Browsing by Author "Zhou, Qi"
Now showing 1 - 20 of 30
Results Per Page
Sort Options
Item Open Access Aerobic Granular Sludge for Treatment of Naphthenic Acids in Semi-Continuous and Batch Modes(2019-01-03) Tiwari, Shubham Sudhindra; Tay, Joo Hwa; Zhou, Qi; Achari, GopalWastewater from the Canadian mining oil sands industry is currently stored in tailings ponds, due to the difficulty in treatment of toxic recalcitrant compounds called naphthenic acids (NAs). The current project aimed at NA treatment using aerobic granular sludge (AGS) in two separate experiments. The first experiment was a proof-of-concept study aimed at assessing the shock response and treatability of commercial NAs over 21 days. It was conducted in three phases, i.e. introduction, starvation and monitoring. Each phase had chemical oxygen demand (COD) removal efficiencies of 54.8%, 23.9% and 96.1%, and NA removal efficiencies of 71.8%, 43.3% and 67.0%, respectively. Specific COD removal rates ranged between 2678 - 6864 g COD/m3/d, whereas specific NA removal rates ranged between 0.5-12.2 g NA/m3/d. These high rates were attributed to higher AGS biomass requiring higher COD consumption, and larger AGS surface area facilitating biodegradation and biosorption. The second experiment subjected mature AGS to three model NA concentrations (10, 50 and 100 mg/L), at three varying supplemental carbon source concentrations (600, 1200 and 2500 mg/L) in batch reactors. Cyclohexane carboxylic acid (CHCA), cyclohexane acetic acid (CHAA) and 1-adamantane carboxylic acid (ACA) were chosen to study structure-based degradation kinetics. The optimal COD was found to be 1200 mg/L. CHCA was removed completely with biodegradation rate constants increasing with lower NA concentrations and lower COD concentrations. CHAA was also removed completely, however, an optimal rate constant of 1.9 d-1 was achieved at NA and COD concentrations of 50 mg/L and 1200 mg/L, respectively. ACA removal trends did not follow statistically significant regressions; however, degradation and biosorption helped remove ACA up to 19.9%. Pseudomonas, Acinetobacter, Hyphomonas and Brevundimonas spp. increased over time, indicating increased AGS adaptability to NAs.Item Open Access Application of Lattice Boltzmann Method for Simulating Stably Stratified Flows past Cylinders(2023-09-17) Maddahi, Navid; Zhou, Qi; Zhou, Qi; He, Jianxun (Jennifer); Chu, AngusThis research looks into the intriguing subject of ambient density-stratified flows, which have long captivated researchers due to their representation of real-world physical phenomena such as diapycnal mixing in oceans driven by environmental influences. The study specifically focuses on the flow past a cylindrical object within such stratified flows, which introduces complexities involving buoyancy and viscous effects. A major focus of this research is the examination of the lattice Boltzmann method as a novel approach to model stratified flows around circular cylinders by solving coupled Navier-Stokes and advection-diffusion equations. The study investigates the impact of stratification on wake characteristics and various flow parameters for a single cylinder at six Reynolds numbers ranging from 10 to 600 and Froude numbers from 2.19 to 7.51. Additionally, the investigation includes the case of two cylinders arranged in tandem at a Reynolds number of 100, with similar Froude numbers. This research demonstrates the suitability and robustness of the lattice Boltzmann method in modeling stratified flows past cylinders. The findings reveal that even moderate levels of stratification can significantly influence the wake pattern, potentially leading to changes in the flow regime. Moreover, the study demonstrates that the introduction of stratification is associated with a reduction in the drag coefficient and shedding frequency, leading to altered flow behaviors. Furthermore, in the case of flow past two cylinders, the presence of stratification increases the critical spacing between the cylinders.Item Open Access Direct simulation of stably stratified wall-bounded turbulence using the lattice Boltzmann method(2023-04-27) Guo, Junwei; Zhou, Qi; Wong, Ron Chik-KwongThe lattice Boltzmann method (LBM) is employed to simulate stratified plane Couette (SPC) flows in their statistically stationary turbulent state. The aim is to assess the suitability of the LBM for direct simulation of wall-bounded, sheared turbulence under the influence of stable stratification. The SPC flow is generated by two parallel plates moving in opposite directions with velocities ± U w, and the buoyancy is fixed at ± b w at the upper and lower plates, respectively. The Reynolds number Re = U w h / ν, where h is the half-gap height, and ν is the kinematic viscosity, varies from 1000 to 3000. The Richardson number Ri = b w h / U w 2 is set to 0 or 0.01. The LBM results are compared to direct numerical simulations using the conventional pseudo-spectral method, and good agreement is found in various turbulence statistics, such as mean and fluctuation velocity and buoyancy, Reynolds stress, turbulent heat flux, dissipation rate, wall fluxes of momentum and heat, and longitudinal and transverse turbulence spectra. The results from grid-sensitivity tests indicate that the uniform isotropic grid spacing Δ x in LBM needs to be no