Papers by Enrico Deusebio
Third order structure function in rotating and stratified turbulence : analytical and numerical results compared with data from the stratosphere
Although turbulence has been studied for more than five hundred years, a thorough understanding o... more Although turbulence has been studied for more than five hundred years, a thorough understanding of turbulent flows is still missing. Nowadays computing power can offer an alternative tool, besides measurements and experiments, to give some insights into turbulent dynamics. In this thesis, numerical simulations are employed to study homogeneous and wall-bounded turbulence in rotating and stably stratified conditions, as encountered in geophysical flows where the rotation of the Earth as well as the vertical density variation influence the dynamics.In the context of homogeneous turbulence, we investigate how the transfer of energy among scales is affected by the presence of strong but finite rotation and stratification. Unlike geostrophic turbulence, we show that there is a forward energy cascade towards small scales which is initiated at the forcing scales. The contribution of this process to the general dynamic is secondary at large scales but becomes dominant at smaller scales wher...
Journal of Fluid Mechanics, 2016
We present a new adaptive control strategy to isolate and stabilize turbulent states in transitio... more We present a new adaptive control strategy to isolate and stabilize turbulent states in transitional, stably stratified plane Couette flow in which the gravitational acceleration (non-dimensionalized as the bulk Richardson number$Ri$) is adjusted in time to maintain the turbulent kinetic energy (TKE) of the flow. We demonstrate that applying this method at various stages of decaying stratified turbulence halts the decay process and allows a succession of intermediate turbulent states of decreasing energy to be isolated and stabilized. Once the energy of the initial flow becomes small enough, we identify a single minimal turbulent spot, and lower-energy states decay to laminar flow. Interestingly, the turbulent states which emerge from this process have very similar time-averaged$Ri$, but TKE levels different by an order of magnitude. The more energetic states consist of several turbulent spots, each qualitatively similar to the minimal turbulent spot. This suggests that the minimal ...
The open-channel version of SIMSON
Although turbulence has been studied for more than five hundred years, a thorough understanding o... more Although turbulence has been studied for more than five hundred years, a thorough understanding of turbulent flows is still missing. Nowadays computing power can offer an alternative tool, besides measurements and experiments, to give some insights into turbulent dynamics. In this thesis, numerical simulations are employed to study homogeneous and wall-bounded turbulence in rotating and stably stratified conditions, as encountered in geophysical flows where the rotation of the Earth as well as the vertical density variation influence the dynamics.In the context of homogeneous turbulence, we investigate how the transfer of energy among scales is affected by the presence of strong but finite rotation and stratification. Unlike geostrophic turbulence, we show that there is a forward energy cascade towards small scales which is initiated at the forcing scales. The contribution of this process to the general dynamic is secondary at large scales but becomes dominant at smaller scales where it leads to a shallowing of the energy spectrum, from k-3 to k-5/3. Two-point statistics show a good agreement with measurements in the atmosphere, suggesting that this process is an important mechanism for energy transfer in the atmosphere.Boundary layers subjected to system rotation around the wall-normal axis are usually referred to as Ekman layers and they can be seen as a model of the atmospheric and oceanic boundary layers developing at mid and high latitudes. We study the turbulent dynamics in Ekman layers by means of numerical simulations, focusing on the turbulent structures developing at moderately high Reynolds numbers. For neutrally stratified conditions, we show that there exists a turbulent helicity cascade in the logarithmic region. We focus on the effect of a stable stratification produced by a vertical positive temperature gradient. For moderate stratification, continuously turbulent regimes are produced which are in fair agreement with existing theories and models used in the context of atmospheric boundary layer dynamics. For larger degree of stratification, we show that laminar and turbulent motions coexist and displace along inclined patterns similar to what has been recently observed in other transitional flows.
Journal of Fluid Mechanics, 2014
First, we review analytical and observational studies on third-order structure functions includin... more First, we review analytical and observational studies on third-order structure functions including velocity and buoyancy increments in rotating and stratified turbulence and discuss how these functions can be used in order to estimate the flux of energy through different scales in a turbulent cascade. In particular, we suggest that the negative third-order velocity–temperature–temperature structure function that was measured by Lindborg & Cho (Phys. Rev. Lett., vol. 85, 2000, p. 5663) using stratospheric aircraft data may be used in order to estimate the downscale flux of available potential energy (APE) through the mesoscales. Then, we calculate third-order structure functions from idealized simulations of forced stratified and rotating turbulence and compare with mesoscale results from the lower stratosphere. In the range of scales with a downscale energy cascade of kinetic energy (KE) and APE we find that the third-order structure functions display a negative linear dependence on...
