Spectral and hyper eddy-viscosity in high Reynolds number turbulence
ABSTRACT: For the purpose of studying the spectral properties of energy transfer between large and small scales in high Reynolds-number turbulence, we measure the longitudinal SGS dissipation spectrum, defined as the co-spectrum of the SGS stress and filtered strain-rate tensors. An array of four closely spaced X-wire probes enables us to approximate a two-dimensional box filter by averaging over different probe locations (cross-stream filtering) and in time (stream-wise filtering using Taylor's hypothesis). We analyze data taken at the centerline of a cylinder wake at Reynolds numbers up to Rl~450. Using the assumption of local isotropy, the longitudinal SGS stress and filtered strain-rate co-spectrum is transformed into a radial co-spectrum, which allows us to evaluate the spectral eddy-viscosity, n(k,kc). In agreement with classical two-point closure predictions, for graded filters, the spectral eddy-viscosity deduced from the box-filtered data decreases near the filter wavenumber k. When using a spectral cutoff filter in the stream-wise direction (with a box-filter in the cross-stream direction) a cusp behavior near the filter scale is observed. In physical space, certain features of a wave-number dependent eddy-viscosity can be approximated by a combination of a regular and a hyper-viscosity term. A hyper-viscous term is also suggested from considering equilibrium between production and SGS dissipation of resolved enstrophy. Assuming local isotropy, the dimensionless coefficient of the hyper-viscous term can be related to the skewness coefficient of filtered velocity gradients. The skewness is measured from the X-wire array and from direct numerical simulation of isotropic turbulence. The results show that the hyper-viscosity coefficient is negative for graded filters and positive for spectral filters. These trends are in agreement with the spectral eddy-viscosity measured directly from the SGS stress-strain rate co-spectrum. The results provide significant support, now at high Reynolds numbers, for the ability of classical two-point closures to predict general trends of mean energy transfer in locally isotropic turbulence. J. Fluid Mech. 421, p. 307. (2000) (© Cambridge University Press) § Archival Journal Publications: Articles may be downloaded for personal use only! |
Charles Meneveau, Department of Mechanical Engineering, Johns Hopkins University, 3400 N. Charles Street, Baltimore MD 21218, USA, Phone: 1-410-516-7802, Fax: 1-(410) 516-7254, email: meneveau@jhu.edu |
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