1002 界面活性剤水溶液の抵抗減少流れにおける乱れ構造の可視化

Abstract

Turbulent structure in the two-dimensional channel flow of drag-reducing surfactant solution is investigated using hydrogenbubble technique. The visual observations are focussed on the coherent motions in the near-wall region; i.e., ejections and quasi-streamwise vortices. It is revealed that the ejections of near-wall fluid are suppressed in the drag-reducing flows. The ejection could not be observed when ‘turbulence reduction – drag’ (TRD) is larger than 75%, where TRD is defined using the friction difference between laminar and turbulent flows (Gasljevic and Matthys [2]). It is found that quasi-streamwise vortices exist even at large TRD above 80%. The average position of the quasi-streamwise vortex centre becomes more distant from the wall as TRD increases. Introduction It is well known that some kinds of surfactant solution cause a remarkable drag reduction in turbulent flows. Many investigations have been conducted concerning this problem [3,12]. The measurements using LDV (laser Doppler velocimetry) show that the Reynolds shear stress is close to zero in surfactant solutions with large drag reduction [3,11]. Warholic et al. [11] conjectured that the quasi-streamwise vortices do not exist near the wall under maximum drag reduction. In our previous experiment [5], we visualized lowspeed streaks in the two-dimensional channel flow of dragreducing surfactant solutions and showed that the low-speed streaks exist even at large drag reduction. It was also revealed that, in the drag-reducing flows, the mean streak spacing reaches a minimum at some distance from the wall and the streaks decrease rapidly with approaching the wall. In order to interpret these experimental evidences, the coherent motions near the wall (ejections and quasi-streamwise vortex motions) are visualized in the present work. Hydrogen-bubble technique is widely used to visualize the coherent motions. Experimental Apparatus and Procedure The experimental apparatus used in this work is the same with the one used before [4]. The two-dimensional channel is made of acrylic plates and the dimensions are the width W = 40 mm, the height H = 400 mm and the length L = 4 m. The pressure drops were measured from static pressure taps situated at 65W~90W downstream from the channel inlet. The velocity measurements and the visual observations of near-wall turbulent structures were made using the hydrogen-bubble technique [6,9]. Figure 1 shows the details of the measuring section. The wire used for generating hydrogen-bubbles was a 30μm stainless-steel wire. A DC pulse generator was used to generate hydrogen-bubbles, and the hydrogen-bubble time-lines or sheets were viewed and recorded using a CCD video camera and a VTR. The individual video frames were converted to a digital array using an imageprocessing system and then analysed by a microcomputer. Th mixt salic impu filter were Exp Pres Fi versu figur