Abstract
Turbulent drag reduction is a promising phenomenon in which the turbulent skin friction can be greatly reduced by various media or approaches. This special issue focuses on the up-to-date progress in this field with 11 research articles from Japan, China, Korea, Malaysia, Oman, Mexico, and Indonesia after a strict peer review process. The topics cover three main areas: drag reduction by additives, drag reduction through surface alteration, and biomimetic drag reduction.
There are 4 articles in the area of drag reduction by additives. The paper “Drag Reduction of a Pipe Flow Using Nata de Coco Suspensions” by S. Ogata et al. investigates a type of biopolymer, nata de coco, exhibiting low mechanical degradation and inducing up to 25% drag reduction at 50 ppm. DR increased as the size of the network of nata de coco increased. The article “Parameters of Drag Reducing Polymers and Drag Reduction Performance in Single-Phase Water Flow” by A. Abubakar et al. presents an experimental investigation about the effect of polymer parameters on the performance of drag reducing water flow. Drag reduction depends on the polymer's ability to form intermolecular associations and/or its flexibility, which can be enhanced by increasing molecular weight, decreasing charge density, and selecting smaller side groups in the main polymer backbone. The article “Experimental and Numerical Study of Water Entry Supercavity Influenced by Turbulent Drag-Reducing Additives” by C.-X. Jiang and F.-C. Li is a study of the configurational and dynamic characteristics of water entry supercavities influenced by turbulent drag reducing additives. The authors find that the surface tension plays an important role in maintaining the cavity and turbulent drag reducing additives have the potential to enhance supercavitation and drag reduction. The article “Time-Dependent Shear-Induced Nonlinear Viscosity Effects in Dilute CTAC/NaSal Solutions: Mechanism Analyses” by N. Xu and J. Wei describes experimental tests of the time-dependent shear-induced nonlinear viscosity effects of a dilute surfactant solution (CTAC/NaSal). The apparent viscosity evolution curve is found to be divided into five stages: weak shear-thickening (Stage I), weak shear-thinning and plateau (Stage II), sharp shear-thickening (Stage III), oscillating adjustment (Stage IV), and rough plateau (Stage V).
There are 4 articles in the area of drag reduction through surface alternation. The article “Large Eddy Simulation of the Subcritical Flow over a U-Grooved Circular Cylinder” by A. Alonzo-García et al. presents a comparative large eddy simulation study of the flow over a smooth circular cylinder and the flow over a U-grooved cylinder. Drag coefficients and secondary vortices were predicted satisfactorily by the LES technique. The article “Drag Reduction by Microvortexes in Transverse Microgrooves” by B. Wang et al. employs a transverse microgrooved surface to reduce the surface drag force by creating slippage in the turbulent boundary layer. The simulation using RNG k-∊ turbulent model shows that the vortexes are formed in the grooves, providing the main reason for the 13% drag reduction. The article “Time Resolved PIV Investigation on the Skin Friction Reduction Mechanism of Outer-Layer Vertical Blades Array” by S. H. Park et al. is the study of outer-layer vertical blades using 2D time resolved PIV measurements. The vortical structures observed are cut and deformed by the blades array and the skin frictional reduction is closely associated with the subsequent evolution of turbulent structures. The article “CFD Study of Drag and Lift of Sepak Takraw Ball at Different Face Orientations” by A. S. A. Mubin et al. investigates the dynamics of the fluid around a static sepak takraw ball at different wind speeds for three different orientations. It is found that although the drag did not differ very much, increasing the wind velocity causes an increase in drag.
One article “Effect of Fluid Viscoelasticity on Turbulence and Large-Scale Vortices behind Wall-Mounted Plates” by T. Tsukahara et al. combines the above two areas. The authors use numerical simulation to study the vortices of the drag reducing flow by polymers or surfactants in channel mounted plates on the wall. They find that turbulent eddies just behind the plates in a viscoelastic fluid decrease in number and in magnitude, but their size increase. The mean flow and small spanwise eddies are influenced by the additional fluid force due to the viscoelasticity and the spanwise component of the fluid elastic force may also play a role in the suppression of fluid vortical motions behind the plates.
There are 2 articles in the area of biomimetic drag reduction. The article “Synthetic Effect of Vivid Shark Skin and Polymer Additive on Drag Reduction Reinforcement” by H. Chen et al. proposes an approach to improve polymer drag reduction by vivid shark skin. The article “Bionic Research on Bird Feather for Drag Reduction” by B. Feng et al. proposes drag reduction technique with a bionic method and obtains significant drag reduction efficiency.