The dynamics of the flow-induced flutter of a thin flexible sheet attached to a streamlined support was experimentally studied in a low-speed wind tunnel. In this study, both the structural dynamics and the fluid dynamics aspects of flutter were considered. The kinematics of the oscillating sheet was investigated using high-speed imaging and the flowfield was examined using hotwire anemometry and particle image velocimetry (PIV). The small-scale perturbation in the flow over the sheet was found to induce a low-amplitude vibration, which changed to a large-amplitude flutter as the wind speed was increased to a critical value. The initiation of flutter occurs with the second mode limit cycle oscillation (LCO), bypassing the first mode, and changes to third mode LCO at a higher wind speed. Based on the behavior of the sheet, five different regimes are identified and discussed in this paper. The natural frequencies of the sheet were found to have a significant role in the initiation of the LCO and its transition to the higher modes. The PIV results show a highly accelerated flow over the curved surface of the oscillating sheet, which induces a lift force that acts as a driving force. The accelerated flow over the sheet separates at its tail and forms a large-scale undulating wake. In the LCO regimes, any large-scale flow separation over the sheet could not be observed and the flow appears to be attached even at high deflection of the sheet.
In the present study, the flow-induced oscillation of an elongated rectangular cylinder (a rigid plate with flat leading and trailing edges), hinged at its leading edge, is studied experimentally. Effects of various parameters such as the Reynolds number, chord, thickness, and moment of inertia on oscillation characteristics of the plate were investigated. Flow visualization and hotwire measurements were carried out to study the nature of the flow field around the plate. The angular deflection and angular velocity of the oscillating plate were obtained using a high-speed camera. There exists a critical chord to thickness ratio, below which the rectangular plate hinged at the leading edge starts oscillating beyond a critical wind speed. The flow-induced oscillation of the plate begins with a small amplitude oscillation which increases with time, and the plate finally attains a limit cycle oscillation. Impinging shear layer vortices, originating at the leading edge, play a key role in the initiation of flow-induced oscillation of the plate. The frequency of the limit cycle oscillation increases almost linearly with the wind speed and decreases with an increase in the chord to thickness ratio of the plate. A dimensional analysis is carried out to identify the important dimensionless parameters, and the same has been used for extracting the scaling for oscillation frequency. Good agreement between the present experimental results and the derived analytical expressions is obtained.
This study presents the combined effect of inclination and internal fins on the melting rate of PCM in a rectangular enclosure (8.89cm×6.35cm). The rectangular enclosure has a hot wall with a temperature of 311K, a cold wall with a temperature of 301.3K and the other two walls are insulated. Gallium with very low Prandtl number is taken as the PCM in this study. First, the individual effects of different inclinations (0⁰, 45⁰ and 90⁰) of the enclosure on melting rate of PCM are studied followed by the study of the effect of internal fins. Then we provide a combined environment of inclination and partial fins and obtain the results of liquid fraction, velocity contours and temperature distributions. Plots for the liquid fraction and average temperatures with respect to time are also obtained. Finally, the results and plots of the combined effect are compared with those of other conditions. From the comparison, we conclude that the combined effect of inclination and partial fins under constant normal gravity condition greatly enhances the heat transfer in PCM.
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