North American Wind Energy Academy 2015 Symposium

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https://hdl.handle.net/10919/52370

The NAWEA 2015 Symposium, which was held June 9-11, 2015 at Virginia Tech in Blacksburg, VA, included technical sessions, panel discussions, a graduate student symposium, a poster session, engineering software workshops, a business meeting, social events, and a tour of the Virginia Tech Stability Tunnel. The Symposium, the second in a series of technical meetings, examined a broad range of topics required to achieve high wind penetration in the North American power-generation sector. In addition to wind energy system science and technology technical tracks, the symposium featured sessions that provided holistic perspectives, overviews, and approaches necessary to maximize future deployment including:

Research and Development, and Technology, including presentations on Aerodynamics, Aeroacoustics, Controls, Innovative Systems and Concepts, Reliability, Offshore, etc.
Electrical Integration, including presentations on issues of achieving high penetration of variable renewables and on energy storage issues
Atmosphere/Turbine/Wake Interactions, including farm/plant interactions and array effects
Atmospheric Science of Wind Characterization, and Forecasting
Public Policy Issues, including presentations, and a panel discussion of issues/problems and how technology can provide solutions, economics of wind
Environmental and Siting Issues, including presentations, and a panel discussion of issues/problems and how technology can provide solutions
Workforce Development and Education, including presentations, and a panel discussion from the Education Committee

- http://www.cpe.vt.edu/nawea/index.html, retrieved 2015-05-13

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An evaluation of power performance for a small wind turbine in turbulent wind regimes
Ward, Nicholas; Stewart, Susan (Virginia Tech, 2015-06)
Large Eddy simulation of Trailing Edge Acoustic Emissions of an Airfoil
Wu, Jinlong; Devenport, William J.; Paterson, Eric G.; Sun, Rui; Xiao, Heng (Virginia Tech, 2015-06)
The present investigation of trailing edge acoustic emission of an airfoil concerns the effects of the broadband noise generated by the interaction of turbulent boundary layer and airfoil trailing edge, and the tonal noise generated by the vortex shedding of trailing edge bluntness. Large eddy simulation (LES) is performed on an NACA0012 airfoil with blunt trailing edge at a Reynolds number Rec = 400; 000 based on the airfoil chord length for three different configurations with different angles of attack. In order to reproduce and compare with the result from experiment in the literature, numerical tripping is tested and chosen to control the boundary layer development to guarantee a similar boundary layer thickness near the airfoil trailing edge. The near wall region inside the boundary layer is directly resolved by LES simulation with Van Driest damping, in order to obtain the instantaneous data in that region. With these instantaneous data from aerodynamic simulation, the acoustic predication is conducted by the Curle's analogy, which is suitable for stationary surface in free ow. To validate the numerical solutions, both ow simulation and acoustic integration results are compared to experimental data and simulation results available in the literature, and good agreement is achieved. The aerodynamic results show that the similar boundary layer development of experimental result can be reproduced by simulation with a suitable choice of numerical tripping, and the similar instantaneous behavior of ow inside the boundary layer is therefore guaranteed, which is vital for the acoustic prediction. The aeroacoustic results show that the acoustic prediction changes with the lift and drag force provided by the airfoil. Basically speaking, it's a result that the unsteady force around the surface is closely related to the mean force provided by an airfoil, which means that the noise control of a given airfoil is coupled with the optimization of its aerodynamic performance. As for the approximation made in the implemetation of Curle's analogy, it is shown in the aeroacoustic results that the airfoil can be treated as a compact point only if low frequency acoustic emission is of interest, and such kind of approximation can cause obvious problem if very high frequency acoustic emission is concerned.
Modifications of the k-kL-ω Transition Model based on Pohlhausen and Falkner-Skan Profiles
Fuerst, Jiri; Islam, Mazhural; Příhoda, J.; Wood, David H. (Virginia Tech, 2015-06)
We will present novel modifications of the three-equation k-kL-ω eddy viscosity model proposed by Walters and Cokljat [1] for the adverse pressure gradient flows that occur on wind turbine blades and airfoils. The original model was based on the k-ω framework with an additional transport equation for laminar kinetic energy which allows the prediction of natural or bypass laminar-turbulent transitions. The model uses only local information and is, therefore, easily implemented in modern CFD codes including Fluent and OpenFOAM. The original model shows very good agreement with experimental data for zero pressure gradient flows (see e.g. [1]) but it delays the transition for adverse pressure gradient flows at low free-stream turbulence levels [2]. Both stability analysis and experiments show that the pressure gradient has a big influence on transition [3].
