Evidence that superstructures comprise of self-similar coherent motions in high ReτRe_{\tau} boundary layers

We present experimental evidence that the superstructures in turbulent boundary layers comprise of smaller, geometrically self-similar coherent motions. The evidence comes from identifying and analyzing instantaneous superstructures from large-scale particle image velocimetry datasets acquired at high Reynolds numbers, capable of capturing streamwise elongated motions extending up to 12 times the boundary layer thickness. Given the challenge in identifying the constituent motions of the superstructures based on streamwise velocity signatures, a new approach is adopted that analyzes the wall-normal velocity fluctuations within these very long motions, which reveals the constituent motions unambiguously. The conditional streamwise energy spectra of the wall-normal fluctuations, corresponding exclusively to the superstructure region, are found to exhibit the well-known distance-from-the-wall scaling in the intermediate scale range. Similar characteristics are also exhibited by the Reynolds shear stress co-spectra estimated for the superstructure region, suggesting that geometrically self-similar motions are the constituent motions of these very-large-scale structures. Investigation of the spatial organization of the wall-normal momentum-carrying eddies also lends empirical support to the concatenation hypothesis for the formation of the superstructures. Association between the superstructures and self-similar motions is reaffirmed on comparing the vertical correlations of the momentum carrying motions, which are found to match with the mean correlations. The mean vertical coherence of these motions, investigated for the log-region across three decades of Reynolds numbers, exhibits a unique distance-from-the-wall scaling invariant with Reynolds number. The findings support the prospect for modelling these dynamically significant motions via data-driven coherent structure-based models.Comment: Manuscript accepted for the Journal of Fluid Mechanics, with 25 pages, 15 figure

Experimental investigation of airfoil-turbulence interaction noise

Airfoil-turbulence-interaction noise, which is created whenever turbulent flow encounters an airfoil, is a major contributor of unwanted noise emitted by aircraft, turbomachinery and alike. The experimental study presented here is the precursor to a larger investigation of the impact of complex turbulence on noise generation at the airfoil's leading-edge and airfoil-wall junction. In the current study, the authors examine links between the experimentally acquired properties of isotropic turbulence and the sound radiation of the immersed airfoil. This is achieved by varying the in-flow turbulence intensity using two different turbulence grids. A NACA0012 airfoil was analysed at a range of geometric angles of attack up to 16 degrees and Reynolds numbers of 1 ∙ 105 up to 3 ∙ 105. Stereoscopic Particle Image Velocimetry (SPIV) was conducted beforehand to capture the turbulence characteristics of the free flow. Additionally, acoustic beamforming with a phased microphone array provides insight into the sound generation at the leading-edge. Pressure taps along the centre chord-line were used to measure the mean static pressure, thereby allowing for an open-jet deflection correction of the angle of attack

Statistics of turbulence in the energy-containing range of Taylor-Couette compared to canonical wall-bounded flows

Considering structure functions of the streamwise velocity component in a framework akin to the extended self-similarity hypothesis (ESS), de Silva \textit{et al.} (\textit{J. Fluid Mech.}, vol. 823,2017, pp. 498-510) observed that remarkably the \textit{large-scale} (energy-containing range) statistics in canonical wall bounded flows exhibit universal behaviour. In the present study, we extend this universality, which was seen to encompass also flows at moderate Reynolds number, to Taylor-Couette flow. In doing so, we find that also the transversal structure function of the spanwise velocity component exhibits the same universal behaviour across all flow types considered. We further demonstrate that these observations are consistent with predictions developed based on an attached-eddy hypothesis. These considerations also yield a possible explanation for the efficacy of the ESS framework by showing that it relaxes the self-similarity assumption for the attached eddy contributions. By taking the effect of streamwise alignment into account, the attached eddy model predicts different behaviour for structure functions in the streamwise and in the spanwise directions and that this effect cancels in the ESS-framework --- both consistent with the data. Moreover, it is demonstrated here that also the additive constants, which were previously believed to be flow dependent, are indeed universal at least in turbulent boundary layers and pipe flow where high-Reynolds number data are currently available.Comment: accepted in J. Fluid Mec