greater than approximately the near-wall viscous length scale δ ν to achieve adequate resolution of stratified wall-bounded turbulence.Item Open Access Direct Simulations of Fluid-Particle Flow in Newtonian and non-Newtonian Fluids Using Coupled Lattice Boltzmann and Discrete Element Methods(2021-09) Guo, Junwei; Wong, Ron Chik-Kwong; Zhou, Qi; Jasso, Martin; Wan, Richard GCoupled lattice Boltzmann and discrete element methods are employed to investigate a suite of fluid-particle flow problems in both Newtonian and non-Newtonian fluids. First, the rheological properties of finer particle suspensions in a Newtonian fluid are investigated, as the particle shape and solid fraction vary. An increase of the relative viscosity is observed when the particles become more oblate, accompanied by an increase of particle contacts and contact distance. Particle reorientation is seen to occur systematically in denser oblate particle suspension subject to the shear flow. A connection between the micro-structure statistics and the suspension viscosity is proposed. Second, the shear rate effects on the finer oblate particle suspension viscosity are studied. The viscosity of the suspension is observed to decrease under a higher shearing rate due to the reduction of the inter-particle friction coefficient, resulting in the shear-thinning behavior of the suspension. Finally, the sedimentation of a granular particle cloud in shear-thinning suspensions is investigated, as the rheology of the suspension, Reynolds number, and particle cloud concentration vary. At higher Reynolds numbers, the particle cloud length grows in the direction of settling and reaches a quasi-steady state. The ratio of the particle cloud quasi-steady settling velocity to the single-particle terminal velocity in the same fluid, increases when the cloud settles in the shear-thinning suspensions. This velocity increase is more significant at a low Reynolds number. At even lower Reynolds numbers, the cloud loses its initial shape and disintegrates while settling, with particles escaping from the cloud due to differential particle settling velocities.Item Open Access Effects of Seasonal Ice Coverage on the Physical Oceanographic Conditions of the Kitikmeot Sea in the Canadian Arctic Archipelago(Taylor and Francis, 2021-08-31) Xu, Chengzhu; Mikhael, Wahad; Myers, Paul G.; Else, Brent; Sims, Richard P.; Zhou, QiThe 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.Item Open Access Effects of volume fraction and particle shape on the rheological properties of oblate spheroid suspensions(AIP Publishing, 2021-08-06) Guo, Junwei; Zhou, Qi; Wong, Ron Chik-KwongCoupled lattice Boltzmann and discrete element methods were employed to investigate the rheological properties of oblate spheroid suspensions in a Newtonian fluid. The volume fraction of the particles is varied along with the particle aspect ratio. As the particle shape is varied from sphere to oblate, we observe an increase in the relative viscosity as well as an increase in the particle contacts and the contact distance. The more oblate particles in denser suspensions are observed to reorient systematically subject to the shear flow. We recast the viscosity data using the Krieger–Dougherty formula and report the modified Einstein coefficients.Item Open Access Evolution of Rayleigh-Taylor instability at the interface between a granular suspension and a clear fluid(American Institute of Physics, 2022-06-19) Guo, Junwei; Zhou, Qi; Wong, Ron C.-K.We report the characteristics of Rayleigh-Taylor instabilities (RTI) occurring at the interface between a suspension of granular particles and a clear fluid. The time evolution of these instabilities is studied numerically using coupled lattice Boltzmann and discrete element methods with a focus on the overall growth rate (σ) of the instabilities and their average wave number (k). Special attention is paid to the effects of two parameters, the solid fraction (0.10{less than or equal to}φ{less than or equal to}0.40) of the granular suspension and the solid-to-fluid density ratio (1.5{less than or equal to}R{less than or equal to}2.7). Perturbations at the interface are observed to undergo a period of linear growth, the duration of which decreases with φ and scales with the particle shear time d/w∞, where d is the particle diameter and w∞ is the terminal velocity. For φ>0.10, the transition from linear to nonlinear growth occurs when the characteristic steepness of the perturbations is around 29%. At this transition, the average wave number is approximately 0.67d-1 for φ>0.10 and appears independent of R. For a given φ, the growth rate is found to be inversely proportional to the particle shear time, i.e., σ ∝(d/w∞)-1; at a given R, σ increases monotonically with φ, largely consistent with a linear stability analysis (LSA) in which the granular suspension is approximated as a continuum. These results reveal the relevance of the time scale d/w∞ to the evolution of interfacial granular RTI, highlight the various effects of φ and R on these instabilities, and demonstrate modest applicability of the continuum-based LSA for the particle-laden problem.Item Open Access Experimental and Numerical Exploration of Electromagnetic-Induced Fracturing of Clay-Shale(2022-09) Chen, Xiaolin; Wan, Richard; Priest, Jeffrey; Wong, Ron; Okoniewski, Michal; Zhou, Qi; Chan, DaveThe