Spanwise Correlation Research Articles

3D LES numerical investigation of vertical vortex-induced vibrations of a 4:1 rectangular cylinder

The aerodynamic characteristics of rectangular cylinders provide valuable references for many engineering structures. Aiming to investigate the characteristics of vortex-induced vibration (VIV) of this kind of bluff section cylinder, the three-dimensional (3D) large-eddy simulations (LES) were conducted on the flow around an elastically mounted rectangular cylinder with an aspect ratio of 4. A novel method for creating 3D meshes was introduced and validated by comparing the aerodynamic parameters and vibration amplitudes with those of experimental data. The mechanism of VIV was then investigated by examining the evolution properties (including the mean and fluctuating pressure, predominant frequency, and spanwise correlation) of distributed pressure at various amplitude-dependent VIV stages. Additionally, the contributing/inhibiting effect of wind load acting on specific regions around the cylinder’s surface on VIV was clarified by investigating the energy input or output. The results show that the vibration amplitude affects both static and fluctuating pressures, with a significant variation in these pressures observed during the amplitude-ascent stage. The distributed aerodynamic force is mainly due to self-excited forces resulting from the cylinder vibration during amplitude-ascent stage, whereas both self-excited forces and forcing forces are prominent during the amplitude-descent stage. Notably, the spanwise correlation coefficient displays its minimum value at the peak amplitude, even lower than that in the non-lock-in region, with the highest value occurring at the onset of the amplitude-ascent stage. The energy analysis revealed that the wind loads acting on the upstream of the cylinder's surface tend to enhance VIV, while those acting downstream tend to inhibit it.

Wake Three-Dimensionality of Profiled Blunt Trailing Edge Bodies with Varying Chord Length

This paper investigates experimentally the structure of turbulent blunt trailing edge body wakes with varying boundary-layer thicknesses controlled through the freestream velocity and the chord length. The intrinsic effect of transition to turbulence on the vortex-shedding frequency, strength, and its three-dimensional structure is explored. At large-scales, the vortex shedding is characterized by significant phase variations along the span, which vary stochastically and are punctuated by vortex dislocations when the phase differences grow large. These features of the vortex shedding are quantitatively examined in a streamwise–spanwise plane of the wake with particle image velocimetry. When the point of transition shifts upstream due to an increasing Reynolds number, greater phase variations in the vortex shedding along the span and more frequent dislocation events are produced. These changes are linked to the spanwise correlation of the streamwise velocity in the wake. In the turbulent boundary-layer regime, it is found that the phase drift does not change significantly. The decline in the spanwise correlation with increasing boundary-layer thickness in this regime is instead linked to the relative strength of the vortex shedding compared to the random turbulent fluctuations in the wake.

Large-eddy simulation of vortex-induced vibration of a circular cylinder at Reynolds number 10 000

The canonical scenario of cross-flow vortex-induced vibration (VIV) of a circular cylinder in the turbulent regime, which has been studied by several physical experiments in the literature, is reexamined in this study through high-fidelity large-eddy simulations (LES) at a Reynolds number 104. The VIV response (including vibration amplitude and frequency) and hydrodynamic coefficients predicted by the present LES agree with the experimental results better than previous numerical attempts. In addition, several phenomena reported by previous experimental studies are confirmed numerically for the first time. After validating against the experiments, new VIV characteristics and physical mechanisms are explored with confidence. First, a collective analysis on the frequency spectra of the displacement, lift, and velocity signals provides a complete picture of the frequency response of the system. In contrast, the use of a single signal may miss certain aspects of the frequency response, so that caution should be exercised. Second, spanwise correlation of primary vortex shedding is examined, where relatively low correlations in the upper and lower branches are likely because the vortex shedding patterns involve complex vortex generation and interaction. Third, the effect of mass ratio (m*) of the cylinder on the VIV response is analyzed with a range of m* (=1.4–3.4) relevant to cylindrical structures used in offshore engineering (such as subsea pipelines). The variations in the amplitude response, frequency response, and hydrodynamic coefficients with m* and reduced velocity are examined in detail. The present results suggest that a lighter pipeline is more susceptible to the onset of VIV.

