Unsteady Wind Research Articles

Numerical investigation of the coupled interaction between an unsteady aerodynamic flow field and a water film coating on a circular cylinder

Rain–wind induced vibration is an aeroelastic phenomenon that occurs on the inclined cables of cable-stayed bridges and arises due to the interaction between the unsteady wind loading and the formation of water rivulets on the cable surface. A new numerical method has been developed at the University of Strathclyde to simulate the influence of the external flow field on the rivulet dynamics and vice versa. The approach is to couple a Discrete Vortex Method solver to determine the external flow field and unsteady aerodynamic loading, and a pseudo-spectral solver based on lubrication theory to model the evolution and growth of the water rivulets on the cable surface under external loading. Results of this coupled model are presented, to provide detailed information on the development of water rivulets and their interaction with the aerodynamic field. In particular, the effect of the initial water film thickness and the angle of attack in plane on the resulting rivulets are investigated. The results are consistent with previous full scale and experimental observations with rivulets forming on the upper surface of the cable only in configurations where rain–wind induced vibration has been observed. Additionally, the thickness of the lower rivulet is found to be self-limiting in all configurations. The results demonstrate that the model can be used to enhance the understanding of the underlying physical mechanisms of rain–wind-induced vibration.

Predicting wind-induced vibrations of high-rise buildings using unsteady CFD and modal analysis

This paper investigates the wind-induced vibration of the CAARC standard tall building model, via unsteady Computational Fluid Dynamics (CFD) and a structural modal analysis. In this numerical procedure, the natural unsteady wind in the atmospheric boundary layer is modeled with an artificial inflow turbulence generation method. Then, the turbulent flow is simulated by the second mode of a Zonal Detached-Eddy Simulation, and a conservative quadrature-projection scheme is adopted to transfer unsteady loads from fluid to structural nodes. The aerodynamic damping that represents the fluid–structure interaction mechanism is determined by empirical functions extracted from wind tunnel experiments. Eventually, the flow solutions and the structural responses in terms of mean and root mean square quantities are compared with experimental measurements, over a wide range of reduced velocities. The significance of turbulent inflow conditions and aeroelastic effects is highlighted. The current methodology provides predictions of good accuracy and can be considered as a preliminary design tool to evaluate the unsteady wind effects on tall buildings.

Influence of operating conditions on unsteady wind performance of vertical axis wind turbines operating within a fluctuating free-stream: A numerical study

A fluctuating free-stream in unsteady wind environment presents a more significant challenge in wind turbine performance. In this paper, a numerical method is presented to investigate the influence of operating conditions on Vertical Axis Wind Turbine (VAWT) of NACA00XX symmetric airfoils with 12% and 22% thickness in unsteady wind condition. The Computational Fluid Dynamics (CFD) numerical method was used to analyze the aerodynamic performance and physics of flow of the VAWT. The VAWT dynamic motion of blades was introduced by sinusoidally oscillating both VAWT blades. Using a validated CFD model, steady wind simulations at Umean=7.00m/s and 11.00m/s were conducted and the results predicted the Power Coefficient (CP) performance for the VAWT scale. The results derived in the numerical analysis show that, within fluctuating free-stream wind conditions, thicker airfoils are desirable. Overall maximum unsteady CP of VAWT with thicker blades reveals positive deviations if the tip speed ratio λ is slightly higher than λ of the steady maximum CP, while thinner blades maximum CP marginally drops from the steady maximum CP for the same λ range. Higher frequencies of fluctuation marginally improve the unsteady wind performance of both VAWT blade profiles. High fluctuation amplitudes reveal overall performance degradation on both VAWT blade profiles more than small fluctuation amplitudes. The findings lend substantially to our understanding of both the kinematic and aerodynamic behavior on VAWT scale blades operating in unsteady wind condition, and the flow physics that causes the behavior.

