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International Journal of Mechanical Engineering and Technology (IJMET) Volume 10, Issue 03, March 2019, pp. 1962–1973, Article ID: IJMET_10_03_198 Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=3 ISSN Print: 0976-6340 and ISSN Online: 0976-6359 © IAEME Publication Scopus Indexed AERODYNAMIC PROFILES FOR APPLICATIONS IN HORIZONTAL AXIS HYDROKINETIC TURBINES J.D. Betancur G, J.G. Ardila M, A. Ruiz S Department of Mechatronics and Electromechanical, Faculty of Engineering. Instituto Tecnológico Metropolitano – ITM. Medellín, Antioquia, 050013, Colombia E.L Chica A Department of Mechanical Engineering, Faculty of Engineering. Universidad de Antioquia- UdeA. Medellin, Antioquia, 050013, Colombia ABSTRACT Research in aerodynamics has allowed the creation of different profiles for certain applications, reliably predicting the lift and drag coefficients by means of simulations or experimental methods. In the field of wind and hydraulic power generation, they have developed turbine rotors with improved performance and greater efficiency; several researchers have evaluated the influence of the use of aero profiles on the operation of hydrokinetic turbines through simulations and experimental tests. The main objective of this review is to show the reader the importance of the proper choice of an aero profile and its influence for the calculation during the design of rotor blades and the performance of hydrokinetic turbines. Key words: Design, Experimental, Hydraulic energy, Renewable energy, Simulation Cite this Article: J.D. Betancur G, J.G. Ardila M, A. Ruiz S, E.L Chica A, Aerodynamic Profiles for Applications in Horizontal Axis Hydrokinetic Turbines, International Journal of Mechanical Engineering and Technology 10(3), 2019, pp. 1962–1973. http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=3 1. INTRODUCTION The environmental and energy problems worldwide have forced researchers and professionals to seek sustainable energy solutions. These have found in renewable sources, such as wind, sun and water, viable alternatives to generate energy in a cleaner and more environmentally friendly way. The energy extracted from water is the cheapest in the world; but, large hydroelectric dams require large dams, this along with the civil works required have caused damage to the environment and ecosystems, for this reason have been criticized [1]. That is why small hydroelectric plants have become one of the most economical and environmentally friendly sources of energy generation in rural areas [2]. Within the small hydroelectric plants, two ways of harnessing the energy of water, hydrostatics and hydrokinetics stand out. The first is the way http://www.iaeme.com/IJMET/index.asp 1962 editor@iaeme.com J.D. Betancur G, J.G. Ardila M, A.Ruiz S, E.L Chica A in which conventionally the energy has been generated by means of reservoirs that generate a pressure head and then by means of a turbo machine the potential energy is extracted [3]. The second form takes advantage of the kinetic energy of different water sources such as channels, tides, oceans and rivers to turn turbo machineries and generate energy. Hydrokinetic turbines are an emerging technology that has begun to gain strength and recognition worldwide because they provide a low-cost energy solution and environmental impact [4]. This technology is still in the development and research stage [5] and studies of hydrodynamic characteristics are limited, for this reason more research is required on the subject [6]. The optimal use of water is paramount for this type of turbines due to the low density of electricity generation, for this reason there are studies that focus on improving the efficiency and operation of hydrokinetic turbines using different aerodynamic profiles and changes in curvature and the thickness [7]–[9]. In addition, the rotor blades are one of the most important pieces for the conversion of energy in hydrokinetic turbines [10], for this reason the design of the blade and the selection of the aero profile are essential for a maximum use of the hydraulic resource [11]. In order to contribute to the development of this technology, this review focuses on showing the importance of the selection of aerodynamic profiles, the ways in which they are studied, due to the impact these profiles have on the performance of hydrokinetic turbines. In addition to the selection of the profile, a review of the design, simulation and experimentation of the hydrokinetic turbines of horizontal axis is carried out, providing the researchers of the subject with a tool that allows them to have a criterion of choosing a profile and to know the different techniques of design, simulation and experimentation of this type of turbines. 