Active and inactive components of the streamwise velocity in wall-bounded turbulence

Townsend (J. Fluid Mech., vol. 11, issue 1, 1961, pp. 97–120) introduced the concept of active and inactive motions for wall-bounded turbulent flows, where the active motions are solely responsible for producing the Reynolds shear stress, the key momentum transport term in these flows. While the wall-normal component of velocity is associated exclusively with the active motions, the wall-parallel components of velocity are associated with both active and inactive motions. In this paper, we propose a method to segregate the active and inactive components of the two-dimensional (2-D) energy spectrum of the streamwise velocity, thereby allowing us to test the self-similarity characteristics of the former which are central to theoretical models for wall turbulence. The approach is based on analysing datasets comprising two-point streamwise velocity signals coupled with a spectral linear stochastic estimation based procedure. The data considered span a friction Reynolds number range is the streamwise/spanwise wavenumber), lending empirical support to the attached eddy model of Perry & Chong (J. Fluid Mech., vol. 119, 1982, pp. 173–217).