Entrainment of Radio Frequency Chaff by Wind as a Function of Surface Aerodynamic Roughness

Abstract

Radio frequency (RF) chaff (~2-cm × 25-µm diameter aluminum-coated glass silicate cylinders) released by military aircraft during testing and training activities has the potential to become entrained by wind upon settling to the Earth’s surface. Once entrained from the surface there is the potential for RF chaff to be abraded and produce PM₁₀ and PM_2.5 , which are regulated pollutants and pose health concerns. A series of portable wind tunnel tests were carried out to examine the propensity of RF chaff to become entrained by wind by defining the relationship between the threshold friction velocity of RF chaff (u _*t RF chaff) and aerodynamic roughness (z _o ) of surfaces onto which it may deposit. The test surfaces were of varying roughness including types near the Naval Air Station (NAS), Fallon, NV, where RF chaff is released. The u _*t of this fibrous material ranged from 0.14 m/sec for a smooth playa to 0.82 m/sec for a rough crusted playa surface with larger cobble-sized (~6–26-cm diameter) rocks rising above the surface. The u _*t RF chaff is dependent on the z _o of the surface onto which it falls as well as the physical characteristics of the roughness. The wind regime of Fallon would allow for chaff suspension events to occur should it settle on typical surfaces in the area. However, the wind climatology of this area makes the probability of such events relatively low.