03.1 - Aerodynamics – CFD Methods and Validation
GRID EFFECT ON M823 BOMB AT TRANSONIC REGIME
J. Jiménez-Varona¹, G. Liaño¹; ¹INTA, Spain
The flow field past an axisymmetric body configuration at high angles of attack is asymmetric, and unsteady. Projectiles, missiles and bombs are composed of an axisymmetric body with a fins set and/or strakes. At subsonic and transonic regimes, the flow may be asymmetric at certain angles of attack, due to instabilities at the tip nose region, amplified by the leeside vortex interaction with the rear fins. The role of grids in such cases is very decisive when numerical calculations are performed. Excessive irregularities in the tip nose region, due to a coarse and/or inaccurate definition of the tip, may lead to important differences of the calculated forces and moments when compared to experimental data. The grid role in terms of resultant forces and moments may be large at high angles of attack, where there are nonlinear effects, some of them due to spurious numerical effects. This effect may also be significant in rolling motion and on the derived Magnus forces and moments at high angles of attack. rnIn this paper, the grid role is studied by using different grids resembling the configuration M823 Bomb at transonic regime and moderate angles of attack. These grids differ on the amount of numerical roughness, which is a way of measurement of the geometrical irregularities regarding the ideal configuration. rnAt the larger values of angle of attack, important differences of the calculated side and normal force and pitching moments are present, due to large irregularities of one of the grids. There are smooth and rough configurations in terms of numerical calculations. The numerical effect in terms of an increment of the side force at larger angles of attack has also been verified in wind tunnel testing with either smooth or rough models. rnTherefore, for subsonic and transonic regime and at high angles of attack conditions, care must be taken in the numerical calculations, due to the grid is absolutely determinant on the accuracy of the solutions. Exper