rapid electromagnetic (EM) heating of saturated clays and shales offers a potential means of selectively fracturing interbedded shales, and thereby enhancing oil production while maintaining the caprock integrity of shaley oil sand reservoirs during thermal production process. It is hypothesized that fracturing will occur in an extremely low permeability porous medium due to a large increase in pore fluid pressure by electromagnetic wave heating. As opposed to classical resistive heating, the EM heating of saturated clays/shales is mainly achieved by the interaction between radiating electromagnetic waves and polar water molecules. The above provides the backdrop for the thesis which aims at understanding the underlying fracture and thermal stress mechanisms of EM heating in a geomaterial. As such, comprehensive laboratory experiments are conducted in a specially designed EM heating apparatus. Challenges of the experimental endeavor are highlighted, given the susceptibility of measurement sensors with electromagnetic radiation. Four scenarios of tests are performed on kaolinite clay samples, a surrogate for clay-shale, whose basic properties are measured prior to EM heating. The use of kaolinite samples allows for the conduct of repeatable tests and provides a benchmarking for future tests on actual clay-shale core samples. Results of undrained tests at different confining conditions show that the clay will lose its structural integrity when the thermally induced excess pore pressure in the sample is higher than its tensile strength plus confining pressure. Alongside with the above lab experiments, a two phase (water-steam) fully coupled Multiphysics (EM wave propagation/thermal/fluid/mechanics) model is developed to study the major mechanisms of rapid EM heating. The finite element method is employed with a segregated numerical strategy to solve this highly nonlinear problem using a commercial software COMSOL Multiphysics. The simulation results indicate that pore water pressures develop mainly due to the high contrast between the fluid and solid phase coefficients of thermal expansion. Numerically predicted zones of fracture initiations and patterns are verified against experimental observations using a Mohr-Coulomb failure envelope combined with a tension cutoff.Item Open Access Experimental Investigation of Vortex-Induced Vibrations using a Cyber-Physical System(2018-09-11) Riches, Graham Paul; Morton, Chris R.; Bauwens, Luc; Zhou, Qi; Wood, David H.The dynamic response and wake of a circular cylinder undergoing vortex-induced vibrations (VIV) is investigated experimentally using a cyber-physical force-feedback system and particle image velocimetry (PIV) measurements. The effects of the structural mass ratio and mass-damping on the VIV amplitude and frequency response are investigated at a constant Reynolds number by manipulating the structural mass, stiffness and damping in the cyber-physical system. The extent of amplitude modulations in VIV is investigated and a new metric is proposed to quantify the extent of amplitude modulations. A methodology for extracting relevant flow physics associated with coherent structures in the flow is presented for VIV data. Low-order models (LOM) are proposed to describe the dynamics of the wake in each of the VIV branches based on the evolution of the modal temporal coefficients obtained from the proper orthogonal decomposition (POD). The low-order modelling approach is re fined by including a phase-averaging term to better model cases where significant spatial oscillations of the velocity field occur.Item Open Access Finite Element-based Methods for Dent Assessment on Pipelines(2023-12-19) zhao, jian; Cheng, Yufeng(Frank); Li, Leping; Tiamiyu, Ahmed Alade; Zhou, Qi; Chen, ZengtaoAs a well-developed form for energy transportation over wide ranges and long distances, pipelines always encounter various threats in service. In recent years, with the strong demand of clean energy, hydrogen transport in existing pipelines induces new challenges to the pipelines. Dent is a common mechanical defect present on pipelines, compromising structural integrity and causing pipeline failures. To date, there have been limited methods available to assess dent, and a dent combined with other types of defects such as corrosion. In this work, novel methods and criteria were developed for assessment of pipeline dent, corrosion in dent and hydrogen distribution at the dent using finite element (FE) modeling. Denting and spring-back processes were modeled and plain dents were created on the pipeline. A new criterion based on ductile damage failure indicator analysis was proposed. Pressure-bearing capacity was assessed on corroded pipelines containing a dent, where the mutual interaction between corrosion and the dent were determined. In addition, a method was developed to assess the corrosion in dent by considering both mechanical and electrochemical forces. For dented pipelines repurposed for transporting hydrogen gas, a FE-based model was developed to determine the stress/strain and H atom concentrations at the dent, where denting, spring-back and cyclic loading processes were modeled. Furthermore, the hydrogen-induced crack initiation on the pipeline subject to denting process was investigated using the