Краткое описание

The objective is to study the effects of mean scalar gradient on the structure functions and power spectrum of the fluctuating scalar field. The scalar field is created by a continuous iso-kinetic release of a passive scalar (S_c ~ 1000) in a turbulent boundary layer at mid-depth of an open channel flow (Re = 1×l0⁴). Measurements of the scalar field are performed using the planar laser induced fluorescence (PUF) technique at four distances downstream of the source in a vertical plane through the plume centerline. Scaling exponents in the inertial-convective range, estimated from the second and higher (even) order structure functions, show dependence on the longitudinal scalar gradient. As the mean longitudinal scalar gradient increases, the longitudinal exponents decrease from the isotropic expectation of n/3. Scaling behavior is also noted in the viscous-convective range of the variance spectrum. The exponent for the dependence on the wavenumber increases in proportion to the logarithm of the mean longitudinal scalar gradient. Overall, the scaling behavior appears to be dependent on the mean field and agrees with isotropic expectations as the mean scalar gradient tends toward zero.