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Drag Reduction Effects in a Turbulent Channel Flow Induced by Spanwise Wall Oscillations
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Wall-Oscillation Conditions for Drag Reduction in Turbulent Channel Flow
No full textThe drag reduction properties of a turbulent channel flow modified by spanwise sinusoidal oscillations of the walls are investigated by direct numerical simulations. The work is based on the linear relation between the drag reduction and the parameter S, function of the maximum wall velocity and the period of the oscillation. This quantity, first determined by Choi et al. [Choi, J.-I, Xu, C.-X., Sung, H. J., 2002. Drag reduction by spanwise wall oscillation in wall-bounded turbulent flows. AIAA J. 40 (5), 842–850] and later studied by Quadrio and Ricco [Quadrio, M., Ricco, P., 2004. Critical assessment of turbulent drag reduction through spanwise wall oscillations. J. Fluid Mech. 521, 251–271], has been found through physical arguments pertaining to the action of the oscillating Stokes layer on the near-wall turbulence dynamics. The predictive potential of the scaling parameter is exploited to gain insight into the drag-reducing effects of the oscillating-wall technique. The period of oscillation which guarantees the maximum drag reduction for a given maximum wall displacement is studied for the first time. The issue of the minimum intensity of wall forcing required to produce a non-zero drag reduction effect and the dependence of the drag reduction on the Reynolds number are also addressed. The drag reduction data available in the literature are compared with the prediction given by the scaling parameter, thus attaining a comprehensive view of the state of the art
Modification of Turbulent Friction Drag by Streamwise-Traveling Waves of Spanwise Velocity
No full textInitial Response of a Turbulent Channel Flow to Spanwise Oscillation of the Walls
No full textThe transient behaviour of a turbulent channel flow suddenly subjected to spanwise harmonic oscillations of the walls is numerically studied by means of direct numerical simulations of the incompressible Navier-Stokes equations. It is well known that this movement of the walls produces a sustained and significant reduction in turbulent friction; in this paper we focus on the early stages of the motion after the start of the oscillations when the fully developed state has not yet established. It is found that at the very beginning of the oscillatory motion the streamwise wall shear-stress remains constant for a short time interval, the length of which depends on the parameters de ning the oscillation. A spanwise velocity pro le starts to develop, almost coincident with the analytical laminar solution for the sudden start-up of harmonic oscillations of the wall. The spanwise flow fully adapts to the new forcing after about one oscillation period, whilst the longitudinal flow is still evolving towards its long-term drag-reducing condition. The duration of the transient for the longitudinal wall shear-stress is significantly longer, and is found to be independent from the oscillation period, at least for the range of periods considered, but to be notably related to the maximum wall velocity. The implications of this last new finding are noteworthy, since it appears that some of the available experimental data concerning drag reduction measurements over an oscillating wall might be biased by transient effects, especially for the highest values of wall velocity. Moreover, we observe that the turbulent wall friction and the turbulence statistics change from their initial condition to their long-term behaviour following a non-monotonic path. The paper concludes with two- and three-dimensional flow visualizations of the turbulent flow fields immediately after the start of the oscillations. The use of a moving reference frame, in motion with a speed that is comparable to the convection velocity of the turbulent structures in the near-wall region, allows a clear appreciation of the dynamics of the turbulent flow, thanks to the removal of the purely convective motion. The laminar spanwise flow is removed from the computed turbulent flow fields, in order to clarify the dynamics of the near-wall turbulent structures. The initial interaction of turbulence with the moving wall is then vividly describe
Critical Assessment of Turbulent Drag Reduction Through Spanwise Wall Oscillations
Get PDFDirect numerical simulations of the incompressible Navier–Stokes equations are employed to study the turbulent wall-shear stress in a turbulent channel flow forced by lateral sinusoidal oscillations of the walls. The objective is to produce a documented database of numerically computed friction reductions. To this aim, the particular numerical requirements for such simulations, owing for example to the time-varying direction of the skin-friction vector, are considered and appropriately accounted for.
A detailed analysis of the dependence of drag reduction on the oscillatory parameters allows us to address conflicting results hitherto reported in the literature. At the Reynolds number of the present simulations, we compute a maximum drag reduction of 44.7%, and we assess the possibility for the power saved to be higher than the power spent for the movement of the walls (when mechanical losses are neglected). A maximum net energy saving of 7.3% is computed.
Furthermore, the scaling of the amount of drag reduction is addressed. A parameter, which depends on both the maximum wall velocity and the period of the oscillation, is found to be linearly related to drag reduction, as long as the half-period of the oscillation is shorter than a typical lifetime of the turbulent near-wall structures. For longer periods of oscillation, the scaling parameter predicts that drag reduction will decrease to zero more slowly than the numerical data. The same parameter also describes well the optimum period of oscillation for fixed maximum wall displacement, which is smaller than the optimum period for fixed maximum wall velocity, and depends on the maximum displacement itself
Streamwise-Travelling Waves of Spanwise Wall Velocity for Turbulent Drag Reduction
Get PDFWaves of spanwise velocity imposed at the walls of a plane turbulent channel flow are studied by direct numerical simulations. We consider sinusoidal waves of spanwise velocity which vary in time and are modulated in space along the streamwise direction.
The phase speed may be null, positive or negative, so that the waves may be either stationary or travelling forward or backward in the direction of the mean flow. Such a forcing includes as particular cases two known techniques for reducing friction drag:
the oscillating wall technique (a travelling wave with infinite phase speed) and the recently proposed steady distribution of spanwise velocity (a wave with zero phase speed). The travelling waves alter the friction drag significantly. Waves which slowly travel forward produce a large reduction of drag that can relaminarize the flow at low values of the Reynolds number. Faster waves yield a totally different outcome, i.e. drag increase (DI). Even faster waves produce a drag reduction (DR) effect again. Backward-travelling waves instead lead to DR at any speed. The travelling waves, when they reduce drag, operate in similar fashion to the oscillating wall, with an improved energetic efficiency. DI is observed when the waves travel at a speed
comparable with that of the convecting near-wall turbulence structures. A diagram illustrating the different flow behaviours is presented
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