHE was occupied with a permeable substance with a high porosity percentage (0.9034 ≤ ɛ ≤ 0.9586 and 0.00015 ≤ dp ≤ 0.00065) and saturated with Ag-water (0 ≤ϕ ≤ 0.06) nanofluid, which has a profound influence on the heat transfer rate of PPHEs. In order to analyze the determinants affecting the heat transfer rate of PPHEs under these conditions, the governing equations were solved using the finite volume method and also the Brinkman-Forchheimer-extended Darcy equation. According to the results, heat transfer is enhanced in PPHE when using a permeable medium with high porosity and a small pore size. That is, PPHEs transfer heat more efficiently when they are placed in a denser porous medium because heat conduction is boosted. Moreover, due to the increased thermal conductivity of the nanoparticles, the application of nanoparticles to the base fluid also enhanced heat transfer and Nusselt number. However, decreases in friction factor and entropy generation were observed with increasing porosity, pore size, and Darcy number, due to reduced flow resistance. A decrease in Richardson number also results in a decrease in friction factor and entropy generation. At the wake region of welding spots, the velocity has reached its lowest values; consequently, this led to a reduction in the heat transfer rate. Nevertheless, within dense porous media, at the lower hydraulic diameter points, the conduction contributes to heat transfer improvement. The porous medium acts as a heat sink and absorbs the heat from the welding spot. This allows the heat to be dissipated away from the welding spot, which reduces the velocity and heat transfer rate. The nanofluid, on the other hand, helps to increase the thermal conductivity of the medium, resulting in more heat being transferred to the surrounding material. Consequently, the results demonstrated that PPHEs' thermal and thermodynamic performance could be significantly improved by using a porous medium saturated with nanofluid.

Investigation of the wake region behind a hemispherical turret via laser Rayleigh scattering.

Planar and volumetric density measurements in the wake region behind a mounted hemispherical turret are obtained using laser Rayleigh scattering. The measurements are conducted in a Mach 2 wind tunnel facility at the Kirtland Air Force Base. Quantitative measurements of density and contour plots with lines of constant density are computed, thus enabling visualization of the turret wake's fluid dynamics. A new, to the best of our knowledge, laser diagnostic methodology and configuration for capturing laser images is also presented. This methodology enables further suppression of background light scattering. Multi-dimensional single-shot and time-average measurements are recorded at multiple axial locations behind the turret. The images acquired reveal turbulent regions of the wake flow, and a discussion of the observed phenomena is presented.

Numerical investigation of hydraulic jumps with USBR and wedge-shaped baffle block basins for lower tailwater

Abstract The stilling basin of the Taunsa barrage is a modified form of the United States Bureau of Reclamation (USBR) Type-III basin, which consists of baffle and friction blocks. Studies revealed uprooting of baffle blocks due to their vertical face. Additionally, the literature highlighted issues of rectangular face baffle blocks: less drag, smaller wake area, and flow reattachment. In contrast, the use of wedge-shaped baffle blocks (WSBBs) is limited downstream of open-channel flows. Therefore, this study developed numerical models to investigate the effects of USBR and WSBB basins on the hydraulic jump (HJ) downstream of the Taunsa barrage under lower tailwater conditions. Surface profiles in WSBB and modified USBR basins showed agreement with previous studies, for which the coefficient of determination (R2) reached 0.980 and 0.970, respectively. The HJ efficiencies reached 57.9 and 58.6% in WSBB and modified USBR basins, respectively. The results of sequent depths, roller length, and velocity profiles in the WSBB basin were found more promising than the modified USBR basin, which further confirmed the suitability of the WSBB basin for barrages. Furthermore, WSBB improved flow behaviors in the basin, which showed no fluid reattachment on the sides of WSBB, increased wake regions, and decreased turbulent kinetic energies.

