Computational and experimental study on flow around a rotationally oscillating circular cylinder in a uniform flow

The effect of rotational oscillation on flow field and fluid forces of a circular cylinder in a uniform flow is studied by three-dimensional computation by large eddy simulation and flow measurement by particle image velocimetry at Reynolds number 2000. The experimental characteristics of the flow indicate the enhancement of vortex shedding at natural frequency, the suppression of vortex shedding at high frequency and the recovery that follows. The above are accurately predicted by numerical simulations except for the slightly smaller estimate of wake suppression effect at high frequency. On the other hand, the fluid force characteristics on the cylinder are studied by numerical simulation, which indicates the drag enhancement and reduction corresponding to the variations of flow field at low and high frequencies, respectively. The drag reduction at high frequency is magnified with an increase in oscillation amplitude, but it can be saturated at higher amplitude. The mechanism of drag reduction is caused by the combined effect of forcing frequency and oscillation amplitude due to the modification of wake flow at high frequency and the delay in flow separation, while the fluctuating lift force slightly increases at a high frequency caused by the small-scale vortex shedding in synchronous with the rotational oscillation.     

The effect of nonharmonic forcing on bluff-body aerodynamics at a low Reynolds number

The aerodynamic response of a circular cylinder to nonharmonic forcing of the inflow velocity is studied by numerically solving the equations of two-dimensional fluid motion on an orthogonal curvilinear mesh. The effect of varying the inflow velocity waveform while maintaining other forcing parameters constant at a Reynolds number of 180 is considered in this study. The forcing frequency is 84% of the natural vortex shedding frequency in the unforced wake while the peak-to-peak amplitude of velocity oscillation is 65% of the reference velocity. Results are reported for the drag and lift coefficients and the flow field in terms of streamline patterns and vorticity distributions. It is shown that the wake is locked-on to the forcing frequency for all cases tested but the aerodynamic response is systematically modified by the imposed changes in the velocity waveform. The magnitude and the phase of the fluctuating drag and lift forces and the mean drag force are affected. These effects are associated with changes in the mechanism of vortex formation and shedding in the wake of the cylinder; it is found that the rolling up of the individual shear layers on both sides can be manipulated to promote shedding of single vortices or vortex pairs.     

A method of reducing drag and fluctuating side force on bluff bodies

Measurements have been made of the forces imposed on typical two-dimensional bluff bodies with a small circular cylinder (here called a “rod”) placed upstream on the stagnation line. In most cases, the interaction was beneficial in that the drag of the overall system was lower with the rod than without it. Fluctuating side forces due to vortex shedding from the main body were also reduced for most rod positions and diameters.Three two-dimensional bluff bodies were investigated: a flat plate, a square and a circular cylinder. Reynolds numbers were in the range of 1 × 10⁴ to 7 × 10⁴.The measured overall drag coefficient at various rod spacing showed a discontinuous “jump” at some critical “jump” spacing. This change corresponded to the elimination of the usual single stagnation point on the bluff body centre line and the appearance of two stagnation points symmetrically placed, close to the lateral edges of the body.Significant drag reductions were obtained for all bluff body shapes when a front rod was located near the “jump” spacing. The fluctuating side force on the circular cylinder was reduced by the rod at high Re. On the square cylinder, the reduction in fluctuating side force was large (up to 90%) for any Re and rod size investigated.     

Drag reduction on a circular cylinder using dual detached splitter plates

Drag reduction on a circular cylinder using dual detached splitter plates is numerically studied. Two splitter plates with the same length as the cylinder diameter (d) are placed along the horizontal centerline; one is upstream of the cylinder and the other is in the near-wake region, respectively. Their positions are described by the gap ratios G₁/d, G₂/d, where G₁ represents the gap between the cylinder stagnation point and the rear edge of the upstream splitter plate, and G₂ denotes the gap between the cylinder base point and the leading edge of the downstream splitter plate. The drag varies with the two gap ratios; it has the minimum value at a certain set of gap ratios for each Reynolds number. The upstream splitter plate reduces the stagnation pressure by friction, while the downstream one increases the base pressure by suppressing vortex shedding. This combined effect causes a significant drag reduction on the cylinder. In particular, the drag sharply increases past the optimum G₂/d; this is related to the restarted vortex shedding in the wake region.     

