Search

The shape of single-side wind fairing, which is the longitudinal triangular prism that tailors the outer side of a bridge deck, is key to the aerodynamic performances of double streamlined box girders used in long-span bridges. Uniform flow past single and double 4:1 rectangular cylinders with various single-side fairings are investigated using large-eddy simulation at a Reynolds number of 1.1 × 104. Various fairing nose angles and heights are compared. The wind loading and flow characteristics of the cylinders are discussed. The upstream fairing shows a larger reduction of mean drag and fluctuating lift on double cylinders than on a single cylinder. The fairing nose angle has a stronger influence on the wind loading than the nose height. By adding the fairing, sharpening the fairing nose, or lifting the fairing nose, the lateral recirculation zones are shortened while the rear recirculation zone barely changes, leading to different influences on the surface pressure. The upstream fairing is effective in reducing the vertical range and complexity of vortex structures around single and double cylinders. [ABSTRACT FROM AUTHOR]

This paper numerically investigates the effects of rigid coupling and the vibration degrees of freedom on the vortex-induced vibrations (VIV) of two tandem circular cylinders for a spacing ratio L/D = 4 at a Reynolds number of 150. Two (translational vibration) and three (translational and rotational vibration) degrees of freedom (RC-2DOF and RC-3DOF, respectively) cases are considered and compared with the case of two freely vibrating cylinders (FC). The dynamic response characteristics, flow structures, and vortex dynamics are analyzed. The results show that the rigid coupling has a significant effect on VIV. Compared with FC, the maximum transverse amplitude of the downstream cylinder decreases by 30% for RC-2DOF and approximately 15% for RC-3DOF. The lock-in region narrows by 40% for RC-2DOF, while it widens by 80% for RC-3DOF. Reattachment and co-shedding regions are observed for FC and RC-3DOF, whereas only the co-shedding region exists for RC-2DOF. We further explore flow forces and fluid–structure-interaction mechanisms in the lock-in region for RC-3DOF. The rotation of the twin-cylinder system triggers some unique vibration characteristics, such as two transverse amplitude peaks of the upstream cylinder and a prominent streamwise amplitude at Vr = 7–9. The vortex-to-vortex and vortex-to-cylinder interactions are complicated and changeable with reduced velocity for RC-3DOF. [ABSTRACT FROM AUTHOR]

Vortex-induced vibration of two rigidly coupled tandem square cylinders with center spacing L = 4B was numerically investigated at a Reynolds number of Re = 150. Both 2 degrees of freedom (DOF)-C (translational vibration) and 3DOF-C (translational and rotational vibration) cases are considered and compared with the case of no rigid connection. The results reveal that the onset of the synchronization region for rigidly coupled cylinders is earlier than that without connection. Compared with the latter, the upstream cylinder with rigid connection displays a lower transverse amplitude within the synchronization region and a higher one outside the region. The transverse amplitude of the downstream cylinder with 2DOF-C is generally lower than that without connection. In contrast, the vibration of the downstream cylinder with 3DOF-C is higher than that without connection, except for the high reduced velocity, in which the maximum transverse amplitude increases by 20%. The synchronization region of the twin uncoupled cylinders appears within the reattachment and co-shedding regimes, while that of the twin coupled cylinders only appears within the co-shedding regime. Although the synchronization region with a similar flow pattern appears within the co-shedding regime in these three cases, it shows different vibration characteristics. [ABSTRACT FROM AUTHOR]

A numerical simulation study is conducted to investigate the effect of the mass ratio (m* = 3, 10, and 20) on vortex-induced vibration (VIV) of two tandem square cylinders at Re = 150. In this study, we mainly focus on the mass ratio effect on the vibration response, force characteristics, wake mode pattern, and fluid–structure-interaction (FSI) mechanism. The results show that mass ratio plays an important role in the VIV response of the two cylinders. With increasing reduced velocity, both the upstream cylinder (UC) and downstream cylinder (DC) at m* = 3 exhibit the soft-lock-in phenomenon (at a lock-in frequency ratio of fy/fn < 1) instead of the typical lock-in phenomenon (at a lock-in frequency ratio of fy/fn ≈ 1). With increase in the mass ratio to m* = 10 and 20, the soft-lock-in phenomenon disappears, while the DC exhibits the typical lock-in phenomenon. The maximum amplitudes of the two cylinders notably decrease with increasing mass ratio. Furthermore, the mass ratio exerts a major impact on the distance between the two cylinders, which may change the flow pattern. The distance sharply decreases in the synchronization region at m* = 3 but remains almost constant at m* = 10 and 20. In addition, the wake mode and FSI mechanism are more diverse at a low mass ratio (m* = 3). [ABSTRACT FROM AUTHOR]

