Model calculations and wind tunnel testing of an isokinetic shroud for high-speed sampling

The direction of airflow streamlines and, therefore, particle trajectories around an aircraft will vary during a flight. A coaxial shroud was designed to align airflow and particle trajectories with the tip of a counterflow virtual impactor (CVI), despite deviations in streamlines at the shroud entrance. Both modeling and wind tunnel studies were employed in order to optimize the shroud dimensions for high-speed use on a DC-8 aircraft. A commercial computational fluid dynamics code was used to determine the approximate dimensions necessary to straighten particle trajectories and to maintain isokineticity within the shroud. Characteristics of one shroud then were studied in the U.S. Air Force Academy’s Trisonic Wind Tunnel. Flow speeds were measured at various locations within the shroud using small pilot-static tubes. Effects of exit to inlet area ratio and angle of attack on shroud flow speeds were studied, and a final shroud design was chosen based on this information. Model simulations were in general agreement with the wind tunnel results.