Kotaro Sato

Numerical analysis of a gas bubble near a rigid boundary in an oscillatory pressure field

The behavior of a gas bubble near a rigid boundary in an oscillatory pressure field is numerically investigated by means of two kinds of methods. The boundary integral method is utilized to simulate the bubble profiles at various times, and the image theory is also applied for solving the differential equations describing the nonlinear oscillations and migrations of a spherical gas bubble. Both calculations are performed, assuming that the liquid around a bubble is inviscid, incompressible, and irrotational and that the gas inside the bubble follows a polytropic gas law. The influence of the boundary proximity on the bubble motion is characterized by the bubble migration and the deformation of the bubble shape, especially by the liquid microjet formation. It is found that the period and the amplitude of the bubble oscillation are related not only to the nature of the time‐dependent pressure in the liquid but also to the bubble location from a rigid wall. Furthermore, the influence of the boundary proximit...

Analysis of Rotating Cavitation in a Finite Pitch Cascade Using a Closed Cavity Model and a Singularity Method

A new method is proposed for the stability analysis of cavitating flow. In combination with the singularity method, a closed cavity model is employed allowing the cavity length freely to oscillate. An eigen-value problem is constituted from the boundary and supplementary conditions. This method is applied for the analysis of rotating cavitation in a cascade with a finite pitch and a finite chordlength. Unlike previous semi-actuator disk analyses (Tsujimoto et al., 1993 and Watanabe et al., 1997a), it is not required to input any information about the unsteady cavitation characteristics such as mass flow gain factor and cavitation compliance. Various kinds of instability are predicted. One of them corresponds to the forward rotating cavitation, which is often observed in experiments. The propagation velocity ration of this mode agrees with that of experiments, while the onset range in terms of cavitation number is larger than that of experiments. The second solution corresponds to the backward mode, which is also found in semi-actuator disk analyses and identified in an experiments. Other solutions are found to be associated with higher order cavity shape fluctuations, which have not yet been identified in experiments.

Study on Flow Characteristics Downstream of Annular Inlet Guide Vanes

The main objectives of the present study are to identify the dominant parameters responsible for the generation of unsteady flow, determine the conditions under which flow oscillations are produced and the relation between the flow characteristics and the number of vanes with identical solidity. The flow instabilities downstream of inlet guide vanes (IGV) are clarified experimentally and by numerical simulation. The conditions for the onset of flow instability, including the number of cells and the oscillation characteristics of the unsteady flow, are discussed based on measured pressure fluctuations and the propagating angular velocity ratio of the instability for various radius ratios (r3/r2). The effectiveness of adjusting the number of vanes to control the flow instabilities is also discussed.Copyright

Vector Control of Synthetic Jets Using an Asymmetric Slot

This is a fundamental study on the jet vectoring control by means of adjusting the synthetic jet Strouhal number (frequency) without slot shape change with time in actuators. This work involved introduction of asymmetric slots with various beak lengths in free synthetic jets for various frequencies, experimentally and numerically. Furthermore, the behavior of synthetic jets was compared with that of the continuous jets. In our experiment, velocity measurements for both types of jets were performed using a hot-wire anemometer with an I-type probe and a traverser, and flow visualization for observing the behavior of the jets was performed using the smoke wire method. Through the experiment, typical flow patterns and time-averaged velocity distributions of synthetic jets for various beak lengths and Strouhal number were demonstrated. The time-averaged velocity was measured at a reference point on the slot centerline to evaluate the degree of the bend of the jets and the obtained data was used in the Strouhal number-beak length map.Copyright

The Influence of an Asymmetric Slot on the Flow Characteristics of Synthetic Jets

The purpose of this paper is to elucidate the influence of an asymmetric slot on the flow characteristics of synthetic jets. In this study, the relation between flow characteristics of two-dimensional synthetic jets and beak-shaped asymmetric slots is investigated experimentally and numerically. Results clarify that the vortex pair with different x-direction locations induced by the asymmetric slot produces a large-scale circulation region on the opposite side of the beak in the case of long beaks. It is also demonstrated that flow fields for synthetic jets depend on asymmetric slot beak length; in contrast, continuous jets are independent of asymmetric slot beak length.

