Reducing skin friction is important in nature and many technological applications. This reduction may be achieved by reducing stresses turbulent boundary layers, for instance tailoring biomimetic rough skins. Here we take a second approach consisting of keeping the layer laminar as long possible forcing small optimal perturbations. Because highly non-normal underlying linearized operator, these perturbations are amplified able to modify mean velocity profiles at leading order. We report...

An experimental and theoretical investigation aimed at describing the nonmodal growth of steady spanwise periodic streamwise streaks in a flat plate boundary layer is presented. Stable laminar are experimentally generated by means array small cylindrical roughness elements fixed on plate. The evolution measured it proved that, except region near elements, they obey scalings. maximum achievable amplitude mainly determined relative height elements. Results compared with numerical simulations...

The linear response to stochastic and optimal harmonic forcing of small coherent perturbations the turbulent channel mean flow is computed for Reynolds numbers ranging from Re τ = 500 20000. Even though linearly stable, it nevertheless able sustain large amplifications by forcing. most amplified structures consist streamwise-elongated streaks that are optimally forced vortices. For structures, energy amplification found be, a first approximation, inversely proportional spanwise wavenumber...

We compute the optimal transient growth of perturbations sustained by a turbulent channel flow following same approach recently used del Álamo and Jiménez [J. Fluid Mech. 559, 205 (2006)]. Contrary to this previous analysis, we use generalized Orr–Sommerfeld Squire operators consistent with investigations mean flows variable viscosity. The are streamwise vortices evolving into streaks. In accordance Jiménez, it is found that for very elongated structures sufficiently large Reynolds numbers,...

It has recently been found by using temporal and spatial numerical simulations that steady optimal streaks of moderate amplitude, i.e., sufficiently large but not exceeding the critical amplitude for inflectional instability, are able to reduce growth Tollmien–Schlichting (TS) waves up their complete suppression. This investigation aims at experimentally verifying this stabilizing effect generating stable symmetric, close sinusoidal, amplitudes (∼12% free-stream velocity) means a spanwise...

The optimal energy growth of perturbations sustained by a zero pressure gradient turbulent boundary is computed using the eddy viscosity associated with mean flow. It found that even if all considered profiles are linearly stable, they support transient growths. most amplified streamwise uniform and correspond to streaks originated vortices. For sufficiently large Reynolds numbers two distinct peaks exist, respectively scaling in inner outer units. structures peak units well probable...

Large-scale motions, important in turbulent shear flows, are frequently attributed to the interaction of structures at smaller scale. Here we show that, a channel Re_{\tau} \approx 550, large-scale motions can self-sustain even when smaller-scale populating near-wall and logarithmic regions artificially quenched. This self-sustained mechanism is not active periodic boxes width than Lz ~ 1.5h or length shorter Lx 3h which correspond well most energetic large scales observed channel.

We compute the optimal response of turbulent Couette mean flow to initial conditions, harmonic and stochastic forcing at Re = 750. The equations for coherent perturbations are linearized near include associated eddy viscosity. is found be linearly stable but it has potential amplify steamwise streaks from streamwise vortices. most amplified structures uniform largest amplifications energy conditions variance realized by large-scale having spanwise wavelengths 4.4 h 5.2 respectively. These...

In this Letter we show by numerical simulation that streamwise streaks of sufficiently large amplitude are able to stabilize Tollmien–Schlichting waves in zero pressure gradient boundary layers at least up Re=1000. This stabilization is due the spanwise averaged part nonlinear basic flow distortion induced and occurs for streak amplitudes lower than critical threshold beyond which secondary inflectional instability observed. A new control strategy implemented using optimal perturbations...

Abstract Magnetorotational dynamo action in Keplerian shear flow is a three-dimensional nonlinear magnetohydrodynamic process, the study of which relevant to understanding accretion processes and magnetic field generation astrophysics. Transition this form subcritical shares many characteristics with transition turbulence non-rotating hydrodynamic flows. This suggests that these different fluid systems become active through similar generic bifurcation mechanisms, both cases have eluded...

We collect and discuss the results of our recent studies which show evidence existence a whole family self-sustaining motions in wall-bounded turbulent shear flows with scales ranging from those buffer-layer streaks to large-scale very-large-scale outer layer. The statistical dynamical features this motions, are associated quasi-streamwise vortices, consistent Townsend’s attached eddies. Motions at each relevant scale able sustain themselves absence forcing larger- or smaller-scale by...