2. FORCES AND EFFECTS OF AERO PROFILES There are different aero profiles suitable for certain speeds and applications [12], the difference between one and the other is geometry. The majority of aerodynamic profiles are composed of the parts. The leading edge or edge of entry is rounded and smooth and smooth; this shape allows the profile to act efficiently at different inclination angles. Opposed to this is the trailing edge or leakage, it is sharp and pointed, this in order to prevent the current from surrounding it, except with an intense detachment of the boundary layer, to direct the current and allow a reduction of the resistance to the advance. On the other hand, the rope is the line that goes from the leading edge to the trailing edge. The line of curvature is equidistant between the extrados and the intrados of the profile, the maximum distance from this to the rope defines the maximum curvature of the profile (𝑐𝑚𝑎𝑥 ), which is usually between 25% and 50% of the rope. When the curvature is less than 15% of the chord, the profile is said to be symmetrical and the thickness distribution is the distance between the extrados and the intrados, normally this line of curvature is a smooth curve between 20% and 40% of the chord [13]. In an aero profile the flow travels the extrados at a higher speed producing a lower pressure in this area; as in the intrados the distance is lower, lower speeds are produced, therefore, higher pressures than in the upper part; this pressure difference generates the lift force (L). In the same direction of flow, with speed 𝑈∞, a drag force (D) is generated. The coordinate vectors, A and N, have as a reference plane the string of the aero profile, which can have an inclination (α) with respect to the flow, called angle of attack. When combining the drag force and lift, a force (F) is produced as seen in the Figure 1. The sustentation (L) is the force that is generated on the aero profile when it crosses a fluid, it is of direction perpendicular to the velocity of the flow; this force is responsible for keeping airplanes in the air and turning the vast majority of wind and hydraulic turbines. The expression (1) allows calculating the lift force. http://www.iaeme.com/IJMET/index.asp 1963 editor@iaeme.com Aerodynamic Profiles for Applications in Horizontal Axis Hydrokinetic Turbines 𝐿= 1 2 𝜌𝑉 𝐴𝐶𝐿 2 (1) Where: L is the lift force [N]. ρ is the fluid density [kg m-3]. V is the velocity [m s-1]. A is the body reference area (also called "wing surface") [m2]. CL is the lift coefficient. Figure 1. Aero profile forces. The drag (D), also called fluid friction, occurs between the aero profile and the fluid through which it moves, acts against the movement and is the sum of all the aerodynamic or hydrodynamic forces in the direction of flow. Equation (2) allows calculating the drag force. 𝟏 𝐃 = 𝛒𝐕 𝟐 𝐀𝐂𝐃 (𝟐) 𝟐 Where: D is the drag force [N]. CD is the drag coefficient. Then the aerodynamic and hydrodynamic profiles are subjected to different loads, and as a result are obtained forces associated with the production or consumption of mechanical energy; Some forces such as lift and drag have been studied by [14]. Yavuz and others, developed a study on two profiles NACA4412-NACA6411, in which they looked for the angle of attack where the relation CL against CD was maximum, as well as points where the CL achieved its highest values, finding that the optimal angles of attack were between 10° and 24° for both profiles [15]. Then they made a comparison between the results obtained experimentally with the turbine designed and those found numerically, obtaining an error lower than 7%. On the other hand, Goundar used an S1210 hydro profile and showed that increasing the curvature by 20% resulted in a new hydro profile named HF-Sx, in which he showed a significant improvement in the lift coefficient at different angles of attack studying it to a Reynolds 190.000. Singh and Choi used the optimal conditions of two hydroprofiles, S914 and DU-91-W2250, to build a new one called MNU26, which had a thickness of 26% with respect to the chord [16]. In another study, Kim, verified the behavior of the drag and lift coefficients for Reynolds from 0,1 to 1.4 million, in profiles NACA63421 and MNU26 to find the optimal relationship that would allow him to choose the best profile and the optimal angle of attack to a given regime [17]. Like the previous ones, other authors have developed their own hydro profiles and have http://www.iaeme.com/IJMET/index.asp 1964 editor@iaeme.com J.D. Betancur G, J.G. Ardila M, A.Ruiz S, E.L Chica A studied the lift and drag coefficients at different angles of attack, comparing their results with commercial hydro profiles, obtaining the optimum L/D ratio with respect to some angle of attack, for example, for Goundar and Ahmed they studied it with respect to a 9 ° angle [18]. Another study, carried out in Mexico, to evaluate the loads on a vane, obtaining the coefficients of lift and drag, experimentally, in a wind tunnel [19]. Others have studied the lift coefficient and the pressure coefficient for an S809 hydro profile, finding that for a Reynolds of one million the best angle of attack was 8.2 °[20]. 