The small-scale structure of passive scalar mixing in turbulent boundary layers

Correlation function;Fractal dimension;Mixing;Passive scalar;Scalar field theory;Spectrum analysis;Structure function;Turbulent boundary layer

Dasi, Lakshmi P. ; Civil and Environmental Engineering

University:Georgia Institute of Technology

Department:Civil and Environmental Engineering

关键词: Correlation function;    Fractal dimension;    Mixing;    Passive scalar;    Scalar field theory;    Spectrum analysis;    Structure function;    Turbulent boundary layer;   

Others  :  https://smartech.gatech.edu/bitstream/1853/7599/1/dasi_lakshmi_p_200412_phd.pdf

美国|英语

来源: SMARTech Repository

【 摘 要 】

The objective is to contribute to several issues regarding the traditional view of the local structure of passive scalar fields: (1) probability density function (PDF) of the scalar concentration and scalar gradient, (2) the scalar power spectrum, (3) the structure functions, and (4) correlation functions and multi-point correlators.In addition, the research provides a geometric description of two-dimensional transects of the passive scalar iso-surfaces using the tools of fractal geometry.The local structure is analyzed as a function of large-scale anisotropy, intermittency factor, Reynolds number, and initial condition of the scalar injection. Experiments were performed in the bed boundary layer produced by a uniform depth open channel flow of water in a tilting flume for Re_lamda= 63, 94, and 120.A small nozzle iso-kinetically delivers a passive scalar of high Schmidt number ( Sc = 1000) at mid-depth to generate the turbulent scalar field.Three nozzle diameters are used to study the effects of the injection length scale.High-resolution planar laser induced fluorescence (PLIF) technique is used to measure the scalar field. The local structure far from isotropic and is influenced even at the smallest scales by large-scale anisotropy, initial injection length scale and the Reynolds number of the flow.The PDF of the scalar fluctuations is non-Gaussian and dependent on large-scale anisotropy.The PDF of scalar gradients show the influence of large-scale anisotropy on the structure at the smallest scales.The spectrum of the scalar field deviates from the in the inertial convection regime and is dependent on large-scale anisotropy, external intermittency, and low Reynolds number.There is no evidence of Batchelors k^-1 scaling law.The scaling exponents of the even-ordered structure functions appear to be inversely correlated with the kurtosis of the scalar fluctuations.The fractal geometry of the two dimensional transects of passive scalar iso-surfaces is scale dependent.The fractal dimension is 1.0 at the smallest length scale and increases in a universal manner in the viscous-convective regime.The coverage length underestimate reflects this universal behavior with practical significance.The lacunarity function shows that the instantaneous scalar field is most in-homogenous around the Kolmogorov scale.