Abdus Samad

The paper highlights the collaborative research between University of Hawaii and the Indian Institute of Technology, Madras, to move forward the design of a compact wave powered desalination unit. Detailed hydrodynamic optimization is undertaken to optimize the weight, volume and overall performance of the unit. A fully coupled hydrodynamics/filtration model, developed with WEC-Sim software enables quick estimation of desalinated water and brine discharge in frequently observed seas. Several design considerations, that arose during the design stage are presented in this paper. We conclude with estimation of performance-desalinated water and brine discharge-in frequently observed ocean conditions.

Gauss–Seidel method is one of the simplest available iterative methods for solving systems of linearized equations. It can effectively reduce high-frequency errors but performs poorly with errors of low frequency. Multigrid (MG) utilizes this quality of the point-wise methods by successively coarsening the grid, so that the lowest frequency errors appear as high frequency and can be easily reduced. In this work, optimization study was performed to lower the CPU time of the Multigrid method. We have considered several parameters, such as the number of grid levels used, the number of inner iterations (iterations at each intermediate grid), the overall coarsening and interpolation cycle (V and W), and the number of these cycles in each iteration. A surrogate model is used to predict optimum value for these parameters. In this chapter, MG is used with a Gauss–Seidel solver for a 2D conduction problem with Dirichlet boundary condition on a 256 × 256 structured grid. The results suggest that a W cycle is more efficient than a V cycle and should be executed to the penultimate grid level during both restriction (coarsening) and prolongation.

Winglets on a turbine blade can modify flow features and improve a marine current turbine (MCT). In this work, to maximize the power coefficient (CP) and torque (T), cant angle (α), and height (h) of a winglet of an MCT were modified. The problem was solved using a high-fidelity solver code Fluent 2019R2 containing Reynolds-Averaged Navier–Stokes (RANS) equations. The flow domain meshed with tetrahedral elements. Nine different designs were produced to fill the design space for optimization. A set of low fidelity models such as second-order regression, kriging, and neural network models were used to approximate the high-fidelity results. The optimal designs further validated with the high-fidelity simulated results. The optimal design, increased CP by 7% for α = 33.2o and h = 2.04% of the turbine radius, reduced the recirculation zone at the trailing edge, increased the pressure gradient, and reduced the tip vortex.

A bidirectional turbine used in an oscillating water column device extracts wave energy from oscillating airflow. To improve its power output, a concept of static extended trailing edge found in the wings of owl and merganser was adopted. The static extended trailing edge with 0–10% of chord length (C) was analyzed for different flow coefficients via Reynolds-averaged Navier–Stokes equation-based computational fluid dynamics (CFD) analysis. ANSYS-CFX 15.0 was used to simulate the flow. Grid convergence index was calculated to obtain optimum mesh, and numerical validation was done with experimental results. The static extended trailing edge with 5%C enhanced relative mean torque by 23.4% and, reduced relative mean efficiency by 5.4%, before stall condition. The modification increased pressure difference between the suction side and the pressure side and enhanced torque. The increased pressure drop reduced the efficiency. A further longer static extended trailing edge showed poorer stall characteristics.

A bidirectional impulse turbine is a self-rectifying air turbine used in an oscillating water column wave energy converters. Most of the study on bidirectional impulse turbines involves steady-state performance analysis; however, the performance under oscillating airflow conditions is necessary to understand its behavior. The objective of this study is to experimentally analyze an optimized and numerically investigated bidirectional impulse turbine with fixed guide vane subjected to oscillating airflow conditions. A bidirectional airflow test facility developed at Indian Institute of Technology Madras, Chennai, India is employed to determine the aerodynamic characterization of the turbine and the dynamics involved in each of the coupling stages. The test rig consists of a piston chamber assembly, which provides different airflow rates by varying the stroke length (SL) and cycle time. Emphasis is made on the pressure and flow rate coefficients of the turbine, turbine-generator coupling, and power developed for different input conditions. The operating range of the turbine is mapped for four different frequencies and three SLs. Different electrical loading and the power output were analyzed for accelerations and decelerations of inflow and outflow. The preliminary dynamic characterization of the turbine with respect to nondimentionalized pressure coefficient and flow coefficient was determined for inflow and outflow. The power output is found to be in strong correlation with the flow rate and angular rotation of the rotor. The turbine analyzed in the experimental test rig will be proceeded with real sea testing on the Indian coast by the National Institute of Ocean Technology, Chennai, India.

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.

This study extracts and reports notable findings on passive micromixers by conducting an exhaustive review of designs, their features, and mixing performance. The study has covered the relevant articles on passive micromixers published from 2010 to 2020. The analysis of filtered and selected articles sums up passive micromixers into four categories: designed inlets, designed mixing-channel, lamination-based, and flow obstacles-based. The prominent mixing channel categories identified in the study are split-and-recombine (SAR), convergent-divergent (C-D), and mixed (SAR, C-D, and others). Moreover, differences in mixing channel designs, number of inlets, and evaluation methods have been used in comparing the mixing performance of passive micromixers. The SAR and the obstacles-based micromixers were found to outperform the others. The designs covered in the present review show significant improvements in the mixing index. However, these studies were conducted in an isolated environment, and most of the time, their fabrication and device integration issues were ignored. The assortment and critical analysis of micromixers based on their design features and flow parameters will be helpful to researchers interested in designing new passive micromixers for microfluidic applications.

The present study attempts to enhance a Wells turbine performance by adopting a leading-edge microcylinder (LEM) as a passive flow control device. The microcylinder is placed near the blade leading edge so that its axis lies on the chord line of the rotor blade. The influence of turbine performance, due to parameters such as microcylinder diameter and the distance between the cylinder and the blade leading edge, is evaluated by solving the steady Reynolds-averaged Navier–Stoke (RANS) equations with the k-ω SST turbulence model. The performance parameters of the microcylinder rotor were compared with the reference rotor. It was found that the pair of counter-rotating and co-rotating vortices shed from the microcylinder feed kinetic energy to the separated flow and re-energize the boundary layer. This phenomenon delays the flow separation and enhances the operating range. Moreover, a parametric investigation of the microcylinder rotor reveals that the diameter and space between the microcylinder and the rotor blade are instrumental in delaying flow separation. It was found that a cylinder diameter equal to 0.02C (C is blade chord) and a distance between the leading edge and the micro cylinder equal to 0.035C resulted in increases in the working range and in the average torque equal to about 22% and 49%, respectively.

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.