Noriyoshi Yonehara

Finite Difference Approximation of Spatial Gradient in Spatio-Temporal Derivative Method for PIV Sub-Pixel Analysis

Some calculation schemes of spatial gradient for the spatio-temporal derivative method, which was used for sub-pixel displacement detection in PIV measurements, were evaluated. It was found that the second-order central finite difference scheme tends to underestimate the gradient of tracer-particles intensity, so that it leads to overestimation of sub-pixel displacements. In this study, a combined upstream and downstream finite difference scheme was proposed and evaluated for one-dimensional flow measurements. Furthermore, the proposed scheme was modified for two-dimensional flow. Two important things for small tracer particle PIV images were shown : 1) the direction of the sub-pixel correction should be taken into account, 2) distant pixels should not be used. It was found that the proposed scheme has high accuracy even if images contain some noises.

Tiny droplets generated by bubbles penetrating a liquid-liquid interface

When bubbles rise through a liquid-liquid (water-oil) interface, the lower liquid is entrained into the upper liquid. Many water droplets are generated in oil layer, and they influence the phenomena of heat and mass transfer between the liquids. In the experiments, silicone oil of 1 cSt is poured on water with 100mm depth. By utilizing a high-speed and hi-resolution video camera, it is found that there are three different processes in generating droplets. The largest class of droplets originated from entrained water columns, while the second class ones were born when penetrating water jets break. The jet appears when water film, which wraps up a bubble, is broken and slips down to a bottom of the bubble. The smallest droplets appear in the vicinity of bubbles when entrained water leaves bubbles.

Measurement of Flow in Micro Channel with partial Gas Wall

Velocities in a micro-channel, 100ƒÊm in width and 30ƒÊm in depth, are measured by using a newly developed micro-PTV system. The experimental channel is straight and its upper and lower sidewalls are partly replaced by air-water interfaces. Stereo pictures are taken by high resolution video cameras. Diameter of tracer particles was 2ƒÊm. This system enables to measure 3D velocity components in a micro-scale space. By utilizing a pair of telecentric lenses, deeper depth of fields and parallel projection pictures can be realized. The depth of fields covers up to 100ƒÊm, and parallel projection pictures make calculations of 3D recovery far simpler than that using perspective pictures.

Development of stereoscopic measurement method based on parallel projection pictures for 3D-micro-PTV

In order to achieve a 3D-micro-PTV, a stereoscopic measurement method using the telecentric lens was developed. Because parallel projection pictures can be obtained by using the telecentric lens; complex calibrations are not needed and stereo-matching become very easy. In this paper, correction method of setting errors for telecentric optical system was explained. Measurement accuracy was evaluated by using simple targets.

Improved Precision of PTV Measurements Through Suppressing Peak Locking Error

The spatio-temporal differential method(gradient method) can be one of the high precision algorithms for PIV analysis. But, when the method is applied to actual PIV pictures, measurement precision cannot be so good as is expected. It is found that inadequate numerical differentiations and peak-locking phenomenon degrade its intrinsic characteristics. The first issue is discussed in a separate paper. In this paper, the peak-locking phenomena for PTV with various picture conditions and influences of interpolation methods on the phenomenon are examined and discussed. Some methods to reduce the peak-locking error are recommended, and the precision of less than 0.05 pixel ambiguity could be realized.

Transient Behaviors of Bubbles Brown from a Bottom Nozzle

Bubble behaviors in liquid have been studied in many engineering fields relating to a power plant, chemical reactor, mixing system, and so on. In those systems, Bubble shapes, motions and surface areas are the most important subjects in order to pursue transport phenomena and to estimate mixing performance. When gases are blown into liquid, bubbles are formed. Once this procedure is observed through high-speed camera, they exhibit anomalous transient shapes and behaviors are found. Similar bubble-shapes had been found in the cavitation, which occurs quite different circumstances from the present case. In this paper, a variety of bubble shapes and behaviors are shown but transient procedures how those features are continuously changing. The transient phenomena play an important role in many gas-liquid two-phase systems, since during the transient period, bubbles are exposed to rushing flow and bubbles cause turbulent flow.

PTV Processing Technique Considering Relative Position of Velocity Data to Deforming Boundaries (Keynote Paper)

In this paper, a post-processing technique for PTV measurement of flow around a deforming object is proposed. In general PTV, a position of velocity vector is defined as the midpoint of a tracer particle’s displacement obtained by two sequential images. This is because this velocity vector is close to the tangent vector of the path line. In this case, however, position of the object boundary at the middle time between two sequential PIV images must be interpolated. In this paper, a technique using three-frame images and Eulerian averaging in order to obtain velocity distribution on the regular grid points is proposed and evaluated. As the result, it is found that smooth and correct velocity distribution on the boundary of a bubble is obtained by the present approach.© 2003 ASME

PTV Measurement Technique around Moving and Deforming Boundaries

Velocity measurements using PIV in the vicinity of solid wall bring some issues relating to the interrogation area and measurement points. In order to avoid complexity associated with the interrogation area in the PIV, the PTV could be more suitable for such measurement. But, velocity measurements in the vicinity of rising bubbles, of which the boundaries are moving and deforming, cause additional problem of the definition of relative position between the boundaries and velocity points. For Example, velocities measured from pictures taken at time 1 and 2 are velocities at time 1.5, whereas existing boundaries in the pictures are at time 1 or 2.This paper proposes a method to define the relative positions between velocity points and boundaries at time 1.

Number and Size of Droplets near Liquid-Liquid Interface Generated by a Penetrating Sphere

This experiment was executed to investigate the deformation of liquid-liquid interface and making droplets by a penetrating sphere. This phenomenon will be related to problem of chemical reaction and contamination. Water and silicon oil under the room temperature are used for two kinds of liquid. An acrylic sphere or a brass sphere which is coated by paint is naturally released upon the oil layer respectively. The scene of penetration is captured by use of two cameras, that are high-speed video camera (500 frame/s) and high-grade video camera (30 frame/s, 1024-1024 pixel). The velocity of falling sphere and the numbers, positions, diameters, migration vectors of droplets are measured by image analyzing. These characteristics values (Table 1) are varied according to the velocity of a penetrating sphere. The time to generate droplets is measured about 0.1 second in this experiment.

Velocity Distributions Close to Surface of a Water Bubble Sinking in Silicone Oil

Boundary flow conditions at an interface between two fluids has been interested in many engineering fields relating to heat and mass transfer phenomena. Authors succeeded in measuring flows adjacent to the interface between a water-bubble and surrounding oil. Since the refraction index of water is less than that of the silicone oil, the inside region of the water bubble can be observed up to the very vicinity of the interface. PTV technique is applied to the measurement. The measuring results dearly show that the flows slip at the boundary and the velocities at both side of the dean interface are coincide with each other. The velocity gradient at each side of the boundary is different on the contrary