phase field method.Item Open Access Hydrological Frequency Analysis under Nonstationary Conditions(2022-08-03) Vidrio-Sahagún, Cuauhtémoc Tonatiuh; He, Jianxun (Jennifer); Wang, Xin; Zhou, QiThe hydrological frequency analysis (HFA) evaluates the recurrence of extreme flow and precipitation events and guides water-related management and risk assessment. The conventional HFA assumes stationarity in the underlying process. However, climate change and other changes in the watersheds may induce nonstationarity in hydrometeorological variables. Under nonstationary scenarios, the nonstationary HFA (NS-HFA) is the theoretical choice. To implement the NS-HFA with more confidence, further understanding of the impacts of nonstationarity on the analysis and advancements of the existing approaches are desired.This dissertation, therefore, aimed to improve the understanding of the HFA under nonstationarity and advance the NS-HFA applications by: (a) investigating the impacts of ignoring the nonstationarity of different patterns and degrees in the stationary HFA (S-HFA); (b) examining the association between the nonstationarity characteristics and estimated flood hazards; (c) improving the determination of the NS-HFA model by proposing a novel procedure based on the decomposition of nonstationary stochastic processes; (d) enhancing the computational efficiency and numerical stability of the profile likelihood (PL) method, which is theoretically superior to other available methods for quantifying the uncertainty in the NS-HFA; and (e) comprehensively assessing the use of the Metastatistical approach and its simplified version to advance the NS-HFA from the perspectives of fitting efficiency, accuracy, and uncertainty. The results demonstrated that: (a) neglecting the nonstationarity in the S-HFA would lead to decreasing the accuracy and increasing the uncertainty of the analysis; (b) the nonstationarity patterns and degrees are strongly associated with the hydrological hazards; (c) the proposed decomposition-based approach based upon the theoretical decomposition of nonstationary stochastic processes advances the model determination in the NS-HFA from both theoretical and practical perspectives; (d) the proposed methods, which incorporate the classical regula-falsi numerical method and the generalized maximum likelihood principle, effectively reduce the computational burden and numerical instability of the PL method, and consequently facilitate its practical applications; and (e) compared to the NS-HFA based on the generalized extreme value distribution, the use of the simplified Metastatistical approach yields improved performance from various perspectives. Therefore, this dissertation improved the understanding of the HFA and advanced the NS-HFA for real-world applications.Item Open Access Impacts of Weather on Urban Bus Performance in the City of Calgary, Alberta(2023-02-23) Mohammadi, Mohammad; He, Jennifer (Jianxun); Saidi, Saeid; Zhou, Qi; Waters, NigelThere is an extensive body of literature on the impacts of adverse weather on traffic performance and transit ridership; however, only a few research have investigated the impacts of adverse weather on urban bus performance. Traffic congestion and unfavourable road conditions caused by adverse weather directly affect the performance of buses. Also, adverse weather impacts buses indirectly by affecting the passengers. This study aims to evaluate the impacts of adverse weather (rainy and snowy weather) on urban bus performance in the City of Calgary. This research focuses on the impacts of rain and snow on seven bus routes in the City of Calgary. Calgary Transit provided the automatic vehicle location and automatic passenger counter data for 2019-2021. The weather data was supplied by the Calgary International Airport and included daily snowfall and 5-minute rainfall data from 16 rain gauges along the bus routes. Statistical tests and public transit performance measures have been employed in this study to assess the impacts of rainy and snowy weather on the buses. The Man-Whitney test was used to identify significant changes in the median of ridership, run time, dwell time and travel time. In addition, Levene’s test was employed to capture significant changes in the variance of run time, dwell time and travel time. Moreover, six measures from four categories of public transit performance measures (schedule adherence, headway, travel time, and wait time) were used to evaluate the impacts of rainy and snowy weather on bus performance. On-time Performance, coefficient of variation of headway, service regularity, coefficient of variation of travel time, 90th-50th percentile travel time, and excess wait time were all used to determine the effect of rain and snow on bus performance. This study found that there is a negative impact of rainy and snowy weather is definite on bus performance. However, the level of impact varies by route and data groups, which suggests considering other influential factors on the performance of buses along with weather for more detailed results.Item Open Access An Investigation of Heat and Fluid Flow for Overhead Conductor Cables(2021-09-15) Abdelhady, Mohamed; Wood, David H.; Martinuzzi, Robert; Morton, Christopher; Zhou, Qi; Sullivan, Pierre E.The flow dynamics and forced convective heat transfer