Unsteady Wind Research Articles

Building a Maxey–Riley framework for surface ocean inertial particle dynamics

A framework for the study of surface ocean inertial particle motion is built from the Maxey–Riley set. A new set is obtained by vertically averaging each term of the original set, adapted to account for Earth’s rotation effects, across the extent of a sufficiently small spherical particle that floats at an assumed unperturbed air–sea interface with unsteady nonuniform winds and ocean currents above and below, respectively. The inertial particle velocity is shown to exponentially decay in time to a velocity that lies close to an average of seawater and air velocities, weighted by a function of the seawater-to-particle density ratio. Such a weighted average velocity turns out to fortuitously be of the type commonly discussed in the search-and-rescue literature, which alone cannot explain the observed role of anticyclonic mesoscale eddies as traps for marine debris or the formation of great garbage patches in the subtropical gyres, phenomena dominated by finite-size effects. A heuristic extension of the theory is proposed to describe the motion of nonspherical particles by means of a simple shape factor correction, and recommendations are made for incorporating wave-induced Stokes drift and allowing for inhomogeneities of the carrying fluid density. The new Maxey–Riley set outperforms an ocean adaptation that ignored wind drag effects and the first reported adaption that attempted to incorporate them.

Position control of a tail-sitter UAV using successive linearization based model predictive control

A successive linearization based model predictive control (SLMPC) method is proposed to control a vertical take-off and landing (VTOL) tail-sitter unmanned aerial vehicle (UAV) in hovering flight. The dynamic model of the vehicle is derived, including a low-fidelity aerodynamic model and a propulsion system model. The position controller is developed by a state–space prediction model augmented with estimated disturbance and feedback integration terms. The time-varying weight in the objective function is included and the velocity of vehicle is considered as reference to improve the performance. The system is first tested in a software-in-loop environment followed by the real-time indoor flight tests. The results demonstrate the vehicle can precisely follow a trajectory and stably hold position under unsteady wind disturbance

Identification of physical nonlinearities of a hybrid aeroelastic\u2013pressure balance

This study has presented an improved method for determining physical nonlinearities of weakly nonlinear spring-suspension system and successfully applied to a novel hybrid aeroelastic–pressure balance (HAPB) system used in wind tunnel, which can be used for simultaneously obtaining the unsteady wind pressure and aeroelastic response of a test model. A nonlinear identification method of equivalent linearization approximation was firstly developed on the basis of the averaging method of Krylov–Bogoliubov to model the physical nonlinearity of a weakly nonlinear system. Subsequently, the nonlinear physical frequency and damping were identified using a modified Morlet wavelet transform method and a constant variant method. Using these methods, the physical nonlinear frequency and damping of the HAPB system with a vertical test model were determined and were validated by a time domain method and the Newmark-beta method. Finally, the nonlinear mechanical frequency and damping of the HAPB system with inclined test models were determined in a similar way. This study has not only provided an identification method for determining physical nonlinearities of weakly nonlinear system, but presented the detail for developing a hybrid aeroelastic–pressure balance used in wind tunnel.

Measurement of unsteady wind loads in a wind tunnel: Scaling of turbulence spectra

Mismatch of turbulence spectra from the corresponding full-scale conditions is a common challenge in wind tunnel modelling of unsteady wind loads on small-scale structures, such as solar panels, heliostats and low-rise buildings. Understanding the effect of this mismatch on the unsteady wind loads is necessary for providing an accurate estimation of wind loads on full-scale structures. The correlation between the turbulence spectra and the unsteady wind loads in wind tunnel measurements is investigated in this study through measurement of unsteady lift and drag forces on horizontal and vertical flat plates. It was found through spectral analysis that the turbulent eddies in the range of reduced frequencies between 0.01 and 1 contributed the most to the unsteady wind loads. An approach for wind tunnel modelling was proposed in which the geometric scaling ratio of each model is determined based on the analysis of the turbulence power spectrum as a function of reduced frequency. The suitable geometric scaling ratio should be then chosen such that the turbulence spectrum as a function of reduced frequency is the closest match to that at full-scale for reduced frequencies between approximately 0.01 and 1.

