Contents lists available at ScienceDirect Applied Ocean Research journal homepage: www.elsevier.com/locate/apor Hydrodynamic performance improvement on small horizontal axis current turbine blade using diferent tube slots confgurations Parikshit Kundu Indian Institute of Technology Kharagpur, Kharagpur, 721302, India ARTICLEINFO Keywords: NACA S1210 hydrofoil Boundary layer control Lift and drag coefcient ABSTRACT Vortex generators are used extensively as a passive fow control devices to delay or remove the boundary layer separation, which afects the hydrodynamic performance of the hydrofoil. In this paper, a new approach is introduced to overcome the boundary layer separation on the NACA S1210 hydrofoil. The outcome of tube slots combination in the S1210 hydrofoil on the boundary layer separation are numerically investigated. The per- formance is compared with respect to the force coefcients and glide ratio. The efects of tube slot inlet positions with diferent diameters on S1210 hydrofoil are presented here. The results show that the smaller diameter tube slots starting near the leading edge improves the hydrodynamics performance of the hydrofoil. 1. Introduction Modern lifestyle heavily relies on electricity, without which it is impossible to imagine today’s world. Generating electricity through the conventional means of using fossil fuels is not enough to meet the ever- growing hunger for energy for a developing country. In addition, burning of fossil fuels is directly related to the emission of the green- house gases (e.g., CO 2 ,CH 4, N 2 O etc.) and the global warming problem. Therefore, many researchers and developers started exploring and ex- ploiting the renewable energy resources that provide a greener path of power generation. In India, several large rivers across the country as well as tidal current at several locations along the coast line present strong potential for generating renewable power by the use of current turbines. Today the majority of the proposed designs for converting ocean current or river current are based on the concept of the Horizontal Axis Current Turbine (HACT) [1,2], which has similar characteristics to those being used in wind energy. In a HACT, hydrofoil sections are used in the blades that convert the fow energy of the fuid into mechanical energy by generating lift force, which in turn creates the torque that rotates the shaft connected to the generator. Although hydrofoils work in a similar manner as an aerofoil, there are signifcant diferences in the design and operation of hydrofoils. These are change in the fuid density (compressibility), Reynolds number (Re), diferent stall char- acteristics, and the possible occurrence of cavitation. It is important to establish a set of parameters to select the proper hydrofoil for a current turbine blade design. Most common ones for HACTs are -surface roughness tolerance, stall initiating from the trailing edge, free from cavitation, no laminar separation bubbles, smooth stall, no noise generation, high structural stifness, high glide ratio at the design point. NACA S1210 hydrofoil is commonly used in a current turbine as it performs better against cavitation inception, and has a high lift to drag ratio (glide ratio) [3]. A modifed version of S1210 hydrofoil is presented in [3] by increasing both the camber and the thickness by 20%, which has shown better performance than the original geometry. This has also been useful to design a stronger blade section that can withstand the thrust experienced in water. In this study, NACA S1210 hydrofoil is used as a baseline profle. The boundary layer separation over a hydrofoil causes energy losses and strong adverse pressure gradient, which afects fuid fow perfor- mances in many typical applications [4–7]. It, is required to bring momentum to the boundary layer to delay and even remove this fow separation so that it can bear the strong opposing pressure gradient [8]. It occurs whenever the change in velocity of the fuid in either mag- nitude or direction, is too large for the fuid to adhere to the solid surface. A favorable pressure gradient is one in which the pressure decreases in the fow direction (i.e., dp/dx < 0); it is called favorable because it tends to overcome the slowing of fuid particles caused by friction in the boundary layer. This pressure gradient arises when the freestream velocity U is increasing with x. On the other hand, an ad- verse pressure gradient is one in which pressure increases in the fow direction (i.e., dp/dx > 0); it is called adverse because it will cause fuid particles in the boundary layer to slow down at a greater rate than that due to boundary-layer friction alone. If the adverse pressure gradient is severe enough, the fuid particles in the boundary layer will actually be brought to rest. When this occurs, the particles will be forced away from https://doi.org/10.1016/j.apor.2019.101873 Received 20 August 2018; Received in revised form 4 June 2019; Accepted 10 July 2019 E-mail address: pkundu@iitkgp.ac.in. Applied Ocean Research 91 (2019) 101873 0141-1187/ © 2019 Elsevier Ltd. All rights reserved. T