Experimental study on microbubble ejection method for frictional drag reduction

The formation of air bubbles ejected through a single hole in a flat plate was observed in uniform flow of 2–10 m/s It was confirmed that the size of the air bubbles was governed by main flow velocity and air flow rate. According to previous experiments, the size of the bubbles is an important factor in frictional drag reduction by microbubble ejection. Usually bubbles larger than a certain diameter (for example 1 mm) have no effect on frictional drag reduction. Three different methods were proposed and tested to generate smaller bubbles. Among them, a 2D convex (half body of an NACA 64-021 section) with ejection holes at the top was the best and most promising. The diameter of the bubbles became about one-third the size of the reference ejection on a flat plate. Moreover, the bubble size did not increase with increasing flow rate. This is a favorable characteristic for practical purposes. The skin friction force was measured directly with a miniature floating element transducer, and decreased drastically by microbubble ejection from the top of the 2D convex shape.     

Locomotion by mechanical pectoral fins

The objectives of this study were the development of a new device for maneuvering an underwater vehicle using the mechanism of a fish swimming, an experimental and theoretical analysis of the hydrodynamic characteristics of the device, and its application to maneuvering a fish robot. Observations and experimental analysis of the pectoral fins of a black bass (Micropterus salmoides) revealed that the locomotion of the fish, such as swimming forward at low speed, swimming backward, and turning in a horizontal plane is generated by using a combination of a feathering motion and a lead-lag motion of the pectoral fins. A mechanical pectoral fin making a feathering motion and a lead-lag motion generates a thrust force in a range of phase differences between both motions. The unsteady vortex lattice method, including the effect of viscosity, can express fairly well the unsteady forces acting on the mechanical pectoral fin in the range of phase differences where it exerts the thrust force. The fish robot, consisting of a model fish body and a pair of mechanical pectoral fins, can not only swim forward and turn in almost the same position, but can also swim in a lateral direction without changing the yaw angle. Translation of an article that appeared in the Journal of The Society of Naval Architects of Japan, vol. 182 (1997): The original article won the SNAJ prize, which is awarded annually to the best papers selected from the SNAJ Journal, JMST, or other quality journals in the field of naval architecture and ocean engineering.     

Effect of microbubbles on the structure of turbulence in a turbulent boundary layer

The time-averaged velocity and turbulence intensity distributions were measured by a laser Doppler velocimeter in a turbulent boundary layer filled with microbubbles. The void fraction distribution was also measured using a fiber-optic probe. The velocity decreased in the region below 100 wall units with an increase in bubble density. This led to a decrease in the velocity gradient at the wall, which was consistent with a decrease in shearing stress on the wall. The turbulence intensity in the buffer layer increased at a low microbubble density, and then began to decrease with an increasing microbubble density. Based on the present measurements, the mechanism of turbulence reduction by microbubbles is discussed and a model is proposed. Received for publication on Dec. 3, 1999; accepted on April 18, 2000     

Estimation of the value of travel time and of travel time reliability for heterogeneous drivers in a road network

Transportation - Travel time reliability has been recognized as an important factor in cost–benefit analysis in a transportation network. To estimate the benefit and cost of travel time...     

A mechanism to enhance bicycle conspicuity and visibility and increase detection distances: New insights into bicycle safety

Poor sensory conspicuity and poor visibility of bicycles are key factors that correlate strongly with bicycle-vehicle accidents. Although researchers have explored how to improve detection distances, i.e., the distances from which bicycles can be recognized by other road users, there is a dearth of research on ways to signal bicyclists' presence on the road. This study investigates how to enhance, at minimum cost, the level of visibility and sensory conspicuity of bicycles; it also considers ways to signal their presence to other road users, without necessitating any active behavior by bicyclists themselves. In the first study, the level of visibility of 6 rear-end components of bicycles was analyzed according to Adrian's model; the sensory conspicuity of these same components was analyzed via respondent perceptions in conditions of sunlight, twilight with no car headlights, twilight with car headlights, and night with car headlights. The level of visibility and sensory conspicuity of the 6 rear-end components were compared with considering angular size of the components under 4 lighting conditions. The level of visibility of the rear fender was good under sunlight and night-time conditions; in other conditions, the level of visibility was directly affected by painting the fender a silver color with reflectivity and also by the fender's angular size. However, the rear tire, among the 6 components tested, had a higher visible area when used with a short fender; it also produced rotational effects during riding conditions with no extra effort by the cyclists. In the second study, adhesive tape with specific patterns and 6 different color combinations were applied to the rear tire of a bicycle under the same lighting conditions, with the aim of creating a strong signal of the bicycle's presence for other road users. Among the 6 combinations, white stripes overlaid on the color red provide an optimal combination in terms of detection distance. The mean detection distance of white stripes on red in sunlight was 138.67 m, 94.67 m in twilight without car headlights, 94 m in twilight with car headlights, and 53.67 m at night with car headlights. In addition, this combination strongly signals the presence of the bicycle to other road users with no extra effort by the cyclists, thereby reducing the likelihood of drivers looking but failing to see bicycles. In sum, the study recommends that bicyclists install white stripes overlaid on red, in order to increase visibility and conspicuity and signal the presence of their bicycles, thereby reducing the likelihood of cyclist-vehicle collisions.     

Field experiments and new design of a spilled oil tracking autonomous buoy

It is important to forecast the location of oil spills to realize effective and adequate oil spill response operations when huge oil spilsl occur. In order to enhance the accuracy of oil drifting simulations, one needs to obtain the meteorological and oceanographic data around the oil slick. In general, the drifting velocity vector of an oil spill contains a wind velocity vector and a water current velocity vector. SOTAB-II was developed for autonomous tracking of oil slicks drifting on the sea surface. It is equipped with a sail whose size and direction are controllable to drift along with the oil slick autonomously. In addition, SOTAB-II transmits its location and necessary measured data around it to the land base in real-time. The results of field experiments using SOTAB-II with a cylindrical hull brought us the effectiveness of the sail and its control. However, the drifting speed of SOTAB-II was lower than a theoretical speed for the oil slick. In order to overcome this problem, SOTAB-II was redesigned. A yacht shape was adopted to reduce the hydrodynamic drag in the water in the advancing direction. Transverse stability, scales of brake board and sail, maneuverability, and performance of tracking spilled oil on the sea surface were considered in the process of the design.     

导流翼片的倾角和长度在降低大后壁车辆气动阻力中的作用

利用1／16缩比的公共汽车模型及不同倾角和长度的导流翼片，在一带有可移动地面和边界层吸收装置的开放型风洞中进行了在车辆后端面的4边上安装导流翼片时车辆后部尾流区的压强恢复及气动阻力降低的实验。对导流翼片的倾角和长度在降低大后壁车辆气动阻力中的作用及机理进行了研究。找到了能使车辆的气动阻力降低20％左右的导流翼片最佳长度及倾角。     

Editorial

From the Editors

Editorial