Study on the Modes of Operation of a Multi-Machine Installations with Mechanical Reduction

The paper gives the description of the original wind electrical installation with mechanical reduction in which the output of vertical axis wind turbine with rather low rotation speed over multiplicator is distributed to a certain number of generators. The number of acting generators is determined by the output of actual operating wind stream at each moment. According to this constructive scheme, it is possible to provide effective and with maximum efficiency installation work in a wide range of wind speeds and under any schedule issued to the consumer of electricity. As there are no any experience in using such complexes, mathematical description of its main elements is given, namely windwheels, generators with electromagnetic excitation of magnetic electrical type, then their interaction with windwheel, and also the results of mathematical modeling of work system regimes under using the offered system of equations. The basis for the mathematical description of the main elements of the installation – synchronous generators – are the system of equations of electrical and mechanical equilibrium in relative units in rotating coordinates without considering saturation of the magnetic circuit. The equation of mechanical equilibrium systems includes torque and brake windwheel electromagnetic moments of generators with taking into account the reduction coefficients and friction. In addition, we specify the alternator rotor dynamics resulting from continuous torque of windwheel fluctuations under the influence of unsteady wind flow and wind speed serving as the original variable is modeled by a set of sinusoids. Model simplification is achieved by equivalization of similar generators and by disregarding these transitions with a small time constant. Calculation the installation with synchronous generators of two types of small and medium capacity taking into account the operational factors allowed us to demonstrate the logic of interactions in the main elements of the reported complex in the process of converting wind flow into the generated active and reactive power. We have shown the possibility of stable system work under changeable wind stream condition by regulating of the plant blade angle and with simultaneous varying of generator number of different types. All these are in great interest for project organizations and power producers.

A Large-Eddy Simulation-Based Assessment of the Risk of Wind Turbine Failures Due to Terrain-Induced Turbulence over a Wind Farm in Complex Terrain

The first part of the present study investigated the relationship among the number of yaw gear and motor failures and turbulence intensity (TI) at all the wind turbines under investigation with the use of in situ data. The investigation revealed that wind turbine #7 (T7), which experienced a large number of failures, was affected by terrain-induced turbulence with TI that exceeded the TI presumed for the wind turbine design class to which T7 belongs. Subsequently, a computational fluid dynamics (CFD) simulation was performed to examine if the abovementioned observed wind flow characteristics could be successfully simulated. The CFD software package that was used in the present study was RIAM-COMPACT, which was developed by the first author of the present paper. RIAM-COMPACT is a nonlinear, unsteady wind prediction model that uses large-eddy simulation (LES) for the turbulence model. RIAM-COMPACT is capable of simulating flow collision, separation, and reattachment and also various unsteady turbulence–eddy phenomena that are caused by flow collision, separation, and reattachment. A close examination of computer animations of the streamwise (x) wind velocity revealed the following findings: As we predicted, wind flow that was separated from a micro-topographical feature (micro-scale terrain undulations) upstream of T7 generated large vortices. These vortices were shed downstream in a nearly periodic manner, which in turn generated terrain-induced turbulence, affecting T7 directly. Finally, the temporal change of the streamwise (x) wind velocity (a non-dimensional quantity) at the hub-height of T7 in the period from 600 to 800 in non-dimensional time was re-scaled in such a way that the average value of the streamwise (x) wind velocity for this period was 8.0 m/s, and the results of the analysis of the re-scaled data were discussed. With the re-scaled full-scale streamwise wind velocity (m/s) data (total number of data points: approximately 50,000; time interval: 0.3 s), the time-averaged streamwise (x) wind velocity and TI were evaluated using a common statistical processing procedure adopted for in situ data. Specifically, 10-min moving averaging (number of sample data points: 1932) was performed on the re-scaled data. Comparisons of the evaluated TI values to the TI values from the normal turbulence model in IEC61400-1 Ed.3 (2005) revealed the following: Although the evaluated TI values were not as large as those observed in situ, some of the evaluated TI values exceeded the values for turbulence class A, suggesting that the influence of terrain-induced turbulence on the wind turbine was well simulated.

