Cavitation risk estimation at orifice spillway based on UVP and dynamic pressure measurements
Orifice spillways might be affected by cavitation if the profile of the inlet structure includes a relevant curvature. At one of the world’s largest dams, such damages were observed on the profiled slab. To limit their progression, an adapted operation regime of the gates had to be defined using physical model tests. The risk estimation was based on dynamic pressure and velocity measurements, indicating that a maximum gate opening ratio of 0.8 should be respected until the damage is repaired and the profile re-shaped. Beside this, the data set allowed to compare the flow characteristics at the profile surface in terms of turbulence intensity and dominant frequencies. The Ultrasonic Velocity Profiler (UVP) provides comparable characteristic values as the dynamic pressure transmitter (DPS).
Use of gas bubbles for ultrasound Doppler flow velocity profile measurement
Abstract Ultrasonic velocimetry based on the Doppler shift effect accurately provides quasi-instantaneous flow fields for fluids with a sufficiently high acoustic scattering level. However, ultrasonic velocity instruments are known to perform poorly in clear water with low acoustic scattering level, which are frequent conditions in laboratory applications. This work confirms a technique to solve the problem by seeding the flow with micro hydrogen bubbles, generated by means of electrolysis. This paper investigates the influence of gas bubbles density on the quality of the ultrasound Doppler based velocity profiles in an open channel flow. The bubbles are generated by electrolysis of water using different magnitudes of electrical current. The estimation of the number of bubbles in the measurement volume confirms that the bubble diameter is similar to that of the wire used for electrolysis. This enables to determine the minimum density of gas bubbles needed to obtain a reasonably good echo and therefore an accurate velocity profile.
Deflector-generated jets
Free jets issued at the end of chutes may be generated rather with deflectors than with flip buckets. The latter jet trajectories were previously investigated, using the standard approach based on the take-off angle. However, few information concerning the air concentration distribution within such jets is presently available. The herein presented study investigates both jet trajectories and the streamwise air concentration development based on 129 hydraulic model tests. The trajectories were normalized using the take-off coordinates and the location of maximum trajectory elevation, to reduce the uncertainties concerning the effective jet take-off angle. The characteristic air concentrations are described as a function of the relative black water jet core length, defined between the take-off section and the location with a minimum of 1% air concentration. An example finally compares the present approach with a former study and illustrates typical air concentration distributions within a deflector-generate...