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The evolution of the secondary vortex arrangement around a foil, performing heaving and pitching motion, is numerically examined for a range of phase offsets (90 ° ≤ ϕ ≤ 270°) and reduced frequency (0.32 ≤ S t c ≤ 0.56), at a Reynolds number of 8000. The wake is dominated by two distinct systems of secondary hairpin-like structures. The first vortex system is associated with an elliptic instability, prompted by the paired primary and secondary leading edge vortices (LEV), which remains persistent within the entire range of Stc. However, the growth of the second system is more closely associated with undulations of the primary LEV at S t c ≥ 0.40, which amplifies as it sheds downstream of the trailing edge. The characteristic presence of the first system is directly linked to the growth of the secondary LEV, formed due to the large-scale interactions under localized adverse pressure gradients. These features promote a streamwise flow compression in neighboring regions of the primary LEV. Subsequently, the stability of these wake arrangements is evaluated using dynamic mode decomposition technique, which identifies a neutrally stable state at the fundamental forcing frequency and its harmonics. However, an increase in the modulus of the first three modes, as Stc increases, coincides with stronger three-dimensionality in the wake. The fundamental mode characterizes the roller undulations, while subsequent modes reveal that the two systems of secondary hairpin-like structures are closely associated with the harmonics of forcing frequency. [ABSTRACT FROM AUTHOR]

This study explored the aerodynamics of a new multi-stage co-axial vertical-axis wind turbine based on bio-inspiration from natural swimming habit of fish. The turbine was formed from a conventional straight-bladed vertical axis turbine (VAWT) with an additional small inner rotor, also of three blades. The azimuthal and radial locations of the inner rotor were varied. Using numerical simulations, performance of the proposed new design was evaluated over a range of tip-speed ratios. The preliminary results identified a 600% increase in power output for multi-stage VAWTs at tip-speed ratios TSR < 3 , and a substantial drop in power coefficient at TSR > 3. 0. The wake dynamics analyses revealed that the increase was due to interactions between the blades of one rotor and the other. This reduced the unsteady separation from the outer rotor, which produced most of the power. A detailed parametric study was also completed, which showed the implications of geometric and kinematic details on the performance of the proposed multistage VAWT. [ABSTRACT FROM AUTHOR]

Inspired by nature, oscillating foils offer viable options as alternate energy resources to harness energy from wind and water. Here, we propose a proper orthogonal decomposition (POD)-based reduced-order model (ROM) of power generation by flapping airfoils in conjunction with deep neural networks. Numerical simulations are performed for incompressible flow past a flapping NACA-0012 airfoil at a Reynolds number of 1100 using the Arbitrary Lagrangian–Eulerian approach. The snapshots of the pressure field around the flapping foil are then utilized to construct the pressure POD modes of each case, which serve as the reduced basis to span the solution space. The novelty of the current research relates to the identification, development, and employment of long-short-term neural network (LSTM) models to predict temporal coefficients of the pressure modes. These coefficients, in turn, are used to reconstruct hydrodynamic forces and moment, leading to computations of power. The proposed model takes the known temporal coefficients as inputs and predicts the future temporal coefficients followed by previously estimated temporal coefficients, very similar to traditional ROM. Through the new trained model, we can predict the temporal coefficients for a long time duration that can be far beyond the training time intervals more accurately. It may not be attained by traditional ROMs that lead to erroneous results. Consequently, the flow physics including the forces and moment exerted by fluids can be reconstructed accurately using POD modes as the basis set. [ABSTRACT FROM AUTHOR]

