Impact of osmotic pressure on the stability of Taylor vortices

We use linear stability analysis and direct numerical simulations to investigate the coupling between centrifugal instabilities, solute transport and osmotic pressure in a Taylor–Couette configuration that models rotating dynamic filtration devices. The geometry consists of a Taylor–Couette cell with a superimposed radial throughflow of solvent across two semi-permeable cylinders. Both cylinders totally reject the solute, inducing the build-up of a concentration boundary layer. The solute retroacts on the velocity field via the osmotic pressure associated with the concentration differences across the semi-permeable cylinders. Our results show that the presence of osmotic pressure strongly alters the dynamics of the centrifugal instabilities and substantially reduces the critical conditions above which Taylor vortices are observed. It is also found that this enhancement of the hydrodynamic instabilities eventually plateaus as the osmotic pressure is further increased. We propose a mechanism to explain how osmosis and instabilities cooperate and develop an analytical criterion to bound the parameter range for which osmosis fosters the hydrodynamic instabilities.

Activity-induced instabilities of brain organoids

We present an analytical and numerical investigation of the activity-induced hydrodynamic instabilities in model brain organoids. While several mechanisms have been introduced to explain the experimental observation of surface instabilities in brain organoids, the role of activity has been largely overlooked. Our results show that the active stress generated by the cells can be a, previously overlooked, contributor to the emergence of surface deformations in brain organoids.

Shock-induced dual-layer evolution

Shock-induced fluid-layer evolution has attracted much attention but remains a challenge mainly because the coupling between layers remains unknown. Linear solutions are first derived to quantify the layer-coupling effect on the shocked dual-layer evolution. Next, the motions of the waves and interfaces of a dual layer are examined based on the one-dimensional gas dynamics theory. Shock-tube experiments on the dual-layer, single-layer and single-mode interface are then performed to validate the linear solutions and investigate the reverberating waves inside the layers. It is proved that the layer-coupling effect destabilises the dual layer, especially when the initial layers are thin, and the reverberating waves impose additional instabilities on all interfaces. Our findings suggest that a slow/fast configuration with a large thickness in a dual layer can facilitate the suppression of hydrodynamic instabilities.

Designing Multi-Shape Dual-Gas Initial Conditions for the Study of Hydrodynamic Instabilities

The flow resulting from the rotation of a series of thin plates that initially separate two gases of different densities is analysed using Direct Numerical Simulations. The ninety degrees plates' rotation forms a vorticity shear layer and a density interface in between the tips of two neighbouring plates. Results of this study show that the shape of these layers strongly depends on the plate tip-based Reynolds number that can be varied thanks to a parametrisation of the plates' opening law. Different regimes are identified corresponding to single- or multi-mode initial interfaces, with or without the occurrence of starting vortices during the formation of the shear layer. The density interfaces resulting from this procedure are particularly well-suited to serve as initial conditions for the study of the Richtmyer-Meshkov instability-induced mixing. Results of this study also provide a description of vortex formation in stratified flows.

Analysis of Premixed Laminar Combustion of Methane With Noble Gases as a Working Fluid

Hydrodynamic and diffusional-thermal instabilities affect the flame dynamics, which result in non-planar flame fronts with self-accelerating cellularities and wrinkles. In premixed flames, the driving mechanism for perturbations is hydrodynamic instabilities, which are associated with thermal expansion. Under high-pressure conditions, such as in spark-ignition engines, the flame curvature and morphology might be influenced by the hydrodynamic instabilities. This study focuses on the replacement of nitrogen with a noble gas (argon and krypton) as the working fluid in the premixed combustion of methane to investigate its effect on flame stability and dynamics. The utilization of noble gases can also enhance the ideal thermal efficiency of internal combustion engines due to the higher specific heat ratio they possess and may also reduce the NOx emissions markedly because of the lack of nitrogen in the working fluid. The experiments are conducted for various equivalence ratios (φ = 0.8, 1.0, 1.2) in a constant volume combustion chamber (CVCC) at atmospheric and elevated initial pressures and atmospheric temperature. As an outcome of this study, to understand the influence of krypton on methane combustion, spherically propagating flames are analyzed in terms of the laminar flame burning velocity, cellular instability, unburned gas Markstein length, and flame morphology utilizing a Z-type Schlieren optical diagnostic technique and fractal analysis, which is a promising approach to analyze flame surfaces. The fractal dimension of the flame fronts is calculated by a box-counting algorithm. The results are compared against the previously examined case studies in which argon was used as the primary working fluid.

