Dr. Fabian Wolfsperger

Wieviel Schnee aus 1 m³ Wasser entsteht ist bei der Beschneiung eine entscheidende Größe. Die Annahme eines realistischen Wertes dieses sogenannten Wasser-Schnee Faktors ist wichtig, um die Produktivität eines Schneeerzeugers sowie den Energieverbrauch (kWh) pro Kubikmeter Schnee korrekt zu bestimmen. Das SLF in Davos führte zu diesen Kennzahlen Studien durch, die bezüglich Planung von Beschneiungsanlagen zu verblüffenden Ergebnissen führen.

Quantifying the density (ρ) and liquid water content (LWC) of snow is crucial for any physical process in the snowpack. A widely used method to measure those quantities involves a ca-pacitance sensor that subjects the snow to an electrical field, an idea first introduced by Denoth (1989). While this dielectric measurement principle is known to be well-working from yearlong use of Denoth prototype sensors, such instruments are not yet commercially available. To this end, we developed a new ρ and LWC sensor with the goal of producing a small batch series in the near future. The sensor is a single coplanar capacitor integrated onto a printed circuit board that measures the relative permit-tivity (ε) of snow. We evaluated the sensor in the field through 13 dry and wet snow profiles and com-pared ε-values obtained from the new sensor with those measured with a Denoth prototype. An addi-tional validation was carried out in the cold lab on homogenous dry and wet snow samples. Field meas-urements deviated on average by 9.7 % (RMSE) and lab measurements by 2.9 % (RMSE) from the reference values. A comparison of the snow density, retrieved from ε using the Denoth inversion, with densities from volume weighing revealed an average deviation of 62 kg m-3 (19.7 %). The new sensor provides a reliable instrument to replace the established, but no longer available Denoth sensors. The accuracy of the new (and likewise the old) sensor may be improved if a better inversion model was available. A thorough understanding of the uncertainties in the dielectric calibration and uncertainties of the empirical permittivity parameterization relating ε, ρ and LWC may therefore lead to improved estimates in the future using the same device.

Objectives: Design parameters and landing impacts for selected snow park jumps in Switzerland were compared with the parameters recommended to increase the jumps’ safety by the Swiss Council for Accident Prevention (BFU). High impact zones were identified to help snow park shapers optimize the design of their jumps. A rough estimate of the influence of snow hardness on landing impacts was also provided.
Design: During the 2020/2021 winter season three-dimensional geometries of 23 jumps were captured using differential global navigation satellite system and terrestrial laser scanning. A point mass model was used to numerically calculate trajectories. The equivalent fall height (eFH) was used to quantify landing impacts and an empiric snow-deformation function was applied to take the effect of snow hardness into consideration. Workshops were held to discuss results and transfer findings.
Methods: 2D-profiles of the jumps were estimated by projecting the captured 3D position data onto the longitudinal cross-section plane. Table and landing geometry were smoothed and interpolated to a spatial resolution of 0.1 ​m, while the kicker was fitted with a 2nd order polynomial. Trajectories were numerically calculated for take-off speeds from 6 to 17.6 ​m ​s⁻¹ including aerodynamic forces using the Runge-Kutta method. The calculated eFH at the landing points were used to divide the landing into low-impact, medium-impact, and high-impact zones.
Results: Medium sized jumps had a low-impact zone of sufficient length (>6 ​m) and eFH smaller than 1.5 ​m throughout the entire table meeting the BFU recommendations. Nevertheless, critical eFH larger than 1.5 ​m, were obtained when take-off speeds increased by only 1.14 ​m ​s⁻¹. Large jumps had low-impact zone lengths in accord with the recommendations (>9 ​m), but high eFH (2.3–3.4 ​m) occurred for table landings. 10 of the 13 XL-jumps had long low-impact zones of approximately 12–15 ​m. Besides the risk of high impact landings towards the end of the landing area, as found similarly for the smaller jumps, portions of XL-jumps had very high eFH (2.6–4.6 ​m) for table landings.
Conclusions: The study confirmed the existing BFU recommendations of size categories, design parameters and landing impacts limits as prevalent and practicable and provided knowledge for future safety recommendations. Modifying table geometries and taking measures to limit the in-run speeds would help reduce landing impacts, and the hazard due to hard snow conditions should also be considered.

Schnee begegnet uns in den verschiedensten Formen. Die großen Unterschiede der Eigenschaften von Schnee werden beim Bearbeiten offensichtlich: Frischer Neuschnee lässt sich mit einem Gebläse wegschaffen. Eine Absprungkante auf einer Rennpiste hingegen kann nur mit der Motorsäge bearbeitet werden. Verantwortlich für diese sehr unterschiedlichen Materialeigenschaften ist die große Spanne der Dichte, die Nähe des Schnees zum Schmelzpunkt und der sehr unterschiedliche Grad der Sinterung, wie im Folgenden erklärt wird.

