Session W05: Aerodynamics: Wind Energy (10:00am - 10:45am CST)

Pseudo-2D RANS model for computationally-efficient simulations of wind farm flows

Performance nowcasting, monitoring and layout optimization of wind farms can be greatly improved by embedding an accurate numerical tool simulating the wind flow and estimating power capture and wake losses. Such tool needs to be computationally efficient to run in real time or operate within an optimization loop. We propose an adaptation of the shallow-water model for the simulation of the 3D flow in a wind farm over a 2D domain. This approach, originally conceived for the investigation of tides and oceanic flow, permits to drop the vertical momentum balance from the set of the Navier-Stokes equations by performing a depth-average. The main hypothesis is that vertical velocities and gradients are negligible compares to the horizontal components. To adapt the shallow-water model to the wind farm case, it is necessary to introduce appropriate corrections near the turbines where vertical velocity magnitude and variability are significant. The corrections leverage the axial symmetry of the near wake to estimate vertical fluxes and dispersive stresses. The tool is validated versus high-fidelity LES simulations.   

Placing Betz on the Improvement of Wind-Turbine Efficiencies through Unsteady Streamwise Motion

The Betz limit represents the theoretical maximum efficiency for power conversion of a fluid-driven energy-harvesting device in steady flow. Dabiri (Phys. Rev. Fluids, 2020) has suggested that this constraint can be circumvented in the case of unsteady flow through the influence of time-varying velocity potentials. We extend this analysis by incorporating analytical models for these velocity potentials, which allow us to directly consider the effects of periodic streamwise motions of an actuator disc on its theoretical efficiency. Various motion profiles for the actuator disc are considered, informed by physical constraints derived from the analytical framework. Cases in which the theory predicts increases in efficiency above the Betz limit are then identified and scrutinized. The results of these theoretical analyses will inform the design of experiments that will evaluate the feasibility of using streamwise motion to improve the efficiency of energy-harvesting devices.   

New insights into the spectral behavior of the power fluctuations of horizontal-axis turbines

The spectral behavior of the turbulence-driven power fluctuations of a single horizontal-axis turbine is investigated both experimentally and through a novel semi-analytical model. The study confirms that the power spectra follow a -11/3 power law over the inertial sub-range (or part of it) as well as that significant spectral energy content may exist around the blade-passing frequency and its higher harmonics. The shape and magnitude of the power spectra are shown to strongly depend on the blade aerodynamics (e.g. lift curve slope), the angular speed of the rotor as well as the integral length scale and distortion level of the approaching turbulence. To gain these new insights, we have derived a novel semi-analytical model which combines the turbulence distortion and blade-element momentum theories, as well as it uses the rotationally sampled spectra technique, to calculate the power spectra. The model is validated using detailed experimental data obtained in the long water flume situated in the laboratory facilities of IFREMER in Boulogne-sur-Mer, France, where a fully instrumented horizontal-axis turbine was deployed, and synchronous measurements of the upstream velocity and the rotor quantities (thrust, torque etc.) were collected for different tip-speed ratios   

On the wake similarity of a utility-scale wind turbine at different yaw angles

This work is dedicated to systematically studying the influence of yaw angle on the wake characteristics of a 2.5 MW wind turbine under fully developed turbulent inflows. The turbine wake is simulated using large-eddy simulation with actuator surface models for turbine blades and nacelle. Four different yaw angles, i.e. $\gamma = 0^o, 10^o, 20^o, 30^o$, are considered. Two sets of scaling factors, which are functions of thrust coefficient and yaw angle, are derived using the one-dimensional momentum theory. The time-averaged velocity, wake width, and turbine-added turbulence kinetic energy from different yaw angles are found being collapsed with each other when normalized using the proposed scaling factors. The wake meandering characteristics are also examined. It is found that the probability distribution functions (PDF) of instantaneous wake center from different yaw angles are symmetric to the wake centerline, and the normalized wake meandering amplitudes are independent of the yaw angle. These findings suggest the feasibility of modeling wake dynamics at different yaw angles based on the wake data at one yaw angle.   

