Numerical Simulations of Internal Solitary and Solitary-Like Waves: Wave Interactions and Instabilities

Internal solitary and solitary-like waves (ISWs) are commonly observed in stably stratified fluids such as the Earth's atmosphere and oceans. As these waves interact with other physical processes and/or move through a varying background environment, they may change their form and possibly become unstable. In this thesis, we study ISWs using high-resolution direct numerical simulations and address three major topics: the interaction of ISWs with short waves, the onset of shear instability in ISWs, and the dynamics of ISWs in a shear background current induced by basin-scale standing internal waves. The first topic examines the behavior of short internal waves as they propagate through large-amplitude ISWs. A key finding is that for waves that are short in comparison to the ISW width, the interaction leads to an almost complete destruction of the short waves, but that longer waves are able to maintain their structure after the interaction. The destruction of short waves occurs primarily due to the velocity shear induced by the ISW, which alters the vertical structure of the short waves so that significant wave activity is found only on the upstream side of the ISW crest (i.e. the deformed pycnocline). These results suggest that through the interaction with waves of relatively smaller length scale, ISWs can provide a means to decrease the power observed in the short-wave band in the coastal ocean. The second topic focuses on the onset and growth of shear instability in ISWs, which is particularly important for the diapycnal mixing in open waters. The complexity of instability onset in ISWs is due to the finite length and the non-parallel structure of the wave-induced high-shear region. We examine large-amplitude ISWs with a flat crest and show that, depending on the ratio of the length of high shear region and the width of the wave, there are cases in which instability can occur spontaneously, cases in which its onset is Reynolds number dependent, and cases in which instability does not occur spontaneously but must be triggered by small, but finite amplitude noise. The amplitude of the noise has a crucial influence on the instability growth, regardless of its spatial structure. In the final topic we study the effect of a shear background current on the dynamics of ISWs by investigating the interaction of ISWs with basin-scale standing internal waves. The ISWs are generated using a lock-release mechanism, while the seiches are created using a tilted tank suddenly returned to the upright position, both of which are readily realizable in a laboratory. In most cases, the wave forms of ISWs in the simulations match those described by the fully nonlinear theory, implying that in laboratory experiments ISWs propagating in a shear background current can be generated in a similar manner. In some circumstances, however, the presence of a shear background current prevents the formation of ISWs, but enables the formation of a finite amplitude dispersive wave train, even when the pycnocline center is not close to the mid-depth

Numerical Simulations of Shoaling Internal Solitary Waves of Elevation

We present high-resolution, two- and three-dimensional direct numerical simulations of laboratory-scale, fully nonlinear internal solitary waves of elevation shoaling onto and over a small-amplitude shelf. The three-dimensional, mapped coordinate, spectral collocation method used for the simulations allows for accurate modelling of both the shoaling waves and the bottom boundary layer. We focus on wave-induced instabilities during the shoaling and de-shoaling processes. The shoaling of the waves is characterized by the formation of a quasi-trapped core which undergoes a spatially growing stratified shear instability at its edge and a lobe-cleft instability in its nose. Both of these instabilities develop and three-dimensionalize concurrently, leading to strong bottom shear stress. During the de-shoaling process, the core breaks up and ejects fluid that forms a vortex-rich region near the down-sloping portion of the shelf. The flow in this region is highly turbulent and the bottom shear stress is extremely strong. Experiments with a corrugated bottom boundary are also performed. Boundary layer separation is found inside each of the corrugations during the wave’s shoaling process. Our analyses suggest that all of these wave-induced instabilities can lead to enhanced turbulence in the water column and increased shear stress on the bottom boundary. Through the generation and evolution of these instabilities, the shoaling and de-shoaling cycles of internal solitary waves of elevation are likely to provide systematic mechanisms for material mixing and sediment resuspension. These mechanisms have significant environmental implications on the near-coastal regions of the world’s oceans

Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

Reduced-order representation of stratified wakes by proper orthogonal decomposition utilizing translational symmetry

Visualizations of reduced-order representations of stratified wakes of Reynolds number Re∈{5,25,100}×103 are presented at a fixed internal Froude number. The reduced-order representations are constructed by applying proper orthogonal decomposition (POD) to numerical datasets that are high-resolution, three-dimensional and time-dependent. Due to the transient nature of the flow, the dynamics to be represented are highly non-stationary, posing a challenge for the effectiveness of POD. The translational symmetry inherent in the computational configuration is utilized for the POD analysis. This technique turns out to be effective in terms of improving the convergence of energy content represented by the POD modes and enhancing the interpretability of the temporal dynamics. Individual POD modes representing distinct dynamics of various scales are visualized. In the turbulent region, visualizations of the reconstructed vertical vorticity fields suggest that the dominant length scale of flow structures decreases with the modal index. For internal wave motions, visualizations of the reconstructed vertical velocity fields show the opposite trend, as the wavelength of internal waves observed in the wake’s ambient increases with the modal index. The temporal coefficients for a given mode are observed to vary minimally between Re=2.5×104 and 105, suggesting a potential asymptote of the large-scale temporal dynamics in terms of Reynolds number.Natural Sciences and Engineering Research Council (NSERC

