Hideharu Makita

Characteristics of internal vortical structures in a merged turbulent spot

Interaction phenomena between two turbulent spots were investigated in a zero pressure-gradient laminar boundary layer. Two types of hotwire rakes, a 16-channel I- and a 30-channel X-probe gave clear instantaneous vortical motion inside the spots, showing that the single spot was an aggregation of many small-scale hairpin vortices accompanied by upwashes and downwashes around their legs. The legs, a pair of counter-rotating longitudinal vortices, were identified by the existence of streaky velocity-defect and -excess regions at the bottom of the spot. As the spot grew downstream, the number of longitudinal vortices increased, though its wingtips were always accompanied by upwashes. When two spots were produced in parallel and merged with each other, the upwash of the low-speed fluid was strongly enhanced in their merged part through the mutual interaction between the longitudinal vortices at their inside wingtips. Resultant unstable inflectional velocity profile gave birth to several spanwise vortices aro...

Experimental Analysis on the Transition Process of Internal Gravity Waves in a Strongly Stably-Stratified Mixing Layer

Internal gravity waves are generated in geophysical flow fields and their collapse produces strong turbulence in them [1]. In the present study, a strongly stably-stratified mixing layer with a large positive temperature gradient was generated in a low turbulence wind tunnel equipped with a thermal stratification generator. Simultaneous measurements were made on temperature and velocity fluctuations for spontaneously induced waves and the waves excited by a thermal fluctuation with three components satisfying Thorpe’s condition [2]. Higher order correlation terms were calculated to clarify the transition process of the internal gravity waves, especially the occurrence of counter-gradient heat flux at their collapse.

Analysis on Energy Transfer Mechanism through the Regulation of Transition Process of Internal Gravity Waves

Energy transfer mechanism of internal gravity waves in their transition process was experimentally investigated. Thermal regulation was conducted by giving small sinusoidal thermal disturbance satisfying the three-wave resonance condition at the initial stage of their streamwise development. For the excited case, the frequencies of their irregular eigen-modes became fixed and the phase difference between vertical velocity and temperature fluctuation components was locked up around -π , Increase in correlation between the velocity and the temperature fluctuations strengthened down-gradient heat flux and the energy of mean temperature gradient was efficiently transferred to encourage the wave motion. The internal gravity waves became to have larger potential energy as they developed downstream. The excited internal gravity waves produced stronger random velocity fluctuations through the cascade process and induced larger counter-gradient heat flux at their collapse.