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Direct numerical simulations of turbulent pipe flow subjected to streamwise-varying wall rotation are performed. This control method is observed to be able to significantly reduce the friction drag and even laminarize the flow under certain control parameters, which are dictated by velocity amplitude and wavelength, for friction Reynolds number Re{tau} =180. Net energy saving is achievable and the variation of wavelength is found to be more efficient than velocity amplitude in reducing the drag. A series of turbulence statistics are discussed in order to elucidate the impact of steady spatially oscillatory forcing, including budgets of transport equation, turbulence intensity, two-point correlation and one-dimensional spectra. An overall assessment of global energy balance identifies a trend toward laminar regime. The control-induced boundary layer, whose thickness is closely related to control wavelength, is shown to induce a streamwise wavy streak pattern, with its orientation governed by the shear force stemming from gradients of mean velocity. Such strong spatial non-homogeneity is found to significantly reduce the streamwise scale of flow structure. The analysis of conditional-averaged fields reveals an emergence of strong transverse advection, which is observed to cause asymmetrical modification of near-wall quasi-streamwise vortex pair, accompanied by the transverse tilt or diffusion of low-speed streaks and the suppression of its surrounding sweep events, leading to the disruption of near-wall quasi-organized flow structure and hence in turn contributing to the decline of turbulent shear stress.
The spectral model of Perry, Henbest & Chong (1986) predicts that the integral length-scale varies very slowly with distance to the wall in the intermediate layer. The only way for the integral length scales variation to be more realistic while keepi
Turbulence is the major cause of friction losses in transport processes and it is responsible for a drastic drag increase in flows over bounding surfaces. While much effort is invested into developing ways to control and reduce turbulence intensities
The aim in the dynamical systems approach to transitional turbulence is to construct a scaffold in phase space for the dynamics using simple invariant sets (exact solutions) and their stable and unstable manifolds. In large (realistic) domains where
This paper presents a method for calculating the wall shear rate in pipe turbulent flow. It collapses adequately the data measured in laminar flow and turbulent flow into a single flow curve and gives the basis for the design of turbulent flow viscom
Highly turbulent Taylor-Couette flow with spanwise-varying roughness is investigated experimentally and numerically (direct numerical simulations (DNS) with an immersed boundary method (IBM)) to determine the effects of the spacing and axial width $s