New Facility for Membrane Fouling Investigations under Customizable Hydrodynamics: Validation and Preliminary Experiments with Pulsating Cross-Flow

Flux reduction induced by fouling is arguably the most adverse phenomenon in membrane-based separation systems. In this respect, many laboratory-scale filtration studies have shown that an appropriate use of hydrodynamic perturbations can improve both performance and durability of the membrane; however, to fully understand and hence appropriately exploit such effects, it is necessary to understand the underpinning flow processes. Towards this end, in this work we propose and validate a new module-scale laboratory facility with the aim of investigating, at very well-controlled flow conditions, how hydrodynamics affects mass transport phenomena at the feed/membrane interface. The proposed facility was designed to obtain a fully developed and uniform flow inside the test section and to impose both steady and pulsating flow conditions. The walls of the facility were made transparent to grant optical accessibility to the flow. In this paper, we discuss data coming from particle image velocimetry (PIV) measurements and preliminary ultrafiltration tests both under steady and pulsating flow conditions. PIV data indicate that the proposed facility allows for excellent flow control from a purely hydrodynamic standpoint. Results from filtration tests provide promising results pointing towards pulsating flows as a viable technique to reduce fouling in membrane systems

Characterisation of drag and wake properties of canopy patches immersed in turbulent boundary layers

The wakes and the drag forces of canopy patches with different densities, immersed in turbulent boundary layers, are investigated experimentally. The patches are circular (with outer diameter D) and are made of several identical circular cylinders (height, H and diameter, d). The bulk aspect ratio of all the patches (AR = H~D) was fixed at 1 and the patch density (? = Ncd2~D2, also referred to as solidity) is altered by varying the number of cylinders (Nc) in the patch. Drag measurements show that the patch drag coefficient increases with increasing density. However, the drag coefficient of the highest investigated density (? =0.25) is greater than the drag coefficient of a solid patch (i.e. ? = 1, which is a solid cylinder with AR = 1). PIV measurements were carried out along streamwise-wall-normal (x - y) plane along the centreline of patch and in the streamwise-spanwise (x - z) plane at its mid height (i.e. y = H~2). Mean velocity fields show that the porosity of the patch promotes bleeding along all directions. It was observed that bleeding along the vertical/horizontal direction increases/decreases with increasing ?. Furthermore, it was also observed that bleeding along the lateral direction dictates the point of flow separation along the sides of the patch and makes it independent of ?. All these aspects make wakes for porous patches markedly different from their solid counterpart and provide a rational basis to explain the observed trends in the drag coefficient

Stability analysis of open-channel flows with secondary currents

This paper presents some results coming from a linear stability analysis of turbulent depth-averaged open-channel flows (OCFs) with secondary currents. The aim was to identify plausible mechanisms underpinning the formation of large-scale turbulence structures, which are commonly referred to as large-scale motions (LSMs) and very-large-scale motions (VLSMs). Results indicate that the investigated flows are subjected to a sinuous instability whose longitudinal wavelength compares very well with that pertaining to LSMs. In contrast, no unstable modes at wavelengths comparable to those associated with VLSMs could be found. This suggests that VLSMs in OCFs are triggered by nonlinear mechanisms to which the present analysis is obviously blind. We demonstrate that the existence of the sinuous instability requires two necessary conditions: (i) the circulation of the secondary currents ω must be greater than a critical value ωc; (ii) the presence of a dynamically responding free surface (i.e. when the free surface is modelled as a frictionless flat surface, no instabilities are detected). The present paper draws some ideas from the work by Cossu, Hwang and co-workers on other wall flows (i.e. turbulent boundary layers, pipe, channel and Couette flows) and somewhat supports their idea that LSMs and VLSMs might be governed by an outer-layer cycle also in OCFs. However, the presence of steady secondary flows makes the procedure adopted herein much simpler than that used by these author

On Escherichia coli Resistance to Fluid Shear Stress and Its Significance for Water Disinfection

