Drag Reduction from Square-Base Afterbodies at High Speeds

Experiments have been carried out in the 0.5-m base flow wind tunnel at high speeds evaluating the drag reduction potential of a family of square base afterbodies including jet flow at the base. Direct afterbody total drag measurements have been made on square bases as well as conventional axisymmetric afterbodies with conical and circular arc boat tails having the same annular base area and jet flow parameters. The results show conclusively that, in the Mach-number range of 0.95‐1.60, the square-base afterbodies have globally minimum drag in the range of jet pressure ratio studied; the total drag reduction observed is about 10‐12% relative to the circular arc afterbodies, which can be of significant value in design applications. Certain broad flow features on square-base afterbodies are discussed based on surface-pressure measurements and surface flow-visualization studies. Nomenclature Ab = annular base area Ab/A f = ratio of annular base area to forebody area, 0.23 A f = forebody cross-sectional area A j = area of jet at exit A j/A f = ratio of jet exit area to forebody area, 0.30 C D = afterbody total drag coefficient; drag force/(q∞∗ A f ) CDb = base-drag coefficient; base force/(q∞∗ A f ) C Dβ = boat-tail profile drag coefficient; boat-tail drag force/(q∞∗ A f ) C p = afterbody surface-pressure coefficient; ( p − p∞)/(0.5γ p∞ M 2 ∞ ) Cpb = base-pressure coefficient; ( pb − p∞)/(0.5γ p∞ M 2 ∞) D = forebody diameter of the model, 127 mm db = base diameter d j = nozzle-exit diameter M∞ = freestream Mach number Poj/ p∞ = jet pressure ratio; ratio of stagnation pressure of jet to the freestream static pressure pb = base pressure p∞ = freestream static pressure q∞ = dynamic pressure (0.5γ p∞ M 2 ∞) β = boat-tail angle, see Fig. 3