Shigeru Nishio

Standard images for particle-image velocimetry

Particle-image velocimetry (PIV) offers lots of advantages for studying fluid mechanics. Many PIV techniques and systems have been developed. However, no standard evaluation tool for evaluating the effectiveness and accuracy of the PIV systems has been established. To popularize PIV practically, for each PIV system there should be some means of evaluating the performance. PIV involves two processes, i.e. capturing the image for visualization and the image analysis. In order to evaluate the image analysis, the use of standard images has been proposed. Using these images, anybody can evaluate the effectiveness and accuracy of the PIV image analysis. The standard PIV images can be grouped into three categories, i.e. standard PIV images for two-dimensional, custom-made images with tunable parameters and images for a transient flow. The standard PIV images that we have developed are distributed via the web site http://www.vsj.or.jp/piv as part of a collaboration with the Visualization Society of Japan. They can be applied to investigate the performance of any PIV technique. The standard PIV images that we have developed have already been accessed by more than 3 000 researchers around the world.

In vivo PIV measurement of red blood cell velocity field in microvessels considering mesentery motion

As endothelial cells are subject to flow shear stress, it is important to determine the detailed velocity distribution in microvessels in the study of mechanical interactions between blood and endothelium. Recently, particle image velocimetry (PIV) has been proposed as a quantitative method of measuring velocity fields instantaneously in experimental fluid mechanics. The authors have developed a highly accurate PIV technique with improved dynamic range. spatial resolution and measurement accuracy. In this paper, the proposed method was applied to images of the arteriole in the rat mesentery using an intravital microscope and high-speed digital video system. Taking the mesentery motion into account, the PIV technique was improved to measure red blood cell (RBC) velocity. Velocity distributions with spatial resolutions of 0.8 x 0.8 microm were obtained even near the wall in the centre plane of the arteriole. The arteriole velocity profile was blunt in the centre region of the vessel cross-section and sharp in the near-wall region. Typical flow features for non-Newtonian fluid were shown. Time-averaged velocity profiles in six cross sections with different diameters were compared.

A highly accurate iterative PIV technique using a gradient method

An iterative PIV technique in which the combination of the iterative cross-correlation technique and three-point Gaussian peak fitting for a sub-pixel analysis has been used can improve the spatial resolution and accuracy of measurement. It is reported that the root-mean-square (RMS) error of the technique is of the order of only 0.04 pixels. However, a large interrogation window, typically 32×32 pixels or larger, should be taken, resulting in a low resolution, in order to achieve the high sub-pixel accuracy. The high accuracy is not compatible with high spatial resolution. In this paper, a new high-resolution PIV technique based on a gradient method is proposed. Initially, the pixel unit displacement is detected by the iterative method. Then, the sub-pixel displacement is evaluated by the use of the gradient method instead of the three-point Gaussian peak fitting technique. The error of the proposed technique is analytically assessed by Monte Carlo simulations. The RMS error is of the order of 0.01 pixels even with a small interrogation window, for instance 13×13 pixels or less. Thus, the method can achieve high sub-pixel accuracy and high spatial resolution compatibility.

Evaluation of the 3D-PIV Standard Images (PIV-STD Project)

Particle Imaging Velocimetry (PIV) offers many advantages for studies of fluid flows. Lots of PIV techniques have been developed and applied to various flow fields. However, there are no standard tools for evaluation of the PIV system effectiveness and accuracy. To popularize the PIV for practical use, the PIV system should have some standard.The Visualization Society of Japan (VSJ) started a project for PIV standardization and popularization in 1996 (PIV-STD Project). The PIV has two processes, i.e., the image capture with visualization and image analysis. In order to evaluate the image analysis system for the PIV, standard images were proposed. Using the images, an evaluation of the effectiveness and accuracy for the PIV system will be carried out. The developed PIV standard images are distributed using the Internet, (http://www.vsj.or.jp/piv). The standard images include the two-dimensional and transient threedimensional images. The correct velocity vectors and correct particle location in three-dimensional field are also provided to evaluate the effectiveness of the target flow field.

Image measurement of flow field using physics-based dynamic model

The PIV technique is generalized from the viewpoint of the identification problem of a flow field. The process of obtaining a velocity distribution as an image measurement can be considered as a parametrization problem of a dynamical system. In almost all conventional techniques, the velocity is evaluated as the parameters in a dynamical system using a linear model without physics. These methods always give some spurious velocity vectors in the case of shear flow or vortex flow, and in the region where the tracer particles are scarce. A new PIV technique based on physics is proposed. The obtained velocity field satisfies the governing equation of the fluid. It is possible that not only the velocity but also the pressure is measured, using the Navier-Stokes equations for the dynamic model. The results of jet flow, cavity flow and flow around a waving airfoil demonstrate the effectiveness of this method.

