Wet cleaning methods using liquids are widely applied in many industrial fields. In such methods, it is first necessary to cover the object to be cleaned with the liquid. However, in structures with small holes, surface tension prevents the deformation of the gas–liquid interface, making it difficult to fill the object with the liquid. We have found that liquid infiltration into such small holes is promoted by the impingement of droplet trains, but the underlying mechanism has not yet been elucidated. In this study, we observed this liquid infiltration process through droplet train impingement into a closed-end hole, and compared the liquid column impact. The filling process was visualized with two high-speed video cameras. Our observations illustrate the importance of the oscillation and deformation of the gas–liquid interface inside the holes following droplet impingement. First, intermittent droplet impingement causes small droplets or large interface deformations to form, and then the gas column inside the hole becomes separated. This separated gas column is then gradually ejected. Therefore, the liquid infiltration can be increased by using a droplet train formed of a small-surface-tension liquid. Furthermore, we investigated the influence of the hole diameter and the uniformity of the droplet train frequency. The results show that droplet train impingement is effective for relatively large holes, although the uniformity of the droplet train frequency has little effect on the liquid infiltration.
When sand particles and volcanic ash are ingested into jet engines, they become molten droplets and are solidified on the surfaces of the turbine blades and endwalls. This is called the deposition phenomenon, and it causes serious problems in the aircraft, e.g., deterioration of the turbine performance and disturbing the cooling flow of the turbine blade. Therefore, the mechanism of the deposition phenomenon should be clarified in order to predict or prevent engine failure. In the present study, we perform three-dimensional numerical simulations of the deposition behavior of a single molten droplet on a cooled substrate using an explicit-moving particle simulation method. The results show a reasonable agreement with the experimental data. We confirm the formation of finger-like-structures that have the characteristic shape of a droplet that has adhered to an edge, and we also investigate effects of the impact angle on the deposition phenomenon.
The present study was performed to evaluate and to improve the measurement accuracies for zeta-potentials of particle and wall. Electric potentials near colloid particle surfaces and channel wall surfaces are defined as the zeta-potential of particle and wall, respectively. Their accurate measurements are important because both zeta-potentials are the key factors to control many microfluidic applications. Electroosmotic flow is used as a means of liquid transport, and its rate is determined by the zeta-potential of wall. In addition, the zeta-potential of particle is also an important parameter to control many properties of colloid particles. The previous studies developed the measurement techniques of zeta-potentials, which are called the current monitoring technique and the closed electrokinetic cell technique. However, the measurement accuracies of both techniques have not been well discussed, and thus, the present study compared the error ratios based on the measurement uncertainties of both techniques. For the measurements of negative zeta-potentials of wall using negatively charged particles, their error ratios of the current monitoring technique and the closed electrokinetic cell technique were 14.2% and 9.6%, respectively. However, when using positively charged particles, it was difficult to measure zeta-potentials due to the adsorption of positively charged particles on negatively charged channel wall surfaces. In order to reduce the adsorption, the surface modification technique was used to alter the electric charge on the channel wall surface. Then, for the measurements of positive zeta-potentials of particle and wall, their error ratios using the current monitoring technique were 31.9% and 31.6%, respectively, and those using the closed electrokinetic cell technique were 13.0% and 17.7%, respectively. It was revealed that the measurement uncertainties of the closed electrokinetic cell technique were superior to those of the current monitoring technique, even if negatively and positively charged particles were used.
