1,805 research outputs found
Stability of a jet in crossflow
We have produced a fluid dynamics video with data from Direct Numerical
Simulation (DNS) of a jet in crossflow at several low values of the velocity
inflow ratio R. We show that, as the velocity ratio R increases, the flow
evolves from simple periodic vortex shedding (a limit cycle) to more
complicated quasi-periodic behavior, before finally exhibiting asymmetric
chaotic motion. We also perform a stability analysis just above the first
bifurcation, where R is the bifurcation parameter. Using the overlap of the
direct and the adjoint eigenmodes, we confirm that the first instability arises
in the shear layer downstream of the jet orifice on the boundary of the
backflow region just behind the jet.Comment: Two fluid dynamics videos, high-resolution 1024x768 (~80MB), and low
resolution 320x240 (~10MB), included in the ancillary file
Recurrent bursts via linear processes in turbulent environments
Large-scale instabilities occurring in the presence of small-scale turbulent
fluctuations are frequently observed in geophysical or astrophysical contexts
but are difficult to reproduce in the laboratory. Using extensive numerical
simulations, we report here on intense recurrent bursts of turbulence in plane
Poiseuille flow rotating about a spanwise axis. A simple model based on the
linear instability of the mean flow can predict the structure and time scale of
the nearly-periodic and self-sustained burst cycles. Rotating Poiseuille flow
is suggested as a prototype for future studies of low-dimensional dynamics
embedded in strongly turbulent environments
Bypass transition and spot nucleation in boundary layers
The spatio-temporal aspects of the transition to turbulence are considered in
the case of a boundary layer flow developing above a flat plate exposed to
free-stream turbulence. Combining results on the receptivity to free-stream
turbulence with the nonlinear concept of a transition threshold, a physically
motivated model suggests a spatial distribution of spot nucleation events. To
describe the evolution of turbulent spots a probabilistic cellular automaton is
introduced, with all parameters directly fitted from numerical simulations of
the boundary layer. The nucleation rates are then combined with the cellular
automaton model, yielding excellent quantitative agreement with the statistical
characteristics for different free-stream turbulence levels. We thus show how
the recent theoretical progress on transitional wall-bounded flows can be
extended to the much wider class of spatially developing boundary-layer flows
Global stability of a jet in crossflow
International audienceA linear stability analysis shows that the jet in crossflow is characterized by self-sustained global oscillations for a jet-to-crossflow velocity ratio of 3. A fully three-dimensional unstable steady-state solution and its associated global eigenmodes are computed by direct numerical simulations and iterative eigenvalue routines. The steady flow, obtained by means of selective frequency damping, consists mainly of a (steady) counter-rotating vortex pair (CVP) in the far field and horseshoe-shaped vortices close to the wall. High-frequency unstable global eigenmodes associated with shear-layer instabilities on the CVP and low-frequency modes associated with shedding vortices in the wake of the jet are identified. Furthermore, different spanwise symmetries of the global modes are discussed. This work constitutes the first simulation-based global stability analysis of a fully three-dimensional base flow. © 2009 Cambridge University Press
Turbulence transition in the asymptotic suction boundary layer
We study the transition to turbulence in the asymptotic suction boundary
layer (ASBL) by direct numerical simulation. Tracking the motion of
trajectories intermediate between laminar and turbulent states we can identify
the invariant object inside the laminar-turbulent boundary, the edge state. In
small domains, the flow behaves like a travelling wave over short time
intervals. On longer times one notes that the energy shows strong bursts at
regular time intervals. During the bursts the streak structure is lost, but it
reforms, translated in the spanwise direction by half the domain size. Varying
the suction velocity allows to embed the flow into a family of flows that
interpolate between plane Couette flow and the ASBL. Near the plane Couette
limit, the edge state is a travelling wave. Increasing the suction, the
travelling wave and a symmetry-related copy of it undergo a saddle-node
infinite-period (SNIPER) bifurcation that leads to bursting and
discrete-symmetry shifts. In wider domains, the structures localize in the
spanwise direction, and the flow in the active region is similar to the one in
small domains. There are still periodic bursts at which the flow structures are
shifted, but the shift-distance is no longer connected to a discrete symmetry
of the flow geometry. Two different states are found by edge tracking
techniques, one where structures are shifted to the same side at every burst
and one where they are alternatingly shifted to the left and to the right.Comment: Conference TSFP8, Poitiers 2013. TSFP-8 conference proceedings 2013,
http://www.tsfp-conference.org/proceedings
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