3. METHODOLOGIES FOR THE STUDY OF AERO PROFILES 3.1. Numerical Methodologies The discretization of the volumes of control is key to the solution of problems by means of finite element programs, when a good discretization is carried out the simulation results are usually more precise and adjusted to the experimental ones. Below, some important aspects for the discretization of the geometries are presented. The mesh is a set of lines that divide an element or geometry creating cells or smaller areas; they are connected with the adjacent elements to form finite element volumes. Meshing is one of the most important steps in the simulation, since the phenomena of interest for the study can be calculated accurately and efficiently through the mesh. For this reason, authors such as Lee, Chung and Baek used hexahedral meshes [21]. Rainbird generated a mesh of 140,000 elements around a NACA0018 profile with an inflow in quadrilaterals near the wall of the aero profile.This was done to capture, in detail, the velocity and pressure gradients that are generated near this area [22], in the periphery of inflation larger triangular elements were generated, this was done in order to decrease the number of elements in areas that are not of interest to the study and thus save computation resources. Table 1 describes different numerical studies performed on profiles, presenting the profiles, their sizes and the density of their meshes. Table 1 Profiles used in different numerical studies and density of their meshes. Author Geometry Chord size [mm] Elements number [Millions] [23] NREL S814 150 2.5 [24] [25] NACA0018/0021 NACA0018 1000 1000 0.51 0.139 [26] NACA0012/ 0015/ 0020/ 0021 1000 0.446 The quality of the elements in a mesh is a factor that influences the precision of the simulation results, for this reason there are several metrics that indicate the elements of a mesh quality, the most important are the obliquity and the elements orthogonality. The obliquity says how similar the mesh cells are to the ideal shape (equilateral prisms and hexahedrons). The quality ranges for the obliquity are of acceptability when the maximum value is between 0.80 and 0.94. The mesh elements must have a maximum acceptable obliquity to facilitate the solution convergence. The second metric indicates the orthogonality between the sides of a face or the faces of a cell, the ranges for orthogonal quality establish a minimum acceptable obliquity between 0.15 and 0.20. The Computational Fluid Dynamics (CFD) programs are used to perform fluid simulations such as water and air, since it is an inexpensive way to obtain results that approximate the real ones, for this reason several authors have used, in their investigations, programs CFD to solve http://www.iaeme.com/IJMET/index.asp 1965 editor@iaeme.com Aerodynamic Profiles for Applications in Horizontal Axis Hydrokinetic Turbines physical phenomena [27] [28] [29] [30]. Turbulence models are mathematical expressions that represent the physical phenomena of flows based on the Navier-Stokes equations. There are three approaches to calculate the turbulence of a flow, the Direct Numerical Simulation (DNS), Large Eddy Simulation (LES) and Reynolds Averaged Navier-Stokes Simulation (RANS) [31], the first two are unworkable and impractical for the industry due to their high computational cost, while the third is widely used for its low computational cost, effectiveness and accuracy. Within the RANS models there are two large groups of turbulence models that are frequently used for accuracy and computational cost, these are k-ε and k-ω, both models have two additional transport equations to the Navier-Stokes model , one for the turbulent kinetic energy (k) and the other for the energy dissipation rate (ε u ω). The turbulence model k-ω SST has shown certain advantages over the k-ε since it solves in a more precise way the phenomena that occur in the viscous sublayer (layer closest to the wall). This attribute is very useful because it allows to capture phenomena such as the detachment of the boundary layer, as well as the pressures and forces that are generated near the wall of the aero profile [10] [32]. Table 2 shows some authors who have used the k-ω SST turbulence model for their investigations. The parameter y+ is a dimensionless number that relates other parameters of the fluid and the flow with the distance that the first node of the mesh must have from the wall so that the turbulence model captures the phenomena that occurred in the boundary layer. Commonly, for the turbulence models, the y+ requirement is specified. The "y" value of the distance between the wall and the centroid of the first mesh element can be calculated and defined according to the expression (3). 