Ricerc@Sapienza

The generalised Kolmogorov equation is used to describe the scale-by-scale turbulence
dynamics in the shear layer and in the separation bubble generated by a bulge at one
of the walls in a turbulent channel flow. The second-order structure function, which
is the basis of such an equation, is used as a proxy to define a scale-energy content,
that is an interpretation of the energy associated with a given scale. Production
and dissipation regions and the flux interchange between them, in both physical
and separation space, are identified. Results show how the generalised Kolmogorov
equation, a five-dimensional equation in our anisotropic and strongly inhomogeneous
flow, can describe the turbulent flow behaviour and related energy mechanisms. Such
complex statistical observables are linked to a visual inspection of instantaneous
turbulent structures detected by means of the Q-criterion. Part of these turbulent
structures are trapped in the recirculation where they undergo a pseudo-cyclic process
of disruption and reformation. The rest are convected downstream, grow and tend to
larger streamwise scales in an inverse cascade. The classical picture of homogeneous
isotropic turbulence in which energy is fed at large scales and transferred to dissipate
at small scales does not simply apply to this flow where the energy dynamics strongly
depends on position, orientation and length scale.