The TU Wien Turbulent Water Channel: Flow control loop and three-dimensional reconstruction of anisotropic particle dynamics.

A horizontal water channel facility was built to study particle dynamics in a turbulent flow. The channel is sufficiently long to produce fully developed turbulence at the test section, and the width-to-height ratio is sufficiently large to avoid the sidewall effect for a large proportion of the cross-section. The system was designed to study the dynamics of complex-shaped particles in wall-bounded turbulence, the characteristics of which can be finely controlled. A maximum bulk velocity of up to 0.8 m s-1 can be achieved, corresponding to a bulk Reynolds number of up to 7 × 104 (shear Reynolds number ≈1580), and flow parameters can be controlled within ±0.1%. The transparent channel design and aluminum structures allow easy optical access, which enables multiple laser and camera arrangements. With the current optical setup, a measurement volume of up to 54 × 14 × 54 mm3 can be imaged and reconstructed with six cameras from the top, bottom, and sides of the channel. Finally, the in-house developed reconstruction and tracking procedure allows us to measure the full motion of complex objects (i.e., shape reconstruction, translational, and rotational motions), and in this instance, it is applied to the case of microscopic, non-isotropic polyamide fibers.

Modelling the direct virus exposure risk associated with respiratory events.

The outbreak of the COVID-19 pandemic highlighted the importance of accurately modelling the pathogen transmission via droplets and aerosols emitted while speaking, coughing and sneezing. In this work, we present an effective model for assessing the direct contagion risk associated with these pathogen-laden droplets. In particular, using the most recent studies on multi-phase flow physics, we develop an effective yet simple framework capable of predicting the infection risk associated with different respiratory activities in different ambient conditions. We start by describing the mathematical framework and benchmarking the model predictions against well-assessed literature results. Then, we provide a systematic assessment of the effects of physical distancing and face coverings on the direct infection risk. The present results indicate that the risk of infection is vastly impacted by the ambient conditions and the type of respiratory activity, suggesting the non-existence of a universal safe distance. Meanwhile, wearing face masks provides excellent protection, effectively limiting the transmission of pathogens even at short physical distances, i.e. 1 m.

Short-range exposure to airborne virus transmission and current guidelines.

After the Spanish flu pandemic, it was apparent that airborne transmission was crucial to spreading virus contagion, and research responded by producing several fundamental works like the experiments of Duguid [J. P. Duguid, J. Hyg. 44, 6 (1946)] and the model of Wells [W. F. Wells, Am. J. Hyg. 20, 611-618 (1934)]. These seminal works have been pillars of past and current guidelines published by health organizations. However, in about one century, understanding of turbulent aerosol transport by jets and plumes has enormously progressed, and it is now time to use this body of developed knowledge. In this work, we use detailed experiments and accurate computationally intensive numerical simulations of droplet-laden turbulent puffs emitted during sneezes in a wide range of environmental conditions. We consider the same emission-number of drops, drop size distribution, and initial velocity-and we change environmental parameters such as temperature and humidity, and we observe strong variation in droplets' evaporation or condensation in accordance with their local temperature and humidity microenvironment. We assume that 3% of the initial droplet volume is made of nonvolatile matter. Our systematic analysis confirms that droplets' lifetime is always about one order of magnitude larger compared to previous predictions, in some cases up to 200 times. Finally, we have been able to produce original virus exposure maps, which can be a useful instrument for health scientists and practitioners to calibrate new guidelines to prevent short-range airborne disease transmission.

Turbulent Drag Reduction by a Near Wall Surface Tension Active Interface.

