Physics

There is overwhelming evidence on SARS-CoV-2 Airborne Transmission (AT) in the ongoing COVID-19 outbreak. It is extraordinarily difficult, however, to deduce a generalized framework to assess the relative airborne transmission risk with respect to other modes. This is due to the complex biophysics entailed in such phenomena. Since the SARS outbreak in 2002, Computational Fluid Dynamics (CFD) has been one of the main tools scientists used to investigate AT of respiratory viruses. Now, CFD simulations produce intuitive and physically plausible colour-coded results that help scientists understand SARS-CoV-2 airborne transmission patterns. In addition to validation requirements, for any CFD model to be of epistemic value to the scientific community; it must be reproducible. In 2020, more than 45 published studies investigated SARS-CoV-2 airborne transmission in different scenarios using CFD. Here, I systematically review the published CFD studies of COVID-19 and discuss their reproducibility criteria with respect to the CFD modeling process. Using a Weighted Scoring Model (WSM), I propose a novel reproducibility index for CFD simulations of SARS-CoV-2 AT. The proposed index (0??R CFD j ??) relies on three reproducibility criteria comprising 10 elements that represent the possibility of a CFD study (j) to be reproduced. Frustratingly, only 3 of 23 studies (13%) achieved full reproducibility index ( R CFD j ??.9) while the remaining 87% were found generally irreproducible ( R CFD j <0.9) . Without reproducible models, the scientific benefit of CFD simulations will remain hindered, fragmented and limited. In conclusion, I call the scientific community to apply more rigorous measures on reporting and publishing CFD simulations in COVID-19 research.

The Note presents an alternative derivation and interpretation of the Germano identity and its error, showing that the Germano identity error directly estimates the residual of the LES equation, i.e., the misfit when evaluating the inexact equation for the exact solution, and therefore represents the source of errors in LES. This has many applications, including for optimal output-based grid/filter-adaptation and uncertainty quantification in LES.

Fuel sprays produce high-velocity, jet-like flows that impart turbulence onto the ambient flow field. The spray-induced turbulence augments fuel-air mixing, which has a primary role in controlling pollutant formation and cyclic variability in engines. This paper presents tomographic particle image velocimetry (TPIV) measurements to analyse the 3D spray-induced turbulence during the intake stroke of a direct-injection engine. The spray produces a strong spray-induced jet in the far field, which travels through the cylinder and imparts turbulence onto the surrounding flow. Planar high-speed PIV measurements at 4.8 kHz are combined with TPIV at 3.3 Hz to evaluate spray particle distributions and validate TPIV measurements in the particle-laden flow. An uncertainty analysis is performed to assess the uncertainty associated with vorticity and strain rate components. TPIV analyses quantify the spatial domain of the turbulence in relation to the SIJ and describe how turbulent flow features such as turbulent kinetic energy, strain rate and vorticity evolve into the surrounding flow field. Access to the full tensors facilitate the evaluation of turbulence for individual spray events. TPIV images reveal the presence of strong shear layers (visualized by high S magnitudes) and pockets of elevated vorticity along the immediate boundary of the SIJ. Values are extracted from spatial domains extending in 1mm increments from the SIJ. Turbulence levels are greatest within the 0-1mm region from the SIJ boarder and dissipate with radial distance. Individual strain rate and vorticity components are analyzed in detail to describe the relationship between local strain rates and 3D vortical structures produced within strong shear layers of the SIJ. Analyses are intended to understand the flow features responsible for rapid fuel-air mixing and provide valuable data for the development of numerical models.

We present an exact analytical solution to the problem of shear dispersion given a general initial condition. The solution is expressed as an infinite series expansion involving Mathieu functions and their eigenvalues. The eigenvalue system depends on the imaginary parameter q=2ik Pe, with k the wavenumber that determines the tracer scale in the initial condition and Pe the Péclet number. Solutions are valid for all Pe, t>0 , and k>0 except at specific values of q= q EP ??called Exceptional Points (EPs), the first occurring at q EP 0 ??.468i . For values of q�?.468i , all the eigenvalues are real, different and eigenfunctions decay with time, thus shear dispersion can be represented as a diffusive process. For values of q�?.468i , pairs of eigenvalues coalesce at EPs becoming complex conjugates, the eigenfunctions propagate and decay with time, and so shear dispersion is no longer a purely diffusive process. The limit q?? is approached by the small Péclet number limit for all finite k>0 , or equally by the large Péclet number limit as long as 2k??/ Pe. The latter implies k?? , strong separation of scales between the tracer and flow. The limit q?��? results from large Péclet number for any k>0 , or from large k and non-vanishing Pe. We derive an exact closure that is continuous in wavenumber space. At small q , the closure approaches a diffusion operator with an effective diffusivity proportional to U 2 0 /κ , for flow speed U 0 and diffusivity κ . At large q , the closure approaches the sum of an advection operator plus a half-derivative operator (differential operator of fractional order), the latter with coefficient proportional to κ U 0 ????????.