Comparison of Offshore Wind Turbine Reliability
Christou, Aristos; McCluskey, Patrick; Lu, Yizshou; Delorm, Tatiana (Virginia Tech, 2015-06)
The results of a comparative probabilistic reliability model applied to offshore wind turbine systems is presented. The model calculations are based on surrogate failure rate data from industrial onshore wind turbine technologies, related marine environment technologies and generic databases. Data are adjusted for the offshore marine environment and integrated with functional as well as reliability block diagrams. The developed models are applied to five generic horizontal-axis offshore wind turbine designs. Predicted subsystem failure rates and total system failure rates are reported and critical reliability limiting sub-assemblies are identified.
Application of Fast Pressure-Sensitive Paint to an Oscillating Wind Turbine Airfoil
Disotell, Kevin J.; Nikoueeyan, Pourya; Naughton, Jonathan W.; Gregory, James W. (Virginia Tech, 2015-06)
In the unsteady flow environment experienced by wind turbine blades, large excursions in local angle of attack and significant three-dimensional flows arise that complicate the prediction of stall onset and unsteady loading. Accurate predictions of the dynamic loads are critical to the pursuit of lightweight, reliable structures to lower the cost of wind energy (Ref. 1). To this end, diagnostic measurement tools capable of fine spatial resolution and high frequency response are important for better understanding unsteady aerodynamic effects on blade pressure distribution. The sparseness of conventional pressure transducers has historically provided motivation for the development of pressure-sensitive paint (PSP), an optical surface pressure measurement technique with inherently fine spatial resolution. Within the last decade, the bandwidth of certain paint formulations has been significantly increased to resolve unsteady flows; a flat frequency response on the order of several kHz is now readily achievable (Ref. 2). PSP consists of luminescent molecules adhered to the test surface by a thin binder layer, typically on the order of 10 microns. The luminophore responds to the local partial pressure of oxygen, which is directly proportional to absolute air pressure. An illumination source such as a light-emitting diode or expanded laser beam excites the luminophore, and the emitted light intensity captured by a scientific-grade camera is converted to absolute pressure via calibration. Each point on the painted surface thus responds as a molecular-sized transducer, with spatial resolution limited by the camera pixel size. Recent advancements in unsteady PSP data acquisition and processing techniques have been developed for rotating blades to account for errors caused by model movement and deformation in nonuniform illumination fields. The single-shot lifetime technique (Ref. 3) and motion capturing technique (Ref. 4) are two methods which have arisen. Due to its straightforward procedure and commonality of required hardware, the single-shot technique has been used across a range of small test facilities (Ref. 5, 6) and large-scale wind tunnels (Ref. 7). PSP measurements have been historically limited to the compressible flow regime to achieve sufficient signal-to-noise ratio in the data images (Ref. 8, 9). Recent wind tunnel testing with unsteady PSP has been geared toward helicopter applications (Ref. 10, 11), although temperature error due to compressibility effects has been noted. Under isothermal conditions or with an accurate temperature correction available, laser-based excitation and highly reflective paints can enable instantaneous PSP measurements at Mach numbers near M ≈ 0.15, corresponding to dynamic pressures of approximately 1.5 kPa (Ref. 6, 12). Unsteady aerodynamic effects can result in even larger pressure differences, considering that peak suction levels on oscillating airfoils can reach factors of the free stream dynamic pressure (Ref. 13). With the present availability of suitable paints, the above considerations present an opportunity for PSP to be deployed as a measurement tool for resolving the global pressure distribution on wind turbine blades.
An Analytical Procedure for Evaluating Aerodynamics of Wind Turbines in Yawed Flow
Rajagopalan, R. Ganesh; Guntupalli, Kanchan; Fischels, Mathew V.; Novak, Luke A. (Virginia Tech, 2015-06)
A new analytical method for evaluating the performance of wind turbines in yawed flow is presented. The method, based on momentum theory, relates the yaw angle to the tip-path-plane orientation of the turbine rotor and the non-dimensional deficit velocity. The analytical calculations are compared with results from Rot3DC, a kernel within the RotCFD software package, an integrated environment for rotors. Rot3DC, a 3-D Navier-Stokes based module, can simulate one or more Horizontal Axis Wind Turbines (HAWT) and uses the concept of momentum sources to compute the turbine performance and flowfield in a self contained manner. Rot3DC simulations are validated against NREL Phase-II experiments. Analytical results for yawed turbines correlate well with Rot3DC and are found to be within 10% error margin for the extreme yaws considered. The developed analytical formulation provides a simple model for quantification of turbine performance in yawed flow and can be used as an input to onboard feedback systems for yaw control.