A comparative study of three aerodynamic devices for suppressing vortex-induced oscillation

Three main types of aerodynamic damping devices, namely, a perforated shroud, helical strakes, and longitudinal slats, have been fitted on a circular cylinder for study in the same wind tunnel and under similar conditions. The drag coefficient of each device has also been determined at Reynolds numbers up to 1.5 × 10⁵.     

Self-similarity of confined flow past a bluff body

Evidence has been gathered from studying the experimental data available in the literature to reveal invariance of the confined flow past an inclined flat plate and a circular cylinder at sub-critical Reynolds numbers. A theoretical wake width is proposed such that the Strouhal number based on it and the velocity at flow separation is independent of blockage effect. By incorporating the proposed wake width and the momentum equation for confined flow into a free-streamline model for unconfined flow, the pressure at separation and the form drag are found as functions of blockage ratio. The predictions are in reasonable agreement with experimental data and comparable with other theoretical methods. The present study also provides a means of deriving the linear relationship between the drag and the base pressure observed empirically. Such a relationship has found applications in various bluff-body flows.     

缆索承重桥并列索尾流致气弹失稳研究

为了研究表面粗糙度和雷诺数对并列索尾流致气弹失稳的影响规律,以圆心间距为4D(D为圆柱直径)的双圆柱为研究对象,通过风洞试验,在风向角α=0°～20°、雷诺数Re=18000～168800,研究下游圆柱发生尾流失稳的起振条件、振动幅度及运动轨迹等振动特性,分析增大阻尼比对尾流失稳的减振效果,探讨了圆柱表面粗糙度和雷诺数对尾流失稳的作用效应。研究表明,下游圆柱在不同的风向角及风速条件下会出现尾流驰振和尾流颤振2种气弹失稳形式;增大阻尼比对尾流驰振有明显的减振效果,但对尾流颤振的影响较小。尾流致气弹失稳有明显的雷诺数效应,随着雷诺数的增大,下游圆柱的振动形式会由尾流驰振转变为尾流颤振。增加上游圆柱表面粗糙度对下游圆柱气弹失稳的影响较小;而增大下游圆柱表面粗糙度,则会明显降低下游圆柱出现尾流失稳的可能性,并会使发散性振动转变为“限幅限速”振动。     

基于气动参数识别技术的钝体雷诺数效应研究

通过有限的风洞试验数据,揭示雷诺数效应的发生、发展机理,关键在于找出能够有效表征雷诺数效应的特征参数。以经典圆柱模型为研究对象,在系列雷诺数效应风洞试验基础上,从边界层分离、转捩和旋涡脱落等方面,研究了表征雷诺数效应的气动参数及其数据识别方法。针对经典圆柱模型,采用压力系数时程和压力梯度提取有关边界层分离和流动转捩的物理信息。研究表明,在分离边界层转捩区间内分离点附近的压力系数时程会出现明显的阶跃现象,并伴有骤增的压力梯度值,可作为判定雷诺数转捩区间的辅助方法。为研究不同雷诺数下模型周围主导旋涡的作用特点,采用谱正交分解法(SPT)来识别旋涡作用及其对脉动风压场的能量贡献等,旨在对类似结构的雷诺数效应研究提供参考。     

Flow patterns and vortex shedding behavior behind a square cylinder

The flow structures, wake-flow characteristics and drag coefficients of a square cylinder at various Reynolds numbers (Re) and incidence angles (θ) were experimentally studied in an open-loop wind tunnel. The cross section of square cylinder is characterized by the aspect ratio (AR) and blockage ratio (BR) of 25% and 4%, respectively. The Reynolds number is changed from 4000 to 36,000 and the incidence angle is adjusted from 0° to 45°. The flow patterns near/behind the square cylinder were determined using the smoke-wire scheme. The global velocity fields and streamline patterns were analyzed using the particle image velocimetry (PIV). Additionally, the flow-topology method was applied to analyze the flow patterns by calculating the separatrices, alleyways and critical points. Experimental results showed that the flow structures around the square cylinder exhibit three modes—leading-edge separation, separation bubble and attached flow. The surface-pressure profile, drag coefficient (C_D), lift coefficient (C_L) and vortex shedding frequency were detected/calculated using a pressure transducer and hot-wire anemometer. The lift coefficient did not significantly vary with Re. The minimum C_D occurs at θ=12°, whereas the minimum C_L occurs at θ=13°. The minimum projected-Strouhal-number (St_d) occurs at θ=0° while the maximum St_d occurs at θ=15°.     

