| Popis: |
High resolution, multi-sensor, hot/cold-wire measurements were made in passively heated, uniformly sheared turbulence in a wind-tunnel. Measurements were focused on terms that are important for modelling of the scalar probability density function (PDF) equation. Unlike previous studies, which considered a single combination of velocity and scalar fields at a time, in this study three different scalar fields were investigated in the same nearly homogeneous turbulence with three passively superimposed temperature fields, namely a transversely homogeneous temperature field with a uniform mean gradient, and two inhomogeneous temperature fields, the plume of a heated line source and a thermal mixing layer. The use of the same uniformly sheared flow allowed the isolation of the effects of scalar inhomogeneity and initial conditions by evaluating the results in the three scalar fields. Thus, the measurements covered a wide range of scalar field conditions and set the ground for a conclusive comparison. For the homogeneous scalar field, results conformed with the literature: the scalar PDF was essentially Gaussian; the conditional expectations of velocities upon the scalar value were approximately linear; and the conditional expectation of the scalar dissipation rate upon the scalar value was mildly anisotropic and had a shape that was similar to those of any of its three parts, which justifies the use of the streamwise part as a surrogate for the total. All these properties behaved very differently in two inhomogeneous scalar fields, the thermal mixing layer and the plume of a heated line source: the scalar PDFs were distinctly sub-Gaussian; the conditional velocity expectations were non-linear functions of the scalar value; and the conditional scalar dissipation rates were very strongly anisotropic, as well as depending on the scalar value in fashions that differed strongly from those of any of their three parts. |
