Offering the largest possible observation field, the turret is the preferred type of aircraft-installed fairing for housing optical or directed energy systems, thus quite frequently present in the configuration of missionized aircraft. But, its blunt shape (hemisphere on top of a cylinder) produces massive, highly unstable separation (Fig. 1), which makes the accurate prediction of the flow about it quite challenging.
Fig. 1 Flow over a hemisphere-cylinder turret (LES [1])
Indeed, steady-state RANS analysis is not adequate for flows about blunt bodies that generate viscous effects–related unsteadiness[2]. Such effects can be properly captured through LES, or at least with a hybrid URANS/LES approach. However, performing LES and even hybrid URANS/LES flow analyses on the complex configuration of a missionized aircraft remains a non-practical option when large numbers of cases need to be treated.
Computation of aerodynamic loads and coefficients/derivatives of missionized aircraft constitutes a major part of the CFD team activity at Bombardier Aerospace CFES. Given its good accuracy-efficiency balance, steady-state RANS analysis is used in virtually all situations, including the configurations that feature turret-type fairings. This raised a question on the quality of the results obtained on turrets from steady-state RANS.
The main objective of this investigation was to assess, against open source experimental or higher-order CFD data, the results provided for high speed flows on turrets via steady-state RANS analysis by the solver CFD++[3] used at CFES.
A quite comprehensive search in the open domain was necessary to find good geometry data and experimental results on a hemisphere-cylinder tested in a higher speed flow (Minf = 0.68)[4]. Based on them, the model of the turret inside the wind-tunnel could be developed and a hybrid (prism + tetra) unstructured mesh, with over 5.25 million elements and at least 18 prism layers near each solid wall, was generated in ICEM CFD within the test section volume. Through a series of test runs, carried out with the SST k-omega turbulence model, the best type of boundary conditions at the domain input and output frontiers were determined. Using them, similar runs were performed with the Spalart-Allmaras and the Realizable k-eps closure models, and their results compared with those obtained with the SST model (Fig. 2), to identify which would be the best choice for conducting steady-state CFD++ flow analyses on turret-type geometries.
Fig. 2 Comparison of experimental[4] and CFD++ - predicted
pressure coefficient variation along turret dome centerline
Since the initial mesh was significantly finer than those normally used at CFES for performing CFD analyses, four additional hybrid unstructured meshes were generated with uniformly decreasing resolution. A mesh-independence study was performed, indicating that the turret surface mesh resolution routinely used in the CFES runs is close to the determined mesh-independence threshold. Because the real flow in turret’s wake is non-symmetrical, but the CFD analyses of symmetric flight cases are conducted on ‘half’ (symmetrical) meshes, the effects caused by forcing the symmetry upon the flow about the turret were also evaluated.
Details about all these phases are presented, along with results that demonstrate through comparison against experimental data the capability of CFD++ to provide fairly accurate aero loads at higher speeds on aircraft-installed turrets.
References
[1] Morgan, P., Visbal, M., Large Eddy Simulation of Flow Around a Turret, Proceedings of the 38th fluid dynamics conference and exhibit, Seattle, WA, June 23–26, 2008. AIAA Paper 2008-3749
[2] Lynch, C.E., Smith, M.J., Extension and Exploration of a Hybrid Turbulence Model on Unstructured Grids, AIAA Journal, vol. 49, no. 11, November 2011, pp 2585-2590
[3] Metacomp Technologies, CFD++ & CAA++ User’s Manual, version 11.1, 2009
[4] Sherer, S.E., Aero-Optics Code Development. Experimental Databases and AVUS Code Improvements, AFRL-RB-TR-2009-3076
Topics: Computational Fluid Dynamics as applied to any of the above, including surface mod