Aerodynamic force correlation of a foundational streamlined box girder under vortex-induced vibration in turbulent flow fields

Study on the vortex-induced vibration (VIV) performance of structures under vibration states in turbulent flow fields is closer to the essence of bridge VIV in real service environments, which has been lacking in existing research. The key parameter used in the VIV performance evaluation, vortex-induced force (VIF) correlation, was focused on in this paper. A foundational streamlined box girder was selected, and free vibration sectional model tests were conducted in turbulent flow fields using synchronous pressure-vibration measurement. The results indicate that turbulence have an impact on the VIV performance of streamlined box girder. The presence of turbulence is likely to decrease the VIV amplitude, but sometimes it may also increase it. The phase difference between the leading and trailing edges of the structural upper surface is approximately 100°, and the vortices on the upper surface do not drift continuously. The streamwise correlation of VIFs decreases to varying degrees with the increase in turbulence intensity. The aerodynamic force spanwise correlation when the structure is stationary is significantly weaker than when it is in a vibrational state. When the structure experiences sustained VIV, the spanwise correlation of VIFs decreases with increasing turbulence intensity, increases with increasing amplitude, and decreases with increasing spanwise distance and tends to be constant at a certain distance. Finally, an extended model based on Ricciardelli's correlation function was proposed, which can better describe the VIF spanwise correlation of streamlined box girders under different amplitudes.

Study on the vortex-induced vibration and flow control of ribbed circular cylinder

This work investigates the vortex suppression performance and mechanism of ribs on high-quality ratio cylinders. Through wind tunnel tests and numerical simulations, the surface wind pressure distribution characteristics and flow separation phenomena of different ribbed cylinders are explored, and the spanwise correlation and nonlinear vibration characteristics of vortex-induced vibrations of ribbed cylinder models are analyzed. The main conclusions are as follows: ribs change the position of the boundary layer separation point, and the difference in size of left and right separated vortices causes a pressure jump phenomenon, altering the wind pressure distribution of the segment model and reducing the wind pressure, resulting in an increase in the locked wind speed of the ribbed cylinder. Complex separated vortices form behind the ribs, affecting the size of the wake vortex and reducing the stability of the segment model at locked wind speeds. Cylinders with four ribs exhibit good vortex-induced vibration suppression performance at 0° and 45° positioning angles. In addition, the cylinder with four installed ribs cylinders exhibits two locked wind speed regions, each showing different motion states: at the primary locked wind speed, they mainly demonstrate quasi-periodic vibrations and degraded quasi-periodic vibrations, while at the secondary locked wind speed, primarily in a chaotic state dominated by high-frequency harmonic components. These research findings have significant implications for future studies and practical engineering applications.

Direct-Noise of an Ultrahigh-Bypass-Ratio Turbofan: Periodic-Sector vs Full-Annulus Large-Eddy Simulations

The tonal and broadband noise of an ultrahigh-bypass-ratio fan stage, which has been developed at École Centrale de Lyon, is studied using large-eddy simulations (LES). Wall-modeled LES of a periodic sector and a 360-deg full fan stage have been performed at approach conditions, which is a relevant operating point for aircraft noise certifications. The fan noise is directly obtained from the fully compressible LES, using a well-refined unstructured mesh, and compared with state-of-the-art analytical models. The impact of the periodic boundary conditions, which are often used for high-fidelity simulations of turbofan engines, is assessed. The results from the 360-deg and periodic-sector LES are compared from aerodynamic and acoustic perspectives, including an analysis of the mean and turbulent flow quantities and sound-pressure spectra. Aerodynamic parameters show similar results for both configurations. However, the fan blade loading is slightly reduced in the 360-deg- LES near the blade tip. Acoustically, lower sound power levels at the intake and exhaust sections of the fan stage are obtained in the 360-deg LES, when compared to the periodic sector LES, particularly at low and middle frequencies. This can be associated with lower coherence levels in the fan wakes and smaller spanwise correlation lengths at the trailing edge of the blades. The modal content of the acoustic field has also been analyzed in detail and shows that the periodic sector LES cannot correctly simulate the modal content of the fan noise.

