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Studies in Homogenous and Nonhomogeneous Free Turbulent Shear Flows

Report Number: AFOSR 69-2151TR
Author(s): Nicholas P. Trentacoste; Pasquale M. Sforza
Corporate Author: Polytechnic Institute of Brooklyn
Laboratory: Air Force Office of Scientific Research
Date of Publication: 1969-09
Pages: 127
Contract: AF 49(638)-1623
Project: 9781
Task: 978101
AD Number: AD0696158

The research effort undertaken in this report concerns itself with an evaluation of the present state-of-the-art of turbulent mixing, by surveying the prevalent concepts and conclusions that have influenced research trends in the field. A critical discussion of the currently favored instruments utilized for obtaining measurements of the time mean properties of a turbulent flow is rendered; in addition, experimental evidence is presented to question the validity of such measurement techniques. A succinct description of the methodology utilized in ascertaining the consistency of experimental measurements with the particular properties of a given flow field is also discussed. An extension of the original Reichart inductive theory of free turbulence is presented. This theory is utilized to solve the problem of a turbulent axisymmetric jet exhausting into a quiescent amnient under arbitrary initial conditions. The theory may be easily generalized to treat the more fashionable two stream mixing problem. Results of the analysis are compared with the mean property measurements of this investigation, and excellent agreement is obtained. Mean property measurements for initially axisymmetric and elliptic jets were obtained under isothemal, nonisothermal, homogeneous and nonhomogeneous conditions. Shadowgraph photography was employed to determine the qualitative effects of changing the initial conditions of a jet flow and to indicate its subsequent development. The most important results of the experimental investigations are listed below: (1) The effect of having the jet's (rho sub j/rho sub a) not equal 1.00, regardless of how produced, is to initiate three-dimensionalities in the subsequent flow field. (2) The jet flow dveleopment is extremely dependent on the initial mean property distribution. It is shown that even small alterations in this distribution can initiate enormous changes in the subsequent flow development. (3) Isothermal elliptic jets are three-dimensional in character, whereas nonisothermal elliptic jets behave rather more like isothermal axisymmetric jets than do nonisothermal axisymmetric jets. (4) Universality of the momentum flux distribution for initially axisymmetric jets, regardless of the ratio (pro sub j/rho sub a), is shown.

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