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In this study, an experimental investigation of separation control using a dielectric barrier discharge plasma actuator was performed on an NACA0015 airfoil over a wide range of Reynolds numbers, angles of attack, and nondimensional burst frequencies. The range of the Reynolds number was based on a chord length ranging from 2.52 × 105 to 1.008 × 106. A plasma actuator was installed at the leading edge and driven by AC voltage. Burst mode (duty-cycle) actuation was applied, with the nondimensional burst frequency ranging between 0.1–30. The control authority was evaluated using the time-averaged distribution of the pressure coefficient Cp and the calculated value of the lift coefficient Cl. The baseline flow fields were classified into three types: (1) leading-edge separation; (2) trailing-edge separation; and (3) the hysteresis between (1) and (2). The results of the actuated cases show that the control trends clearly depend on the differences in the separation conditions. In leading-edge separation, actuation with a burst frequency of approximately F+= 0.5 creates a wide negative pressure region on the suction-side surface, leading to an increase in the lift coefficient. In trailing-edge separation, several actuations alter the position of turbulent separation.

A floater serve in volleyball is a technique of serving a non-rotating or low-rotating ball, which is difficult to return because the flight path of the ball changes irregularly. On the other hand, the randomness of the trajectory makes it difficult for the ball to fall on the target. Players are required to serve taking into account the variability of the trajectory. In previous studies using wind tunnels, it was shown that aerodynamic characteristics such as drag force and lateral force applied to the ball vary depending on the type of ball and the orientation of the panel. Therefore, in order to control the flight trajectory, it is necessary to understand the aerodynamic characteristics of each ball. Since the velocity of the ball and the fluid force applied to the ball changes during flight, it is important to measure not only the fluid force at a steady state in the wind tunnel but also the actual flight distance of the ball. In this study, to provide valuable information for precise control of floater serves, we measured the drag force applied to the ball in a wind tunnel and the flight distance of the ball using an ejection machine, and clarified the effects of the type of ball and the panel face. In the drag force measurement, the drag force on three types of balls, V200W, MVA200, and FLISTATEC, was measured in the wind speed range of 4 m/s to 30 m/s. In the ejection measurement, the ball flight distances were measured while changing the orientation of the panel using an ejection machine. Basically, the FLISTATEC, MVA200, and V200W, in that order, were more likely to increase the distance and the variability, but it was shown that the drop point could be adjusted slightly by selecting the panel face. This result was also obtained when a human player actually served the ball, indicating the tactical importance of the player consciously controlling the direction of the panel. The tactical importance of the player’s conscious control of the direction of the panel was demonstrated. We also proposed receiver positions that would be effective based on the characteristics of each ball.

The panel patterns of soccer balls that change with each World Cup have a significant impact on the balls’ aerodynamic and flight characteristics. In this study, the aerodynamic forces of eleven types of soccer ball with different panel patterns were measured in a wind tunnel experiment. We characterized the panel shapes of soccer balls by the length, cross-sectional area, and the panel grooves’ volume. The results confirmed that the drag and drag crisis characteristics are dependent on the groove length and volumes. Flow separation points were visualized by an oil film experiment and particle image velocimetry (PIV) measurement to understand the drag crisis of the soccer balls. The results showed that the panel shape of the ball significantly changes the position of the separation point near the critical region, where the drags crisis occurs. In the critical region, laminar and turbulent flows coexist on the ball. On the other hand, the effect of panel shape on the separation point position is small in subcritical and supercritical states.

The soccer ball panel pattern, which changes every World Cup, greatly affects the ball’s aerodynamics and flight characteristics. In this study, the fluid force of 11 soccer balls with different panel patterns was measured by wind tunnel tests. The drag crises with different Reynolds numbers were confirmed depending on the panel shape. To understand this, the shapes of panel grooves were measured and the relationship between them was investigated. The flow separation point was also visualized by the oil film method and the particle image velocimetry (PIV) analysis. The separation points were confirmed to be different depending on the panel groove by the oil film method in a supercritical Reynolds region. The flow separation points were found to be almost the same position in the subcritical and supercritical state and to be partly different around the Reynolds number of drag crisis.

Volleyball is a sport that starts with a serve, so effective service is essential to win the game. The trajectory of the ball is complicatedly affected by the fluid force, which depends on the speed, spin speed, and panel shape. To understand the aerodynamic characteristics of the ball and to propose an ideal serve method, we measured the fluid force and flight trajectory. The fluid force applied to the ball was measured at a wind speed of 4–30 m/s in the wind tunnel. The fluid force on the ball was strongly dependent on the ball type and orientation of the panel. In the flight trajectory measurement, the trajectory of the ball was measured using a high-speed camera under controlled speed and spin speed using a shotting machine. The effect of the panel orientation shown by the fluid force measurement was consistent with the results of the trajectory analysis, clarifying the importance of the panel orientation in serving.

Recent advances in the microfabrication technology have made it possible to control surface properties at micro- and nanoscale levels. Functional surfaces drastically change wettability and condensation processes that are essential for controlling of heat transfer. However, the direct observation of condensation on micro- and nanostructure surfaces is difficult, and further understanding of the effects of the microstructure on the phase change is required. In this research, the contact angle of droplets with a wall surface and the initial condensation process were analyzed using a molecular dynamics simulation to investigate the impact of nanoscale structures and their adhesion force on condensation. The results demonstrated the dependence of the contact angle of the droplets and condensation dynamics on the wall structure and attractive force of the wall surface. Condensed water droplets were adsorbed into the nanostructures and formed a water film in case of a hydrophilic surface.