On Turbulence Measurements in Hydraulic Jumps with ADV Techniques

ROMAGNOLI, M.; LOPARDO, R. A.; GARCIA, C. M.

Lake Place, New York

Congreso; HYDRAULIC MEASUREMENTS & EXPERIMENTAL METHODS; 2007

IWA

A hydraulic jump is a rapidly varied transition from supercritical to subcritical flow and it is characterized by the presence of high flow turbulence levels and air entrainment. Hydraulic jumps has been studied by many researchers in the past, but most of the works have focused on an integral analysis and little is known about internal flow and turbulence structures due to the adverse experimental conditions for water velocity measurements within the hydraulic jump. This work explores issues on turbulence measurements in hydraulic jumps using acoustic Doppler velocimeters (ADV) techniques. Previous studies showed that ADVs are capable of reporting accurate mean values of water velocity in three directions, even in low flow velocities. In addition, ADVs have proved to yield a good description of turbulence when certain conditions are satisfied. These restrictions are related to the instrument configuration (sampling frequency and noise energy level) and flow conditions (convective velocity and turbulence scales in the flow). This work explores and analyzes these restrictions as well as other issues on turbulence measurements using ADVs in hydraulic jumps. A micro acoustic Doppler velocimeter (micro ADV, SonTek) has used in this work to record time signals of flow velocities in a free hydraulic jumps generated downstream a vertical sluice gate in a horizontal rectangular flume. Measurements have been performed using ADVs in different locations within the hydraulic jump for different flow conditions represented by different Froude numbers. It was found that in the regions of high flow turbulence levels and air entrainment, the computed power spectrums of the velocity signals showed a flat plateau which is characteristic of Doppler noise. Furthermore, the restrictions previously mentioned to yield a good description of the flow turbulence were not always satisfied. This suggests that the capability of the acoustic Doppler velocimeter to resolve flow turbulence in these regions is questionable. However, the ADV performance significantly improved in the transition region between the end of the hydraulic jump and the flow conditions downstream, where both the air concentration and the turbulence intensity decrease.