Vortex shedding from bluff cylinders in strongly sheared turbulent streams

Vortex shedding frequencies across circular and rectangular cylinders in uniformly sheared turbulent flow have been measured at high subcritical Reynolds numbers and large shear parameters and turbulence intensities. The results confirm the occurence of spanwise cells with nearly constant shedding frequency. The presence of strong shear and turbulence appears, in general, to cause a strong spanwise variation of the local base pressure coefficient and an appreciable decrease of the average Strouhal number.     

Drag coefficients and Strouhal numbers of a port crane boom girder section

This work reports the drag coefficients (C_d) for three wind directions measured in low turbulence flow and in turbulent flow with characteristics of the atmospheric boundary layer wind, and the Strouhal numbers (St) of an approximately trapezoidal flanged section, used in the boom girder of a 100 m high port crane. These experimental results were obtained at the Boundary Layer Wind Tunnel of the Dept. Aeronautica at the Universidad Nacional de La Plata, in the range of Reynolds numbers between 30,000 and 180,000. The drag coefficients in the three studied directions showed a reduction for turbulent flow. Further measurements were carried out for the model with an inclination of 80° relative to the flow direction, the position of the crane boom when out of service, giving practically the same drag coefficients.     

Computational study on aerodynamic mitigation of wind-induced, large-amplitude vibrations of stay cables with strakes

Modifications of circular cylinder surfaces, such as strakes and helical wires, effectively mitigate Kármán vortex-induced vibrations normal to flow and have been applied to the reduction of large-amplitude vibrations of stay cables in bridges, which occur under wind oblique to a cable with or without rainfall. This aerodynamic control method cannot be fully effective without understanding the behavior of the flow around and the associated forces on oblique cables. To address this issue, flow around a yawed cylinder with various strake patterns was studied using three-dimensional detached eddy simulation (DES) at Reynolds number of 1.4×10⁵. Results demonstrated that strake patterns strongly influence the development of flow structures around a yawed cylinder and therefore the associated forces on the cylinder. The results suggest that particular strake patterns can mitigate large-amplitude and low-frequency vibrations of stay cables induced by oblique wind.     

Two-degree-of-freedom inclined cable galloping—Part 2: Analysis and prevention for arbitrary frequency ratio

It has previously been shown that inclined cables and other slender structures can experience galloping instabilities in the critical Reynolds number region, due to the changes in force coefficients, and that coupling of the vibrations in two orthogonal planes can significantly affect the behaviour. It is shown in this paper that for more than a few percent detuning of the natural frequencies, the structural damping required to prevent galloping vibrations tends towards solutions for uncoupled single-degree-of-freedom (DOF) systems. However, the coupling can cause the trajectory of cable motion to be elliptical. Using measured force coefficients for an inclined circular cylinder in the critical Reynolds number range, calculated results of the effective damping and the cable trajectory have been obtained in good agreement with a separate full-scale dynamic cable model test that exhibited galloping behaviour under certain conditions. The special case of sub-critical Reynolds numbers is also addressed. The 1DOF and perfectly tuned 2DOF solutions are considered for inclined circular cables in a horizontal wind from any direction, leading to simple guidelines for the structural damping required in practice to prevent dry inclined cable galloping, for any frequency ratio. Finally, other factors in the behaviour and similarity to rain-wind excitation are discussed.     

Effect of a T-shaped plate on reduction in fluid forces on two tandem cylinders in a cross-flow

The effect of a T-shaped plate on reduction in fluid forces on two tandem circular cylinders in a uniform cross-flow was examined at a Reynolds number of 6.5×10⁴. The T-shaped plate, with a head-width of 5 mm (one-tenth of cylinder diameter), is used as the control object to control the approaching flow onto the upstream cylinder, resulting in reduction in fluid forces on the cylinders. The trail length of the T-shaped plate was varied in order to bring the head of the T-shaped plate in action at different positions so that an optimum position of the head of the T-shaped plate for reducing fluid forces could be identified. From the result, it was found that, when the T-shaped plate with a trail length of 0.70-1.00 times the cylinder diameter is used, an optimum reduction in fluid forces occurs, and the interference effect of the downstream cylinder on the upstream cylinder is completely suppressed.     