Computational Optimization of Adaptive Hybrid Darrieus Turbine: Part 1

Darrieus-type Vertical Axis Wind Turbines (VAWT) are promising for small scale decentralized power generation because of their unique advantages such as simple design, insensitive to wind direction, reliability, and ease of maintenance. Despite these positive aspects, poor self-starting capability and low efficiency in weak and unsteady winds deteriorate further development. Adaptive Hybrid Darrieus Turbine (AHDT) was proposed by the author in the past study as a potential solution to enhance low wind speed characteristics. The objective of the current research is to optimize the parameters of AHDT. AHDT integrates a dynamically varying Savonius rotor with a Darrieus rotor. A fully detailed 2D numerical study employing Reynold-Averaged Navier Stokes (RANS) is carried out to investigate the impact of the Darrieus rotor diameter (DR) on the Savonius rotor (DT) with regard to hybrid turbine performance. The power coefficient of the Darrieus rotor is evaluated when the Savonius rotor is in the closed condition (cylinder) of various diameters. The influence of Reynolds number (Re) on the torque coefficient is examined. Power loss of 58.3% and 25% is reported for DR/DT ratio of 1.5 and 2 respectively for AHDT with solidity 0.5 at 9 m/s. The flow interaction between the Savonius rotor in closed configuration reveals the formation of von Karman vortices that interact with Darrieus blades resulting in flow detachment. An optimum diametrical ratio (DR/DT) of 3 is found to yield the maximum power coefficient of the Darrieus rotor.

Smart Rotor With Trailing Edge Flap Considering Bend–Twist Coupling and Aerodynamic Damping: Modeling and Control

Aerodynamic damping and bend–twist coupling significantly affect the dynamic response of wind turbines. In this paper, unsteady aerodynamics, aerodynamic damping, and bend–twist coupling (twist-towards-feather) are combined to establish a smart rotor model with trailing edge flaps (TEFs) based on a National Renewable Energy Laboratory (NREL) 5 MW reference horizontal-axis wind turbine. The overall idea is to quantitatively evaluate the influence of aerodynamic damping and bend–twist coupling on the smart rotor and to present the control effect of the TEFs under normal wind turbine operating conditions. An aerodynamic model considering the dynamic stall and aerodynamic damping as well as a structural bend–twist coupling model with the influence of gravity and centrifugal force are incorporated into the coupling analysis. The model verification shows that the present model is relatively stable under highly unsteady wind conditions. Then, a robust adaptive tracking (RAT) controller is designed to suppress fluctuations in both the flapwise tip deflection and output power. The simulations show an average reduction of up to 63.86% in the flapwise tip deflection power spectral density (PSD) of blade 1 at the 1P frequency, with an average reduction in the standard deviation of the output power of up to 34.33%.

Corona Poling Conditions for Barium Titanate/Epoxy Composites and their Unsteady Wind Energy Harvesting Potential

The objective of this paper is to design and fabricate barium titanate (BTO)/epoxy composites, investigate the effect of poling conditions on their piezoelectric properties, and obtain their unsteady energy harvesting potential. The poling condition test deals with the effect of the poling conditions on the piezoelectricity of BTO/epoxy composites. The specimens are polarized by corona poling under different conditions, and their piezoelectric coefficient d33 is measured. The distribution of the piezoelectric particles in the matrix is studied using scanning electron microscopy (SEM), and the microstructure of the particle is investigated using X‐ray diffraction (XRD). A wind harvesting test is then performed, and the output voltage is measured for the composite sheets under unsteady air flow. This study opens the way for the development of flexible energy harvesting devices.