Numerical investigation on cavitation and induced noise reduction mechanisms of a three-dimensional hydrofoil with leading-edge protuberances

In this work, a National Advisory Committee for Aeronautics 66 hydrofoil with leading-edge protuberances is designed. The large eddy simulation combined with the Schnerr–Sauer cavitation model is used to obtain a satisfactory result as compared with the experimental measurement, integrating the permeable Ffowcs Williams–Hawkings equation for cavitation-induced noise analysis. It is found that the special leading-edge geometric structure deflects the incoming flow, creating two counter-rotating streamwise vortices at the peak shoulders. These lead to upwash and downwash effects and alter the pressure distribution on the suction side. The low pressure localized in the trough facilitates the advancement of the leading-edge cavitation while severely limiting the spanwise development of the cloud cavity, shortening the cavitation evolution by about 20% and reducing the maximum cavitation volume by about 35%. Analysis using the vorticity transport equation indicates that different vorticity transport equation splitting terms play dominant roles at different stages of cavitation evolution. Although the cavitation induces disturbances in the primary vortex, the effect is limited. Acoustic simulation shows that the bionic structure can reduce the total sound pressure level by 7.8–8.3 dB. The spherical noise reduction is not as effective as expected due to the similar cavitation volume acceleration processes of the two hydrofoils. However, the pressure fluctuation caused by the collapse of the cloud cavity is reduced by cavitation suppression, which reduces the linear noise. In addition, the protuberances suppress the generation of large-scale vortex systems and transform them into smaller ones, resulting in reduced spanwise correlation and coherence of the shedding vortices. This is a critical factor in noise reduction. Finally, we hypothesize that the unstable noise reduction is related to the streamwise vortices in the trough regions. These vortices increase the momentum exchange within the boundary layer, affecting its stability and weakening the acoustic feedback loop.

Effect of fluid motions on finite spheres released in turbulent boundary layers

This paper extends the work in Tee et al. (Intl J. Multiphase Flow, vol. 133, 2020, 103462) to investigate the effect of turbulent fluid motions on the translation and rotation of lifting and wall-interacting spheres in boundary layers. Each sphere was released from rest in smooth-wall boundary layers with $Re_\tau =670$ and 1300 ( $d^+=56$ and 116, respectively) and allowed to propagate with the incoming fluid. Sphere and surrounding fluid motions were tracked simultaneously via three-dimensional particle tracking velocimetry and stereoscopic particle image velocimetry in streamwise–spanwise planes. Two-point correlations of sphere and fluid streamwise velocities yielded long positive regions associated with long fast- and slow-moving zones that approach and move over the spheres. The related spanwise correlations were shorter due to the shorter coherence length of spanwise fluid structures. In general, spheres lag the surrounding fluid. The less-dense lifting sphere had smaller particle Reynolds numbers varying from near zero up to 300. Its lift-offs coincided with oncoming fast-moving zones and fluid upwash. Wall friction initially retarded the acceleration of the denser sphere. Later, fluid torque associated with approaching high-velocity regions initiated forward rotation. The rotation, which was long-lived, induced sufficient Magnus lift to initiate repeated small lift-offs, reduce wall friction, and accelerate the sphere to higher sustained velocity. Particle Reynolds numbers remained above 200, and vortex shedding was omnipresent such that the spheres clearly altered the fluid motion. Spanwise fluid shear occasionally initiated wall-normal sphere rotation and relatively long-lasting Magnus side lift. Hence the finite sphere size contributed to multiple dynamical effects not present in point-particle models.