To investigate the flow field evolution and resultant aerodynamic characteristics of a square cylinder during flow acceleration, large-eddy simulations were undertaken to simulate flow with a dimensionless acceleration rate ap of 0.0048. The adopted ap resembles that is likely to be experienced during a severe downburst wind storm. The tested incidence angle α ranges from 0° to 45° and the time-dependent Reynolds number Re [Re = U(t)D/v, where U(t) is the time-dependent inflow velocity and D and v are the cylinder length and the kinematic viscosity, respectively] varies from 0 to 5 × 104 throughout the simulation. Results revealed that the flow around a square cylinder in accelerated flow evolves through three distinct temporal flow phases: namely, Phase I (Ia and Ib) where no vortex shedding (the instantaneous lift force through this period is zero) is observed, Phase II, where periodic vortex shedding and associated lift force oscillations are initiated and enhanced, and Phase III where stable vortex shedding occurs. Phase I can be further broken down into Phase Ia, which is characterized by laminar shear layer separation and the existence of steady recirculation vortices in the near wake region, and Phase Ib (only present at α = 0° and 45°) which is distinguished by the presence of quasi-steady recirculation vortices behind the cylinder. Aside from the variation of flow patterns, the transient lift and drag coefficients and the wind pressure distributions, as well as the spatial evolution of three-dimensional flow structures and pressure distributions, are also elucidated in detail. [ABSTRACT FROM AUTHOR]

To investigate the flow characteristics of the biased gap flow and the flip-flopping phenomenon at high Reynolds numbers, large-eddy simulation is adopted to simulate the crossflow around two side-by-side circular cylinders at Re = 1.4 × 105. The center-to-center pitch ratio P/D varies from 1.1 to 4. Three flow patterns, i.e., single bluff body regime, biased gap flow regime, and coupled vortex regime, have been successfully simulated. A tri-stable flow pattern is found at P/D = 1.1; namely, a single bluff body regime and a biased gap flow regime deflected to the upper or lower cylinder occur spontaneously and intermittently. In contrast to the results obtained at low Reynolds numbers, two distinct biased gap flow structures are observed at P/D = 1.2 and 1.5, respectively. For P/D = 1.2, the gap-side shear layer separated from the narrow wake cylinder further reattaches to the cylinder; namely, a series of separation bubbles would develop along the gap-side surface of the cylinder. These unsteady separation bubbles induce the inward-directed lift force on the cylinder. For P/D = 1.5, the gap-side shear layer separates directly without further reattachment, and both cylinders experience outward lift forces. It is revealed that the flip-flopping of the biased gap flow is associated with the interaction of the gap vortices and the vortex shed from the outer-side shear layer of the wide wake cylinder. Furthermore, the flip-flopping phenomenon arises initially at one particular cross section of the cylinder, rather than occurring simultaneously along the spanwise direction of the cylinder. [ABSTRACT FROM AUTHOR]