Evaluation of actuator disk model on predicting turbine wakes for different inflows

Advanced turbine models have been developed in the literature, such as the actuator surface models for turbine blades and nacelle. However, the actuator disk (AD) model is still preferred for simulating large utility-scale wind farms because of its less requirements on grid resolution. In this work, we evaluate the capability of the AD model in predicting the velocity deficits, turbulence kinetic energy and the DMD (dynamic mode decomposition) modes by comparing the simulation results from the AD model with those from the actuator surface model for uniform inflow and fully developed turbulent inflow. For the uniform inflow cases, the predictions from the AD model are significantly different from those from the AS model in terms of time-averaged velocity, turbulence kinetic energy, dominant DMD frequencies and DMD modes. For the turbulent inflow cases, on the other hand, the differences in the time-averaged quantities predicted by the AS and AD models are not significant especially at far wake locations. As for the DMD modes, despite differences in dominant DMD frequencies, similarities on the spatial patterns of the dominant DMD modes are observed.   

Dynamic overset large eddy simulations of NREL PHASE VI wind turbine

Large eddy simulations of the National Renewable Energy Laboratory (NREL) phase VI wind turbine are performed using a non-inertial frame of reference dynamic overset grid method. The flow over the rotating parts, including blades and hub is modeled in a non-inertial frame on rotating grids, and flow over the tower and nacelle is solved in an inertial one on fixed grids. The simulations are carried out for a pitch angle of 3 degrees, and wind speeds of 7 m/s and 15m/s. For both wind speeds, the total power, force coefficient, and local pressure on the blade are in good agreement with the experimental results. Moreover, the role of a dynamic sinusoidal pitch with amplitude of 3 degrees and different frequencies ranging from 2.4/s to 7.2/s is investigated on the performance and wake of the turbine. The results of simulations indicate that a dynamic sinusoidal pitch cannot enhance the aerodynamic performance of the turbine but it can modify the wake of the turbine. This work was partly supported by the National Science Foundation (NSF) CAREER Grant CBET 1453982, and the High Performance and Research Center (HPRC) of Texas A{\&}M University.   

Wake and Power Characteristics of a Fixed Bottom Offshore Wind Turbine

Wind turbine energy production and what specific conditions drive the efficiency, life cycle, and placement of the turbines is becoming more detailed. A scaled wind turbine model was fixed to a large water tank with a wave generator upstream and a damping beach downstream. A standard 2D PIV set-up was used to capture the wave profile of the turbine at different conditions. Three image planes were used to capture the full development of the wake from near the turbine to far downstream. Wake momentum, Reynolds stresses, and power production using standard PIV and power measurement averaging are performed with the intent of addressing how the power production and wake of a fixed bottom wind turbine behave based on the interaction between specific wave conditions and the wake.   

Control of flow past a vertical axis wind turbine at low tip speed ratio by a bio-inspired device

We investigate the flow characteristics of a vertical axis wind turbine (VAWT) and control the flow with a bio-inspired device using large eddy simulation. The device is a passive control device inspired by the secondary feather of a bird, called automatic moving deflector (AMD) by Kim et al. (2016, Bio {\&} Bio). The performance of a VAWT is improved in the range of tip speed ratio below 1.2 when AMD is applied to the inner surface of each blade. The AMD automatically pops up when the leading edge separation occurs in the upwind region. As a result, the relative location of the leading edge vortex with respect to the blade is changed. The effect of its position is considered at the tip speed ratio of 0.8. When AMD is located near the leading edge of the blade, it alters the path of the leading edge vortex towards the front part of the blade. The low pressure region due to this vortex enhances the tangential force in the direction of rotation, thus increasing the torque and power coefficient. On the other hand, the power loss increases due to AMD after the leading edge vortex detaches from the blade. Overall, the power coefficient increases with AMD.   

Turbulent Decay of the Yawed Wind Turbine Counter-Rotating Vortex Pair

Yawed wind turbines generate a counter-rotating vortex pair (CVP) that deflects and deforms the turbine's wake downstream. Informed by large eddy simulations (LES) of yawed wind turbines in the atmospheric boundary layer (ABL) and the airplane trailing vortex literature, we develop a model for the shed vorticity and circulation of the CVP. Analytical integration of a simplified form of the vorticity transport equation yields analytical equations that do not require costly numerical integration. We apply an eddy-viscosity model with the ABL friction velocity and width of the vortex sheets representing the velocity and length scales, respectively. Comparisons of the analytical model to LES measurements of the maximum vorticity and circulation magnitude show considerable agreement. These results indicate that cross-diffusion dominates the CVP decay as the vorticity cancels along the wake's line of symmetry.   

