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## Scholarly Works (618 results)

Throughout their life, plants typically remain in one location utilizing sunlight for the synthesis of carbohydrates, which serve as their sole source of energy as well as building blocks of a protective extracellular matrix, called the cell wall. During the course of evolution, plants have repeatedly adapted to their respective niche,which is reflected in the changes of their body plan and the specific design of cell walls. Cell walls not only changed throughout evolution but also are constantly remodelled and reconstructed during the development of an individual plant, and in response to environmental stress or pathogen attacks. Carbohydrate-rich cell walls display complex designs, which together with the presence of phenolic polymers constitutes a barrier for microbes, fungi, and animals. Throughout evolution microbes have co-evolved strategies for efficient breakdown of cell walls. Our current understanding of cell walls and their evolutionary changes are limited as our knowledge is mainly derived from biochemical and genetic studies, complemented by a few targeted yet very informative imaging studies. Comprehensive plant cell wall models will aid in the re-design of plant cell walls for the purpose of commercially viable lignocellulosic biofuel production as well as for the timber, textile, and paper industries. Such knowledge will also be of great interest in the context of agriculture and to plant biologists in general. It is expected that detailed plant cell wall models will require integrated correlative multimodal, multiscale imaging and modelling approaches, which are currently underway.

Wakes of bluff bodies in a stratified environment are common in oceanic and atmospheric flows. Some examples are marine swimmers, underwater submersibles and flow over mountains and islands. The first part of the research in stratified wakes concerns temporal/spatial simulations of turbulent self-propelled/towed wakes without including a body. Direct numerical simulations are performed to contrast the influence of the mean velocity profile with that of the initial turbulence on the subsequent evolution of velocity and density fluctuations in a stratified self-propelled wake. It is also verified that results of temporal simulations matches with that of the spatial simulations when the initial near-wake condition of the temporal approximation is chosen to match the inflow of the spatially evolving model. Typically, the wake of a body develops in the presence of external fluctuations, motivating a study of wake evolution under the influence of various intensities of external turbulence. The stratified wake was found to decay substantially faster than its unstratified counterpart for same intensity of the external turbulence. Theoretical arguments and additional simulations were performed to show that the level of external turbulence relative to wake turbulence is a key governing parameter in both stratified and unstratified backgrounds.

The second part of this research focuses on flow past a sphere in a stratified fluid at a sub-critical Reynolds number of 3,700 and for a range of Froude numbers U/ND \in [0.025,1]. The conservation equations are solved in a cylindrical coordinate system and an immersed boundary method is employed to represent the sphere. The prime objective of this investigation is to understand the statistical response of the near, intermediate and far wake of a sphere at sub-critical Re under the influence of buoyancy. It is observed that buoyancy leads to the inhibition of vertical motion resulting in faster decay of r.m.s. velocity in the vertical direction as compared to the horizontal r.m.s. velocity, collapse of the wake, propagation of internal gravity waves and the organization of the primarily horizontal flow into coherent vortical structures. Unprecedented with respect to previous studies, the time averaged turbulent kinetic energy budget is closed for the unstratified and stratified cases. A novel finding of this research is the regeneration of turbulent fluctuations in the near wake when the stratification increases beyond a critical level (Fr decreases beyond a critical value) which is in contrast to the previous results at lower Re that suggest monotone suppression of turbulence with increasing stratification. Vorticity evolution, energy spectra and the turbulence energy equation explain turbulence regeneration. Another objective of this study is to quantify the distinction between the body and turbulence generated internal waves, in terms of the amplitude, frequency, potential energy distribution and propagation angles. With a decrease in Fr, the body generation mechanism become stronger and waves exhibit upstream propagation.

The dissertation investigates buoyancy effects in turbulent bluff-body wakes that evolve in stratified fluids. The investigation utilizes high-resolution numerical simulations and employs a body-inclusive approach to describe the flow from the body into the far wake unlike the usual temporal-model approximation of most prior stratified-wake simulations. The dissertation is composed of three main parts. The first part focuses on the dynamics of vorticity that accounts for the unexpected regeneration and increase of turbulence in the near-to-intermediate wake when stratification increases in the regime of low body Froude numbers. The second part characterizes buoyancy effects on the evolution of turbulent kinetic energy in a sphere wake at moderate Froude number and an intermediate Reynolds number. The third part concerns the decay of a disk wake at relatively high Reynolds number and a wide range of Froude numbers, constitutes the major contribution of this thesis, and is summarized below.

Large-eddy simulations (LES) of flow past a disk are performed at Re = UbLb/ν = 50,000 and at Fr = Ub/NLb = ∞,50,10,2; Ub is the free-stream velocity, Lb is the disk diameter, ν is the fluid kinematic viscosity, and N is the buoyancy frequency.

