## Abstract

To study the mixing of a passive scalar in nearly isotropic turbulence, experiments have been made in isotropically mixed thermal fields with thermal mesh size M_{θ} (a) equal to the momentum mesh size M, (b) larger than M (obtained by heating only alternate rods of the turbulence generating grid), and (c) smaller than M. This last condition was achieved by inserting a fine heating screen with M_{θ} < M, at locations downstream of the turbulence grid. The heating screen was designed to produce negligible statistical change in the velocity field a short distance downstream. In all the heated grid experiments, for a given initial configuration of the thermal field, the intensity of temperature fluctuations θ normalized by the mean temperature rise ΔT, and the decay rate of [formula omitted] were both independent of the temperature of the grid. The principal effect of having M_{θ} > M was an increase in the relative intensity of temperature fluctuations compared with the M_{θ} = M case, and a marginal increase in their decay rate; contrary to expectation, the ratio R of temperature to velocity integral scales in the region of approximate homogeneity did not differ from that corresponding to M_{θ} = M. In heated screen experiments, the relative decay rate was independent of M_{θ}/M and ΔT. For the three locations of the heating screen used in these experiments, the decay rate was also independent of the relative distance x_{s} of the heating screen from the turbulence generating grid; however, larger x_{s} was associated with larger relative intensity of fluctuations. To a first approximation, the ratio R approached unity according to the empirical relation R = 1 − A exp [− αx_{θ}/(UT_{0})], where x_{θ} is downstream distance measured from the heating screen, and T_{0} is a characteristic turbulence decay time scale at x_{0} = 0. It was also verified that the skewness of the streamwise temperature derivative is approximately zero sufficiently downstream of the heating screen. Where the present study overlaps with previous measurements, an extensive comparison reveals several points of agreement as well as departure.

Original language | English (US) |
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Pages (from-to) | 597-621 |

Number of pages | 25 |

Journal | Journal of Fluid Mechanics |

Volume | 100 |

Issue number | 3 |

DOIs | |

State | Published - Oct 1980 |

## ASJC Scopus subject areas

- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering