Circumferential Oscillation Research Articles

Amplification of flow-induced vibrations of a circular cylinder by an oscillating minute attachment

The presence and circumferential oscillation of rivulets can excite large-amplitude vibrations for cables. Inspired by this phenomenon, a minute attachment was employed to simulate the upper-rivulet motion and successfully excited a large-amplitude vibration for a circular cylinder in a wind tunnel. By minute attachment, we mean a stainless-steel strip exhibiting morphological and dimensional characteristics analogous to the rivulet. The role of the circumferentially oscillating minute attachment on the flow-induced vibration of a circular cylinder was explored by detailed flow and structural measurements. The experimental results demonstrated that as wind velocity increased, the vibration amplitude of the model progressively increased, and the impact of the attachment on aerodynamics increased as well. It was also found that the wake behavior shifted from alternating vortex shedding to periodic “expansion–contraction” flapping in the boundary layers with increasing wind velocity. In particular, the oscillating minute attachment could control the boundary layer separation and form a “low velocity zone” on the upper surface of the model that varied with the attachment. Then, the aerodynamic lift force was synchronized with the attachment oscillation frequency, which was equal to the natural frequency of the vibrating circular cylinder. As a consequence, the vibration amplitude was rapidly developed.

Direct numerical simulation of turbulent flows through concentric annulus with circumferential oscillation of inner wall

Periodic wall oscillations in the spanwise or circumferential direction can greatly reduce the friction drag in turbulent channel and pipe flows. In a concentric annulus, the constant rotation of the inner cylinder can intensify turbulence fluctuations and enhance skin friction due to centrifugal instabilities. In the present study, the effects of the periodic oscillation of the inner wall on turbulent flows through concentric annulus are investigated by the direct numerical simulation (DNS). The radius ratio of the inner to the outer cylinders is 0.1, and the Reynolds number is 2 225 based on the bulk mean velocity Um and the half annulus gap H. The influence of oscillation period is considered. It is found that for short-period oscillations, the Stokes layer formed by the circumferential wall movement can effectively inhibit the near-wall coherent motions and lead to skin friction reduction, while for long-period oscillations, the centrifugal instability has enough time to develop and generate new vortices, resulting in the enhancement of turbulence intensity and skin friction.

Three-phase coupled modelling research on rain–wind induced vibration of stay cable based on lubrication theory

Cables of cable-stayed bridges may vibrate with large amplitude under wind and rain, which is known as rain–wind induced vibration (RWIV). According to the pervious researches, the formations and oscillations of rivulets on stay cable surface play important roles in RWIV. In this paper, four different 2D models are presented based on lubrication theory, and the best way of simulating RWIV through lubrication theory is confirmed by the comparisons of rivulet motions and cable vibration responses between these four models and pervious researches. On this basis, the relations among rivulet motions, cable aerodynamic forces and vibration responses are investigated to reveal the mechanism of RWIV. Numerical simulation results show that when RWIV occurs, the periodic oscillations of rivulets around cable lead to the periodic fluctuations of cable lift and drag, whose frequencies are almost equal and close to cable natural frequency. Under the periodically fluctuant lift and drag, cable vibrates with large amplitude in across-wind and along-wind directions, which may further enhance the circumferential oscillations of rivulets conversely. These confirm the conclusion that the resonance between rivulets and cable oscillation may be one of the main reasons for RWIV.

Flow and solute uptake in a twisting tube

We investigate flow in a tube driven by axial and radial wall oscillations that are periodic in time. Furthermore, the walls undergo periodic circumferential oscillations, the amplitude of which increases with axial distance along the tube; this wall motion results in a new mechanism for the generation of axial flows within the tube. The axial steady-streaming flows that arise as a result of circumferential oscillations are opposed to those due to the axial and radial pulsations, and are potentially as significant. We then consider the uptake of a passive solute through the walls of such a tube into an adjacent medium in which the solute diffuses and is consumed at a constant rate. The tube ends are open to well-mixed fluid containing the solute. The effect of the streaming flows on the solute flux into the tube is determined. The solute disperses along the tube due to the interaction between advection and transverse diffusion, and the time-mean solute distribution throughout the tube is determined for a wide range of parameters.