Investigation of block foundations resting on soil–rock and rock–rock media under coupled vibrations

In the present study,the dynamic response of block foundations of different equivalent radius to mass(R_o/m) ratios under coupled vibrations is investigated for various homogeneous and layered systems.The frequency-dependent stiffness and damping of foundation resting on homogeneous soils and rocks are determined using the half-space theory.The dynamic response characteristics of foundation resting on the layered system considering rock-rock combination are evaluated using finite element program with transmitting boundaries.Frequencies versus amplitude responses of block foundation are obtained for both translational and rotational motion.A new methodology is proposed for determination of dynamic response of block foundations resting on soil-rock and weathered rock-rock system in the form of equations and graphs.The variations of dimensionless natural frequency and dimensionless resonant amplitude with shear wave velocity ratio are investigated for different thicknesses of top soil/weathered rock layer.The dynamic behaviors of block foundations are also analyzed for different rock-rock systems by considering sandstone,shale and limestone underlain by basalt.The variations of stiffness,damping and amplitudes of block foundations with frequency are shown in this study for various rock—rock combinations.In the analysis,two resonant peaks are observed at two different frequencies for both translational and rotational motion.It is observed that the dimensionless resonant amplitudes decrease and natural frequencies increase with increase in shear wave velocity ratio.Finally,the parametric study is performed for block foundations with dimensions of 4 m × 3 m × 2 m and 8m×5m×2m by using generalized graphs.The variations of natural frequency and peak displacement amplitude are also studied for different top layer thicknesses and eccentric moments.     

Flow interference between three equispaced cylinders when subjected to a cross flow

The drag and lift coefficients occurring on one cylinder in a group of three lying with their axes perpendicular to a uniform stream flow have been measured at a flow Reynolds number of 3 × 10⁴ at various inclination angles to the free stream. The cylinders were arranged with their centres equidistant from each other (an equilateral triangle), the spacing ratio of their centres being in the range 1.25 < S/D < 5.From the force coefficients for the individual cylinders, total force coefficients for the group as a whole and the direction in which the resultant force acts were determined. It was found that at certain inclination angles the force coefficients were similar to those pertaining to two-cylinder flow, but that in general the effect of the third cylinder was significant. The results presented should aid in the establishment of standard design codes for flow interference around groups of cylinders.     

Measurements in the wake flow behind finite circular cylinders fitted with slat devices

In this paper the influence of a slat device upon the wake flow behind a finite circular cylinder is investigated. Frequency and cross-correlation (in time) measurements are carried out at a Reynolds number of 31000 and at several streamwise and lateral positions in the near wake of a plain circular cylinder and a circular cylinder fitted with the slat device. The resulting modifications of the wake pattern due to the slat device are discussed. It is shown that the presence of the slat device increases the length of the vortex formation region and thus delays the formation of a periodic vortex wake to relatively long distances behind the body. It is this delay that is largely responsible for the effectiveness of the slat device in suppressing large amplitude vortex-induced oscillations of circular cylindrical bodies.     

Aerodynamic forces on bluff structures in a wake

An experimental investigation into mean aerodynamic forces on a cylindrical structure in a wake of another parallel cylinder is carried out. Measurements of the aerodynamic response were taken for various configurations of streamwise and transverse spacings over a range of 12 000–73 000 of subcritical Reynolds number. On the basis of the geometric parameters and the flow Reynolds number dealt with in this study, empirical correlations of the normalized symmetric variations of drag coefficients as well as of the anti-symmetric behaviour of lift coefficients are presented. The surface pressure distributions lead to additional information about the roles of the main parameters involved in this complex flow situation. The data presented yield further information about the effects of turbulent shear flows on bluff structures and the formation of associated wakes.     

Free Vibration Analysis of Thin Circular Cylindrical Shell with Closure Using Finite Element Method

Vibration analysis of a thin circular cylindrical shell with closure is conducted using finite element method (FEM). Theoretically, shell vibrates in different axial modes, m; circumferential modes, n; and any of their combinations with corresponding modal frequencies. The present FEM results are verified by the results reported in the literature using various shell theories. The eigenvalues of the shell are extracted using block Lanczos and subspace iteration methods, in order to investigate their computational efficacy. Further, the effect of adding various types of closures at one end of the circular cylindrical shell such as flat, cone, and dome, on the modal frequencies are investigated. The two aspect ratios (length to radius ratio) of shell with closure, broad, and slender are considered for this study. The effect of the ratio of the thickness of the closure to the thickness of shell wall on the frequency is also investigated. For the shell with the closure, the vibration modes can be cylinder, closure, or combined cylinder and closure. The modal frequency of the cylindrical shell is significantly affected by the closure. The lowest frequency is observed in the flat type of closure in both the broad and slender cylindrical shells in comparison to the non-closure, dome, and cone type of the closures.