𝐲+𝛍 𝐲= (𝟑) 𝟏 √𝟐 𝟐 (𝟎.𝟎𝟓𝟖𝐑𝐞−𝟎.𝟐 )𝛒𝐔∞ 𝛒 𝛒 The y+ depends on the turbulence model chosen as mentioned above, the density (ρ) and the fluid viscosity (μ) are known, the Reynolds number (Re) is given by properties of the fluid and the flow, and the length feature is the rope (L), as defined in (4). 𝛒𝐕𝐋 𝐑𝐞 = (𝟒) 𝛍 The authors that used the family of the turbulence model k-ε used values to y+ (3) called less than 300. On the other hand, those that used the family of the model k-ω reported values lower than 1. Lee, Chung and Baek used a model of turbulence of this family called SST k-ω to correct the lift coefficient of a S809 profile [21]. As Rainbird [22], in addition to maintaining the value of y+ below 1 they used a growth rate of 1.1 for their study of aero profiles used in a wind turbine. Table 2 reports the turbulence models and the y+ used by several researchers. Table 2 Turbulence model k-ω and Y+ in different investigations. Author Geometry http://www.iaeme.com/IJMET/index.asp Turbulence model 1966 Y+ Fluid editor@iaeme.com J.D. Betancur G, J.G. Ardila M, A.Ruiz S, E.L Chica A [33] S809 k-ω SST <1 Air [34] S809 k-ω SST 30 < Y+ < 300 Air [29] S809 k-ω SST <3 Air [35] S809, NASA ls421, NACA 4415 k-ω SST <1 Air [36] NACA63-815 k-ω SST <1 Water [37] HAWT k-ω SST <1 Air [12] AF300 k-ω SST <1 Air [38] SD8000 k-ω SST < 1,2 Air [39] NACA0021/0015 k-ω SST < 1,3 Air [40] NACA0022 k-ω SST <1 Air [41] S1210/HF-Sx k-ω SST <1 Water [42] NACA63-4XX k-ω SST <1 Air 3.2. Experimental methodologies Experimental methodologies are widely used due to their high reliability in the results to obtain results, which allow predicting the power generated and observe the behavior of aerodynamic profiles for the design of wind turbine blades. Different authors have used wind tunnels, also, to characterize geometries that may be hydro profiles of the hydro turbines. Researchers have carried out studies in wind tunnels to choose the hydro profile and its optimum angle of attack and later, when the turbine is built; hydraulic channel tests are carried out to obtain power coefficients and other useful curves. At the University of Newcastle there is a closed wind tunnel 1.2 m wide and 0.8 m high in which wind speeds up to 8 m / s are developed, this tunnel has a Cussons R102 scale that was developed at this same university and measures loads by breaking them into three components [23]. The wind tunnel of the USNA has a 185kW fan controlled by a frequency inverter to produce maximum speeds of 100m/s, it also has a balance of forces AEROLAB that is responsible for measuring the lift and drag on aero profiles with a 0.002% accuracy [14]. Lee and others used a closed-circuit wind tunnel with 36.5m long, 4m wide and 2.6m high, where the maximum wind speed can be 36.5 m/s. This tunnel has a torque wrench where the aerodynamic profile and a Pitot tube to establish the velocity profile; they built a 1m diameter turbine with a three blades SD8000 profile and with the wind tunnel they obtained power factor curves against TSR for two different wind speeds of 8 and 10 m/s [38]. Jessica M and others used a wind tunnel to obtain the CL and CD curves of a NACA63-618 2D airborne of a 0.23m rope and a length of 0.79m [14]. In the Emerson Cavitation Tunnel (ECT) of the University of Newcastle, several profiles were characterized. They used a Dantec Dynamics Stereo PIV speed sensor and a National Instruments data acquisition card, the data was analyzed instantaneously by LabVIEW, for each point they were made three tests and the averages of each result were compared obtaining standard deviations lower than 1.1% for the drag and lift coefficients [23]. In the Engineering Laboratory Design (ELD) experiments were developed in a wind tunnel to find pressure coefficient, drag and lift curves against attack angle for different Reynolds of an HF-Sx hydro profile [7]. 