The purpose of the study is to investigate the Reynolds number dependency of a circular arc airfoil at Reynolds numbers from 1×10³ to 10×10³ by CFD. A block-structured Cartesian-mesh CFD was employed to precisely predict the vortex flowfields in the low Reynolds number region. The calculated aerodynamic coefficients were reasonably matched with wind tunnel tests at different Reynolds numbers. The flow goes along upper surface at lower angles of attack, but separation on the upper surface occurs at the trailing edge as the angle of attack increases when the Reynolds number is 1×10³. On the other hand, 2D vortices are formed on the upper surface at low angles of attack for Reynolds numbers of 5×10³ and 10×10³, and then the laminar separation bubbles (long bubbles) are formed as the angle of attacks increases. This causes the nonlinear increase of lift coefficient.

Cavitation is a phenomenon that degrades the performance of fluid machinery, whereas the thermodynamic self-suppression effect of cavitation is prominent in pumps transporting cryogenic fluids because the effect suppresses cavity development. The thermodynamic self-suppression effect is caused by the temperature decreasing inside the cavity by the evaporation and the reduction of saturation pressure inside the cavity; therefore, the cavity temperature is essential to understand the thermodynamic self-suppression effect. The temperature measurement inside the cavity has been limited in the quasi-steady cavitation because it is difficult to accommodate the fast responsiveness of a temperature sensor to respond to the temperature variation of unsteady cavitating flow varying from 10–100 Hz. This study aims to develop the estimation method for the cavity temperature from unsteady temperature response, visualized cavity image, and lumped capacitance model, which is applicable even if the responsiveness of the sensor cannot be increased. The temperature in the unsteady cavitating flow of water at 100 ℃ and 10 m/s with various cavitation numbers were measured, although the temperature responsiveness was insufficient for the unsteadiness of the cavity. In addition, the phase index, which is the parameter to distinguish the fluid phase around the thermistor, was determined from high-speed video images. The cavity temperature was estimated using the lumped capacitance model with the time variation of the temperature and phase index. As the result of the estimation, the temperature fluctuation of the thermistor and the estimated temperature fluctuation were in general agreement by adjusting the cavity temperature and heat transfer coefficient in the estimation equation. The estimated cavity temperature was smaller than the averaged value and the minimum value of the temperature fluctuation of the thermistor at each cavitation number. Additionally, the estimated heat transfer coefficient in the cavity is of nearly the same order of magnitude as that in the liquid phase.

We aimed to derive a phase diagram in the parameter space of wall temperature T_w and Weber number We for the spreading behaviors of water drops impacting a heated sapphire plate. Focusing on hydrodynamic instabilities, we performed drop impact experiments to investigate spreading behaviors with varying wall temperatures (T_w = 60 to 150 ℃) and Weber numbers (We = 29.5 to 443) using high-speed infrared (IR) imaging. A phase diagram was established accommodating three different regimes: the gentle spreading regime, rim instability regime, and contact-line instability regime. The gentle spreading regime occurs under low-We and low-T_w conditions. In this regime, the drop spreads while maintaining a round shape with no perturbations. With an increasing Weber number at any wall temperature, the spreading behavior undergoes a transition to the rim instability regime, where a finger like perturbation can be observed along the lamella rim of the spreading drop. For low-We and high-T_w conditions, instead of rim instability, we observed a transition to the contact-line instability regime, where finger like perturbations form along the contact line. Additionally, we explored the temperature dependency of the maximum spreading ratio, dimensionless perimeters, and number of fingers for the contact-line instability and rim instability regimes. The experimental results indicated that intense evaporation around the contact line has a significant impact on the growth of the observed hydrodynamic instabilities, leading to changes in spreading behaviors. To the best of our knowledge, ours is the first study to observe the contact line instability induced by thermal effects. Considering the occurrence of contact line instability only at high T_w values, we expect that intense evaporation around the contact line is a key factor in the underlying mechanism of contact line instability.

Boundary-layer transition on swept wings is dominantly caused by the crossflow instability, which is expected to be suppressed by placing artificial roughness elements near the leading edge. It is however difficult to find the optimal roughness shape by using direct numerical simulation (DNS), because a lot of computations are required for assessing a suppression effect due to one roughness shape. In this study, we develop an efficient method to evaluate the suppression effect for a series of roughness shapes by changing a shape parameter quasi-statically and observing the subsequent change of the crossflow mode at a downstream position. Since the mode grows spatially as convective instability, we need to allow for the delay time for the change in the shape to cause the change in the mode. This method is demonstrated for optimizing the height and angle of sinusoidal roughness elements. By applying a volume penalization (VP) method, the height and angle are changed slowly in DNS, where the initial values, rates of change and permeability of the VP method should be chosen appropriately to reproduce the correct results for the fixed shapes. The method developed here shows that the suppression (or laminarizing) effect tends to be improved as the height is increased, but there is a critical height at which flow tripping occurs. Both the laminarization effect and the critical height vary greatly depending on the angle. This result suggests the optimal roughness shape, considering the effectiveness and robustness. For laminar flow control, this method is useful for optimizing the wall roughness shape.