Fluctuating Lift Forces Research Articles

Hydrodynamic fluctuations near a Hopf bifurcation: Stochastic onset of vortex shedding behind a circular cylinder.

We investigate hydrodynamic fluctuations in the flow past a circular cylinder near the critical Reynolds number Re_{c} for the onset of vortex shedding. Starting from the fluctuating Navier-Stokes equations, we perform a perturbation expansion around Re_{c} to derive analytical expressions for the statistics of the fluctuating lift force. Molecular-level simulations using the direct simulation Monte Carlo method support the theoretical predictions of the lift power spectrum and amplitude distribution. Notably, we have been able to collect sufficient statistics at distances Re/Re_{c}-1=O(10^{-3}) from the instability that confirm the appearance of non-Gaussian fluctuations, and we observe that they are associated with intermittent vortex shedding. These results emphasize how unavoidable thermal-noise-induced fluctuations become dramatically amplified in the vicinity of oscillatory flow instabilities and that their onset is fundamentally stochastic.

Two-dimensional spanwise flow regime influenced by tip vortex around a ground-mounted square cylinder in low turbulence uniform flow

The time series data of the lift force acting on a ground-mounted square cylinder in low-turbulence uniform flow reveal a distinctive pattern characterized by a predominant high-amplitude process modulated by intermittent low-amplitude fluctuations. This behavior arises from the intricate interplay between tip and Kármán vortices. However, conflicting interpretations persist, with some findings even presenting contradictory conclusions, particularly regarding the presence of symmetric shedding in the fluctuating lift force process with low amplitude. Furthermore, a clear consensus regarding the mechanism governing the interaction between the tip vortex and the spanwise vortex remains elusive. This study aims to elucidate the two-dimensional flow regime in the spanwise direction influenced by the tip vortex of a ground-mounted square cylinder in low-turbulence uniform flow through experimental investigation. Multiple-point synchronous pressure measurement and particle image velocimetry systems are utilized to measure wind pressure on the side walls and the corresponding flow field at 2/3 of the cylinder's height. The analysis confirms the presence of two distinct types of lift force coefficient behavior: low-amplitude fluctuation (LAF) characterized by longer durations and high-amplitude fluctuation (HAF) occurring in shorter intervals. Subsequently, the flow regime in the near wake corresponding to each mode of the lift force coefficient is discussed. It is observed that the LAF regime corresponds to symmetrical vortex shedding with a prolonged shear layer, maintaining nearly constant curvature. Conversely, for HAF, a pronounced Kármán vortex street is evident. This study conclusively demonstrates the existence of symmetrical vortex shedding, which predominantly contributes to the LAF component of the lift force.

Numerical study on influence of surface vegetation on aerodynamics of high-rise buildings

Vegetation on the surface of buildings is becoming popular in modern society. Besides their environmental and visual effect, vegetation can also affect the flow around them. In this study, the impact of surface planted trees on the aerodynamics of high-rise buildings under atmospheric boundary flow is investigated. Large eddy simulation is employed for the quantitative study. The results indicate that fluctuating lift force of building with both continuous ribs and fully planted trees can be 15.3% lower than that with ribs only. A closer examination on the model's surface pressure indicates that trees primarily decrease the mean and fluctuating wind pressures on the model's sidewalls by up to 20% and 26%, respectively. The mean flow field shows the separation bubble of the models with trees are significantly compressed and pushed away from the side face. Such flow structure contributes to the reduction of mean pressure coefficient at the vortex core of the separation bubble and side faces comparing to the reference model. Additionally, results also show that trees can significantly reduce flow fluctuations near the side faces of model, which is responsible of reduction of the fluctuating wind pressure coefficients on the model surfaces.

Control of vortex shedding and acoustic resonance of a circular cylinder in cross-flow

This study experimentally investigates the effectiveness of a control rod in suppressing self-excited acoustic resonance within a range of Reynolds numbers (Re) spanning from 2.1 × 104 to 1.6 × 105. The investigation focuses on specific parameters, including diameter ratio (d/D) values of 0.1, 0.2, and 0.3; gap ratio (G/D) values of 0.05, 0.1, and 0.2; and angular positions (θ) ranging from 0 to 180 degrees. Comparative analyses are conducted between cases featuring the control rod and a reference case (base case) without it. The near-wake flow field is characterized using Particle Image Velocimetry (PIV), and aeroacoustic response measurements are employed to quantify the aeroacoustic noise emission, particularly during self-excited acoustic resonance. Simultaneous measurements of fluctuating lift force and aeroacoustic response measurements, facilitate the quantification of energy transfer from the flow field to the acoustic field during self-excited acoustic resonance. The results reveal that the control rod’s placement significantly impacts the Strouhal periodicity, with outcomes heavily dependent on the rod’s angular orientation. At certain angular positions, the control rod reduces the sound pressure level (SPL) generated during acoustic resonance excitation. However, at different angular positions, the rod exacerbates resonance excitation. This variability is attributed to the control rod’s profound influence on the vortex core formation and the energy transfer mechanism during acoustic resonance.

