There are mainly two types of motion for a soft particle moving in a fluid: tank-treading and tumbling motions. This transition from one mode to the other is very important for the rheological characteristics of the suspension, due to the significant changes in its macroscopic viscosity. In this study, we focused on the critical internal and external viscosity contrast, which is one of the parameters that determine the motion of a soft particle. The critical internal and external viscosity contrast was defined at the transition point between the motion modes. Consequently, the effects of capillary number and reduced area on the critical internal and external viscosity contrast were investigated. Our results confirmed that the effect of capillary number was minor, while the effect of the reduced area was significant.

In this study, the liquid film thickness of slug flow in a microchannel was numerically investigated. Particularly, we focused on a flow regime with high Reynolds and capillary numbers; these dimensionless number were independently modified to investigate their effect on the liquid film thickness. The results based on the turbulence model confirmed that the liquid film thickness tends to remain constant in the low Reynolds number region; whereas it sharply increases when the Reynolds number reaches a certain value. However, this increase stops once a certain value of Reynolds number is reached, and the liquid film thickness tends to remain constant in the high Reynolds number flow regime. These results tend to agree with the experimental results from previous studies. Moreover, the liquid film thickness calculated under the laminar condition was similar compared to that based on the turbulence model, except in the high Reynolds number region. Under the laminar condition, the liquid film thickness in the high Reynolds number region did not converge to a constant value but kept increasing. The results of this study suggest that the shear stress acting on the bubble interface, which appears in the form of Reynolds stress, seem to prevent the liquid film thickness from increasing.

We introduced a core-flooding test equipment with the triaxial cell having been made available to in situ computed tomography (CT) scanning. Water flow-through tests were conducted on 38 mm-diameter fractured granite cores containing either saw-cut or rough fractures, subjected to a confining pressure varying from 5 to 20 MPa in a single loading-unloading cycle. We put a particular focus on fracture deformation and hydraulic performance of multiple fractures that were artificially assembled to form different fracture interconnection. Results show that in all the cases the permeability of the fractured granite cores nonlinearly decreases with increasing confining pressure, and was reduced by 80.90% when the confining pressure was increased to 20 MPa. Permeability recovery is very limited (10.30%) when the specimen was unloaded to the initial confining pressure of 5 MPa. Poorly-connected fractures show approximately half permeability of the well-connected fractures. Nonuniform deformation including changes in aperture and contact area in natural fractures were observed through CT image analysis. In the end, advantages and limitations of in situ imaging for quantitatively evaluating hydro-mechanical behavior of granite fractures at core-scale were discussed.

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.

In the numerical simulations of the supersonic flow past a cylinder with an isothermal wall, the shorter displacement d_IP of the image point from the wall provided a better estimation of the wall heat flux. Furthermore, the wall heat flux distributions and the amount of heat converged with grid refinement.

In the numerical simulations of the supersonic flow past a wedge, the minimum grid resolution of the Cartesian cut-cell method for wall flux estimation was suggested. Furthermore, the results of the isothermal wall condition, the adiabatic wall condition, and the inviscid condition were compared.

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.