Alternative water treatment techniques are needed to overcome the limitations of chemical disinfectants. Stemming from recent findings which point to high levels of shear stress induced by flow as the cause of microbial removal in water, we conducted systematic experiments on bacterial solutions in well-controlled hydrodynamic conditions to evaluate the effect of different levels of shear stress on the viability of Escherichia coli. We investigated a wide range of shear stresses (57–4240 Pa) using viscous substrates prepared by mixing a bacterial solution with thickeners (2-hydroxyethyl cellulose and/or guar gum). Substrate samples were tested for up to 60 min in a laminar shear flow at a constant temperature using a rotational rheometer equipped with a cone-plate measuring system so that the whole sampling volume was exposed to the same shear stress. Results show that, contrary to previous studies, high shear stresses (i.e., of order 103 Pa) do not induce inactivation or lysis of E. coli, even for prolonged exposure times. Stemming from our results and a thorough discussion of the literature on E. coli mechanical lysis and modeling cell dynamics, we infer that E. coli can resist high shear forces because of stress relaxation in a wide range of hydrodynamic conditions

On the Very-Large-Scale Motions in Smooth-Bed Open-Channel Flows

Bimodal distribution of the streamwise velocity pre-multiplied spectra in canonical turbulent flows (pipe, channel and boundary layer flow) is well documented in the literature. The two peaks of this distribution are associated with eddies of a defined size and they are called Large-Scale Motion (LSM) and Very-Large-Scale Motion (VLSM). These eddy structures are very important inasmuch they contain a significant fraction of the total kinetic energy of the flow. The LSMs and VLSMs size is proportional to the characteristic outer length scale of the flow (i.e. the radius, the channel half width and the boundary layer thickness); the former’s length is a few while the latter is some tens of . However, little is known about their size and scaling in open-channel flows. The knowledge of LSMs and VLSMs in open-channel flows (i.e. rivers, tides and marine currents) is important, not only from a theoretical point of view, but most of all for their impact on key transport and mixing processes occurring in many geophysical flows (e.g., sediment dynamics, transport of nutrients, microorganisms movement, etc.). The present study aims to shed light into the dynamics of LSMs and VLSMs in open-channel flow through a laboratory study. The experiments were conducted in a recirculating open-channel flume 50 m long, 0.61 m wide and 1 m deep with a smooth concrete bed. During the experiments, the instantaneous velocity in the streamwise and bed-normal directions was measured with the aid of a 2D Laser Doppler Anemometer (LDA). The conditions in every experiment were that of fully developed smooth turbulent flow. The experiments were designed in order to highlight the influence of various relevant non-dimensional groups (presumably) involved on the LSMs and VLSMs dynamics. The main results are that the evolution of LSMs and VLSMs seems not to be affected by the Von Karman and the Froude Number (in the range of conditions analysed). As suggested also in the literature, the results hint that the non-dimensional parameter that mostly influences these vortices seems to be the aspect ratio. For values of the aspect ratio below 5 (that represent a condition of 3D motion, with the instauration of secondary flows in the flume), the size of these vortices is reduced by more than half with respect to a situation of an aspect ratio greater than 5

Aerodynamic Roughness Length of Fresh Snow

This study presents the results from a series of wind-tunnel experiments designed to investigate the aerodynamic roughness length z 0 of fresh snow under no-drift conditions. A two-component hot-film anemometer was employed to obtain vertical profiles of velocity statistics in a zero pressure gradient turbulent boundary layer for flow over naturally deposited snow surfaces. The roughness of these snow surfaces was measured by means of digital photography to capture characteristic length scales that can be related to z 0. Our results show that, under aerodynamically rough conditions, the mean value of the roughness length for fresh snow is $${\langle{z}_{0}\rangle= 0.24}$$ mm with a standard deviation σ(z 0)= 0.05 mm. In this study, we show that variations in z 0 are associated with variations in the roughness geometry. The roughness measurements suggest that the estimated values of z 0 are consistent with the presence of irregular roughness structures that develop during snowfalls that mimic ballistic deposition processe