Investigating Instantaneous Wind-Driven Infiltration Rates using the CO2 Concentration Decay Method

Carbon dioxide has already been recognized as a potential tracer gas for estimating the mean air exchange rates of a room or building. The wind direction and mean wind velocity have also been identified as critical factors that affect the air infiltration. In this paper, the indoor CO2 concentration has been logged at three specific points in an office room for seven selected measurement-periods. The decay method was used to estimate the infiltration rates (ac/h). In parallel, an ultrasonic anemometer was used outdoors for monitoring wind characteristics such as the direction angle and the instantaneous velocity components. The results of the calculated ac/h varied from 0.32 h-1 to 0.75 h-1 from measurement to measurement and thus an investigation was carried out from the perspective of unsteady wind conditions. The wind direction angle and the relevant velocity components seem inadequate to entirely explain the differences in ac/h and thus the wind turbulence intensity was calculated and a spectral analysis of the wind measurements was applied. The corresponding power spectra Syy(f) were correlated to the mean ac/h of the room, giving a better understanding of wind-driven infiltration and depicting the role of the wind fluctuation frequency. In addition, a hypothesis of using the indoor spatial distribution of CO2 concentration as an airflow pattern tracer is presented respective to the location of the leakages. Finally, the local CO2 fluctuations are discussed and the respective standard deviations are presented as indicators of leakage detection.

Power Optimization for Adaptive Wind Turbine: Case Study on Islanded and Grid Connected

Wind energy is not commonly used in Indonesia although it has great potential. The main problem of wind power generation is the uncertain level of power that can be generated because of unsteady wind condition. To minimize that problem, this paper will estimate the wind speed using ANN with Levenberg-Marquardt backpropagation method, based on wind speed data from the previous studies. After obtaining the result of wind speed prediction, the value of pitch angle and turbine rotor speed will be established to make output wind turbine optimal. The method of optimization process is GA which the value of blade pitch angle and turbine rotor speed is spread out randomly. This random calculation will provide blade pitch angle and turbine rotor speed parameter when the power output is optimal. The case study in this research is using wind turbine in islanded and grid connected.

Detached eddy simulation of flow past an isolated inclined solar panel

Detached Eddy Simulations (DES) were carried out to analyze the influence of ground clearance and to estimate the unsteady wind loading on solar panels. A generic type of solar panel was considered at three different ground clearances. For the range of ground clearance values considered in the study, the results show that an increase in the clearance produces stronger vortex shedding fluctuations, higher shedding frequencies, and larger mean and unsteady wind loading. For the smallest ground clearance, an irregular vortex shedding was observed. The flow structures formed in the wake are analyzed using various indicators of turbulence characteristics including the vorticity and the λ2 criterion. The flow structures suggest a complex formation and subsequent shedding of vortices. The absence of regular vortex shedding at the smallest ground clearance and the presence of the same at the mid-ground clearance value suggests a reference for minimizing the unsteady wind loading of solar panels. Furthermore, the unsteady results reveal that the lower sections of the panel are more vulnerable to wind loading.

Wind Effects, Unsteady Behaviors, and Regimes of Propagation of Surface Fires in Open Field

The subject of this article concerns the unsteady effects (fire intensity, wind) upon the propagation and, more generally, the behavior of surface fires in open fields. The study focused on two sources of unsteadiness: the first one resulting from the regime of propagation (wind driven or plume dominated), which can affect greatly the behavior of the flame front and consequently the fire intensity, the second one resulting from the wind gusts associated with the conditions of flow of wind in real conditions. The study was based on numerical simulations, using a multiphase formulation, and on spectral analysis of the time evolution of the fire line intensity. The calculations were performed in 2D for a homogeneous vegetation layer (grassland) and for a large interval of wind conditions (10 m open wind velocity U10 ranged between 1 m/s and 25 m/s). The results have highlighted the link between the unsteady character of flame front behavior and the regime of propagation (plume dominated, wind driven). A particular interest was focused on the role played by two potential sources of instabilities, namely the Kelvin-Helmholtz instability (wind effects) and the thermo-convective instability (plume effects), upon the behavior of fires. A second set of simulations has been carried out using unsteady wind conditions, reproduced using sinusoidal boundary conditions for the streamwise velocity, with a frequency ranging between 0.5 Hz and 3 Hz.

Variable geometry wind turbine for performance enhancement, improved survivability and reduced cost of energy

AbstractThe conceptual design and proof‐of‐concept testing of a furling vertical axis wind turbine, suited to large‐scale offshore deployment, is described. Through the implementation of variable geometry capabilities, extreme storm loads can be reduced, and unsteady flow‐related fatigue loads can be minimized thereby reducing capital (structural) and maintenance costs. Moreover, annual power generation can be optimized in real‐time to account for unsteady wind effects related to weather and siting thus improving efficiency and annual power generation. Copyright © 2014 John Wiley & Sons, Ltd.

