Recent advances in the subcritical transition to turbulence
Recent years have witnessed a profound change in our understanding
of the route to turbulence in wall-bounded shear flows
such as pipes, ducts, and channels. These lectures will review
our current knowledge of the dynamics of transitional turbulence
on a wide range of scales. Considerable focus will be given to
quantifying the complex spatiotemporal intermittency observed
in experiments and numerical simulations. A theoretical underpinning
of the route to turbulence in subcritical shear flows will
be presented. Finally, lectures will include a discussion of outstanding
Turbulence closure modelling in the coastal ocean: the essential effect of stable stratification on vertical mixing
Small-scale turbulent mixing in the ocean is highly variable, with eddy viscosities and diffusivities ranging over several orders of magnitudes. While turbulence in the nearly unstratified surface and bottom boundary layers is generally high and only bounded geometrically by the thickness of the boundary layers, turbulence in the stratified interior is strongly suppressed. Specifically in the coastal ocean, temporal variability is high and boundary layers may occupy a substantial part of the water column. In this presentation, turbulence closure models are introduced which account for this spatial and temporal variability. Two-equation turbulence closure models are argued to be an optimal compromise between efficiency and accuracy for the purpose of calculating vertical fluxes of momentum, heat and tracers in coastal ocean modelling. They provide enough degrees of freedom to be calibrated to the most prominent properties of coastal ocean mixing, but are still numerically robust and computationally efficient. One essential ingredient for a working turbulence cosure in the ocean is the proper calibration of the suppression of vertical mixing by stratification. Major implementational and numerical aspects are presented. Some focus will be on the inherent problem of numerically-induced mixing which together with the physically-induced mixing gives the effective mixing in ocean models. Vertically adaptive coordinates are presented as one possibility to reduce numerical mixing. Some examples for thermocline mixing in the Northern North Sea, physically and numerically induced mixing in the Western Baltic Sea as well as basinwide mixing in the Central Baltic Sea are presented. All three examples highlight the importance of using well-calibrated turbulence closure models together with vertically adaptive coordinates.
Limits to turbulent transport & dissipation:
bounding solutions to equations of fluid mechanics & other dynamical systems
Turbulent solutions to fundamental equations of fluid mechanics
— and more generally complex and chaotic solutions to
nonlinear dynamical systems — are often usefully characterized
statistically, i.e., in terms of space and/or time averages of physically
relevant quantities. Even then exact results are rare and
in practice the goal is frequently to obtain reliable predictive
estimates of some sort. These may be from direct numerical simulations,
from theoretical approximations, or from mathematical
analysis producing rigorous limits on averages of the variables of
interest. In these lectures we will develop some effective methods
for bounding bulk turbulent transport and dissipation in the
Navier-Stokes and related equations for some canonical flow configurations.
The focus will be on nonlinear energy stability theory
and its extension to the background method relevant for developed
flows. We will also present some recent research that generalizes
the background method opening the door to the search
for sharp optimal bounds.
We will scrutinize the emergence and some properties of spatiotemporal
chaos, i.e. mild turbulence, in systems with dimensions
large when compared to intrinsic scales generated by instabilities.
The key characteristic is the slow dynamics in time and
space that results from the proximity of a bifurcation and from
the continuous symmetries typical of the unbounded-domain limit.
A brief review of relevant elements of stability theory and bifurcation
analysis will first be given. A key feature is the continuous
or discontinuous nature of the primary bifurcation away from
the system’s base state of interest. Considered first, the globally
supercritical transition scenario takes place when the primary
instability is continuous. This case is amenable to analysis via
multiple-scale expansions that introduce envelopes and phases
as natural tools apt to account for pattern formation and phase
turbulence. The subcritical case implies the coexistence of concurrent
locally stable states in both phase space and physical
space. In extended systems, this leads to a spin-like reduction to
be studied within the framework of statistical physics, hence an
analysis of, e.g., spatiotemporal intermittency in terms of phase
transitions and critical phenomena. To conclude, some general
properties of far-from-equilibrium systems will be discussed.
The 6th Bremen Winter School and Symposium "Dynamical systems and turbulence", March 12-16 2018.