The Structure of Turbulent Shear Flow By Dr. A. A. Townsend. Pp. xii + 315. 8¾ in. × 5½ in. (Cambridge: At the University Press.) 40s.

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The Structure of Turbulent Shear Flow

Although the literature on university–industry links has begun to uncover the reasons for, and types of, collaboration between universities and businesses, it offers relatively little explanation of ways to reduce the barriers in these collaborations. This paper seeks to unpack the nature of the obstacles to collaborations between universities and industry, exploring influence of different mechanisms in lowering barriers related to the orientation of universities and to the transactions involved in working with university partners. Drawing on a large-scale survey and public records, this paper explores the effects of collaboration experience, breadth of interaction, and inter-organizational trust on lowering different types of barriers. The analysis shows that prior experience of collaborative research lowers orientation-related barriers and that greater levels of trust reduce both types of barriers studied. It also indicates that breadth of interaction diminishes the orientation-related, but increases transaction-related barriers. The paper explores the implications of these findings for policies aimed at facilitating university–industry collaboration.

Investigating the factors that diminish the barriers to university-industry collaboration

This article reviews evidence concerning the cornerstone dissipation scaling of turbulence theory: � = CU 3 /L, with C� = const., � the dissipation rate of turbulent kinetic energy U 2 ,a ndL an integral length scale characterizing the energy-containing turbulent eddies. This scaling is intimately linked to the Richardson-Kolmogorov equilibrium cascade. Accumulating evidence shows that a significant nonequilibrium region exists in various turbulent flows in which the energy spectrum has Kolmogorov's −5/3 wave-number scaling over a wide wave-number range, yet C� ∼ Re m /Re n ,w ithm ≈ 1 ≈ n, Re I a global/inlet Reynolds number, and ReL a local turbulence Reynolds number.

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Dissipation in Turbulent Flows

: The objective of this project is to improve our fundamental knowledge of turbulent flows over rough surfaces. Specifically, we hope to investigate the manner in which roughness affects the near-wall drag-producing turbulent structures, and to what extent surface roughness affects the outer part of rough-wall boundary layers. Ultimately we hope to use this knowledge to propose control strategies to reduce momentum loss in rough-wall boundary layers.

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA426193&Location=U2&doc=GetTRDoc.pdf

Rough-Wall Turbulent Boundary Layers

The relationship between the upstream boundary layer and the low-frequency, large-scale unsteadiness of the separated flow in a Mach 2 compression ramp interaction is investigated by performing wide-field particle image velocimetry (PIV) and planar laser scattering (PLS) measurements in streamwise–spanwise planes. Planar laser scattering measurements in the upstream boundary layer indicate the presence of spanwise strips of elongated regions of uniform momentum with lengths greater than 40?. These long coherent structures have been observed in a Mach 2 supersonic boundary layer (Ganapathisubramani, Clemens & Dolling 2006) and they exhibit strong similarities to those that have been found in incompressible boundary layers (Tomkins & Adrian 2003; Ganapathisubramani, Longmire & Marusic 2003). At a wall-normal location of y/?=0.2, the inferred instantaneous separation line of the separation region is found to oscillate between x/?=?3 and ?1 (where x/?=0 is the ramp corner). The instantaneous spanwise separation line is found to respond to the elongated regions of uniform momentum. It is shown that high- and low-momentum regions are correlated with smaller and larger size of the separation region, respectively. Furthermore, the instantaneous separation line exhibits large-scale undulations that conform to the low- and high-speed regions in the upstream boundary layer. The low-frequency unsteadiness of the separation region/shock foot observed in numerous previous studies can be explained by a turbulent mechanism that includes these elongated regions of uniform momentum

Effects of upstream boundary layer on the unsteadiness of shock induced separation

The wall-normal gradient of Reynolds shear stress, denoted ∂ T+/∂ y+, may be thought of as a momentum source or sink term (depending on its sign). This quantity represents a critical driver of the dynamics of wall-turbulence that give rise to the mean velocity distribution. While there have been a number of studies concerning the statistics of ∂ T+/∂ y+, there remains much to discover regarding the flow features and their interactions that contribute to ∂ T+/∂ y+. The aim of the present work is to investigate the velocity-vorticity correlations that comprise the Reynolds shear stress gradient. Results show velocity-vorticity correlations rapidly change behaviour up to the location of peak Reynolds shear stress. Beyond this point, the qualitative behaviour remains similar throughout the turbulent flow. Interestingly, it was found from two-point correlations that the mean Reynolds stress gradient at any wall-normal location results from only slight asymmetry in the velocity-vorticity correlations. This suggests that the mean momentum balance is extremely sensitive to the structural features and/or interactions that are responsible for vorticity transport throughout the turbulent layer.

