Sedat F. Tardu

Active control of near-wall turbulence by local oscillating blowing

The effect of time-periodical blowing through a spanwise slot on the near-wall turbulence characteristics is investigated. The blowing velocity changes in a cyclic manner from 0 to 5 wall units. The frequency of the oscillations is nearly equal to the median frequency of the near-wall turbulence. The measurements of the wall shear stress and the streamwise velocity are reported and discussed. The flow field near the blowing slot is partly relaminarized during the acceleration phase of the injection velocity which extends 40 wall units downstream. The imposed unsteadiness is confined to the buffer layer, and the time-mean structural parameters under unsteady blowing are found to be close to those of isotropic turbulence in this region. The relaminarized phase is unstable and gives way to a coherent spanwise structure that increases the shear from 80 to 300 wall units downstream of the slot in a predictable way. This phenomenon is strongly imposed-frequency dependent.

Realization and simulation of wall shear stress integrated sensors

In this paper we present the impact of hot wire geometry and fluid characteristics on the efficiency of a hot wire to be used as a wall shear stress sensor. Finite differences and finite elements modelling based simulators were used in order to evaluate the thermal performances of hot wire wall shear stress sensor. Several structures were explored including simple conductor or suspended above a cavity (free and sealed with an oxide membrane). It is found out that wire length, wire section and the dimensions of the micro-cavity lying below the hot wire are of fundamental importance on the thermal exchanges occurring between the hot wire and the fluid.

Near wall turbulence control by local time periodical blowing

Abstract The effect of time periodical blowing through a slot on the spatio-temporal characteristics of the wall shear stress is experimentally investigated. It is shown that the imposed unsteadiness considerably affects the fine structure of the near wall turbulence. The drag is reduced by steady blowing, essentially because of a different redistribution of the quadrant events compared with the unmanipulated boundary layer. The unsteady blowing affects the vorticity generation mechanism, without appreciably interacting with this redistribution.

Experiments and Modeling of an Unsteady Turbulent Channel Flow

The k–ω model is combined with the rapid distortion scheme to develop effective unsteady closures in nonequilibrium wall flows subject to oscillating shear. The phase-averaged eddy viscosity is related to the modulation of the effective strain parameter whose distribution is obtained from the steady turbulent channel flow data. New experimental data on the wall and Reynolds shear stresses are reported. The model predicts the time-mean and phase-averaged flow quantities in a satisfactory way when the imposed frequency is smaller than the median frequency of the near-wall turbulence in the relaxation regime. It is particularly successful at the wall and in the low buffer layer.

Cyclic Modulation of Reynolds Stresses and Length Scales in Pulsed Turbulent Channel Flow

Recent data obtained in an unsteady turbulent channel flow is reviewed. Results concerning the modulation characteristics of the Reynolds shear stresses, of the structural parameters and of the length scales inferred from unsteady spatial correlations are discussed. The close examination of both the amplitude and the phase shifts of the Reynolds shear stresses confirms the existence of three distinct inposed frequency regimes, namely the quasi-steady regime, the relaxation regime, in which the amplitudes decrease and which is accompanied by large time lags, and a subsequent third regime wherein the modulation characteristics change considerably. The fine structure of the near-wall turbulence response through quadrant analysis reveals large cyclic variations of the contributions of ejections and sweeps to the Reynolds shear stress. The reaction of the spanwise extent of the near-wall structures is investigated through the spanwise correlation coefficient between the wall shear stress and the streamwise velocity, and the resulting length scales. A temporal filtering of both signals shows that the inactive motions respond uniformly in the whole imposed frequency regime. A strong correlation is found between the modulation characteristics of the streak spacing and the ejection frequency.

Coherent structures and riblets

This work deals with the effect of the riblets on the coherent structures near the wall. The emphasis is put on the genesis of the quasi-streamwise vortices in the presence of the riblets. The quasi-streamwise vortices regenerate by the tilting of wall normal vorticity induced by prevailing structures. This requires a mechanism which leads to a temporal streamwise dependence near the elongated flow structures and to a subsequent formation of new wall normal vorticity. It is suggested here that the action of existing quasi-streamwise vortices on the sidewalls of wall normal vorticity may create a local, streamwise dependent spanwise velocity and therefore, a secondary wall normal vorticity field. A preliminary analysis of the set-up and the time and space development of this secondary three-dimensional flow associated with the regeneration mechanism, is given. An attempt is made, in order to explain the drag reduction performed by the riblets through an intermittent model, based on the protrusion height. Logical estimates of the amount of drag reduction are obtained. The differences between the mechanism suggested here and those based on forced control experiments are also discussed.

