Statistics Of Scalar Field Research Articles

High-frequency simultaneous measurements of velocity and concentration within turbulent flows in wind-tunnel experiments

We analyse the metrological aspect related to systems devoted to the simultaneous measurements of (one point) velocity and scalar concentration statistics. We consider systems based on the coupling of an anemometer (laser Doppler or hot wire) with a flame ionization detector. In particular, we focus on the estimate of the cross-correlation velocity-concentration. Key aspects in the reliability of such measurements are related to the identification of an optimal distance between the two probes, the time lag with which the velocity and concentration signals are acquired, and, eventually, the influence of seeding particles (when using a laser Doppler anemometer) on the concentration measurements. We investigate these aspects by performing wind-tunnel experiments on the scalar dispersion downwind a ground-level source in a turbulent boundary layer. This analysis allowed us to identify a time delay and a convenient distance between the signals provided by the anemometer and the concentration detector. Results also show that the seeding used for the laser Doppler anemometer measurements modifies the cut-off frequency in the concentration spectra and induces a slightly larger uncertainty on the statistics of scalar field. The results show the reliability of both systems in estimating (horizontal and vertical) turbulent mass fluxes.

Statistics of Scalar Fields in the Atmospheric Boundary Layer Based on Large-Eddy Simulations. Part II: Forced Convection

Statistics of Scalar Fields in the Atmospheric Boundary Layer Based on Large-Eddy Simulations. Part II: Forced Convection

Statistics of Scalar Fields in the Atmospheric Boundary Layer Based on Large-eddy Simulations. Part 1: Free Convection

Forced convection in a quasi-steady atmospheric boundary layer is investigated based on a large-eddy simulation (LES) model. The performed simulations show that in the upper portion of the mixed layer the dimensionless (in terms of mixed layer scales) vertical gradients of temperature, humidity, and wind velocity depend on the dimensionless height z/zi and the Reech number Rn. The peak values of variances and covariances at the top of the mixed layer, scaled in terms of the interfacial scales, are functions of the interfacial Richardson number Ri. As a result expressions for the entrainment rates, in the case when the interfacial layer has a finite depth, and a condition for the presence of moistening or drying regimes in the mixed layer, are derived. Profiles of dimensionless scalar moments in the mixed layer are proposed to be expressed in terms of two empirical similarity functions Fm and Fi, dependent on dimensionless height z/zi, and the interfacial Richardson number Ri. The obtained similarity expressions adequately approximate the LES profiles of scalar statistics, and properly represent the impact of stability, shear, and entrainment. They are also consistent with the parameterization proposed for free convection in the first part of this paper.

Diffusion equation coupled to Burgers' equation

Scalar diffusion in one-dimensional Burgers' flow is considered. When the Prandtl number is unity, the diffusion equation with convective term is reduced to a simple diffusion equation by a generalized Cole-Hopf transformation. An exact solution of an initial value problem is obtained in a closed form. When the Prandtl number is arbitrary, a similar analytical treatment is possible for limited classes of Burgers' flow (expansion wave and single shock). The statistics of scalar field are discussed briefly.