1,330 research outputs found
Creep of a fracture line in paper peeling
The slow motion of a crack line is studied via an experiment in which sheets
of paper are split into two halves in a ``peel-in-nip'' (PIN) geometry under a
constant load, in creep. The velocity-force relation is exponential. The
dynamics of the fracture line exhibits intermittency, or avalanches, which are
studied using acoustic emission. The energy statistics is a power-law, with the
exponent . Both the waiting times between subsequent
events and the displacement of the fracture line imply complicated stick-slip
dynamics. We discuss the correspondence to tensile PIN tests and other similar
experiments on in-plane fracture and the theory of creep for elastic manifolds
Puff turbulence in the limit of strong buoyancy
We provide a numerical validation of a recently proposed phenomenological theory to characterize the space-time statistical properties of a turbulent puff, both in terms of bulk properties, such as the mean velocity, temperature and size, and scaling laws for velocity and temperature differences both in the viscous and in the inertial range of scales. In particular, apart from the more classical shear-dominated puff turbulence, our main focus is on the recently discovered new regime where turbulent fluctuations are dominated by buoyancy. The theory is based on an adiabaticity hypothesis which assumes that small-scale turbulent fluctuations rapidly relax to the slower large-scale dynamics, leading to a generalization of the classical Kolmogorov and Kolmogorov-Obukhov-Corrsin theories for a turbulent puff hosting a scalar field. We validate our theory by means of massive direct numerical simulations finding excellent agreement. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'
Recommended from our members
Passive control of the flow around unsteady aerofoils using a self-activated deployable flap
Self-activated feathers are used by many birds to adapt their wing characteristics to the sudden change of flight incidence angle. In particular, dorsal feathers are believed to pop-up as a consequence of unsteady flow separation and to interact with the flow to palliate the sudden stall breakdown typical of dynamic stall. Inspired by the adaptive character of birds feathers, some authors have envisaged the potential benefits of using of flexible flaps mounted on aerodynamic surfaces to counteract the negative aerodynamic effects associated with dynamic stall. This contribution explores more in depth the physical mechanisms that play a role in the modification of the unsteady flow field generated by a NACA0020 aerofoil equipped with an elastically mounted flap undergoing a specific ramp-up manoeuvre. We discuss the design of flaps that limit the severity of the dynamic stall breakdown by increasing the value of the lift overshoot also smoothing its abrupt decay in time. A detailed analysis on the modification of the turbulent and unsteady vorticity field due to the flap flow interaction during the ramp-up motion is also provided to explain the more benign aerodynamic response obtained when the flap is in use
The dynamics of fibers dispersed in viscoelastic turbulent flows
This study explores the dynamics of finite-size fibers suspended freely in a
viscoelastic turbulent flow. For a fiber suspended in Newtonian flows, two
different flapping regimes were identified previously by Rosti et al (2018).
Here we explore, how the fiber dynamics is modified by the elasticity of the
carrier fluid by performing Direct Numerical Simulations of a two-way coupled
fiber-fluid system in a parametric space spanning different Deborah numbers,
fiber bending stiffness and the linear density difference between fiber and
fluid. We examine how these parameters influence various fiber characteristics
such as the frequency of flapping, curvature, and alignment with the fluid
strain and polymer stretching directions. Results reveal that the
neutrally-bouyant fibers, depending on their flexibility, oscillate with large
and small time scales transpiring from the flow, but the smaller time-scales
are suppressed as the polymer elasticity increases. Polymer stretching is
uncommunicative to denser-than-fluid fibers, which flap with large time scales
from the flow when flexible and with their natural frequency when rigid. Thus,
the characteristic elastic time scale has a subdominant effect when the fibers
are neutrally-bouyant, while its effect is absent when the fibers become more
inertial. Additionally, we see that the inertial fibers have larger curvatures
and are less responsive to the polymer presence, whereas the neutrally-bouyant
fibers show quantitative changes. Also, the neutrally-bouyant fibers show a
higher alignment with the polymer stretching directions compared to the denser
ones. In a nutshell, the polymers exert a larger influence on neutrally-bouyant
fibers compared to the denser ones. The study comprehensively addresses the
interplay between polymer elasticity and the fiber structural properties in
determining its response behaviour in an elasto-inertial turbulent flow
Turbulent channel flow over an anisotropic porous wall - drag increase and reduction
The effect of the variations of the permeability tensor on the close-to-the-wall behaviour of a turbulent channel flow bounded by porous walls is explored using a set of direct numerical simulations. It is found that the total drag can be either reduced or increased by more than 20 % by adjusting the permeability directional properties. Drag reduction is achieved for the case of materials with permeability in the vertical direction lower than the one in the wall-parallel planes. This configuration limits the wall-normal velocity at the interface while promoting an increase of the tangential slip velocity leading to an almost ‘one-component’ turbulence where the low- and high-speed streak coherence is strongly enhanced. On the other hand, strong drag increase is found when high wall-normal and low wall-parallel permeabilities are prescribed. In this condition, the enhancement of the wall-normal fluctuations due to the reduced wall-blocking effect triggers the onset of structures which are strongly correlated in the spanwise direction, a phenomenon observed by other authors in flows over isotropic porous layers or over ribletted walls with large protrusion heights. The use of anisotropic porous walls for drag reduction is particularly attractive since equal gains can be achieved at different Reynolds numbers by rescaling the magnitude of the permeability only
- …