We investigate behavior of bouncing and rupturing air bubbles on solid surfaces experimentally. We focus our attention to how the hydrophilicity of the solid surface alters the rupture process. We observe motion of the single bubble of a fixed diameter on several flat glass plates using high-speed cameras. In this experiment, we use two kinds of plate whose contact angles are different from each other. The bubble rises and bounces on the glass surface several times without touching the plate. It is found that, on the weakly hydrophilic glass plate whose contact angle is 65 degree, the liquid film between the bubble and the solid surface ruptures within about a hundred milliseconds after bouncing. The rupture starts at a single site. In contrast, on a highly hydrophilic glass whose contact angle is 7.6 degree, rupture time becomes much longer, i.e. more than 30 minutes. The rupture starts at several sites simultaneously. Note that in the both cases bounce times, bounce intervals, and bounce distance, are quite similar. Quite large difference in onset of rupture indicates that the existence of surface nanobubbles on the weakly hydrophilic glass enhances the rupture of liquid film. In the case of highly hydrophilic glass, the penetration of air into the liquid film after the rupture exhibits the pattern similar to viscous fingering.
The purpose of this study is to develop a high performance propulsive mechanism with high efficiency at high sailing speed by using a mechanism of rotary reciprocating wing motion based on the two-dimensional Weis-Fogh model. The sailing tests by both towing the model ship and its self-propulsion were carried out. In the towing test, the thrust of the model ship, the drag acting on the wing, and the wing opening angle in motion were measured for various parameters such as the wing moving speed and the sailing speed of the model ship. On the other hand, in the self-propulsion test the sailing speed of the model ship was measured for various frequency of wing reciprocating motion. The following results were obtained. The change of the wing opening angle in the mechanism of rotary reciprocating wing motion is obviously different from that of the wing opening angle in the mechanism of linear reciprocating wing motion. It was confirmed that the former is effective for the speeding up of the wing motion in comparison with the latter.
For the further introduction of wind power generation, it is necessary to solve the problems with output power fluctuation and early failure of wind turbine components. These problems are caused by the fluctuation of wind which is the energy source of wind power generation. In this paper, to solve these problems, the wind turbine controls that suppress the fluctuations of wind turbine power and the rotor thrust by using the inflow wind observed by LiDAR and ultrasonic anemometer are developed. The feedforward control for the pitch system is demonstrated with a 100 kW test wind turbine. For the feedforward control, the blade pitch angle is set to suppress the fluctuation of power or the rotor thrust according to the inflow wind velocity. The target pitch angle is determined by referring to the inflow wind velocity, wind turbine conditions, and the database constructed by using numerical analysis. The time series of the pitch angle command is fitted to the timing of inflow wind arrival to the rotor plane. A LiDAR installed downstream of the wind turbine rotor on the ground and an ultrasonic anemometer on a reference mast installed upstream of the rotor are used as observation devices for the inflow wind velocity. The suppression effect for the power or the rotor thrust fluctuation by feedforward control with pitch control was verified for either LiDAR or ultrasonic anemometer. Then, the demonstration results are compared with the LiDAR's control system and the ultrasonic anemometer's control system and evaluated. The feedforward controls assisted by LiDAR or ultrasonic anemometer are able to suppress the power and the rotor thrust exceeding the target value. Ultrasonic-anemometer-assisted feedforward control is able to suppress the fluctuation of power and load with high accuracy because the ultrasonic anemometer can accurately observe the inflow wind velocity.
Dielectric Barrier Discharge Plasma Actuator (DBDPA) is one of active flow control devices, which generates a wall-surface jet utilizing atmospheric discharge. However, the enhancement of the jet is indispensable for application to high Reynolds number flow because the flow speed is typically only up to several meters per second. In recently, it was reported that a Pulsed-DC voltage waveform can drastically improve the jet thrust (McGowan et al., 2016). In the Pulsed-DC waveform, the high DC voltage rapidly drops to zero and gradually recover. In the original idea, the DC high voltage is applied to the top electrode and the Pulsed-DC waveform is applied to the bottom electrode. In this study, firstly, we conduct experiments using the same method proposed by the previous study, and next, the DBDPA, in which the top electrode is powered by the Pulsed-DC waveform and the bottom electrode is electrically grounded, is investigated. Finally, we propose a new waveform which is a sinusoidal voltage with periodical pulsed-earthing. As a result, the DBDPA with Pulsed-DC voltage waveform cannot generate strong wall-surface jet, and on the other hand, the proposed waveform generates a significant jet. Even though its strength is the same magnitude as that by the simple sinusoidal waveform, it is confirmed that the jet thrust is enhanced at phases before and after the pulsed-earthing; the tentative thrust increment is up to 32 %.
