An Innovative Methodology for Measuring the Free Surface Profile in Two-phase High-speed Flows

The investigation of the flow over hydraulic structures like spillways and weirs is challenging and represent a key point in the hydraulic design. The shape of the free flow over these hydraulic structures depends on several factors, among which: the interaction between air and water, the turbulence of the flow and the particular geometry of the hydraulic structure. In particular, air-water two-phase flows have been observed in a large number of hydraulic structures, such as hydraulic jumps, spillways, jets and plunge pools. Since numerical models cannot be trusted in these conditions, physical models are often used to investigate the related flow characteristics.

The present study proposes an innovative experimental method for the identification of the two-phase free surface profile downstream of a plunge pool of a dam physical model (Gurzia Dam, Italy). The physical model operates under the Froude similitude. In order to keep the physical models within economic dimensions and to minimize the discharges, a scale factor of 40:1 is applied. The new experimental technique involves the use of a photo camera and laser sheets. The water flow is vertically sectioned by a green laser sheet emitted by a 200 mW power line laser source (fan angle 30°). Although the water is transparent, the refraction of the laser light due to air and water particles allows to clearly detect the two-phase free surface profile. To highlight the contrast of the laser beam on the two-phase flows, a dark environment is necessary. A Canon EOS500d, situated on a horizontal bar at a distance of 2 m from the test section, is used to image the two-phase water surface profile. A calibration grid is used in the post-processing to transform the detected water level from pixel coordinates to metric coordinates. To precisely detect the position of the water surface, it’s crucial to choose the suitable camera parameters. A mean value of the water profile is obtained averaging out the detected profile of a high number of pictures. Once set-up, this method is rapid and enables to determine accurate results of the water surface profile both in time and space dimensions.