 Mechanical Engineering  Research Publications
Mechanical Engineering  Research Publications
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ItemEvidence that superstructures comprise selfsimilar coherent motions in high Reynolds number boundary layersDeshpande, R ; de Silva, CM ; Marusic, I (Cambridge University Press, 20230811)We present experimental evidence that the superstructures in turbulent boundary layers comprise smaller, geometrically selfsimilar coherent motions. The evidence comes from identifying and analysing instantaneous superstructures from largescale 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 analyses the wallnormal velocity fluctuations within these very long motions, which reveals the constituent motions unambiguously. The conditional streamwise energy spectra of the Reynolds shear stress and the wallnormal fluctuations, corresponding exclusively to the superstructure region, are found to exhibit the wellknown distancefromthewall scaling in the intermediatescale range. It suggests that geometrically selfsimilar motions are the constituent motions of these verylargescale structures. Investigation of the spatial organization of the wallnormal momentumcarrying eddies, within the superstructures, also lends empirical support to the concatenation hypothesis for the formation of these structures. The association between the superstructures and selfsimilar motions is reaffirmed on comparing the vertical coherence of the Reynoldsshearstresscarrying motions, by computing conditionally averaged twopoint correlations, 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 distancefromthewall scaling invariant with Reynolds number. The findings support modelling of these dynamically significant motions via datadriven coherent structurebased models.

ItemNo Preview AvailableModelling the downstream development of a turbulent boundary layer following a step change of roughnessLi, M ; de Silva, CM ; Chung, D ; Pullin, D ; Marusic, I ; Hutchins, N (CAMBRIDGE UNIV PRESS, 20220923)In this study, we develop an analytical model to predict the turbulent boundary layer downstream of a stepchange in the surface roughness where upstream flow conditions are given. We first revisit the classical model of Elliott (Trans. Am. Geophys. Union, vol. 39, 1958, pp. 1048–1054), who modelled the velocity distribution within and above the internal layer with a simple piecewise logarithmic profile, and evolved the velocity profile using the streamwise momentum equation. Elliott's model was originally developed for an atmospheric surface layer, and to make the model applicable to a spatially developing turbulent boundary layer with finite thickness, we propose a number of more physical refinements, including adding a wake function to the velocity profile, considering the growth of the entire boundary layer in the streamwise direction, and using a more realistic shear stress profile in the momentum equation. In particular, we implement the blending model (Li et al., J. Fluid Mech., vol. 923, 2021, p. A18) to account for the deviation of the mean flow within the internal layer from a canonical velocity profile based on the local wall condition. These refinements lead to improved agreement between the prediction and the measurement, especially in the vicinity of the roughtosmooth change.

ItemNo Preview AvailableInvestigation of coldwire spatial and temporal resolution issues in thermal turbulent boundary layersXia, Y ; Rowin, WA ; Jelly, T ; Marusic, I ; Hutchins, N (ELSEVIER SCIENCE INC, 20220125)

ItemNo Preview AvailableNavierStokesbased linear model for unstably stratified turbulent channel flowsMadhusudanan, A ; Illingworth, SJ ; Marusic, I ; Chung, D (AMER PHYSICAL SOC, 20220406)

ItemAn extensional strain sensing mechanosome drives adhesionindependent platelet activation at supraphysiological hemodynamic gradientsAbidin, NAZ ; Poon, EKW ; Szydzik, C ; Timofeeva, M ; Akbaridoust, F ; Brazilek, RJ ; Lopez, FJT ; Ma, X ; Lav, C ; Marusic, I ; Thompson, PE ; Mitchell, A ; Ooi, ASH ; Hamilton, JR ; Nesbitt, WS (BMC, 20220324)BACKGROUND: Supraphysiological hemodynamics are a recognized driver of platelet activation and thrombosis at highgrade stenosis and in blood contacting circulatory support devices. However, whether platelets mechanosense hemodynamic parameters directly in free flow (in the absence of adhesion receptor engagement), the specific hemodynamic parameters at play, the precise timing of activation, and the signaling mechanism(s) involved remain poorly elucidated. RESULTS: Using a generalized Newtonian computational model in combination with microfluidic models of flow acceleration and quasihomogenous extensional strain, we demonstrate that platelets directly mechanosense acute changes in freeflow extensional strain independent of shear strain, platelet amplification loops, von Willebrand factor, and canonical adhesion receptor engagement. We define an extensional strain sensing "mechanosome" in platelets involving cooperative Ca2+ signaling driven by the mechanosensitive channel Piezo1 (as the primary strain sensor) and the fast ATP gated channel P2X1 (as the secondary signal amplifier). We demonstrate that type II PI3 kinase C2α activity (acting as a "clutch") couples extensional strain to the mechanosome. CONCLUSIONS: Our findings suggest that platelets are adapted to rapidly respond to supraphysiological extensional strain dynamics, rather than the peak magnitude of imposed wall shear stress. In the context of overall platelet activation and thrombosis, we posit that "extensional strain sensing" acts as a priming mechanism in response to threshold levels of extensional strain allowing platelets to form downstream adhesive interactions more rapidly under the limiting effects of supraphysiological hemodynamics.