Research on the dynamic characteristics of wind turbine gearboxes under the spatiotemporal inhomogeneous in the wake

In response to the problem of increasing fatigue load and frequent faults caused by the wake effect, the dynamic characteristics and fatigue life analysis of wind turbine gearbox under the Spatial-temporal non-uniformity of wake region were carried out in this paper. A rigid-flexible coupling model of gears transmission system for a 2 MW wind turbine was established. Based on the three-dimensional Station-temporal inhomogeneous wake model proposed in our previous work, the wind speed distribution of six typical locations in the wake region was obtained. Results show that, the spatiotemporal inhomogeneous effects of the wake expansion have great impact on the dynamic characteristics of the gearboxes for the downwind turbines located in different positions. Vibration energy representing the magnitude of impact energy during the operation of the gearbox is significantly higher under wake conditions than under upstream conditions; meanwhile, the gearbox vibration signal contains more impact component in the wake. The velocity deficit and increased turbulence intensity results in less intense but more complex vibration non-smoothness in the turbine transmission system. As the downwind distance downwind increases, the vibration frequency distribution of the transmission system at each moment is larger, but the vibration energy is reduced. Under the 3D spatiotemporal inhomogeneous effects of the wake expansion, the downstream turbine with staggered arrangement is less affected by wake and has a smoother vibration than the tandem arrangement. At the same downstream distance, the gear life is better in a tandem arrangement than in a staggered arrangement (a maximum value happens at the downstream distance of 6D). Research in this paper can provide references for the study of the dynamic characteristics of large wind turbine transmission systems under the wake effects as well as for the micro-siting of wind farms.

Machine Learning Approach for Flow Fields Over a Circular Cylinder Based on Particle Image Velocimetry Measurements

Flow around a circular cylinder has been experimentally studied at Reynolds numbers of Re = 4 × 103 and Re = 8 × 103 using Particle Image Velocimetry (PIV). The Artificial Neural Network (ANN) method, a subset of machine learning, has been implemented to capture complex flow patterns and relationships within data to estimate the experimental results. Experimental and estimated results regarding instantaneous contour graphics for streamwise (u) and cross-stream (v) velocity components with velocity vector fields have been compared. Furthermore, drag coefficient (CD) and Strouhal number (St) have been validated. In flow features, instantaneous streamwise velocity components indicated a lower pressure zone in the wake region of the cylinder owing to flow separation. The ANN method has predicted the positions of the minimum velocity clusters with 3.2 % for Re = 8 × 103 as a mean relative error compared to the experimental results. Moreover, the average relative difference of 10.82 % between the experimental and the estimated results for the clusters with the minimum streamwise velocity components has been attained at Re = 4 × 103 and Re = 8 × 103. Regarding the cross-stream velocity components, the mean relative difference values have been observed for the clusters with maximum values of 6.83 % and the clusters with minimum values of 3.66 %. Drag coefficients have also been obtained, and these values are very close as CD = 1.090 and CD = 1.084 for PIV and ANN methods at Re = 8 × 103, respectively. Furthermore, St values of the circular cylinder are St = 0.216 at Re = 4 × 103 and St = 0.211 at Re = 8 × 103, which agree with the results of the literature.

Reynolds Number Effects on Turbulent Wakes Generated by Rectangular Cylinders With Streamwise Aspect Ratios Between 1 and 4

Abstract The effects of streamwise aspect ratio and Reynolds number on the separated shear layer and near wake of rectangular cylinders in uniform flow are investigated experimentally using a particle image velocimetry system. Four length-to-height ratios (AR = 1, 2, 3, and 4) were examined at Reynolds numbers (based on freestream velocity and cylinder height) of 3000, 7200, 14,700, and 21,000. The results show that the separated shear layer is either shed directly into the wake region (AR1 and AR2) or reattaches onto the cylinder (AR4), regardless of the Reynolds number. Meanwhile, a transitional regime occurs for AR3 where mean flow reattachment on the cylinder is highly dependent on the Reynolds number. The peak magnitudes of the Reynolds stresses, turbulent kinetic energy, turbulence production, and its transport are highest for AR1 owing to stronger vortex shedding. Aspect ratio and Reynolds number also have significant effects on shear layer transitioning from laminar to turbulence but the transition lengths, when normalized by the corresponding value at Re = 3000, follow a universal power decay law. The wake characteristics, including the recirculation length and wake formation lengths, are independent of Reynolds number for AR1 but decrease with Reynolds number for the longer cylinders, while AR2 shows the largest values. The probability density functions and joint probability density functions are used to examine the effects of Reynolds number on the fluctuating velocities and momentum transport in the shear layer and near-wake region.

