Turbulence in stably stratified boundary layers

In this thesis, turbulence dynamics for a stably stratified boundary layer is studied. The processes by which stable boundary layers are formed through strong surface cooling imposed on neutrally stratified wall-bounded turbulence is explored first using high-resolution direct numerical simulation at a moderate Reynolds number. The adjustment of the flow to the imposed strong surface cooling is investigated. Secondly, characteristics of near-wall turbulence at quasi-stationarity under strong wall cooling are studied. It is shown that if turbulence reaches quasi-stationarity, the characteristics of quasi-stationary near-wall turbulence, even with the strongest wall cooling rate, are generally similar to the weakly stratified case. The effects of strong stable stratification on the characteristics of near-wall turbulence are transient. It is shown that among mechanisms that contribute to the budget of turbulent kinetic energy, transfer and pressure-work are more dependent on the stratification if turbulence reaches quasi-stationarity. Buoyancy destruction has a considerable effect on the evolution of turbulence producing eddies but not on production itself at stationarity. Relevant length scales are also discussed in detail. In summary, it is found that the behaviour of near-wall turbulence at quasi-stationarity is approximately similar to weakly stratified cases, regardless of the choice of upper boundary condition. Finally, the kinetic energy cascade in a stably stratified boundary layer is investigated. A mathematical framework to incorporate vertical scales into the conventional kinetic energy spectrum and its budget is introduced. It is shown that energetic streamwise scales ($\lambda_x$) become larger with increasing vertical scale. For the strongest stratification, for which the turbulence becomes intermittent, the energetic streamwise scales are suppressed, and energy density resides in $\lambda_x$ of the size of the domain. It is shown that in the unstratified case, vertical scales of the size comparable to the height of the logarithmic layer connect viscous regions to the outer layer. By contrast, in stratified cases, such a connection is not observed

Paper 4: Instabilities in downslope propagating gravity currents

In this paper, the stability of a gravity current propagating down an inclined channel is explored. The gravity current is generated through a partial lock release. A local analysis of the flow about the lock is conducted using classical methodologies in two-layer shallow water theory, which is here developed to allow for drag and an angled channel. A global analysis of the flow is done by exploring the long-time forced steady state, using non-linear simulations solving the full three-dimensional Navier-Stokes equations. A steady state is forced through the inclusion of a so-called mask function. For sufficiently large slope angles, wave-like instabilities are observed on the density interface (pycnocline) of the generated gravity current. Spectral Proper-Orthogonal Decomposition of the time-series data provides evidence that instability arises via the interaction of two asymmetric vorticity waves, occurring on either side of a critical layer, akin to a spatially evolving Kelvin-Helmholtz instability. The work in this paper is exploratory in nature and motivates further investigation

CIVE 4143 senior design project

This reports details upgrades to the Alligator Creek wastewater treatment plant in Texas to meet new flow requirements and various permit limits. We evaluated the current performance of the plant systems to determine if they needed to be enhanced and designed the necessary changes. Alternatives taking into account advantages and disadvantages were also evaluated to determine the best solution, which oftentimes resulted in no changes or simply adding additional units to an already existing process

Wall turbulence response to surface cooling and formation of strongly stable stratified boundary layers