Assessment of an improved Random Flow Generation method to predict unsteady wind pressures on an isolated building using Large-Eddy Simulation

A wide variety of improved or modified Random Flow Generation (RFG) methods have recently been proposed for simulating the Atmospheric Boundary Layer (ABL) in a Large-Eddy Simulation (LES) framework. A distinct advantage of RFG methods over precursor-successor methods is the significant savings that can be made in terms of setup and computational costs. A review of the literature indicates that RFG methods are mostly evaluated in terms of velocity statistics and spectra. However, many applications in computational wind engineering ultimately aim to simulate the wind loads acting on a building. It would therefore be useful to evaluate the real capabilities of RFG methods for predicting unsteady wind pressures acting on various shapes of building. In this paper, one of the available RFG methods based on a summation of Fourier harmonic functions is assessed for various shapes of isolated building (a full-scale cube, a wind-tunnel cube, a high-rise building and a low-rise building) in terms of pressure statistics and spectra. Comparisons between measurements and simulations show that the RFG method tested here could be particularly relevant for industrial applications, provided that the inlet turbulence intensity profile is adapted.

Numerical Investigation of Passive Vortex Generators on a Wind Turbine Airfoil Undergoing Pitch Oscillations

Passive vortex generators (VGs) are widely used to suppress the flow separation of wind turbine blades, and hence, to improve rotor performance. VGs have been extensively investigated on stationary airfoils; however, their influence on unsteady airfoil flow remains unclear. Thus, we evaluated the unsteady aerodynamic responses of the DU-97-W300 airfoil with and without VGs undergoing pitch oscillations, which is a typical motion of the turbine unsteady operating conditions. The airfoil flow is simulated by numerically solving the unsteady Reynolds-averaged Navier-Stokes equations with fully resolved VGs. Numerical modelling is validated by good agreement between the calculated and experimental data with respect to the unsteady-uncontrolled flow under pitch oscillations, and the steady-controlled flow with VGs. The dynamic stall of the airfoil was found to be effectively suppressed by VGs. The lift hysteresis intensity is greatly decreased, i.e., by 72.7%, at moderate unsteadiness, and its sensitivity to the reduced frequency is favorably reduced. The influences of vane height and chordwise installation are investigated on the unsteady aerodynamic responses as well. In a no-stall flow regime, decreasing vane height and positioning VGs further downstream can lead to relatively high effectiveness. Compared with the baseline VG geometry, the smaller VGs can decrease the decay exponent of nondimensionalized peak vorticity by almost 0.02, and installation further downstream can increase the aerodynamic pitch damping by 0.0278. The obtained results are helpful to understand the dynamic stall control by means of conventional VGs and to develop more effective VG designs for both steady and unsteady wind turbine airfoil flow.

An approach of calculation and analysis of pulsating flow rate of single-sided ventilation due to unsteady wind pressure

Natural ventilation of buildings has received considerable attention aiming to reduce the energy consumption and CO2 emissions. The flow rate of a single-sided ventilation building due to unsteady wind pressure consists of two elements: mean ventilation rate and fluctuating ventilation rate. To design an efficient ventilation system, an accurate model that is capable of considering the variation of fluctuating flow needs to be developed. In this paper, a novel and simple model was developed that can predict the fluctuating ventilation rate due to the pulsating flow of single-sided, wind-driven natural ventilation in buildings. The model of the pulsating ventilation rate is derived based on the theory of wind engineering, the air compressibility, and inertia of air at the opening. The indoor and outdoor fluctuating wind pressure was constructed using a transfer function. It is found that there was an acceptable relative error of 13.9% compared with Haghighat’s model, which proved that the calculation method is reasonable. Finally, the model had also been used to analyze the effect of influencing parameters such as opening area, room volume, mean wind speed, turbulence intensity of the wind, and power spectrum density analysis on the pulsating ventilation rate.

