MODELLING OF WIND FLOW PATTERN AFFECTING SNOWDRIFT

Abstract

The modelling of wind flow pattern has a significant role in the assessment of snow transportation and the height of snowpack for avalanche warning. The wind-driven snow forms snow cornices which trigger avalanches from a high mountainous slope. These avalanches result in enormous loss of human life and property every year. The mountainous region has a high potential to generate wind energy. Thus, the study of wind flow pattern is significant to mitigate the snowdrift problem and to assess the wind energy potential in different parts of a country. The research work was carried out to model the wind flow pattern affecting the snowdrift. The field experiments at Banihal top (Jammu and Kashmir, India), wind tunnel experiments and numerical analysis of wind flow around the hill and across the snowdrift control structures were carried out to meet the objectives of the study. The observations of high wind activities ( maximum wind speed 22 m/s) and heavy snow precipitation (maximum height of snowpack, HS equal to 3.5m) at Banihal top (Jammu and Kashmir, India) helped to select it as the experimental site and to carry out snowdrift experiments at the site. The wind and snow data were collected for the period from 2014 to 2018 and the data were analysed to find the wind flow pattern and snowdrift activities at the site. The wind sensors were installed at windward and leeward sides of a hill ridgeline near the observatory to observe the wind flow patterns around the hill ridgeline. It was found that the effect of lee slope was more prominent in the formation of snow cornices as compared to the windward slope. The Computational Fluid Dynamics (CFD) analysis of wind flow across the hill models was carried out to simulate the wind flow pattern with a CFD tool namely Ansys Fluent. The lee slope terrain has high turbulent kinetic energy zone in compare to the windward side. The simulation of wind flow over the hill model was carried out with the snow roughness length equal to 0.0002 m. The wind flow CFD analysis showed the separation zones and wind low patterns and they were visualised with the help of the velocity vectors and velocity contours. The experimental field wind data were used to validate the simulated results of the wind flow patterns around snowdrift control structures. Computational Fluid Dynamics (CFD) analysis of wind flow over 2D hill models was carried out using an RNG-based 𝑘 − 𝜖 turbulence model. The wind flow patterns were simulated over the lee slope of different hill models in the domain size more than 30Hx 10H (where H is the height of the hill model). They were observed on the scaled model of the experimental site and the simulated results were validated with the experimental data. The adaptation of the wind field to steep terrain was modelled for specific initial and boundary conditions. The wind profile in the hilly terrain was much more complex. The wind blew smoothly on the windward side in comparison to the leeward side. The wind was speeded up on the windward side of the hilly terrain whereas it observed high turbulence on the leeward side due to high wind shear and flow separation. It was observed that the size of the separation zone developed in the leeward side of the hill model varied with the lee slope of the hill and the roughness length. Effects of hill lee slopes from 1:2 to 1:6 on wind flow pattern were analysed to find the separation zone on the hill with the fixed windward slope. It was found that there is a condition of a lee slope at which the separation zone ceased to exist for a hill with a particular windward slope. The critical lee slope equal to 1:4 was found for the experimental site. The onset of flow separation was highly sensitive to initial and boundary conditions, slope angle, and surface roughness. The results of the comparison between the model simulations and measurements on the experimental site show that typical wind field characteristics were well reproduced. A facility was created for flow and deposition measurement of foam beads (Expanded Polystyrene) in a closed re-circulating wind tunnel to model the snowdrift experimentally. The geometrical and flow similarities between the model (foam beads) and prototype (snow particles) were established for the wind tunnel experiments. The wind tunnel was instrumented with the electronic regulator, anemometer and newly developed flux measuring sensors along with a data logger to carry out the experiments for different simulated terrain conditions. An array of optical sensors for measurement of foam beads flux was calibrated with the known quantity of the material and used to find the flux variation with wind speed inside the tunnel. All three modes of drifting transportation namely creeping, saltation and suspension could be observed through the transparent acrylic wall for data collection. The threshold velocity of foam bead movement was 4.05 m/s in the horizontal section and same was 2.7 m/s in the inclined section at an angle of 30° downward (a critical angle of avalanche starting zone). The process of snow deposition by the snow fence was simulated with the foam beads deposition around the scaled model (1:50) of a snow fence in the wind tunnel. The experiments helped to find a flow measuring technique for a low-density drifting material and the modelling of drifting snow with the help of foam beads. The snowdrift control structures like snow fence and Jet-roof modified the wind flow pattern. They are used to deposit the drifted snow mass at the desired location to mitigate the snowdrift problems. A snow fence of 4 m height and 50 % porosity was installed at the experimental site (Banihal top) in 130 m running length. CFD analysis of wind flow pattern around the structures was carried out using the RNG based k- ε model. The results of the simulated wind flow pattern around the snow fence model were found close to the field observation. The backward wind flow on the lee slope of a hill supports to form a snow cornice. The phenomenon of snow cornice formation was explained with the help of velocity vectors on the lee slope. The wind flow direction is aligned downward with the lee slope of the hill because of a jet roof. The effects of a jet roof were presented with velocity contours and velocity vectors during the CFD analysis of wind flow across a jet roof. The research study helped to understand the effect of wind on the snowdrift and the performance of the structures to mitigate the snowdrift problems.