Numerical simulation of wind-induced turbulence over precipitation gauges

Abstract To better quantify measurement uncertainty associated with solid precipitation gauges, time averaged and transient turbulent flow over gauges is studied using a Computational Fluid Dynamic (CFD) approach. By numerically solving the Reynolds-Averaged Navier-Stokes ( RANS ) equations, vortex formation in the bulk structure of the flow is examined. To study transient effects, Large-Eddy Simulation ( LES ) quantified the turbulence and eddy dynamics. The simulated gauge is a Geonor T200B gauge shielded with Alter slats for a wind velocity ranging from 1 to 10 m/s. A multi-layer mesh generation technique allows the capture of a wide range of turbulent length scales. Gauge exposure effects on the wind characteristics are investigated for unshielded and shielded cases. Updrafting results in an effective collection area of only 30% of the orifice for capturing precipitation. The RANS results show wind momentum reduction above the gauge orifice from shielding with vertical slats assists catch efficiency of the gauge. However, tilting the slats in high winds degenerates shielding benefits. LES results show considerable production of turbulence kinetic energy near the inlet orifice area. Transient analysis with LES demonstrates that the shields intensify the turbulence near the gauge. With the addition of wind shields, the flow momentum above the gauge is reduced.