No abstract

Screeching behavior and the closure mechanism of a feedback loop for the flapping mode in under-expanded supersonic jet are investigated by schlieren imaging and near-field acoustic measurements for a high aspect ratio elliptic nozzle. Near-field measurements revealed a single screech frequency for the measured Mach number range. The cross spectrum of pressure signals shows that the upstream propagating wave is out of phase, which was identified as the asymmetric flapping mode. The proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) methods were applied to the time-resolved schlieren data to extract the coherent information associated with screech and its harmonics. The first three spatial POD modes reveal the periodic flapping of the jet and the asymmetric upstream propagation of acoustic waves. POD modes 4 and 5 identify the flow structures and acoustic wave patterns associated with the harmonics of the fundamental screech tone. The spatial DMD mode corresponds to the frequency of ∼10 kHz and exhibits the radial distortion of the jet shear layer and the acoustic radiation pattern. The interaction of the acoustic waves with the jet shear layer indicates that the upstream-traveling guided jet mode is responsible for closing the feedback loop of the flapping mode.

No abstract

No abstract

Understanding the behavior of an aeroelastic system beyond the critical point is essential for effective implementation of any active control scheme since the control system design depends on the type of instability (bifurcation) the system encounters. Previous studies had found the aeroelastic system to enter into chaos beyond the point of instability. In the present work, an attempt has been made to carry out an experimental study on an aeroelastic model placed in a wind tunnel, to understand the behavior of aerodynamics around a wing section undergoing classical flutter. Wind speed was increased from zero until the model encountered flutter. Pressure at various locations along the surface of wing and acceleration at multiple points on the wing were measured in real time for the entire duration of experiment. A Leading Edge Separation Bubble (LSB) was observed beyond the critical point. The growing strength of the LSB with increasing wind speed was found to alter the aerodynamic moment acting on the system, which forced the system to enter into a second bifurcation. Based on the nature of the response, the system appears to undergo periodic doubling bifurcation rather than Hopf-bifurcation, resulting in chaotic motion. Eliminating the LSB can help in preventing the system from entering chaos. Any active flow control scheme that can avoid or counter the formation of leading edge separation bubble can be a potential solution to control the classical flutter.

One of the simplest problems involving external vorticity in boundary layer flows is the flow over a semi-infinite plate under a stream of uniform shear. We study the transient growth phenomenon in this flow to investigate the role of freestream shear on energy amplification, and analyse the differences with the Blasius flow. The initial optimal disturbance which triggers the maximum growth is found to be streamwise vortices, as in other shear flows. Compared to the Blasius boundary layer, higher optimum energy and larger spanwise wavelength of streamwise vortices have been observed. We provide scaling laws for the maximum optimal amplification, which is found to increase exponentially with the freestream shear gradient.

No abstract

An experimental apparatus is designed and realized in order to analyze the behavior of the kinetics and the dynamics of methanol droplet evaporation. Two experimental techniques are used. One is optical and relies on video recording and image processing and the other one is a thermal technique based on heat flux measurements with a heat flux sensor. The case of pure methanol, a more volatile liquid, seems to be more complicated and more intriguing than that of water. Indeed, for that case, the evaporation kinetics is very fast and instabilities appear during evaporation. 96% of the droplet volume evaporate in the first 61% of the droplet lifetime where the contact line is first pinned then the contact radius decreases sharply. The droplet contracts to form a smaller droplet with a higher contact angle. The capillary effects are substantial and produce instabilities. The remaining 4% of the volume evaporate in a second step lasting for 39% of the droplet lifetime with decreasing contact angle, height and contact radius until the drop disappears. The effects of the substrate temperature on the different stages of wetting and the physical mechanisms related to the evaporation of methanol have been identified and discussed. The use of optical method to evaluate the evaporation kinetics based on the elementary volume variation during evaporation showed its limits in the transient stage characterized by the flattening of the drop to form a liquid film and the appearance of instabilities in the liquid-gas interface. On the other hand, the thermal method showed its efficiency and validity in this stage for estimating the mass and heat transfer rate exchanged during all the process of evaporation.