Stefan Horender
Leibniz Association
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Publication
Featured researches published by Stefan Horender.
Water Resources Research | 2013
Benjamin Walter; Stefan Horender; Cb Christof Gromke; Michael Lehning
[1] Fluorescent Particle Tracking Velocimetry (FPTV) measurements of the pore-scale water flow through the pore space of a wet-snow sample are presented to demonstrate the applicability of this measurement technique for snow. For the experiments, ice-cooled water seeded with micron sized fluorescent tracer particles is either sprinkled on top of a snow sample to investigate saturated and unsaturated gravity-driven flow or supplied from a reservoir below the snow sample to generate upward flow driven by capillary forces. The snow sample is illuminated with a laser light sheet and the fluorescent light of the particles transported with the water in the pore space is recorded with a high-speed camera equipped with an optical filter. Tracking algorithms are applied to the images to obtain flow paths and flow velocities. A flow loop found in a pore space for the case of saturated gravity flow together with the tortuosity of the particle trajectories indicate the three-dimensionality of the water flow in wet snow. The average vertical flow velocities in the pore spaces were 11.2 mm s � 1 for the downward saturated gravity flow and 9.6 mm s � 1 for the upward flow that is driven by capillary forces for the limited cases presented as examples of the measurement technique. In the case of unsaturated gravity-driven flow, the average and the maximum flow velocities were found to be 30 times smaller than for the saturated gravity flow. Velocity histograms show that the fraction of the total water flowing against the main flow direction was about 3–5%, and that the horizontal velocities average to zero for both the saturated gravity-driven and the capillary flow.
Geophysical Research Letters | 2014
Benjamin Walter; Stefan Horender; Christian Voegeli; Michael Lehning
A widely used, yet thus far unproven, fluid dynamical hypothesis originally presented by P. R. Owen 50years ago, states that the surface shear stress induced by a fluid on the ground during equilibrium sediment saltation is constant and independent of the magnitude of the fluid velocity and consequently the particle mass flux. This hypothesis is one of the key elements in almost all current model descriptions of sediment erosion. We measured the surface shear stress in a drifting-sand wind tunnel and found Owens hypothesis being merely an approximation of the real situation. A significant decrease of the fluid stress with increasing wind velocities was measured for low to intermediate particle mass fluxes. For high particle mass fluxes, Owens hypothesis essentially holds, although a slight increase of the fluid stress was measured.
Boundary-Layer Meteorology | 2016
P. Crivelli; Enrico Paterna; Stefan Horender; Michael Lehning
We compare two of the most common methods of quantifying mass flux, particle numbers and particle-size distribution for drifting snow events, the snow-particle counter (SPC), a laser-diode-based particle detector, and particle tracking velocimetry based on digital shadowgraphic imaging. The two methods were correlated for mass flux and particle number flux. For the SPC measurements, the device was calibrated by the manufacturer beforehand. The shadowgrapic imaging method measures particle size and velocity directly from consecutive images, and before each new test the image pixel length is newly calibrated. A calibration study with artificially scattered sand particles and glass beads provides suitable settings for the shadowgraphical imaging as well as obtaining a first correlation of the two methods in a controlled environment. In addition, using snow collected in trays during snowfall, several experiments were performed to observe drifting snow events in a cold wind tunnel. The results demonstrate a high correlation between the mass flux obtained for the calibration studies (
Journal of Glaciology | 2014
Cb Christof Gromke; Stefan Horender; Benjamin Walter; Michael Lehning
Boundary-Layer Meteorology | 2014
C. D. Groot Zwaaftink; Marc Diebold; Stefan Horender; Jan Overney; G. Lieberherr; Marc B. Parlange; Michael Lehning
r \geqslant 0.93
The Cryosphere | 2016
Rebecca Mott; Enrico Paterna; Stefan Horender; Philipp Crivelli; Michael Lehning
Boundary-Layer Meteorology | 2017
Benjamin Walter; Christian Voegeli; Stefan Horender
r⩾0.93) and good correlation for the drifting snow experiments (
International Snow Science Workshop Grenoble – Chamonix Mont-Blanc - October 07-11, 2013 | 2013
Stefan Horender; Christine D. Groot Zwaaftink; Beni Walter; Michael Lehning
Bulletin of the American Physical Society | 2015
P. Crivelli; Enrico Paterna; Stefan Horender; Michael Lehning
r \geqslant 0.81
Geophysical Research Letters | 2014
Benjamin Walter; Stefan Horender; Christian Voegeli; Michael Lehning