Measurements of flow-induced vibration of a flexible splitter plate mounted on a cylinder in free stream flow

We experimentally study flow-induced vibration (FIV) of a thin, cantilevered, flexible plate attached on the lee side of a circular cylinder in a free stream airflow. The Reynolds number (ReD) based on the diameter of the cylinder was varied in the range of (4 × 103, 5×104). The plates of different lengths and the same span were tested with varying airflow velocities (U) in a wind tunnel. The reduced velocity (UR) based on the length and the second natural frequency of the plate was in the range of (1, 11). We describe the plate dynamics showing displacement, frequency, phase plane, amplitude spectral density (ASD), oscillation envelope, modal energy contribution, and flow structures. Based on the motion of the plate, three FIV regimes are observed, namely, initial excitation, transition, and lock-in. During lock-in, the plate attains self-sustained limit cycle oscillation. We observe the presence of small-amplitude higher harmonics in the ASD plot. The second mode is dominant around the onset of lock-in, while the first mode is dominant later in the lock-in regime. The dimensionless displacement increases with an increase in mass ratio (Ms) in the lock-in regime. With a decrease in Ms, the slope of the dimensionless frequency with UR increases. To explain measurements, we develop a wake oscillator model (WOM) in which the plate and fluctuating lift force are modeled as an Euler–Bernoulli cantilever beam and van der Pol oscillator, respectively. The coupled WOM qualitatively and quantitatively predict the measured amplitude and frequency response, respectively, in the lock-in regime.

Flow modulation mechanism in a cylinder with corrugated surfaces

Flow modulation mechanism in a cylinder with corrugated surfaces is investigated by wind tunnel experiments at Reynolds number Re=25 600. Experimental results include surface pressure measurement for the cylinder wall and particle image velocimetry (PIV) for the wake flow. A pair of corrugated surfaces are symmetrically installed on the cylinder wall. Corrugated surfaces are distributed at three different locations on the cylinder wall, i.e., the windward part (case 1), leeward part (case 2), and lateral part (case 3). Experimental results show that corrugated surfaces can modify surface pressure, aerodynamic forces, and vorticity evolution of the cylinder flow. Compared with the natural cylinder (baseline case), the mean drag and fluctuating lift forces of case 1 are reduced by 58% and 82%, which are optimal among all test cases. Flow modulation effects of case 2 on global cylinder wake flow are unobvious and that of case 3 are between cases 1 and 2. Corrugated surfaces can also modify modal properties of proper orthogonal decomposition (POD) of the cylinder wake. Moreover, characteristics of recirculation bubbles, velocity deficits, turbulence kinetic energy, and Reynolds stresses in the wake are all modulated. The main flow modulation mechanism is that shear-layer shapes and streamline distributions near the corrugated surfaces are changed based on zoom-in PIV results on the cylinder near-wall region.

