 Mechanical Engineering  Research Publications
Mechanical Engineering  Research Publications
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ItemObservations on high Reynolds number turbulent boundary layer measurementsHAFEZ, SHM ; MARUSIC, I ; CHONG, MS ; JONES, MB (The University of Sydney, 2004)

ItemSpanwise periodicity and the existence of very large scale coherence in turbulent boundary layersHutchins, N ; Ganapathisubramani, B ; Marusic, I (Begellhouse, 20051201)

ItemDominant spanwise Fourier modes, and the existence of very large scale coherence in turbulent boundary layersHutchins, N. ; Ganapathisubramani, B. ; Marusic, I. ( 2004)Multiple plane stereo PIV results and data from a rake often hotwire probes are used to investigate the largest scalestructures in a zeropressuregradient turbulent boundary layer.Instantaneous vector fields from stereo PIV in spanwisestreamwiseplanes reveal long low and highspeed regions,with a length that often exceeds the viewing window (> 2d).Also evident is a remarkable degree of spanwise organisation,that manifests as a persistent spanwise stripiness in the u componentof the PIV vector field. Almost all trace of such spanwiseorganisation is lost in the mean statistics, presumably dueto the multitude of scales naturally present in wallbounded turbulence.This can be overcome by ‘dejittering’ the instantaneousvector fields. By sorting the data according to dominantspanwise fourier modes, and then applying simple statisticaltools to the sorted subsets, we are able to extract a clear viewof spanwise organisation. Results are confirmed in the variousPIV datasets. Since the PIV fails to adequately capture the fullstreamwise extent of the lowspeed regions, a rake of hotwireprobes is also employed to capture a continuous view of thespanwise coherence. It is found that the lowspeed regions arein fact extremely persistent in the streamwise direction, oftenexceeding 20 d in length. The fact that these long features meanderappreciably in the spanwise direction will limit the overallstreamwise lengthscale as witnessed by a single probe or singlepoint statistic. For instance, premultiplied onedimensionalspectra of the streamwise velocity (kxFuu) at this z/d show apeak contribution for characteristic lengthscales of 57d.

ItemThree dimensional structure characterization and visualization in a turbulent boundary layerGanapathisubramani, B ; Longmire, E ; MARUSIC, I ; Urness, T ; Interrante, V (CIMNE  International Center for Numical Methods in Engineering, 2004)

ItemInclined crossstream stereo particle image velocimetry measurements in turbulent boundary layersHutchins, N. ; Hambleton, W. T. ; MARUSIC, IVAN (Cambridge University Press, 2005)This work can be viewed as a reprise of Head & Bandyopadhyay’s (J. Fluid Mech. vol. 107, 1981, p. 297) original boundarylayer visualization study although in this instance we make use of stereo particle image velocimetry (PIV), techniques to obtain a quantitative view of the turbulent structure. By arranging the laser lightsheet and image plane of a stereo PIV system in inclined spanwise/wallnormal planes (inclined at both 45° and 135° to the streamwise axis) a unique quantitative view of the turbulent boundary layer is obtained. Experiments are repeated across a range of Reynolds numbers, Reτ ≈690–2800. Despite numerous experimental challenges (due to the large outofplane velocity components), mean flow and Reynolds stress profiles indicate that the salient features of the turbulent flow have been well resolved. The data are analysed with specific attention to a proposed hairpin eddy model. Inplane twodimensional swirl is used to identify vortical eddy structures piercing the inclined planes. The vast majority of this activity occurs in the 135° plane, indicating an inclined eddy structure, and BiotSavart law calculations are carried out to aid in the discussion. Conditional averaging and linear stochastic estimation results also support the presence of inclined eddies, arranged about lowspeed regions. In the 135° plane, instantaneous swirl patterns exhibit a predisposition for counterrotating vortex pairs (arranged with an ejection at their confluence). Such arrangements are consistent with the hairpin packet model. Correlation and scaling results show outerscaling to be the correct way to quantify the characteristic spanwise length scale across the log and wake regions of the boundary layers (for the range of Reynolds numbers tested). A closer investigation of twopoint velocity correlation contours indicates the occurrence of a distinct tworegime behaviour, in which contours (and hence streamwise velocity fluctuations) either appear to be ‘attached’ to the buffer region, or ‘detaching’ from it. The demarcation between these two regimes is found to scale well with outer variables. The results are consistent with a coherent structure that becomes increasingly uncoupled (or decorrelated) from the wall as it grows beyond the logarithmic region, providing additional support for a wall–wake description of turbulent boundary layers.

ItemNew evolution equations for turbulent boundary layers in arbitrary pressure gradientsPerry, A. E. ; Marusic, I. ; Jones, M. B. (Indian Academy of Sciences, 1998)A new approach to the classical closure problem for turbulent boundary layers is presented. This involves using the wellknown meanflow scaling laws such as Prandtl's law of the wall and the law of the wake of Coles together with the mean continuity and the mean momentum differential and integral equations. The important parameters governing the flow in the general nonequilibrium case are identified and are used for establishing a framework for closure. Initially, closure is done here empirically from the data but the framework is most suitable for applying the attached eddy hypothesis in future work. How this might be done is indicated here.

