Measuring interfacial waves on film flowing down tube inner wall using laser focus displacement meter

To elucidate details of the fascinating nonlinear phenomena of waves on a film, spatial temporal knowledge of the interfacial waves is essential. This paper presents a new method for measuring waves on a film flowing down the inner wall of a vertical tube, using a laser focus displacement meter. The purpose of the study was to clarify the effectiveness of the new method for obtaining detailed information of film thickness or wave characteristics, and to investigate the effect of film entry length on the phenomena. With this method, accurate measurements of film thickness in real time, with sensitivity of 0.2 μm and 1 kHz, were achieved. The error caused by refraction of the laser beam passing through an acrylic tube, whose outer wall surface was flat, and water was estimated. As the entry length increased, the ripple on the flowing film grew to a solitary wave of about 0.5 mm in height, then the wave became a two-wave system. In the entry region, the wave amplitude decreased as the flow rate increased, in the same manner as that in a film flowing down a plate wall.