Experimental Investigations on the Effect of a Wavy Surface on Hydrodynamic Instabilities in a Taylor-Couette System

In this work, we propose an experimental study of the effect of surface roughness of the internal cylinder Couette-Taylor system in order to investigate the hydrodynamic instabilities of the flow. During experiments, the inner cylinder, which presents a rough surface with u cylinder corrugations, rotates at a given angular speed and the outer cylinder, which is smooth, is kept fixed. The main objective of the study is to demonstrate the effect of geometric parameters on the flow (the shape of the roughness). Experimental results have shown that the shapes of the surface irregularities have an effect on the appearance of the first instabilities, which strongly depend on the size, shape and nature of the roughness. In fact, the nature of surface roughness not only affects the friction on the wall, but also strongly influences the transport of mass and momentum in a given flow regime. The flow therefore evokes more friction when the inner (rotating) cylinder has a rough surface. This friction, which slows the speed of the fluid particles, strongly depends on the surface nature in contact with the fluid. The movement of the particles in these irregularities will therefore, be damped as a function of the shape of the roughness. In addition, the results also showed that once Couette-Taylor vortices are present, surface roughness can promote continued flow disturbance. The resulting flow then becomes less slow in the troughs of surface irregularities; thus, leads to less friction.

Validation of models for shock-driven hydrodynamic instabilities

The propagation of fluids through space-time is a truly beautiful and mysterious marvel that humankind has spent nearly all our existence trying to comprehend, understand, manipulate, and master. From waves over water to the Sun and the stars in the sky; fluids prove to be as elementary as they are esoteric, as calming as they are chaotic, and as delicate as they are detrimental. The levity in which fluids propagate can be as swift as the milliseconds it takes to observe hydrodynamic instability in say a shock tube facility, to the hundreds if not thousands of years over which a cosmological event's hydrodynamic instability may evolve. Comprehending, studying, manipulating, and mastering the propagation of fluids, specifically within the realm of fluid mechanics, s.c., hydrodynamic instability (HI), is of paramount prominence to the success of humankind. Today, a group of personnel within the scientific and academic community study the evolution and propagation of hydrodynamic instabilities (HIs) through a vast multitude of avenues for a plethora of applications; the two main avenues being experimentally and computationally. However, the ability to experimentally generate, for example, Asymptotic giant branch (AGB) star within a laboratory is as unattainable as the multiple lifetimes for its hydrodynamic instabilities take to develop and evolve, and study. The necessity of generating numerical simulations which match the experimental results of the growth and morphological evolution of hydrodynamic instabilities is a perfectly idealized way to address the capacious and enduring time scales of the hydrodynamic instabilities mentioned. The goal of this dissertation work is to compare the numerical results of the evolution of HIs with experimental results, generate qualitative and quantitative analyses of how the results differ, and improve upon the numerical methods in which the simulation results are generated. To achieve the goal of this dissertation, the evolution and morphology of the two-dimensional hydrodynamic Shock-Driven Multiphase Instability (SDMI) is investigated through experimental measurements obtained within a shock tube facility. The experimental results are then used to validate the results achieved through simulations which utilize identical initialized parameters to model the experiment. The simulations were performed in the open source software FLASH, which is employed to solve the Multi-Phase Particle-in-Cell (MP-PIC) method with the Piecewise Parabolic Method (PPM) for the SDMI's multispecies gas flow. To gather data on the SDMI's morphological evolution experimentally, the planar laser Mie scattering (PLMS) technique was used to illuminate a cylindrical particle-laden flow field (interface), in 2-D, where high-resolution charged-coupled device (CCD) camera captured cross-sectional images of the interface's evolution. The gas flow itself consisted of a mixture of three different species: nitrogen, air, and water vapor; while the dispersed phase consists of water droplets in gas mixture. Utilizing a Mach number, M [subscript alpha] of 1.67, equivalent to a shock wave velocity, v [subscript sh] of 570 (ms [superscript -1]), data was obtained for two different effective Atwood numbers (particles concentrations), A [subscript t] of 0.0479 and 0.0184, at three time intervals for comparison of the experimental data to the computationally acquired data. The results obtained from the computational and experimental data show good quantitative agreement. For example, average dispersed phase speed measured experimentally is 99.5 [percent] of average calculated speed numerically, also, shape wise numerical distance between two developed vortices in dispersed phase is 93.5 [percent] of those measured experimentally. Qualitatively, the morphology of the dispersed phase shows same evolution in both simulated and experimental results. SDMI can also be seen in the circumstellar medium with the infinite number of morphologies due to the complexity of the hydrodynamics evaluations near AGB stars. An attractive solution shows the pulsation of the AGB star producing hot bubble combined with a shock wave and then interacting with dust shell making different types of instabilities.