Sustainability of the Wind Turbine Blade Manufacturing Process: A Bio-Based Alternative
Ramirez, Katerin; Turcotte, David (Virginia Tech, 2015-06)
Content : Globally, more than 23,000 wind turbines were manufactured in 2011 and there were 225,000 operational wind turbines by the end of 2012. United States' installed wind capacity will need to increase from 11.6GW to 300 GW to achieve the 20% wind production goal by 2030. To meet the increasing demand, not only more blades are being manufactured, but also longer blades of up to 100 meters long are being produced. The current stock of blades and the manufacturing process use petroleum based thermoset composites as the primary materials. The anticipated influx in disposal and manufacturing leads to health and environmental concern, given that the industry does not manufacture blades in a sustainable manner and has no practical way to recycle these blades. The main objective of this research project is to create an energy pathway for the sustainability of wind energy. In particular, this research is trying to develop a bio-based epoxy system for the wind turbine blade manufacturing process, in which the toughness, stiffness, durability and costs are comparable with petroleum based epoxy systems. The bio-derived thermoset will allow the industry to substitute a fossil fuel for vegetable oil, a more readily available feedstock, in the blade manufacturing process. In addition to the bio-based system, we want to realize the property of reworkability in both conventional and bio-based epoxies. The property of reworkability means that we could be able to thermally reprocess any anhydride-cured epoxy system with the addition of a special catalyst. It should allow for an effective recycling at the end of the blades' life cycle, presumably without the negative economic, health and environmental consequences that current disposal methods for thermoset composite impose. Overall, the expectation is to produce a high performance bio-based reworkable system which should enable manufacturing of blades that would be easily repaired, reworked or ground up for reuse with another purpose. The combination of these complementary technologies should improve sustainability both at the beginning and the end of the life cycle of a typical wind turbine blade. Approach to evaluation To evaluate the environmental, health and economic implications of replacing the petroleum based epoxy with a bio based epoxy, we use two different methodologies. The first one is a Life Cycle Cost Analysis (LCCA) to compare conventional and bio-based manufactured blades. The aim of the LCCA is to compare different alternatives in the manufacturing process and determine which one is the least costly while maintaining the same performance. We expect the bio-based manufacturing process to be very similar in terms of blade performance, but to differ in terms of the cost over the life cycle of the blades. The present value of all recurring costs and residual value of the blade at the moment of being disposed are calculated in order to determine the best long term manufacturing process.
Flare Reduction Technique for Near-Surface Airfoil Boundary Layer Measurements with Laser Diagnostics
Shin, Dongyun; Cadel, Daniel R.; Lowe, K. Todd (Virginia Tech, 2015-06)
Co-location of Wind and Solar Power Plants and Their Integration onto the US Power Grid
Pattison, Chris (Virginia Tech, 2015-06)
A number of research and development groups and several renewable project operators have examined combining wind power production with on-site solar power production. Past research has been devoted to small, off-grid applications only. In the absence of actually building a utility-scale project, short time scale (5 minutes) estimates of combined power production are difficult to simulate due to the lack of hub-height wind data combined with on-site solar insolation data available in similar time scales. This presentation will present hub-height, high-fidelity, wind data from the Texas Tech University's 200-meter meteorological tower combined with a co-located solar pyranometer to estimate short-term (5-minute) power production data. Recent reduced costs associated with solar-PV may make this option more attractive in the future. This analysis addresses fixed-plate, single- and dual-axis PV arrays. This presentation also includes an plant-level and grid-level economic analysis of a wind-only, solar-only, and combined wind-solar power plant. Over the past few years, renewable energy has entered the electrical grid at an exponential rate. To reduce the uncertainties for the grid operator and wind power plants owner/operators, "firm" production has a direct impact on the power purchase agreements (PPA's). Since wind power is traditionally best at night and solar power is only during the day, by combining their synergies, uncertainty is reduced and higher PPA's are possible. This analysis will present economic estimates of the ability of plant operators to secure higher purchase prices for power by raising the "firm" production level and reducing risk.