In the axisymmetric wake in a homogeneous fluid, it is found that the mean streamwise velocity deficit (U0) decays in two stages; U0 ∝ x−0.9 during 10 < x/Lb < 65 followed by U0 ∝∼ x−2/3. Consequently, none of the simulated stratified wakes is able to exhibit the classical 2/3 decay exponent of U0 in the interval before buoyancy effects set in. The turbulent characteristic velocity, taken as K1/2 with K the turbulent kinetic energy (TKE), satisfies K1/2 ∝∼ x−2/3 after x/Lb ≈ 10. Turbulent wakes are affected by stratification within approximately one buoyancy time scale (Ntb ≈ 1) after which, provided that RehFrh2 ≥ 1, we find 3 regimes: weakly stratified turbulence (WST), intermediately stratified turbulence (IST), and strongly stratified turbulence (SST). The regime boundaries are delineated by the turbulent horizontal Froude number Frh = u′h/NLHk; here, u′h and LHk are r.m.s horizontal velocity and TKE- based horizontal wake width. WST begins when Frh decreases to O(1), spans 1 < Ntb < 5 and, while the mean flow is strongly affected by buoyancy in WST, turbulence is not. Thus, while the mean flow transitions into the so-called non-equilibrium (NEQ) regime, turbulence remains approximately isotropic in WST. The next stage of IST, identified by progressively increasing turbulence anisotropy, commences at N tb ≈ 5 once F rh decreases to O(0.1). During IST, the mean flow has arrived into the NEQ regime with a constant decay exponent, U0 ∝ x−0.18, but turbulence is still in transition. The exponent of 0.18 for the disk wake is smaller than the approximately 0.25 exponent found for the stratified sphere wake. When F rh decreases by another order of magnitude to F rh ∼ O(0.01), the wake transitions into the third regime of SST that is identified based on the asymptote of turbulent vertical Froude number (Frv = u′h/Nlv) to a O(1) constant. During SST that commences at Ntb ≈ 20, turbulence is strongly anisotropic (u′z ≪ u′h), and, both u′h and U0 satisfy x−0.18 decay signifying the arrival of the NEQ regime for both turbulence and mean flow. Turbulence is patchy and temporal spectra are broadband in the SST wake.

Energy budgets reveal that stratification has a direct and positive influence on the prolongation of wake life. During the WST/early-IST stage, energy budgets show that the mean buoyancy flux acts to augment the MKE before the additional augmentation by reduced turbulent production. On the other hand, during WST/early-IST, the decay of TKE is faster than the unstratified case because of negative buoyancy flux (a sink that serves to increase turbulent potential energy) and increased dissipation and, additionally, also by the reduced production. In the late-IST/early-SST stages, production is enhanced and, additionally, there is injection from turbulent potential energy so that the TKE decay slows down. Only in the SST stage, when NEQ is realized for both the mean and turbulence, the turbulent buoyancy flux becomes negative again, acting as a sink of TKE.

Mixing from turbulence is key to the distribution of oxygen, salt, and heat in the ocean. Climate models which do not appropriately represent this mixing cannot accurately interpret present or future climate. Topographic features with steep slope on the ocean bottom are sites of significant energy conversion from the oscillating tide to internal waves. Such sites can also host intense turbulence and reportedly are the primary source of deep ocean mixing. In this research, We investigate the internal wave dynamics, and magnitude and spatial distribution of turbulence at realistic topographies as well as isolated model obstacles using three-dimensional, high-resolution numerical simulations to develop physical parameterizations of conversion and dissipation rates in the near-field. Direct Numerical Simulations (DNS) and Large Eddy Simulation (LES) are performed on model ridges and realistic ocean topographies to investigate the effect of topographic and flow properties such as Reynolds number (Re), excursion number (Ex) and criticality (e) on internal wave fields, turbulence mechanisms and energy budget terms. These simulations close the energy budget, match with observations and illustrate significant local energy loss generated from mechanisms including Lee waves breaking during flow reversal, downslope jets, critical slope boundary layer, internal wave beams, off-slope lee-wave breaking, and valley flows.\

The physical scales of processes driving mixing during the internal waves generation in the ocean spans several orders of magnitude from the outgoing low-mode internal tide (vertical scale of order 1 km, horizontal of order several tens of km, time of order hours) to the nonlinear formation of higher wavenumber modes to, finally, turbulence events with spatial scale of order meters and time scale of order minutes. This range of scales poses a severe constraint on realistic simulations. I am involved in development of a comprehensive multiscale tool with a novel hierarchical approach that combines Large Eddy Simulation (LES) at small scales with the Stratified Ocean Model with Adaptive Refinement (SOMAR) for the large scales. These simulations are used to assess the accuracy of inferred estimates of turbulent dissipation using density overturn-based methods. This method is commonly used by oceanographer due to the complexity and cost of direct microstructure measurements. Result of this work have shown bias in the magnitude of dissipation at locations with high convectively-driven turbulence. To address this, we have introduced an alternative density overturn-based model for such situation.