In this work we study the turbulence modulation in a viscosity-stratified two-phase flow using Direct Numerical Simulation (DNS) of turbulence and the Phase Field Method (PFM) to simulate the interfacial phenomena. Specifically we consider the case of two immiscible fluid layers driven in a closed rectangular channel by an imposed mean pressure gradient. The present problem, which may mimic the behaviour of an oil flowing under a thin layer of different oil, thickness ratio h2/h1 = 9, is described by three main flow parameters: the shear Reynolds number Reτ (which quantifies the importance of inertia compared to viscous effects), the Weber number We (which quantifies surface tension effects) and the viscosity ratio λ = ν1/ν2 between the two fluids. For this first study, the density ratio of the two fluid layers is the same (ρ2 = ρ1), we keep Reτ and We constant, but we consider three different values for the viscosity ratio: λ = 1, λ = 0.875 and λ = 0.75. Compared to a single phase flow at the same shear Reynolds number (Reτ = 100), in the two phase flow case we observe a decrease of the wall-shear stress and a strong turbulence modulation in particular in the proximity of the interface. Interestingly, we observe that the modulation of turbulence by the liquid-liquid interface extends up to the top wall (i.e. the closest to the interface) and produces local shear stress inversions and flow recirculation regions. The observed results depend primarily on the interface deformability and on the viscosity ratio between the two fluids (λ).

Turbulent breakage of ductile aggregates.

In this paper we study breakage rate statistics of small colloidal aggregates in nonhomogeneous anisotropic turbulence. We use pseudospectral direct numerical simulation of turbulent channel flow and Lagrangian tracking to follow the motion of the aggregates, modeled as sub-Kolmogorov massless particles. We focus specifically on the effects produced by ductile rupture: This rupture is initially activated when fluctuating hydrodynamic stresses exceed a critical value, σ>σ(cr), and is brought to completion when the energy absorbed by the aggregate meets the critical breakage value. We show that ductile rupture breakage rates are significantly reduced with respect to the case of instantaneous brittle rupture (i.e., breakage occurs as soon as σ>σ(cr)). These discrepancies are due to the different energy values at play as well as to the statistical features of energy distribution in the anisotropic turbulence case examined.

Upscale energy transfer and flow topology in free-surface turbulence.

Free-surface turbulence, albeit constrained onto a two-dimensional space, exhibits features that barely resemble predictions of simplified two-dimensional modeling. We demonstrate that, in a three-dimensional open channel flow, surface turbulence is characterized by upscale energy transfer, which controls the long-term evolution of the larger scales. We are able to associate downscale and upscale energy transfer at the surface with the two-dimensional divergence of velocity. We finally demonstrate that surface compressibility confirms the strongly three-dimensional nature of surface turbulence.

Time persistence of floating-particle clusters in free-surface turbulence.

We study the dispersion of light particles floating on a flat shear-free surface of an open channel in which the flow is turbulent. This configuration mimics the motion of buoyant matter (e.g., phytoplankton, pollutants, or nutrients) in water bodies when surface waves and ripples are smooth or absent. We perform direct numerical simulation of turbulence coupled with Lagrangian particle tracking, considering different values of the shear Reynolds number (Re_{τ}=171 and 509) and of the Stokes number (0.06

Protocols to compare infusion distribution of wound catheters.

Multi-holed wound catheters are increasingly used in clinical practice to administer analgesic/anaesthetic locally to the painful region. The distribution of flow infused during controlled (continuous or intermittent) administration of medication is believed to be an important issue for successful pain relief. Nevertheless, this information is not available from the literature. In this paper, we propose protocols to screen the performance of wound infusion catheters in the laboratory environment. Four wound infusion systems (PAINfusor by Baxter, OnQ Pump with Soaker catheter by I-Flow, PolyFuser Polymedic by Temena and Infiltralong by Pajunk) have been tested. Test results demonstrate that the distribution of the infused flow is different for the four catheters and closely connected to the catheter design (i.e. hole size and position, lumen diameter). Catheters characterized by small size holes (e.g. Baxter, Pajunk) distribute the flow more homogeneously than catheters characterized by large size holes (e.g. I-Flow, Temena). The distribution of infused flow does not change significantly during continuous or intermittent infusion.

Water quality control in the river Arno.

In this work, we analyzed pollution in the river Arno using a non-steady advection-dispersion-reaction equation (ADRE) calibrated on experimental data. We examined the influence different pollution control strategies have on dissolved oxygen (DO). We considered (i) flow rate variation; (ii) local oxygenation at critical points; (iii) dynamic modification of wastewater load. Results indicate first, that reservoir management is effective in reducing pollution; second, that local oxygenation is necessary to ensure that DO does not fall below safety levels; and finally, that tuning wastewater loads appears to be impractical to manage the river quality given the stringent limitations it would impose on the industrial effluents.