Jack Legrand

Experimental Study on Oscillatory Couette-Taylor Flows Behaviour

In the simplest and original case of study of the Taylor–Couette TC problems, the fluid is contained between a fixed outer cylinder and a concentric inner cylinder which rotates at constant angular velocity. Much of the works done has been concerned on steady rotating cylinder(s) i.e. rotating cylinders with constant velocity and the various transitions that take place as the cylinder(s) velocity (ies) is (are) steadily increased. On this work, we concentrated our attention in the case in which the inner cylinder velocity is not constant, but oscillates harmonically (in time) clockwise and counter-clockwise while the outer cylinder is maintained fixed. Our aim is to attempt to answer the question if the modulation makes the flow more or less stable with respect to the vortices apparition than in the steady case. If the modulation amplitude is large enough to destabilise the circular Couette flow, two classes of axisymmetric Taylor vortex flow are possible: reversing Taylor Vortex Flow (RTVF) and Non-Reversing Taylor Vortex Flow (NRTVF) (Youd et al., 2003; Lopez and Marques, 2002). Our work presents an experimental investigation of the effect of oscillatory Couette-Taylor flow, i.e. both the oscillation frequency and amplitude on the apparition of RTVF and NRTVF by analysing the instantaneous and local mass transfer and wall shear rates evolutions, i.e. the impact of vortices at wall. The vortices may manifest themselves by the presence of time-oscillations of mass transfer and wall shear rates, this generally corresponds to an instability apparition even for steady rotating cylinder. On laminar CT flow, the time-evolution of wall shear rate is linear. It may be presented as a linear function of the angular velocity, i.e. the evolution is steady even if the angular velocity is not steady. At a “critical” frequency and amplitude, the laminar CT flow is disturbed and Taylor vortices appear. Comparing to a steady velocity case, oscillatory flow accelerate the instability apparition, i.e. the critical Taylor number corresponds to the transition is smaller than that of the steady case. For high oscillation amplitudes of the inner cylinder rotation, the mass transfer time-evolution has a sinusoidal evolution with non equal oscillation amplitudes. If the oscillation amplitude is large enough, it can destabilize the laminar Couette flow, Taylor vortices appears. The vortices direction can be deduced from the sign of the instantaneous wall shear rate time evolution.Copyright

Flow Behaviour Around Square and Circular Obstacles in 2D and 3D Configurations Using Lattice Boltzmann Method

The investigation of wakes of bluff bodies in a channel is still relevant despite the large number of works devoted on it, in both experimental and numerical studies. This attractiveness is mainly due to its related applications and practical interest in varied engineering fields.The understanding of dynamic flow behavior and the topology of the instability structures occurring in the wake is essential in order to optimize the obstacle shape according to the desired objectives.A confined laminar flow around a square and a circle, placed in a channel is numerically investigated in this work using Lattice Boltzmann method. The study is then extended to 3D computations with horizontal cylinder within a square then a circular cross-section mounted inside a rectangular duct.The Reynolds number (Re), based on the maximum velocity and the cross-section height varies between 50 and 120 and the blockage ratio is r=1/3. This geometry is representative of a passive method to enhance mixing in the laminar channel flow. LBM was built up on the D2Q9 and D3Q19 model for respectively 2D and 3D computations. The single relaxation time approach called the lattice-BGK method was adopted.The topology of the vortex-shedding phenomena and wake behavior according the Reynolds numbers, for both geometries of the obstacle are focused. The effect of wall confinement on the flow transition to the vortex shedding regime is discussed. Velocity profiles and integral parameters such as recirculation length and Strouhal number were investigated.The numerical results are supported by literatures works results for the same configuration showing the performance of LBM as numerical tools simulation for such kind of flows.Copyright

Use of the POD and the Coherent Structure Detection Criteria to Study the Flow Dynamics Downstream a Confined Square Obstacle

Studying the concept of the vortex is an essential tool in the comprehension of fluid dynamics. Despite this, it is still very difficult to find a universal definition of a vortex. Several methods of detection and characterization of vortex structures has been developed and performed. Specifically tailored for PIV data, they present an important topic in modern experimental fluid mechanics.In fact, the performance of the method is related to its effectiveness in the velocity data analysis and to successfully detect and locate the vortices as well as calculate the characteristic vortex parameters.In the present study, we explore the efficiency of different vortex detection algorithms such as the vorticity ω, Γ2 and Q criteria by studying the vortices structures in the wake of a confined square obstacle.Proper orthogonal decomposition (POD) of the velocity fields was used to extract the energetic contribution of the different instabilities modes.These methods were firstly applied to an experimentally two-dimensional instantaneous velocity fields obtained by Particle Image Velocimetry (PIV) technique. Then, in a second step, we tested these criterions on a numerical velocity fields determined from Lattice Boltzmann simulations and compared to experimental results.Copyright

Use of Lattice-Boltzmann Method in Mass Transfer for the Wall Shear Stress Calculation in an Unsteady Laminar Flow Downstream of a Cylinder Located in a 2D Rectangular Channel

A Lattice Boltzmann method (LBM) is proposed to study the flow and mass transfer in the parietal zone of a channel containing a blocking circular obstacle at low Reynolds number. For such configuration, the variation of Re based on cylinder diameter leads to different regime that fluid may occur during it flowing. Hence, this varied behaviour of flow downstream of the obstacle affects the mass transfer rate in the parietal zone of the channel.A sensor with zero concentration on the surface is placed at different locations on the channel wall downstream of the cylinder. Flow velocity and concentration profile of diffusing species on the sensor were evaluated and analyzed for different Reynolds. The Sherwood numbers are calculated and compared with available experimental data.For the present simulation, LBM is based on the D2Q9 lattice model and the single relaxation time approach called BGK method.The challenge of this work is to extend the use of the mesoscopic method (LBM) for a flow problem to studying the enhancement of the rate of mass transfer to channel wall downstream of the obstacle. The numerical results are in good agreement with the benchmark result available in the literature. The highlight of calculation is the flexibility to deal with the boundary conditions for such a problem.The result indicates that LBM is useful for simulation of fluid flows with mass transfer as well as heat transfer.© 2012 ASME

Lattice Boltzmann Method Used to Simulate an Unsteady Flow Around an Obstacle in Laminar Regime

This work deals with the application of the lattice Boltzmann method to simulate the unsteady laminar flow around a confined square obstacle. For this configuration, we can observe some regimes that fluid may occur during its flowing. We have determined numerically the flow behavior for linear and stable regime. The variable aspect of the flow observed depends on the Reynolds number. In this study, we determine the velocity fields for a various Reynolds numbers by resolving the Navier-Stokes equations using the Lattice Boltzmann Method with BGK schema. This method is a recent extension of the LB method which demonstrated its potential for describing incompressible flow around an obstacle. A numerical study of 2D and 3D problem around a square obstacle using the Lattice Boltzmann Method with BGK schema is presented for an unsteady flow in laminar regime. The flow behavior in a horizontal channel with a rectangular cross-section, where a squared obstacle is placed in the middle, is discussed. In the 2D simulation, the obtained numerical results show a good agreement with experimental results [18]. Then we extend the ability of this method to solve the 3D problem. Numerical results behind the obstacle, obtained for various Reynolds numbers, are also analyzed and discussed.Copyright