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.

In the oil and gas industry, upstream and downstream hydrocyclones are used extensively to separate heavy or dense particles from the formation water/reservoir fluids. These hydrocyclones, after a long period of operation, can fail as a result of wear-initiated leakage, thereby needing maintenance or replacement. A detailed investigation of this failure was carried out using computational fluid dynamics (CFD). One-way and two-way coupling of a discrete phase model was used along with the Reynolds stress turbulence model (RSM). Experimental studies were conducted to understand the flow dynamics within the hydrocyclone and to validate the computational model. Key findings, such as bifurcation of the inlet flow, local acceleration of fluid within the hydrocyclone, the impact of the sand drain pipe on fractional efficiency, and the impact of multiple particle sizes and density interactions on the degree of particle entrapment, are discussed in detail. The approach and results presented in this work provide useful insights and a systematic basis for improving the service life and separation efficiency of the hydrocyclone.

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 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.

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.

This paper presents numerical study of the surface curvature effects on the performance of a 3D lab scale tidal turbine (E387) using Eppler 387 airfoil. The prescribed surface curvature distribution blade design method is used to remove the surface curvature discontinuity of E387 turbine and the redesigned turbine is denoted as A7 turbine. The two turbines are analysed using in-house BEM code and CFD RANS. The performance of E3787 turbine obtained from BEM and RANS match well with the experimental results from reported literature. The A7 turbine has mildly better performance at low tip speed ratio (1-4.25) at which the blade is partly or fully stalled.