Dynamic Behavior and Gust Simulation in an Unsteady Flow Wind Tunnel

Rapid oscillations of test-section flow velocity can be generated in a wind tunnel using flow-control louvers installed in the wind-tunnel circuit; however, control of the test-section velocity must still account for the dynamic behavior of the wind tunnel itself. In this paper, the dynamic behavior of a low-speed closed-circuit wind tunnel in response to dynamic louver excitation is examined using an experimentally validated method-of-characteristics modeling approach. The model is used to develop and test a method of controlling the louver motions to simulate longitudinal gusts with a von Kármán gust spectrum for length scales from 0.1 to 10 m. Results showing improvement of the wind-tunnel dynamic behavior using a Helmholtz resonator are also presented.

Effects of oscillation amplitude on motion-induced forces for 5:1 rectangular cylinders

While the 5:1 rectangular cylinder is a benchmark section, studied extensively, there are limited experimental studies commenting on any amplitude-dependence of its motion-induced forces. To this goal, such a cylinder is tested in wind tunnel through a forced vibration protocol for extracting distributed simultaneous pressure measurements under smooth flow conditions and for different heaving, pitching and coupled motion amplitudes. Ordinary flutter derivatives are extracted, and discrepancies due to oscillation amplitude are scrutinized. Spectral analysis is performed for the developing motion-induced forces, and it is found that torsional amplitudes above a threshold would increase higher harmonic frequency content. The phenomenon was also confirmed by means of Probability Density Functions and (PDFs) the Proper Orthogonal Decomposition (POD) of the unsteady wind force. In order to understand the link between the observed amplitude dependence and the flow field variation, the movement of the reattachment point on the cylinder surface is investigated by interpreting statistics of the recorded pressure measurements. The response in terms of instantaneous angle of attack is proven to be incompatible with respect to observations, since equal amplitudes of this variable result to different motion-induced forces.

Cross Wind Effects on Vehicle–Track Interactions: A Methodology for Dynamic Model Construction

An efficient and accurate model for the dynamic assessment of vehicle-track behavior subjected to cross wind actions is developed in this paper, where the wind–vehicle–track interaction is regarded as a coupled vibration system. First, a vehicle–track interaction model is proposed by taking the hypotheses of wheel/rail rigid contact and displacement complementarity. Unlike explicit force-based methods, the vehicle-track systems are wholly coupled by interaction matrices and load vectors, which are computationally more efficient than most of the existing methods and fairly accurate in low frequency vibrations. Then, the fluctuating cross winds are simulated by the fast Fourier transform technique from spectral representations with the consideration of spatial correlation of multipoint wind time histories and vehicle movement. The unsteady cross wind forces are obtained by introducing weighting function. Finally, a modeling framework, with the coupled interactions between cross winds, vehicle, and the tracks included, is built effectively. Through the validated dynamic model, the cross wind effects on vehicle-track dynamic performance can be fully revealed. Besides, it is concluded that the dynamic performance of vehicle-track systems differs significantly in various excitation modes, i.e., average cross wind, fluctuating cross wind, and track irregularities.

Wind turbine aerodynamics scale-modeling for floating offshore wind platform testing

Wave tank testing is a crucial step during the development of floating offshore wind concepts. Scaled prototypes are engineered to represent the most important phenomena related to the fluid-structure interaction. Nevertheless, in the case of floating wind turbines, the interaction between turbine aerodynamics and platform hydrodynamics presents important challenges that need to be considered. A new hybrid system focused on floating wind turbine tank testing is presented in this paper. It is based on a variety of fans, a multi-fan, which allows the high-fidelity reproduction of a wide range of wind turbine aerodynamics. One of the main advantages of the new methodology proposed, is the possibility to include other loading aside from thrust and wind turbine torque. The most important aerodynamic loads are represented. In fact, it has been demonstrated that wind turbine thrust can be captured with less than 0.5% error compared to the theoretical value. It is also possible to simultaneously model the thrust, torque and shear moments on the rotor within an error below 2%. The multi-fan system deploys a fast rate of change, up to 35N/s, guaranteeing the scaled dynamics. In unsteady wind speeds using 4% of atmospheric turbulence, 94% of the total energy is captured.

