Wind Tunnel Tests of Simulated Spherical Hailstones with Variable Roughness

Wind tunnel test data on the relationship between drag coefficient and Reynolds number are presented for solid spherical models with different roughness parameters. Using a wind tunnel with minimal free-stream turbulence it is found that spheres with spherical roughness elements designed to simulate hailstones with rough rime coats exhibit an unexpectedly sharp drop in drag coefficient at critical Reynolds number. For a spherical hailstone with roughness elements as large as 0.02 inch, the transition to the lower drag coefficient could occur when the stone is as small as 1.6 inch in diameter (for a smooth sphere the corresponding transition does not occur unless its diameter exceeds 4 inches).