Previous research has shown that friction between ski and snow can vary substantially due to changes in snow conditions. The variation of friction affects the speed a freestyle skier or snowboarder (athlete) reaches during the in-run of a jump. Athletes risk severe injuries if their take-off speed is not within the right margin to land in the "sweet spot" zone. To reduce the risk of injury, snow park designers and competition managers need to calculate the speed athletes reach during the in-run. However, despite multiple attempts over the last decades, to date no model can predict ski-snow friction from snow physical quantities. Hence, simulations of in-run speeds suffer from insufficient validity. For the first time, this work combines kinematic athlete data and comprehensive snow surface measurements to infer the coefficient of friction of freestyle skis and snowboards across a wide range of snow conditions. Athletes' point mass kinematics were recorded at more than 200 straight gliding runs with differential global navigation satellite systems. The subjects' air drag and lift were deployed from wind tunnel measurements. Along with the kinematic data and data from wind measurements, a mechanical model of the athlete was established to solve the equation of motion for the coefficient of friction between ski/snowboard and snow. The friction coefficients for ski (snowboard) ranged from 0.023 ± 0.006 (0.026 ± 0.008) to 0.139 ± 0.018 (0.143 ± 0.017) and could be explained well (R_adj² = 0.77) from the measured snow parameters using a multivariate statistical model. Our results provide a new quantitative tool for practitioners to predict the friction of skis and snowboard on snow of various conditions, which aims to increase athletes’ safety in slopestyle and big air.

Objectives: This study aimed to investigate air drag and lift during the in-run and flight phase of ski and snowboard slopestyle and big air, to allow more valid modeling of jumps and hence reduce injury risk.
Design: We present an experimental, multiple single athlete study based on wind tunnel measurements of 4 skiers and 3 snowboarders.
Methods: Measurements were carried out in a closed loop wind tunnel, measuring airflow speed and 3D forces acting on the athletes. Athletes performed trials in typical postures at 35, 60 and 85 km/h wearing slim-, regular- and wide fit apparel. Drag and lift area (c_DA; c_LA) were calculated and analyzed using linear and multiple regression to describe their dependencies on posture, apparel and speed.
Results: c_DA values were higher than earlier assumed and ranged from 0.3 to 0.95 m² for skiers and from 0.35 to 0.55 m² for snowboarders, primarily dominated by posture, and followed by apparel. c_LA ranged from −0.1 to 0.45 m² for skiers and from 0.04 to 0.17 m² for snowboarders. To facilitate more valid jump modeling posture- and apparel-dependent formulations for air drag coefficients were provided and the consequences of sport specific differences on modeling were highlighted.
Conclusions: Applying the air drag coefficients and relationships determined in this study will help to improve validity of jump modeling in big air and slopestyle. The variability in aerodynamic forces in slopestyle and big air is caused by differences between sports, posture and apparel.

To maximize performance in elite winter sports, considerable effort has been made to find gliding surfaces, which decrease snow friction to a minimum. Shifting away the focus from elite sport to mass winter sport equipment, this study aimed to evaluate materials providing acceptable gliding performance and in the same time high structural strength. Therefore, we investigated wet and dry snow friction of known engineering polymers with different surface configurations. Static and kinetic friction coefficients (COF) on snow of twelve gliders made of different polymers were measured on a linear friction tester placed in a cold chamber. Of each polymer, two gliders were built and tested: one with a smooth surface, another one with a stone grinded surface. Roughness parameters and dynamic contact angles were measured to characterize the surfaces. Dry and wet snow samples were prepared by grinding ice into powder followed by sieving, compressing and natural snow sintering. The snow surface temperature was measured before each experiment with a pyrometer. Liquid water content was generated directly before each experiment by applying infrared light. COF on wet and dry snow were correlated with mean R_a-values and mean contact angles. Mean static COF ranged from 0.05 to 0.375 on dry, and from 0.133 to 0.674 on wet snow. On dry snow, only a moderate negative relationship between static COF and contact angle was found (r_pearson = -0.6), whereas on wet snow a strong negative relationship was found (r_pearson = -0.92). Static COF and Ra showed a moderate relationship only on dry snow (r_pearson = 0.62). Excluding HDPE and ABS, smooth samples showed distinctly lower static friction than structured samples. The study showed that snow friction is a complex interplay of surface topography, hydrophobicity and mechanical properties, especially on wet snow. The typical texture for HDPE ski bases did not sufficiently decrease snow friction of the tested engineering polymers. On wet snow, hydrophobicity was the most important factor for good gliding. On dry snow, surface roughness had a stronger influence than hydrophobicity. Snow had clearly the strongest influence on the polymer - snow friction because it is a highly variable material with quickly changing physical properties.