Computational fluid dynamics analysis of pollutant dispersion around a high-rise building: Impact of surrounding buildings

Understanding the detailed mechanism of pollutant dispersion in urban areas is a critical aspect of air quality control in residential areas. In this study, wind tunnel measurements and high-resolution computational fluid dynamic (CFD) based on large eddy simulation (LES) were used for the detailed analysis of the pollutant dispersion around a high-rise building for an isolated case and also a case when the central high-rise building was surrounded by several low-rise buildings. The pollutant was released on the leeward façade of the high-rise building at the pedestrian height. It was shown that the presence of the surrounding buildings significantly alters the pollutant transfer mechanism around the high-rise building. In the surrounded case, the pollutant in the wake region shifted vertically toward the roof of the high-rise building. As a result, at a location of 2/3 of the high-rise building height, the mean concentration is five times larger than the isolated case, which means the residents on higher floors experience more pollution concentration in the case of the surrounded case even though they are located much higher than the canyon rooftop and the pollutant source. In both cases, the contribution of the turbulent diffusive flux in the streamwise direction is comparable with the mean convective flux while they have opposite directions. Detailed analyses of the pollutant mass transfer rate budget around different control volumes around the high-rise buildings highlighted the effect of the surrounding buildings. It is concluded that there is a risk of underestimating pollutant concentrations around a high-rise building, especially near the upper floors if the surrounding buildings are not properly considered.

Numerical simulation of flow characteristics of falling-film evaporation of R32/R134a non-azeotropic refrigerant outside a horizontal tube

Horizontal-tube falling-film evaporator is regarded as an effective alternative to full-liquid evaporator because of its higher heat transfer coefficient and lower refrigerant charge. In the process of falling-film evaporation, the thin liquid film thickness (δ) is an important parameter for characterizing the hydrodynamics, and accurate prediction of δ can effectively prevent the reduction in heat transfer efficiency caused by local wall dryout. In this study, non-azeotropic refrigerant R32/R134a was taken as the research object. By incorporating the multi-component phase change model and contact angle model into the governing equations, a two-dimensional numerical model of falling-film evaporation outside a horizontal circular tube was established. The effects of the spray height (H), tube diameter (d), Reynolds number (Re), inlet temperature (Tinlet), and mass fraction of R32 in the liquid-phase mixture (MR32_liquid) on δ were analyzed under sheet flow. The results show that an increase in Tinlet, H, and d is favorable for increasing the average δ (δave), whereas an increase in MR32_liquid and Re leads to a reduction in δave. Besides, the decreasing rate of the δave decreases gradually as H and d increase. When H is 4 mm, the local δ (δlocal) first decreases and then increases as the circumferential angle (Φ) increases. When H ranges from 6 mm to 13 mm, as Φ increases, δlocal first increases slightly, then decreases, and finally increases significantly near the wake region. The δlocal of a larger tube diameter is thicker than that of a smaller tube diameter near the impact region, whereas in the subsequent regions, the δlocal of a larger tube diameter is thinner than that of a smaller tube diameter. The minimum δlocal occurs at Φ between 130° and 140°, and with an increase in H and d, Φ corresponding to the minimum δlocal decreases.