High Performance Computing Analysis of Wind Turbine Response to Atmospheric Eddy Passage
Brasseur, James G. (Virginia Tech, 2015-06)
The Impact of Offshore Wind Turbines on Underwater Ambient Noise Levels
Glegg, Stewart (Virginia Tech, 2015-06)
The underwater sound levels generated by offshore wind turbine farms is a concern because of the possible environmental impact on marine mammals. This paper will consider how sound generated by a wind turbine is transmitted into a shallow water channel. It is shown that the underwater sound levels can be calculated for a typical offshore wind turbine by using the theory of Chapman and Ward (1990) combined with aeroacoustic models of trailing edge noise on the wind turbine blades. A procedure is given for estimating the underwater sound levels from a wind turbine whose airborne noise levels are known. The results indicate that the sound levels are strongly modulated at the blade passing frequency, which leads to infrasound that is more easily detected than a continuous sound source of the same level.
Analysis of Tower Shadow Effects on the UAE Rotor Blades
Noyes, Carlos; Loth, Eric; Qin, Chao (Virginia Tech, 2015-06)
A leading obstacle hindering the development of wind turbines to extreme scale is the structural integrity of the blades. Downwind rotors have been shown to give structural advantages for larger systems. However, there is an added aerodynamic complication from the tower shadow. This paper presents and analyzes a previously unpublished subset of data collected by NREL during an extensive wind tunnel campaign for Unsteady Aerodynamic Experiment Phase VI (UAE Phase VI). The experimental data includes relative flow fields, aerodynamic blade forces, and root blade flapwise bending moments, from upwind turbines, downwind turbines and downwind turbines with the use of an aerodynamic tower fairing. It is shown that the tower shadow can have a severe and negative effect on these variables, leading to higher bending stresses. The use of a tower fairing can greatly reduce these detrimental effects. To better interpret this data, predictions using an aeroelastic wind turbine code, FAST, was used to model the experimental conditions. The differences between the experimental data and the computational predictions are attributed to unsteady effects of the wake. This suggest that wake modeling for downwind turbines may require modifications to capture physically realistic tower shadow effects.
A Wind Tunnel Study on the Aeromechanics of Dual-Rotor Wind Turbines
Wang, Zhenyu; Tian, Wei; Sharma, Anupam; Hu, Hui (Virginia Tech, 2015-06)
In the present study, we report our recent efforts to develop a novel dual-rotor wind turbine (DRWT) concept to improve aerodynamic efficiency of isolated turbines as well as wind farms. The DRWT concept employs a secondary, smaller, co-axial rotor with two objectives: (1) mitigate losses incurred in the root region of the main rotor by using an aerodynamically optimized secondary rotor, and (2) mitigate wake losses in DRWT wind farms through rapid mixing of turbine wake. Mixing rate of DRWT wake will be enhanced by (a) increasing radial shear in wind velocity in wakes, and (b) using dynamic interaction between primary and secondary rotor tip vortices. Velocity shear in turbine wake are tailored (by varying secondary rotor loading) to amplify mixing during conditions when wake/array losses are dominant. The increased power capacity due to the secondary rotor can also be availed to extract energy at wind speeds below the current cut-in speeds, in comparison to conventional single-rotor wind turbine (SRWT) design. For a DRWT system, the two rotors sited on the same turbine tower can be set to rotate either in the same direction (i.e., co-rotation DRWT design) or at opposite directions (i.e., counter-rotating DRWT design). It should be noted that a counter-rotating rotor concept (i.e., the rotors rotate at opposite directions) has been widely used in marine (e.g., counter-rotating propellers used by Mark 46 torpedo) and aerospace (e.g., Soviet Ka-32 helicopter with coaxial counter-rotating rotors) applications to increase aerodynamic efficiency of the systems. The recent work Ozbay et al. (2015) reveal that, with the two rotors in counter-rotating configuration (i.e., counter-rotating DRWT design), the downwind rotor could benefit from the disturbed wake flow of the upwind rotor (i.e., with significant tangential velocity component or swirling velocity component in the upwind rotor wake). As a result, the downwind rotor could harvest the additional kinetic energy associated with the swirling velocity of the wake flow. With this in mind, the effects of relative rotation direction of the two rotors on the aeromechanics performances of DRWTs (i.e., co-rotation DRWT design vs. counter-rotating DRWT design) and the turbulent mixing process in the DRWT wakes are also evaluated in the present study. The experimental study was performed in a large-scale Aerodynamics/Atmospheric Boundary Layer (AABL) Wind Tunnel located at the Aerospace Engineering Department of Iowa State University. Scaled DRWT and SRWT models were placed in a typical Atmospheric Boundary Layer (ABL) wind under neutral stability conditions. In addition to measuring the power outputs of the DRWT and SRWT systems, static and dynamic wind loads acting on the test models were also investigated to assess the effects of the secondary, smaller, co-axial rotor in either counter-rotating (rotors rotate at opposite directions) or co-rotating (rotors rotate at same direction) configuration on the power production performance and the resultant dynamic wind loads (both aerodynamic forces and bending moments) acting on the DRWT models. Furthermore, a high-resolution stereoscopic Particle Image Velocimetry (Stereo-PIV) system was also used to make both "free-run" and "phase-locked" measurements to quantify the transient behavior (i.e., formation, shedding and breakdown) of unsteady wake vortices and the flow characteristics behind the DRWT and SRWT models. The detailed flow field measurements were correlated with the power output data and dynamic wind loading measurements to elucidate underlying physics for higher total power yield and better durability of wind turbines operating in turbulent non-homogenous atmospheric boundary layer (ABL) winds.