Wind Tunnel Test Investigation on Unsteady Aerodynamic Coefficients of Iced 4‐Bundle Conductors

Iced conductor motion is induced by the aerodynamic instability of these conductors. The unsteady aerodynamic characteristics are different from the steady aerodynamic characteristics. The unsteady aerodynamic coefficients of typical iced conductors’ models under torsional motion are measured by the unsteady wind tunnel test. The unsteady aerodynamic coefficients of crescent‐shape and sector‐shape iced 4‐bundle conductors under different torsional motion frequencies, wind velocities, and ice thicknesses are obtained. Wind test results show that there are significant differences between the unsteady and steady aerodynamic coefficients. The unsteady aerodynamic coefficients curve is a loop which is different from the steady aerodynamic coefficients. In addition, the obvious differences exist between unsteady aerodynamic coefficients of crescent‐shape and sector‐shape iced bundle conductors. Critical parameters, including torsional motion frequencies, wind velocity, ice shape, and ice thickness, have significant influences on unsteady aerodynamic coefficients. It shows that the wind tunnel experiment results are able to provide necessary data for the investigation of iced bundle conductor motion and their prevention techniques.

Corona Poling Conditions for Barium Titanate/Epoxy Composites and their Unsteady Wind Energy Harvesting Potential

Corona Poling Conditions for Barium Titanate/Epoxy Composites and their Unsteady Wind Energy Harvesting Potential

Wind Tunnel Tests on Aerodynamic Forces of Road Vehicles Under Unsteady Wind Conditions

This paper describes the results obtained from a wind tunnel test on the aerodynamic characteristics of road vehicles subjected to unsteady wind. In order to study the aerodynamic response of vehicles under atmospheric fluctuations, the velocity of the wind has been simulated in a pulsating state, where vehicles at a constant speed are driving in air with large wind oscillation. On the other hand, we consider three types of vehicles: two types of the Ahmed model, with slant angles of 0° and 30°, and the basic rectangular-prism model. The effect of wind oscillation on the drag and lift forces acting on the vehicle models under a pulsating wind condition has been extracted by comparing it to the forces under steady wind conditions. The oscillation of the wind exerts a significant effect on the fluid forces, and the drag and lift forces change with time in a peculiar way, depending on the body shape of the vehicle.

Reduced-order modeling of pressure excitation on automotive front side window

The pressure excitation on automotive front side window acts as an indicator of the unsteady flow and wind noise in the front side window region. The complex unsteady flow in this area generates a wider range of vortex structures resulting in the nonhomogeneous and complex pressure excitation on side glass. The description of the pressure field, which can consider the nonhomogeneous in the exact space, is needed to better solve the vibration and noise problems. The turbulent pressure excitation on side window was achieved by the incompressible improved delayed detached-eddy simulation, which is validated by the wind tunnel experiment. The reduced-order modeling methods, including the proper orthogonal decomposition and the dynamic mode decomposition, were employed to describe the pressure excitation on side glass. The dynamic mode decomposition modes separate the pressure excitation into three parts just corresponding to three main flow structures in the front side window region: the vortex shedding of the side mirror (lower frequency range), pedestal vortex (middle frequency range), and A-pillar vortex (higher frequency range). The turbulent pressure excitation generated by the vortex shedding of the side mirror contributes most of the vibration of the side glass and then the wind noise in the cabin in the low-frequency range. (The characteristic frequency is around 60 Hz, which is close to both the measured coincidence frequency and the theoretical derivation value.) The dynamic mode decomposition analysis with the unique and exact frequency for each mode, considering the nonhomogeneous of the pressure excitation, has potential to understand and solve the vibration and wind noise problems.