Interfacial Electrokinetic Effect on the Microchannel Flow Linear Stability

The electrostatic double layer (EDL) effect on the linear hydrodynamic stability of micro-channel flows is investigated. It is shown that the EDL destabilizes the Poiseuille flowconsiderably. The critical Reynolds number decreases by a factor five when the non-dimensional Debye-Huckel parameterkis around ten. Thus, the transition may be quiterapid for microchannels of a couple of microns heights in particular when the liquidcontains a very small number of ions. The EDL effect disappears quickly for k>150corresponding typically to channels of heights 400 mm or larger. These results mayexplain why significantly low critical Reynolds numbers have been encountered in someexperiments dealing with microchannel flows. @DOI: 10.1115/1.1637927#

Bursting and structure of the turbulence in an internal flow manipulated by riblets

The buffer layer of an internal flow manipulated by riblets is investigated. The distributions of the ejection and bursting frequency from the beginning to the middle part of the buffer layer, together with high moments of the fluctuating streamwise velocity, u’, and its time derivative are reported. The profiles of the ejection and bursting frequency are determined and compared using three single point detection schemes. The effect of the riblets on the bursting mechanism is found confined in a localized region in the buffer layer. The multiple ejection bursts are more affected than the single ejection bursts. The skewness and flatness factors of the u’ signal are larger in the manipulated layer than in the standard boundary layer. That, also holds true for the flatness factor of the time derivative, but the Taylor and Liepman scales are not affected. The spectrum of the u’ signal is altered at the beginning part of the viscous sublayer.

Stochastic synchronization of the near wall turbulence

We investigate the characteristics of the instantaneous phases and amplitudes of the wavelet coefficients applied to the fluctuating wall shear stress and longitudinal velocity in the low buffer layer of a fully developed turbulent boundary layer. We show that the instantaneous phase exhibits long quiescent periods of constant values separated by sudden phase jumps. We establish a similarity with the stochastic synchronization of chaotic systems in the presence of noise that plays a role similar to the incoherent turbulence. We analyze the statistical characteristics of the constant phase periods and show the existence of type-I intermittency of the constant phase lengths related to a saddle-node bifurcation of the unstable periodic orbit embedded in the wall turbulent attractor. The period of the later is closely related to that of the cyclic regeneration of shear stress producing eddies.

Breakdown of Kolmogorov's first similarity hypothesis in grid turbulence

Kolmogorovs first similarity hypothesis (or KSH1) stipulates that two-point statistics have a universal form which depends on two parameters, the kinematic viscosity ν and the mean energy dissipation rate ⟨ε⟩. KSH1 is underpinned by two assumptions: the Reynolds number is very large and local isotropy holds. To disentangle the intricacies of these two requirements, we assess the validity of KSH1 in a flow where local isotropy is a priori tenable, i.e. decaying grid turbulence. The main question we address is how large should the Reynolds number be for KSH1 to be valid over a range of scales wider than, say, five Kolmogorov scales. To this end, direct numerical simulations based on the lattice Boltzmann method are carried out in low Reynolds number grid turbulence. The results show that when the Taylor microscale Reynolds number Rλ drops below about 20, the Kolmogorov normalised spectra deviate from those at higher Rλ; the deviation increases with decreasing Rλ. It is shown that at Rλ ≃ 20, the contribution of the energy transfer in the scale-by-scale energy budget becomes smaller than the contributions from the viscous and (large-scale) non-homogeneous terms at all scales, but never vanishes, at least for the range of Reynolds investigated here. A phenomenological argument based on the ratio N between the energy-containing timescale and the dissipative range timescale leads to the condition for KSH1 to hold. The numerical data indicate that N = 5, yielding Rλ ≃ 20, thus confirming our numerical finding. The present results show that KSH1, unlike the second Kolmogorov similarity hypothesis (KSH2,) does not require the existence of an inertial range. While it may seem remarkable that KSH1 is validated at much lower Reynolds numbers than required for KSH2 in grid turbulence (Rλ ≥ 1000,), KSH1 applies to small scales which include both dissipative scales and inertial range (if it exists). One can expect that, as the Reynolds number increases, the dissipative scales should satisfy KSH1 first; then, as the Reynolds number attains very high values, the inertial range is established in conformity with KSH2.