Despite offering promising opportunities for wind energy harvesting in urban environments, vertical axis wind turbines face limitations in terms of poor starting characteristics. In this study, we focus on analyzing improvements offered by dual-stage turbines for a range of wind velocities. Numerical simulations are performed for different phase angles between the rotors (a measure of relative angular positions of the blades in the two rotors) to quantify the response time for their starting behavior. These simulations rely on a through sliding mesh technique coupled with flow-induced rotations. We find that for U ∞ = 4 m / s , the phase angles of 30 ∘ and 90 ∘ substantially reduce starting time in comparison to a single-stage turbine. Dual-stage turbines with a phase angle of 90 ∘ exhibit similar or better starting behavior for other wind speeds. The phase angle of 0 ∘ in double-rotor turbines shows the poorest starting response. Moreover, it is revealed that stabilization of shear layers generated by the blades passing through the windward side of the turbine, vortex-entrapment by these rotating blades, and suppressing of flow structures in the middle of the wake enhance the capacity of VAWTs to achieve faster steady angular speed. [ABSTRACT FROM AUTHOR]

Alterations to the unsteady wake dynamics imposed by abrupt changes in the phase angle between two pitching side-by-side foils are computationally examined at the Reynolds number of 1000 and 4000 and Strouhal number of 0.25 – 0.5. Four hybrid modes are considered in this study inspired by the swimming habits of red nose tetra fish and burst-and-coast swimming phenomenon. At the higher Strouhal number of 0.50, abrupt changes in the phase angle result in the formation and growth of a secondary vortex street between the two primary streets, which enable and maintain a split-wake configuration. Furthermore, phase switching alters pressure levels on the top and bottom surfaces of both foils to similar levels, which attribute to lowering the side-force. The growth rate of the secondary vortex street remains consistent for all four hybrid modes. At lower Strouhal numbers (0.25 – 0.4), however, the abrupt change in the phase angle converts the wake to a single vortex street. Thus, this indicates that the wake reactions for such cases in synchronization substantially change at lower Strouhal number. Although a different behavior of total side force production is observed at a lower Reynolds number for Strouhal number of 0.50, the wake dynamics implies that phase alterations act as a similar flow control mechanism to stabilize the wake. Finally, it is identified that the suspension of oscillations significantly limits the implications of initiation of oscillations on wake dynamics and performance following abrupt changes in the phase angle. [ABSTRACT FROM AUTHOR]

Understanding the connection between physiology and kinematics of natural swimmers is of great importance to design efficient bio-inspired underwater vehicles. This study looks at high-fidelity three-dimensional numerical simulations for flows over an undulating American eel with prescribed anguilliform kinematics. Particularly, our work focuses on why natural anguilliform swimmers employ wavelengths shorter than their bodylengths while performing wavy kinematics. For this purpose, we vary the undulatory wavelength for a range of values generally observed in different aquatic animals at Strouhal numbers 0.30 and 0.40. We observe that our anguilliform swimmer is able to demonstrate more suitable hydrodynamic performance for wavelengths of 0.65 and 0.80. For longer wavelengths, the swimmer experiences large frictional drag, which deteriorates its performance. The wake topology was dominated by hairpin-like structures, which are closely linked with the underlying physics of anguilliform swimming found in nature. [ABSTRACT FROM AUTHOR]

In this paper, we present a novel approach based on shifted Gegenbauer wavelets to attain approximate solutions of some classed of time‐fractional nonlinear problems. First, we present the approximation of a function of two variables u(x,t) with help of shifted Gegenbauer wavelets and then some novel operational matrices are proposed with the help of piecewise functions to investigate the positive integer derivative (Dx and Dt), fractional‐order derivative (Pxα and Ptβ), fractional‐order integration (Jxα and Jtβ) and delay terms (Dba and Ddc) of approximated function u(x,t). In order to transform the discussed nonlinear problem into linear problem Picard iterative scheme has been adopt. The current scheme converts the discussed highly nonlinear time‐fractional problem into system of linear algebraic equation the help of developed operational matrices and Picard idea. Analysis on the error bound and convergence to authenticate the mathematical formulation of the computational algorithm. We solve various test problems, such as the van der Pol oscillator model, generalized Burger–Huxley, neutral delay parabolic differential equations, sine‐Gordon, parabolic integro‐differential equation and nonlinear Schrödinger equations to show the efficiency and accuracy of linearized shifted Gegenbauer wavelets method. A comprehensive comparative examination shows the credibility, accuracy, and reliability of the presently proposed computational approach. Also, this scheme can be extended conveniently to other multi‐dimensional physical problems of highly nonlinear fractional or variable order of complex nature. [ABSTRACT FROM AUTHOR]

