In the outer region of fully developed turbulent pipe flow very large-scale motions reach wavelengths more than 8$R$–16$R$ long (where $R$ is the pipe radius), and large-scale motions with wavelengths of $2R$–$3R$ occur throughout the layer. The very-large-scale motions are energetic, typically containing half of the turbulent kinetic energy of the streamwise component, and they are unexpectedly active, typically containing more than half of the Reynolds shear stress. The spectra of the $y$-derivatives of the Reynolds shear stress show that the very-large-scale motions contribute about the same amount to the net Reynolds shear force, d$\overline{-u'v'}/{\rm d}y$, as the combination of all smaller motions, including the large-scale motions and the main turbulent motions. The main turbulent motions, defined as the motions small enough to be in a statistical equilibrium (and hence smaller than the large-scale motions) contribute relatively little to the Reynolds shear stress, but they constitute over half of the net Reynolds shear force.
Simultaneous streamwise velocity measurements across the vertical direction obtained in the atmospheric surface layer (Re τ 5 × 10 5 ) under near thermally neutral conditions are used to outline and quantify interactions between the scales of turbulence, from the very-large-scale motions to the dissipative scales. Results from conditioned spectra, joint probability density functions and conditional averages show that the signature of very-large-scale oscillations can be found across the whole wall region and that these scales interact with the near-wall turbulence from the energycontaining eddies to the dissipative scales, most strongly in a layer close to the wall, z + . 10 3 . The scale separation achievable in the atmospheric surface layer appears to be a key difference from the low-Reynolds-number picture, in which structures attached to the wall are known to extend through the full wall-normal extent of the boundary layer. A phenomenological picture of very-large-scale motions coexisting and interacting with structures from the hairpin paradigm is provided here for the high-Reynolds-number case. In particular, it is inferred that the hairpin-packet conceptual model may not be exhaustively representative of the whole wall region, but only of a near-wall layer of z + = O(10 3 ), where scale interactions are mostly confined.
Measurements of the instantaneous wake flow from a model wind turbine placed in a turbulent boundary layer were obtained by wall-parallel oriented particle image velocimetry (PIV) in the St. Anthony Falls Laboratory wind tunnel. PIV velocity vector fields were used to investigate mean (expansion angle, wavelength, and wake velocity) and higher order statistics (local slope, curvature, and correlation) describing meandering motions in the turbine wake. These statistics were used to compare the wakes produced by four different wind turbine operating configurations, which include a single turbine operating at two different tip-speed ratios and two turbines aligned with the mean flow. The origin of meandering motions was identified for all cases in the hub vortex signature, which evolved into a stretched or compressed low speed meander in the wall parallel plane, depending on the turbine operating conditions and on the interaction with the wake shear layer. Finally, both autocorrelation and scale-dependent statistics on the velocity minima fluctuations about the meander signature suggest that small scale vortices, found in the hub shear layer and in the wake shear layer, interact with the hub vortex and govern its spatial evolution into large scale wake meandering.
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