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

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

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

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

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

La arquitectura vernácula como legado de criterios bioclimáticos de diseño para viviendas de interés social

El objetivo de este trabajo es presentar una propuesta de investigación de cátedra incorporada al plan de acción a tres años exigido en la convocatoria a concurso docente de la universidad en noviembre de 2005. La propuesta se incorpora como parte de los trabajos prácticos de la asignatura Ecología y Medio Ambiente que los alumnos cursan en cuarto año de la carrera durante el segundo cuatrimestre; por lo cual se encuentra actualmente en etapa de búsqueda de datos y selección de casos de estudio. La metodología propuesta tiene como objetivo demostrar la importancia de considerar aspectos bioclimáticos de la arquitectura vernácula, en la definición de criterios de diseño del hábitat social e incorporarlos como normativa para el desarrollo de planes de vivienda, apuntando a una mayor identificación cultural y apropiación de las mismas por parte de los usuarios.The objective of this project is to present a proposal of research included in the plan for the following three years, which is required in November 2005 by the academic University concourse. The proposal is related to the "Ecología y Medio Ambiente" assignments. This subject belongs to the fourth year of the degree course in architecture during its second period. Therefore this research is in the stage of compilation and study cases selection. The objective of methodology applied is to demonstrate the importance of considering bioclimatic aspects of vernacular architecture in the design criteria definition for social housing and to incorporate them as a regulation its development, in order to obtain cultural identification and appropriation by the users.Asociación Argentina de Energías Renovables y Medio Ambiente (ASADES

La arquitectura vernácula como legado de criterios bioclimáticos de diseño para viviendas de interés social

El objetivo de este trabajo es presentar una propuesta de investigación de cátedra incorporada al plan de acción a tres años exigido en la convocatoria a concurso docente de la universidad en noviembre de 2005. La propuesta se incorpora como parte de los trabajos prácticos de la asignatura Ecología y Medio Ambiente que los alumnos cursan en cuarto año de la carrera durante el segundo cuatrimestre; por lo cual se encuentra actualmente en etapa de búsqueda de datos y selección de casos de estudio. La metodología propuesta tiene como objetivo demostrar la importancia de considerar aspectos bioclimáticos de la arquitectura vernácula, en la definición de criterios de diseño del hábitat social e incorporarlos como normativa para el desarrollo de planes de vivienda, apuntando a una mayor identificación cultural y apropiación de las mismas por parte de los usuarios.The objective of this project is to present a proposal of research included in the plan for the following three years, which is required in November 2005 by the academic University concourse. The proposal is related to the "Ecología y Medio Ambiente" assignments. This subject belongs to the fourth year of the degree course in architecture during its second period. Therefore this research is in the stage of compilation and study cases selection. The objective of methodology applied is to demonstrate the importance of considering bioclimatic aspects of vernacular architecture in the design criteria definition for social housing and to incorporate them as a regulation its development, in order to obtain cultural identification and appropriation by the users.Asociación Argentina de Energías Renovables y Medio Ambiente (ASADES

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