273 research outputs found
The phase-locked mean impulse response of a turbulent channel flow
We describe the first DNS-based measurement of the complete mean response of
a turbulent channel flow to small external disturbances. Space-time impulsive
perturbations are applied at one channel wall, and the linear response
describes their mean effect on the flow field as a function of spatial and
temporal separations. The turbulent response is shown to differ from the
response a laminar flow with the turbulent mean velocity profile as base flow.Comment: Accepted for publication in Physics of Fluid
Experimental assessment of drag reduction by traveling waves in a turbulent pipe flow
We experimentally assess the capabilities of an active, open-loop technique
for drag reduction in turbulent wall flows recently introduced by Quadrio et
al. [J. Fluid Mech., v.627, 161, (2009)]. The technique consists in generating
streamwise-modulated waves of spanwise velocity at the wall, that travel in the
streamwise direction.
A proof-of-principle experiment has been devised to measure the reduction of
turbulent friction in a pipe flow, in which the wall is subdivided into thin
slabs that rotate independently in the azimuthal direction. Different speeds of
nearby slabs provide, although in a discrete setting, the desired streamwise
variation of transverse velocity.
Our experiment confirms the available DNS results, and in particular
demonstrates the possibility of achieving large reductions of friction in the
turbulent regime. Reductions up to 33% are obtained for slowly
forward-traveling waves; backward-traveling waves invariably yield drag
reduction, whereas a substantial drop of drag reduction occurs for waves
traveling forward with a phase speed comparable to the convection speed of
near-wall turbulent structures.
A Fourier analysis is employed to show that the first harmonics introduced by
the discrete spatial waveform that approximates the sinusoidal wave are
responsible for significant effects that are indeed observed in the
experimental measurements. Practical issues related to the physical
implementation of this control scheme and its energetic efficiency are briefly
discussed.Comment: Article accepted by Phys. Fluids. After it is published, it will be
found at http://pof.aip.or
Streamwise-traveling waves of spanwise wall velocity for turbulent drag reduction
Waves 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 traveling 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
traveling wave with infinite phase speed) and the recently proposed steady
distribution of spanwise velocity (a wave with zero phase speed).
The traveling 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. Even faster waves produce a drag
reduction effect again. Backward-traveling waves instead lead to drag reduction
at any speed.
The traveling waves, when they reduce drag, operate in similar fashion to the
oscillating wall, with an improved energetic efficiency. Drag increase 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 behaviors is presented
Spanwise forcing for drag reduction: recent progresses at PoliMi, applications and understanding
Numerical Simulation of Turbulent Flow in a Pipe Oscillating Around Its Axis
The turbulent flow in a cilindrical pipe oscillating around its lungitudinal axis is studied via direct numerical solution of the Navier-Stokes equations, and compared to the reference turbulent flow in a fixed pipe and in a pipe with steady rotation. The maximum amount of drag reduction achievable with appropriate oscillations of the pipe wall is found to be of the order of 40%, hence comparable to that of similar flows in planar geometry. The transverse shear layer due to the oscillations induces substantial modifications to the turbulence statistics in the near-wall region, indicating a strong effect on the vortical structures. These modifications are illustrated, together with the implications for the drag-reducing mechanisms. A conceptual model of the interaction between the moving wall and a streamwise vortex is discussed
An Almost Subharmonic Instability in the Flow Past Rectangular Cylinders
The three-dimensional instability of the flow past a 5 :1 rectangular cylinder is investigated via Floquet analysis and direct numerical simulations. A quasi-subharmonic (QS) unstable mode is detected, marking an important difference with the flow past bodies with lower aspect ratio and/or with smooth leading edge. The QS mode becomes unstable at Reynolds number (based on the cylinder thickness and free-stream velocity) Re approximate to 480; its spanwise wavelength is approximately three times the cylinder thickness. The structural sensitivity locates the wavemaker region over the longitudinal sides of the cylinder, indicating that the instability is triggered by the mutual inviscid interaction of vortices generated by the leading edge shear layer
Coherent Near-Wall Structures and Drag Reduction by Spanwise Forcing
The effect of streamwise-traveling waves of spanwise wall velocity (StTW) on the quasistreamwise vortices (QSV) populating the near-wall region of turbulent channels is studied via a conditional averaging technique applied to flow snapshots obtained via direct numerical simulation. The analysis by Yakeno, Hasegawa, and Kasagi [Phys. Fluids 26, 085109 (2014)], where the special case of spatially uniform wall oscillation (OW) was considered, is extended to the general case of StTW, which yield both reduction and increase of turbulent skin-friction drag. StTW are found to significantly impact the wall-normal distribution of the vortex population. The conditionally averaged velocity field around the vortices shows that the contributions of the QSV to the quadrant Reynolds shear stresses change significantly during the control cycle. On the one hand, as for OW, the suppression of Q2 events (with upwelling of low-speed fluid away from the wall) dominates the drag-reduction process. On the other hand, the enhancement of Q2 and also Q4 events (with downwelling of high-speed fluid toward the wall) is related to drag increase. Based on the link identified between the phase changes of the Reynolds stresses and the principal directions of the rate-of-strain tensor induced by the StTW, a predictive correlation for drag reduction by StTW is proposed which uses physically significant parameters to overcome the shortcomings of existing models
Study of energetics in drag-reduced turbulent channels
Changes in integral power budgets and scale energy fluxes as induced by certain active flow control strategies for turbulent skin-friction drag reduction are studied by performing Direct Numerical Simulation of turbulent channels. The innovative feature of the present study is that the flow is driven at Constant total Power Input (CtPI), which is a necessary enabling choice in order to meaningfully compare a reference unmanipulated flow with a modified one from the energetic standpoint. Spanwise wall oscillation and opposition control are adopted as model strategies, because of their very different control input power requirements. The global power budget show that the increase of dissipation of mean kinetic energy is not always related to drag reduction, while the preliminary analysis of the scale energy fluxes through the generalized Kolmogorov equation shows that the space- and scale properties of the scale energy source and fluxes are significantly modified in the near-wall region, while remain unaltered elsewhere
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