Atmospheric and oceanic flows are strongly affected by rotation and stratification. Rotation is e... more Atmospheric and oceanic flows are strongly affected by rotation and stratification. Rotation is exerted through Coriolis forces which mainly act in horizontal planes whereas stratification largely affects the motion along the vertical direction through buoyancy forces, the latters related to the vertical variation of the fluid density. Aiming at a better understanding of atmospheric and oceanic processes, in this thesis the properties of turbulence in rotating and stably stratified flows are studied by means of numerical simulations, with and without the presence of solid walls. A new code is developed in order to carry out high-resolution numerical simulations of geostrophic turbulence forced at large scales. The code was heavily parallelized with MPI (Message Passing Interface) in order to be run on massively parallel computers. The main problem which has been investigated is how the turbulent cascade is affected by the presence of strong but finite rotation and stratification. As opposed to the early theories in the field of geostrophic turbulence, we show that there is a forward energy cascade which is initiated at large scales. The contribution of this process to the general dynamic is secondary at large scales but becomes dominant at smaller scales where leads to a shallowing of the energy spectrum. Despite the idealized setup of the simulations, two-point statistics show remarkable agreement with measurements in the atmosphere, suggesting that this process may be an important mechanism for energy transfer in the atmosphere. The effect of stratification in wall-bounded turbulence is investigated by means of direct numerical simulations of open-channel flows. An existing fullchannel code was modified in order to optimize the grid in the vertical direction and avoid the clustering of grid points at the upper boundary, where the solid wall is replaced by a free-shear condition. The stable stratification which results from a cooling applied at the solid wall largely affects the outer structures of the boundary layer, whereas the near-wall structures appear to be mostly unchanged. The effect of gravity waves is also studied, and a new decomposition is introduced in order to separate the gravity wave field from the turbulent field.
Journal of Fluid Mechanics, 2014
We study the turbulent Ekman layer at moderately high Reynolds number, $\def \xmlpi #1{}\def \mat... more We study the turbulent Ekman layer at moderately high Reynolds number, $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}1600 < \mathit{Re} = \delta _{E}G/\nu < 3000$, using direct numerical simulations (DNS). Here, $\delta _{E} = \sqrt{2\nu /f}$ is the laminar Ekman layer thickness, $G$ the geostrophic wind, $\nu $ the kinematic viscosity and $f$ is the Coriolis parameter. We present results for both neutrally, moderately and strongly stably stratified conditions. For unstratified cases, large-scale roll-like structures extending from the outer region down to the wall are observed. These structures have a clear dominant frequency and could be related to periodic oscillations or instabilities developing near the low-level jet. We discuss the effect of stratification and $\mathit{Re}$ on one-point and two-point stati...
Well-resolved large-eddy simulations of passive control of the laminar-turbulent transition proce... more Well-resolved large-eddy simulations of passive control of the laminar-turbulent transition process in flat-plate boundary-layer flows are presented. A specific passive control mechanism is studied, namely the modulation of the laminar boundary-layer profile by a periodic array of steady boundary-layer streaks. This has been shown experimentally to stabilise the exponential growth of Tollmien-Schlichting (TS) waves and delay transition to turbulence. Here we examine the effect of the steady modulations on the amplification of different types of disturbances such as TS-waves, stochastic noise and free-stream turbulence. In our numerical simulations, the streaks are forced at the inflow as optimal solutions to the linear parabolic stability equations (PSE), whereas the additional disturbances are excited via volume forcing active within the computational domain. The simulation results show, in agreement with experimental and theoretical studies, significant damping of unstable two-dimensional TS-waves of various frequencies when introduced into a modulated base flow: The damping characteristics are mainly dependent on the streak amplitude. A new phenomenon is also identified which is characterised by the strong amplification via nonlinear interactions of the second spanwise harmonic of the streak when the streak amplitude is comparable to the TS amplitude. Furthermore, we demonstrate that control by streaks can be effective also in case of stochastic two-dimensional noise. However, as soon as a significant three-dimensionality is dominant, as in e.g. oblique or bypass transition, control by streaks leads often to premature transition. Visualisations of the flow fields are used to highlight the different vortical structures and their interactions that are relevant to the various transition scenarios and the corresponding control by streamwise streaks. 