An Aerodynamic Load Correction Method for HFFB Technique Based on Signal Decoupling and an Intelligent Optimization Algorithm

In high-frequency force balance (HFFB) wind tunnel tests, the aerodynamic wind loads at the base of the building model are usually amplified by the model-balance system. This paper proposes a new method for eliminating such an amplification effect. Firstly, the measured base bending moment signals are decoupled into independent components. Then, an optimization model is established to represent the problem of identifying the natural frequencies and damping ratios for the different modes of the model-balance system. Finally, the genetic algorithm (GA) is employed to seek the solution to the optimization problem, and the base bending moment is corrected through the identified dynamic parameters of the model-balance system. Compared to the conventionally used knocking method, the proposed method requires no extra knocking tests and can take the aerodynamic damping of the model-balance system into account. An engineering case, the Guangzhou East Tower (GZET), is taken as an example to show the effectiveness of the method

Characteristics and Development Trends of Ocean Scientific Drilling: Comparative analysis based on the scientific framework of International Ocean Discovery Plan

Scientific ocean drilling program is the longest and most effective international scientific cooperation program in the field of earth science. Since 1968, it has experienced four stages: Deep Sea Drilling Project, Ocean Drilling Program, Integrated Ocean Drilling Program and International Ocean Discovery Program. Entering the International Ocean Discovery Program, the IODP Working Group released the scientific framework twice, and this paper provided a comparative analysis of the two scientific frameworks. The characteristics and development trend of scientific ocean drilling were to persist in solving scientific problems, expand from the earth to the universe, attach importance to technology and big data, and expand social influence and publicity. This had provided enlightenment for China to launch the international big science program. Attention should be paid to scientific and technological innovation and big data and other advanced technologies that would be used firstly, to strengthen scientific management and promote openness and sharing secondly, and pay attention to enhancing influence and expanding publicity and influence finally

Photocatalytic degradation of gaseous benzene with H3PW12O40/TiO2/palygorskite composite catalyst

H3PW12O40/TiO2/palygorskite composite photocatalysts were prepared by the sol–gel method and characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and N2 adsorption–desorption measurement. These results indicated that the self-made composite photocatalysts had excellent catalytic performance of degradation of gaseous benzene. In the case of the benzene concentration at 100 mg/m3, over the composite catalysts H3PW12O40/TiO2/palygorskite with mass ratio of 1:5:5 calcined at 350 °C, reached 96.3% after 210 min UV irradiation. The gaseous benzene photo-catalytic degradation was mainly caused by strong oxidizing property of HPW, the TiO2 electron–holes and oxygen synergy effect. The degradation followed pseudo-first-order reaction process and the main products were CO2 and H2O

Magnetron Sputtering Construction of Nano-Al Metallized Wood and Its Functional Research

The magnetron sputtering method was used to deposit nano-Al film on the wood surface of Pinus sylvestris L. var. mongholica Litv., and the material structure, electrical conductivity, mechanical properties and wetting properties were tested and characterized. When the sputtering time was 60 min, the average cross-grain sheet resistance of metallized wood was 695.9 mΩ, and the average along-grain sheet resistance was 227.2 mΩ. Load displacement decreased by more than 88%,elastic modulus increased by 49.2 times, hardness increased by 46 times andsurface hydrophobic angle was close to 130°. The grain size of the Al film on the wood surface was presented as nanoparticles, and the arrangement was uniform and dense. The results indicate that without any burden on the environment, the use of magnetron sputtering can quickly and efficiently achieve Al metallization on wood surfaces, so that the wood surface can obtain conductivity and hydrophobic properties. The elastic modulus and hardness of the wood surface were improved, the mechanical properties of the wood were effectively improved and the functional improvement of the wood was realized. This study provides a feasible method and basis for the study of the simple, efficient and pollution-free modification of wood

Effects of Seasonal Ice Coverage on the Physical Oceanographic Conditions of the Kitikmeot Sea in the Canadian Arctic Archipelago

The Kitikmeot Sea is a semi-enclosed, east–west waterway in the southern Canadian Arctic Archipelago (CAA). In the present work, the ice conditions, stratification, and circulation of the Kitikmeot Sea are diagnosed using numerical simulations with a 1/12° resolution. The physical oceanographic conditions of the Kitikmeot Sea are different from channels in the northern CAA due to the existence of a substantial ice-free period each year. The consequences of such ice conditions are twofold. First, through fluctuations of external forcings, such as solar radiation and wind stress, acting directly or indirectly on the sea surface, the seasonal ice coverage leads to significant seasonal variations in both stratification and circulation. Our simulation results suggest that such variations include freshening and deepening of the surface layer, in which salinity can reach as low as 15 during the peak runoff season, and significantly stronger along-shore currents driven directly by the wind stress during the ice-free season. The second consequence is that the sea ice is not landfast but can move freely during the melting season. By analyzing the relative importance of thermodynamic (freezing and/or melting) and dynamic (ice movement) processes to the ice dynamics, our simulation results suggest that there is a net inflow of sea ice into the Kitikmeot Sea, which melts locally each summer. The movement of sea ice thus provides a significant freshwater pathway, which contributes approximately 14 km3 yr−1 of fresh water to the Kitikmeot Sea, on average, equivalent to a third of freshwater input from runoff from the land