Velocities and turbulent stresses of free-surface skimming flows over triangular cavities

Velocity distributions in supercritical open-channel flows over stepped cavities have traditionally been described using a power-law approach or theoretical solutions of plane mixing layers. These approaches were found to be valid either above the step edges or above/within step cavities, but no generalized model is available. In this study, a four-layered velocity model is proposed, which combines different physical concepts, including the mixing layer, log-layer, wake function, and free-stream layer. This multilayer model was applied to previous experimental stepped chute data, providing novel opportunities to comparatively assess the relative contribution of individual physical effects on the velocity profile. Model parameters provided insights into flow hydrodynamics, comprising mixing layer length scales and shear velocities. Equations for Reynolds shear stresses within the different layers were formulated using an eddy viscosity concept, while normal stresses and turbulent kinetic energy compared well to semitheoretical open-channel flow equations

Surface localization of gas sources on comet 67P/Churyumov-Gerasimenko based on DFMS/COPS data

We reconstruct the temporal evolution of the source distribution for the four major gas species H2O, CO2, CO, and O2 on the surface of comet 67P/Churyumov-Gerasimenko during its 2015 apparition. The analysis applies an inverse coma model and fits to data between August 6th 2014 and September 5th 2016 measured with the Double Focusing Mass Spectrometer (DFMS) of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) and the COmet Pressure Sensor (COPS). The spatial distribution of gas sources with their temporal variation allows one to construct surface maps for gas emissions and to evaluate integrated productions rates. For all species peak production rates and integrated productions rates per orbit are evaluated separately for the northern and the southern hemisphere. The nine most active emitting areas on the comet's surface are defined and their correlation to emissions for each of the species is discussed.Comment: 11 page

Seasonal changes of the volatile density in the coma and on the surface of comet 67P/Churyumov-Gerasimenko

Starting from several monthly data sets of Rosetta's COmetary Pressure Sensor we reconstruct the gas density in the coma around comet 67P/Churyumov-Gerasimenko. The underlying inverse gas model is constructed by fitting ten thousands of measurements to thousands of potential gas sources distributed across the entire nucleus surface. The ensuing self-consistent solution for the entire coma density and surface activity reproduces the temporal and spatial variations seen in the data for monthly periods with Pearson correlation coefficients of 0.93 and higher. For different seasonal illumination conditions before and after perihelion we observe a systematic shift of gas sources on the nucleus.Comment: 11 pages, 7 figures, accepted in MNRAS (2017

Linking turbulent waves and bubble diffusion in self-aerated open-channel flows: Two-state air concentration

High Froude-number flows become self-aerated when the destabilizing effect of turbulence overcomes gravity and surface tension forces. Traditionally, the resulting air concentration profile has been explained using single-layer approaches that invoke solutions of the advection-diffusion equation for air in water, i.e., bubbles' dispersion. Based on a wide range of experimental evidences, we argue that the complete air concentration profile shall be explained through the weak interaction of different canonical turbulent flows, namely a Turbulent Boundary Layer (TBL) and a Turbulent Wavy Layer (TWL). Motivated by a decomposition of the streamwise velocity into a pure wall flow and a free-stream flow [Krug et al., J. Fluid Mech. (2017), vol. 811, pp. 421--435], we present a physically consistent two-state formulation of the structure of a self-aerated flow. The air concentration is mathematically built upon a modified Rouse profile and a Gaussian error function, resembling vertical mass transport in the TBL and the TWL. We apply our air concentration theory to over 500 profiles from different data sets, featuring excellent agreement. Finally, we show that the turbulent Schmidt number, characterizing the momentum-mass transfer, ranges between 0.2 to 1, which is consistent with previous mass-transfer experiments in TBLs. Altogether, the proposed flow conceptualization sets the scene for more physically-based numerical modelling of turbulent mass diffusion in self-aerated flows.Comment: 47 pages, 7 figures, includes supplemental material, accepted for publication in Journal of Fluid Mechanic

Outgassing induced acceleration of comet 67P/Churyumov-Gerasimenko

Cometary activity affects the orbital motion and rotation state due to sublimation induced forces. The availability of precise rotation-axis orientation and position data from the Rosetta mission allows one to accurately determine the outgassing of comet Churyumov-Gerasimenko/67P. We derive the observed non-gravitational acceleration of 67P directly from the Rosetta spacecraft trajectory. From the non-gravitational acceleration we recover the diurnal outgassing variations and study a possible delay of the sublimation response with respect to the peak solar illumination. This allows us to compare the non-gravitational acceleration of 67P with expectations based on empirical models and common assumptions about the sublimation process. We use an iterative orbit refinement and Fourier decomposition of the diurnal activity to derive the outgassing induced non-gravitational acceleration. The uncertainties of the data reduction are established by a sensitivity analysis of an ensemble of best-fit orbits for comet 67P. We find that the Marsden non-gravitational acceleration parameters reproduce part of the non-gravitational acceleration but need to be augmented by an analysis of the nucleus geometry and surface illumination to draw conclusions about the sublimation process on the surface. The non-gravitational acceleration follows closely the subsolar latitude (seasonal illumination), with a small lag angle with respect to local noon around perihelion. The observed minor changes of the rotation axis do not favour forced precession models for the non-gravitational acceleration. In contrast to the sublimation induced torques, the non-gravitational acceleration does not put strong constraints on localized active areas on the nucleus. We find a close agreement of the orbit deduced non-gravitational acceleration and the water production independently derived from Rosetta in-situ measurement.Comment: 6 pages, 3 figures, accepted for A&A special issue Rosetta

Dust and gas emission from cometary nuclei: the case of comet 67P/Churyumov-Gerasimenko

Comets display with decreasing solar distance an increased emission of gas and dust particles, leading to the formation of the coma and tail. Spacecraft missions provide insight in the temporal and spatial variations of the dust and gas sources located on the cometary nucleus. For the case of comet 67P/Churyumov-Gerasimenko (67P/C-G), the long-term observations from the Rosetta mission point to a homogeneous dust emission across the entire illuminated surface. Despite the homogeneous initial distribution, a collimation in jet-like structures becomes visible. We propose that this observation is linked directly to the complex shape of the nucleus and projects concave topographical features into the dust coma. To test this hypothesis, we put forward a gas-dust description of 67P/C-G, where gravitational and gas forces are accurately determined from the surface mesh and the rotation of the nucleus is fully incorporated. The emerging jet-like structures persist for a wide range of gas-dust interactions and show a dust velocity dependent bending.Comment: 17 pages, with 7 figures. To appear in Advances in Physics X (2018

Safety criteria for the trafficability of inundated roads in urban floodings

The probability of unexpected urban flood hazards is steadily increasing due to global warming and climate change. Consequently, there is a growing need for safety criteria determining the trafficability of inundated roads to ensure a fast and safe evacuation of people in case of such events. In order to determine those criteria, experimental investigations on the stability of two scaled watertight vehicle models and of one prototype passenger car are conducted in a laboratory flume and a steel tank.The conducted flume experiments clearly show a dependency of vehicle stability on the flow angle, whereas the prototype experiments indicate that floating water depths are higher in prototype than in model scale, which is due to the use of a watertight vehicle model. Based on both experiments, a constant total head is proposed as decisive parameter for determining trafficability. This parameter approximates the measured stability curves and can be easily adopted in practice. Furthermore, it is in accordance with fording depths evaluated from relevant literature or by means of manufacturer inquiry. The recommended safety criteria for passenger cars and emergency vehicles are total heads of h(E) =0.3 m =const. and h(E)=0.6 m=const., respectively. (C) 2016 Elsevier Ltd. All rights reserved