1 INTRODUCTION The reduction and control of the viscous drag force exerted on thin bodies moving in a fluid is of great technical interest. Drag reduction can be achieved by delaying the onset of a turbulent flow as well as quenching turbulence itself. Several active and passive methods to reduce the drag associated to a turbulent flow and/or achieve a delay of laminar-turbulent transition in the boundary layer have been developed in the past. Due to the highly local nature of turbulent events and the rapid nature of the breakdown a sensor-less (open-loop) strategy may be preferable, since it prevents the necessity of large numbers of fast sensor/actuator combinations (recent progress in feedback control of boundary layer instabilities can be found in e.g. [1, 2]). Thus far, the success of the control strategies for boundarylayer flows is limited and for bypass transition, i.e. transition in boundary layers subject to high levels of external perturbations, none of the strategies has been successful. Attention has been given to control via wall blowing/suction in the form of traveling waves. Du and Karniadakis [3] first showed drag reduction for control in the form of spanwise traveling waves in turbulent channel flow, actually implemented using volume forcing. These waves sustain streaky
Physical Review E, 2014
In this work we investigate, by means of direct numerical hyperviscous simulations, how rotation ... more In this work we investigate, by means of direct numerical hyperviscous simulations, how rotation affects the bidimensionalization of a turbulent flow. We study a thin layer of fluid, forced by a two-dimensional forcing, within the framework of the "split cascade" in which the injected energy flows both to small scales (generating the direct cascade) and to large scale (to form the inverse cascade). It is shown that rotation reinforces the inverse cascade at the expense of the direct one, thus promoting bidimensionalization of the flow. This is achieved by a suppression of the enstrophy production at large scales. Nonetheless, we find that, in the range of rotation rates investigated, increasing the vertical size of the computational domain causes a reduction of the flux of the inverse cascade. Our results suggest that, even in rotating flows, the inverse cascade may eventually disappear when the vertical scale is sufficiently large with respect to the forcing scale. We also study how the split cascade and confinement influence the breaking of symmetry induced by rotation.
Journal of Fluid Mechanics, 2014
Helicity, which is defined as the scalar product of velocity and vorticity, $\def \xmlpi #1{}\def... more Helicity, which is defined as the scalar product of velocity and vorticity, $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}{\mathcal{H}} = {\boldsymbol {u}} \boldsymbol {\cdot }\boldsymbol{\omega}$, is an inviscidly conserved quantity in a barotropic fluid. Mean helicity is zero in flows that are parity invariant. System rotation breaks parity invariance and has therefore the potential of giving rise to non-zero mean helicity. In this paper we study the helicity dynamics in the incompressible Ekman boundary layer. Evolution equations for the mean field helicity and the mean turbulent helicity are derived and it is shown that pressure flux injects helicity at a rate $ 2 \varOmega G^2 $ over the total depth of the Ekman layer, where $ G $ is the geostrophic wind far from the wall and $ {\boldsymbol{\Omega}} = \varOmega {...
Physical Review Letters, 2011
We hypothesize that the observed wave number spectra of kinetic and potential energy in the atmos... more We hypothesize that the observed wave number spectra of kinetic and potential energy in the atmosphere can be explained by assuming that there are two related cascade processes emanating from the same large-scale energy source, a downscale cascade of potential enstrophy, giving rise to the k À3 spectrum at synoptic scales and a downscale energy cascade giving rise to the k À5=3 spectrum at mesoscales. The amount of energy which is going into the downscale energy cascade is determined by the rate of system rotation, with negligible energy going downscale in the limit of very fast rotation. We present a set of simulations of a system with strong rotation and stratification, supporting these hypotheses and showing good agreement with observations.