The following article appeared in (Atoufi, A., Scott, K. A., & Waite, M. L. (2019). Wall turbulence response to surface cooling and formation of strongly stable stratified boundary layers. Physics of Fluids, 31(8), 085114. https://doi.org/10.1063/1.5109797) and may be found at (https://doi.org/10.1063/1.5109797). This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing.This paper investigates the processes by which stable boundary layers are formed through strong surface cooling imposed on neutrally stratified wall-bounded turbulence using high-resolution direct numerical simulation at a moderate Reynolds number. The adjustment of the flow to the imposed strong surface cooling is investigated. We further focus on a strongly stable case where turbulence partially collapses. We show that, due to a significant reduction in turbulence production, turbulence becomes patchy, with a band of turbulence coexisting with quiet regions. The nature of the quiet regions, which are often characterized as laminar, is investigated and shown to be consistent with viscously coupled stratified turbulence. The one-dimensional longitudinal streamwise velocity spectrum exhibits −5 and −3 behavior in the buffer and logarithmic layers, respectively, adjacent to an active region of three-dimensional turbulence with a −5/3 spectrum. Scenarios for turbulence recovery from such a patchy state are also discussed. We show that the presence of outer layer turbulence above z+ ≈ 300 is a key requirement for recovery. For higher values of stratification, it is shown that inner layer turbulence is damped entirely and outer layer turbulence is damped subsequently.Funder 1, This research was enabled in part by support provided by the Shared Hierarchical Academic Research Computing Network (SHARCNET) || funder 2, Compute/Calcul Canada || Funder 3, the Natural Sciences and Engineering Research Council of Canada (K.A.S.: Grant No. RGPIN-418344-2012; and M.L.W.: Grant No. RGPIN-386456-2015) || Funder 4, and the Canadian Foundation for Innovation

Geometry of stratified turbulent mixing: local alignment of the density gradient with rotation, shear and viscous dissipation

We introduce a geometric analysis of turbulent mixing in density-stratified flows based on the alignment of the density gradient in two orthogonal bases that are locally constructed from the velocity gradient tensor. The first basis connects diapycnal mixing to rotation and shearing motions, building on the recent 'rortex-shear decomposition' in stratified shear layers (Jiang et al., J. Fluid Mech. 947, A30, 2022), while the second basis connects mixing to the principal axes of the viscous dissipation tensor. Applying this framework to datasets taken in the stratified inclined duct laboratory experiment reveals that density gradients in locations of high shear tend to align preferentially (i) along the direction of minimum dissipation and (ii) normal to the plane spanned by the rotex and shear vectors. The analysis of the local alignment across increasingly turbulent flows offers new insights into the intricate relationship between the density gradient and dissipation, and thus diapycnal mixing

Characteristics of quasistationary near-wall turbulence subjected to strong stable stratification in open-channel flows

The author has the right to post and update the article on a free-access e-print server using files prepared and formatted by the author. Any such posting made or updated after acceptance of the article for publication by APS should include a link to the online APS journal article abstract. In all cases, the appropriate bibliographic citation and notice of the APS copyright must be included.Characteristics of near-wall turbulence at quasistationarity under strong wall cooling are studied using direct numerical simulation of open-channel flow. It is shown that if turbulence reaches quasistationarity, then the characteristics of quasistationary near-wall turbulence, even with the strongest wall cooling rate, are generally similar to the weakly stratified case. The effects of strong stable stratification on the characteristics of near-wall turbulence are transient. The effect of stratification on several characteristics of stratified near-wall turbulence, including first-, second-, and higher-order statistics, turbulent kinetic energy budget, and mechanisms involved in the evolution of turbulence producing eddies, are discussed. It is shown that among mechanisms that contribute to the budget of turbulent kinetic energy, transfer and pressure-work are more dependent on the stratification if turbulence reaches quasistationarity. The buoyancy destruction term influences the budget for the tangential Reynolds stress more than the budget for the turbulent kinetic energy. Relevant length scales are also discussed in detail. The Corrsin and Ellison scales are smaller than the Ozmidov scales and are sensitive to stratification in the upper logarithmic layer and in the outer layer. The Corrsin scales in the lower half of the buffer layer and fine-scale structures of wall-normal velocity in the viscous sublayer are smaller than the Kolmogorov scale. Finally, the effect of heat entrainment from the upper boundary and computational domain size are also examined. In summary, it is found that the behavior of near-wall turbulence at quasistationarity is approximately similar to weakly stratified cases, regardless of the choice of upper boundary condition.This research was enabled in part by support provided by the Shared Hierarchical Academic Research Computing Network (SHARCNET), Compute/Calcul Canada, the Natural Sciences and Engineering Research Council of Canada (KAS: Grant No. RGPIN-418344-2012; and MLW: Grant No. RGPIN-386456-2015), and the Canadian Foundation for Innovation

