Mechanical Engineering - Research Publications
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ItemForm-induced stress in turbulent flow over riblets(Australasian Fluid Mechanics Society, 2018-01-01)We carry out direct numerical simulation of minimal openchannel flow over riblets. Several riblet geometries are simulated, namely symmetric triangular, asymmetric triangular, blade and trapezoidal, and with this unprecedented high-fidelity dataset, we are able to obtain broad insights into the flow physics of riblets. We find that the roughness sublayer thickness, above which the flow is statistically homogeneous, is proportional to the square root of the riblet groove cross-sectional area ℓ+ g in both the drag-reducing and the drag-increasing regime, consistent with the ability of this parameter to collapse the roughness function corresponding to different groove geometries. Large grooves are associated with mean secondary velocities and they carry additional stress that contributes up to 40% of the total shear stress at the crest, comparable to the contribution from the turbulent fluctuations.
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ItemKelvin–Helmholtz rollers in turbulent flow over riblets(Australasian Fluid Mechanics Society, 2018-01-01)Structures resulting from a Kelvin–Helmholtz instability have been shown to contribute to skin-friction drag in turbulent flow over blade-shaped riblets [4]. Using Direct Numerical Simulation (DNS) data, the present survey of several different riblet shapes reveals that the contribution to wall-shear stress due to the Kelvin–Helmholtz instability depends on riblet shape, in addition to a previously known dependence on riblet size. For a given drag change, sharp triangular and blade riblets promote development of the instability whilst blunt triangular and trapezoidal riblets appear to suppress it.
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ItemContribution of dispersive stress to skin friction drag in turbulent flow over riblets(Darmstadt University of Technology, 2019)We carry out direct numerical simulations (DNSs) of minimal open-channel flow over riblets, which are streamwise-aligned grooves that modify the near-wall flow for drag reduction. Several riblet sizes and cross-sectional geometries are simulated, namely symmetric triangular, asymmetric triangular, blade and trapezoidal. With this unprecedented breadth and detail afforded by the DNS data, we are able to obtain more general insights into the flow physics of riblets. A generalization of the Fukagata–Iwamoto–Kasagi (FIK) identity is used to isolate the different contributions to skin friction drag changes. We show that, in the nonlinear regime of large riblet size, the dispersive contribution is comparable or larger than the turbulent one, representing an important mechanism to the breakdown of drag reduction.