Flow-induced vibrations of ten tandem cylinders at low Reynolds number

Flow-induced vibrations of ten cylinders in tandem arrangement are numerically investigated by using the immersed boundary method with a low Reynolds number (Re = 100). Seven spacing ratios L/D (where L = center–center spacing between tandem cylinders and D = diameter of the cylinder) are selected from 1.1 to 4.0, and the reduced velocity Ur ranges from 2.0 to 13.5 with an increment of 0.5. Small (L/D < 2.0) and large (L/D > 2.0) spacing ranges are identified, both including two types of responses: wake-induced vibration (WIV; Ur = 2.0–9.0∼10.5 for a small L/D and Ur = 2.0–6.0∼6.5 for a large L/D) and wake-induced galloping (WIG; Ur > 9.0∼10.5 for a small L/D and Ur > 6.0∼6.5 for a large L/D). The largest vibration amplitude of each cylinder is obtained in the WIG region for the small L/D condition. The presence of downstream cylinders suppresses the vortex shedding of upstream cylinders and thus postpones the vibration of upstream cylinders at a small Ur, whereas the downstream cylinder enhances the vibration at a larger Ur due to the wake interference. For a small L/D, three flow regimes with the extended-body, reattachment, and co-shedding patterns are successively presented as Ur increases. For a large L/D, four types of flow regimes, namely, EB-2S (the extended-body with “2S” pattern), RT-2S (the reattachment with “2S” pattern), TR-2S (two-row vortex street with “2S” pattern), and CS-VS (co-shedding with variation shedding), are classified. Two new vortex shedding patterns, “2G (two counter-rotating vortices shed from each side per vibration cycle)” and “2C (two co-rotating vortices shed from each side per vibration cycle),” have been identified. In the WIV region, there is only one dominant vibration frequency for upstream cylinders (C1–C7), while a sub-harmonic frequency emerges and dominates C8–C10 when L/D is large. The fluctuating lift force spectra show a broad-band frequency distribution due to the irregular positions of the vortex generation and merging, and the dominant frequency in the WIG region decreases consecutively from C1 to C10.

Experimental investigation on the optimal control of vortex shedding of a circular cylinder with rotating rods at moderate Reynolds numbers

This paper presents an experimental study of fluid–structure interaction conducted at Reynolds numbers around 104 with scale models in a water channel. A circular cylinder was equipped with eight control rods positioned around its perimeter to interact with the external flow. Each rod could be driven independently to rotate about its axis. The controlled rotation of the rods interfered with the vortex generation mechanism mitigating the formation of a coherent wake. The results showed that it is possible to simultaneously reduce the mean drag and fluctuating lift forces when the rods rotate at a uniform speed or with each rod set at a different rate. A multi-objective genetic algorithm was employed to find the optimum rotation speeds. The optimum rotation produced a more significant reduction in both objectives and also consumed less energy. The contribution of each rod depends significantly on its angular position around the body and the flow conditions resulting from the upstream control rods. The present work clarifies the physical principles of the phenomenon and paves the way for the development of technological applications.

Wall-proximity effects on vortex-induced vibrations of a circular cylinder

Vortex-induced vibrations (VIV) of an elastically supported circular cylinder (of diameter D) undergoing a wall interference are investigated for a mass-damping ratio 0.31, Reynolds numbers 1.77 × 103–1.24 × 104, and wall-cylinder gap height ratios e* = e/D = 0.1–1.6. Measurements of vibration responses and vortex shedding frequencies are done using a laser vibrometer and a hotwire anemometer, respectively, while particle image velocimetry techniques are employed to capture flow fields. This investigation aims to assimilate the effect of e* on vibration response, frequency response, VIV range, quality factor of VIV, added mass, added damping, lift force, and flow structure. A smaller e* is observed to increase the added mass, reduce the maximum vibration amplitude, and shorten the VIV range while enhancing the quality factor of VIV. The increase in added mass and the decrease in the maximum vibration amplitude in the VIV range are dramatic when e* is decreased from 0.6. Decreasing e* significantly impacts the flow structures and vortex shedding modes, resulting in intensified asymmetry in pressure distributions. The interference between the wall and the cylinder is further reflected by increasing time-mean lift forces and decreasing fluctuating lift forces with decreasing e* < 0.6.

Wake of wavy elliptic cylinder at a low Reynolds number: wavelength effect

Effects of the spanwise wavelength (λ) of a sinusoidal wavy cylinder with elliptic cross-section on wake structures and fluid forces are numerically investigated at a Reynolds number Re = 100. A wide range of the wavelength, $0.43 \le \lambda /{D_m} \le 8.59$ , is considered with a wave amplitude of a/Dm = 0.048, where Dm is the hydraulic diameter of the wavy cylinder. Based on vortical structures, Strouhal number (St) and wake closure length (Lc), fluid forces, streamline topologies and spatio-temporal evolutions of the near wake, five distinct flow patterns (I–V) are identified depending on λ/Dm. The drag force reaches its minimum in pattern III, the fluctuating lift force is zero in flow patterns III and IV. Distinct from the classical flow where alternate vortex shedding occurs synchronously over the entire cylinder span, flow pattern IV has alternate vortex shedding over a one half-wavelength of the wavy elliptic cylinder, antiphased with that over the other half-wavelength, thus leading to zero fluctuating lift over one complete wavelength. A thorough comparison of the wakes is made between the wavy elliptic cylinder and wavy circular or square cylinder, distinguishing the underlying flow physics behind the salient behaviours observed.

