Flow Visualization and analysis by PIV Measurement System using Micro-bubbles as Tracer in Ship Model Basin-Measurement in Still Water and Regular Waves -
To measure and analyze the flow around ship model, use of micro-babbles as tracer is proposed here. The system has two high speed cameras for stereo photography with continuous laser light sheet. Using continuous laser light sheet flow around ship can be observed directly. To make validation of the present system flow analysis of uniform flow and regular waves in ship model basins is carried out. From the comparison between flow velocity analyzed by the present system and that measured by current meter in different three ship model basins, it is found that the correlation factors of the basins are over 0.99 for flow velocity of ship advance direction. In regular waves the trajectory of the wave motion is compared to small amplitude wave theory and analyzed value. From the comparison it is found that the agreement is well for different wave frequencies. Continuously, flow measurement using a ship model of large tanker has performed in still water and in regular head waves. From the analysis it is found that the difference of the flow vector between still water and regular head waves can be evaluated qualitatively. Thereafter to understand the flow around propeller the tank tests in self propelling condition of the ship model are performed. From the comparison between the tests with propeller and without propeller, the effect of propeller suction is revealed. From the various comparisons the effectiveness of the PIV system using micro-bubbles to obtain flow information around ship is confirmed.
Optimization of Orifice Shape and Oscillatory Displacement of an Ultrasonic Microbubble Generator to Increase Microbubble Yield
Microbubbles have various properties such as low rising velocity and large surface area per unit volume. Owing to these favorable properties, there has been a recent increase in the application of microbubbles in the agricultural and medical fields, including the development of a microbubble generator, which employs a hollow cylindrical ultrasonic horn. In this device, gas is supplied through a path in the horn to create a gas-liquid interface in water. Thereafter, ultrasonic oscillation is irradiated to the surface to generate microbubbles. However, to realize practical applications of microbubbles, a higher yield is required. This study aims to increase the microbubble yield by modifying the shape of the orifice at the horn end and evaluating the resulting microbubble yield by measuring the concentration of oxygen dissolved in water. The horn end contains side orifices and releases bubbles between walls; it achieved an increase of up to 60% in the generation of microbubbles. This increase is likely because this modified design enables better reception of strong ultrasonic oscillations, as compared to conventional straight orifices. The effect of ultrasonic amplifications were also evaluated.
Iterative image deformation methods in PIV
Image processing methods for particle image velocimetry (PIV) are reviewed. The discussion focuses on iterative methods aimed at enhancing the precision and spatial resolution of numerical interrogation schemes. Emphasis is placed on the efforts made to overcome the limitations of the correlation interrogation method with respect to typical problems such as in-plane loss of pairs, velocity gradient compensation and correlation peak locking. The discussion shows how the correlation signal benefits from simple operations such as the window-offset technique, or the continuous window deformation, which compensates for the in-plane velocity gradient. The image interrogation process is presented within the discussion of the image matching problem and several algorithms and implementations currently in use are classified depending on the choice made about the particle image pattern matching scheme. Several methods that differ in their implementations are found to be substantially similar. Iterative image deformation methods that account for the continuous particle image pattern transformation are analysed and the effect of crucial choices such as image interpolation method, displacement prediction correction and correlation peak fit scheme are discussed. The quantitative performance assessment made through synthetic PIV images yields order-of-magnitude improvement on the precision of the particle image displacement at a sub-pixel level when the image deformation is applied. Moreover, the issue of spatial resolution is addressed and the limiting factors of the specific interrogation methods are discussed. Finally, an attempt for a flow-adaptive spatial resolution method is proposed. The method takes into account the local velocity derivatives in order to perform a local increase of the interrogation spatial density and a refinement of the local interrogation window size. The resulting spatial resolution is selectively enhanced. The method's performance is analysed and compared with some precursor techniques, namely the conventional cross-correlation analysis with and without the effect of a window discrete offset and deformation. The suitability of the method for the measurement in turbulent flows is illustrated with the application to a turbulent backward facing step flow.