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CHAPTER 6
F-16 ANALYSIS RESULTS
Introduction
In order to emulate the torsional and bending nature of a limit cycle oscillation (LCO)
mechanism, the first step in the flow-field break-down is to perform forced, rigid-body pitch and
roll oscillations. Cobalt’s built in oscillation motion-type is utilized for this purpose. This method
applies sinusoidal motions to the grid, and can include both translational and rotational
oscillations. The form of the applied motion is given by
() t = θ i ± A×
( π × f × t)
θ sin 2
(6-1)
where θ () t is the angle motion with respect to time in degrees, θ i is the initial angle in degrees,
A is the amplitude in degrees, f is the frequency in Hertz (Hz), and t is solution time in seconds.
Cobalt also incorporates a ramp-up time (the time at which the displacement reaches 99% of its
amplitude) in order to ramp displacement from 0.0. The unsteady oscillations are simulated using
the delayed detached eddy simulation (DDES) version 120,121 of the Spalart-Allmaras (SA) one-
equation turbulence model with rotation correction (DDES-SARC) to predict the effects of fine
scale turbulence. Fully turbulent flow is assumed. The outer (physical) time step is set to Δt =
0.00025 sec., corresponding to a non-dimensional time step of Δt* = 0.01. The number of
Newton sub-iterations is set to 5. The temporal damping coefficients for advection and diffusion
are set to 0.05 and 0.0, respectively. The unsteady numerical simulations are initialized by 3,000
iterations of steady-state solutions computed with the DDES-SARC turbulence model. An
additional 7,000 iterations are run for a steady, time-accurate solution, for convergence to a
sufficiently steady-state.
All computations are run on up to 256 CPUs on the ‘Jaws’ supercomputing system at the
Maui High Performance Computing Center (MHPCC). ‘Jaws’ is a Dell PowerEdge 1955 blade
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