| The present study is focused on the effect of variation of the size of the large scale structure on the mixing of a passive scalar in nearly homogeneous grid-generated turbulence. A passive scalar is introduced into the flow by means of a grid consisting of closely separated horizontal wires. A set of ten wires is heated by means of an electrical current passed thorough the wires. A temperature change of about 0.7°C is observed just downstream of the wires, which is low enough so the heated fluid can be considered a passive scalar. The mean velocity is held constant at about 6m/s. Free-stream turbulence is produced by means of biplane grids with a mesh size, Mu = 2.54cm, 5.08cm, and 10.16cm which leads to corresponding grid Reynolds numbers of 8401, 16802, and 33605, respectively. It is found that the normalized mean temperature plotted as a function of the vertical distance from the centerplane normalized by the half wake width are nearly similar in shape far downstream of the wire grid. The profiles of the normalized root mean square temperature, normalized temperature skewness and kurtosis plotted as a function of the vertical distance from the centerplane normalized by the half wake width are not similar even far downstream of the wire gird for all grids. At moderate distances downstream of the wire grid (for example x/Mu = 10 for Mu = 2.45cm), the Probability Density Functions show the presence of ambient temperature fluid on the centerplane far downstream of the wire-grid. This can occur when the thermal wake produced by the wire grid "flaps", i.e. the heated wake moves vertically with time without significant mixing. |