This study numerically investigates two-dimensional incompressible flows over elastically mounted foils, undergoing semi-passive and fully passive motion. In our strongly coupled numerical models, we employ linear and cubic stiffness and damping terms in order to examine their highly nonlinear response. The undamped model of the fully passive system exhibits various responses from periodic to chaotic and then to flip-over for the reduced velocity, ranging from 1 to 10. However, introducing the cubic damping terms causes a significant decrease in the magnitude of plunging and pitching amplitudes without affecting the onset point of bifurcation. Also, plunging and pitching amplitudes are altered significantly after the point of onset. Furthermore, the performance metrics of each passive system are computed for power generation applications to demonstrate that semi-passive system attain efficiency up to 20% for a pitching amplitude of 50° with the excitation frequency in the narrow range of 0.15 to 0.20. On the other hand for a fully passive system, the efficiency of around 34% is obtained near the onset point of a bifurcation with a low mass ratio and linear damping terms. However, introducing cubic damping terms causes degradation in efficiency to bring it down to 14 − 20 % for a wide range of reduced velocity. • Nonlinear dynamical models are strongly coupled with an in-house CFD-based solver. • Nonlinear damping and stiffness are introduced in the computational models. • For the semi-passive dynamical system, an efficiency of 20% is attained. • The fully passive system leads to an efficiency of up to 17% for low mass ratios. • Introducing cubic damping degrades the efficiency to 8%–9% for fully-passive foils. [ABSTRACT FROM AUTHOR]

Numerical simulations have been used to analyze the equivalence of pitching and plunging motions found in a flapping NACA0012 airfoil. Two-dimensional incompressible Navier–Stokes equations are solved at Reynolds number of 103 over a range of Strouhal numbers. A novel criterion based on the Strouhal number is proposed which provides equivalence of pitching and plunging motions using the length scale traversed by the trailing edge in each case. Aerodynamic coefficients are found to match well for both the kinematics in temporal as well as spectral domains. Detailed analysis provides contribution of different mechanisms, such as vortex shedding, added mass, interaction of leading and trailing edge vortices, in the overall aerodynamic forces produced by a pitching or plunging airfoil. Wake deflection is observed for a plunging airfoil at high Strouhal numbers resulting in a bias in the lift coefficient. Further investigation reveals the dominance of second harmonic of the fundamental frequency in the lift spectrum emphasizing the role of quadratic nonlinearity in the observed phenomenon. [ABSTRACT FROM AUTHOR]

Straight-bladed vertical-axis hydrokinetic turbines have a number of advantages for power extraction from river, tidal, and ocean currents. They are simple to construct, the generator does not need to be submerged, and they can have good power extraction efficiency. Among their potential disadvantages is that when the load is lost, or when they start with no load, they can reach high instantaneous blade speeds before returning to a "steady" runaway speed. These high speeds can cause high loads on the blades and must, therefore, be fully understood. This paper describes a computational investigation of the effect of inertia as a turbine starts from rest with no load and reaches runaway. Turbine inertia is modified by altering the blade density while the turbine geometry is not altered. At the minimum inertia, the added mass contributes significantly to the dynamics, and the highest overshoot occurs in blade speed. Increasing inertia damps the peak but slows the acceleration. The added mass depends on blade mass but is constant for the whole starting sequence and is independent of water speed. The results give guidance for the design of turbines to balance the minimization of the overshoot and starting time. • The influence of turbine inertia on the starting performance of a H-Darrieus hydrokinetic turbine are investigated. • A computational investigation of the effect of inertia as a turbine starts from rest with no load and reaches runaway. • The dependency of added mass on blade mass are investigated. • The turbine inertia has a significant effect on the starting performance of vertical-axis hydrokinetic turbines. • The added mass is constant during starting and independent of water speed. [ABSTRACT FROM AUTHOR]

