The flow around single and two side-by-side infinite cylinders is numerically modelled using dynamic Smagorinsky large eddy simulation (LES). For the single cylinder, the Reynolds number based on the diameter and the free stream velocity is 3900. A complete sensitivity study was conducted based on the extrusion length in the span-wise direction, the grid refinement at the wall, the convection scheme, and the sub-grid scale (SGS) model. It was found that the mean solution is not influenced by the extrusion length beyond $4$ diameters or by $1\%$ up-winding. However, coarsening the mesh in the wall normal direction or switching off the sub-grid scale model led to drastic effects on the recirculation length and on the underlying velocity field. The two side-by-side cylinders were tested for a range of pitch to diameter ratios ($T/D=1.0,1.25\le T/D\le 5.0$ ) at a Reynolds number of 3000. For the intermediate pitch to diameter ratios ($1.25\le T/D\le 1.75$ ), multiple shedding frequencies were detected with a biased wake flow deflection. Furthermore, this biased flow deflection was found to be bistable, i.e., it changes the direction (flipping over) intermittently from one side to the other. This behavior was found to be consistent with reported experimental measurements. During the flip-over from one stable mode to the other, the intermittent gap vortex shedding was found to be stronger than for a stable mode, with in-phase vortex shedding. However, for the higher pitch ratio cases ($T/D\ge 2$ ), a symmetrical wake behavior with anti-phase vortex shedding was observed.

A wide range of engineering industrial applications require both the thermal and optical efficiencies of the system to be maximized with a reasonable low penalty for the friction factor and subsequently low losses in pressure. Among the family of concentrated solar power systems, parabolic trough collectors (PTCs), which have recently received significant attention, face similar challenges. The current work presents an extensive review of the PTC systems comparing recent and past technologies, which are widely being used to improve and enhance the thermal and optical efficiencies. Furthermore, the techniques used for single and two‐phase flow modeling in numerical simulations, design variables, and experimental processes have been discussed in detail. The article also presents different numerical methods and analytical approaches of implementing the nonuniform solar distribution with different design parameters. Four main technologies are comprehensively addressed to effectively enhance the thermal performance of the PTCs; changing working heat transfer fluids, replacing the working fluids by nanofluids (single and hybrid) that have higher thermal–physical properties than those of base working fluids, inserting different tabulators with various design configurations, and finally combining the advantages of nanofluids and swirl generators in the same application. The article also critically summarizes the studies investigating the enhancement of thermal performance: use of novel design of PTCs and passive heat transfer enhancement techniques. Finally, a wide range of numerical and experimental studies are proposed for the future work related to the aforementioned main technologies.