A two-degree-of-freedom aeroelastic model for the vibration of dry cylindrical body along unsteady air flow and its application to aerodynamic response of dry inclined cables

A two-degree-of-freedom aeroelastic model is proposed to study wind-induced response of bridge stay cables in the context of a rigid circular cylinder model oscillating in an arbitrary orientation with respect to the oncoming unsteady flow. Compared to the existing analytical models and wind tunnel studies, the proposed model can take into account the unsteady characteristics of natural wind and predict the evolving history of cable response. The validity of the proposed model is verified by the results from an earlier wind tunnel study. The rationality of defining a cable aerodynamic stability criterion using solely the Ur–Sc relation is discussed. Two different unsteady wind models are used to investigate the impact of flow unsteadiness on the wind-induced response of stay cables. A case study is presented to evaluate the aerodynamic behavior of a real bridge stay cable in unsteady/turbulent natural wind. Results show that while the emergence of critical Reynolds number regime is crucial for the onset of dry inclined cable galloping, the spanwise correlation of aerodynamic forces acting on a cable and the sustained duration of critical flow condition are the determining factors for the continuous growth of unstable cable response and the eventual occurrence of this type of aerodynamic instability.

Saltation transport rate in unsteady wind variations.

Wind flow in the atmospheric boundary layer is usually turbulent. The gusty wind significantly influences the saltation transport which is treated as equilibrium saltation. This study performs one-dimension numerical simulations of unsteady sand saltation to discuss the effects of parameters of periodical wind variations on saltation response and sand transport rate prediction. The results show that unsteady transport rates are larger than steady rates of equivalent mean wind velocity. The ratio of unsteady/steady transport rates increases with the increase of amplitude and frequency. For the average wind velocities much larger than the threshold value, the errors of transport rates predicted by unsteady and steady model are about 10%, while for a wind velocity slightly larger than saltation threshold, the errors will be more than 200%. The sand transport rates are not zero even though the average wind velocity equals (is even smaller than) the threshold value, whereas Q must be zero in the steady model. Finally, an unsteady transport rate prediction formula is proposed which takes mean velocity, fluctuating intensity and period as independent variables.

Unsteady Airflows and Their Impact on Small Unmanned Air Systems in Urban Environments

Existing unmanned air system platforms currently do not lend themselves well to autonomous operation within complex, highly variable aerodynamic environments. As such, there is a need for accurate high-fidelity urban airflow models to help reduce the risk of failure of urban unmanned air system surveillance and engagement missions. Although urban aerodynamics are exceptionally complicated because of complex interactions between geometry, physical conditions, and varying meteorology, high-fidelity computational fluid dynamics models exist that capture these interactions effectively. Using sufficient resolution, these large-eddy simulation models provide a viable means to characterize urban airflow environments when wind-tunnel testing and field trials are too expensive or impossible. This paper presents a simulation tool that captures unsteady aerodynamics of aircraft flight in an urban environment for the study of vehicle–environment interactions. By combining a high-resolution model of the terrain/buildings with a precomputed large-eddy simulation wind field throughout the urban environment and the closed-loop dynamics of a vehicle flying a desired trajectory, the simulation computes in real time the response of the vehicle linear and angular velocity to the unsteady wind field. The paper describes the mathematical development and implementation of several new environment–vehicle interaction models that could, through the unmanned air system simulation tool, revolutionize the understanding of these interactions in urban environments.

A Numerical Study on the Impact of Wind Gust Frequency on Air Exchanges in Buildings with Variable External and Internal Leakages

Wind-driven air infiltration has been recognized among the major reasons for energy loss in buildings, and the impact to energy efficiency under steady conditions has been reported and issued as part of many building codes. The nearly zero-energy building demand makes uncontrolled leakage paths even more undesired and creates the need for further investigation of their behavior under unsteady wind conditions. The present numerical study examines the role of wind gustiness on instantaneous infiltration rates of a low-rise building. For this purpose, two levels of gust frequency Ω have been simulated, expressed as a sinusoidal factor in the wind profile formula. In parallel, a ratio α is employed to represent seven different cases of external leakages distribution, while five scenarios of compartmentalization and internal leakages shows the impact of the latter on the dynamics of building air exchange rates. The results indicate that higher wind gustiness results in higher ACH, marking out gusts as a potential critical factor under unsteady climate conditions. The infiltration rates shown in relation to the leakage distribution ratio α provide arguments for the importance of the detailed detection of external leakages while the comparison of the different internal-volume-scenario highlights the key-role of internal leakages control towards a drastic reduction of infiltration rates.