of stranded cables and circular cylinders were studied, numerically and experimentally, for Reynolds number, Re, of up to 3,900. The effects of Constant Temperature (CT) and Constant Heat Flux (CHF) thermal boundary conditions on forced convection were assessed for circular cylinders. In addition, particle image velocimetry and Large Eddy Simulations (LES) were used to investigate in detail the heat and fluid flow of stranded cables and how they differ from circular cylinders. Besides, the effects of free-stream turbulence on a circular cylinder placed close to a turbulence grid was investigated using LES. The results showed that the thermal boundary condition significantly affects forced convection, contrary to the common assumption; the overall Nusselt number is ~15% higher for CHF boundary condition than the CT. In addition, the results revealed that stranded cable wakes are dominated by alternatively shed Kármán vortices, at a frequency similar to the cylinder. However, the Reynolds stresses, shape factor, and the details of vortex shedding showed substantial alterations associated with the cable strands. Proper orthogonal decomposition and phase averaging were used to triply decompose into mean, coherent, and incoherent components. It is shown that stranded cable coherent fields are different from the cylinder; and that the strands of the cable affect the transport and production of turbulent kinetic energy for the total, coherent, and incoherent fields. The stranded cable surface parameters, like Nusselt number, surface pressure, and skin friction, are altered significantly by the cable strands which are associated with three-dimensional mean flow and heat transfer. The cable gaps have near-stagnant flow, which decreases the local heat transfer rate between the strands, but the overall Nusselt number is only slightly lower than for the cylinder. The cable coherent vorticity and temperature fields were noticeably different from the cylinder. Besides, the coherent and incoherent heat energy transport of the cylinder and stranded cables, showed that, in general, the highest heat energy transport takes place close to the cable rear side. Finally, the cylinder experiencing free-stream turbulence had 43% higher convective cooling relative to non-turbulent free-stream.Item Open Access An Investigation on Methane Flux in Landfills and Correlation with Surface Methane Concentration(2020-04-29) Irandoost, Erfan; Hettiaratchi, Joseph Patrick A.; He, Jianxun; Zhou, QiWith growing concerns over greenhouse gas emissions increase on one hand, and methane’s high global warming potential on the other, direct methane emission measurement techniques from area sources such as landfills are receiving increased scrutiny. The static enclosure chamber method is the only technique that allows direct measurement of landfill gas fluxes. However, due to the large footprint of landfills, as well as the temporal and spatial variability of landfill methane emissions, the static enclosure method may not be the best option under some situations because it is time-consuming and labor-intensive. Collecting surface methane concentration (SMC) data through the instantaneous emission measurement (IEM) technique is relatively easy and inexpensive, however it is merely a qualitative means of evaluating surface methane emissions. This study investigated the development of a relationship between SMCs and methane flux across the soil-atmosphere boundary in a small-scale test cell under a partially controlled environment, in an attempt to translate SMC data into quantitative estimates. In addition, the study investigated the effect of wind speed on surface and flux measurements and the correlation between the two. The results demonstrated a significant positive correlation between SMCs and surface flux measurements. However, a better correlation was achieved when the analysis was performed under calm wind conditions and mild-to-moderate wind conditions separately. Under calm wind conditions, a linear correlation was found between SMCs and flux measurements with a resulting R2 of 0.94 and 0.90 for regression through origin and regression with intercept, respectively. These findings were in agreement with those of various researchers who have suggested surface flux has a positive and, in some cases, strong relationship with methane concentrations. The results also suggested that the presence of wind caused a decrease in average measured flux for the majority of inlet flowrates. It also significantly decreased concentrations measured in the test cell, while shifting the gas to defuse from areas that are further away from the wind source. Under windy conditions, the results of statistical analysis showed that SMCs have a linear correlation with flux divided by wind speed with a resulting R2 of 0.88, and other independent variables were found to be statistically insignificant. This finding was in agreement with the findings of researchers who observed an inverse relationship between SMCs and wind speeds. It is also in line with the Gaussian steady-state dispersion model which shows a direct relationship between SMC and emission rate divided by wind velocity.Item Open Access Large-scale characteristics of stratified wake turbulence at varying Reynolds number(2019-08-09) Zhou, Qi; Diamessis, Peter