Wind Farm Aerodynamic Simulation Using Prescribed and Free-Wake Vortex Methods

We present an analysis of the Graphically Accelerated Vortex Lattice Library (GAVLL) to simulate the aerodynamics of a wind farm. Vortex based methods have been used extensively to simulate the aerodynamics of single turbines. GAVLL extends these methods to allow for the simultaneous aerodynamic simulation of multiple turbines within a single computational domain. This extension allows users to predict the aerodynamic interactions between independent wind turbine rotors which is critical for realistic wind-farm and multi-rotor wind turbine simulations. This work presents a detailed methodology of the approach using both free and prescribed wake models followed by a preliminary validation study. The validation study includes the power output predictions of GAVLL for a single turbine compared with other computational methods as well as experimental data. Building on that, the methodology is scaled to the farm level to predict the total power production of a utility scale wind farm in comparison with field data from a real wind farm. -/a   

Dependence of wind farm performances on the terrain topography

Atmospheric turbulence affects wind turbine performances in terms of both power production and blade loads. One of the main sources of turbulence for onshore wind farms is the presence of terrain topography. The purpose of this study is to correlate the characteristics of the terrain topography, represented as sinusoidal roughness, with wind farm performances. To reproduce the effect of different terrain topographies, a set of precursory large eddy simulations has been performed with a wavy terrain of varying streamwise and spanwise wavelengths. The terrain is modeled with the immersed boundary method. A cross-section from the precursor simulations was used as inflow condition upstream of a wind farm made by an array of $4 \times 4$ turbines. The rotating actuator disk has been used to reproduce the turbines. Turbulence properties, integral scales and coherent structures have been analyzed before and after impinging on the wind farm. The wake recovery and the overall wind farm power production performances have been then correlated with the turbulence and with the upstream characteristics of the terrain.   

Wind turbine wakes under high thrust coefficients

Wind turbines wakes are typically characterized in terms of the thrust coefficient. The thrust coefficient is a non-dimensional number that describes the force exerted by the turbine in the axial direction to the incoming momentum of the flow. In the case of a wind turbine with a high-thrust coefficient, the wake becomes turbulent very close to the rotor and the recovery is enhanced. In this work, we study the behavior of wind turbine wakes under high thrust coefficients using large-eddy simulations and propose a simple model to predict the wake deficit of a wind turbine with a high-thrust coefficient.   

Pulsed Coand\u{a} Effect for Wind Energy Generation.

The pulsed Coand\u{a} effect was used alternately on the sides of a spring-stabilized circular cylinder to extract energy from the wind in the form of reciprocating motion. This radically new wind-energy generation concept is ideally suited to high-pressure positive displacement pumping used for reverse-osmosis desalination, because it eliminates electrical generator and motor inefficiencies. Flow visualization experiments were performed to identify the time-scales associated with dynamic flow attachment and separation. Subsequently, a proof-of-concept scale-model technology demonstrator was constructed, which consisted of a vertically-mounted, spring-stabilized Coand\u{a} circular cylinder that was equipped with two span-wise blowing slots. It was connected, via a sting, to a one-degree-of-freedom pivot at its lower end and loaded with a positive displacement pump. Measurements included the static loads generated by the Coand\u{a} effect and system power performance evaluations. The former were used, together with a dynamical system model, for performance predictions. Direct system performance measurements demonstrated a positive net power output, while model predictions indicated efficiencies of approximately 20{\%}. System efficiency can be greatly improved by using two blowing slots on either side of the cylinder or by allowing two-degree-of-freedom oscillations.   

Dynamic Stall for Wind Energy Generation

Although insects fly by exploiting the phenomenon of dynamic stall, designers consider it to be both inefficient and potentially damaging. In a break with convention, we show that for a large chord-radius ratio (c/R) vertical axis wind turbine, dynamic stall can be exploited for efficient wind energy generation. Physically, the lift overshoot effects resulting from the dynamic stall vortex (DSV) generate positive torque when the blade dynamic pressure is high while shedding of the DSV occurs when the blade dynamic pressure is low. Blades also experience varying virtual camber and chordwise favorable pressure gradients. An analytical model was developed based on a momentum balance that included the effects of dynamic pitching and virtual cambering together with an empirical data set. Complimentary wind tunnel experiments were performed on a small-scale (0.4m$^{\mathrm{2}})$ turbine. Both the analysis and the experiments illustrated the impact of c/R and the chordwise blade connection point on the effects of dynamic stall. Despite the small scales and low Reynolds numbers, power coefficients of up to $16\% $ were measured and flow visualization confirmed that dynamic stall, with the associated DSV, is the mechanism driving the turbine to maximum power.   