Effects of Tip Injection and Mie Vanes on the Performance of a Model Wind Turbine Rotor
Abdulrahim, Anas; Anik, Ezgi; Uzol, Oguz (Virginia Tech, 2015-06)
This paper presents the results of an ongoing experimental investigation of the effects of tip injection, as well as V-shaped Mie Vanes on the performance of a model wind turbine rotor. Experiments are conducted by placing a three-bladed horizontal axis wind turbine rotor at the exit of an open-jet wind tunnel facility. It is observed that tip injection has significant effects on the power and thrust coefficient variations especially at higher TSR values beyond max CP TSR. Results of the effects of tip injection as well as Mie vanes on the power and thrust coefficient variations with Tip Speed Ratio (TSR) for several wind speeds and comparisons with baseline data will be presented in the final paper.
Impact of Hurricane Wind/Wave Misalignment on the Analyses of Fixed-Bottom Jacket Type Offshore Wind Turbine
Wei, Kai; Arwade, Sanjay R.; Myers, Andrew T.; Valamanesh, Vahid; Pang, Weichiang (Virginia Tech, 2015-06)
A high risk of hurricane is threatening the development of offshore wind energy in the east coast of the United States. Hurricane loads on an offshore wind turbine, namely wind and waves, not only exert large demands but also have rapidly changing characteristics, especially wind and wave directions. Waves are, in general, inert to rapid changes, whereas wind can change its properties within very short time scales. Misalignment of local winds and propagating ocean waves occurs regularly in a hurricane environment. It is a common practice to design monopile support structures for offshore wind turbines (OWTs) under extreme conditions by the highest wind/wave loads when they are assumed to come from the same direction. However, this co-directional wind/wave assumption can be hazardous for non-axisymmetric fixed bottom support structures for deeper water such as jackets due to their sensitive capacity to loading directions. The goal of this work is to examine the impact of wind/wave misalignment on the extreme loads and structural response under hurricanes. We select a fixed-bottom jacket type offshore wind turbine located in a water depth of 50m as the example structure. The hurricane induced wind and wave loads on the structure system are calculated from a reduced set of 1000 simulated full-track hurricane events, selected from a database of 200,000 years of simulated hurricanes, to represent the hazard of Nantucket, Massachusetts. The meteorological ocean (met-ocean) conditions and wind/wave directions for each hurricane are identified from the track data by physical models. The wind direction, wave direction at different time and location and the orientation of structure are included to capture the misalignment impact on the structural analyses over a wide range of possible engineering designs and conditions. It will let us clearly understand the impact of wind/wave misalignment on the analyses of a jacket-type support structure. Summary : This work, (1) predicts environmental conditions and directions from a reduced set of synthetic hurricane catalogue based on the analytical models; (2) calculates wind and wave forces on a jacket-type offshore wind turbine example (3) studies impact of wind/wave misalignment on the structural analyses over a wide range of possible engineering designs and conditions
Noise and Vibration Issues of Wind Turbines and Their Impact – A Review
Samanta, Biswanath; Saavedra, Rudolf (Virginia Tech, 2015-06)
This paper presents a systematic review of current literature on the issues of noise and vibration of wind turbines and their impact on human health and wild life. The paper reviews the literature on the issues of noise and vibration in wind turbines, the generation mechanisms, the propagation, the impact on human health and wild life. The current status of technology and future developments to mitigate the health and environmental impacts of wind turbine noise and vibration are also reviewed. The paper includes a review of current standards on measurement of acoustic noise of wind turbines and data analysis.