Assessment of Aerodynamic Plates Subjected to Von Kármán Vortex Street for Enhancing the Wind Energy Generation in Blade-Less Devices

This study explores the feasibility of using an oscillating plate downstream of a cylindrical body to produce mechanical energy from a Von Kármán vortex street under sub-critical flow conditions (Re = 72,500). The study aims to quantify the impact of the plate length, its separation from the cylinder, and a machine damping factor on the power coefficient and the blade’s displacement to identify the optimal configuration. This preliminary assessment assumes that the plate oscillation is small enough to avoid changes in the vortex dynamics. This assumption allows the construction of a surrogate model using Computational Fluid Dynamics (CFD) to evaluate the effect of plate length and separation from the cylinder on the fluctuating lift forces over the plate. Later, the surrogate model, combined with varying machine damping factors, facilitates the description of the device’s dynamics through the numerical integration of an angular momentum equation. The results showed that a plate with a length of 0.52D, a separation of 5.548D from the cylinder, and a damping factor of 0.013 achieved a power coefficient of 0.147 and a perpendicular displacement of 0.226D. These results demonstrate a substantial improvement in the performance of blade-less generators.

Flow structure and aerodynamic forces of finned cylinders during flow-induced acoustic resonance

This experimental study investigated the impact of self-excitation of acoustic resonance on the fluctuating lift force, flow structure, and aeroacoustic energy transfer of spirally finned cylinders with crimps in cross-flow. Three different finned cylinders with varying fin pitch-to-root diameter ratios (p/Dr) were compared with their equivalent diameter (Deq) bare cylinders. The study found that self-excitation of acoustic resonance occurred at a lower reduced flow velocity for the finned cylinders. During acoustic resonance excitation, finned cylinders with lower p/Dr experienced an abrupt increase in the fluctuating lift force coefficient and acoustic pressure. The decomposition of the fluctuating lift force with respect to the acoustic pressure showed that the earlier onset of resonance and the abrupt increase in the lift force and acoustic pressure were due to the increased susceptibility to flow-induced acoustic resonance. Phase-locked particle image velocimetry (PIV) measurements of the vorticity field supported this finding by showing a well-organized vorticity field in the wake of these finned cylinders. However, the fins and crimps had a significant effect on the distribution of the acoustic particle velocity field compared to the bare cylinder. The presence of the fins and crimps caused the acoustic particle velocity magnitude to be concentrated within the fins and rapidly decline away from the cylinder base. As a result, the acoustic power generated was lower in the case of the finned cylinders. This ultimately resulted in a lower peak acoustic pressure and fluctuating lift force being generated in the case of the crimped spirally finned cylinders.

Flow control of a circular cylinder by self-adaptive furry microfibers

In the present study, two columns of self-adaptive furry microfibers (nylon wires) are placed near the separation points of a cylinder to manipulate the wake vortex shedding and improve its aerodynamics performances. The effect of extension length on the control efficiency of the self-adaptive nylon wires is experimentally investigated at a subcritical Reynolds number of Re = 2.67×104. The unsteady aerodynamic forces of the cylindrical model with and without control are estimated by the pressure distributions around the model surface, and the flow structures are visualized by the high-speed particle image velocimetry measurement system and the smoke-wire technique. The results demonstrate that the nylon wires can significantly suppress the fluctuating lift forces acting on the cylindrical model and modify the wake-flow dynamics. The instantaneous results show that the nylon wires can stretch the unsteady shear layer with both sides of the cylindrical model, thus increasing the vortex formation length and pushing the vortex structure further downstream. Furthermore, the nylon wires with an appropriate length are found to suppress completely the wake vortex shedding pattern of the von Kármán vortex street.

