Flow and Acoustic Properties of Underexpanded Elliptic-Slot Jets

An experimental program was conducted to investigate the effect of aspect ratio on the e ow and acoustic properties of unheated jets issuing from elliptic slots. The aspect ratios investigated ranged from low (2:1) to moderatevalues (3:1and4:1). Further,passivecontrolorthemixingandnoisecharacteristicsisdemonstrated.The experimental conditions chosen ranged from fully expanded (Mj =1.0) to underexpanded jets with an equivalent Machnumberof2.0.Themean e owstudyrevealsthatin moderateaspect-ratiojettheazimuthaldeformationstake longer to evolve than in low aspect-ratio jet, resulting in lower bulk mixing. Notches, as passive control devices, enhance mixing in each aspect ratio. The acoustic properties of underexpanded jets show great variation with aspect ratio. Minor-axis sides of 3:1 and 4:1 plain jets radiate shock noise levels that are 4 and 10 dB, respectively, higher than those by 2:1 plain jet. The presence of notches alters the azimuthal emission characteristics of these jets. For a 2:1 jet notches bring down the noise levels along the major-axis plane by 5 dB, whereas for 3:1 and 4:1 jets notches effect the noise intensity along minor-axis sides and show a reduction of 3.5 and 7 dB, respectively, with a repeatability of 2%. Nomenclature bn = local unit vector in the direction of the binormal De = equivalent diameter of noncircular slot La = equivalent semimajor axis length of plain elliptic slot Lavg = average shock-cell length M = local e ow Mach number Mj = Mach number obtained by correctly expanding an underexpanded jet Pa = atmospheric pressure