Fitzgerald, J. G., & Farrell, B. F. (2014). Mechanisms of Mean Flow Formation and Suppression in Two-Dimensional Rayleigh-Benard Convection. Physics of Fluids , 26, 054104. Publisher's VersionAbstract

Two-dimensional laminar roll convection is capable of generating energetic horizontal
mean flows via a well-understood process known as the tilting instability. Less wellunderstood
is the physical mechanism behind the strong dependence of this effect
on the horizontal lengthscale of the convection pattern. Mean flows of this type have
been found to form for sufficiently large Rayleigh number in periodic domains with a
small aspect ratio of horizontal length to vertical height, but not in large aspect ratio
domains.We demonstrate that the elimination of the tilting instability for large aspect
ratio is due to a systematic eddy-eddy advectionmechanism intervening at linear order
in the tilting instability, and that this effect can be captured in a model retaining two
nonlinearly interacting horizontal wavenumber components of the convection field.
Several commonly used low-order models of convection also exhibit a shutdown of
the tilting instability for large aspect ratio, even though thesemodels do not contain the
eddy-eddy advection mechanism. Instability suppression in such models is due to a
different mechanism involving vertical advection.We showthat this vertical advection
mechanism is excessively strong in the low-order models due to their low resolution,
and that the instability shutdown in such models vanishes when they are appropriately

Farrell, B. F., & Ioannou, P. J. (2014). Statistical State Dynamics: a new perspective on turbulence in shear flow. arXiv , 1412.8290v1.Abstract

Traditionally, single realizations of the turbulent state have been the object of study in shear  flow
turbulence. When a statistical quantity was needed it was obtained from a spatial, temporal or
ensemble average of sample realizations of the turbulence. However, there are important advantages
to studying the dynamics of the statistical state (the SSD) directly. In highly chaotic systems
statistical quantities are often the most useful and the advantage of obtaining these statistics directly
from a state variable is obvious. Moreover, quantities such as the probability density function (pdf)
are often dicult to obtain accurately by sampling state trajectories even if the pdf is stationary.
In the event that the pdf is time dependent, solving directly for the pdf as a state variable is the
only alternative. However, perhaps the greatest advantage of the SSD approach is conceptual:
adopting this perspective reveals directly the essential cooperative mechanisms among the disparate
spatial and temporal scales that underly the turbulent state. While these cooperative mechanisms
have distinct manifestation in the dynamics of realizations of turbulence both these cooperative
mechanisms and the phenomena associated with them are not amenable to analysis directly through
study of realizations as they are through the study of the associated SSD. In this review a selection
of example problems in the turbulence of planetary and laboratory
ows is examined using recently
developed SSD analysis methods in order to illustrate the utility of this approach to the study of
turbulence in shear  flow.

Constantinou, N. C., Lozano-Duran, A., Farrell, B. F., Ioannou, P. J., Nikolaids, M. - A., & Jimenez, J. (2014). Turbulence in the highly restricted dynamics of a closure at second order: comparison with DNS. Journal Of Physics , (506).Abstract

Abstract. S3T (Stochastic Structural Stability Theory) employs a closure at second order to obtain the dynamics of the statistical mean turbulent state. When S3T is implemented as a coupled set of equations for the streamwise mean and perturbation states, nonlinearity in the dynamics is restricted to interaction between the mean and perturbations. The S3T statistical mean state dynamics can be approximately implemented by similarly restricting the dynamics used in a direct numerical simulation (DNS) of the full Navier–Stokes equations (referred to as the NS system). Although this restricted nonlinear system (referred to as the RNL system) is greatly simplified in its dynamics in comparison to the associated NS, it nevertheless self-sustains a turbulent state in wall-bounded shear flow with structures and dynamics comparable to those observed in turbulence. Moreover, RNL turbulence can be analysed effectively using theoretical methods developed to study the closely related S3T system. In order to better understand RNL turbulence and its relation to NS turbulence, an extensive comparison is made of diagnostics of structure and dynamics in these systems. Although quantitative differences are found, the results show that turbulence in the RNL system closely parallels that in NS and suggest that the S3T/RNL system provides a promising reduced complexity model for studying turbulence in wall-bounded shear flows.

