Physics

The design and application of an instrumented particle for the lagrangian characterization of turbulent free surface flows is presented in this study. This instrumented particle constitutes a local measurement device capable of measuring both its instantaneous 3D translational acceleration and angular velocity components, as well as recording them on an embarked removeable memory card. A lithium ion polymer battery provides the instrumented particle with up to 8 hours of autonomous operation. Entirely composed of commercial off the shelf electronic components, it features accelerometer and gyroscope sensors with a resolution of 16 bits for each individual axis, and maximum data acquisition rates of 1 and 8 kHz, respectively, as well as several user programmable dynamic ranges. Its ABS 3D printed body takes the form of a 36 mm diameter hollow sphere, and has a total mass of (19.6 ± 0.5) g. Controlled experiments, carried out to calibrate and validate its performance showed good agreement when compared to reference techniques. In order to assess the practicality of the instrumented particle, we apply it to the statistical characterization of floater dynamics in experiments of surface wave turbulence. In this feasibility study, we focused our attention on the distribution of acceleration and angular velocity fluctuations as a function of the forcing intensity. The IP's motion is also simultaneously registered by a 3D particle tracking velocimetry (PTV) system, for the purposes of comparison. Beyond the results particular to this study case, it constitutes a proof of both the feasibility and potentiality of the IP as a tool for the experimental characterization of particle dynamics in such flows.

In an earlier paper we showed that the combination of azimuthal magnetic fields and super-rotation in Taylor-Couette flows of conducting fluids can be unstable against non-axisymmetric perturbations if the magnetic Prandtl number of the fluid is Pm≠1 . Here we demonstrate that the addition of a weak axial field component may allow axisymmetric perturbation patterns for Pm of order unity depending on the boundary conditions. The axisymmetric modes only occur for magnetic Mach numbers (of the azimuthal field) of order unity, while higher values are necessary for non-axisymmetric modes. The typical growth time of the instability and the characteristic time scale of the axial migration of the axisymmetric mode are long compared with the rotation period, but short compared with the magnetic diffusion time. The modes travel in the positive or negative z -direction along the rotation axis depending on the sign of B ϕ B z . We also demonstrate that the azimuthal components of flow and field perturbations travel in phase if | B ϕ |≫| B z | , independent of the form of the rotation law. Within a short-wave approximation for thin gaps it is also shown (in an Appendix) that for {\em ideal} fluids the considered helical magnetorotational instability (HMRI) only exists for rotation laws with negative shear.

A Numerical Wave Tank (NWT) is developed within the fully nonlinear potential flow theory in two dimensions. It uses a combination of the Harmonic Polynomial Cell (HPC) method for solving the Laplace problem on the wave potential and the Immersed Boundary Method (IBM) for capturing the free surface motion. This NWT can consider either submerged or surface-piercing bodies or arbitrary shape. To compute the flow around the body and associated pressure field, a multi overlapping grid method is implemented. Each grid having its own free surface, a two-way communication is ensured between the problem in the body vicinity and the larger scale wave propagation problem. Nonlinear loads on the structure are computed from an accurate pressure field obtained thanks to a boundary value problem formulated on the time derivative of the potential, at the cost of a second matrix inversion at each time step. The mathematical formalism and the numerical methods of the NWT are first presented. Then a focus is made on both the stability and convergence properties of the solver. Then, the NWT is tested against both numerical and experimental results, analyzing forces acting on different bodies in various wave conditions. Each selected test-case exhibits a particular difficulty, from large steepness waves to very small water gap, and even sharp corners of the body. Nonlinear effects of various magnitude are considered and compared. A dedicated experimental campaign is also performed in a wave flume with a floating barge of rectangular cross-section in order to test the model against a practical engineering case under many different regular wave conditions.

The nuclear community has coupled several three-dimensional Computational Fluid Dynamics (CFD) solvers with one-dimensional system thermal-hydraulic (STH) codes. This work proposes to replace the CFD solver by a reduced order model (ROM) to reduce the computational cost. The system code RELAP5-MOD3.3 and a ROM of the finite volume CFD solver OpenFOAM are coupled by a partitioned domain decomposition coupling algorithm using an implicit coupling scheme. The velocity transported over a coupling boundary interface is imposed in the ROM using a penalty method. The coupled models are evaluated on open and closed pipe flow configurations. The results of the coupled simulations with the ROM are close to those with the CFD solver. Also for new parameter sets, the coupled RELAP5/ROM models are capable of predicting the coupled RELAP5/CFD results with good accuracy. Finally, coupling with the ROM is 3-5 times faster than coupling with the CFD solver.

Multiphase flows are commonly found in chemical engineering processes such as distillation columns, bubble columns, fluidized beds and heat exchangers. Physical boundaries in numerical simulations of multiphase flows are generally defined by a mesh that conforms to the physical boundaries of the system. Depending on the complexity of the physical system, generating the conformal mesh can be time-consuming and the resulting mesh could potentially contain a large number of skewed elements, which is undesirable. The diffuse-interface approach allows for a structured mesh to be used while still capturing the desired solid-fluid boundaries. In this work, a diffuse-interface method for the imposition of physical boundaries is developed for two-fluid incompressible flow systems. The diffuse-interface is used to define the physical boundaries and the boundary conditions are imposed by blending the conservation equations from the two-fluid model with that of the solid. The results from the diffuse-interface method and mesh-defined boundaries are found to be in good agreement at small diffuse-interface widths.

