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U.S. Army Research Laboratory,
Aberdeen Proving Ground, Maryland
The U.S. Army Research Laboratory (ARL), Aberdeen Proving Ground, Md.,
recently demonstrated an approach for using Navier-Stokes computational
fluid dynamics (CFD) simulations to accurately compute the supersonic
flowfield and aerodynamic forces on a missile with grid fins. A grid fin
is an unconventional lifting and control surface that consists of an outer
frame supporting an inner grid of intersecting planar surfaces of small
chord. The advantages of grid fins are effective aerodynamic control at
a high angle of attack and a high Mach number, small hinge moments, and
compact storage, which make them applicable to highly maneuverable, smart
munitions.
Pressure coefficient contours on the symmetry plane at a 10° angle of
attack
The coupled, implicit solver in FLUENT 5 was used to calculate the steady
state, viscous flowfield, and aerodynamic coefficients on a 13-caliber,
generic missile with grid fins. Three-dimensional parallel calculations
at Mach 2.5 and several angles of attack between 0° and 20° were performed.
An unstructured mesh consisting of 3.2 million predominantly hexahedral
cells was used. A small region consisting of tetrahedral and transition
pyramid cells was used ahead of the missile to take care of the point
singularity at the nose. Mesh generation was the challenge for the problem
of resolving the boundary layer flow around the missile body and the individual
fin surfaces while keeping the model size manageable on up to six processors
of an SGI Onyx 2 high-performance computer. The Spalart-Almaras one-equation
turbulence model was used with wall functions. The missile geometry and
a set of experimental wind tunnel data for validation of the computations
were supplied by the Defense Evaluation and Research Agency (DERA), United
Kingdom.
 Normal force coefficient on grid fins: calculated vs. DERA wind tunnel
measurements
The calculations accurately predicted the missile aerodynamic coefficients.
The normal force and pitching moment coefficients were within 7 percent
of the measured data. The axial force coefficient was within 11 percent
at a 0° angle of attack and within 6 percent at higher angles. The calculations
also accurately predicted the normal force coefficient on the individual
grid fins. The negative normal force on the leeward fin at higher angles
of attack was captured by the simulation. The latter effect is due to
the leeward fin’s location in the flow separation region, giving an effective
negative angle of attack. As the angle of attack of the missile increases,
the size of the recirculating flow in the separation region increases.
This leads to more of the leeward fin having an effective negative angle
of attack and a larger negative force
Pressure coefficient contours on the symmetry plane showing shock structure
on the leeward fin
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