Correlation and modal analysis techniques for the study of the VIV response of a twin-box deck based on 3D LES simulations

AbstractTwin-box decks are prone to vortex-induced vibration (VIV) at moderate wind speeds, and previous studies have shown a complex interplay between the flow and the two parallel box girders. OpenFOAM is used to model the aerodynamic and aeroelastic responses of the twin-box deck of the Stonecutters Bridge by means of 3D LES simulations. Two cases are thoroughly analysed under smooth incoming flow: (i) static deck and ii) heave peak amplitude response at VIV excitation. The computational data provided by the CFD simulations have been exploited aiming at better understanding the complex aerodynamic and aeroelastic phenomena taking place. First, spanwise correlation analyses are applied to understand the level of organisation of the forcing flow actions, and identify the regions of the deck affected by different flow structures. Second, three different mode decomposition techniques (POD, SPOD and DMD) are applied to the time-dependent pressure distributions, revealing the pivotal role played by the leeward box in the twin-box deck response under wind action. The modal analyses are of utmost interest in the development of Reduced Order Models (ROM) for the VIV response of twin-box long-span bridges.

Influence of wind directionality on the three-dimensional distortion of buffeting force on a streamlined bridge deck: Insight into the pylon interference effect

The present work aims to experimentally investigate the influence of wind directionality on the three-dimensionality of buffeting force on a streamlined bridge deck, considering the pylon interference effect (PIE). The pylon has an obvious impact on the downstream flow structure under the skew wind, influencing the aerodynamic loads acting on the bridge deck. The results show that the most dangerous wind direction exists at 30° for the downstream bridge deck, indicating the invalidation of the traditional “cosine rule.” It indicates that the lift force will be amplified due to the PIE, depending on the yaw angle of the oncoming flow. When turbulence passes through the pylon, the large-scale eddies in the wake region will be broken into small-scale eddies, attenuating the integral length scale in comparison with that of the free-stream turbulence. Consequently, the lift spectrum and corresponding three-dimensional aerodynamic admittance (AAF) increase in the high-frequency domain, resulting in the decrease in the spanwise correlation under the skew wind. Notably, the two-dimensional (2D) AAF indicates that the 2D distortion effect of turbulence on the lift increases in the wake region, causing the 2D AAF to decay rapidly in the high-frequency domain.

Trailing-edge far-field noise and noise source characterization in high inflow turbulence conditions.

Airframe noise currently is a bottle neck in various applications, e.g., wind energy, maritime applications, and aircraft. Airframe noise is significantly increased by the presence of inflow turbulence. High inflow turbulence influences the boundary layer and wall-pressure fluctuations close to the trailing edge of airfoils. In this research, measurements of boundary layer and wall-pressure fluctuations near the trailing edge of an airfoil are conducted to investigate how the inflow turbulence affects the trailing-edge noise generation mechanism. Far-field noise measurements of additional three airfoils are shown to understand the role of the airfoil geometry in the dominant noise source for the cases of inflow turbulence and to generalize the observed increase in trailing-edge noise. Inflow turbulence leads to an increase in both the wall-pressure spectrum and spanwise correlation length. Trailing-edge noise increases due to the inflow turbulence in the entire frequency range at least 2 dB up to more than 15 dB for all the cases. The contribution of leading- and trailing-edge noise to the total noise varies with the airfoil geometry and inflow velocity, with the trailing-edge noise dominating in a larger frequency range for the thickest airfoil and for lower velocities.

Examining error-contaminated, long-span bridge buffeting vibration by Stochastic Differential Equations

This paper summarizes the development of an analytically-based and numerically-implemented algorithm for the analysis of uncertainty propagation in aeroelastic systems. The term “uncertainty” is utilized to describe either simplifications in the modeling of the fluid–structure interaction mechanism or the effect of experimental errors, for example emerging during wind-tunnel tests. The algorithm is applied to study the dynamic response of long-span bridges. This study provides, by theoretical derivations, estimation of the stationary joint-probability distribution of the generalized bridge response, accounting for perturbations in the buffeting loads. The formulation includes physical states, fluid–structure interaction states and a random variable, which quantifies the errors in the span-wise correlation of the buffeting deck load. A 1200m span bridge is used for illustration of the numerical algorithm.

