Shigehira Ozono

The effects of turbulence on a separated and reattaching flow

The effect of free-stream turbulence on the mean pressure distribution along the separation bubble formed on a flat plate with rectangular leading-edge geometry is investigated experimentally in a wind tunnel using turbulence-producing grids. Emphasis is placed on finding the effect of turbulence scale. The ratio of turbulence scale to plate thickness investigated was about 0.5 to 24 for two values of turbulence intensity of about 7 and 11%. The Reynolds number based on plate thickness was approximately (1.4–4.2) × 10 4 . It is found that the main effect of free-stream turbulence is to shorten the separation bubble. It is progressively shortened with increasing turbulence intensity. The mean pressure distribution along the shortened separation bubble is insensitive to changing turbulence scale up to a scale ratio of about 2. With further increase in the scale ratio it asymptotes towards the smooth-flow distribution. There is no trace of interaction between turbulence and vortex shedding (the impinging-shear-layer instability) in the mean pressure distribution.

Experimental and numerical analysis of vortex shedding from elongated rectangular cylinders at low Reynolds numbers 200-103

Abstract Experimental and numerical investigation on vortex shedding from elongated rectangular cylinders at low Reynolds numbers 200-10 3 was performed in this paper. The experiment was made in a low speed wind tunnel, and the numerical analysis was based on the finite difference method applicable to the 2D Navier-Stokes equations. The side ratio of rectangular cylinders tested ranged from 3 to 16 in the experiment, and 3 to 10 in the numerical computation. Good agreement was obtained between experiment and numerical analysis of the Strouhal number. With increasing Reynolds number, transition was observed in the type of vortex shedding, i.e., from the Karman type shedding to the impinging-shear-layer instability, where a single separated shear layer can be unstable in the presence of a sharp trailing edge corner. The transitional Reynolds number was roughly equal to 300.

Vortex suppression of the cylinder wake by deflectors

The flow around a circular cylinder with a few interference elements shifted along the wake was investigated. This paper is mainly concerned with the case where a circular cylinder of the same diameter as that of the main cylinder was used as an interference element. In fact, this situation coincides with the flow around two circular cylinders in staggered arrangement that had been studied extensively. In the present study, however, a different point of view was adopted. Attention was devoted to a critical nature of the base pressure of the upstream fixed cylinder. As the downstream cylinder was shifted upstream, the base suction exhibited a critical fall at a certain gap for different levels off the wake centerline. The spatial trace of the critical gaps was compared with those of the experiments using thin splitter plates as an interference element.

In-plane dynamic interaction between a tower and conductors at lower frequencies

Abstract Transmission line system can be represented by several analytical models composed of towers and conductors. The contribution of normal modes of a conductor to the dynamic tension force transmitted to a tower is obtained by using a frequency response function of tensions devised for the coupled systems. The in-plane dynamic interaction between a tower and conductors is investigated with varying support stiffness. A representation of the mechanism of the in-plane free vibration of transmission line systems is proposed which covers a wide frequency range.

The effects of turbulence on bluff-body mean flow

Abstract This paper describes the results of our recent investigation on the effects of turbulence on the mean flow past 3D and 2D bluff bodies. Emphasis is placed on finding possible effects of turbulence scale. It is shown that turbulence can significantly influence bluff-body mean flow at two main scales. Small-scale turbulence causes the separated shear layer to reattach earlier on the side face of a bluff body through enhanced mixing. Large-scale turbulence interacts with vortex shedding from a bluff body, thus changing the mean flow considerably.

Characteristics of in-plane free vibration of transmission line systems

Abstract A numerical procedure for the dynamic analysis of tower-and-conductor coupled systems is introduced. Characteristics of in-plane free vibration are investigated by using two series of models. Of particular interest is the phenomenon in which a group of the natural frequencies of the system are seen near each natural frequency of the same freestanding tower. This dynamic behaviour is found to be similar to that of a simplified analogue model.

A numerical study on the flow around flat plates at low Reynolds numbers

The unsteady viscous flow around flat plates is investigated through a finite-difference computation of the incompressible two-dimensional Navier-Stokes equations. The side ratio of the plates d/h ranges from 3 to 10 with Reynolds numbers Re equal to 200 and 400, where d is the depth, h is the height, and Re is based on h. It was pointed out in our previous experiment[1] performed at Re=1000 that vortex shedding from the plates is characterized by the impinging-shear-layer instability, since the Strouhal number based on d increases stepwise with increasing d/h, as was confirmed in our numerical analysis[2]. In the present paper, the flows at two lower Reynolds numbers, Re=200 and 400, are numerically analyzed and compared with the results for Re=1000.

Stably stratified flow around a horizontal rectangular cylinder in a channel of finite depth

Abstract This paper describes a numerical study of the two-dimensional flow of a linearly stably-stratified Boussinesq fluid around a rectangular cylinder in a channel of finite depth. To attain highly stratified flow, a difference scheme combining two grids was applied. Attention was focused upon the interaction between the vortex shedding and the internal gravity waves. A parameter K is used, which is the ratio of the fastest linear internal-wave speed to the mean flow speed. As K increases, the symmetric configuration with the cylinder placed at the mid-depth first stimulates the symmetric vertical dominant mode of the columnar disturbance at K ∼ 2, but the non-symmetric configuration with the cylinder displaced stimulates the non-symmetric mode at K ∼ 1. Once a columnar disturbance of a certain mode is created, accompanied by the lee waves, the wave speed of the columnar disturbance is not characterized by cylinder conditions, but by linear theory. A critical change in the Strouhal number is observed, shortly after appearance of the lee wave, which probably influences the dynamics of vortex formation. The likely flow alteration is thought to occur when the lee waves grow in amplitude and the wavelength becomes sufficiently short so that the wake cavity is suppressed/biased by the first troughs of the lee waves.

A wind tunnel for studying density-stratified flows

Abstract A wind tunnel was designed to investigate the effects of density stratification on fluid motions appearing in the atmosphere. By filling the tunnel with a different density mixture of air and a heavy gas, we can easily create stratified flows with the desired density profiles in a short time. A wide range of the density gradient can be generated, therefore, the experiments at low Froude numbers and at relatively high Reynolds numbers can be performed.