Performance Characteristics of a Fan Using Synthetic Jets

Studies of synthetic jets generated by compact actuators have been conducted for various applications such as drag reduction, boundary layer separation control, and multi-flow mixing. In this paper, a jet fan that utilizes synthetic jets is proposed. The flow patterns inside the jet fan are revealed, and performance curves are drawn. Static pressure distributions in the fan duct are also analyzed to discuss the process of static pressure recovery.

Influence of an Upstream Obstacle on the Flow Characteristics of Axial-Flow Fans

System environments vary for axial-flow fans, which are often used under unsuitable operating conditions. In the recent high density design of such equipment, obstacles are being more frequently distributed upstream of the fan to determine a smaller fan size. In this study, an attempt is made to clarify the influence of an obstacle on the flow characteristics of axial-flow fans. In our experiment, blockage disks with various diameters were placed upstream of a typical cooling axial-flow fan. The performance curves of the axial-flow fan with the blockage disks were measured. In addition, the flow instabilities and their cell structures were investigated under typical conditions. The main objectives of the present study are as follows: (1) to evaluate the performance degradation when obstacles are placed upstream of the test fan, (2) to elucidate the causes of a positive slope and reverse flow, and (3) to clarify the generation mechanism of flow instabilities when the blockage disks, which have a larger diameter than the test fan, are used.Copyright

Flow Characteristics Downstream of Swirl Flow Generators

An attempt was made, through experiments and numerical simulations, to clarify the mechanisms associated with the flow characteristics downstream of both inlet guide vanes (IGVs) with different vane geometries and circulation control wings (CCWs) with different jet sheet momentum coefficients. The novelty of the present study lies in the proposal of a swirl flow generator which can control the flow angle without changing geometrically. The main objectives were: (1) to revalidate the numerical analysis method employed in the present study, which was used to assess the flow characteristics downstream of the IGVs and (2) to suppress flow oscillations through tangential blowing. Characteristic flow patterns downstream of flat-plate vanes and circular cylinders with tangential blowing, with identical exit flow angles, are presented, and the unsteady flow characteristics downstream of the IGVs are compared with those of the CCWs. Furthermore, the influence of the radius ratio of the device outlet to the vane exit on the unsteady flow characteristics downstream of the IGVs and CCWs is discussed based on the measured pressure and velocity fluctuations.Copyright

A Study on the Coanda Effect and the Thrust Characteristics of Synthetic Jets

Although there have recently been various studies on synthetic jets, many issues remain to be clarified, including details of the structure, Coanda effect and thrust characteristics. The present study clarifies some fundamental flow characteristics of synthetic jets near a rigid boundary by experiments and numerical simulations. In addition, a thruster model using the Coanda effect of synthetic jets is proposed and thrust characteristics are evaluated. As the main results, the flow of a synthetic jet near a rigid boundary is visualized and the trajectory of a vortex pair is demonstrated. It is confirmed that the flow patterns of the synthetic jets near a rigid boundary depend on H/b0 (offset ratio, normalized step height). The behavior of the asymmetric synthetic jets caused by the presence of the rigid wall was observed experimentally and the results were compared with numerical data. Furthermore, typical flow patterns around the proposed synthetic jet thruster and its thrust characteristics curves are shown.Copyright

Fundamental Study on Oil Mist Separation in Swirl Flow

Recently, the rotation rates used in fluid machines have been increasing as such machines continue to be downsized. However, these high rotation rates lead to the possibility of lubricant oil mist formed inside the compressor becoming mixed with the refrigerant gas, which can lead to a decrease in the heat exchange efficiency. The present study attempted to clarify the separation mechanism for refrigerant gas-lubricant oil mist two-phase flow during swirl flow in order to improve the oil mist separation efficiency. Both experiments and numerical simulations were carried out to investigate the relation between the oil mist separation efficiency and the flow characteristics in a test chamber containing a rotor. It was found that the flow characteristics were determined by the swirl number Sw, and the flow pattern was quite different for Sw = 1.0, 1.5 and 3.0. In addition, the oil mist separation efficiency increased with Sw and also showed a dependence on the chamber aspect ratio. The centrifugal force acting on a virtual particle in the chamber was also evaluated under various conditions.Copyright