Noureddine Latrache

Defect-mediated turbulence in ribbons of viscoelastic Taylor-Couette flow.

Transition to defect-mediated turbulence in the ribbon patterns observed in a viscoelastic Taylor-Couette flow is investigated when the rotation rate of the inner cylinder is increased while the outer cylinder is fixed. In four polymer solutions with different values of the elasticity number, the defects appear just above the onset of the ribbon pattern and trigger the appearance of disordered oscillations when the rotation rate is increased. The flow structure around the defects is determined and the statistical properties of these defects are analyzed in the framework of the complex Ginzburg-Landau equation.

Destabilization of inertio-elastic mode via spatiotemporal intermittency in a Couette-Taylor viscoelastic flow

We have characterized the transition to turbulence in a flow with semi-dilute solutions (concentration of 0.07%) of polyethylene oxide (PEO) in the Couette-Taylor system with rotating only the inner cylinder. The first instability mode occurs via a supercritical Hopf bifurcation in form of interacting right and left spirals. For higher values of the rotation velocity, turbulent spots appear in the flow and coexist with the spirals. This spatio-temporal intermittency (STI) regime has been characterized by the turbulent fraction and the statistical properties of the laminar domains.

Numerical Study of the Fluid Characteristics Effect on the Penetration of a Negatively Buoyant Jet

The behavior of a negatively buoyant jet in laminar conditions that result from the injection of lighter fluids downwards into a large container of homogeneous fluid of denser density was studied numerically using OpenFoam with the finite volume method. The fluid characteristics effect on the evolution of the pure water injected in the tank full of salt water has been investigated particularly the molecular diffusivity that affects the mixing layer, the relative difference of density between two liquids, and the kinematic viscosity of the salt water that has an important effect in the transient phase as well as the subsequent steady state in terms of stationary penetration depth and jet profile.

Experimental Study of the Injection of a Pure Water Jet in Miscible Salt Water

In this chapter, some experiments were performed to observe the evolution of a pure water jet injected with negatively buoyant conditions in a miscible surrounding salt water in a laminar regime. The effect of the flow rate and the relative difference of density between the pure water jet and the surrounding salt water were examined. In these conditions, the increase of the penetration depth of the jet is obtained by the increase of the flow rate and the decrease of the density difference between the two liquids. A good agreement was found between the numerical and the experimental results.

Radial Propagation of the Instability Modes Observed in a Viscoelastic Couette–Taylor Flow

Experimental investigation of the flow of a high-molecular-mass polymer solution in the Couette–Taylor system with fixed outer cylinder was performed using visualization and particle image velocimetry (PIV) techniques. Spatiotemporal diagrams of the reflected light intensity and of velocity data allow to describe the flow dynamics in the meridional cross section. When the elasticity and inertia effects are comparable (inertioelastic regime), the circular Couette flow bifurcates to standing waves—in the axial direction—called ribbons. These critical waves also propagate in the radial direction toward the outer cylinder. The higher instability mode manifests in form of domains with disordered oscillations separated by fluctuating walls characterized by strong radial inflow.

Experimental Study of the Effect of the Spreading Buoyant Gravity Current on the Coastal Environment

The experiments of injection gravity currents were completed in a rectangular basin by release the fresh water over a salty water surface at rest. The surface flow of lighter liquids horizontally can be considered as a special case of a two dimensional gravity current, and the spreading law of the fresh water is validated by using image processing technique to record the progress of the injection current. The jet fluid was colored and the digital video can have a variation of light intensity in order to obtain the shape of the outer boundary of the gravity current and its distance from the source point with relation to time, initial, or flow parameters of the jet source.