 Mechanical Engineering  Research Publications
Mechanical Engineering  Research Publications
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ItemIdentifying regions of importance in wallbounded turbulence through explainable deep learning.Cremades, A ; Hoyas, S ; Deshpande, R ; Quintero, P ; Lellep, M ; Lee, WJ ; Monty, JP ; Hutchins, N ; Linkmann, M ; Marusic, I ; Vinuesa, R (Springer Science and Business Media LLC, 20240513)Despite its great scientific and technological importance, wallbounded turbulence is an unresolved problem in classical physics that requires new perspectives to be tackled. One of the key strategies has been to study interactions among the energycontaining coherent structures in the flow. Such interactions are explored in this study using an explainable deeplearning method. The instantaneous velocity field obtained from a turbulent channel flow simulation is used to predict the velocity field in time through a Unet architecture. Based on the predicted flow, we assess the importance of each structure for this prediction using the gametheoretic algorithm of SHapley Additive exPlanations (SHAP). This work provides results in agreement with previous observations in the literature and extends them by revealing that the most important structures in the flow are not necessarily the ones with the highest contribution to the Reynolds shear stress. We also apply the method to an experimental database, where we can identify structures based on their importance score. This framework has the potential to shed light on numerous fundamental phenomena of wallbounded turbulence, including novel strategies for flow control.

ItemNo Preview AvailableNearWall Flow Statistics in HighRe_{τ} DragReduced Turbulent Boundary LayersDeshpande, R ; Zampiron, A ; Chandran, D ; Smits, AJ ; Marusic, I (SPRINGER, 20230101)

ItemPressure drag reduction via imposition of spanwise wall oscillations on a rough wallDeshpande, R ; Kidanemariam, AG ; Marusic, I (Cambridge University Press, 20240111)The present study tests the efficacy of the wellknown viscous drag reduction strategy of imposing spanwise wall oscillations to reduce pressure drag contributions in transitional and fully rough turbulent wall flow. This is achieved by conducting a series of direct numerical simulations of a turbulent flow over twodimensional (spanwisealigned) semicylindrical rods, placed periodically along the streamwise direction with varying streamwise spacing. Surface oscillations, imposed at fixed viscousscaled actuation parameters optimum for smooth wall drag reduction, are found to yield substantial drag reduction ( $\gtrsim$ 25 %) for all the rough wall cases, maintained at matched roughness Reynolds numbers. While the total drag reduction is due to a drop in both viscous and pressure drag in the case of transitionally rough flow (i.e. with large interrod spacing), it is associated solely with pressure drag reduction for the fully rough cases (i.e. with small interrod spacing), with the latter being reported for the first time. The study finds that pressure drag reduction in all cases is caused by the attenuation of the vortex shedding activity in the roughness wake, in response to wall oscillation frequencies that are of the same order as the vortex shedding frequencies. Contrary to speculations in the literature, this study confirms that the mechanism behind pressure drag reduction, achieved via imposition of spanwise oscillations, is independent of the viscous drag reduction. This mechanism is responsible for weakening of the Reynolds stresses and increase in base pressure in the roughness wake, explaining the pressure drag reduction observed by past studies, across varying roughness heights and geometries.

ItemOn the relationship between manipulated interscale phase and energyefficient turbulent drag reductionDeshpande, R ; Chandran, D ; Smits, AJ ; Marusic, I (CAMBRIDGE UNIV PRESS, 20230926)We investigate the role of interscale interactions in the highReynoldsnumber skinfriction drag reduction strategy reported by Marusic et al. (Nat. Commun., vol. 12, 2021). The strategy involves imposing relatively lowfrequency streamwise travelling waves of spanwise velocity at the wall to actuate the drag generating outer scales. This approach has proven to be more energy efficient than the conventional method of directly targeting the drag producing inner scales, which typically requires actuation at higher frequencies. Notably, it is observed that actuating the outer scales at low frequencies leads to a substantial attenuation of the major drag producing inner scales, suggesting that the actuations affect the nonlinear inner–outer coupling inherently existing in wallbounded flows. In the present study, we find that increased drag reduction, through imposition of spanwise wall oscillations, is always associated with an increased coupling between the inner and outer scales. This enhanced coupling emerges through manipulation of the phase relationships between these triadically linked scales, with the actuation forcing the entire range of energycontaining scales, from the inner (viscous) to the outer (inertial) scales, to be more in phase. We also find that a similar enhancement of this nonlinear coupling, via manipulation of the interscale phase relationships, occurs with increasing Reynolds number for canonical turbulent boundary layers. This indicates improved efficacy of the energyefficient drag reduction strategy at very high Reynolds numbers, where the energised outer scales are known to more strongly superimpose and modulate the inner scales. Leveraging the interscale interactions, therefore, offers a plausible mechanism for achieving energyefficient drag reduction at high Reynolds numbers.

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, 202204)

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.