Assessing the Structural Impact of Low Level Jets over Wind Turbines
Gutierrez, Walter; Araya, Guillermo; Kiliyanpilakkil, Praju; Basu, Sukanta; Ruiz-Columbie, Arquimedes; Tutkun, Murat; Castillo, Luciano (Virginia Tech, 2015-06)
Low Level Jets (LLJs) are defined as regions of relatively strong winds in the lower part of the atmosphere. They are a common feature over the Great Plains in the United States. It has been reported that 75 % of LLJs in the Great Plains occur at night and with seasonal patterns, affecting significantly the wind energy production. Present results have corroborated some of the LLJ's known characteristics. LLJs develop due to the formation of stable stratification in the lower atmosphere. This paper is focused on the determination of the static/dynamic impact that real LLJs produced in West Texas have over wind turbines. High-frequency (50Hz) observational data from the 200-m tower data (Reese, Texas) have been input as inflow conditions into the NREL FAST code in order to evaluate the structural impacts of LLJ's on a typical wind turbine. Due to the higher levels of wind speed, the potential for power increase proportional to the cube of the velocity. It has been observed that during an LLJ event the level of turbulence intensities and TKE are significantly much lower than those during unstable conditions; as a result, cyclical aerodynamic loads on turbine blades are different. Low-frequency oscillations prevail in stable conditions with formation of LLJ, as opposed to high-frequency oscillations more prevalent in unstable conditions. The turbulent kinetic energy is lower in LLJ but the energy concentrates in particular frequencies that can stress the turbine. From the point of view of the wind turbine loads/stresses, we have detected frequencies that can be correlated with those from the incoming wind.
Detection of Wake Impingement in Support of Wind Plant Control
Bottasso, Carlo L.; Cacciola, Stefano; Schreiber, Johannes (Virginia Tech, 2015-06)
This paper describes a methodology to detect on a wind turbine wake impingement by an upstream machine. The detection is based on the use of rotor loads. As rotor load sensors are becoming routinely available on modern wind turbines, for example to enable individual blade pitch control, no additional sensors or equipment is necessary for the implementation of the present method. The new wake detector is used for driving wake deflection strategies by active wind turbine yaw.
Windtelligence: The Development of a Wind Farm Performance Management System
Baert, Todd; Carriveau, Rupp; McKay, Phil; Ting, David S-K; Kent, Robert (Virginia Tech, 2015-06)
Experimental Study of Turbulence Influence on Wind Turbine Performance
Talavera, Miguel; Shu, Fangjun (Virginia Tech, 2015-06)
Regarding the issue about unmatched Reynolds number for downscaled wind turbine tests in wind tunnels, a study of the performance characteristics of a model wind turbine operating in the wake of another turbine of the same model under laminar and turbulent inflow was performed. The distance between the two turbines was set at 5, 10, and 15 turbine diameters. In the laminar inflow case, due to the low recovery rate in the wake of the front turbine, the efficiency of the rare turbine has been greatly reduced even when the distance was 15 diameters. To address this issue, turbulent inflow was created using an active grid system installed between the contraction and test-section of the wind tunnel; the maximum turbulence intensity can reach 20%. Velocity fields upstream and in the wake of the turbine were measured using a 2D-PIV system; 1000 pairs of images were acquired for each location to achieve statistical convergence. It was found that by using turbulent inflow the efficiency of both the upstream and the downstream turbine was highly improved. Also, it was found that the efficiency of both turbines is highly related to the turbulence intensity in the inflow. At a constant tip speed ratio for the upstream turbine of 10.3, and a distance of 5 diameters between them, the efficiency for the downstream turbine was 4.1 times higher than in laminar case; for 10 and 15 diameters with the same conditions it was 2.71, and 2.48 times higher respectively. The maximum efficiencies reached for the downstream turbine were 38.5%, 34.5%, and 24.6% for 15, 10, and 5 diameters of distance between respectively. Therefore, despite the low Reynolds number, a high efficiency close to the field was reached using turbulent flow created by an active grid system.

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