In most subcritical planar shear flows, the transitional regime features oblique large-scale laminar-turbulent patterns. So far, such laminar-turbulent patterns have only been investigated in flows over perfectly smooth walls and little attention has been devoted to cases with rough surfaces as found in most practical engineering, urban applications, and in nature. In this study, we investigate laminar-turbulent patterns in plane Couette flow with one rough wall by means of direct numerical simulation, as a function of the Reynolds number and of the roughness height. The roughness is modeled using a force term in the Navier–Stokes equations. The focus of this study is on a new regime featuring non-oblique turbulent bands transverse to the motion of the walls, and separated by arbitrary long laminar gaps. This regime is found when the wall is sufficiently rough. This transverse turbulent band occurs at Reynolds numbers just below the onset of self-sustained turbulence found in the smooth wall case. The localized turbulence patches have a streamwise extent as large as 50–180 gap widths, decreasing with decreasing Reynolds number. The turbulent fraction as well as the band width show a linear relationship with the Reynolds number.
An inflatable membrane reentry vehicle has been developed as one of the innovative reentry technologies. A suborbital reentry demonstration using a sounding rocket was carried out in 2012. Contrary to the result of a preliminary study, the vehicle always had an angle of attack (AoA) during its reentry. In addition, the amplitude of AoA gradually increased as altitude decreased, and the vehicle rotated vertically under Mach number of 0.1 (M0.1). As a first step to clarify the cause of attitude instability and vertical rotation, the aerodynamic characteristics, that concern static stability, are numerically investigated. Numerical simulations were carried out for the cases of Mach 0.9 (M0.9), 0.6 (M0.6), 0.3 (M0.3), and 0.1 (M0.1) and pitching moment coefficients (CM) were obtained. Analysis software “RG-FaSTAR” for M0.9, and “FrontFlow/red” for M0.6, M0.3 and M0.1, are used, respectively. Large eddy simulation (LES) was performed using the standard Smagorinsky model to resolve highly unsteady flow features. Because the slope of CM with respect to AoA was negative for all cases, it was found that the vehicle is statically stable. For M0.9, M0.6 and M0.3 cases, absolute values of CM were almost the same. On the other hand, for M0.1, CM had a particularly large value, because the surface pressure distribution on rear side of the vehicle was different from the other cases. This difference was attributed to the separation point on the lower torus moving backward and turbulence in wake being enhanced with a decrease in Mach number and an increase in the Reynolds number.
The present purpose is to reveal the mechanism of a flying pipe from an aerodynamic point of view. At first, we conduct field observations of a flying pipe using a pair of high-speed video cameras, together with three-dimensional motion analyses. In addition, we conduct numerical analyses by a finite difference method based on the MAC scheme. As a result, the observed orbit is approximated to be not an obvious parabolic curve but rather a straight line, after an initial instable and complicated curve. The stable flight with this approximately-straight orbit suggests the importance of aerodynamics in flying mechanism. More specifically, the model is in an unstable and complicated flight during an initial flight, afterwards becomes in a stable and approximately-straight flight. In the initial instable and complicated flight, the model flies fluctuating its posture upward, downward, left-ward and right-ward. As flight distance increases, the absolute value and the amplitude of moment becomes small to zero. During such a decaying and stabilising process, the gyroscopic effect plays a primary role balancing not angular acceleration of the model but aerodynamic fluid moment. In the stable and approximately-straight flight, the flow in the stable and approximately-straight flight is nearly the velocity-potential one, and accompanies very-small drag force. And, we could ignore the influence of model's rotation upon the flow and the orbit. In this context, the model's rotation is only to stabilise its posture, and gives negligible contribution upon its aerodynamics.