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4th Thermal and Fluids Engineering Conference

ISSN: 2379-1748

FREQUENCY CHARACTERISTICS OF A NATURAL CONVECTION BOUNDARY LAYER OF Pr=0.7 ADJACENT TO A VERTICAL ISOTHERMAL SURFACE

Peng Zhao
School of Hydraulic, Energy and Power Engineering, Yangzhou University, Jiangsu, 225127, China

J. Cao
School of Hydraulic, Energy and Power Engineering, Yangzhou University, Jiangsu, 225127, China

Y. Xu
School of Hydraulic, Energy and Power Engineering, Yangzhou University, Jiangsu, 225127, China

Yongling Zhao
School of Hydraulic, Energy and Power Engineering, Yangzhou University, Jiangsu, 225127, China; Centre for Wind, Waves and Water, School of Civil Engineering The University of Sydney, Sydney, NSW 2006, Australia

DOI: 10.1615/TFEC2019.fmi.027563
pages 955-963


KEY WORDS: Disturbances, Boundary layer, Natural convection, Direction numerical simulation

Abstract

The instability and resonance characteristics of the natural convection boundary layer of Pr = 0.7 and Ra = 109 are investigated by direct numerical simulations. To understand the streamwise evolution of the frequencies in the boundary layer, local small-amplitude temperature disturbances at random frequencies were introduced into the leading edge of the boundary layer adjacent to an isothermally heated vertical surface. Power spectral analyses of the temperature time series obtained at various streamwise locations suggest that a low-frequency band is distinct in the upstream, which disappears gradually in the further downstream region, whereas a high-frequency band becomes dominant in the downstream region. The dominant high-frequency band is considered to be the characteristic-frequency band of the boundary layer and the peak frequency within the band is recognized to be the characteristic frequency of the boundary layer, which is determined to be 1.273 Hz for the boundary layer under the present examination. To validate the characteristic frequency of the boundary layer, temporal sinusoidal temperature disturbances of a singlefrequency were introduced into the boundary layer to understand the response of the boundary layer. The strongest response of the boundary layer was observed when the disturbances at the single frequency of 1.273 Hz were introduced into the boundary layer. The results obtained in the present study lead to profound implication for heat transfer manipulation.

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