Use of isotope dilution method to predict bioavailability of organic pollutants in historically contaminated sediments.

Many cases of severe environmental contamination arise from historical episodes, where recalcitrant contaminants have resided in the environment for a prolonged time, leading to potentially decreased bioavailability. Use of bioavailable concentrations over bulk chemical levels improves risk assessment and may play a critical role in determining the need for remediation or assessing the effectiveness of risk mitigation operations. In this study, we applied the principle of isotope dilution to quantify bioaccessibility of legacy contaminants DDT and PCBs in marine sediments from a Superfund site. After addition of 13C or deuterated analogues to a sediment sample, the isotope dilution reached a steady state within 24 h of mixing. At the steady state, the accessible fraction (E) derived by the isotope dilution method (IDM) ranged from 0.28 to 0.89 and was substantially smaller than 1 for most compounds, indicating reduced availability of the extensively aged residues. A strong linear relationship (R2=0.86) was found between E and the sum of rapid (Fr) and slow (Fs) desorption fractions determined by sequential Tenax desorption. The IDM-derived accessible concentration (Ce) was further shown to correlate closely with tissue residue in the marine benthic polychaete Neanthes arenaceodentata exposed in the same sediments. As shown in this study, the IDM approach involves only a few simple steps and may be readily adopted in laboratories equipped with mass spectrometers. This novel method is expected to be especially useful for historically contaminated sediments or soils, for which contaminant bioavailability may have changed significantly due to aging and other sequestration processes

Instantaneous Pressure Determination from Unsteady Velocity Fields Using Adjoint-based Sequential Data Assimilation

A sequential data assimilation (DA) method is developed for pressure determination of turbulent velocity fields measured by particle image velocimetry (PIV), based on the unsteady adjoint formulation. A forcing term F, which is optimized using the adjoint system, is added to the primary Navier–Stokes (N–S) equations to drive the assimilated flow toward the observations at each time step. Compared with the conventional unsteady adjoint method, which requires the forward integration of the primary system and the backward integration of the adjoint system, the present approach integrates the primary-adjoint system all the way forward, discarding the requirement of data storage at every time step, being less computationally resource-consuming, and saving space. The pressure determination method of integration from eight paths [J. O. Dabiri et al., “An algorithm to estimate unsteady and quasi-steady pressure fields from velocity field measurements,” J. Exp. Biol. 217, 331 (2014)] is also evaluated for comparison. Using synthetic PIV data of a turbulent jet as the observational data, the present DA method is able to determine the instantaneous pressure field precisely using the three-dimensional velocity fields, regardless of the observational noise. For the two-dimensional three-component (3C) or two-component (2C) velocity fields, which are not sufficient for pressure determination by the integration method due to the lack of off-plane derivatives, the present DA method is able to reproduce pressure fields whose statistics agree reasonably well with those of the referential results. The 3C and 2C velocity fields yield quite similar results, indicating the possibility of pressure determination from only planar-PIV measurements in turbulent flows. The tomography PIV measurements are also used as observational data, and a clear pressure pattern is obtained with the present DA method

Dynamics of the Jet Flow Issued from a Lobed Nozzle: Tomographic Particle Image Velocimetry Measurements

This study focuses on the dynamics of the three-dimensional jet flow issued from a lobed nozzle. Flow field measurements were performed using a split-screen dual-camera tomographic particle image velocimetry (tomo-PIV) system. The lobed nozzle was constructed using three-circle configuration at the nozzle exit, where the ratio of the circular centre offset to the circle radius was a⁄b= 0.8. Two high-speed cameras were used with an appropriate combination of prismatic and planar mirrors to split each camera into two views. The Reynolds number was fixed at Re= 2400, and the circular jet at the same Reynolds number was also measured for comparison. Fourier mode decomposition was used to identify the large-scale structures superposed in the unsteady flow field. The lobe trough in the jet decreased the jet’s width due to the smooth connection between the pipe section and the lobed exit. Turbulence was intensified in the jet shear layer, and the passing fluid puffs in the circular jet instantaneous flow field were also broken in the lobed jet. The successively passing fluid puffs in the circular jet were observed to be the axisymmetric large-scale structure in Fourier modes dominating at St= 0.39. However, breakdown of the axisymmetric structures was induced by the lobed nozzle at St= 0.51 and 0.65, and the double helical modes arose at St= 0.28 and 0.38 (also 0.4) in the shear layer of the jet potential core and interacted with the entire jet column

An experimental study of turbulent vortex rings using particle image velocimetry

In this dissertation, the early development of turbulent vortex rings at two Reynolds numbers is studied using two-dimensional and Stereoscopic Particle Image Velocimetry. In the late 1980s’ a similarity theory of turbulent vortex rings was proposed and tested primarily using a twochannel tracking Laser Doppler Velocimeter. However, due to the limitations of the experimental technique the tests were inconclusive and important assumptions could not be checked. Since single-point measurements were used, turbulent vortex ring structures could only be inferred using a complex signal-analysis technique. In the present study, two-dimensional and stereoscopic Particle Image Velocimetry techniques provide spatial and temporal resolved measurements of the full field of the cross-section of turbulent vortex rings, from which a more rigorous investigation of the similarity theory is possible. Since the region over which the similarity theory appears to hold starts at about 2.5 orifice diameters downstream, this study focusses on the early development region from this point to ten diameters downstream. Finally, the ensembleaveraged turbulent ring velocity contours, vorticity contours, pressure field contours, as well as Reynolds stresses and turbulence production contours, are presented. The effects of the turbulent vortex ring position dispersion and tilting angle variation on the measurement results are also studied and quantified. An effort is also made to reconstruct a three-dimensional turbulent vortex ring velocity field by adopting Taylor’s hypothesis. Some important features are successfully captured. An azimuthal-averaging method is also developed in an attempt to estimate the turbulence quantities in cylindrical coordinates. However, because of various limitations, the three-dimensional reconstruction method is not perfect, and room for future improvement is discussed