ItemMean dynamics of transitional channel flowElsnab, J ; Klewicki, J ; Maynes, D ; Ameel, T (CAMBRIDGE UNIV PRESS, 20110701)The redistribution of mean momentum and vorticity, along with the mechanisms underlying these redistribution processes, is explored for postlaminar flow in fully developed, pressure driven, channel flow. These flows, generically referred to as transitional, include an instability stage and a nonlinear development stage. The central focus is on the nonlinear development stage. The present analyses use existing direct numerical simulation data sets, as well as recently reported highresolution molecular tagging velocimetry measurements. Primary considerations stem from the emergence of the effects of turbulent inertia as represented by the Reynolds stress gradient in the mean differential statement of dynamics. The results describe the flow evolution following the formation of a nonzero Reynolds stress peak that is known to first arise near the critical layer of the most unstable disturbance. The positive and negative peaks in the Reynolds stress gradient profile are observed to undergo a relative movement toward both the wall and centreline for subsequent increases in Reynolds number. The Reynolds stress profiles are shown to almost immediately exhibit the same sequence of curvatures that exists in the fully turbulent regime. In the transitional regime, the outer inflection point in this profile physically indicates a localized zone within which the mean dynamics are dominated by inertia. These observations connect to recent theoretical findings for the fully turbulent regime, e.g. as described by Fife, Klewicki & Wei (J. Discrete Continuous Dyn. Syst., vol. 24, 2009, p. 781) and Klewicki, Fife & Wei (J. Fluid Mech., vol. 638, 2009, p. 73). In accord with momentum equation analyses at higher Reynolds number, the present observations provide evidence that a logarithmic mean velocity profile is most rapidly approximated on a subdomain located between the zero in the Reynolds stress gradient (maximum in the Reynolds stress) and the outer region location of the maximal Reynolds stress gradient (inflection point in the Reynolds stress profile). Overall, the present findings provide evidence that the dynamical processes during the postlaminar regime and those operative in the high Reynolds number regime are connected and describable within a single theoretical framework.

ItemMean dynamics of transitional boundarylayer flowKlewicki, J ; Ebner, R ; Wu, X (CAMBRIDGE UNIV PRESS, 20110901)The dynamical mechanisms underlying the redistribution of mean momentum and vorticity are explored for transitional twodimensional boundarylayer flow at nominally zero pressure gradient. The analyses primarily employ the direct numerical simulation database of Wu & Moin (J. Fluid Mech., vol. 630, 2009, p. 5), but are supplemented with verifications utilizing subsequent similar simulations. The transitional regime is taken to include both an instability stage, which effectively generates a finite Reynolds stress profile, −ρuv(y), and a nonlinear development stage, which progresses until the terms in the mean momentum equation attain the magnitude ordering of the fourlayer structure revealed by Wei et al. (J. Fluid Mech., vol. 522, 2005, p. 303). Selfconsistently applied criteria reveal that the third layer of this structure forms first, followed by layers IV and then II and I. For the present flows, the fourlayer structure is estimated to be first realized at a momentum thickness Reynolds number Rθ = U∞ θ/ν ≃ 780. The firstprinciplesbased theory of Fife et al. (J. Disc. Cont. Dyn. Syst. A, vol. 24, 2009, p. 781) is used to describe the mean dynamics in the laminar, transitional and fourlayer regimes. As in channel flow, the transitional regime is marked by a nonnegligible influence of all three terms in the mean momentum equation at essentially all positions in the boundary layer. During the transitional regime, the action of the Reynolds stress gradient rearranges the mean viscous force and mean advection profiles. This culminates with the segregation of forces characteristic of the fourlayer regime. Empirical and theoretical evidence suggests that the formation of the fourlayer structure also underlies the emergence of the mean dynamical properties characteristic of the highReynoldsnumber flow. These pertain to why and where the mean velocity profile increasingly exhibits logarithmic behaviour, and how and why the Reynolds stress distribution develops such that the inner normalized position of its peak value, ym+, exhibits a Reynolds number dependence according to $y_m^+ {\,\simeq\,} 1.9 \sqrt{\delta^+}$.

ItemLargescale amplitude modulation of the smallscale structures in turbulent boundary layersMathis, R ; Hutchins, N ; Marusic, I (CAMBRIDGE UNIV PRESS, 20090610)In this paper we investigate the relationship between the large and smallscale energycontaining motions in wall turbulence. Recent studies in a highReynoldsnumber turbulent boundary layer (Hutchins & Marusic, Phil. Trans. R. Soc. Lond. A, vol. 365, 2007a, pp. 647–664) have revealed a possible influence of the largescale boundarylayer motions on the smallscale nearwall cycle, akin to a pure amplitude modulation. In the present study we build upon these observations, using the Hilbert transformation applied to the spectrally filtered smallscale component of fluctuating velocity signals, in order to quantify the interaction. In addition to the largescale logregion structures superimposing a footprint (or mean shift) on the nearwall fluctuations (Townsend, The Structure of Turbulent Shear Flow, 2nd edn., 1976, Cambridge University Press; Metzger & Klewicki, Phys. Fluids, vol. 13, 2001, pp. 692–701.), we find strong supporting evidence that the smallscale structures are subject to a high degree of amplitude modulation seemingly originating from the much larger scales that inhabit the log region. An analysis of the Reynolds number dependence reveals that the amplitude modulation effect becomes progressively stronger as the Reynolds number increases. This is demonstrated through three orders of magnitude in Reynolds number, from laboratory experiments at Reτ ~ 103–104 to atmospheric surface layer measurements at Reτ ~ 106.

ItemUnravelling turbulence near wallsMarusic, I (CAMBRIDGE UNIV PRESS, 20090710)Turbulent flows near walls have been the focus of intense study since their first description by Ludwig Prandtl over 100 years ago. They are critical in determining the drag and lift of an aircraft wing for example. Key challenges are to understand the physical mechanisms causing the transition from smooth, laminar flow to turbulent flow and how the turbulence is then maintained. Recent direct numerical simulations have contributed significantly towards this understanding.