A CFD study of the influence of turbulence on undercatch of precipitation gauges

- Turbulence effects on undercatch of gauges are quantified.
- Eddy dynamics have stronger influences on smaller precipitates.
- Unfavorable eddies cannot be adequately damped by single-shield gauges.
- Fluid mixing profits double-shield catchments of small precipitates.
- Double shielding is inevitable for acceptable performance in high winds.

The response of precipitation to turbulent fluctuations near gauges is studied using time-averaged (RANS) and unsteady (LES) turbulence modeling. Updrafting effects on catch performance are analyzed for unshielded and shielded gauges. The effective precipitation catchment area of the gauge for both wind-induced effects and snowflake characteristics is found to reduce significantly for small particles in high winds but can be partly recovered by shielding. The variation in the amount of precipitation caught is quantified for different free-stream wind speeds using LES and RANS. The fluctuations, captured with LES are analyzed to determine the local structure of eddies near the orifice plane. Wind-induced drag on precipitates are modeled for a wide range of particle Reynolds numbers from low speed Stokes flow condition to high speed flows with inertial effects. Results show noticeable effect of drag-force model on catch performance calculation of precipitation gauges with uncertainties of up to 40% in high winds and large snowflake sizes. Finally, particle-wall collision on the catch performance is studied for different restitution conditions. These simulations have differences of up to 5% in catch performance for large particle sizes in high winds, dependent on whether the particles undergo elastic or plastic collisions. Comparing RANS and LES results, turbulence fluctuations show a considerable influence on shielding performance degeneration at high winds. Double shielding the gauge can improve efficiency by maintaining a lower fluctuation-to-mean catch ratio as wind speed increases.