Natural instability of free shear layers

Under controlled small-amplitude excitation, an initially laminar free shear layer experience maximum growth rate at a Strouhal number St// theta of 0. 017 (consistent with theory) and maximum growth at St// theta congruent 0. 011, while the natural instability frequency St// theta //n (of an unexcited shear layer) is found to have an intermediate value. Investigations in both axisymmetric and plane shear layers in a number of independent facilities reveal that the St// theta //n value falls in the range 0. 0125 to 0. 0. 155, depending on the exit boundary-layer fluctuation level and the spanwise radius of curvature. The StD theta //n value decreases with increasing jet diameter or exit boundary-layer fluctuation level, but it is not a direct function of the exit momentum thickness Reynolds number ReD theta .