This study examines the behavior of two-dimensional synthetic jets generated from asymmetric stepped slots, focusing on the formation of recirculation regions and jet deflection near the slot. The effects of step length and frequency on the behavior of the jets were investigated. Experiments were conducted at a constant Reynolds number (Re = 1000), with varying frequencies. A speaker was used to generate the synthetic jet. Particle image velocimetry (PIV), which employed images captured via the smoke-wire method, was used to analyze flow patterns. To evaluate jet deflection, velocity distributions were measured using a hot-wire anemometer in selected cases. This study discusses the onset conditions of the recirculation region and the deflection mechanism, examining their relationship with jet deflection, dimensionless frequency, and dimensionless step length. The key findings indicate that jet deflection, associated with the recirculation region size, is influenced by both the dimensionless step length and frequency. The jet exhibits a straight flow when the dimensionless step length is either extremely small or large. Moreover, to minimize the size of the recirculation region, larger dimensionless step lengths require smaller dimensionless frequencies. Additionally, the study establishes a flow similarity rule within the tested conditions, employing the stroke length of the synthetic jet as a representative length.

Several studies have focused on jet vectoring and mixing enhancement using continuous jets for the secondary flow. In recent years, studies have applied synthetic jets to secondary flows, wherein the driving source can be easily downsized. However, a method to simultaneously control both the deflection angle and the degree of mixing of the generated flow has not yet been established. Thus, this study proposes a novel nozzle that actively controls the direction and turbulence of the primary jet simultaneously by replacing the facing control port of a conventional flip-flop jet nozzle with compact speakers. Specifically, the control factors considered were the phase difference between the opposing synthetic jets generated by the speakers and the dimensionless frequency of the synthetic jets based on the velocity of the primary flow. The flow field was measured via two-dimensional particle image velocimetry analysis and hot-wire anemometer. Consequently, the influence of the phase difference and dimensionless frequency on the deflection angle and turbulence intensity of the generated flow was investigated. The results indicated that the deflection angle of the generated flow could be controlled by adjusting only the phase difference even for the same dimensionless frequency under specific conditions. Furthermore, optimizing the dimensionless frequency by introducing the phase difference facilitated efficient operation with reduced power consumption. In addition, while controlling the deflection angle by the dimensionless frequency and phase difference, the cross-stream distribution of turbulence intensity could be selected at the set deflection angle, subject to limited restrictions. Thus, the results of this study confirm the feasibility of simultaneously controlling the deflection angle and turbulence intensity of the generated flow.

The drag reduction effect of sinusoidal superhydrophobic surfaces (SHSs) in turbulent channel flow is investigated by means of direct numerical simulations. The microgrooves and micro-ridges in SHS are represented by free-slip and no-slip conditions, respectively. The simulations are performed under constant pressure gradient condition at two friction Reynolds numbers of 180 and 395. A parametric study shows that the drag reduction rate increases and gradually decreases as increasing the wavelength in sinusoidal SHSs. The sinusoidal SHSs, except in the short wavelength case, increase the drag reduction rate more than conventional straight SHSs with streamwise uniform microgrooves. We analyzed the contribution of the skin-friction drag by the FIK identity equation. The contributions from the slip velocity and the coherent RSS are significantly smaller than that of the laminar flow and random RSS. For the shorter wavelength cases, the contribution of the slip velocity depends on the Reynolds number. On the other hand, the contribution of the random RSS is scaled by the wavelength in the wall unit and the Reynolds number dependency is small.

Suspension flows are frequently found in our daily lives, and their rheological properties are critical issues in many fields. In numerical simulations of suspension flows based on the two-way coupling approach, it is important to treat the no-slip boundary condition. The immersed boundary method (IBM) and interpolated bounce-back (IBB) scheme are representative schemes for satisfying the no-slip boundary condition in the lattice Boltzmann method. In the regard, the virtual flux method (VFM) has also been recently developed. In this study, the effect of pressure interpolation with arbitrary precision on numerical convergence was investigated and various methods were compared. Simulations of flows around a fixed cylinder, inertial migration of a single particle, and suspension flow were performed to compare the VFM with the multi direct forcing immersed boundary method and single-node second-order bounce-back scheme. Consequently, in the simulation of flows past a circular cylinder, we demonstrated that the convergence rate improved slightly when the pressure interpolation precision was increased. Moreover, in the suspension flow, using the VFM and IBB scheme reduced the computing time compared with using the IBM and allowed for more stable analysis at higher area fractions. The VFM can improve the numerical convergence rate by changing the pressure interpolation precision.