Alpine as well as Nordic skiing tourism strongly depend on the production of machine-made snow for the timely opening of the winter season. However, it is likely that sublimation, evaporation, wind drift, and the discharge of unfrozen water to the ground will result in the loss of significant parts of the water used. The relation between these water losses and the ambient meteorological conditions is poorly understood. We present results from a series of 12 detailed snow-making field tests performed in a ski resort near Davos, Switzerland. Water inflows, measured at the snow machine, are related to the mass of snow deposited on the ground. Snow amounts are calculated from accumulated volumes, measured with terrestrial laser scanning (TLS), and manually sampled snow densities. Additionally, samples of liquid water contents (LWCs) of the produced snow are presented. We find that 7 to 35 ± 7% (mean 21%) of the consumed water was lost during snow-making and that the loss is strongly related to the ambient meteorological conditions. Linear regression analysis shows that water losses increase with air temperature (TA). Combining our data with observations from earlier field measurements shows similar correlations.

Snow sports remain very popular. As well as classic skiing on slopes in traditional winter sports destinations like the Alps, new and often fun alternatives are now available in snow parks. Meanwhile, populous countries in Asia are discovering the potential of skiing as a recreational sport. All these developments are imposing increasingly stringent requirements on snow preparation. At the same time, climate change is presenting additional challenges for managers of slopes and cross-country ski trails. [...]

Competitions in Paralympic alpine skiing are complex because athletes with different disabilities compete against each other in the same categories. There are three categories (visually impaired (VI); standing; sitting) subdivided into 19 sport classes (SCs) (International Paralympic Committee, 2016).

There are various exogenous and endogenous factors influencing an athlete's performance in a competition. In non-disabled competitive alpine skiing, examples of exogenous factors include slope characteristics, course setting, and snow conditions. Certain slope and course-setting characteristics provoke larger time differences among athletes than other characteristics in non-disabled competitive alpine skiing (Spörri, Kröll, Arnesberger, Blake, & Müller, 2012; Waibel, Huber, & Spitzenpfeil, 2009).

Unfortunately, research into slopes, course setting, and snow conditions and their influence on athletes' performance in competitive Paralympic alpine skiing is rare.

The primary aim of this study was to describe slope characteristics, course setting, and snow conditions for a complete run and for intermediate sections in giant slalom (GS) and slalom (SL) World Cup races in the 2014-2015 and 2015-2016 seasons. The secondary aim was to characterize, specifically, the men's inter-SC differences in performance regarding steepness, course setting, and snow conditions.

Summer storage of snow for tourism has seen an increasing interest in the last years. Covering large snow piles with materials such as sawdust enables more than two-thirds of the initial snow volume to be conserved. We present detailed mass balance measurements of two sawdust-covered snow piles obtained by terrestrial laser scanning during summer 2015. Results indicate that 74 and 63% of the snow volume remained over the summer for piles in Davos, Switzerland and Martell, Italy. If snow mass is considered instead of volume, the values increase to 83 and 72%. The difference is attributed to settling and densification of the snow. Additionally, we adapted the one-dimensional, physically based snow cover model SNOWPACK to perform simulations of the sawdust-covered snow piles. Model results and measurements agreed extremely well at the point scale. Moreover, we analysed the contribution of the different terms of the surface energy balance to snow ablation for a pile covered with a 40 cm thick sawdust layer and a pile without insulation. Short-wave radiation was the dominant source of energy for both scenarios, but the moist sawdust caused strong cooling by long-wave emission and negative sensible and latent heat fluxes. This cooling effect reduces the energy available for melt by up to a factor of 12. As a result only 9% of the net short-wave energy remained available for melt. Finally, sensitivity studies of the parameters "thickness of the sawdust layer", "air temperature", "precipitation" and "wind speed" were performed. We show that sawdust thickness has a tremendous effect on snow loss. Higher air temperatures and wind speeds increase snow ablation but less significantly. No significant effect of additional precipitation could be found as the sawdust remained wet during the entire summer with the measured quantity of rain. Setting precipitation amounts to zero, however, strongly increased melt. Overall, the 40 cm sawdust provides sufficient protection for mid-elevation (approx. 1500m.a.s.l.) Alpine climates and can be managed with reasonable effort.

