In 1998 the Air Force Seek Eagle office proposed the store separation challenge to CFD codes. The configuration chosen was the release of a Joint Direct Attack Munition (JDAM) off an F-18C. Previous work [1] used unstructured grids to compute the static carriage loads, and isolated JDAM calculations. The carriage loads were blended with the steady isolated JDAM calculations and input into a six degree of freedom model (6-DOF) model (NAVSEP) to predict the separation in a decoupled fashion. The present application uses overset grids and a 6-DOF model to directly compute the store separation in a fully-coupled fashion. The work was performed in support of NAVAIR and served to validate the new overset module that can be used with Cobalt V4.0 (see News)
This simulation covers the M=0.962 case. A fully viscous solution using the Spalart-Allmaras turbulence model was computed. A steady-state solution of the JDAM in carriage was obtained in 500 iterations. The release portion was simulated using rigid-body-motion (RBM), overset and an ejector model. A time-step of 0.001 sec was used over the 500 iterations of motion. The trajectory portion of the simulation was obtained using 286 PEs and required 11.5 hours of wall clock time. The two grids contain a total of 13.2 million cells.
Figure 1 shows sample results of the grid assembly process. The main grid is the F-18C and is shown in blue. The overlapped grid is the JDAM and is shown in red. The grid assembly time in carriage required 32 seconds. At the end of the motion, the grid assembly time had decreased to 11 seconds per iteration.
A user can input ejector force vs. time parameters into Cobalt’s 6-DOF module. NAVAIR provided this data for the main vertical ejectors. As per NAVAIR suggestions, the aft ejector force was increased to 150% of the supplied values to better control pitch. Additionally, there exists a large yawing moment on the JDAM in carriage. Physically, this yawing moment is constrained by the ejectors at onset of release. Since the NAVAIR ejector force data did not counteract this moment, we modified the ejector forces to do so for the first 0.025 seconds of release. Various runs showed the predicted trajectory to be very sensitive to the ejector force vs. time values.
Plots for pitch, yaw, x-displacement, and z-displacement are presented in Figures 2-5. The agreement to the telemetry data is fairly good. An animation of the final release is shown in Video 1. This case shows that with Overset, Cobalt can rapidly predict store separations of complex configurations.
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Figure 1: Resulting Assembled Grids - Front View.
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Figure 2: Pitch Angle vs. Time
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Figure 3: Yaw Angle vs. Time
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Figure 4: X-displacement vs. Time
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Figure 5: Z-displacement vs. Time