Large eddy simulation of turbulence and solute transport in a forested headwater stream

The large eddy simulation (LES) module of the Virtual StreamLab (VSL3D) model is applied to simulate the flow and transport of a conservative tracer in a headwater stream in Minnesota, located in the south Twin Cities metropolitan area. The detailed geometry of the stream reach, which is _135 m long, _2.5 m wide, and _0.15 m deep, was surveyed and used as input to the computational model. The detailed geometry and location of large woody debris and bed roughness elements up to _0.1 m in size were also surveyed and incorporated in the numerical simulation using the Curvilinear Immersed Boundary approach employed in VSL3D. The resolution of the simulation, which employs up to a total of 25 million grid nodes to discretize the flow domain, is sufficiently fine to directly account for the effect of large woody debris and small cobbles (on the streambed) on the flow patterns and transport processes of conservative solutes. Two tracer injection conditions, a pulse and a plateau release, and two cross sections of measured velocity were used to validate the LES results. The computed results are shown to be in good agreement with the field measurements and tracer concentration time series. To our knowledge, the present study is the first attempt to simulate via high-resolution LES solute transport in a natural stream environment taking into account a range of roughness length scales spanning an order of magnitude: From small cobbles on the streambed (_0.1 m in diameter) to large woody debris up to _3 m long. © 2016. American Geophysical Union. All Rights Reserved

Physical complexity to model morphological changes at a natural channel bend

This study developed a two‐dimensional (2‐D) depth‐averaged model for morphological changes at natural bends by including a secondary flow correction. The model was tested in two laboratory‐scale events. A field study was further adopted to demonstrate the capability of the model in predicting bed deformation at natural bends. Further, a series of scenarios with different setups of sediment‐related parameters were tested to explore the possibility of a 2‐D model to simulate morphological changes at a natural bend, and to investigate how much physical complexity is needed for reliable modeling. The results suggest that a 2‐D depth‐averaged model can reconstruct the hydrodynamic and morphological features at a bend reasonably provided that the model addresses a secondary flow correction, and reasonably parameterize grain‐sizes within a channel in a pragmatic way. The factors, such as sediment transport formula and roughness height, have relatively less significance on the bed change pattern at a bend. The study reveals that the secondary flow effect and grain‐size parameterization should be given a first priority among other parameters when modeling bed deformation at a natural bend using a 2‐D model

Instantaneous transport of a passive scalar in a turbulent separated flow

The results of large-eddy simulations of flow and transient solute transport over a backward facing step and through a 180° bend are presented. The simulations are validated successfully in terms of hydrodynamics and tracer transport with experimental velocity data and measured residence time distribution curves confirming the accuracy of the method. The hydrodynamics are characterised by flow separation and subsequent recirculation in vertical and horizontal directions and the solute dispersion process is a direct response to the significant unsteadiness and turbulence in the flow. The turbulence in the system is analysed and quantified in terms of power density spectra and covariance of velocity fluctuations. The injection of an instantaneous passive tracer and its dispersion through the system is simulated. Large-eddy simulations enable the resolution of the instantaneous flow field and it is demonstrated that the instabilities of intermittent large-scale structures play a distinguished role in the solute transport. The advection and diffusion of the scalar is governed by the severe unsteadiness of the flow and this is visualised and quantified. The analysis of the scalar mass transport budget quantifies the mechanisms controlling the turbulent mixing and reveals that the mass flux is dominated by advection

Reflexivity and children’s exploration of moral aesthetics in creative writing

In this paper, we show how children’s creativity manifests in literary aesthetics and how they explore moral subjects as a result. Literary aesthetics involve a range of artistic elements or expressions within a textual work. Writers may use aesthetic properties to express individual appreciation or how people view the world, often in relation to moral values. This paper takes an interdisciplinary approach to understanding the relation between aesthetics and moral value in children’s writing. First, we situate our discussion within the context of reflexivity theory, which framed the larger study from which these data are drawn. Second, we apply appraisal theory within the lens of reflexivity to analyse children’s writing samples for their aesthetic representations of moral qualities. As a result of our study, we show how teachers may improve and complicate the way children practice literary aesthetics in their writing. We argue it is important for students to learn about aesthetics in writing as aesthetics are a key quality of literary expression and may educate readers on the perception and moral values of others

Assessment of Parshall flumes for discharge measurement of open-channel flows: A comparative numerical and field case study

This paper presents a series of numerical and field studies to examine the accuracy of two field-scale Parshall flumes, which are employed to measure the flow rate of a wastewater system in the city of Minneapolis, Minnesota. The numerical studies were carried out using the large-eddy simulation (LES) and level-set methods to compute the turbulent flow of sewers under two-phase flow (wastewater and air) conditions. Flow rate measurements in the field were conducted using the dye dilution approach and an in-site flow measuring tool. Using the combination of field measurements and numerical simulations, this research aims at quantifying (i) the margin of error of the filed-scale Parshall flumes and (ii) the water surface fluctuations within the Parshall flumes. The LES turbulent model, coupled with the level-set method, allowed for resolution of instantaneous water surface variations and uncertainty quantification of the flow rate measurements