Ivan Marusic

Evidence of very long meandering features in the logarithmic region of turbulent boundary layers

This is a publisher’s version of an article published in Journal of Fluid Mechanics

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 Karman “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.

Characteristics of vortex packets in turbulent boundary layers

Stereoscopic particle image velocimetry (PIV) was used to measure all three instantaneous components of the velocity field in streamwise–spanwise planes of a turbulent boundary layer at Re τ =1060 ( Re θ =2500). Datasets were obtained in the logarithmic layer and beyond. The vector fields in the log layer ( z + =92 and 150) revealed signatures of vortex packets similar to those proposed by Adrian and co-workers in their PIV experiments. Groups of legs of hairpin vortices appeared to be coherently arranged in the streamwise direction. These regions also generated substantial Reynolds shear stress, sometimes as high as 40 times − uw . A feature extraction algorithm was developed to automate the identification and characterization of these packets of hairpin vortices. Identified patches contributed 28% to − uw while occupying only 4% of the total area at z + =92. At z + =150, these patches occupied 4.5% of the total area while contributing 25% to − uw . Beyond the log layer ( z + =198 and 530), the spatial organization into packets is seen to break down.

Large-scale influences in near-wall turbulence

Hot-wire data acquired in a high Reynolds number facility are used to illustrate the need for adequate scale separation when considering the coherent structure in wall-bounded turbulence. It is found that a large-scale motion in the log region becomes increasingly comparable in energy to the near-wall cycle as the Reynolds number increases. Through decomposition of fluctuating velocity signals, it is shown that this large-scale motion has a distinct modulating influence on the small-scale energy (akin to amplitude modulation). Reassessment of DNS data, in light of these results, shows similar trends, with the rate and intensity of production due to the near-wall cycle subject to a modulating influence from the largest-scale motions.

Large-scale amplitude modulation of the small-scale structures in turbulent boundary layers

© 2009 Cambridge University Press. Online edition of the journal is available at http://journals.cambridge.org/action/displayJournal?jid=FLM

Hot-wire spatial resolution issues in wall-bounded turbulence

Careful reassessment of new and pre-existing data shows 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 the Reynolds number and viscous-scaled wire length l + . An empirical expression is given to account for these effects. 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 the Reynolds number. We also investigate the appearance of the so-called outer peak in the broadband streamwise intensity, found by some researchers to occur within the log region of high-Reynolds-number boundary layers. We show that the ‘outer peak’ is consistent with the attenuation of small scales due to large l + . For turbulent boundary layers, in the absence of spatial resolution problems, there is no outer peak up to the Reynolds numbers investigated here ( Re τ = 18830). Beyond these Reynolds numbers – and for internal geometries – the existence of such peaks remains open to debate. Fully mapped energy spectra, obtained with a range of l + , are used to demonstrate this phenomenon. We also establish the basis for a ‘maximum flow frequency’, a minimum time scale that the full experimental system must be capable of resolving, in order to ensure that the energetic scales are not attenuated. It is shown that where this criterion is not met (in this instance due to insufficient anemometer/probe response), an outer peak can be reproduced in the streamwise intensity even in the absence of spatial resolution problems. It is also shown that attenuation due to wire length can erode the region of the streamwise energy spectra in which we would normally expect to see k x −1 scaling. In doing so, we are able to rationalize much of the disparity in pre-existing literature over the k x −1 region of self-similarity. Not surprisingly, the attenuated spectra also indicate that Kolmogorov-scaled spectra are subject to substantial errors due to wire spatial resolution issues. These errors persist to wavelengths far beyond those which we might otherwise assume from simple isotropic assumptions of small-scale motions. The effects of hot-wire length-to-diameter ratio ( l / d ) are also briefly investigated. For the moderate wire Reynolds numbers investigated here, reducing l / d from 200 to 100 has a detrimental effect on measured turbulent fluctuations at a wide range of energetic scales, affecting both the broadband intensity and the energy spectra.

Predictive Model for Wall-Bounded Turbulent Flow

Elucidating Turbulent Flow When needing to mix two fluids rapidly, turbulent flow can be beneficial. However, in most cases, the churning and tumbling motions of a fluid during turbulent flow reduce the efficiency of a device or process. When fluid flows past a solid object, the bulk of the turbulent motion is concentrated at the surface boundary, but it is unclear to what extent these inner motions are influenced by flow far from the boundary. Marusic et al. (p. 193; see the Perspective by Adrian) demonstrate a nonlinear connection between inner-layer motions and the large-scale outer-layer motions in wind tunnel experiments. A simple model was able to describe the relationship mathematically while accurately mapping the experimental data. A nonlinear relationship is developed for turbulent flow past an object. The behavior of turbulent fluid motion, particularly in the thin chaotic fluid layers immediately adjacent to solid boundaries, can be difficult to understand or predict. These layers account for up to 50% of the aerodynamic drag on modern airliners and occupy the first 100 meters or so of the atmosphere, thus governing wider meteorological phenomena. The physics of these layers is such that the most important processes occur very close to the solid boundary—the region where accurate measurements and simulations are most challenging. We propose a mathematical model to predict the near-wall turbulence given only large-scale information from the outer boundary layer region. This predictive capability may enable new strategies for the control of turbulence and may provide a basis for improved engineering and weather prediction simulations.

Taking the “Waste” Out of “Wastewater” for Human Water Security and Ecosystem Sustainability

Humans create vast quantities of wastewater through inefficiencies and poor management of water systems. The wasting of water poses sustainability challenges, depletes energy reserves, and undermines human water security and ecosystem health. Here we review emerging approaches for reusing wastewater and minimizing its generation. These complementary options make the most of scarce freshwater resources, serve the varying water needs of both developed and developing countries, and confer a variety of environmental benefits. Their widespread adoption will require changing how freshwater is sourced, used, managed, and priced.

A wall-wake model for the turbulence structure of boundary layers. Part 1. Extension of the attached eddy hypothesis

©Cambridge University Press. Perry, A.E. & Marusic, Ivan. (1995) A wall-wake model for the turbulence structure of boundary layers. Part 1. Extension of the attached eddy hypothesis. Journal of fluid mechanics, 298, 361-388. http://journals.cambridge.org/action/displayJournal?

A comparison of turbulent pipe, channel and boundary layer flows

© 2009 Cambridge University Press. Online edition of the journal is available at http://journals.cambridge.org/FLM