Rosettes & Ribbons: Some Recent Accomplishments of Note at the School

Developing a simple produces for efficient derivation of motor neurons (MNs) is essential for neural tissue engineering studies. Stem cells with high capacity for neural differentiation and scaffolds with the potential to promote motor neurons differentiation are promising candidates for neural tissue engineering. Recently, human olfactory ecto-mesenchymal stem cells (OE-MSCs), which are isolated easily from the olfactory mucosa, are considered a new hope for neuronal replacement due to their neural crest origin. Herein, we synthesized conducting hydrogels using different concentration of chitosan-g-aniline pentamer, gelatin, and agarose. The chemical structures, swelling and deswelling ratio, ionic conductivity and thermal properties of the hydrogel were characterized. Scaffolds with 10 chitosan-g-aniline pentamer/gelatin (S10) were chosen for further investigation and the potential of OE-MSCs as a new source for programming to motor neuron-like cells investigated on tissue culture plate (TCP) and conductive hydrogels. Cell differentiation was evaluated at the level of mRNA and protein synthesis and indicated that conductive hydrogels significantly increased the markers related to motor neurons including Hb-9, Islet-1 and ChAT compared to TCP. Taken together, the results suggest that OE-MSCs would be successfully differentiated into motor neuron-like cells on conductive hydrogels and would have a promising potential for treating motor neuron-related diseases. © 201

Stratified inclined duct: direct numerical simulations

The stratified inclined duct (SID) experiment consists of a zero-net-volume exchange flow in a long tilted rectangular duct, which allows the study of realistic stratified shear flows with sustained internal forcing. We present the first three-dimensional direct numerical simulations (DNS) of SID to explore the transitions between increasingly turbulent flow regimes first described by Meyer \& Linden (\textit{J. Fluid Mech.} \textbf{753}, 242-253, 2014). We develop a numerical set-up that faithfully reproduces the experiments and sustains the flow for arbitrarily long times at minimal computational cost. We recover the four qualitative flow regimes found experimentally in the same regions of parameter space: laminar flow, waves, intermittent turbulence, and fully-developed turbulence. We find good qualitative and quantitative agreement between DNS and experiments and highlight the added value of DNS to complement experimental diagnostics and increase our understanding of the transition to turbulence, both temporally (laminar/turbulent cycles) and parametrically (as the tilt angle of the duct and the Reynolds number are increased). These results demonstrate that numerical studies of SID -- and deeper integration between simulations and experiments -- have the potential to lead to a better understanding of stratified turbulence in environmental flows

Evaluation of the Effect of Sodium Hypochlorite Irrigant on Pull-out Bond Strength of FRC Posts Using Different Resin Cements

Introduction: To evaluate the effects of using sodium hypochlorite irrigant on pull-out bond strength of self-etch and self-adhesive resin cements to dentin. Methods: Sixty intact premolars were decoronated and the root canals were prepared by step-back technique. The teeth were divided into two groups according to the irrigant used during root canal treatment: 5.25% sodium hypochlorite and normal saline. The canals were obturated and after 48 hours storage, 8mm depth post space was prepared with application of normal saline as irrigant. Then, each group was divided into two subgroups depending on the type of the cements used for cementation of fiber reinforced composite (FRC) posts (Panavia F2or Embrace). The pull-out bond strength test was evaluated. Statistical analysis was performed by Two-way AONVA. Results: The type of cement had no statistically significant effect on the bond strength; however, the type of irrigant was statistically effective. There was no interaction between two independent variables. The application of sodium hypochlorite significantly decreased the pull-out bond strength in Embrace cement in comparison with the use of normal saline. Conclusion: The type of irrigants used in endodontic treatment may affect on bond strength of FRC posts cemented by self-adhesive cements.