Experimental and Computational Results of a Simplified C-130 Shape Depicting Airflow Influence on Airdrop

Abstract

The following work was conducted as part of the Airflow Influence on Airdrop (AIA) project. The AIA Project, started in July 2002, is an international project involving US, UK, France and German partners. The AIA Project aims at developing a better understanding of airdrop issues while validating state-of-the-art computational fluid dynamics (CFD) methods like Detached Eddy Simulation (DES) to predict the aerodynamic loads and the flow pattern around the air vehicle. The project is comprised of wind tunnel tests, CFD simulations, and flight tests of a full scale C-130. This paper documents the wind tunnel campaign and the companion CFD simulations of a simplified C-130 with an accurate aft portion that includes the horizontal tail as well as the rear door in both closed and open configurations. The experiments of the simplified model included determination of the lift and drag forces as a function of incidence as well as Particle Image Velocimetry (PIV) flow visualization to better understand the relevant aerodynamics in airdrop activities. The resulting experimental data was then used to better evaluate the capability of current CFD methods. The wind tunnel experiments were conducted by the French team of ENSICA while the numerical simulations were performed by the United States Air Force Academy. CFD efforts incorporated the use of Reynolds Averaged Navier-Stokes (RANS) methods for steady-state simulations and the promising Detached Eddy Simulation (DES) method for unsteady simulations. Simulations were performed using unstructured meshes with accommodation for viscous boundary layers. Results of the simulations demonstrate excellent comparison between experiments and simulation for both loads and off-body vorticity. The experimental lift and drag forces were compared to both RANS and DES simulations with excellent results. The PIV results were also compared with the CFD results to determine if the detailed flow-field phenomena are also in agreement. The PIV results were post-processed into cross-planes of vorticity and compared to the CFD results in the same planes using the same contour levels. Excellent agreement between the experiment and CFD results are shown for several planes aft of the rear door for a vortex pair. Results show that detached eddy simulations were less dissipative and compared more favorably to the PIV results than the RANS results.