Wind Tunnel Measurements of the Response of Hot-Wire Liquid Water Content Instruments to Large Droplets

Abstract

Wet wind tunnel tests were performed on more than 23 cloud liquid water content (LWC) probes and drop spectrometers at the NASA Icing Research Tunnel, with a main objective to characterize their response to large-droplet conditions. As a part of this study, the LWC and median volume diameter (MVD) reference values of the tunnel were examined, and accuracies were estimated and reported herein. Alternative MVDs were calculated from measurements conducted during the study. MVD accuracy was particularly difficult to estimate due to the lack of accepted standards, and MVD estimates were quite sensitive to the complement of instruments used to generate composite droplet spectra. Four hot-wire LWC probes were tested to characterize the LWC response as a function of cloud MVD. A response reduction was observed with increasing MVD for all three probes with cylindrical hot wires, most significant for the probe with the smallest wire diameter. The response of the Nevzorov total water content (TWC) probe, with its relatively large-diameter conical hot wire, did not roll off appreciably within the range of MVDs tested, although significantly larger inertial collision efficiency corrections were predicted for smaller droplets. The results corroborate previous studies that suggest re-entrainment of collected water likely creates a rolloff of the response of small-diameter cylindrical hot-wire probes, but also suggest that the aerodynamic design of the Nevzorov TWC probe inhibits re-entrainment by trapping droplets that enter its sample volume.