Wernher Brevis

Experimental and large eddy simulation study of the flow developed by a sequence of lateral obstacles

In this paper we provide a description of the three-dimensional flow induced by a sequence of lateral obstacles in a straight shallow open-channel flow with flat bathymetry. The obstacles are modelled as rectangular blocks and are located at one channel wall, perpendicular to the main stream direction. Two aspect ratios of the resulting dead zones are analysed. The flow structure is experimentally characterised by particle image velocimetry measurements in a laboratory flume and simulated using three-dimensional Large Eddy Simulations. Good agreement between experimental measurements and numerical results is obtained. The results show that the effect of the obstacles in the main channel is observed up to one obstacle length in the spanwise direction. The spacing between obstacles does not seem to have a large influence in the outer flow. The mean flow within the dead zone is characterised by a large recirculation region and several additional vortex systems. They are discussed in the paper, as well as the mean and root-mean-square wall shear-stresses.

Shallow-flow visualization analysis by proper orthogonal decomposition

The identification of the spatial characteristics and the shedding frequencies of coherent structures in shallow flows usually involves the use of sophisticated equipment for velocity measurements such as laser Doppler anemometry or particle image velocimetry. In this work, a simple and low-cost alternative for the quantitative characterization of quasi two-dimensional shallow coherent structures is presented. The technique is based on the image pre-processing of flow visualizations and post-processing by means of the proper orthogonal decomposition. As an illustration of the method, the analysis of a shallow flow in the wake of a cylinder with a vortex-street pattern is presented. The method is able to discriminate concentration patches attached to the vortices and their advection, providing frequencies in excellent agreement with a previous study.

A rapid non-iterative proper orthogonal decomposition based outlier detection and correction for PIV data

The present work proposes a novel method of detection and estimation of outliers in particle image velocimetry measurements by the modification of the temporal coefficients associated with a proper orthogonal decomposition of an experimental time series. Using synthetic outliers applied to two sequences of vector fields, the method is benchmarked against state-of-the-art approaches recently proposed to remove the influence of outliers. Compared with these methods, the proposed approach offers an increase in accuracy and robustness for the detection of outliers and comparable accuracy for their estimation.

Experimental and Modelling Investigations of Air Exchange and Infection Transfer due to Hinged-Door Motion in Office and Hospital Settings

Abstract Occupants spend a significant amount of time indoors where temperature and air quality has an important impact on their comfort, health and work performance. Understanding the role of airflow exchange between spaces is crucial to describe the processes of mixing and transport of substances driven by air motion and therefore essential for evaluating indoor air quality. This work presents the results of field measurements and laboratory experiments designed to characterise door operation and to quantify its influence on air volumes exchanged between rooms due to door motion. The field study was conducted to identify typical total door cycle times in single person offices. The laboratory experiments were conducted in a scale model to investigate the exchange flow between two generic rooms. The model consisted of a water filled tank divided into two equal rooms, which were connected by a computer-controlled hinged door. Flow visualisations were used to describe flow patterns and concentration measurements of Rhodamine WT were performed to quantify exchange volumes. With hold open times of between 0s and 26.67s the total fluid volume exchanged was found to be between 67% and 98% of the total volume swept. Based on the exchange volume found in these experiments combined with the Wells-Riley equation the effect of ventilation rate on the probability of occupants in an adjacent room becoming infected was investigated. With ventilation rates for a medium air quality the risk of infection is low (<0.05). However, the probability of infection quickly rises with lower ventilation rates.

Numerical simulation of the hydrodynamics and turbulent mixing process in a drinking water storage tank

ABSTRACT Jet-mixing and residence time in a rectangular water storage tank with a constant water level are investigated using the tools of Computational Fluid Dynamics (CFD). A set of Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations using a realisable k-ε model for different inlet configurations has been used. Numerical simulations were validated by means of experimental measurements. A saline inflow was simulated and the computed salinity in the outflow was compared with the measured values, with the aim of improving the tank performance based only on simple modifications of the inlet position and inflow rate. The results show that the URANS technique is able to adequately capture the experimental dilution curve measured at the outlet of the tank. The residence time is mainly influenced by advective transport. Modifications of the horizontal angle and Reynolds number of the inflow jet produce changes in the mixing characteristics when different performance indexes are compared.