Direct numerical simulations (DNS) are performed to study the turbulent shear flow of an electrically conducting fluid in a cylindrical container. The flow is driven by the interaction between the radial electric currents ( I ) injected through a large number of small electrodes at the bottom wall and an axial magnetic field. All the numerical parameters, including the geometry of the container, the total injcected currents and the magnetic field, are in line with the experiment performed in J. Fluid Mech. 456, 137-159. First, witth laminar Hartmann layers, three dimensional simulations recover experimentally measured quantities (global angular momentum, velocity profiles). The variation laws for the wall shear stresses, the energy spectra and visualizations of flow structures near the side wall highlight separation and turbulence within the side wall layers. Furthermore, a parametric analysis of the flow reveals that Ekman recirculations have significant influence on the vortex size, the free shear layer, and the global dissipation. Second, we recover the scaling laws of the cutoff scale that separate the large quasi-two-dimensional scales from the small three-dimensional ones (J. Fluid Mech. 118, 507-518), and thus establish their validity in sheared MHD turbulence. Furthermore, we find that three-componentality are and the three-dimensionality appear concurrently and that both the two-dimensional cutoff frequency and the mean energy associated to the axial component of velocity scale with N t , respectively as 0.063 N 0.37 t and 0.126 N ??.92 t .

Many materials, processes, and structures in science and engineering have important features at multiple scales of time and/or space; examples include biological tissues, active matter, oceans, networks, and images. Explicitly extracting, describing, and defining such features are difficult tasks, at least in part because each system has a unique set of features. Here, we introduce an analysis method that, given a set of observations, discovers an energetic hierarchy of structures localized in scale and space. We call the resulting basis vectors a "data-driven wavelet decomposition". We show that this decomposition reflects the inherent structure of the dataset it acts on, whether it has no structure, structure dominated by a single scale, or structure on a hierarchy of scales. In particular, when applied to turbulence---a high-dimensional, nonlinear, multiscale process---the method reveals self-similar structure over a wide range of spatial scales, providing direct, model-free evidence for a century-old phenomenological picture of turbulence. This approach is a starting point for the characterization of localized hierarchical structures in multiscale systems, which we may think of as the building blocks of these systems.

In fully-developed pressure-driven flow, the spreading of a dissolved solute is enhanced in the flow direction due to transverse velocity variations in a phenomenon now commonly referred to as Taylor-Aris dispersion. It is well understood that the characteristics of the dispersion are sensitive to the channel's cross-sectional geometry. Here we demonstrate a method for manipulation of dispersion in a single rectangular microchannel via controlled deformation of its upper wall. Using a rapidly prototyped multi-layer microchip, the channel wall is deformed by a controlled pressure source allowing us to characterize the dependence of the dispersion on the deflection of the channel wall and overall channel aspect ratio. For a given channel aspect ratio, an optimal deformation to minimize dispersion is found, consistent with prior numerical and theoretical predictions. Our experimental measurements are also compared directly to numerical predictions using an idealized geometry.

This study outlines a model for injected fluid flow in a vertically confined porous aquifer with mechanical dispersion. Existing studies have investigated the behaviour and geometry of immiscible fluid flow in this setting, where the injected fluid displaces the resident fluid, forming a sharp interface between the two. The present study extends analytical solutions to include mechanical dispersion of the interface. The solutions are inverted to solve for time as a function of position (r,z), giving each position in the aquifer an intersection time corresponding to the moment the travelling interface intersects a point of interest. The set of (r0,z0) positions which share an intersection time are treated as dummy variables that represent an 'effective surface' and are integrated over to solve for the velocity field within the aquifer. Using this velocity field, the concentration profile resulting from mechanical dispersion can be found this http URL is shown that the concentration of the injected fluid smoothly decays around the position of the interface from immiscible solutions, allowing for the injected fluid to be present in detectable quantities beyond the extent of these interfaces. This concentration spread should be considered in defining outer boundaries on fluids in injection well applications such as carbon capture and storage or groundwater applications.

Scaling and structural evolutions are contemplated in a new perspective for turbulent channel flows. The total integrated turbulence kinetic energy remains constant when normalized by the friction velocity squared, while the total dissipation increases linearly with respect to the Reynolds number. This serves as a global constraint on the turbulence structure. Motivated by the flux balances in the root turbulence variables, we also discover dissipative scaling for u2 and v2, respectively through its first and second gradients. This self-similarity allows for profile reconstructions at any Reynolds numbers based on a common template, through a simple multiplicative operation. Using these scaled variables in the Lagrangian transport equation derives the Reynolds shear stress, which in turn computes the mean velocity profile. The self-similarities along with the transport equations render possible succinct views of the turbulence dynamics and computability of the full structure in channel flows.