In this study, the receptivity of a flat-plate boundary layer was studied by introducing a thin sheet-type disturbance. An airfoil-shaped device was used to generate a thin disturbance without velocity deficit in which a steady jet was ejected from its trailing edge to the downstream. Despite the absence of strong disturbances in the freestream outside the boundary layer, streaky structures similar to an ordinary bypass transition were generated. They meandered slowly in the spanwise direction where their downstream parts were oscillating in a delayed fashion. Turbulent spots were formed in the further downstream region. Consequently, the energy growth of the low frequency band in the velocity fluctuation spectrum was found to originate from this meandering motion of the streaks, whereas the growth of the middle- and high-frequency bands was attributed to the appearances of the turbulent spots.

Stochastic flow simulation methods based on the polynomial chaos expansion (PCE) are developed and verified to quantify the propagation of a geometric uncertainty of a quasi-one dimensional flow in a supersonic wind tunnel. The effect of uncertainty in the area of diffuser throat, i.e. second throat, on the wind tunnel starting problem is focused on, where a slight change in the area can cause a large jump of the shock wave resulting in a breakdown of the supersonic test conditions. Two major numerical techniques in our intrusive PCE are the multi-wavelet (MW) basis and the multi-element (ME) PCE, in order to properly deal with discontinuous responses of output variables, which are caused by the shock wave and its jump at started/unstarted mode change. Single-element spectral PCE using Legendre basis and the Haar-wavelet are also included as special cases of the MW, and the methods are all compared with Monte-Carlo Simulations (MCS) executed by the deterministic code. Response surfaces of the pressure by the employed PCEs qualitatively agree with the result of MCS except the spectral PCE. Furthermore, from quantitative evaluations by the probability density function (PDF) of the output on a rather complicated response surface with several discontinuities, the ME-PCE best agrees with the MCS at much lower computation costs.

The present study aims to develop an ultrasound acoustic streaming actuator for flow control. The driving force can be derived from the continuity equation and the Navier-Stokes equation for the viscous compressible flow. Commercially available transducers are used as an ultra sound source, and the acoustic and induced flow characteristics for a single and multiple transducer configurations are examined. The sound pressure distribution indicates the strong acoustic pressure fluctuation near the transducer. For the multiple transducer cases, the region of the strong pressure fluctuation is widened due to the superposition of the waves. The distributions of the induced velocity are evaluated using particle image velocimetry. It is revealed that the maximum flow velocity is about 0.04 m/s for the single transducer case, and the maximum velocity is observed slightly downstream of the high sound intensity region. Since the driving force is proportional to the square of the sound pressure intensity, the higher flow velocity can be achieved using more transducers. A transducer array having 100 transducers has been applied in a turbulent boundary layer. It is confirmed that the flow velocity near the wall increases in the case with the control, and turbulence intensity augments by approximately 17% compared with the case without the control.

An immersed boundary method of discrete type is tested as a tool for direct numerical simulation of aeroacoustic sound. The numerical method consists of the WENO scheme, the immersed boundary method by Chaudhuri et al. (J. Comp. Phys. Vol. 230, 1731–1748 (2011)), and the perfectly matched layer together with the dyadic mesh refinement and the Runge-Kutta method. The accuracy of the method is shown to be sufficient for four basic problems: propagation of acoustic waves, aeroacoustic sound generation in a flow past a fixed circular cylinder, in a flow past an oscillating square cylinder, and from a vortex pair passing through a circular cylinder. The results confirm that the developed method can deal with moving bodies and it is accurate not only for viscous flows but also for inviscid flows.

We consider a Couette flow of a rarefied Ar gas with heat transfer between two wall surfaces and investigate the scattering behavior of gas molecules reflected either at a clean Pt surface or at a surface contaminated with adsorbates. Water molecules abundantly present in the atmosphere were adopted as the adsorbates. The reflection of gas molecules on the lower wall surface was simulated by Molecular Dynamics (MD) method to obtain accommodation coefficients and velocity distribution functions of gas molecules. We applied the modified reflection model of gas molecule and investigated the velocity distribution functions of the model by comparing the MD results to verify the validity. The accommodation coefficients obtained by the MD method depend on the number of adsorbed water molecules on the lower wall surface. Specifically, tangential momentum accommodation coefficient (TMAC) tended to increase and then decrease with the increase in adsorbed water molecules, but normal momentum accommodation coefficient (NMAC) tended to decrease monotonically. The velocity distribution functions of the modified reflection model approximately show the good agreement with the MD calculation but the degree of coincidence depends on the speed difference between the upper and lower wall surfaces, and the number of adsorbed water molecules on the surface.

The objective of this study is to clarify the effect of a wake on liquid column drag and deformation for a high-Weber-number (We) flow. A simulation was performed for a liquid column with a high We behind a shock wave, using a ghost fluid method as a two-phase flow solver. The simulated We were 500, 1000, 2000, 3000, and 4000. Relatively large oscillation of the drag coefficients was observed for We = 500 and 1000. This feature of the drag coefficients was possibly caused by varying pressure on the downstream interface. The pressure variation is derived from the wake. In addition, it was suggested that such varying pressures could contribute to the flattening of the liquid column.