4. APPLICATION OF AERO PROFILES: HYDROCINETIC TURBINES 4.1. Hydraulic turbines http://www.iaeme.com/IJMET/index.asp 1967 editor@iaeme.com Aerodynamic Profiles for Applications in Horizontal Axis Hydrokinetic Turbines The classification of hydro turbines starts with two groups, action and reaction turbines. The first are those that by means of nozzles transform the potential energy of water into kinetics to form a jet that hits the blades, such as the Pelton, Turgo and Michell-Banki turbines. The reaction turbines the water pressure applies a force on the faces of the blade that decreases as it moves through the turbine. These are classified according to the flow as mixed or axial. Within the turbines of mixed flow is the Francis and axial flow are the turbines of propeller, Kaplan, bulb and annular (used in tidal generation) [43]. Hydrokinetic turbines are essentially reactive and are classified by the direction of flow with respect to the axis as: cross flow turbine, vertical axis and horizontal axis, the latter being the most used worldwide [44]. The idea of hydrokinetic turbines (HKT) has been adopted from wind turbines, although both turbines are similar in form, there are many differences [16]. The majority of HKT used today only use a hydro profile for the entire length of the blade, unlike large wind turbines that use different aero profiles with decreasing thicknesses from the hub to the tip [45]. One of the biggest challenges facing HKT is cavitation, because the working fluid is water, this brings about problems of vibration, increased hydrodynamic drag, noise and erosion on solid surfaces. [16]. The operation of THK or wind turbines depends mainly on the characteristics of the main components such as the generator, the shaft, the gearbox, and the rotor [46]. The rotor blades are one of the most important parts for the conversion of energy in wind and hydrokinetic turbines [10], for this reason the design of the blade and the selection of the aero profile is essential [11]. 4.2. Design of horizontal axis hydrokinetic turbines The use of algorithms and mathematical models has facilitated the design of the rotors in HKT; some authors have used semi-analytical models to improve performance characteristics [47] or the Blade Element Momentum (BEM) theory to simplify phenomena and optimize the construction of rotors. El Khchine and Sriti considered the losses at the end of the vane in the BEM model to design a wind turbine of 10 m in diameter with an S809 profile. As a result, the curves of drag and torque against each vane section as well as the angle of attack and torsion for each local radio [48]. Other authors have also used this sizing and optimization methodology to design blades and their results have been validated with simulation programs and / or experimental results [49]. Hassanzadeh, and others, make use of BEM theory in conjunction with genetic algorithms to optimize chord distribution and turn angle at each local radius to maximize the power of a 10 m diameter wind turbine using an S809 profile, achieving an increase of 8.51% of the annual energy produced [50]. Lanzafame and Messina in 2007 carried out a study in which they used induction factors based on the BEM theory with Glauert correction and compared the power and torque coefficients with experimental results validating the numerical model [51]. In 2015 they carried out an optimal design study for a rotor of a horizontal axis wind turbine where the rope, the torsion angle and the thickness of the profile varied in different local radii until finding the points that produced the highest power coefficient [52]. Arramach, and others, used two codes in which the BEM theory is applied to evaluate the behavior of a wind rotor comparing the theoretical and experimental results obtaining low errors for wind speeds between 5 m/s and 12 m/s [53]. Authors such as Walker, and others, used the BEM methodology to design the rotor of a 0.8 m diameter hydraulic turbine, using a NACA 63-618 profile for a water velocity of 1.68 m / s, this methodology was validated with experimental results showing power coefficient curves against different speeds at the end of the blade (Tip Speed Ratio - TSR) [14]. Some authors have used different programs to improve the performance of profiles [7] [46] and others have http://www.iaeme.com/IJMET/index.asp 1968 editor@iaeme.com J.D. Betancur G, J.G. Ardila M, A.Ruiz S, E.L Chica A used evolutionary algorithms to design the proper geometry avoiding cavitation and obtaining maximum power [54][55]. 