Coherent Vortices Properties of Partially Coherent Elegant Laguerre-Gaussian Beams in the Free Space

The analytical expression of the cross-spectral density of partially coherent elegant Laguerre-Gaussian beams propagating in free space has been derived. The coherence vortex properties of such beams have been investigated. The effect of the beam parameters, including the topological charge, radial mode index and coherence length on the coherent vortex, is analyzed. The results show that the higher order (lth) of coherent vortices split to lth first order of coherent vortex. New coherent vortices of opposite sign appear, and then pairs of coherent vortices form. The propagation distance and coherence length affect the generation of coherent vortices, while the radial mode index doesn’t affect the coherent vortices. These results might be helpful for potential application of such beams in optical communication.

Partially coherent vortex beams:Fundamentals and applications

It has been over 30 years since the concept of optical vortices was first proposed by Coullet et al.in 1989,and the field of structured beams has grown extremely.In the last two decades,partially coherent vortex beams(PCVBs)have received increasing interest in the fields of optical manipulation,optical communication,optical imaging,etc.,and great progress has been made in the area of the coherence singularities,generation methods,topological charge measurements,and promising applications of PCVBs.In this review,we firstly outline the basic concepts of PCVBs.We explicate the relationship between the coherence vortices and optical vortices,and the evolution behavior of optical vortices to coherence vortices is summarized in detail.We discuss a special form of coherence singularity,ring dislocation,mathematically and physically.The ring dislocation in the correlation functions under low coherence is dependent on the mode indices,which provide a feasible approach to measure mode indices of PCVBs.Subsequently,we summarize the various methods for measuring the topological charge of PCVBs,highlight the measurement method based on the cross-correlation function,and a physical explanation on the relation between ring dislocation and topological charge is given.After that,we review the recent advances on experimental generation of several kinds of PCVBs.Lastly,we give an overview on the potential applications of PCVBs.

Comparison of Reynolds Averaged Navier-Stokes Based Simulation and Large-eddy Simulation for One Isothermal Swirling Flow

The flow structure of one isothermal swirling case in the Sydney swirl flame database was studied using two numerical methods. Results from the Reynolds-averaged Navier-Stokes (RANS) approach and large eddy simulation (LES) were compared with experimental measurements. The simulations were applied in two different Cartesian grids which were investigated by a grid independence study for RANS and a post-estimator for LES. The RNG k-ε turbulence model was used in RANS and dynamic Smagorinsky-Lilly model was used as the sub-grid scale model in LES. A validation study and cross comparison of ensemble average and root mean square (RMS) results showed LES outperforms RANS statistic results. Flow field results indicated that both approaches could capture dominant flow structures, like vortex breakdown (VB), and precessing vortex core (PVC). Streamlines indicate that the formation mechanisms of VB deducted from the two methods were different. The vorticity field was also studied using a velocity gradient based method. This research gained in-depth understanding of isothermal swirling flow.