Micro Air Vehicle’s Attitude Control Using Real-Time Pressure and Shear Information

It has been observed and experimentally validated that birds and bats can obtain airflow information with mechanoreceptors on their wing and body surfaces to achieve ultrastable, highly maneuverable, and energy efficient flights. Inspired by this observation, an attitude control method using the real-time flow (pressure and shear) information is studied. Previous research has demonstrated the effectiveness of using pressure information only for a micro air vehicle’s pitching control in both simulation and wind tunnel tests. In this paper, the new concept is extended to three-axis attitude controls. A new three-axis attitude motion model is developed, relating the pressure and shear information to attitude states. A nonlinear controller, which is robust with respect to bounded uncertainties, is implemented to track the attitude commands. A delta wing configuration with 20 sensor couples on the wing surface and one sensor couple on the rudder surface is proposed, designed, and tested in a simulated environment. The simulation results show that the pressure and shear information based attitude control approach can help micro air vehicles achieve accurate and robust attitude tracking control in both steady and unsteady wind conditions with a small amplitude lateral damping.

A wind tunnel study of aeolian sediment transport response to unsteady winds

Although moderate attention has been paid to the response of the aeolian mass transport rate to wind gusts, it is still unclear how the particle size and volumetric concentration affect this relation. Very little is known about the response time of the particle speed, and specifically, how the sensor scale and elevation affect measurements of this variable. The present study addresses this knowledge gap through a series of wind tunnel experiments in which a gusty wind was generated by programming the fan motor to adjust to a randomly selected rpm every 10s. Beds consisting of either medium or coarse sand were investigated through synchronous, co-located measurements of the local wind speed and particle speed/count rate obtained via a customized laser Doppler anemometry (LDA) system. The vertically integrated sand transport rate (Q) and the wind speed in the freestream were quantified using a passive sand trap and pitot tube, respectively. The results of the experiments indicate that the response of the aeolian transport system to wind gusts is generally faster in terms of the particle speed than the mass transport rate, while the degree of correlation is found to vary with the sensor elevation, as well as with the particle size and volumetric concentration. In essence, the coupling within the transport system is demonstrated to be strongly scale dependent.

Galloping of a Wind-Excited Tower Under Internal Parametric Damping

The effect of harmonic internal parametric damping (IPD) on the amplitude and the onset of the periodic galloping of a tower is investigated in the presence of steady and unsteady wind. The structure is modeled by a lumped single degree of freedom (sdof) equation and attention is focused on the cases where the unsteady (turbulent) wind activates the external excitation, the parametric one, or both. A perturbation analysis is performed to approximate periodic solutions and the effect of the IPD on the amplitude and the onset of periodic galloping is examined in different cases of loading. It is shown that the IPD substantially improves the reduction in the galloping amplitude for all cases of loading and it has no influence on the galloping onset.

Dynamic Stability of Euler Beams under Axial Unsteady Wind Force

Dynamic instability of beams in complex structures caused by unsteady wind load has occurred more frequently. However, studies on the parametric resonance of beams are generally limited to harmonic loads, while arbitrary dynamic load is rarely involved. The critical frequency equation for simply supported Euler beams with uniform section under arbitrary axial dynamic forces is firstly derived in this paper based on the Mathieu-Hill equation. Dynamic instability regions with high precision are then calculated by a presented eigenvalue method. Further, the dynamically unstable state of beams under the wind force with any mean or fluctuating component is determined by load normalization, and the wind-induced parametric resonant response is computed by the Runge-Kutta approach. Finally, a measured wind load time-history is input into the dynamic system to indicate that the proposed methods are effective. This study presents a new method to determine the wind-induced dynamic stability of Euler beams. The beam would become dynamically unstable provided that the parametric point, denoting the relation between load properties and structural frequency, is located in the instability region, no matter whether the wind load component is large or not.

