Strouhal number of naturally-oscillating triangular and circular jets

Abstract

A nozzle consisting of an abrupt expansion into a short open- ended tube can produce a naturally-excited oscillating-jet flow. The characteristics of the oscillating jet depend on jet-orifice to chamber expansion ratio (D/d1), chamber length-to-diameter ratio (L/D), and shape of the jet orifice. In experiments using water as a flow medium, air-bubble visualisation and signals from a pressure transducer show that a triangular-jet orifice produces aperiodic oscillation without a spectral peak. In contrast, oscillation of the circular jet has clearly visible periodicity and the spectrum has a broad peak. The circular and triangular orifices produce completely different dependence of Strouhal number on expansion ratio. For a circular orifice, Strouhal number is inversely proportional to (D/d1−1). For a triangular orifice, Strouhal number is directly proportional to (D/d1−1). The two curves intersect at an expansion ratio of 4.8, which is approximately the minimum possible expansion ratio for a circular oscillating jet.