Sweeping jets for flow control over propellers and wind turbine blades in a gust and shear environment from a fan array

The performance of propellers and wind turbine blades instrumented with sweeping jets have been tested in spatially and temporally varying relative-wind conditions. Sweeping jets or fluidic oscillators allow Active Flow Control on an airfoil surface. A test bench was designed and built in order to channel air flow through a rotating hub and into three separate blades in a controlled manner. The blades were instrumented with a number of sweeping jets along their span. This research was enabled by the use of a windshaper, a new family of wind-generating facilities, which consists of an array of a large number of fans (wind-pixels) that may be arranged in various patterns and activated on demand. It is in some ways a digital wind facility that can be programmed to generate arbitrary winds of variable intensity and directions, such as uniform flows, gusts, and shear flows. In particular, sweeping jets were triggered independently on each blade depending on local flow conditions at the blade location. Force coefficients, torque coefficients, and efficiencies were evaluated in various flow conditions.   

Spectral and phase average analysis of a model horizonal-axis wind turbine (HAWT) wake at high Reynolds numbers

The interactions between HAWT wake-flows and downstream turbines can affect wind farm power output. However, our ability to model such flows and interactions is limited.~ A reason for the limited understanding is a lack of detailed wake studies at field-similar Reynolds numbers, which can be in excess of 100 million. Experimental data acquired in the wake of a HAWT model (20cm diameter)~obtained within the High Reynolds Number Testing Facility (HRTF) at Princeton University, will be presented. The HRTF is a high-pressure wind tunnel, that can pressurize the working fluid, air, up to 238 bar, while maintaining flow speeds up to 10m/s. Streamwise velocity measurements at Reynolds numbers up to 7.2x10$^{\mathrm{6}}$ at five different downstream locations,~ from 0.77 to 5.52 diameters, were acquired. Flow measurements were obtained using the nanoscale thermal anemometry probe (NSTAP), which offers spatial and temporal resolution well beyond conventional hot-wires capabilities. Wake-flow characterization will be presented through an investigation of streamwise velocity variance profiles, spectral characteristics, and phase averaged mean velocity profiles. From these profiles, discussions on tip vortex evolution and vortex structure population will be made.   

Simulation of Collection Efficiency on Airfoil Profiles Using Different Particle Tracking Methods

In many aerodynamic applications, wind turbines and aircraft for example, airfoil profiles are exposed to airflow carrying water particles. The key parameter that influences the accuracy of simulation of phenomena occurring on the surface of the aerodynamic bodies, ice accretion, for instance, is collection efficiency. This work aims to simulate water particles impinging on airfoil profiles and collection efficiency distribution over the profiles using Lagrangian and Eulerian particle tracking. After simulating water droplets impingement, the accuracy of collection efficiency and computational time are compared to show the advantages and disadvantages of each method. The simulations are done using an in-house code developed within the OpenFOAM framework to simulate background flow and particle tracking. After that, simulation cases, the resulting collection efficiency distribution will be validated with values resulting from wind tunnel experiments available in many published literature.   

Characterizing Performance and Unsteady Flow Dynamics of AeroMINE Energy Harvesting Foils

Commercial wind-energy turbines successfully supply grid-level power production but are not well suited to smaller distributed production because of scaling, building integration and reliability challenges. The AeroMINE system harnesses wind energy by using a rigid two-airfoil assembly that avoids rotating parts and reduces the wake disturbance issues of adjacent devices. The use of AeroMINEs on warehouse-sized structures can provide significant distributed power. Wind tunnel experiments demonstrate increased lift as the angle-of-attack (AoA) of the airfoils increases, corresponding to an increase in power generation. However, if the AoA is too large flow instabilities occur that reduce the harvesting efficiency. We characterize this system using wind tunnel experiments at Reynolds numbers of O(100,000) and high-fidelity simulations at lower Reynolds numbers, O(1,000). We provide performance maps that indicate harvesting potential across the parameter space and describe performance-deteriorating instabilities by correlating dynamics of key flow structures to those of the forces on the airfoil system.