Characteristics of momentum sources and sinks in turbulent channel flow

High-resolution particle image velocimetry data obtained in rough-wall boundary layer experiments are re-analysed to examine the influence of surface roughness heterogeneities on wind resource. Two different types of heterogeneities are examined: (i) surfaces with repeating roughness units of the order of the boundary layer thickness (Placidi & Ganapathisubramani. 2015 J. Fluid Mech.782, 541-566. (doi:10.1017/jfm.2015.552)) and (ii) surfaces with streamwise-aligned elevated strips that mimic adjacent hills and valleys (Vanderwel & Ganapathisubramani. 2015 J. Fluid Mech.774, 1-12. (doi:10.1017/jfm.2015.228)). For the first case, the data show that the power extraction potential is highly dependent on the surface morphology with a variation of up to 20% in the available wind resource across the different surfaces examined. A strong correlation is shown to exist between the frontal and plan solidities of the rough surfaces and the equivalent wind speed, and hence the wind resource potential. These differences are also found in profiles of [Formula: see text] and [Formula: see text] (where U is the streamwise velocity), which act as proxies for thrust and power output. For the second case, the secondary flows that cause low- and high-momentum pathways when the spacing between adjacent hills is beyond a critical value result in significant variations in wind resource availability. Contour maps of [Formula: see text] and [Formula: see text] show a large difference in thrust and power potential (over 50%) between hills and valleys (at a fixed vertical height). These variations do not seem to be present when adjacent hills are close to each other (i.e. when the spacing is much less than the boundary layer thickness). The variance in thrust and power also appears to be significant in the presence of secondary flows. Finally, there are substantial differences in the dispersive and turbulent stresses across the terrain, which could lead to variable fatigue life depending on the placement of the turbines within such heterogeneous terrain. Overall, these results indicate the importance of accounting for heterogeneous terrain when siting individual turbines and wind farms.This article is part of the themed issue 'Wind energy in complex terrains'.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.2016.0109

Wind resource assessment in heterogeneous terrain

This report documents a large scale joint research project with the aim of improving the efficiency of ship operations and management by providing a methodology and technology that can quantify the emission and fuel usage penalty due to bio-fouling on ship hull. This can be obtained through better understanding of turbulent boundary layer flows over rough surfaces that cause skin friction drag. Here six different institutions from four countries (Australia, Denmark, Indonesia, and UK) that consist of universities, a passenger ship company, a manufacturer of anti-fouling coatings, and the Indonesian Classification Society are formed. They represent three fields, namely: academic, industrial, and an independent party that supports policy makers. Each of them has different objectives and interests that are interconnected. The research collaboration uses an in-situ laser-based measurement technique of the water flow over the hull of an operating ship combined with under-water image-based surface scanning techniques. The shipboard experiments are accompanied by detailed laboratory experiments to provide further validation. This paper will discuss the importance and challenges of managing such collaboration and the significance of satisfying individual objectives from each three fields in order to achieve the overarching aim.

Managing international collaborative research between academics, industries, and policy makers in understanding the effects of biofouling in ship hull turbulent boundary layers

Abstract Based on experimental data acquired with particle image velocimetry, we examine turbulent boundary layers that are subjected to an abrupt change in wall roughness in the streamwise direction. Three different sandpapers (P24, P36 and P60) together with a smooth wall are used to form a number of different surface transition cases, including both R $\rightarrow$ S (where upstream surface is rough and second surface is either smooth or smoother compared with the upstream surface) and S $\rightarrow$ R (where upstream surface is smoother compared with the downstream surface; both surfaces are rough). This enables us to investigate the effect of the surface transition strength ($M = \ln [y_{02}/y_{01}]$, where $y_{01}$ and $y_{02}$ are the roughness lengths of the upstream and downstream surfaces, respectively) on the growth of the internal boundary layer (IBL) and the corresponding flow structure. The results show that for each surface transition group (i.e. R $\rightarrow$ S and S $\rightarrow$ R), the thickness of the IBLs is proportional to the strength of the surface transition, and that the IBLs are thicker for the S $\rightarrow$ R cases compared with their R $\rightarrow$ S counterparts for similar $|M|$, when normalised by the initial boundary layer thickness ($\delta _0$). The results also show that the growth rates of the IBLs could be represented by a power law, consistent with the previous studies. However, despite a wide range of scatter in the literature for the power-law exponent, an average value of $0.75$ (varies between $0.71$ and $0.8$ with no clear trend) is obtained in the present study considering all the surface transition cases. The pre-factor for the power-law fit, on the other hand, is found to be related to the strength of the surface transition. In addition to the variations in the velocity defect and diagnostic plots with the growth of the IBLs, sweep and ejection events appear to differ significantly (depending on the type of the step change). Two-point spatial correlations, moreover, show that the structure is more elongated in the wall-normal and streamwise directions, as the flow accelerates over the downstream surface (i.e. R $\rightarrow$ S cases). For the reverse transition cases (i.e. S $\rightarrow$ R, where the flow decelerates over the downstream rougher surface), however, the correlation coefficients shrink in size in both directions.

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Experimental observations on turbulent boundary layers subjected to a step change in surface roughness

A novel study on assessing the drag penalty due to hull roughness from a recently cleaned and painted ship hull is reported. Here we estimate the rough surface drag penalty by measuring the velocity profile directly over the hull of an operating ship under steady cruising using a Laser Doppler Anemometer (LDA). The novel technique is non-intrusive and requires only optical accessand does not require a full hull-penetration. For this experiment a small glass window is placed on the double-bottom hull of an operating ship, allowing the LDA to measure the velocity gradient in the turbulent boundary layer formed over the hull (during steady sailing) across some traversable distance from close to the hull surface, to at least the end of the logarithmic region. Preliminary initial results show that there is an approximate 37% increase in skin-friction drag for a recently cleaned ship-hull compared to the hydrodynamically smooth surface

B. Ganapathisubramani

Papers

Characteristics of momentum sources and sinks in turbulent channel flow

Wind resource assessment in heterogeneous terrain

Managing international collaborative research between academics, industries, and policy makers in understanding the effects of biofouling in ship hull turbulent boundary layers

Experimental observations on turbulent boundary layers subjected to a step change in surface roughness

In-situ turbulent boundary layer measurements over freshly cleaned ship-hull under steady cruising