Galloping of wind-excited tower under external excitation and parametric damping

This paper investigates the influence of combined fast external excitation and fast parametric damp- ing on the amplitude and the onset of galloping of a tower submitted to steady and unsteady wind flow. A lumped single degree of freedom model is considered and the cases where the turbulent wind activates either external ex- citation, parametric one or both are studied. The methods of direct partition of motion and the multiple scales are used to drive a slow dynamic near primary resonance. The influence of the combined excitations on the galloping is examined showing the beneficial effect on the amplitude and the onset of galloping when the excitations are introduced together.

A numerical investigation into the influence of unsteady wind on the performance and aerodynamics of a vertical axis wind turbine

Numerical simulations using RANS-based CFD have been utilised to carry out investigations on the effects of steady and unsteady wind on the performance of a wind tunnel scale VAWT. Using a validated CFD model, steady wind simulations at U∞=7m/s were conducted and results have shown a typical performance curve prediction for this particular VAWT scale.Very detailed understandings of the flow physics are discussed showing the importance of stall and flow re-attachment on the performance of the turbine with unsteady winds. The three blades of the VAWT experience very different flow regimes as they rotate during a single periodic oscillation of the wind speed. When the VAWT operates in periodically fluctuating wind conditions, overall performance slightly improves if the following are satisfied: the mean tip speed ratio is just above the λ of the steady performance maximum, the amplitude of fluctuation is small (<10%), and the frequency of fluctuation is high (>1Hz). Operation at a mean λ that is lower than λ for peak performance coefficient causes the VAWT to run in the λ band with deep stall and vortex shedding, to the detriment of the VAWT performance coefficient. Large fluctuations in wind speed causes the VAWT to run in λ conditions that are drag dominated, thus reducing the performance of the wind turbine. Within realistic conditions, higher frequencies of fluctuation marginally improve the performance of the VAWT.

Lift correction model for local shear flow effect on wind turbine airfoils

Wind turbines operate under various wind conditions in which turbulence virtually always exists. Therefore, unsteady wind turbine simulation methods to estimate wind loading in turbulent inflow conditions are very important for developing optimally designed wind turbines. Several methods have been developed for this purpose and are usually based on the blade element momentum theory (BEMT), which is used for calculation of the wind loading on turbine blades. The local shear flow effect induced by turbulence, however, is not explicitly considered in the popular BEMT-based simulations. Extreme situations can occur in a large-scale wind farm where the inflow field of a wind turbine may contain strong tip vortices generated from upstream turbines. In this study, the effects of idealized local shear flows around a two-dimensional airfoil, S809, on its aerodynamic characteristics were analyzed by CFD simulations. Various parameters including reference inflow velocity, shear rate, angle of attack, and cord length of the airfoil were examined. From the simulation results, several important characteristics were found. The shear rate in a flow causes some changes in the lift coefficient depending on its sign and magnitude, while the angle of attack does not have a distinguishable influence. The chord length and reference inflow also cause proportional and inversely proportional changes in the lift coefficient, respectively. Based on these observations, we adopted an analytic expression for the lift coefficient from the thin airfoil theory and proposed a lift correction model, which is easily applicable to the traditional load analysis procedure based on the BEMT.

On the quasi-instantaneous aerodynamic load and pressure field measurements on turbines by non-intrusive PIV

An experimental technique is presented to non-intrusively measure the quasi-instantaneous aerodynamic loads and surrounding pressure field for a turbine by using particle image velocimetry (PIV). The PIV measurements provide the velocity flow field needed to calculate the pressure field around the turbine using three different methods. In the first method, the quasi-instantaneous and mean pressure fields are obtained by solving the Poisson equation and by calculating the boundary conditions from the Navier–Stokes equations. In the second method, the pressure at the boundaries is determined by spatial integration of the pressure gradient. In the third method, the pressure is calculated using the Bernoulli equation. The experimental results are compared to aerodynamic load theoretical predictions from the Blade Element Momentum theory (BEM). An analysis of the experimental results showed the importance of the local acceleration, convective and pressure terms when calculating the forces and the pressure field in a stationary reference frame. Only the Poisson method includes all these terms, and had a small standard deviation between the calculated instantaneous forces. Furthermore, the Poisson method results are independent of the control volume size investigated while the other two experimental methods are affected. This experimental technique could be used to simultaneously replace instrumentation such as force balance and pressure taps while providing for the first time quasi-instantaneous information about the surrounding flow in any turbine immersed in an incompressible flow. In addition, it could be applied to evaluate unsteady wind loads and aerodynamic stall and also provide much needed information for validating computational studies.