Olivier Crumeyrolle

Experimental study of inertioelastic Couette–Taylor instability modes in dilute and semidilute polymer solutions

Instabilities appearing in the circular Couette flow with a dilute or semidilute solution of high molecular weight polyethyleneoxide in water have been investigated when the outer cylinder is kept at rest. The shear-thinning behavior of solutions was determined through low shear viscosity measurements. We have found that, depending on concentration, different flow structures can appear at the onset of instability. For dilute concentrations, the critical mode is the stationary and axisymmetric Taylor vortex flow, which bifurcates to time periodic wavy vortex flow for a higher shear rate. The oscillation amplitude of wavy vortex flow decreased with the increasing shear rate. For sufficiently semidilute solutions, the critical mode occurs in the form of standing waves, the frequency of which decreases with the shear rate. The critical Taylor number increases for solutions without a shear-thinning effect and decreases for solutions exhibiting shear thinning.

Dielectrophoretic force-driven thermal convection in annular geometry

The thermal convection driven by the dielectrophoretic force is investigated in annular geometry under microgravity conditions. A radial temperature gradient and a radial alternating electric field are imposed on a dielectric fluid that fills the gap of two concentric infinite-length cylinders. The resulting dielectric force is regarded as thermal buoyancy with a radial effective gravity. This electric gravity varies in space and may change its sign depending on the temperature gradient and the cylinder radius ratio. The linear stability problem is solved by a spectral-collocation method. The critical mode is stationary and non-axisymmetric. The critical Rayleigh number and wavenumbers depend sensitively on the electric gravity and the radius ratio. The mechanism behind the instability is examined from an energetic viewpoint. The instability in wide gap annuli is an exact analogue to the gravity-driven thermal instability.

Numerical investigation of the heat transfer in cylindrical annulus with a dielectric fluid under microgravity

Three-dimensional (3D) flow driven by thermal convection in a dielectric liquid confined in the gap between two coaxial cylinders is investigated by direct numerical simulations. The inner surface is warmer than the outer one and a high frequency alternating electric tension is applied to the cylinders. The fluid is therefore subjected to a radial dielectrophoretic force which plays the role of a buoyancy force that can generate a thermal convection. We have performed 3D simulations using periodic boundary conditions. The transition from the base state to convective flow occurs via a supercritical bifurcation and leads to helicoidal stationary vortices. The behavior of the heat transfer by convective flow is investigated for different values of the radius ratio, Prandtl number, and electric Rayleigh number.

Heat transfer in the thermo-electro-hydrodynamic convection under microgravity conditions

This article deals with the thermal convection in a dielectric fluid confined in a finite-length plane capacitor with a temperature gradient under microgravity conditions. The dielectrophoretic force resulting from differential polarization of the fluid plays the role of buoyancy force associated with an electric effective gravity. It induces the convection when the Rayleigh number based on this electric gravity exceeds a critical value. Two-dimensional numerical simulation for a geometry with a large aspect ratio is used to determine the convective flow in the saturated state. The Nusselt number Nu is computed for a wide range of Prandtl number (0.01 ≤ Pr ≤ 103) and its dependence on the distance from the critical condition is determined. A correlation between Nu and Pr in the vicinity of criticality is obtained and compared with that of the Rayleigh-Bénard convection. The behavior of the convection is analyzed in detail from an energetic viewpoint: electrostatic energy, power inputs by different components of the electric gravity and viscous and thermal dissipations are computed.Graphical abstract

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.

Velocity field of the spiral vortex flow in the Couette-Taylor system

Spiral vortex flow in the counter-rotating Couette-Taylor system with a large aspect ratio and an intermediate gap has been investigated using Particle Image Velocimetry (PIV). From data of velocity components, we have determined nonlinear properties (anharmonicity, mirror symmetry, axial and radial flow rates) of spiral vortices and compared them to those of Taylor vortices. The velocity field around a spatio-temporal defect has been measured. There is a good agreement between these experimental results with available results from numerical simulations.Graphical abstract

Thermo-electro-hydrodynamic convection under microgravity: a review

Recent studies on thermo-electro-hydrodynamic (TEHD) convection are reviewed with focus on investigations motivated by the analogy with natural convection. TEHD convection originates in the action of the dielectrophoretic force generated by an alternating electric voltage applied to a dielectric fluid with a temperature gradient. This electrohydrodynamic force is analogous to Archimedean thermal buoyancy and can be regarded as a thermal buoyancy force in electric effective gravity. The review is concerned with TEHD convection in plane, cylindrical, and spherical capacitors under microgravity conditions, where the electric gravity can induce convection without any complexities arising from geometry or the buoyancy force due to the Earth’s gravity. We will highlight the convection in spherical geometry, comparing developed theories and numerical simulations with the GEOFLOW experiments performed on board the International Space Station (ISS).

Instabilities with polyacrylamide solution in small and large aspect ratios Taylor-Couette systems

We have investigated the stability of viscoelastic polyacrylamide solution in Taylor-Couette system with different aspect ratios. The first instability modes observed in a Taylor-Couette system with Γ = 10 were TVF and WVF, as for Newtonian fluid. At higher Taylor numbers moving vortices occur, a wavy mode with non-stationary vortex size. In the Taylor-Couette system with Γ = 45.9 we note a coexistence of various instability modes. In addition to TVF, counterpropagating waves developed at the transition from the base state flow. At higher Taylor number values Taylor vortices of different sizes occurred. Reduced amplitude Wavy vortex flow has also been observed.

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.

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.