Antonio Criscione

Shedding of Water Drops from a Surface under Icing Conditions

A sessile water drop exposed to an air flow will shed if the adhesion is overcome by the external aerodynamic forces on the drop. In this study, shedding of water drops were investigated under icing conditions, on surfaces with different wettabilities, from hydrophilic to superhydrophobic. A wind tunnel was used for experiments in a temperature range between -8 and 24.5 °C. Results indicate that the temperature has a major influence on the incipient motion of drop shedding. The critical air velocity (U(c)) at which a drop first starts to shed generally increases under icing conditions, indicating an increase in the adhesion force. The contact angle hysteresis (CAH) and the drop base length (L(b)) are found to be the controlling factors for adhesion. A correlation was also developed to deduce the drag coefficient, C(D) for the drop. It was found that C(D) can decrease under icing conditions. In general, a lower C(D) and higher adhesion together lead to a higher critical air velocity. However, there are systems such as water on Teflon for which the critical air velocity remains practically unaffected by temperature because of similar adhesion and C(D) values, at all temperatures tested.

Preliminary Numerical Assessment of Turbulent Flow Control with Plasma Actuators

A numerical investigation of damping of near-wall spanwise velocity fluctuations in a turbulent channel flow is carried out. Spanwise damping is realized with a body force which mimics the characteristic force distribution of plasma actuators. This force distribution is implemented in a direct numerical simulation. The body force is triggered by the signal of a sensor in upstream location. The control loop is applied with different actuator configurations, considering a spatially continuous and discontinuous body force distribution at the wall. The influence of distributed sensors and actuators of finite size on the control loop performance is investigated. The results show a reduction of the skin friction at the wall with maximum power saving rates up to 20%.