DES Grid Resolution Issues for Vortical Flows on a Delta Wing and an F-18C

Abstract

An assessment of unstructured grids for use in Detached-Eddy Simulations (DES) of vortical flowfields over two configurations, a 70 degree delta wing and an F-18C are presented. The role of the grid in detached eddy simulations of vortical flowfields, including complex features such as vortex breakdown, is assessed on a delta wing with comparison to wind tunnel data. Adaptive mesh refinement is applied to the delta wing grid to improve the focus region aft of the vortex breakdown where massively separated flow exists and unsteady pressures are generated that could impact the loads on vertical tails of more complex configurations. The adaptively refined mesh is compared to the baseline mesh to determine the advantage of the adaptive mesh refinement approach for vortex breakdown. The focus region grid resolution is then applied to an F-18C in the region of the vortex generat ed from the leading edge extension (LEX). The resulting unsteady tail loads are compared to flight test data from the NASA F-18 HARV database. This paper represents one of the first times adaptive mesh refinement will be applied to a detached eddy simulati on of a flight vehicle configuration.

High Resolution Turbulence Treatment of F/A-18 Tail Buffet

“…This characteristic time step was found adequate in previous studies of vortex breakdown and massively separated flows. 2,24,44 Figure 7a depicts a top view of the surface mesh and 7b depicts a cross-plane at a station 450 inches aft of the origin for both the baseline grid and the AMR grid. The AMR grid has enhanced resolution in the core of the LEX vortex and over the wing.…”

High Resolution Turbulence Treatment of F/A-18 Tail Buffet

“…A time-averaged SADES solution was used to produce an AMR grid with 3.9 million cells by following the approach outlined in Ref. 44. All time-accurate simulations were run for over 10,000 iterations with second-order temporal and spatial accuracy, three Newton sub-iterations, and a time step of 0.0005 seconds.…”

“…Figure 10 is a well known plot in the literature of the streamwise location of the LEX vortex breakdown as a flap settings and diverter slot being uncovered. 44 The effect of the various turbulence methods on computing the loads for these massively separated flows is demonstrated in Figs. 11a and b.…”

Flow Control

“…These and other applications illustrate the capability of DES to accurately resolve chaotic unsteady features in the separated regions along with a rational treatment of the attached boundary layers. Recent DES predictions of the flow around complex configurations (all using Cobalt) include the massively separated flow around an F-15E at 65 o angle of attack reported by Forsythe et al [2004] (this simulation was the first eddy-resolving simulation of flow around a full aircraft configuration), transonic shock-separated flow over an F/A-18E by Forsythe and Woodson [2003], and vortex breakdown on an F-18C by Morton et al [2003Morton et al [ , 2004.…”