Offshore wind turbines are becoming increasingly popular due to their higher wind energy harnessing capabilities and lower visual pollution. Researchers around the globe have been reporting significant scientific advancements in offshore wind turbines technology, addressing key issues, such as aerodynamic characteristics of turbine blades, dynamic response of the turbine, structural integrity of the turbine foundation, design of the mooring cables, ground scouring and cost modelling for commercial viability. These investigations range from component-level design and analysis to system-level response and optimization using a multitude of analytical, empirical and numerical techniques. With such wide-ranging studies available in the public domain, there is a need to carry out an extensive yet critical literature review on the recent advancements in offshore wind turbine technology. Offshore wind turbine blades' aerodynamics and the structural integrity of offshore wind turbines are of particular importance, which can lead towards system's optimal design and operation, leading to reduced maintenance costs. Thus, in this study, our focus is to highlight key knowledge gaps in the scientific investigations on offshore wind turbines' aerodynamic and structural response. It is envisaged that this study will pave the way for future concentrated efforts in better understanding the complex behavior of these machines. [ABSTRACT FROM AUTHOR]

Micro-power generators are increasingly becoming popular to meet the power requirements of micro-electromechanical systems, such as small sensors. One such resource of harnessing energy is through exploiting flow instabilities found in vortex-induced vibrations, flutter, etc. In this work, we numerically investigate the hydrodynamic performance of fully forced flapping foils with the goal to exploit their underlying physical mechanisms for the development of micro-power generators. We consider prescribed combination of plunging and pitching motions imposed to a NACA-0012 airfoil. We conduct a parametric study by varying the Strouhal number and the amplitude of the pitching angle to identify two operational flow regimes: power generation and thrust-producing propulsion using the feathering criterion. In the latter regime, the foil performs positive work on the surrounding fluid and therefore, the positive propulsive efficiency can be attained as long as the horizontal hydrodynamic force remains negative. For the power generation regime, the product of the lift force and plunging velocity is found mostly positive over the oscillating cycle, which indicates that the flowing fluid carries out work on the foil. The parametric study reveals that the foil can reach up to 42% power generation efficiency when setting the pitching amplitude in the range of 60° to 70°. For foils operating in the power generation regime, we present a piezoelectric energy harvester that can efficiently harness usable electric power from high fluid pressure regions. We identify two core locations based on the pressure field at which the attachment of piezoelectric patches can lead to significant energy harvesting. As such, the present study provides guidance for the design enhancement of micro-power generators relying on the interactions of flapping foils with the surrounding fluid. • Develop a computational model to simulate incompressible flows over moving bodies. • Analyze the hydrodynamic characteristics and propulsive efficiency of flapping foils. • Identify the flow regimes of flapping foils: power generation and thrust-producing propulsion. • Propose a design of a piezoelectric energy harvester. [ABSTRACT FROM AUTHOR]

This study introduces novel physics-based scaling laws to estimate the propulsive performance of synchronously pitching foils in various schooling configurations. These relations are derived from quasi-steady lift-based and added mass forces. Hydrodynamic interactions among the schooling foils are considered through vortex-induced velocities imposed on them, constituting the ground effect. Generalized scaling equations are formulated for cycle-averaged coefficients of thrust and power. These equations encompass both the pure-pitching and induced velocity terms, capturing their combined effects. The equations are compared to computational results obtained from two-foil systems, exhibiting foil arrangements over a wide range of parameter space, including Strouhal number (0.15 ≤ St ≤ 0.4), pitching amplitude ([Formula: see text]) and phase difference ([Formula: see text]) at Re = 1000-10 000. The individual contributions of pure-pitching and induced velocity terms to propulsive performance elucidate that solely relying on the pure-pitching terms leads to inadequate estimation, emphasizing the significance of the induced velocity terms. The validity of the approach is further assessed by testing it with three-foil and five-foil configurations, which displays a collapse of estimated and measured results. This indicates that the scaling laws are applicable to multi-foil arrangements.