Constantinou, N. C., Farrell, B. F., & Ioannou, P. J. (2014). Emergence and equilibration of jets in beta-plane turbulence. J. Atmos. Sci. , 71 (5), 1818-1842.Abstract

Stochastic structural stability theory (S3T) provides analytical methods for understanding the emergence and equilibration of jets from the turbulence in planetary atmospheres based on the dynamics of the statistical mean state of the turbulence closed at second order. Predictions for formation and equilibration of turbulent jets made using S3T are critically compared with results of simulations made using the associated quasi-linear and nonlinear models. S3T predicts the observed bifurcation behavior associated with the emergence of jets, their equilibration, and their breakdown as a function of parameters. Quantitative differences in bifurcation parameter values be- tween predictions of S3T and results of nonlinear simulations are traced to modification of the eddy spectrum which results from two processes: nonlinear eddy-eddy interactions and formation of discrete nonzonal struc- tures. Remarkably, these nonzonal structures, which substantially modify the turbulence spectrum, are found to arise from S3T instability. Formation as linear instabilities and equilibration at finite amplitude of multiple equilibria for identical parameter values in the form of jets with distinct meridional wavenumbers is verified, as is the existence at equilibrium of finite-amplitude nonzonal structures in the form of nonlinearly modified Rossby waves. When zonal jets and nonlinearly modified Rossby waves coexist at finite amplitude, the jet structure is generally found to dominate even if it is linearly less unstable. The physical reality of the manifold of S3T jets and nonzonal structures is underscored by the existence in nonlinear simulations of jet structure at subcritical S3T parameter values that are identified with stable S3T jet modes excited by turbulent fluctuations.

Farrell, B. F., & Ioannou, P. J. (2012). Dynamics of streamwise rolls and streaks in wall-bounded shear flow. J. Fluid Mech. , 708, 149-196.Abstract

Streamwise rolls and accompanying streamwise streaks are ubiquitous in wall-bounded shear flows, both in natural settings, such as the atmospheric boundary layer, as well as in controlled settings, such as laboratory experiments and numerical simulations. The streamwise roll and streak structure has been associated with both transition from the laminar to the turbulent state and with maintenance of the turbulent state. This close association of the streamwise roll and streak structure with the transition to and maintenance of turbulence in wall-bounded shear flow has engendered intense theoretical interest in the dynamics of this structure. In this work, stochastic structural stability theory (SSST) is applied to the problem of understanding the dynamics of the streamwise roll and streak structure. The method of analysis used in SSST comprises a stochastic turbulence model (STM) for the dynamics of perturbations from the streamwise-averaged flow coupled to the associated streamwise-averaged flow dynamics. The result is an autonomous, deterministic, nonlinear dynamical system for evolving a second-order statistical mean approximation of the turbulent state. SSST analysis reveals a robust interaction between streamwise roll and streak structures and turbulent perturbations in which the perturbations are systematically organized through their interaction with the streak to produce Reynolds stresses that coherently force the associated streamwise roll structure. If a critical value of perturbation turbulence intensity is exceeded, this feedback results in modal instability of the combined streamwise roll/streak and associated turbulence complex in the SSST system. In this instability, the perturbations producing the destabilizing Reynolds stresses are predicted by the STM to take the form of oblique structures, which is consistent with observations. In the SSST system this instability exists together with the transient growth process. These processes cooperate in determining the structure of growing streamwise roll and streak. For this reason, comparison of SSST predictions with experiments requires accounting for both the amplitude and structure of initial perturbations as well as the influence of the SSST instability. Over a range of supercritical turbulence intensities in Couette flow, this instability equilibrates to form finite amplitude time-independent streamwise roll and streak structures. At sufficiently high levels of forcing of the perturbation field, equilibration of the streamwise roll and streak structure does not occur and the flow transitions to a time-dependent state. This time-dependent state is self-sustaining in the sense that it persists when the forcing is removed. Moreover, this self-sustaining state rapidly evolves toward a minimal representation of wall-bounded shear flow turbulence in which the dynamics is limited to interaction of the streamwise-averaged flow with a perturbation structure at one streamwise wavenumber. In this minimal realization of the self-sustaining process, the time-dependent streamwise roll and streak structure is maintained by perturbation Reynolds stresses, just as is the case of the time-independent streamwise roll and streak equilibria. However, the perturbation field is maintained not by exogenously forced turbulence, but rather by an endogenous and essentially non-modal parametric growth process that is inherent to time-dependent dynamical systems.

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