Understanding the detailed flame transport in IC engines is important to predict ignition, rate of heat release and assess engine performance. This is particularly important for RANS and LES engine simulations, which often struggle to accurately predict flame propagation and heat release without first adjusting model parameters. Detailed measurements of flame transport are required to guide model development. This work introduces an experimental dataset designed to study the detailed flame transport and flame/flow dynamics for SI engines. Simultaneous dual-plane OH-LIF and stereoscopic PIV is used to acquire 3D measurements of unburnt gas velocity, flame displacement speed and overall flame velocity during the early flame development. Experiments are performed in an optical engine operating at 800 and 1500 RPM with premixed C8H18-air mixtures. Analysis reveals several distinctive flame/flow configurations that yield a positive or negative flame displacement for which the flame progresses towards the reactants or products, respectively. For the operating conditions utilized, Sd exhibits and inverse relationship with flame curvature; a strong correlation between negative Sd and convex flame contours is observed. Trends are consistent with thermo-diffusive flames, but have not been quantified in IC engines. Flame wrinkling is more severe at the higher RPM, which broadens Sd distribution towards higher positive and negative velocities. Spatially-resolved distributions of Ugas and Sd describe in-cylinder locations where convection or thermal diffusion is the dominating mechanism contributing to flame transport. Findings are discussed in relation to common engine flow features, including flame transport near solid surfaces. Findings are designed to support engine simulation validations.

Most biological fluids are viscoelastic, meaning that they have elastic properties in addition to the dissipative properties found in Newtonian fluids. Computational models can help us understand viscoelastic flow, but are often limited in how they deal with complex flow geometries and suspended particles. Here, we present a lattice Boltzmann solver for Oldroyd-B fluids that can handle arbitrarily-shaped fixed and moving boundary conditions, which makes it ideally suited for the simulation of confined colloidal suspensions. We validate our method using several standard rheological setups, and additionally study a single sedimenting colloid, also finding good agreement with literature. Our approach can readily be extended to constitutive equations other than Oldroyd-B. This flexibility and the handling of complex boundaries holds promise for the study of microswimmers in viscoelastic fluids.

An useful approximation for the displacement of two immiscible fluids in a porous medium is the Hele-Shaw model. We consider several liquids with different constant viscosities, inserted between the displacing fluids. The linear stability analysis of this model leads us to an ill-posed problem. The growth rates (in time) of the perturbations exist iff some compatibility conditions on the interfaces are verified. We prove that these conditions cannot be fulfilled.

Numerical modelling of flow in stepped spillways is considered, focusing on a highly economical approach combining interface capturing with explicit modelling of air entrainment. Simulations are performed on spillways at four different Froude numbers, with flow parameters selected to match available experimental data. First, experiments using the model developed by Lopes et al. (Int. J. Nonlin. Sci. Num., 2017) are conducted. An extensive simulation campaign is used to carefully evaluate the predictive accuracy of the model, the influence of various model parameters, and sensitivity to grid resolution. Results reveal that, at least for the case of stepped spillways, the number of parameters governing the model can be reduced. A crucial identified deficiency of the model is its sensitivity to grid resolution. To improve the performance of the model in this respect, modifications are proposed for the interface detection algorithm and the transport equation for the volume fraction of entrained air. Simulations using the improved model formulation demonstrate better agreement with reference data for all considered flow conditions. A parameter-free criterion for predicting the inception point of air entrainment is also tested. Unfortunately, the accuracy of the considered conventional turbulence models proved to be insufficient for the criterion to work reliably.

To isolate the multiscale dynamics of the logarithmic layer of wall-bounded turbulent flows, a novel numerical experiment is conducted in which the mean tangential Reynolds stress is eliminated except in a subregion corresponding to the typical location of the logarithmic layer in channels. Various statistical comparisons against channel flow databases show that, despite some differences, this modified flow system reproduces the kinematics and dynamics of natural logarithmic layers well, even in the absence of a buffer and an outer zone. This supports the previous idea that the logarithmic layer has its own autonomous dynamics. In particular, the results suggest that the mean velocity gradient and the wall-parallel scale of the largest eddies are determined by the height of the tallest momentum-transferring motions, implying that the very large-scale motions of wall-bounded flows are not an intrinsic part of logarithmic-layer dynamics. Using a similar set-up, an isolated layer with a constant total stress, representing the logarithmic layer without a driving force, is simulated and examined.

Describing effects of small but finite inertia on suspended particles is a fundamental fluid dynamical problem that has never been solved in full generality. Modern microfluidics has turned this academic problem into a practical challenge through the use of high-frequency oscillatory flows, perhaps the most efficient way to take advantage of inertial effects at low Reynolds numbers, to precisely manipulate particles, cells and vesicles without the need for charges or chemistry. The theoretical understanding of flow forces on particles has so far hinged on the pioneering work of Maxey and Riley (MR in the following), almost 40 years ago. We demonstrate here theoretically and computationally that oscillatory flows exert previously unexplained, significant and persistent forces, that these emerge from a combination of particle inertia and spatial flow variation, and that they can be quantitatively predicted through a generalization of MR.