High-frequency Measurement Of Vorticesin Open Channel Flow With Particle Image Velocimetry

Open channel flow occurs in numerous natural and engineeringsystems.Like other wall-bounded turbulence, open channel flowencompasses various coherent structures, among which, filamentary vortices has been postulated to be the sinews and muscles of turbulence.Experimental investigation of the dynamics of filamentary vortices necessitatestwo fundamental steps, i.e., accurate multi-point measurement of the velocity fieldsand adequate extraction of vortical structures with proper methods.To achieve multi-point velocity measurement, ahigh-frequency PIV(particle image velocimetry) system has been developed. The system includesa continuous-wave laser for illumination and a high-frame-rate COMS camera for image acquisition. The system was validated by applying standard PIV evaluationprocedures. Measurements in steady, uniform open channel flows verified that the precesion, temporal resolution, and spatial resolution of the system are sufficient for studying vortices in the outer region of open channel flows.To achieve adequate extraction of vortices from two-dimensonal velocity fields, an improved swirling-strength method has been proposed based on normalizing the vortex indicator for prograde and retrograde vortices, respectively, with its conditional mean. The improved method selects a single, universal threshold for both prograde and retrograde vortices andeliminates the influence of wall-normal dependence, rotation sense, and Reynolds number.Experiments were conducted in an open channel to investigate the characteristics of spanwise vortices,including their population density, mean radius, mean strength, and convection velocities. Results show that prograde vortices and retrograde vortices follow different wall-normal distributions. At fixed wall-normal positions, prograde vorticesexceed retrograde vortices in quantify, size, and strength. The free surface acts to influence vortices in the near-surface region(y/H≥0.7), resulting in an increase of population density, a reduction of the fraction of retrograde vortices, and adecrease of mean vortex size.Spanwise vorticesevolve from the growth stage to the decay stage. In the growth stage, spanwisevortices are small and weak but they gain strength and grow in size. The vortex size continues to grow in the early period of the decay stage before turning to decrease in the mid-late period. In the entire decay stage,the vortex strength drops constantly due to viscous dissipation and diffusion.The Reynolds stress is closely related to the generation of turbulent kinematic energy, and its gradient, defined as the net force,represents the mean effect of turbulent transport on the mean flow.In the core region of open channel flow, the induced motions of spanwise vortices generate much of the Reynolds stress, and the upward movement of spanwise vortices induced by these motions contributes significantly to the net force. These findings reveal that spanwise vortices are very important in shaping the mean flow and sustaining the turbulence.