Narrow Results

It is argued that PHENIX collaboration observed for the first time the radiation of the longitudinal wake oscillations formed behind the parton penetrating the quark–gluon medium. It shifts the maximum of a hump in two-particle correlations and changes its width in the case of some special orientation of the trigger particle.

Particle image velocimetry (PIV) is a non-intrusive optical diagnostic and must be made of the seedings (foreign particles) instead of the fluid itself. However, reliable PIV measurement of turbulence requires sufficient numbers of seeding falling in each interrogation window of image. A gray-level criterion is developed in this work to check the attainment of statistically stationary status of turbulent flow properties. It is suggested that the gray level of no less than 0.66 is used as the threshold for reliable PIV measurements in the present near-wake turbulent regions.

In this study, we illustrate the fractal nature of the wake shed by a periodically flapping filament. Such wake structure is a combination of primary vortex shedding resulting in the von Kármán vortex street, a series of concentrated vortex dipoles formed when the trailing edges of filaments reach their maximum amplitudes and small eddies form along the shear layer connected with the concentrated vortices due to the shear layer instability. The vortex dynamics of the flapping filament are visualized and imaged experimentally using a soap-film flow tunnel with a high-speed camera and a low pressure sodium lamp as a light source. The wake fractal geometry is measured using the standard box-counting method and it is shown that the fractal dimension of the soap pattern boundaries in the wake is D = 1.38 ± 0.05, which agrees well with those measured for fully developed turbulences and other shear flow phenomena. The invariant of the fractality in the wake induced by the flapping filament thus provides another illustration of the geometrical self-similarity and nonlinear dynamics of chaotic fluid flows.

To research the wake structure of horizontal axis marine current turbine, numerical model that based on actuator disc theory and CFD is established, which could simulate characteristics of the wake. This study focused on the far wake that influenced by turbine arrays, the model applied a momentum source term to the momentum conservation equations that represent current turbine region, successfully parameterizing the turbine influence in the Reynolds-averaged Navier-Stokes equations. The SST k – ω turbulent model is applied in the model. The velocity deficit value of different cross section downstream is compared between model and experiment, the result performs good agreement, shows that the numerical model is credible in predicting the far wake velocity deficit. The velocity distribution of far wake is analysed in details, the result shows that the 12D downstream centerline velocity is 81% relative to the inflow velocity, the influence of transverse distance to the velocity deficit is analysed as well. The numerical model in this study is more efficient in predicting the far wake velocity deficit and occupies less computing resources, can give velocity deficit value more quickly that is credible to some extent, and is more feasible in engineering application. This study can provide reference for horizontal marine current turbine arrays application.