Increase of trailing-edge noise due to inflow turbulence

Altough the regulatory imposition of low-noise limits, airframe noise currently limits the design space of several applications, such as, wind energy, marine, and aeronautics. The noise of those applications is significantly increased by the presence of inflow turbulence. It has been believed that only the leading-edge component is increased. However, recent results show that the trailing-edge noise is also increased because of the free-stream turbulence penetration in the airfoil boundary layer. This research measures the increase of trailing-edge noise due to inflow turbulence for NACA 63018 and NACA 0008 airfoils. Noise measurements were conducted for velocities ranging from 15 to 40 m/s, with and without inflow turbulence. A rod installed upstream of the airfoils generated an inflow turbulence of ≈ 12%. The wall-pressure fluctuations were measured close to the trailing edge to analyze the near-field phenomenon. The inflow turbulence increases the wall-pressure spectrum level in the entire frequency range and the spanwise correlation length in the low-frequency range. The trailing-edge noise increases due to the inflow turbulence in the entire frequency range at least 2 db up to more than 15 dB for all the cases. The contribution of leading- and trailing-edge noise to the total noise varies with the airfoil geometry, with the trailing-edge noise dominating in a larger frequency range for the thickest airfoil.

The influence of surface roughness on postcritical flow over circular cylinders revisited

This work investigates the effect of surface roughness on cylinder flows in the postcritical regime and reexamines whether the roughness Reynolds number ( $Re_{k_s}$ ) primarily governs the aerodynamic behaviour. It has been motivated by limitations of many previous investigations, containing occasionally contradictory findings. In particular, many past studies were conducted with relatively high blockage ratios and low cylinder aspect ratios. Both of these factors appear to have non-negligible effects on flow behaviour, and particularly fluctuating quantities such as the standard deviation of the lift coefficient. This study employs a 5 % blockage ratio and a span-to-diameter ratio of 10. Cylinders of different relative surface roughness ratios ( $k_s/D$ ), ranging from $1.1\times 10^{-3}$ to $3\times 10^{-3}$ , were investigated at Reynolds numbers up to $6.8 \times 10^5$ and $Re_{k_s}$ up to 2200. It is found that the base pressure coefficient, drag coefficient, Strouhal number, spanwise correlation length of lift and the standard deviation of the lift coefficient are well described by $Re_{k_s}$ in postcritical flows. However, roughness does have an effect on the minimum surface pressure coefficient (near separation) that does not collapse with $Re_{k_s}$ . The universal Strouhal number proposed by Bearman (Annu. Rev. Fluid Mech., vol. 16, 1984, pp. 195–222) appears to be nearly constant over the range of $Re_{k_s}$ studied, spanning the subcritical through postcritical regimes. Frequencies in the separating shear layers are found to be an order of magnitude lower than the power law predictions for separating shear layers of smooth cylinders.

Spanwise correlation of unsteady lift due to vortex shedding on the square cylinder: Perspectives provided by a time–frequency analysis

Spanwise correlation of unsteady lift due to vortex shedding is of particular interest for slender structures. In this paper, Hilbert transform and Wavelet transform were applied as time–frequency analysis tools to the sectional lift of a square cylinder. For Wavelet transform, the complex Morlet wavelet and the generalized Morse wavelet were considered as the mother wavelets, and the synchrosqueezing technique was used for post-processing. At high Reynolds number and in smooth flow, Hilbert transform and Wavelet transform mutually confirm that vortex shedding is characterized by an instantaneous shedding frequency fluctuating in real-time. As an example, at Reynolds number 3.9×104, the results of Wavelet transform suggest a standard deviation of the instantaneous shedding frequency of at least 4% about the conventional Strouhal frequency. The extracted instantaneous frequency is found stochastic, showing reasonably good Gaussian properties for its probability density function. The instantaneous frequencies simultaneously extracted at two cross sections of a spanwise separation are correlated, being the correlation coefficient lower than that calculated with fluctuating lifts. The shape of the coherence of the instantaneous frequency is found quite similar to that of turbulence, and the magnitude overall decreases with the increase of spanwise separation. The fluctuation of the instantaneous vortex shedding frequency, especially the low-frequency part (lower than about one-half of the Strouhal frequency), is believed critical in determining the spanwise correlation of unsteady lift, which was confirmed reconstructable via the extracted instantaneous shedding frequency. Finally, attention in this paper has also been paid to a reliable extraction and quantification of the concerning time–frequency characteristics. For instance, tests of the used time–frequency tools first on the purposely prepared artificial signal suggest, that the high-frequency fluctuation in the instantaneous vortex shedding frequency (higher than the Strouhal frequency) can actually not be reliably extracted.