Spatio-Temporal Surface Shear-Stress Variability in Live Plant Canopies and Cube Arrays

This study presents spatiotemporally-resolved measurements of surface shear-stress τ s in live plant canopies and rigid wooden cube arrays to identify the sheltering capability against sediment erosion of these different roughness elements. Live plants have highly irregular structures that can be extremely flexible and porous resulting in considerable changes to the drag and flow regimes relative to rigid imitations mainly used in other wind-tunnel studies. Mean velocity and kinematic Reynolds stress profiles show that well-developed natural boundary layers were generated above the 8m long wind-tunnel test section covered with the roughness elements at four different roughness densities (λ=0, 0.017, 0.08, 0.18). Speed-up around the cubes caused higher peak surface shear stress than in experiments with plants at all roughness densities, demonstrating the more effective sheltering ability of the plants. The sheltered areas in the lee of the plants are significantly narrower with higher surface shear stress than those found in the lee of the cubes, and are dependent on the wind speed due to the plants ability to streamline with the flow. This streamlining behaviour results in a decreasing sheltering effect at increasing wind speeds and in lower net turbulence production than in experiments with cubes. Turbulence intensity distributions suggest a suppression of horseshoe vortices in the plant case. Comparison of the surface shear-stress measurements with sediment erosion patterns shows that the fraction of time a threshold skin friction velocity is exceeded can be used to assess erosion of, and deposition on, that surfac

On Shear-Driven Ventilation of Snow

A series of experiments have been made in a wind tunnel to investigate the ventilation of snow by shear. We argue that the zero-plane displacement can be used as a convenient indicator of ventilation, and that this can be obtained from measurements of mean velocity profiles in conditions of zero pressure gradient. Measurements made over a natural snow surface show a zero-plane displacement depth of less than 5mm, but practical considerations preclude extensive use of snow for these measurements. Instead, the influence of permeability is investigated using reticulated foams in place of snow. We demonstrate that the foam and snow have similar structure and flow-relevant properties. Although the surface of the foam is flat, the roughness lengths increase by two orders of magnitude as the permeability increases from 6 × 10−9 to 160 × 10−9 m2. The zero-plane displacement for the least permeable foams is effectively zero, but more than 15mm for the most permeable foams. Our data compare well to the few studies available in the literature. By analogy to conditions over snow surfaces, we suggest that shear-driven ventilation of snow is therefore limited to the upper few millimetres of snow surface

Turbulence structure of open channel flows over permeable and impermeable beds : A comparative study

Peer reviewedPublisher PD

Dynamics of bubbles under stochastic pressure forcing

Several studies have investigated the dynamics of a single spherical bubble at rest under a nonstationary pressure forcing. However, attention has almost always been focused on periodic pressure oscillations, neglecting the case of stochastic forcing. This fact is quite surprising, as random pressure fluctuations are widespread in many applications involving bubbles (e.g., hydrodynamic cavitation in turbulent flows or bubble dynamics in acoustic cavitation), and noise, in general, is known to induce a variety of counterintuitive phenomena in nonlinear dynamical systems such as bubble oscillators. To shed light on this unexplored topic, here we study bubble dynamics as described by the Keller-Miksis equation, under a pressure forcing described by a Gaussian colored noise modeled as an Ornstein-Uhlenbeck process. Results indicate that, depending on noise intensity, bubbles display two peculiar behaviors: when intensity is low, the fluctuating pressure forcing mainly excites the free oscillations of the bubble, and the bubble's radius undergoes small amplitude oscillations with a rather regular periodicity. Differently, high noise intensity induces chaotic bubble dynamics, whereby nonlinear effects are exacerbated and the bubble behaves as an amplifier of the external random forcin