Abdus Samad

A Gurney flap (GF) placed at pressure side of the trailing edge of an airfoil and perpendicular to the chord line enhances lift in aircraft wings, helicopter rotors, and wind turbines, etc. In this article, the GF concept was introduced for Wells turbine blade used to harvest wave energy with special consideration as the blades are having symmetric airfoil and faces bidirectional flow. Hence, the flap was extended to both pressure and suction sides of the trailing edge (TE) to maintain blade symmetry, and the turbine performance was evaluated using opensource computational fluid dynamics code OpenFOAM 4.0. Different GF-lengths (0.5–3% chord length) were considered, and the performance parameters such as non-dimensional torque, pressure drop and efficiency were evaluated. The GF blades produced a counter-rotating vortex pair behind the TE which modified the TE Kutta condition and increased the circulation and lift. In addition, the GF blades increased the blade loading and enhanced the torque generated. However, the increased pressure drop lead to decrement in efficiency.

Optimization of wave energy converters (WECs) through robust controls is mandatory to maximize the energy absorption in the face of sea waves’ stochastic nature. Single optimization criterion predicted by linear models is rendered ambiguous when modulated by viscous forces, dominant at the controlled conditions. An efficient hydrodynamic model capable of identifying suitable parameters for nonlinear controller design is desirable but missing. Hence, this paper proposes a superposed hydrodynamic model (SPHM) to optimize the power output of a scaled-down point absorber WEC. Two variants of SPHM are considered to evaluate the differences between linear and nonlinear viscous models. Optimization is guided by a non-predictive latch control strategy. The model is numerically solved using the fourth Runge-Kutta method to obtain the time domain response of buoy. The nonlinear SPHM reveals a new optimization parameter based on the maximum velocity criterion. At off-resonant states, the controller enhances the system power by eleven times.

Delaying a stall improves the performance of any turbomachinery system. TC (tip clearance), which is used in a bi-directional flow Wells turbine of an ocean wave energy device, changes the flow pattern on the turbine blade suction surface, while changing or modifying the TC zone can help obtaining a delayed stall. In the present work, a new tip grooving scheme is introduced and the performance is compared for different tip groove depths and TCs of a Wells turbine. The performance is defined in terms of wider operating range or stall delay, power production and efficiency. The problem was solved by a numerical analysis technique. A multi-block meshing scheme was employed to generate structured and hexahedral elements in the computational domain and the flow was solved in ANSYS CFX® v14.5 by solving Reynolds-averaged Navier Stokes equations. It was found that the grooves improve the turbine operating range and power production as compared to those of the turbine without a groove. The groove depth of 3% of the chord length produced highest power and widest operating range. Using the circumferential groove, 26% increase in turbine power output for a particular operating point is achieved.

The present paper investigates the leading edge (LE) undulations of a Wells turbine blade through numerical analysis. The aspiration for this modification came from humpback whales, which have uneven protrusions at the LE of their pectoral flippers. The flippers help whales to maneuver during swimming. The work is performed by using three-dimensional steady, incompressible Reynolds Averaged Navier-Stokes (RANS) equations with turbulent closer model. The LE of the turbine blades is modified with undulations of three different amplitudes: 1mm, 2.5mm, and 4mm. The results show that the undulation changes the turbine performance. The amplitude 2.5mm gives the peak performance. The comparison between blades with different amplitudes and the reference blade has been discussed throughout this study.

The present work focuses multi-objective optimization of blade sweep for a Wells turbine. The blade-sweep parameters at the mid and the tip sections are selected as design variables. The peak-torque coefficient and the corresponding efficiency are the objective functions, which are maximized. The numerical analysis has been carried out by solving 3D RANS equations based on k-w SST turbulence model. Nine design points are selected within a design space and the simulations are run. Based on the computational results, surrogate-based weighted average models are constructed and the population based multi-objective evolutionary algorithm gave Pareto optimal solutions. The peak-torque coefficient and the corresponding efficiency are enhanced, and the results are analysed using CFD simulations. Two extreme designs in the Pareto solutions show that the peak-torque-coefficient is increased by 28.28% and the corresponding efficiency is decreased by 13.5%. A detailed flow analysis shows the separation phenomena change the turbine performance.