The ski manufacturing industry is characterized by short product life cycles and high innovation pressure in order to meet customer's expectations of progressive improvements of skis. To reduce development time a FE model was developed and validated to simulate the influence of changes of the ski construction on its bending and torsional stiffness. Moreover, the study aimed to evaluate the feasibility of a novel ski sandwich construction with a load dependent bending stiffness. Therefore, the FE model was used to define required strain-stiffening of a single construction layer material to reach a perceptible change of the overall ski's bending stiffness. Two existing skis with identical geometry but distinctive differences of their torsional and bending stiffness were modelled using ANSYS Workbench. The sandwich construction was modelled by 18 solid bodies with bonded contacts: core, sidewalls, edges, upper and lower face with 13 layers including 3 additional resin interlayers. Orthotropic linear elastic material properties were taken from data sheets of suppliers and from material tests of the manufacturer. Sweep meshing was used to create an all-hex mesh of solid elements. Static structural simulations of a 3-point bending test and a torsion test were run in accordance to existing laboratory tests to validate the model. For both skis, the FE model showed good agreement with the experimental data for ski A and B. The simulated overall bending stiffness (center spring constant) was, respectively 2.8% and 3.2% higher compared to the experiments. The elastic curves revealed the model as slightly too stiff at the afterbody and too soft at the forebody. The simulated torsional stiffness (torsional spring constant) was, respectively 2.1% and 4.2% lower than found in the experiments. In the development process, for example, the model was then used to quantify the influence of edge profile heights on the ski's overall bending stiffness (3% per 0.1 mm edge height). The application of strain-stiffening materials to realize a load depending ski stiffness turned as not feasible due to too small strains within the ski structure. A realistic representation of the different construction layers of an alpine ski is still challenging, especially due to the heterogeneity of the fibre compound layers and the resin distribution. Using bulk properties of the upper and lower face of the sandwich construction is not an alternative. To virtually test the influence of new materials and layups every single layer has to be represented. Using two skis for validation and additional resin interlayers for calibration appeared appropriate in order to achieve adequate model results.

Quantifying snow grain size is crucial to analyze radiative transfer and mechanical interactions in the snow cover. We present a nondestructive method for fast measurements of snow optically equivalent diameter (OED). The method consists of diffuse near-infrared reflectance measurements by a compact integrating sphere setup to derive OED. This principle is realized in the handheld InfraSnow instrument. The correlation between snow OED and reflectance is calculated by applying Monte Carlo ray tracing to a 3-D implementation of the measurement geometry. Including the geometrical boundary conditions is essential to obtain a good agreement between modeled and measured InfraSnow reflectance values. In addition to InfraSnow reflectance, snow density is required as second input parameter to the OED analysis. Our InfraSnow OED measurements agree with reference OED measurements by micro computed tomography (micro-CT) within 25% for seven of the ten tested snow blocks. Furthermore, the relative differences between both measurement methods are close to the estimated uncertainties of the InfraSnow methodology. If density is measured by micro-CT and then used as InfraSnow model input to derive OED, an average agreement with the reference micro-CT OED values within 13% is found. If density is measured by a permittivity sensor, the average agreement is within 20%.

One-piece binding plates in snowboard racing have initially been introduced at the Olympics in 2006 by the later gold medalist. Today different types of such binding plates are used and play an important role to setup the equipment according to individual preferences. This study aims to quantify to what extent different binding plates modify the mechanical and dynamical properties of racing snowboards. The mechanical and dynamical properties of five racing snowboards with and without binding plates were characterized by laboratory measurements of the following parameters: Overall bending stiffness, bending stiffness distribution, torsional stiffness of rear and front body, force distribution along the running base, natural frequencies and their damping ratios. An increase in bending stiffness of 3.5% to 10.7% was measured for the different binding plates. Concerning torsional stiffening, large differences were found between the tested items of 0% to 19.7% for the front body and 2.6% to 35.1% for the rear body of the snowboard-binding plate systems. Free vibration tests showed a strong increase in damping for 4 of 5 binding plates while one plate damped distinctively less, which was also the only binding plate causing a clear change of the force distribution along the running base. While all plates cause relatively low bending stiffening, indicating a consensus about what a binding plate in snowboard racing should provide, the role of torsional stiffening and damping is probably considered controversially among manufactures and athletes as strong differences were found for these properties between the tested binding plates. It could be shown that current binding plates do partly modify the mechanical and dynamical properties of the snowboard - binding plate system to an extent that is larger than the differences between the analyzed racing snowboards itself.