We consider the influence of slip boundary conditions on modal and nonmodal stability pressure-driven channel flows. In accordance with previous results by Gersting [“Hydrodynamic plane porous flow,” Phys. Fluids 17, 2126 (1974)] but in contradiction recent investigation Chu [“Instability Navier flow liquids,” C. R. Mec. 332, 895 (2004)], we show that increases significantly value critical Reynolds number for linear instability. The analysis, however, reveals has a very weak maximum...

It has recently been shown that large-scale and very-large-scale motions can self-sustain in turbulent channel flows even the absence of input from at smaller scales. Here we show also intermediate scales, mainly located logarithmic layer, survive when scales are artificially quenched. These elementary self-sustained involve bursting regeneration sinuous streaks. This is a further indication full range autonomous processes exists with ranging those buffer layer streaks to large scale outer layer.

Large-scale motions in wall-bounded turbulent flows are frequently interpreted as resulting from an aggregation process of smaller-scale structures. Here, we explore the alternative possibility that such large-scale themselves self-sustained and do not draw their energy activated buffer layers. To this end, it is first shown Couette flow at Re=2150 self-sustain even when active processes smaller scales artificially quenched by increasing Smagorinsky constant Cs large eddy simulations. These...

The ability of linear stochastic response analysis to estimate coherent motions is investigated in turbulent channel flow at the friction Reynolds number $\text{Re}_{\unicode[STIX]{x1D70F}}=1007$ . performed for spatial scales characteristic buffer-layer and large-scale by separating contributions different temporal frequencies. Good agreement between measured spatio-temporal power spectral densities those estimated means resolvent found when effect stresses, modelled with an eddy-viscosity...

We examine the linear stability of Ginzburg-Landau operator with spatially varying coefficients, which mimics strongly nonparallel open flows such as wakes, jets, and boundary layers. The streamwise non-normality global eigenmodes explains observed large transient growths, classically interpreted in terms local convective instability. use pseudospectra provides an exact measure spatial amplification aids determination when entrance noise dominates open-flow dynamics.

The optimal amplifications of small coherent perturbations within turbulent pipe flow are computed for Reynolds numbers up to one million. Three standard frameworks considered: the growth an initial condition, response harmonic forcing and Karhunen-Lo\`eve (proper orthogonal decomposition) analysis stochastic forcing. Similar analyses plane channel boundary layer, it is found that streaks elongated in streamwise direction can be greatly amplified from quasistreamwise vortices, despite linear...

Understanding the origin of large-scale structures in high-Reynolds-number wall turbulence has been a central issue over number years. Recently, Rawat et al. ( J. Fluid Mech. , vol. 782, 2015, pp. 515–540) have computed invariant solutions for turbulent Couette flow at $Re_{\unicode[STIX]{x1D70F}}\simeq 128$ using an overdamped large-eddy simulation with Smagorinsky model to account effect surrounding small-scale motions. Here, we extend this approach Reynolds numbers order magnitude higher...

The objective of the study is to determine absolute/convective nature secondary instability experienced by finite-amplitude streaks in flat-plate boundary layer. A family parallel streaky base flows defined extracting velocity profiles from direct numerical simulations nonlinearly saturated optimal streaks. computed impulse response then determined as a function streak amplitude and streamwise station. Both temporal spatio-temporal properties are directly retrieved wave packet, without...

The nature of dynamo action in shear flows prone to magnetohydrodynamc instabilities is investigated using the magnetorotational Keplerian flow as a prototype problem. Using direct numerical simulations and Newton's method, we compute an exact time-periodic solution three-dimensional dissipative incompressible magnetohydrodynamic equations with rotation shear. We discuss physical mechanism behind cycle show that it results from combination linear nonlinear interactions between large-scale...

Abstract. The energy produced by wind plants can be increased mitigating the negative effects of turbine–wake interactions. In this context, axial-induction control and wake redirection control, obtained intentionally yawing or tilting rotor axis away from mean direction, have been subject extensive research but only very few investigations considered their combined effect. study we compute power gains that are operating tilted yawed rotors at higher axial induction means large-eddy...