Wells turbine is a self-rectifying axial flow reaction turbine used to harvest energy from the ocean waves. It suffers from a premature stall at higher flow rates. The present study discusses a comparative performance analysis with a turbine-blade leading-edge (LE) microcylinder (LEM) and D-cylinder (LED). The space between the LE and the cylinder was fixed as 1.5% of chord length (c). The sizes of the cylinder were varied from 0.5% to 0.75% of the chord. The unstructured tetrahedral mesh elements were used to discretize the computational flow domain that consists of a single blade passage with periodic boundary conditions. The Reynolds-Averaged Navier-Stokes equations with the k-? shear stress transport (SST) turbulence equations were solved in a commercial CFD code Ansys CFX 18.1. The flow was considered incompressible. The present numerical study was compared with available open literature. The modified rotor blades showed a significant performance enhancement compared to the reference turbine. The peak efficiency was improved by 11.29% at a particular flow coefficient in 0.5%c radius LED-turbine. The presence of the cylinders delayed the flow separation and enhanced the operating range up to 11.11%.

The present work describes the design and analysis of a compact and modular wave-powered desalination (WPD) system. The design of the unit is influenced by the United States Department of Energy (DOE) Waves to Water competition rules. The system consists of a flap-type wave surge converter that converts wave energy into hydraulic piston force through a double-acting cylinder working as a power take-off (PTO) device. The dimensions of the flap are determined through frequency domain analysis under different sea states and the required discharge of desalinated water. Further, the whole system, including the reverse osmosis (RO) module, is modeled using WEC-Sim (an open-source code) and Simscape/SimHydraulics toolbox in MATLAB. The analysis of the system under six different sea states shows encouraging results for freshwater production. Considering the sea states studied here, the designed system can generate more than 50 bar pressure of the seawater feed discharge and produce up to 100 L/h of freshwater with TDS less than 500 mg/L. It was also observed that the use of an accumulator reduces the fluctuation in feed flow and feed pressure at the inlet of the RO module, resulting in improved system efficiency.

In this article, a Wells turbine geometry is created with the modification of multiple geometric parameters, i.e., blade sweep/skew, endplate, casing groove, and guide vane. The experiment of a bidirectional turbine is conducted at the wave and fluid engineering laboratory, IIT Madras. The preliminary objective of the study is to measure the starting characteristics and corresponding flow velocity, revolution per minute (rpm), differential pressure of higher and lower pressure sides of the turbine. The output parameters are measured at different cycle times and the stroke length of a piston-cylinder combination, which simulates different wave conditions. After starting the Wells turbine, rotational and axial speeds increase for some time. After that, it will fluctuate between a specific range, and pressure is prepositional to the airflow rate. The wave energy can be converted into pneumatic energy with the help of wave energy converting(WEC) devices, i.e., oscillating water column(OWC) that can be further converted into mechanical energy and then into electrical energy with some appropriate devices. In this article, an experimental analysis of the turbine geometry is reported.

Oscillating water column based wave energy extracting system has a low efficiency due to the poor performance of its principal power extracting component, the bidirectional turbine. In the present work, flow over a bidirectional impulse turbine was simulated using CFD technique and optimized using multiple surrogates approach. The surrogates being problem dependent may produce unreliable results, if a wrong surrogate is selected. Hence, multiple surrogates such as response surface approximation, radial basis function, Kriging and weighted average surrogates were incorporated in this problem. Same design points were used to generate multiple optima via multiple surrogates to enhance the robustness of the optimization process. Numbers of guide vanes and rotor blades were chosen as the design variables, and the objective was to maximize the blade efficiency. Reynolds-averaged Navier-Stokes equations were solved for analyzing the flow physics. The computed results were used to train the surrogates and find the optimal points via hybrid genetic algorithm. The surrogates were further applied to find the optimal flow parameters by changing flow velocity and turbine speed. The relative efficiency enhancement through our present approach was about 16%. Detailed methodologies, analysis of the results and surrogate applicability have been presented in this paper.

Wave energy harvesting systems mostly have low power production capability because of unoptimized design of the system components. A bidirectional flow impulse-turbine used in such a system has efficiency less than 40%, and it is required to design the turbine for a higher efficiency. Present work finds an optimal design and shows design-variable sensitivity to the turbine efficiency. The problem is solved using numerical analysis technique. The flow through the turbine was analyzed by solving the Reynolds-averaged Navier-Stokes equations (RANSE). The design variables; namely number of rotor blades and number of guide vane, guide vane angle and guide vane profile were modified to maximize the turbine efficiency. Using the Latin hypercube sampling technique, sample points were selected from a design space defined by lower and upper limits of the variables. Then, several surrogates were constructed using the RANSE calculated results, and the turbine performance was optimized. The results show that guide vane angle is the most sensitive parameter, while the guide vane profile has negligible effect on efficiency. A hybrid genetic algorithm searched the optimal design point. The relative mean efficiency enhancement over a wide range of flow coefficient was approximately 24%, while it was 28% at maximum efficiency point.