Measurement of a velocity field in microvessels using a high resolution PIV technique.

Abstract: Because endothelial cells are subject to flow shear stress, it is important to determine the velocity distribution in microvessels during studies of the mechanical interactions between the blood and the endothelium. Particle image velocimetry (PIV) is a quantitative method for measuring velocity fields instantaneously in experimental fluid mechanics. The authors have developed a high‐resolution PIV technique that improves the dynamic flow range, spatial resolution, and measurement accuracy. The proposed method was applied to images of the arteriole in the rat mesentery, using an intravital microscope and high‐speed digital video system. Taking the mesentery motion into account, the PIV technique was improved to measure red blood cell (RBC) velocity. Velocity distributions with spatial resolutions of 0.8 3 0.8 mm were obtained even near the wall in the center plane of the arteriole. The arteriole velocity profile was blunt in the center region of the vessel cross‐section and sharp in the near‐wall region. Typical flow features for non‐Newtonian fluid are shown.

A Comparative Study of the PIV and LDV Measurements on a Self-induced Sloshing Flow

Particle Imaging Velocimetry (PIV) and Laser Doppler Velocimetry (LDV) measurements on a self-induced sloshing flow in a rectangular tank had been conducted in the present study. The PIV measurement result was compared with LDV measurement result quantitatively in order to evaluate the accuracy level of the PIV measurement. The comparison results show that the PIV and LDV measurement results agree with each other well in general for both mean velocity and fluctuations of the velocity components. The average disagreement level of the mean velocity between PIV and LDV measurement results was found to be within 3% of the target velocity for the PIV system parameter selection. Bigger disagreements between the PIV and LDV measurement results were found to concentrate at high shear regions. The spatial resolution and temporal resolution differences of the PIV and LDV measurements and the limited frames of the PIV instantaneous results were suggested to be the main reasons for the disagreement.

Evaluation of cavitation in injector nozzle and correlation with liquid atomization

Present paper describes the correlation between occurrence of cavitation inside injector nozzle and progress of atomization in discharged liquid jet flow. A transparent two-dimensional shape nozzle was designed and it was used for flow visualization. The visualized images both of cavitation and atomization were obtained simultaneously. The cavitation was evaluated by applying image analysis. The quantitative analysis on length, thickness and their fluctuations of cavitation was made, and the correlation with atomization was examined. The authors proposed an evaluation method of atomization in the previous study, and it was combined with evaluation method of cavitation developed in this study. It is found that the change of cavitation and progress of atomization are synchronized, and that a strong correlation between them exists. The best performance of atomization may be obtained by controlling the cavitation inside nozzle, and obtained aspects with this study will be utilized for the design of fuel injection nozzle of internal combustion engines.Graphical abstract

Study on application of shock waves generated by micro bubbles to the treatment of ships’ ballast water

This paper reports the fundamental study for development of a ships’ ballast water clarifying treatment technology using shock pressures generated by the collapse of micro bubbles. From the microscopic observation, it was confirmed that micro bubbles less than 65 μm in diameter became small in size and disappeared, and the shock wave generation by their collapse was observed by optical visualization method and pressure sheets. The bio-experiments using a marine Vibrio sp. were carried out under the three conditions: (1) only use of micro bubbles, (2) only use of 20 kHz ultrasonic waves, and (3) simultaneous use of them. The number of survival cells was investigated under the respective conditions for 12 hours. The inactivation effect on the marine Vibrio sp. by simultaneous use of micro bubbles and ultrasonic waves indicated better inactivation than that by the only use of micro bubbles or ultrasonic waves.

Simulation of Deepwater Horizon Oil Spill using Atmosphere-Ocean General Circulation Model

The Deepwater Horizon accident discharged a staggering amount of oil (about 780,000 KI) into the Gulf of Mexico. In other forecast simulations based on the ocean-only models, spilled oil reach the Atlantic Ocean by the Gulf Stream. In this study, we simulated 3-dimenSiOnal movement of oils using the real-time oceanic and atmospheric conditions obtained by the coupled MSSG (Multi-Scale Simulator for the Geoenvironment) model. Our simulation show that the amounts of oil discharge into the Atlantic is very limited due to the trade winds. The atmospheric coupling to the ocean model is essential in the forecast simulation of spilled oils.