Wind tunnel tests are conducted to study the aerodynamic characteristics of two identical square cylinders with variations in the incidence angle at a Reynolds number of Re = 8.0 × 104. The center-to-center spacing (P) ranges from 1.25 to 5.0 times the width of the cylinders (B), and the incidence angle (α) varies from 0° to 90°. The spacing ratio significantly influences the aerodynamics. When the two cylinders are very close, they can be treated as one bluff body for which the vortex sheds only at the wake of the downstream cylinder. With an increasing spacing ratio, the flow finally separates individually from the two cylinders and creates two related vortex shedding patterns. The interference effects of the two cylinders on the aerodynamics changes with an increasing spacing ratio. According to the aerodynamics, the arrangement of two identical square cylinders can be classified by the spacing ratio into three states: closely spaced (P/B < 1.5), moderately spaced (1.5 ≤ P/B ≤ 3) and widely spaced (3 < P/B ≤ 5). The characteristics of the aerodynamics and mechanism of interference effects in the three states are discussed, considering the influence of the incidence angles. • The aerodynamic forces of two identical square cylinders at various incidence angle were investigated in wind tunnel. • Three states were revealed by the spacing ratio:closely spaced state, moderately spaced state and widely spaced state. • The mechanism of interference is discussed based on the aerodynamic forces, pressure distributions, and Strouhal numbers. [ABSTRACT FROM AUTHOR]

Shape modification effectively reduces the wind loading and wind-induced vibrations of slender structures. Large-eddy simulation is used to investigate the flow around multiple square-like cylinders at a Reynolds number of 22,000. Based on a standard square cylinder, six square-like cylinders are simulated with sharp/rounded corners and straight/concave/convex sides. The combination of rounded corners and convex sides enables the best aerodynamic performance. Furthermore, a parametric study on the round-convex cylinder is conducted considering a fixed corner radius and various side curvatures B / K (B : nominal side length; K : side radius). The results show that both the aerodynamic characteristics of and the flow features around the cylinder change considerably with varying B / K and that three distinct flow regimes are observed. In Stage I (0 ≤ B / K ≤ 0.71), the flow separates at the front corners, and the aerodynamic forces decrease gradually. In Stage II (0.71 < B / K < 1.00), the shear layer separating at the front corner-side region reattaches to the lateral sides. The fluctuating coefficients rise at B / K = 0.74; thereafter, the aerodynamic forces drop and are minimized at B / K = 1.00. In Stage III (1.00 ≤ B / K ≤ 1.77), flow separates at the lateral sides, and the aerodynamic forces recover gradually. • Rounded corners and convex sides reduce the aerodynamic forces. • Three flow regimes are identified for the round-convex cylinder. • The side curvature B / K = 1.00 enables the best aerodynamic performance. [ABSTRACT FROM AUTHOR]

Wind tunnel experiments on flow interference of adjacent (spacing ratio P / B ​= ​1.75) twin square cylinders in various attack angles α were conducted at a Reynolds number of 8.0 ​× ​104 considering both horizontal and diagonal arrangements of models. Investigations on the mean and fluctuating aerodynamic force coefficients, mean and fluctuating wind pressure coefficients, spanwise correlations, power spectrum densities (PSD) of lift, and Strouhal numbers of two cylinders, as well as the inter-cylinder correlations between them, are presented. The results show that the aerodynamic characteristics of two diagonal cylinders differ significantly from those of horizontal ones. Compared with the single-cylinder, the pressure distributions on adjacent cylinders are dramatically changed, leading to the negative drag, attractive lift, or repulsive lift on the downstream cylinder. At small α , diagonal cylinders present stronger mean drag and higher fluctuating forces than horizontal cylinders do. Besides, a stronger spanwise correlation is found for the diagonal ones. The vortex shedding pattern can be divided into four stages for the horizontal arrangement and six for the diagonal case. PSD curves and Strouhal numbers are similar between two cylinders for both arrangements, suggesting that the two closely spaced cylinders act like one bluff body in the vortex shedding. • Horizontal and diagonal cylinders have quite different aerodynamic properties. • Two closely spaced square cylinders act like one bluff body in vortex shedding. • Negative drag and attractive lift on the downstream cylinder exist at α ​≤ ​30°. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of the flow around two static staggered circular cylinders at a high subcritical Reynolds number of Re = 1.4 × 105. The center-to-center pitch ratio (P/D) varied from 1.5 to 4, and the incidence angle (β) increased from 0° to 90°. Three critical aerodynamic phenomena at the downstream cylinder—negative drag force, inner lift force, and outer lift force—are successfully simulated. New explanations for the phenomena are proposed. The negative drag force appears at approximately P/D = 1.5–2 and β = 0°–10°, which is ascribed to a recirculation pair with opposite directions and a high-speed gap flow between the cylinders in the tandem and staggered configurations, respectively. The inner lift force happens at about P/D = 1.5–3 and β = 10°, which is due to the combined effects of a stagnation point shift, high-speed gap flow, and rising of the gap-side unsteady separation bubble at the downstream cylinder. The outer lift force occurs at about β = 20° and P/D = 3–4, which is attributed to a gap-side separation point shift from windward to leeward and interactions between the vortices shed from the upstream cylinder and the shear layer of the downstream cylinder. • The flow around two staggered circular cylinders is simulated at a high subcritical Reynolds number of 1.4 × 105. • The relationship between the aerodynamic characteristics and the flow features is established. • New explanations are proposed for the negative drag force, the inner lift force and the outer lift force of downstream cylinder. • The unsteady separation bubble contributes a lot to the inner lift force of the downstream cylinder. • The lower separation point of the downstream cylinder shifts downwards, which contributes to the outer lift force. [ABSTRACT FROM AUTHOR]