4.3. Simulation of horizontal axis hydrokinetic turbines The Computational Fluid Dynamics (CFD) programs are used to perform fluid simulations such as water and air, since it is an inexpensive way to obtain results that approximate the real ones, for this reason several authors have used, in their investigations, programs CFD to solve physical phenomena [28] [29]. Tian and others performed a simulation of a three-bladed hydrokinetic turbine with a CFD module of the ANSYS® program, using the turbulence model Shear Stress Transport (SST) k-ω, a y+ in the blades of 0.02 mm, the velocity of the fluid was 1.73 m / s and made variations in the TSR from 5 to 10. As a result a curve of power coefficient vs TSR, velocity contours of the blade at different local radii and stelae produced by the turbine at different TSR [56]. Lee and others use a CFD module from ANSYS® to simulate the blades of a THK with a fluid velocity of 1.3 m/s. As a result, pressure profiles were obtained in the blade, vorticity in the end of the vane showing that when rounding the tip of the vane the vorticity diminishes in magnitude. In addition, power coefficient curves are obtained against TSR compared with experimental and other numerical results [27]. On the other hand, the interaction of the fluid blades, built with aero-fiber shapes, generates stresses and deformations, and manages to cause problems of aeroelastic instability, which can have a devastating impact on the entire system [57], For this reason, studies are carried out on them, generally by means of simulation programs. Singh and Choi carried out a study of stress deformation and pressure on two different blades, one with shorter rope length in contrast to the typical blade for sea current turbines [16]. For the study, they used two different materials for each blade, aluminum and steel, finding that the effort and deformation were three times greater for the typical blade, regardless of the material with which it was constructed. Payne, Stallard and Martinez used the finite element stress analysis on a blade, obtaining errors less than 6% for the deflection [58]. 4.4. Experimental tests of horizontal axis hydrokinetic turbines HKT have been tested experimentally in different ways. Hydraulic test channels are used to obtain experimental turbine curves and observe their behavior. The testing channel of the United States Naval Academy (USNA) has a length of 116 m, a width of 7.9 m and a depth of 4.9 m. The torque measurements are taken with a reference dynamometer R46-01. They studied a 0.8 m diameter turbine built with a NACA63-618 profile. In this study, they obtained rotational speed measurements with an HS35 model encoder for flows between 5 and 11 m/s for different roughness on three blades. The acquisition of the data was made by means of a card in real time, the results obtained had an uncertainty lower than 1.3% for the coefficients of lift and pressure [14]. In the Ata Nutku Ship Model Testing Laboratory of the Technical University of Istanbul, there is a hydraulic channel of 160 m in length, 3.4 m deep and 4 m wide; in which the water can reach speeds of 6 m / s. This test bench has a torque meter DRBK model that can measure up to 200 Nm. They developed a rotor of 1.2 m in diameter with three blades built with a NACA 4412 profile, which obtained results of torque, speed, voltage and current; for data acquisition they used a QUANTUMX MX840 card [46]. Tian and others, after having made the design of a THK and having manufactured it, conducted tests in a static flow channel with a speed of 0.25 m / s, the channel is 132 m long, a width of 10.8 my 2 m depth, obtaining as results the power coefficient curve against TSR between 4 and 11 [56]. On the other hand, authors such as Atcheson and others developed a catamaran type structure to test 1:20 scale HKT in a static fluid at low speeds between 0.9 and 1.2 m / s. The results show the power coefficients for TSR http://www.iaeme.com/IJMET/index.asp 1969 editor@iaeme.com Aerodynamic Profiles for Applications in Horizontal Axis Hydrokinetic Turbines between 0.5 and 4 comparing them with the same graph obtained through BEM [59]. A similar study was carried out by Jeffcoate and others, where they use similar fluid structure and conditions to obtain the curve of the power coefficient against TSR between 0.5 and 4.5 [60]. 5. CONCLUSIONS This article has reviewed the different techniques of evaluation of aerodynamic profiles, some design methodologies of hydrokinetic turbines of horizontal axis and different methods of evaluation by means of simulation and experimentation. The aerodynamic and hydrodynamic profiles play a fundamental role in the operation of the hydrokinetic turbines, for this reason more studies with different profiles must be carried out to observe the performance behavior. The creation of new profiles specially designed for hydrokinetic turbines can improve performance, taking into account that nowadays there are computational tools that allow the researcher to evaluate the behavior without having to manufacture it, thus saving time and materials. Hydrokinetic turbines are a new technology for the use of renewable energy sources that, up to now, have low efficiency and need to be studied more thoroughly in order to better understand the parameters that can improve their performance. 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