Effect of upstream flow characteristics on the wake topology of a square-back truck

The influence of upstream flow characteristics on the bi-stable flow structure in the wake region of a simplified square-back heavy vehicle model at a Reynolds number of 2.7 × 104 was investigated by using the improved delayed detached eddy simulation method. The asymmetric wake structure of this model and its corresponding aerodynamic response were examined, aiming to identify the effect mechanism of three inlet profiles on the asymmetric wake structure of the named ground transportation system (GTS) model in simulations. The accuracy of the numerical method used in this study was validated by comparison with wake structure data, including the flow states, vortex core's location, and aerodynamic drag obtained from previous large eddy simulations and water channel experiments. The numerical results show that different turbulent inlet velocity profiles lead to different wake topologies. When the turbulent velocity profile with a turbulence intensity of 15% generated by TurbSim, a stochastic inflow turbulence tool for generating turbulent velocity inlet on an atmospheric boundary layer profile, is used, the expected bi-stable flow topology is still observed, but it is not shown in the case by means of the turbulence generator incorporated into ANSYS Fluent. Those turbulent inlet velocity profiles contribute to the increase in GTS model's aerodynamic drag forces. Compared to the uniform velocity profile, the TurbSim velocity profile can achieve a drag increase in 7.23%. In addition, this turbulent profile intensifies the flow fluctuations in the wake region and enhances the transient response frequency of the wake region. Thus, when assessing the vehicle aerodynamic performance in open air, especially under crosswinds, the real turbulence velocity profile, e.g., the profile generated by TurbSim in the current study, is recommended to be used for a more accurate prediction in numerical simulations.

STUDY OF AERODYNAMIC WAKE ANALYSIS OF SUV CARS FOR IMPROVING FUEL CONSUMPTION

SUV cars are known to be some of the most fuel-inefficient vehicles on the road. This is due in part to their large size and shape, which create a lot of aerodynamic drag. The wake region behind an SUV car is particularly turbulent, which further increases drag. The aim of this study of aerodynamic wake analysis of SUV cars for improving fuel consumption is to identify and quantify the factors that contribute to aerodynamic drag in SUV cars, and to develop strategies for reducing drag and improving fuel economy. Identify the factors that contribute to aerodynamic drag in SUV cars. This will involve understanding the flow dynamics of the wake region behind SUV cars and identifying the key features of the wake that contribute to drag. Develop strategies for reducing the size and strength of the wake. This will involve developing methods for modifying the shape of the SUV car or the flow around it in order to reduce drag. Assess the impact of wake reduction on fuel economy. This will involve conducting wind tunnel tests or CFD simulations to determine how changes in the wake affect the overall drag of the SUV car. This study researches the streamlined wake of SUV vehicles to distinguish amazing open doors for diminishing drag. Wind tunnel tests and CFD analysis are utilized to examine the stream designs around and behind an assortment of SUV models. The outcomes show that the streamlined wake of SUV vehicles is described by two enormous vortices. These incorporate diminishing the detachment point at the back of the vehicle, streamlining the backside of the vehicle, and utilizing streamlined gadgets like spoilers and diffusers. In this comparative study we designed and developed a prototype of a conventional SUV body with square back to analyse the actual drag and wake behind the vehicle using CFD analysis and Wind Tunnel test. After performing all the tests for conventional SUV, results are documented to compare the results determined with modified body. KEYWORDS: SUV; CFD; CAD Model; Drag; Base Bleed;

Impact of Lateral Gap on Flow Distribution, Backwater Rise, and Turbulence Generated by a Logjam

AbstractLogjams may form at natural obstructions and are also used as nature‐based solutions for river restoration and natural flood management. Previous research has described backwater rise due to logjams that span the full channel cross‐section and logjams with a gap between the lower edge of the logjam and the bed. Logjams that fill the channel depth, but not its width, leaving a lateral gap between the logjam and the channel bank, are also common natural formations and the focus of this study. The flow distribution between the logjam and the lateral gap, backwater rise, and wake turbulence are key factors in determining the ecologic and flood risk impact of a logjam. Specifically, relative to a channel‐spanning logjam, the introduction of a lateral gap can reduce backwater rise and increase the potential for trapping particles, such as nutrients or microplastics, within the wake region, but may also promote erosion in the gap. The choice of logjam and gap widths can be used to maximize flow and habitat diversity in rivers, while reducing erosion risk. We present experimental results demonstrating that the flow distribution between the logjam and the lateral gap can be predicted by assuming equal resistance through the logjam and gap sections. Further, we show that backwater rise can be determined from the predicted discharge through the logjam using a momentum balance developed for channel‐spanning logjams. Finally, turbulence generated within the jam was observed directly downstream of the logjam, and, for the densities considered, increased with jam density.