Vortex evolution in the near wake behind polygonal cylinders

The near wake of the polygonal cylinder with the side number N = 3~∞ is systematically studied using particle image velocimetry (PIV) at Re = 1.6 × 104. The proper orthogonal decomposition (POD) analysis is carried out to extract the large-scale coherent vortex structures and their evolution. It has been found that the vortex circulation grows to the maximum at the vortex formation length by entraining the vorticity from the separated shear layer and then undergoes a two-stage decay. The maximum circulation scales with the wake width, defined as the vertical distance between the two peaks of streamwise velocity fluctuation at vortex formation length. The vortex center trajectory indicates that the vortices move towards the centerline first and then away, with the vortex size monotonically increasing over the examined streamwise range. The vortex size at the maximum circulation also scales with the wake width. The vortex convection velocity increases gradually in the streamwise direction, and the ratio of the lateral and streamwise components of the vortex convection velocity, when scaled by wake width and vortex formation length respectively, approaches asymptotically 0.18 in the downstream, irrespective of the cylinder orientation or N

A 4D-flow cardiovascular magnetic resonance study of flow asymmetry and haemodynamic quantity correlations in the pulmonary artery

Objective: In this paper we elucidate the asymmetric flow pattern and the haemodynamic quantity distributions and correlations in the pulmonary artery (PA) vasculature in healthy adults having structurally normal hearts, to provide reference on the flow characteristics in the PA and the right ventricle. Approach: Velocity data are acquired non-invasively from 18 healthy volunteers by 4D-Flow magnetic resonance imaging, resolved to 20 phases with spatial resolution 3 × 3 × 3 mm3. Interpolation is applied to improve the accuracy in quantifying haemodynamic quantities including kinetic energy, rotational energy, helicity and energy dissipation rate. These quantities are volumetrically normalised to remove size dependency, representing densities or local intensity. Main results: Flow asymmetry in the PA is quantified in terms of all the flow dynamic quantities and their correlations. The right PA has larger diameter and higher peak stroke velocity than the left PA. It also has the highest rotational energy intensity. Counter-rotating helical streams in the main PA appear to be associated with the unidirectional helical flow noticed in the left and the right PA near the peak systole. Significance: This study provides a fundamental basis of normal flow in the PA. It implies the validity to use these flow pattern-related quantitative measures to aid with the identification of abnormal PA flow non-invasively, specifically for detecting abnormalities in the pulmonary circulation and response to therapy, where haemodynamic flow is commonly characterised by increased vortical and helical formations

A Polynomial Model with Line-of-Sight Constraints for Lagrangian Particle Tracking Under Interface Refraction

This paper introduces an improvement of the "Shake-The-Box (STB)" (Schanz, Gesemann, and Schröder, Exp. Fluids 57.5, 2016) technique using the polynomial calibration model and the line-of-sight constraints (LOSC) to overcome the refractive interface issues in Lagrangian particle tracking (LPT) measurement. The method (named LOSC-LPT) draws inspiration from the two-plane polynomial camera calibration in tomographic particle image velocimetry (Tomo-PIV) (Worth and Nickels, Thesis, 2010) and the STB-based open-source Lagrangian particle tracking (OpenLPT) framework (Tan, Salibindla, Masuk, and Ni, Exp. Fluids 61.2, 2019). The LOSC-LPT introduces polynomial mapping functions into STB calibration in conditions involving gas-solid-liquid interfaces at container walls exhibiting large refractive index variations, which facilitates the realization of particle stereo matching, three-dimensional (3D) triangulation, iterative particle reconstruction, and further refinement of 3D particle position by shaking the LOS. Performance evaluation based on synthetic noise-free images with a particle image density of 0.05 particle per pixel (ppp) in the presence of refractive interfaces demonstrates that LOSC-LPT can detect a higher number of particles and exhibits lower position uncertainty in the reconstructed particles, resulting in higher accuracy and robustness than that achieved with OpenLPT. In the application to an elliptical jet flow in an octagonal tank with refractive interfaces, the use of polynomial mapping results in smaller errors (mean calibration error 1.0 px). Moreover, 3D flow-field reconstructions demonstrate that the LOSC-LPT framework can recover a more accurate 3D Eulerian flow field and capture more complete coherent structures in the flow, and thus holds great potential for widespread application in 3D experimental fluid measurements

A review of experiments on stationary bluff-body wakes

Experimental studies dealing with the wake of isolated stationary bluff-bodies are reviewed. After briefly recalling the pioneering works in this domain, the paper focuses on recent research conducted with the latest experimental methods and techniques. The review encompasses a range of topics, including, the effects of bluff-body geometry (non-circular cross sections and nonuniformity in spanwise direction), steady and unsteady (periodic and non-periodic) inflow conditions; surface proximity (rigid wall, confinement and water free surface) and non-Newtonian fluids. Focus is brought to the flow physics of the wakes, including especially the complex threedimensional and oscillatory behaviours induced by the periodic vortex shedding phenomenon. The paper aims to offer a critical and systematic review of new knowledge and findings on the subject area, as well as emerging? and the most frequently adopted experimental techniques. The review also helps identifying knowledge gaps in the literature that need to be addressed in future investigations