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.

This study proposes a novel approach for the wall-boundary immersed flow simulation, wherein the Navier-Stokes equation is modified to include a level-set definition of a solid body in fluid flow by an approach of viscosity solution of thickened interface. Following to the previous work for slip wall condition (Oshima, 2023a), a model of wall shear stress on no-slip condition is additionally proposed. One-, Two- and Three-dimensional simulations validate accuracy and applicability of wall model for theoretical and practical application.

A new device for direct measurement of the local wall shear stress in boundary layers has been developed with a square friction surface and higher accuracy. The uncertainties were estimated from experiments in boundary layers at moderate Reynolds numbers. The analysis states that the main sources of uncertainty are temperature variation, gap size, and misalignment. The expected errors or uncertainties are represented as a function of gap size and misalignment. The use of the material Super Invar in the force sensor considerably reduces the uncertainty due to temperature variation by a factor of 6. Regarding the direct measurement of the local shear stress in a flat-plate turbulent boundary layer, there are no differences in the skin-friction coefficient measured by circular and square shaped floating elements. In this experiment, the maximum relative standard uncertainty of the developed direct measurement device is 0.65% for the local skin friction coefficient.

The streamwise velocity at two points in a turbulent boundary layer under zero pressure gradient was measured using two hot-wire probes separated in the wall-normal direction. The time trace of the fluctuating velocity difference, probability density function, fluctuating vorticity, structure function, and the spatial correlation coefficient was obtained. First, the physical importance of structure functions was explained in some detail. Then, the results of the measurements were considered. When the spatial separation between two points is small, the fluctuating velocity difference decreases monotonically as the two points move away from the wall. On the other hand, as the spatial separation increases, the similarity between the two fluctuating velocities decreases. In addition, the fluctuating velocity at the nearer point from the wall becomes dominant. When moving away from the wall, the fluctuating velocity difference first increases, then reaches a maximum, and finally decreases. When both positions are within the logarithmic region or defect region, the velocity difference is made between similar fluctuating velocities. Hence, the probability density function distribution shape is symmetrical with respect to positive and negative values, close to the Gaussian distribution. The fluctuating vorticity decreases monotonically as it moves away from the wall, regardless of the spatial separation. The structure function increases as the separation increases. Away from the wall, the length over which the correlation is maintained increases, resulting in a large eddy.

CO₂-separation technology is required in various industrial plants to reduce CO₂ emissions. A method for promoting adsorption via acoustic excitation is proposed, and the effects of acoustic excitation on CO₂ physisorption are investigated using a monolith coated with zeolite. The monolith is placed in a flow duct, where a CO₂ + N₂ gas mixture is flowed and acoustic resonance occurs due to acoustic excitation by speakers. Observations show that the time required to achieve a certain amount of adsorption can be reduced. Moreover, the mass transfer coefficient estimated based on a linear driving-force model increases as acoustic excitation is intensified, which indicates the promotion of adsorption. In particular, the promotion is intensified when the monolith is placed at the anti-node of the velocity fluctuations of the acoustic resonance. The measured velocity field in the wake of a monolith cell shows flat and high velocity profiles occurring periodically under acoustic excitation, which indicates that high-velocity CO₂ gas flows near the cell wall in the monolith. Moreover, the effects of acoustic excitation on the CO₂ concentration field in a planar jet comprising a CO₂ + N₂ gas mixture are investigated using a background-oriented Schlieren method. The result shows varying distributions of CO₂ concentration with time under acoustic excitation, where the CO₂ concentration around the jet exit is indicated to become periodically larger compared with the baseline case without sound. This indicates the possibility of enhanced CO₂ concentration in the monolith with acoustic excitation, which can contribute to promotion of CO₂ adsorption.

Improving the thermal efficiency of internal combustion engines requires a high compression ratio, a high thermal load, and high mechanical strength in the combustion chambers. However, they will promote excessive temperatures within the combustion chamber and increase the likelihood of knocking. As the piston reciprocates, the engine oil injected into the piston crown oil gallery can oscillate within the gallery to reduce the piston temperature. However, the interaction between the ambient air and the interface of the axial jet liquid column results in unstable fluctuations, significantly affecting the filling rate of the oil gallery. In this study, the fluctuation of the jet interface is analyzed by changing the Reynolds number (Re) and bending radius R of the nozzle. Two-dimensional/two-component and two-dimensional/three-component particle image velocimetry are used to measure the flow fields in the longitudinal section of two bent nozzles with small (R = 15 mm) and large (R = 60 mm) curvature radii and horizontal cross-sections at 4D, 2.5D, and 1D from the exit of each bent nozzle, respectively, to explore the causes of the fluctuating internal flow. By calculating the secondary flow intensity Se on the inner and outer wall sides of the nozzle, the inner and outer jet interface width changes, and the different fragmentation mechanisms of the jet interface, it is shown that the higher frequency of the high-speed core region at the outer wall side of the nozzle interior can drive the waveform on the outside of the jet interface to break and produce small-diameter droplets easily.