Wind tunnel tests are conducted to investigate the wake-induced vibrations (WIVs) of two circular cylinders with a centre-to-centre spacing of 4 times the diameter and attack angles varying from 0°to 20°for Reynolds numbers between 18,000 and 168,800. Two types of circular cylinders with different surface roughnesses are adopted, and the effects of the surface roughness and Reynolds number on the WIVs are examined. The results show that increasing the surface roughness of a cylinder changes the dynamic characteristic of the WIVs significantly. Two divergent types of wake-induced instability phenomena of distinct dynamic characteristics, namely oscillation trajectories, maximum amplitudes, and evolution of the vibrations, are observed. Changing the roughness of the upstream cylinder only has a smaller effect on wake-induced instabilities than changing the roughness of the downstream cylinder only. For a downstream cylinder with a rough surface, the cylinder only oscillates in a limited velocity range and exhibits "velocity-restrained vibrations" rather than divergent vibrations. Furthermore, it is found that the Reynolds number has strong effects on the wake-induced instabilities of the downstream cylinder, and the possibility that a cylinder experience instabilities is much lower at higher Reynolds numbers (Re >80,000) than at lower Reynolds numbers (Re <80,000). [ABSTRACT FROM AUTHOR]

Vortex-induced vibration (VIV) of twin square cylinders is numerically investigated at Re = 150 with tandem, staggered and side-by-side configurations. Three VIV categories are identified due to dynamic responses, flow-induced forces and flow fields. For Case-Ⅰ (α = 0°), two cylinders only display the typical synchronization region (the reduced velocity of the synchronization region, V r ≈ 5–6). The maximum transverse amplitudes of twin cylinders are 2 times and 2.4 times that of the single one (A y ,max = 0.3 B , B is cylinders' characteristic length), respectively. There is also sizable streamwise oscillation for Cylinder-2 (A x ,max = 0.48 B). Compared to Case-Ⅰ, streamwise and transverse amplitudes of twin cylinders systematically rise for Case-Ⅱ (α = 15°, 30°), where A y ,max and A x ,max of two cylinders is approximately 1.9 times and 2.9 times those in Case-Ⅰ, respectively. Two cylinders exhibit not only a typical synchronization region (V r ≈ 5–6) but also a non-typical synchronization region at high V r ≈ 10–12. Unlike the typical one, Cylinder-2 within the non-typical region shows stronger vortexes, more pronounced vibrations and lower fluctuating lift. For Case-Ⅲ (α = 45°–90°), the flow interference of twin square cylinders mainly occurs in the wake area and marginally affects the dynamic responses. The VIV characteristics of twin cylinders are similar to those of an isolated cylinder. • Configuration effect on the VIV of twin square cylinders enriches the relevant study. • The dynamic response, flow-induced force and Fluid-structure-interaction with staggered cases are analyzed systematically. • Twin cylinders are allowed to oscillate in streamwise and transverse directions freely. [ABSTRACT FROM AUTHOR]