An optimal performance film cooling implies a high cooling effectiveness along with a wide surface cooling coverage. During the past decades, film cooling effectiveness has been well defined with a specific formula to evaluate the quality of a cooling system. However, despite numerous research on film cooling, there is still no explicit parameter to quantify how well a coolant film is spread over a hot surface. This work introduces a new coefficient namely cooling uniformity coefficient (CUC) to evaluate how well a coolant film is spread over a surface that is being cooled. Four different cases at two blowing ratios of 0.5 and 1.5 are studied. Three cases are ordinary single jets with different cross sections whereas the fourth one is a novel combined-triple-jet, sometimes called anti-vortex-holes, introduced in a previous work. The cross sections of all four cases were kept the same to have equal coolant mass flow rate. This article suggests that CUC is a necessary parameter in order to have a well performance film cooling system, in addition to the film cooling effectiveness. Indeed, CUC evaluates how well coolant films are spread over hot surfaces which is very important from thermal loads and thermal stress points of view.

This study was conducted to investigate the flow structures of turbulent flow passing over a short finite cylinder in which a rigid flag is attached to the rear of the cylinder, in wake region. The length-to-diameter ratio of the cylinder was chosen to be L/D = 2, whereas the rigid flag had a width-to-diameter ratio of W/D = 1.5. Wall-adapted large-eddy simulation (LES-WALE) was used to resolve unsteady turbulent flow structures. The far field Reynolds number based on cylinder diameter was chosen to be 20,000. The results were compared with the regular case wherein no flag was attached to the cylinder. Results revealed that the flow pattern behind the cylinder with flag was totally different in comparison with the regular case one. However, top free end of the cylinder was not influenced by the flag in contrast with the wake region. At far downstream from the cylinder, most of the flow structures in both cases appeared the same. The horseshoe vortices in both cases appeared to be an unsteady phenomenon, with slightly different patterns. Moreover, in the case of flag attachment, the pressure coefficient was smaller than that of with no flag. Finally, it was shown that the main and secondary Strouhal numbers locations were different in both cases.

In nature, most microorganisms have flexible micro/nanostructure tails, which help them create propulsion, reduce drag, or search for food. Previous studies investigated these flexible structures mostly from the propulsion creation perspective. However, the drag reduction and the underlying physical mechanisms of such tails are less known. This scientific gap is more significant when multi-polymeric/hierarchical structures are used. To fill the gap, we use the dissipative particle dynamics (DPD) method as a powerful fluid–polymer interaction technique to study the flexible tails' influences on drag reduction. Note that the flow regime for these microorganisms is in the range of laminar low Reynolds number; hence, the effects of both pressure and viscous drag forces are crucial. On the other hand, in the DPD method, only the total drag force is obtained. Therefore, this paper first proposes a way to determine the contribution of viscous and pressure drags and then investigates their effects on the body of the micro-robot separately. As a bioinspired-templated micro-robot simulation, the flow over a circular cylinder with an attached flexible tail is investigated. The problem is carried out for the Reynolds numbers from 10 to 25 for different polymer lengths (single/multi) and hierarchical structure tails. Our results show that long polymer tails strongly affect pressure drag, such that the longer polymeric tails (single/multi), the more drag reduction, particularly the pressure drag. Moreover, the hierarchical structures (containing short and long tails) caused the total drag reduction mainly by decreasing the viscous drag rather than the pressure one.