Quasi-coherent structures in the marine atmospheric boundary layer
Boppe, Ravi Shankar
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Turbulence research in the laboratory over the past three decades has confirmed the existence of quasi-coherent structures amidst the chaos of a turbulent boundary layer. It has been observed that a quasi-periodic phenomena called "bursting" accounts for a major contribution to the turbulent Reynolds stress and the production of turbulent kinetic energy. Bursting is the term used for a sequence of events, where a low-speed streak of fluid from the near wall region lifts away from the wall, slowly at first, and then rapidly moves away from the wall as it convects downstream where it becomes unstable and breaks up violently upon interaction with the outer flow. This ejection of low speed fluid into the mean flow is responsible for locally high values of turbulent kinetic energy. Though a great deal is known about these structures in laboratory flows, little has been done to investigate their existence in the turbulent air flow over the ocean. It would seem, intuitively, that such structures, if present in the marine atmospheric boundary layer, would playa major role in the transfer of momentum, mass and heat across the air-sea interface. The present study is aimed at identifying the existence of burst structures in the marine atmospheric boundary layer. The standard ejection detection schemes like the quadrant, the VITA and the modified u-level techniques were applied to the turbulent wind data measured over the ocean. It was found that the proportion of contribution to the Reynolds stress from the four quadrants of the u'w' plane is in close agreement with the corresponding contributions for a laboratory flow. Ejection detection followed by the grouping of ejections into bursts yielded a mean burst period of 47 sec., at a height of 8.2 m above the water surface, where the mean wind velocity was 6.74 m/s. This burst period corresponds well with the peaks obtained from the autocorrelation of the streamwise velocity signal and the first moment of the stress spectrum. Furthermore, phase averages of these events show a structure which is similar to the structure of the events detected in the laboratory flows.
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