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Unformatted text preview: National Aeronautics and Space Administration Langley Research Center Hampton, Virginia 236812199 NASA/CR1998206909 ICASE Report No. 985 A Kinematically Consistent TwoPoint Correlation Function J. R. Ristorcelli ICASE Institute for Computer Applications in Science and Engineering NASA Langley Research Center Hampton, VA Operated by Universities Space Research Association January 1998 Prepared for Langley Research Center under Contracts NAS197046 & NAS119480 A KINEMATICALLY CONSISTENT TWOPOINT CORRELATION FUNCTION J. R. RISTORCELLI * Abstract. A simple kinematically consistent expression for the longitudinal twopoint correlation func tion related to both the integral length scale and the Taylor microscale is obtained. On the inner scale, in a region of width inversely proportional to the turbulent Reynolds number, the function has the appropriate curvature at the origin. The expression for twopoint correlation is related to the nonlinear cascade rate, or dissipation , a quantity that is carried as part of a typical singlepoint turbulence closure simulation. Con structing an expression for the twopoint correlation whose curvature at the origin is the Taylor microscale incorporates one of the fundamental quantities characterizing turbulence, , into a model for the twopoint correlation function. The integral of the function also gives, as is required, an outer integral length scale of the turbulence independent of viscosity. The proposed expression is obtained by kinematic arguments; the intention is to produce a practically applicable expression in terms of simple elementary functions that allow an analytical evaluation, by asymptotic methods, of diverse functionals relevant to singlepoint turbulence closures. Using the expression devised an example of the asymptotic method by which functionals of the twopoint correlation can be evaluated is given. Key words. twopoint correlation functions, turbulence modeling, functionals Subject classification. Fluid Mechanics, Aeroacoustics 1. Introduction. In singlepoint turbulence closures one is often in the position of needing to approx imate functionals integrals of functions of the twopoint correlation function Batchelor [1], Kraichnan [2], Ristorcelli [3]. In aeroacoustical developments in which acoustic radiation due to turbulence is related to the statistics of the turbulent source field one is in a similar position: the acoustic radiation is a function of diverse twopoint integrals, Proudman [4], Ribner [5], Lilley [6]. Batchelor [1], for example, in order to finish his development for the pressure variance in an isotopic turbulence needed to evaluate the two functionals I ( f ) = R x ( f ) 2 dx and I ( f ) = R x 1 ( f ) 2 dx . The function f ( x ) is the twopoint longitudinal correlation function. Lilley [6] in an investigation of noise radiated from isotropic turbulence required an evaluation of I ( f ) = R x 4 ( f ) 2 dx . Proudman [4] in his statistical application....
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