Analytical study on growth mechanism of rain vibration of cables

The fundamental growth mechanism of rain vibration of cable in cable-stayed bridges are studied analytically. It has been reported by Hikami and Shiraishi (1987) that the rain vibration is a large amplitude oscillation caused by the combined influenced of rain and wind, and that the rain water rivulet formed along the upper surface of cable is the origin of rain vibration. Therefore, a cylinder of figure-8 cross section is adopted as an analytical model for the rain vibration of cable with upper rivulet. The characteristics of unsteady wind forces of the figure-8 section model are investigated first through wind tunnel experiments. Two possible mechanisms are, then, checked for rain vibration by means of quasi-steady galloping analysis; one is Den Hartog mechanism of cable oscillation and another is the two-degree-of-freedom instability, i.e. the torsional mechanism, caused primarily by the circumferential oscillation of the rivulet along cable surface. It is found that the oscillation of upper rivulet, the fundamental frequency of which could coincide with the natural frequency of cable motion at the wind speed around 10 m/s, is aerodynamically coupled with the flexural oscillation of cable, and that this coupled motion of rivulet makes the modal aerodynamics damping negative. That is, it is concluded that the circumferential oscillation of the upper rivulet is indispensable to the growth of rain vibration.

Analytical Study on Growth Mechanism of Rain Vibration of Cables

The fundamental growth mechanism of rain vibration of cable in cable-stayed bridges are studied analytically. It has been reported by Hikami and Shiraishi [1987] that the rain vibration is a large amplitude oscillation caused by the combined influence of rain and wind, and that the rain water rivulet formed along the upper surface of cable is the origin of rain vibration. Therefore, a cylinder of figure-8 cross section is adopted as an analytical model for the rain vibration of cable with upper rivulet. The characteristics of unsteady wind forces of the figure-8 section model are investigated first through wind tunnel experiments. Two possible mechanisms are, then, checked for rain vibration by means of quasi-steady galloping analysis; one is Den Hartog mechanism of cable oscillation and another is the two-degree-of-freedom instability, i.e. the torsional mechanism, caused primarily by the circumferential oscillation of the rivulet along cable surface. It is found that the oscillation of upper rivulet, the fundamental frequency of which could coincide with the natural frequency of cable motion at the wind speed around 10 m/s, is aerodynamically coupled with the flexural oscillation of cable, and that this coupled motion of rivulet makes the modal aerodynamic damping negative. That is, it is concluded that the circumferential oscillation of the upper rivulet is indispensable to the growth of rain vibration.

Observations of magnetopause geometry and waves at the lunar distance

Magnetic observations at the lunar distance of the magnetopause, or boundary between the geomagnetic tail and the magnetosheath, are surveyed. The boundary surfaces are shown to have normal vectors from which an average tail aberration induced by the Earth's heliocentric motion of 9 ± 5° and a flaring half-angle of ∼9° is found. The boundary is assumed to be a tangential discontinuity. The average ecliptic diameter of the tail at lunar distance is 50 Earth radii. Using 21 normal vectors, a statistical variation transverse to the tail axis three times that along the axis is shown. This may correspond to magnetic perturbations induced by the Kelvin-Helmholtz instability; the variations of the unit normals are consistent with circumferential oscillations having wavelengths smaller by 1 3 to 1 10 than those of waves moving in the downstream direction. The circumferential oscillations appear to give evidence of fluting of the tail surface. Several distinct types of boundary signature are identified. Boundary speeds which usually exceed typical spacecraft velocities of ∼1 km sec −1 are deduced from simple models of boundary motion. Implied boundary thicknesses are usually ∼1000 km, but perhaps are as low as ∼30 km in some instances. Use of K p as an indicator of solar wind conditions does not reveal correlation with the number of multiple crossings or the changes in magnetic field magnitude across the boundary.