ItemAn energyefficient pathway to turbulent drag reductionMarusic, I ; Chandran, D ; Rouhi, A ; Fu, MK ; Wine, D ; Holloway, B ; Chung, D ; Smits, AJ (NATURE PORTFOLIO, 20211004)Simulations and experiments at low Reynolds numbers have suggested that skinfriction drag generated by turbulent fluid flow over a surface can be decreased by oscillatory motion in the surface, with the amount of drag reduction predicted to decline with increasing Reynolds number. Here, we report direct measurements of substantial drag reduction achieved by using spanwise surface oscillations at high friction Reynolds numbers ([Formula: see text]) up to 12,800. The drag reduction occurs via two distinct physical pathways. The first pathway, as studied previously, involves actuating the surface at frequencies comparable to those of the smallscale eddies that dominate turbulence near the surface. We show that this strategy leads to drag reduction levels up to 25% at [Formula: see text] = 6,000, but with a power cost that exceeds any dragreduction savings. The second pathway is new, and it involves actuation at frequencies comparable to those of the largescale eddies farther from the surface. This alternate pathway produces drag reduction of 13% at [Formula: see text] = 12,800. It requires significantly less power and the drag reduction grows with Reynolds number, thereby opening up potential new avenues for reducing fuel consumption by transport vehicles and increasing power generation by wind turbines.

ItemHeat Transfer Coefficient Estimation for Turbulent Boundary LayersWang, S ; Xia, Y ; Abu Rowin, W ; Marusic, I ; Sandberg, R ; Chung, D ; Hutchins, N ; Tanimoto, K ; Oda, T (The University of Queensland, 20201211)Convective heat transfer in rough wallbounded turbulent flows is prevalent in many engineering applications, such as in gas turbines and heat exchangers. At present, engineers lack the design tools to accurately predict the convective heat transfer in the presence of nonsmooth boundaries. Accordingly, a new turbulent boundary layer facility has been commissioned, where the temperature of an interchangeable test surface can be precisely controlled, and conductive heat losses are minimized. Using this facility, we can estimate the heat transfer coefficient (Stanton number, St), through measurement of the power supplied to the electrical heaters and also from measurements of the thermal and momentum boundary layers evolving over this surface. These methods have been initially investigated over a shorter smooth prototype heated surface and compared with existing St prediction models. Preliminary results suggest that we can accurately estimate St in this facility.

ItemAn investigation of coldwire spatial resolution using a DNS databaseXia, Y ; Rowin, W ; Jelly, T ; Chung, D ; Marusic, I ; Hutchins, N (The University of Queensland, 20201211)The effect of spatial resolution of coldwire anemometry on both the variance and energy spectrum of temperature fluctuations is analyzed through the use of a numerical database. Temperature fluctuation snapshots from a direct numerical simulation (DNS) of a heated smoothwall turbulent channel flow are spatially averaged in the spanwise direction to simulate the wire filtering. The results show that the wire length does not affect the mean temperature while it significantly attenuates the variance of temperature fluctuations, particularly in the vicinity of the wall. As the filter length grows, the peaks of the one and twodimensional energy spectrograms are further attenuated. Limited attenuation is seen when the filter length is smaller than 30 wall units in the vicinity of the wall, whereas a complete suppression of the nearwall energetic peak is observed when the filter length exceeds 100 wall units.

ItemEvolution of the turbulent/nonturbulent interface of an axisymmetric turbulent jetKhashehchi, M ; Ooi, A ; Soria, J ; Marusic, I (SPRINGER, 20130101)

ItemOn the universality of inertial energy in the log layer of turbulent boundary layer and pipe flowsChung, D ; Marusic, I ; Monty, JP ; Vallikivi, M ; Smits, AJ (SPRINGER, 20150701)