Effects of free-stream turbulence on the near wake flow and aerodynamic forces of a square cylinder

Despite a large volume of preceding studies on the effects of free-stream turbulence (FST) on the aerodynamic forces of a square cylinder, limited knowledge is available regarding the evolutionary characteristics of flow with the change of turbulence integral length scale and turbulence intensity. This paper presents a systematic experimental study of the near wake flow and aerodynamic forces of a two-dimensional (2D) square cylinder in grid-generated turbulent flows and smooth flow. Particular attention is devoted to exploring the respective influences of turbulence integral length scale and turbulence intensity. The effect of FST with a higher-level turbulence intensity than previous investigations is also investigated. The near wake flow and aerodynamic forces are studied by particle image velocimetry (PIV) and synchronous pressure measurements. The proper orthogonal decomposition (POD) analysis is also performed to further understand the effects of FST on the coherent structures of fluctuating pressures around the square cylinder. The results show that the FST with a larger turbulence integral length scale and higher turbulence intensity can cause an increase in the vortex formation length, weaker vortex shedding process and interactions between the oppositely signed vortex rows. These changes lead to significant reductions in Reynolds normal and shear stresses, turbulence kinetic energy and fluctuating lift forces. A completely different flow pattern featured with flow reattachment and reseparation is interestingly captured on the square cylinder in a highly turbulent flow with a turbulence intensity of 20%. This flow pattern results in an obvious mean pressure recovery on the side surfaces and much larger pressure fluctuations near the leading edges and fluctuating torsional moments compared to those in smooth flow and turbulent flows with lower turbulence intensities.

Two identical tandem square prisms with rounded edges and hard marine fouling at incidence in cross-flow: Effect of spacing and Reynolds number on unsteady fluid dynamics

Because of their long submerged columns, deep-draft semi-submersible floating structures are particularly susceptible to vortex-induced motions. This paper focuses on the steady and unsteady forces that act on such a column pair, as well as on the frequency and strength of the shed vortices behind the downstream column. Wind tunnel experiments were performed on two rounded and lightly rough square-section prisms in cross-flow for Reynolds numbers from 100,000 up to 7 million. They are arranged inline at centre-to-centre distances of S/D = 2.8, 4, and 5.6 and at 0° or 45° incidence angle. The trend of the drag curve for the upstream prism deviates at S/D = 2.8 and 4 sharply from that for a single prism. The drag force on the downstream prism strongly depends on both S/D and α. At S/D = 2.8, a thrust force occurs either in certain (α = 0°) or in all (α = 45°) flow states. Larger spacing values lead to an overall higher drag and a flattening of the Cd2 (ReD) curve. The highest fluctuating lift forces on the downstream prism are obtained at α = 45°. Regarding the vortex shedding frequency, differences between both incidence angles occur at S/D = 5.6 only.

Effects of streamwise gust amplitude on the flow around and forces on two tandem circular cylinders

Three-dimensional large eddy simulations are carried out to investigate the effect of streamwise gust amplitude ratio AR on the flow structure around and dynamic forces on two tandem circular cylinders at a subcritical Reynolds number of Re = 1 × 103. The center-to-center space L between the two cylinders is fixed at 3.5D, where D is the cylinder diameter. The velocity U of sinusoidal streamwise gust is set as U = U0 (1+ARsin2πfut), where the velocity amplitude ratio AR is varied from 0 to 0.25, U0 is the mean velocity, fu is the frequency of the sinusoidal streamwise gust, and t is the time. In order to enhance the understanding of the streamwise gust effect on the physical properties of the flow around the two tandem cylinders, the shear-layer reattachment, flow separation, wake recirculation, Strouhal number (St), unsteady forces, and phase lag (φ) between the fluctuating lift forces of the two cylinders are explicated. The results show that the flow structure around the cylinders is sensitive to AR. A change in AR thus leads to modification of reattaching flow (AR ≤ 0.15) to co-shedding flow (AR ≥ 0.20). The St, fluctuating forces and φ thus jumps at the boundary between the reattaching and co-shedding regimes. A stronger gust effect appears on the fluctuating drag of the upstream cylinder than that of the downstream cylinder. Besides the dominant component of the fluctuating lift induced by the vortex shedding, two other components are observed with the frequencies equaling to the superposition and subtraction of the gust and vortex-shedding frequencies, respectively. In addition, two more cases with L/D = 1.5 and 6.0 are conducted for deeper understanding of AR effects.