J.We analyze a large-eddy simulation data set of wakes of a towed sphere of diameter D at speed U in a uniformly stratified Boussinesq fluid with buoyancy frequency N and kinematic viscosity ν. These temporally evolving wakes are simulated using a spectral multidomain penalty-method-based incompressible Navier-Stokes solver for Fr≡2U/ND∈{4,16,64} and Re≡UD/ν∈{5×103,105,4×105, enabling a systematic examination of stratified wakes at three different values of Re sufficiently separated in magnitude. As such, particular attention is paid to the effects of varying Re on the evolution of large-scale characteristics of stratified wake turbulence. We examine the evolution of horizontal and vertical integral length scales (ℓh and ℓv), horizontal and vertical fluctuation velocities (U and W), local vertical shear, as well as the resulting dimensionless parameters based on the above quantities. In particular, the vertical turbulent Froude number Fr★v≡2πU/Nℓv is found to be of order unity, a signature of the dynamics in the strongly stratified regime where shear instabilities develop between anisotropic flow layers. The horizontal turbulent Reynolds number Reh≡Uℓh/ν stays approximately constant in time and the horizontal turbulent Froude number Frh≡U/Nℓh decays in time as (Nt)−1, consistent with scaling analysis of freely decaying turbulence. We characterize the transitions between distinct stratified flow regimes and examine the effects of body-based parameters Re and Fr on these transitions. The transition from the weakly to the strongly stratified regime, which is marked by Fr★v decaying to unity, occurs when Frh≃O(0.01). We further show that the initial value of Reh at which the flow completes the above transition scales as ReFr−2/3, which provides a way to predict the possibility of accessing the strongly stratified regime for a wake of given Re and Fr. The analysis reported here constitutes an attempt to obtain the predictive capability of stratified wake turbulence in terms of Reynolds number Re, applying select elements of strongly stratified turbulence theory, so far typically utilized for homogeneous turbulence, to a canonical inhomogeneous turbulent free-shear flow.Item Open Access Modeling and Analysis of Stably Stratified Wall-Bounded Turbulent Flows(2022-10) Cen, Haoyang; Zhou, Qi; Korobenko, Artem; Wong, Ron Chik-Kwong; Wood, DavidStably stratified wall-bounded turbulent flows have been drawing a great deal of research interest. The profound driver is the underlying physics in the topic that concerns many problems in industry as well as the natural environment, e.g., stratified mixing efficiency in topographically complex boundary regions and closure parameterization in operational atmospheric models. In this dissertation, stratified turbulence in wall-bounded flows is studied and modeled via a series of numerical simulations. Under sufficiently strong stratification, fully developed wall-bounded turbulence could transition to intermittent flows in which laminar and turbulent patches coexist. Using direct numerical simulations (DNS), I explore the boundary at which such a transition occurs in the parameter space for stably stratified channel flow (SCF). A range of friction Reynolds (Reτ ) and Richardson (Riτ ) numbers, parameters that are observed to control the dynamics, are covered by the numerical simulations. For each Reτ investigated, the stratification level is varied incrementally from moderate to relatively strong, leading the fully turbulent flows to transition to intermittently turbulent. My results show that depending on Riτ and Riτ , SCF could exhibit intermittency in the near-wall and/or the channel core. At low-Reτ -high-Riτ , intermittency spans the entire channel depth, whereas at high-Reτ -low-Riτ , intermittency is confined in the channel core. Within the tested range, I identify the near-wall intermittency boundary by quantifying the volume fraction of turbulent patches in each simulation. The applicability of various dimensionless parameters for predicting the onset of near-wall intermittency is examined. My results suggest that near-wall intermittency in SCF occurs for Nusselt number, N u ≲ 3. A first-order closure model based on a K-profile type parameterization is developed for SCF. The model is shown to have good agreement with predicted mean profiles of velocity and potential temperature closely matching their DNS counterparts. The boundary at which near-wall intermittency occurs in SCF is delineated using the developed model based on the critical value N u = 3. The second topic of this dissertation concerns with the development of a computational framework for modeling stably stratified turbulent flows over flat boundaries. I examine the performance of a turbulence modeling framework consisting of residual-based variational multiscale method (RBVMS) and isogeometric analysis (IGA) applied to two canonical numerical experiments, namely stably stratified channel flows at Reτ = 180, 550, and a stable boundary layer (SBL). In the SCF cases, the framework is implemented with two augmentation companion features, namely weak imposition of Dirichlet boundary conditions (WD) and a new subgrid-scale (SGS) model. The performance of the modeling framework, as well as its interaction with the two companion features, are assessed in both weakly