Numerical study on three-dimensional flow around a cylinder with perforated shrouds at different Reynolds numbers

The perforated shrouds have been proposed to control cylinder flows, while the effects and mechanisms at different Reynolds numbers (Res) remain unclear. Three-dimensional numerical simulations are conducted in this paper to compare the aerodynamic performance of the flow around a smooth cylinder and a shrouded cylinder at Re of 3900 and 1.4×105. The results indicate that the drag of the perforated shrouded cylinder is reduced by 30.8% at the high Re, while increased by 26% at Re of 3900 compared with the smooth cylinder. Differently, the lift oscillations of the cylinder are greatly weakened by 83.3% at the Re of 3900 and 98.5% at the Re of 1.4×105, which implies the wake oscillations are nearly eliminated at the Re of 1.4×105. Further analysis exhibits the near wake region is elongated along the mainstream, with significantly recovered pressure. Especially, the greater pressure loss owing to outer shrouds even leads to the negative drag of the inner cylinder at the high Re. In addition, the incoming flow is broken up by outer shrouds, and different flow patterns appear in the gap. The discernible vortex pairs occur in the gap at the Re of 3900, while as Re increases to 1.4×105, the quantities of small-scale vortex weaken the impact on the inner cylinder. The shear layer characteristics are elucidated by Lamb vector curl and Kelvin–Helmholtz instabilities. The vorticity stretching and tilting in the shear layer of the shrouded cylinder is much weaker at the high Re. Generally, the energy for shear layer instabilities at low frequencies is diminished with the presence of perforated shrouds. However, the energy at higher frequencies is strengthened at the low Re.

Wake interaction of aligned wind turbines over two-dimensional hills

An experimental investigation was carried out to explore the interaction and wake statistics of model wind turbines operating individually and in pairs over two-dimensional hills with varying heights. The hills shared a sinusoidal shape and extended L/D=20 in the streamwise direction, where D represents the diameter of the turbine rotor. The peak heights of the hills were H/D=0, 0.5, 1, and 1.5. The first turbine was located at the beginning of the hill development, and the second turbine was positioned halfway between the first and the hill's peak, downwind. The flow in the intermediate wake regions was characterized using particle image velocimetry, focusing on the recovery mechanisms of streamwise momentum on the windward side of the hills, ranging from gentle to steep-up slopes. The results indicate that the advection terms play a more significant role than turbulence in the wake recovery mechanism with steeper hill slopes. Associated reduced turbulence levels are attributed to flow acceleration, which led to a higher power availability at the top of the hills.

Wake and power prediction of horizontal-axis wind farm under yaw-controlled conditions with machine learning

The main objective of this study is to employ the Extreme Gradient Boosting (XGBoost) machine learning algorithm to predict the power, wake, and turbulent characteristics of horizontal-axis wind farms under yaw-controlled conditions. For this purpose, a series of high-fidelity numerical simulations using LES method are performed over tandem NREL-5 MW wind turbines to generate the input data for training and testing in machine learning analysis. It is observed that XGBoost is more accurate for wake prediction of the yaw-controlled wind farms compared to ANN, which was used in previous studies. The results illustrate that XGBoost can predict the power with a mean deviation of 0.94 % for different yaw angles, while ANN can estimate the power generation with a mean deviation of 2.15 % for various tested yaw angles. At far wake regions (X > 2000 m) of the second wind turbine, the deviations reach below 1 %. Moreover, XGBoost requires a much shorter training time, 87.5 % faster than ANN. The power production of both wind turbines can be predicted more accurately with XGBoost compared to ANN. The wake prediction time of XGBoost is just 0.105 sec, while this time is 4.480 for the ANN model. In conclusion, XGBoost provides a significant reduction in error and training time compared to ANN and deep learning algorithms over yaw-misaligned wind farms.