On the surface flow patterns and associated aerodynamics of an asymmetrical flat-box section

In this study, wind tunnel experiments were carried out in smooth flow to examine the nexus between flow patterns around a flat-box section and its aerodynamics with the variation of the wind angle of incidence (α). The model cross section closely represents the overall asymmetric geometry and attendant aerodynamics of a class of flat-box decks frequently employed in long-span bridges. To explore the flow around and associated aerodynamic properties of this model that are distinct from a typical symmetric section (e.g., rectangular), detailed wind tunnel tests involving smoke-wire based flow visualizations and synchronous multipoint pressure measurements were carried out. Experimental results were analyzed to identify flow patterns, pressure distributions and associated Strouhal numbers, power spectral densities, aerodynamic force coefficients, proper orthogonal decomposition (POD) analyses, and span-wise correlation. Three typical flow patterns previously observed around symmetric bluff bodies with varying α were also observed in the asymmetric cross section, i.e., the trailing-edge vortex shedding (TEVS), the impinging leading-edge vortices (ILEV), and the alternate-edge vortex shedding (AEVS). However, due to the chordwise asymmetry of the section, the three flow patterns identified are distinct from the classical ones associated with symmetric sections in terms of flow separation points, the static “stall” angle of the section, and characteristics of leading-edge vortices. The roles played by these flow patterns in the aerodynamics of the flat-box section have been identified. It is observed that, if the ILEV or AEVS governs the flow topology, the aerodynamic force coefficients vary nonlinearly with the increase in α, in contrast with the linear variation in the TEVS-dominant case. For cases in which the TEVS dominates, the POD mode participation factors of the fluctuating pressure field display energy in higher modes. In this case, the reconstruction process requires that higher POD modes are retained. Finally, in the TEVS flow pattern, a weaker spanwise correlation of the fluctuating forces is noted as compared to the ILEV and AEVS-type flows. This study is limited to the fundamental examination of asymmetric section aerodynamics in smooth flows, which should be extended in case the influences of turbulence and aeroelastic effect are of interest.

Investigations on lock-in vortex-induced vibration of an airfoil at a high angle of attack based on detached eddy simulation

Large-scale modern wind turbines at standstill are prone to vortex-induced vibration (VIV). In this study, coupled fluid–solid dynamics of the wind turbine airfoil at a 90° attack angle are performed using the detached eddy simulation. The fully developed vibration responses with different structural dampings are explored in detail. The frequency lock-in regime is determined, and the corresponding phase differences between the lift and displacement are presented with the Lissajous curve. The dominant surface pressure mode and wake flow exhibit significant three-dimensional flow characteristics in unlock-in conditions, while a strong spanwise correlation in lock-in conditions is detected. The pressure fluctuation on the suction side in the lock-in state is observed to be more significant than in the unlock-in state. The effect of the distributed airfoil surface pressure on VIV is evaluated by considering the contribution value and the cyclic aerodynamic work density. With the decrease in structural damping, the aerodynamic work near the leading edge gets enhanced and the negative work region is reduced, leading to a higher amplitude of VIV. The beat vibration and hysteresis behavior at the critical reduced velocity are also analyzed in both the time domain and frequency domain.