and strongly stratified regimes. In comparison to existing direct numerical simulation (DNS) data, my study reveals that RBVMS–IGA framework is able to faithfully capture the flow structures and one-point statistics in SCF simulation with relatively coarse grid resolution. The framework also demonstrates its capability of replicating intermittent flow dynamics under strong stratification. Such dynamics are reproduced robustly when the modeling framework is enhanced with WD and the new SGS model, features that are shown to generally improve numerical accuracy of simulations for the cases tested. My results confirm the computational efficiency as well as the robustness of RBVMS–IGA framework in modeling stratified wall-bounded flows. In addition, we develop a wall-function-based weak imposition of Dirichlet boundary condition (WFWD) for stably stratified flows. The performance of WFWD is validated with SCF at Reτ = 550 and in a stable atmospheric boundary layer, demonstrating its effectiveness and potential in mitigating the effects of under-resolved boundary layer on stratified wall-bounded flow modeling. Comparisons are made against results of the original formulation of WD, as well as direct or large-eddy simulations whenever available. My results show that WFWD with a smooth wall function offers improved accuracy over its WD counterpart in predicting one-point statistics of SCF at various degrees of stratification. Furthermore, on account of adopting a rough wall function WFWD successfully predicts the occurrence of super-geostrophic jet as well as statistics that are in good agreement with highly-resolved large-eddy simulations. My findings suggest that formulating the weak imposition of Dirichlet boundary condition based on wall functions could mitigate shortcomings of WD when factors like roughness play a significant role. The final portion of the dissertation focuses on modeling stably stratified wall-bounded turbulent flows over complex boundaries. The performance of the developed computational framework is validated against observations in a laboratory experiment on strongly stratified flow past a three-dimensional bell-shaped hill. Good agreement is observed for qualitative flow physics, with the predicted occurrences of flow separation, recirculation, and hydraulic jump closely matching those in the experiment. In addition, the dividing-streamline height and the wavelength of lee wave computed from the present framework compare well to theoretical predictions. I show that the present framework is able to tackle various degrees of stratification in wall-bounded flows. The effect of weak imposition of Dirichlet boundary condition on the performance of the framework is also examined. The dissertation is concluded with an outlook toward applying the present framework to modeling stratified flow past real-world terrains at microscale (∼10 m) by simulating stratified flow past a two-dimensional environmental terrain.Item Open Access Modeling and Control of Dynamic Stall Loads on Smart Airfoil(2020-08-26) Mohamed, Ayman Salah Ashry; Wood, David H.; Pieper, Jeffery Kurt; Morton, Chris R.; Ziadé, Paul; Zhou, Qi; Johnson, David AndrewWind turbines operate under challenging inflow conditions, which can affect the turbine loads greatly. The turbine blades subjected to these excessive, fluctuating aerodynamic loads can go into dynamic stall, during which the flow separation is delayed and intense vortex structures are developed. There is no closed-form solution for a dynamic stall, and the present state-of-the-art still shows limitations in modeling and controlling the dynamic stall loads. The present work aims to improve the modeling of dynamic stall loads and to provide simulation tools for mitigating the variation in these loads with the assistance of a trailing edge flap (TEF). A modified version of the extended ONERA dynamic stall model is proposed for predicting unsteady forces, with an emphasis on modeling the effects of the dynamic stall vortex (DSV). The modifications include modeling the chord-axis forces instead of the wind-axis forces that the model was originally developed for. A novel approach for defining the onset of a dynamic stall in the time-marching solution is proposed based on the events taking place during a dynamic stall in the axial force without correlating them empirically. An extensive validation has been implemented on experimental data of different airfoils relevant to wind turbine applications. The results show an excellent correlation with the experimental data, particularly in deep dynamic stall scenarios, during which large fluctuations in the aerodynamic loads appear. The present work included direct, unsteady force measurements on a NACA 643-618 airfoil equipped with TEF (smart airfoil) in both a wind tunnel and water channel. Measurements in the wind tunnel have been used to expand the application of the new dynamic stall model in terms of predicting and controlling the unsteady loads on smart airfoils. The experiment utilizing the water channel investigated the impact of controlling the TEF hinge moment on unsteady loads. In addition, particle image velocimetry (PIV) measurements were carried out to explore the influence of the TEF actuation on the flow field around the smart airfoil. The modified model demonstrates an