Mechanical Engineering - Research Publications

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Author: Chung, Daniel × Author: Marusic, Ivan × Author: Rouhi, Amirreza × Type: Journal Article ×

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    An energy-efficient pathway to turbulent drag reduction
    Marusic, I ; Chandran, D ; Rouhi, A ; Fu, MK ; Wine, D ; Holloway, B ; Chung, D ; Smits, AJ (NATURE PORTFOLIO, 2021-10-04)
    Simulations and experiments at low Reynolds numbers have suggested that skin-friction 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 small-scale 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 drag-reduction savings. The second pathway is new, and it involves actuation at frequencies comparable to those of the large-scale 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.
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    Coriolis effect on centrifugal buoyancy-driven convection in a thin cylindrical shell
    Rouhi, A ; Lohse, D ; Marusic, I ; Sun, C ; Chung, D (Cambridge University Press (CUP), 2021-03-10)
    We study the effect of the Coriolis force on centrifugal buoyancy-driven convection in a rotating cylindrical shell with inner cold wall and outer hot wall. This is done by performing direct numerical simulations for increasing inverse Rossby number Ro−1 from zero (no Coriolis force) to 20 (very large Coriolis force) and for Rayleigh number Ra from 107 to 1010 and Prandtl number Pr=0.7, corresponding to air. We invoke the thin-shell limit, which neglects the curvature and radial variations of the centripetal acceleration. As Ro−1 increases from zero, the system forms an azimuthal bidirectional wind that reaches its maximum momentum at an optimal Ro−1opt, associated with a maximal skin-friction coefficient Cf and a minimal Nusselt number Nu. Just beyond Ro−1opt, the wind weakens and an axial, quasi-two-dimensional cyclone, corotating with the system, begins to form. A local ‘turbulence’ inverse Rossby number (non-dimensionalised by the eddy turnover time) determines the onset of cyclone formation for all Ra, when its value reaches approximately 4. At Ro−1≫Ro−1opt, the system falls into the geostrophic regime with a sudden drop in Nu. The bidirectional wind for Ro−1≤Ro−1opt is a feature of this system, as it hastens the boundary layer transition from laminar to turbulent, towards the ultimate regime. We see the onset of this transition at Ra=1010 and Ro−1≃Ro−1opt, although the mean flow profile has not yet fully collapsed on the Prandtl–von Kármán (logarithmic) law.