Direct numerical simulations of stratified open channel flows
Journal of Physics: Conference Series, 2011
We carry out numerical simulations of wall-bounded stably stratified flows. We mainly focus on ho... more We carry out numerical simulations of wall-bounded stably stratified flows. We mainly focus on how stratification affects the near-wall turbulence at moderate Reynolds numbers, i.e. Reτ = 360. A set of fully-resolved open channel flow simulations is performed, where a stable stratification has been introduced through a negative heat flux at the lower wall. In agreement with previous studies, it
Journal of Fluid Mechanics, 2013
We investigate the route to dissipation in strongly stratified and rotating systems through high-... more We investigate the route to dissipation in strongly stratified and rotating systems through high-resolution numerical simulations of the Boussinesq equations (BQs) and the primitive equations (PEs) in a triply periodic domain forced at large scales. By applying geostrophic scaling to the BQs and using the same horizontal length scale in defining the Rossby and the Froude numbers, $\mathit{Ro}$ and $\mathit{Fr}$, we show that the PEs can be obtained from the BQs by taking the limit ${\mathit{Fr}}^{2} / {\mathit{Ro}}^{2} \rightarrow 0$. When ${\mathit{Fr}}^{2} / {\mathit{Ro}}^{2} $ is small the difference between the results from the BQ and the PE simulations is shown to be small. For large rotation rates, quasi-geostrophic dynamics are recovered with a forward enstrophy cascade and an inverse energy cascade. As the rotation rate is reduced, a fraction of the energy starts to cascade towards smaller scales, leading to a shallowing of the horizontal spectra from ${ k}_{h}^{- 3} $ to ${...
Energy transfer in geostrophic turbulence at finite Rossby and Froude number
ABSTRACT Energy transfer in geostrophic turbulence at finite Rossby and Froude number
Pathways to dissipation in strongly rotating and stratified turbulent systems 1 ENRICO DEUSEBIO, ... more Pathways to dissipation in strongly rotating and stratified turbulent systems 1 ENRICO DEUSEBIO, ERIK LINDBORG, Linné FLOW Centre, Royal Institute of Technology, KTH -Geophysical flows are strongly affected by rotation and stratification at very large scales (≈ 10 3 km). As the flow scale is reduced, first rotation (at ≈ 10 2 km) and then also stratification (at ≈ 1 km) become of secondary importance. Understanding the transitions between different regimes is crucial in order to evaluate the global circulating models which nowadays start to resolve them. We mainly focus on how energy is transferred from the large scales, at which it is injected, to the small-scales, where it is dissipated, in strongly rotating and stratified systems by means of numerical simulations of the Boussinesq equations. The large resolution employed, N x = N y = N z = 1024, allows us to resolve more than one dynamical regime. Large scale dynamic closely resembles quasi-geostrophic dynamics. However, departure from a quasi-geostrophic regime may also be recognized. We show the presence of a leakage of energy which starts from the largest scales and it is entirely supported by a non-geostrophic dynamics, which is possibly stratified turbulence. Despite the idealized set considered in the study, the results surprisingly agree with observations in the atmosphere, suggesting that the presented mechanism may play a crucial role in geophysical dynamics.
Although turbulence has been studied for more than five hundred years, a thorough understanding o... more Although turbulence has been studied for more than five hundred years, a thorough understanding of turbulent flows is still missing. Nowadays computing power can offer an alternative tool, besides ...
This dataset contains HDF5 files with three-dimensional flow fields from several key times in the... more This dataset contains HDF5 files with three-dimensional flow fields from several key times in the reported model simulations. The initial condition is provided, and the code is open access, allowing the full time-dependent model simulations to be reproduced. In addition, several 3D fields from the subsequent model evolution are included which were used to make figures in the published manuscript. The HDF5 file format is self-describing with header metadata and is readable in a wide variety of available software packages.
Isolating turbulent spots in stratified Plane-Couette flow John R. Taylor 1 , Enrico Deusebio, Co... more Isolating turbulent spots in stratified Plane-Couette flow John R. Taylor 1 , Enrico Deusebio, Colm-cille Caulfield 2,1 and Rich Kerswell 3 Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Centre for Mathematical Sciences, Willberforce Road, CB3 0WA, Cambridge BP Institute, University of Cambridge, Madingley Road, CB3 0EZ, Cambridge School of Mathematics, University of Bristol, BS8 1TW, Bristol Abstract We present a new adaptive control strategy to isolate and stabilize turbulent states in intermittent stably-stratified plane Couette flow in which the gravitational acceleration (non-dimensionalized as the bulk Richardson number Ri) is adjusted in time to maintain the turbulent kinetic energy (TKE) of the flow. We demonstrate that applying this method at various stages of decaying stratified turbulence halts the decay process and allows a succession of intermediate turbulent states of decreasing energy to be isolated and stabilized. Once the energy ...
Spots and stripes: Isolating the building blocks of intermittent stratified turbulence in plane Couette flow
Bulletin of the American Physical Society, 2015
Intermittent dynamics in stably stratified plane Couette flows
Bulletin of the American Physical Society, 2014
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Papers by Enrico Deusebio