excellent correlation to the unsteady loads and the associated fluctuation on the smart airfoil despite the change in the effective angle of attack and apparent camber in response to the TEF actuation. The measurements also show that the TEF has a remarkable ability to reduce load variations despite the excessive effort required to control it in the presence of a substantial laminar separation bubble (LSB) and the development of DSVs. Additionally, the results indicate that the TEF hinge moment can be utilized as a localized sensor for unsteady loads and DSV shedding on the smart rotor. Controlling the hinge moment of TEF provides a promising reduction in the variation in the unsteady normal force.Item Open Access Numerical Investigation of Diapycnal Mixing in the Kitikmeot Sea, Canadian Arctic Archipelago(2022-09) Afsharipour, Yasaman; Zhou, Qi; Else, Brent; Huang, WendyThe Kitikmeot Sea, located in the southern Canadian Arctic Archipelago, has particular features distinguishing it from the northern parts of the Archipelago. Substantial ice-free period, massive freshwater input from rivers, limited water exchange due to its surrounding narrow straits and shallow sills, can influence the local ocean dynamics, in particular, the mixing and transport in this sea. In this thesis, diapycnal mixing is investigated by analyzing the output data from a numerical simulation of the Kitikmeot Sea, with 1/12◦ horizontal resolution, during years 2003 to 2019. Mixing strength has been quantified in terms of diapycnal diffusivity values derived from a volume-averaged advection-diffusion equation for fluid density. Spatial and temporal variability of mixing in the Kitikmeot Sea is investigated. Furthermore, the contributions from a number of energy sources to the mixing process have been estimated in order to identify the main driving mechanism for mixing. Investigation of the temporal variability in diffusivity reveals seasonal patterns which can be attributed to the annual cycle of sea-ice coverage. It was found that wind stirring and convection due to sea-ice forming and sea-surface cooling make significant energy contributions to mixing in the Kitikmeot Sea.Item Open Access Numerical investigation of particle cloud sedimentation in power-law shear-thinning fluids for moderate Reynolds number(Elsevier, 2021-09-03) Guo, Junwei; Zhou, Qi; Wong, Ron Chik-KwongA series of numerical simulations are performed for the sedimentation process of a particle cloud in shear-thinning fluids using lattice Boltzmann and discrete element methods. The initial particle concentration, , and the power-law index of the fluid, n, and Reynolds number, , are varied in these simulations. For , the particle cloud size grows in the longitudinal direction as the cloud settles, leading to reduced particle concentration and a quasi-steady settling velocity, . The velocity ratio, , where is the corresponding single-particle terminal velocity, is found to decrease with both n and . This velocity ratio is only weakly dependent on the initial concentration () due to particle dispersion. For , the cloud loses its initial shape and disintegrates while settling, with particles escaping from the cloud due to differential particle settling velocities.Item Open Access Numerical Simulation of Interbedded Shale Failure in Oilsand Reservoirs by Electromagnetic Wave Excitation(2019-09-13) Shi, Longyang; Wan, Richard; Wan, Richard; Wong, Ron Chik Kwong; Zhou, QiThis thesis work offers an exploratory study of selective heating and fracturing of a shale layer embedded into an oilsand material. A three phase (oil-water-steam) thermal-fluid flow formulation enriched with electromagnetic physics is developed to serve as a basic model to investigate whether selective heating is possible, and thereafter verify the mechanism of shale fracturing by solving a reservoir geomechanics problem. The finite element method is employed to solve numerically this highly nonlinear multiphysics (thermal/fluid/wave propagation) problem in porous media following a staggered scheme. This is coined as the EMTH (Electromagnetic-Thermo-Hydro) modeling framework. It has been found that the configuration of EM sources such as density in the form of interval distances between point-dipoles, and most importantly phase angle, control the electromagnetic field pattern and intensity that determine the efficacy of directing heating towards a specified target. The proper characterization of electrical properties of multiphasic-capillary-porous media is another outstanding issue to address for fully understanding the radiation and heating pattern of electromagnetic wave excitation. A synthetic reservoir geomechanics model for verifying the potential of fracturing is constructed and interfaced with the EMTH framework in a loosely coupled fashion. As such, the evolutions of temperature and pore pressure fields under electromagnetic excitation are treated as parametric inputs into the geomechanics model. Tensile failure within the interbedded shale as expected for fracturing is achieved at a specific combination of initial water/oil saturation setup. Other cases are investigated to help reveal a full image of correlations between electromagnetic excitation, temperature/pore pressure escalation, geomechanical constraints, and natural properties of reservoir.