Cross-flow aerodynamics of bridge cables with wire meshes

Adverse weather conditions in the northern regions during winter might lead to accumulation of ice or snow on bridge cables and consequently trigger ice shedding. The falling ice poses a risk for the traffic and pedestrians below, which often results in bridge closures and/or insurance claims. A passive surface-modifying device in the form of a steel wire mesh tautened on the surface of a cable sheath was previously found to substantially reduce the risk associated with falling ice through improved ice retention. In this paper, the cross-flow aerodynamic behaviour of bridge cables fitted with steel wire meshes is investigated in detail. The wire mesh generally resulted in higher levels of aerodynamic drag when compared with a plain cable sheath, although the Reynolds number dependence was highly reduced. Several angles of attack were investigated and no instability issues with respect to the across-wind vibrations were observed for any of the tested wire mesh configurations based on the quasi-steady theory. Even though vortex shedding was still present in all cases, the intensity of the fluctuating lift forces was much lower than that of the plain cable. To investigate basic aspects of the flow structure produced by the wire mesh, two flow visualisation techniques, combined with streamwise wind speed measurements in the near wake, were further used and are discussed.

Effects of facade rib arrangement on aerodynamic characteristics and flow structure of a square cylinder

Ribs on building façade can reduce wind loads acting on high-rise buildings and modify the corresponding flow structures. The present study investigates the effect of vertical ribs with different arrangements on the flow field and aerodynamic characteristics of a square cylinder under uniform flow by using LES. The results show that the windward face ribs and downstream ribs of sidewall can obviously affect the behavior of the separated shear layer by introducing the local recirculation. The upstream ribs of sidewall and leeward face ribs, however, show insignificant effect on the flow filed and aerodynamic force. The multi-location combination of ribs generally exhibits better aerodynamic performance than the single-location ribs. The optimal combination can significantly modify the shear layer flow, and reduce the surface pressure, mean drag force and fluctuating lift force. The four-face-ribs arrangement is the most practical arrangement with considering real application. By using this arrangement, the mean drag and fluctuating lift force can be reduced up to 38% and 70%, respectively. The near-façade flow becomes more complicated with more local re-circulations when model is equipped with densely arranged ribs. Correspondingly, the wake vortex is obviously shortened accompanying with increase of wind force. The space in between the ribs is also found to be an essential factor for modifying the flow structure and wind load acting on the structure.

Effects of end condition and aspect ratio on vortex-induced vibration of a 5:1 rectangular cylinder

A series of wind tunnel tests were carried out to study the end condition and aspect ratio effects of a sectional model on the vortex-induced vibration (VIV) of a 5:1 rectangular cylinder. Five aspect ratios varying from 9 to 32 as well as eight endplates with different sizes were examined in the tests. Static and elastically mounted sectional models were employed to measure the surface pressure and VIV response of the cylinder, respectively. Sectional fluctuating lift forces, shedding frequency, as well as their spanwise distribution and correlation on the sectional model were obtained and discussed. Results indicated that the aspect ratio had a significant effect on VIV amplitudes, and small aspect ratios tended to remarkably underestimate the VIV amplitudes. There existed an end interference region with a length of 6–8 D, where the sectional lift forces could be, on average, half the value for the non-interference region. Besides, the aspect ratio was positively related to the spanwise correlation of sectional lift forces. Both end condition and VIV had little effect on the length of the end interference region. In the VIV ‘lock-in’ range, with the increasing reduced wind velocity the fluctuating lift force first increased and reached a maximum value earlier than the VIV amplitude, and after that further increase of reduced wind velocity led to decreased fluctuating lift force (toward a value even smaller than that of the stational model). This complicated variation of sectional lift forces implied that vortex shedding was highly nonlinear and self-limiting. Finally, an empirical method based on the test results was explored to account for the influence of aspect ratio on VIV amplitude.

Experimental study on proximity interference induced vibration of two staggered square prisms in turbulent boundary layer flow

Slender structures placed close to each other could greatly magnify the dynamic response due to interference effects. This paper presents the results of experimental studies of the proximity interference induced vibration of two identical square prisms in a turbulent boundary layer to clarify its excitation mechanism. The upstream interfering prism was static, while the principal prism was dynamic and free to vibrate only in the crosswind direction. The divergent vibration behavior at the critical interfered location was examined in detail. Based on the time-resolved particle image velocimetry (TR-PIV) technique and unsteady pressure measurement, the vortex shedding process and corresponding pressure distribution of the dynamic principal prism were described by examining their correlation with the motion time histories. The biased gap flow between the two prisms was found to control the vibration of the dynamic principal prism. The interaction between the vortex shedding in the wake of the dynamic principal prism and the gap flow is dramatically intensified with increasing wind velocity. Such effects create a large fluctuating lift force on the dynamic principal prism at high reduced wind velocity, compared with measurement taken with two static prisms, which drives the prism to undergo a divergent crosswind response.