As skin-friction drag in wall turbulence has a considerable environmental impact, a feedback control technique is applied to provide the drag reduction effect. Specifically, opposition control, a feedback control technique, is used, which involves applying blowing and suction from the wall to cancel out turbulent vortical structures in the region near the wall. Opposition control typically involves the use of detected velocity immediately as the control input. However, in the present study, the near-wall velocity is predicted by an orbit-instability-based forecasting method based on deterministic chaos in low-Reynolds-number flow. We performed direct numerical simulations of turbulent channel flows to investigate the drag reduction performance. The maximum drag reduction rate was found to be comparable to that of the original opposition control. Although the drag reduction rate decreased slightly with increasing prediction time in the orbit-instability-based forecasting method, a positive drag reduction rate was achieved even when the prediction time was more than 100 times the time resolution of direct numerical simulation.

This study investigates the accuracy of dynamic models in predicting model constants within inviscid flow fields, such as those used in wind farm flow analysis, from the perspectives of identity and turbulent energy conservation accuracy. Specifically, results are compared between tensor-level and vector-level identities, the latter of which includes the calculation of model constants taking into account the errors of differential approximation. The subgrid-scale models employed include the Smagorinsky model and the coherent structure model. The analysis focuses on inviscid flow fields within a three-dimensional periodic domain. Fourth- or second-order spatial accuracy was applied to a coarse computational grid. The results yielded values of a model constant that compensated the resulting energy conservation errors to zero. The statistics of the velocity fluctuation derivatives in the flow fields where the energy conservation errors were compensated were examined. Dynamic model predictions for both identities were then computed for the Smagorinsky and coherent ctructure models and compared with the correct values. The results show that the dynamic model predictions are largely independent of the energy conservation errors, and that the predictions based on the tensor-level identity deviate significantly more from the correct values than those based on the vector-level identity.

The growth and collapse of a laser-induced bubble near an agarose gel were observed with a high-speed video camera in which 0.7% (Young's modulus E = 13 kPa), 1.5% (E = 55 kPa), 3.0% (E = 200 kPa), and 5.0% (E = 570 kPa) agarose gels were used as tissue-mimicking phantoms. The effects of Young's modulus of agarose and the dimensionless bubble-boundary distance γ on the dynamics of laser-induced bubbles, i.e., bubble oscillation time, bubble migration, bubble shape, jetting, and penetration of the liquid jet into the elastic wall were investigated. It was shown that as Young’s modulus of the wall increased, the amount of migration of the bubble centroid toward the wall increased. The results also showed that as γ decreased, the bubble shape in the late collapse stage changed from a spherical shape, a cone shape, and to a mushroom shape when E = 13, 55, and 200 kPa, while, when E = 570 kPa, it changed from a cone shape, a mushroom shape, and to a volcano shape: the threshold values of γ, where the bubble shape changed from a cone shape to a mushroom shape and from a mushroom shape to a volcanic shape, increased as E increased. These bubble shapes in the late collapse stage were determined from the bubble shapes at the maximum expansion. In the case of mushroom-shaped bubbles, the jet velocity was higher than volcano-shaped bubbles. The value of γ at the onset of the penetration of the liquid jet into the elastic wall increased with an increase in E. The penetration area of bubbles into the elastic wall took the maximum value when the bubble shape in the late collapse stage changed from a mushroom shape to a volcano shape. The difference between the bubble behavior near the agarose wall and that near the PAA wall was also discussed.

Engineering Division.This four-year cycle stimulated international collaboration, bringing the global fluids engineering community together.

Optical fiber liquid film sensors utilizing the reflection intensity at the gas-liquid interface exhibit excellent environmental robustness. However, this technique needs to improve its sensitivity when measuring liquid film thicknesses approaching the fiber core diameter. In this study, we propose a method for accurately measuring liquid film thickness around the fiber core diameter by changing the spatial intensity distribution of laser light emitted from the fiber. We employ stepped-index-type optical fibers (SI-type) instead of the commonly used graded-index-type fibers (GI-type) to alter the spatial distribution and investigate the relationship between reflection intensity and liquid film thickness. The reflection intensity of SI-type optical fibers exhibits a linear response over a range approximately eight times the core radius, significantly enhancing sensitivity for thin films. In addition, we use a theoretical model for the spatial intensity distribution of light emitted from SI-type optical fibers, demonstrating a close match between the predictions and experimental results, and discussing the linear response. Finally, we demonstrate the liquid film thickness measurement of the liquid jet impinging surface.

a professor of Nihon University in Japan, for the consistent aim to explore the fundamental characteristics and related advanced applications of Jets, Wakes and Separated Flows by discussing matters not only in the academic fields, but also in the industrial and practical fields.To fulfill this aim, the chair invited Prof.

In this study, the ground effect in flapping flight was investigated by using a two-dimensional flapping mechanism that make the flapping angle and twisting angle of the wings moving in a sinusoidal function. The experiment employed a height-adjustable ground platform to test the aerodynamic force variation with/without ground effect. The results indicated that when the distance to the ground is more than double the wing’s chord length, the lift coefficient remains unaffected by the ground for various Reynolds numbers and flapping amplitudes. Conversely, when the distance to the ground is less than double the chord length, the lift coefficient increases dramatically. The Reynolds number also plays a crucial role in the ground effect, as the lift coefficient grows less than 10% when the distance to the ground is the closest in the low Reynolds number (Re=5000), but increases by over 25% in the high Reynolds number (Re=20,000). Although the increase curve of the lift coefficient about each movement is similar for different flapping wing amplitudes, the lift coefficient decreases when the minimum distance is double the chord length. The analysis of the position of the center of pressure on the wing's surface showed that the wing with ground effect shifts the center of pressure toward the wing root and trailing edge, indicating an increase in the effective angle of attack. PIV measurement verified that the ground effect in the middle wing stroke increased the effective angle of attack. This study expanded our understanding of the ground effect theory in flapping aerodynamics under various flight conditions and provided valuable references for the take-off and landing manipulation of flapping-MAV.

We conducted measurements of a butterfly’s motion in forward flight and numerical simulations using a computational model reflecting its motion. We measured the motion of a cabbage white butterfly (Pieris rapae), and then we constructed a computational model composed of a thorax, an abdomen, and four wings (i.e., left and right wings with fore and hind parts). Furthermore, we calculated the flow field and aerodynamic force and torque generated by the butterfly model using the immersed boundary–lattice Boltzmann method. In this simulation, we considered two types of periodic motions corresponding to slightly-descending and ascending forward flights. As a result, we found that the wing-tip and leading-edge vortices are formed on the wings and then released backward and downward in both flights. The major difference between the two flights is the flapping amplitude, indicating that the butterfly changes the flapping amplitude for each period and increases it to ascend. In addition, we considered a chimera model whose motion is based on the slightly-descending flight but partly given by the ascending flight. As a result, we found that the pitching angle and the angle of attack determine the traveling direction, but simply changing these angles does not achieve the ascending flight due to insufficient lift force. Thus, the butterfly should adjust the flapping and lead–lag angles in response to the pitching angle and the angle of attack to change the flight mode.

The Cartesian cut-cell method evaluates wall-fluid interactions by using the fluxes through a wall. This wall treatment by the Cartesian cut-cell method is expected to overcome the mass and energy conservation problems of the immersed boundary method. A wall flux evaluation using the image point is proposed in this study to avoid volume center estimation. The Cartesian cut-cell method that uses the proposed wall flux evaluation is validated by the numerical simulations of supersonic flows past simple shapes.