Mizuyasu Koide

Influence of cross-sectional configuration on Kármán vortex excitation

Three cylindrical bodies with different cross sectional configurations, i.e. a circular, semi-circular and triangular cylinder, are used as the test cylinders, in order to investigate the influence of movement of separation point on the Karman vortex excitation. The cylinders were supported elastically by plate springs. The synchronization of Karman vortex shedding occurs on all three cylinders over almost equal ranges of oscillation amplitude and frequency given by the mechanical oscillator. However, the Karman vortex excitation behavior differs drastically among the three cylinders in spite that the cylinders are supported elastically with virtually equal structure parameters.

The Vibration Response of a Cantilevered Rectangular Cylinder in Cross-Flow Oscillation

It is well known that a large-amplitude oscillation called galloping @1–3# is generated, in addition to Karman vortex excitation, for rectangular cylinders supported perpendicularly to a uniform flow when the slenderness b/d is in the range of 0.6 to 2.8 ~d5height, b5streamwise length of the rectangular cross-section!. The basic aerodynamic excitation mechanism of the cross-flow galloping of a rectangular cylinder is explained by the quasisteady nonlinear aerodynamic theory developed by Parkinson et al. @4#. In this theory, the relative attack angle plays an important role in the excitation mechanism of galloping. Also, as was shown by Deniz and Staubli @5#, the attack angle of a fixed rectangular cylinder strongly affects the vortex shedding frequency and lift. Although the mechanism of pure cross-flow oscillation for a circular and rectangular cylinder is becoming clear, as seen in the recent paper @6#, a slight difference in support conditions may affect strongly the oscillation behavior. The specific aim of this

Performance Test of Electricity Generation Utilizing Vortex Induced Vibration

The Karman vortex induced vibration (KVIV) is observed over a wide range of conditions, and has been regarded as a negative phenomenon until now since it has caused many accidents. Therefore a lot of researches have been conducted to predict and to avoid it. Recently, however, KVIV is regarded as a process to convert energy of natural flows into mechanical energy, and techniques for electricity generation utilizing it are proposed. The electric power of this method is smaller than that of wind and water turbine generations, but this method has possibility to become a smaller and more maintenance-free apparatus than rotary machines. In earlier works, we found that the trailing vortex shed periodically from a cruciform two-circular-cylinder system, and that it induces a cross flow vibration on the upstream cylinder (TVIV) over a wide velocity range, which becomes broader by replacing the downstream cylinder by a strip-plate. Because of this character, an electricity generator utilizing TVIV should be effectively applied to rivers of which velocity usually varies largely. The purpose of this work is to develop a technique to generate electricity utilizing TVIV in water flow. Experiments using a water tunnel and an open-surface water channel are conducted to know conditions of the maximum electric power and to test the performance in a river. The optimum gap-to-diameter ratio is 0.22 since the cylinder vibration amplitude is largest. The optimum resistance of the circuit is the value which makes the virtual damping due to electricity generation nearly equal to the structure damping. The performance test in the water channel shows that the open surface and the turbulence in flow have little influences on the cylinder vibration amplitude and the synchronization velocity range of KVIV. However, TVIV is not observed, maybe because of the large aspect ratio.Copyright

A Novel Technique for Hydroelectricity Utilizing Vortex Induced Vibration

The target of our present research is to develop a river monitoring instrument equipped with electricity power generator to supply for its own demand by utilizing energy of flow of the river. To apply this instrument to natural river, its generator should be simple, small and durable with wide flow velocity range of operation. In this paper, the vibration induced by the trailing vortex (T-VIV) on a cruciform circular-cylinder/strip-plate system is firstly introduced compared with the well-known Karman vortex excitation (K-VIV). Secondly, a design for a generator utilizing the T-VIV is presented. Thirdly, an estimation technique for electricity power extracted from the vortex induced vibration (VIV) and a design guide for power generation module obtaining the maximum power are proposed. Finally, using the estimation technique to the T-VIV in air, an estimation of oscillation behavior and power generation performance of an T-VIV generator module applied to water flow are presented.Copyright

Interference Effect of Wake Body on the Cross Flow Oscillation of a Square Cylinder in Uniform Flow

Wind tunnel experiments are carried out to investigate interference effect of wake body on cross flow vibration of a square cylinder in uniform flow. The side length d of the square cylinder is 26∼40 mm and the length le = 315 mm. As the wake body, a strip-plate of width w = d is set downstream the square cylinder with a gap s in cruciform arrangement. The length of the plate ld is varied from infinity, i.e. full measuring section height, to ld /d = 1. Both the Karman vortex excitation (K-VIV) and the galloping are suppressed by the ld /d = ∞ plate in the non-dimensional gap range of 1.6<s/d ≦ 4, although the mechanisms are completely different between the two oscillations. The longitudinal vortex excitation (L-VIV) found in the previous work is confirmed to be induced by the plate at around s/d = 1.4 for the systems with various dimensions and structure parameters. The K-VIV suppression effect is virtually the same for the wake plates with ld /d≧10, and becomes less definite for shorter plates when ld /d ≦ 6. The galloping suppression effect persists up to the shortest wake plate of ld /d = 1 at s/d<2. The L-VIV is observed for plates of ld /d≧6, with weaker degree for shorter plates. The K-VIV seems to be enhanced by ld /d = 2, 4, 6 plates at 1≦s/d≦2 (EK-VIV). By setting the wake plate with ld /d = 1 or 2 at s/d around 0.3, a new type of fluid-elastic vibration is induced as an interference effect of wake body (WBI-FEV). A method is presented to predict fluid-elastic vibrations to which the Van der Pol equation does not apply. The prediction by this method agrees well with measured WBI-FEV for the ld /d = 2 plate.Copyright

Interference of Downstream Body on the Oscillation Behavior of a Circular and a Square Cylinder in Uniform Flow

Wind tunnel experiment showed that the Karman vortex excitation of a circular and a square cylinder is effectively suppressed by setting another cylindrical body downstream in a cruciform arrangement with a certain gap between them, and that the galloping of a square cylinder is also completely suppressed by the downstream cylinder. However, a new oscillation is induced over a velocity range higher than that of the Karman vortex excitation, caused by longitudinal vortices which shed periodically around the crossing. These interference effects of the downstream cylinder on the upstream cylinder oscillation are different depending on the cross-sectional configuration of the cylinders.Copyright

Effect of Attack Angle Fluctuation Superimposed on the Cross-Flow Oscillation of a Rectangular Cylinder in Uniform Flow

The cross-flow oscillation of a rectangular cylinder supported by a cantilever plate-spring system was investigated using a wind tunnel, in order to reveal the effect of attack angle fluctuation superimposed on cross-flow oscillation. Three kinds of support systems were tested in order to give a different phase-shift between attack angle and cylinder displacement. Three rectangular cylinders with the slenderness of 0.5, 1.0 and 2.0 were used to investigate effect of attack angle both on the Karman vortex excitation and the galloping. The effect of attack angle fluctuation on oscillation behavior markedly differed among the three rectangular cylinders of different slenderness. When the slenderness was 0.5, attack angle fluctuation had no influence on the oscillation behavior of the cylinder. When the slenderness was 1.0 and 2.0, the oscillation behavior was quite different compared with pure cross-flow oscillation, depending on the support system and slenderness.Copyright