A new design of fuel injector for diesel engines is expected to enhance fuel spray atomization at the end of the fuel injection for the reduction in particulate matter emission. It was pointed out that string cavitation may occur in a multi-hole mini-sac injector and increase spray angle. However, the complicated velocity distributions in the injectors have not been understood yet, which make it difficult for us to reveal the overall string cavitation phenomena. In this study, visualization of string cavitation in a transparent three-hole mini-sac injector and spray at various needle seat gaps and particle image velocimetry (PIV) analysis of the flow in a plane perpendicular to the axis of an orifice are carried out to clarify the cavitation and flow structure as well as spray characteristics. The results clarify that (1) at the low needle lift of Z/D = 1 twin string cavitation flow appears intermittently and (2) at the very low needle lift of Z/D = 0.5 single string cavitation flow with a hollow-cone spray occurs, which significantly increases spray angle.

The influences of surface corrugations on zero-pressure-gradient boundary-layer instability were examined numerically by solving the two-dimensional compressible Navier-Stokes equations. The immersed interface method with compact filter and Navier-Stokes characteristic boundary condition (NSCBC) was newly proposed and applied. Two kinds of computation were made: a channel flow to validate the numerical scheme used and a zero-pressure-gradient boundary layer over a corrugated surface whose amplitude was less than 10% of the boundary layer thickness and wavelength was ranged from half to double the T-S wavelength. Linear stability analyses based on the parabolized stability equations (PSE) were also made to see how critically the corrugation modified the neutral stability curves. The results showed that the characteristics of the T-S waves in the channel with the corrugated wall were in excellent agreement with the linear stability theory, thus verifying the numerical accuracy of the present scheme. The surface corrugation destabilized the boundary layer significantly due to positive energy production occurring just behind the corrugation crest. The PSE analyses well described the development of T-S waves over the corrugated surface. When the corrugation height was 3%, 5% and 7% of the boundary-layer thickness, the critical Reynolds number based on the displacement thickness was about 4%, 14% and 19% lower than that for the smooth surface, respectively, while destabilization effect was weakly dependent of the corrugation wavelength. It was also found that the transient distance of change in surface geometry was much shorter for boundary-layer flows than for channel flow; for corrugation whose height was 4% the boundary layer thickness and wavelength was the same order as that of TS wave, the instability nature became the same as that for the case of corrugation over entire surface at the distance of about 4 times the corrugation wavelength from the beginning of corrugation, which was about 1/5 of that for the channel.

Numerical simulations of particle laden and fully developed turbulent channel flows with rectangular riblet were performed to investigate the influence of particle adhesion on the drag reduction effect. We employed the one-way coupling method: the flow affects the particle, but the particle does not affect the flow. We obtained particle adhesion distribution on the riblet and grooves. Most of the particles adhered to the top wall and upper half side wall surface of the riblets owing to the rotating flow motion in the groove. The riblet shape then was modified based on the particle adhesion distribution and numerical simulation of the turbulent channel flow over the particle-adhered riblet was performed. The results show that the skin-friction drag reduction effect decreases (or the drag increases) as compared with that of in the initial riblet case.

A flapping wing is considered as one of the most effective aerodynamic systems for micro air vehicles (MAVs). Many numerical studies have been attempted to investigate the flow field around a flapping wing; nevertheless, the long-term flow characteristics, which can cause a non-negligible effect on long-term hovering operations of MAVs have not been adequately clarified owing to the high computational cost involved. This study numerically investigates the long-term flow characteristics around a flapping flat-plate wing during hovering flight at a chord-based Reynolds number of 2.5 × 10⁴. Based on the finite-volume method with the arbitrary Lagrangian-Eulerian (ALE) method, two-dimensional laminar flow analyses were performed for 40 periods of flapping motions with stroke inversion angles of β = 30°, 45°, and 60°. The results showed that the lift coefficient C_L was not completely periodic despite the periodic motion. To identify the C_L characteristics for each β case, a half-stroke-period-based phase-average of C_L was calculated over different time segments. Then, the phase-averaged C_L using the fifth to 30th periods sufficiently provided converged aerodynamic characteristics: the β = 30° and 45° cases had a single peak of C_L, whilst the β = 60° had double peaks; the second peak taking the maximum C_L in the β = 60° case was delayed compared to others. The results of this study provide a guideline for the number of periods required in the numerical estimation of the C_L characteristics and associated flow fields around flapping-type MAVs, which contributes to their further improvement of them.