To reduce the drag of a cylinder, numerical simulations and experiments for both smooth cylinder and circular cylinder with the dimpled surface are carried out in this paper. The numerical simulation focuses on the variation of pressure coefficient, skin friction coefficient, and vortex shedding strength of the smooth cylinder and the circular cylinder with the dimpled surface. It is found that the dimpled structure can effectively reduce the drag of the cylinder within a specific range of Reynolds number, and the maximum drag reduction rate reaches up to 19%. Another conclusion is that the pressure drag and skin friction drag have an essential influence on the total drag of the circular cylinder with the dimpled surface. On the other hand, the strength of vortex shedding also decreases with the decrease of cylinder drag. Then, the flow field of both cylinders is measured using the particle image velocimetry (PIV) technique, confirming that the dimpled structure can affect the velocity field, the release of vortices and the scale of the vortex. More specifically, the velocity recovery of the circular cylinder with the dimpled surface is faster than that of the smooth cylinder, and the dimpled structure delays the release of the vortex at a specific range of Reynolds number.

In this study, the drag reduction effect is studied for a cylinder with different V-groove depths on its surface using a k-ω/SST (Shear Stress Transport) turbulence model of computational fluid dynamics (CFD), while a particle image velocimetry (PIV) system is employed to analyze the wake characteristics for a smooth cylinder and a cylinder with different V-groove depths on its surface at different Reynolds numbers. The study focuses on the characteristics of the different V-groove depths on lift coefficient, drag coefficient, the velocity distribution of flow field, pressure coefficient, vortex shedding, and vortex structure. In comparison with a smooth cylinder, the lift coefficient and drag coefficient can be reduced for a cylinder with different V-groove depths on its surface, and the maximum reduction rates of lift coefficient and drag coefficient are about 34.4% and 16%, respectively. Otherwise, the vortex structure presents a complete symmetry for the smooth cylinder, however, the symmetry of the vortex structure becomes insignificant for the V-shaped groove structure with different depths. This is also an important reason for the drag reduction effect of a cylinder with a V-groove surface.

In order to improve the efficiency and safety of vertical hydraulic transport systems for non-spherical particles, a new pipeline transport system with a tangential jet inlet is adopted in this study, and a modified non-spherical drag coefficient model is used to analyze the liquid–solid flow characteristics based on the CFD-DEM (Computational Fluid Dynamics-Discrete Element Method) coupling method. The focus of the study is on the influence of different tangential flow proportions in terms of the velocity distribution, the vorticity, the total pressure, the concentration and drag force of particles of various shapes. The conveying efficiency is measured according to the fluid velocity distribution and the particle concentration, and the safety of conveying is evaluated according to the flow structure and drag force of the particles. The result shows that the velocity of the swirling pipes is significantly higher than the straight pipe. With the increase of the tangential flow proportion, the swirling number and the vorticity magnitude increase, and the vortex core is broken and merged more quickly. Furthermore, the concentration gap and axial drag force gap between particles of various shapes are reduced with the effect of swirling flow, the particle concentration increases, and the particles of each component are uniformly mixed and transported.

The near-wake structure resulting from the interaction between the water and the cylinder is researched. The wake characteristics for the smooth cylinder and the cylinder with different numbers of grooves are measured by high-speed particle image velocimetry (PIV) at a Reynolds number of 7400. From the measured data, it is observed that the v-groove increases the Reynolds shear stress and the turbulent kinetic energy; thus, the formation of the Karman-like vortices is prevented and the recirculation region is reduced. The number of grooves has a limited effect on the wake characteristics. In addition, a one-dimensional orthogonal wavelet decomposes the fluctuation velocities in the cylindrical wake region to study the multi-scale wake characteristics behind the v-groove cylinders. The large-scale structures show that the Karman-like vortices of the grooved cylinder are closer to the trailing edge of the cylinder. The intermediate-scale structures show more vortices behind the 32-groove cylinder. The small-scale structures show that the phase difference between the upper and lower oscillation positions has become smaller. Meanwhile, the large-, intermediate- and small-scale Reynolds shear stress distributions suggest that the shear layer instability in the near-wake region creates vortices, which increase the turbulent kinetic energy and narrow the wake region.

In order to understand the wake characteristics for the circular cylinder with the dimpled structure, particle image velocimetry (PIV) and proper orthogonal decomposition (POD) method are used to measure and analyze the flow field velocity and the flow characteristic of the wake flow of the circular cylinder with the dimpled surface. This study focuses on the distribution of Reynolds stress and turbulent kinetic energy, the velocity profiles and recirculation zones, and the velocity fluctuating characteristics of flow field with POD technology. It is found that the equivalent high-intensity Reynolds stress and turbulent kinetic energy regions of the circular cylinder with the dimpled surface are smaller, and the peak values are lower, and the velocity gradient in the wake region of the circular cylinder with the dimpled surface is larger. Otherwise, the energy contained by the dominant modes of the smooth cylinder is larger than that contained by the dominant modes of the circular cylinders with the dimpled surface, which means the energy of the dimpled cylinder is more distributed. At the same time, it is observed that the dimpled structure will decrease the vortex shedding intensity, but may increase the vortex shedding frequency, and destroy the inherent flow mode of the flow field around the cylinder.

Flow around cylinders is widespread in marine engineering projects such as marine risers, marine pipelines, and tension leg. To understand the wake characteristics of the circular cylinder with different roughness, at a Reynolds number of 7400, a circulation water tunnel is used for experimental PIV measurements to compare the wake characteristics among the smooth cylinder, the original grooved cylinder, and the secondary grooved cylinder. The results revealed that the secondary grooved reduced the recirculation region, the flow-direction velocity gradient, the Reynolds shear stresses, and turbulent kinetic energy. Both small-scale and large-scale vortices are present in the wake vortex shedding. The instantaneous large-scale vortices behind the grooved cylinders are dispersed into several relatively small-scale vortices. Furthermore, the spike of the secondary grooved cylinder is a vortex generator, and directly impacts the generation of small eddies and the dissipation of large vortices.