Wall-pressure spectra, spanwise correlation, and far-field noise measurements of a NACA 0008 airfoil under uniform and turbulent inflows

This paper discusses unique measurements of wall-pressure fluctuations (WPFs), spanwise correlation length, and far-field noise of a NACA 0008 airfoil subjected to uniform, non-turbulent and turbulent inflows. From these measurements, the near-field characteristics of trailing-edge (TE) and leading-edge (LE) noise of a thin airfoil are analyzed. The competing nature of the LE and TE noise mechanisms of an airfoil subjected to a turbulent inflow is also investigated. Experiments were performed in the Aeroacoustic Wind Tunnel of the University of Twente for a chord-based Reynolds number ranging from 3.2×105 to 9×105 for a uniform inflow and a rod-generated turbulent inflow with a turbulence intensity of approximately 18%. The effective angles of attack ranged from -5∘ to 5∘. Measurements of the boundary layer at the TE, the WPFs along the chord and span, and the far-field radiated noise were performed. For the uniform inflow case, laminar-boundary-layer noise, turbulent-boundary-layer (TBL) noise, and blunt TE noise sources are identified. The far-field noise spectrum presents similar components in the frequency domain as the WPF spectrum and the spanwise correlation length at the TE. The angle of attack mainly affects the WPF spectrum and spanwise correlation length for chord-based Strouhal numbers St<10. The angle of attack slightly affects the TE noise with a maximum variation of 3 dB for 10<St<60. For the turbulent inflow case, it is observed that the turbulence significantly affects the WPFs and spanwise correlation length along the chord, increasing considerably compared to the case of uniform inflow. The highest WPF spectral level and the larger spanwise correlation length occur at positions close to the LE. The results show that the turbulent inflow yields a higher WPF spectral level and larger spanwise correlation length at the TE, resulting in higher levels of TE noise. However, LE noise is still the dominant noise source for St<25.2. For higher frequencies, the TE noise level is expected to become higher than the LE noise level.

Across-wind fluctuating aerodynamic force acting on large aspect-ratio rectangular prisms

This work addresses the aerodynamic characteristics and spatial structure of the across-wind fluctuating aerodynamic force and its influence on wind-induced responses of rectangular prisms. Seven rectangular rigid models with different side ratios (1/4–4) are constructed, and simultaneous pressure tests are conducted in turbulent boundary layer flows. According to the measured data, separation and reattachment characteristics, span-wise correlation coefficients, and coherences are calculated, and then an empirical coherence function model is obtained. By comparing this model with traditional coherence models, the main factors affecting the coherence of the across-wind fluctuating aerodynamic force are presented and discussed. The proposed normalized coherence model takes into account factors such as the vortex shedding, side ratio, building feature size, the spatial location, and frequency correction. The non-dimensional joint acceptance function is presented, based on which the influence of the coherence function model on across-wind fluctuating wind load evaluation and wind-induced response analysis is compared and analyzed.

Large-scale sinusoidal gust effect on aerodynamic pressures and forces on a square cylinder

Aerodynamic pressures and forces on a two-dimensional square cylinder were experimentally investigated in large-scale sinusoidal gusts. The effects of streamwise and transverse gusts with a wide range of gust amplitudes were studied at a Reynolds number of Re = 2.1 × 104. To enhance our understanding of the large-scale sinusoidal gust effect, the pressure fields and global aerodynamic forces of the square cylinder in large-scale gusts were compared with those in smooth and grid-generated turbulent flows. This study shows that both large-scale streamwise and transverse gusts can promote early pressure recovery, resulting in larger mean pressures on the side and leeward surfaces. The root mean square (RMS) of the pressures on the side surface increased and decreased with an increase in the streamwise and transverse gust amplitudes, respectively. The mean drag on the square cylinder was reduced under the effects of large-scale streamwise and transverse gusts. The increase in the streamwise and transverse gust amplitudes significantly affected the RMS of the drag and lift on the square cylinder, respectively. A proper orthogonal decomposition (POD) analysis showed that the contribution of the first POD mode switched from fluctuating lift to drag when the streamwise gust amplitude was very high. The spectral analysis demonstrated that the alternating von Kármán vortex-shedding process still dominated the fluctuating lift under the effect of large-scale gusts. In addition, the spanwise correlations of drag were significantly strong under the effects of large-scale streamwise gusts. However, the spanwise correlations of lift were reduced under the effects of large-scale gusts with high gust amplitudes.