Nobumasa Sekishita

Characteristics of Temperature Fields and Flow Fields in a Heated Street Canyon by Scale Modeling

The street canyon phenomenon has a negative impact on air quality. Flow, temperature, and the concentration fields of pollutants affect street canyon. Many studies have focused on the concentration and flow fields; model experiments on the temperature field have been less examined. This experiment investigates the effect of model height and placement of flow and temperature fields. Temperature and flow fields are visualized using a smoke-wire method, one of the methods used to visualize a flow field. Visualization images were investigated in comparison to several other images. In turbulent flow conditions, temperature change of the street is verified when the turbulence parameter has changed. As a result of the experiment, data showing a high cooling effect was obtained. Furthermore, a trend was verified for cooling effects under turbulent flow conditions.

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.