Implications of the selection of a particular modal decomposition technique for the analysis of shallow flows

ABSTRACT This work deals with the capabilities of two synoptic modal decomposition techniques for the identification of the spatial patterns and temporal dynamics of coherent structures in shallow flows. Using two different experimental datasets it is shown that due to the linear behaviour of large-scale, quasi-two-dimensional flow structures, there are almost no differences in the identification of dominant modes between the results obtained from a traditional proper orthogonal decomposition and the more recently developed dynamic mode decomposition. However, it is also shown that nonlinear dynamics can arise in the transition of these structures to a quasi-two-dimensional behaviour, which can result in the proper orthogonal decomposition identifying structures composed of multi-frequencies, a sign of a convoluted dynamics. Thus dynamic mode decomposition is recommended instead for the analysis of such phenomena. In addition, this paper introduces a simple ranking methodology for the use of the dynamic mode decomposition technique in shallow flows, which is based on the results of the proper orthogonal decomposition.

Assessment of 3D-RANS models for the simulation of topographically forced shallow flows

Abstract In this work the performance of Reynolds Averaged Navier-Stokes (RANS) simulations to predict the flow structure developed by the presence of a sidewall obstacle in a uniform open-channel shallow flow is discussed. The tested geometry was selected due to its important role in several fluvial applications, such as the control of riverbank erosion and the creation of improved ecological conditions in river restoration applications. The results are compared against experimental laboratory velocity fields obtained after Large Scale Particle Image Velocimetry (LSPIV) measurements. It is shown that the length of reattachment of the separated shear layer generated by the obstacle is well predicted by a Reynolds Stress Model, while classical two-equation models show important limitations. All the performed RANS simulations are unable to properly predict the formation of a secondary gyre region, which develops immediately downstream the obstacle.

A multiaxial notch fatigue methodology to estimate in-service lifetime of corroded cast iron water pipes

The present paper summarises an attempt of using the so-called Modified Wöhler Curve Method (MWCM) to estimate fatigue damage in pitted cast iron water pipes subjected to in-service variable amplitude multiaxial fatigue loading. In this setting, pits are treated as hemispherical/hyperbolic notches whose depth increases over time due to conventional corrosion processes taking places in buried cast-iron pipes. The validity of such an approach is proven by showing, through a case study, that, under particular circumstances, the combined effect of corrosion and fatigue can remarkably shorten the in-service lifetime of cast-iron pipes as observed in the case study.

Discharge Coefficients for Baffle-Sluice Gates

AbstractIn this work, the discharge coefficients of four well-defined operation stages of baffle-sluice gates are presented and modeled using the individual contributions of weir and sluice flow components. Existing empirical relationships proved effective to model the discharge. The presence of submerged baffles has an insignificant effect on the module discharge. One of the most important differences, compared with the constant value recommended in the literature, was found in the case of the submerged discharge coefficient, which grew linearly with the bottom opening. Other differences in the discharge coefficient were also found for the sluice gate flow; however, they did not significantly affect the magnitude of the discharge coefficients. The study is useful for refining the design of baffle-sluice irrigation module.

A comparative study of manhole hydraulics using stereoscopic PIV and different RANS models

Flows in manholes are complex and may include swirling and recirculation flow with significant turbulence and vorticity. However, how these complex 3D flow patterns could generate different energy losses and so affect flow quantity in the wider sewer network is unknown. In this work, 2D3C stereo Particle Image Velocimetry measurements are made in a surcharged scaled circular manhole. A computational fluid dynamics (CFD) model in OpenFOAM® with four different Reynolds Averaged Navier Stokes (RANS) turbulence model is constructed using a volume of fluid model, to represent flows in this manhole. Velocity profiles and pressure distributions from the models are compared with the experimental data in view of finding the best modelling approach. It was found among four different RANS models that the re-normalization group (RNG) k-ɛ and k-ω shear stress transport (SST) gave a better approximation for velocity and pressure.