Evidence of large-scale amplitude modulation on the near-wall turbulence

The relationship between large- and small-scale motions remains a poorly understood process in wall-bounded turbulence. Such misunderstanding is perhaps, in part, due to the limited scale separation typical of many laboratory-scale facilities. A recent investigation performed by Hutchins and Marusic [11] in a high Reynolds number turbulent boundary layer has qualitatively shown the existence of a modulating influence of the large-scale log region motions on the small-scale near-wall cycle. For this study we build upon these observations, using the Hilbert transformation applied to the spectrally filtered smallscale component of fluctuating velocity signals, in order to quantitatively determine the degree of amplitude modulation imparted by the large-scale structures onto the near-wall cycle

Drag forces on a bed particle in open-channel flow : Effects of pressure spatial fluctuations and very-large-scale motions

The study has been supported by two EPSRC/UK grants, ‘High-resolution numerical and experimental studies of turbulence-induced sediment erosion and near-bed transport’ (EP/G056404/1) and ‘Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification’ (EP/K041088/1). I.M. also acknowledges the support of the Australian Research Council.Peer reviewedPublisher PD

Analysis of scale energy budgets in wall turbulence using dual plane PIV

The scale energy budget in the near wall region is a subject of great interest in turbulent flows since it combines concepts from independent analysis in physical space and scale space. Earlier, this energy budget was studied numerically using Direct Numerical Simulation (DNS) data and experimentally using low resolution dual plane Particle Image Velocimetry (PIV) data. It was observed that the low resolution PIV data were not sufficient to accurately capture the dynamics of the energy balance and hence high resolution experiments were conducted in similar experimental conditions. The results from these high resolution data conducted in two locations of the logarithmic layer of the boundary layer indicate that the resolution of these experiments is sufficient to capture the scale energy budget in the near wall region. Predictions of the cross-over scale, which is related to the relative importance of production and transfer of turbulent kinetic energy, are found to match expected trends, and illustrate that the experimental technique provides a powerful tool for the scale energy budget analysis

Wall-bounded turbulent flows at high Reynolds numbers: Recent advances and key issues

Wall-bounded turbulent flows at high Reynolds numbers have become an increasingly active area of research in recent years. Many challenges remain in theory, scaling, physical understanding, experimental techniques, and numerical simulations. In this paper we distill the salient advances of recent origin, particularly those that challenge textbook orthodoxy. Some of the outstanding questions, such as the extent of the logarithmic overlap layer, the universality or otherwise of the principal model parameters such as the von Kármán “constant,” the parametrization of roughness effects, and the scaling of mean flow and Reynolds stresses, are highlighted. Research avenues that may provide answers to these questions, notably the improvement of measuring techniques and the construction of new facilities, are identified. We also highlight aspects where differences of opinion persist, with the expectation that this discussion might mark the beginning of their resolution

The Influence of Spatial Resolution due to Hot-Wire Sensors on Measurements in Wall-Bounded Turbulence.

Reassessment of compiled data reveal that recorded scatter in the hot-wire measured near-wall peak in viscous-scaled streamwise turbulence intensity is due in large part to the simultaneous competing effects of Reynolds number and viscous-scaled wire-length l ( lUt n, where l is the wirelength, Ut is friction velocity and n is kinematic viscosity). These competing factors can explain much of the disparity in existing literature, in particular explaining how previous studies have incorrectly concluded that the inner-scaled near-wall peak is independent of Re. We also investigate the appearance of the, so-called, ‘outerpeak’ in the broadband streamwise intensity, found by some researchers to occur within the log-region of high Reynolds number boundary layers. We show that this ‘outer-peak’ is most likely a symptom of attenuation of small-scales due to large l . Fully mapped energy spectra, obtained with two different l , are presented to demonstrate this phenomena. The spatial attenuation resulting from wires with large l effectively filters small-scale fluctuations from the recorded signal

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

Hydraulic resistance in open-channel flows over self-affine rough beds

Acknowledgements The authors wish to express their gratitude to Stephan Spiller for advice regarding the silicone moulds, to Cameron Scott for assisting with manufacturing of the roughness elements and Davide Collautti for help with conducting experiments.Peer reviewedPublisher PD

Flow development in rough-bed open channels : mean velocities, turbulence statistics, velocity spectra, and secondary currents

Acknowledgements: The authors wish to express their gratitude to Roy Gillanders for the help provided in the laboratory and to the School of Engineering of the University of Aberdeen for the support. The comments and suggestions of the Associate Editor and two anonymous reviewers helped to improve the final version of the paper and are much appreciated. Funding The study has been supported by three Engineering and Physical Sciences Research Council/UK grants: “High-resolution numerical and experimental studies of turbulence-induced sediment erosion and near-bed transport” (EP/G056404/1), “Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification” (EP/K041088/1) and “Secondary currents in turbulent flows over rough walls” (EP/V002414/1). Publisher Copyright: © 2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.Peer reviewedPublisher PD