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Free compressible jet nozzle investigation

The behaviour of a supersonic compressible jets originated from a convergent/divergent nozzle issuing in still air was investigated at supersonic Mach number Mj=1.5. The experiment was designed in order to achieve significant effect of the compressibility on the free shear flow. A dedicated nozzle was designed and built in order to obtain a jet Mach exit equal to Mj=1.5 to get a convective Mach number of Mc=0.6. The nozzle jet was characterised for three different conditions: isentropic expansion where the nozzle exit pressure (pe) is equivalent to the ambient pressure (pa) (pe=pa adapted nozzle), overexpanded nozzle characterised by nozzle exit pressure lower than the ambient pressure (pepa). Carbon dioxide was used to generate the jet flow in order to evaluate the additional effect of the density on the shear layer. Particular care was devoted in measuring the shear layer flow characteristics by selecting the appropriate nozzle diameter, by selecting special miniaturised Pitot and total temperature probes, by selecting appropriate seeding. The investigated test conditions are summarised in Table 1. The feeding gas was monitored and recorded at the inlet of the nozzle test case. The Static pressure and temperature were measured together with the mass flow rate. These measurements allowed calculating the gas speed of sound at the nozzle exit using the isentropic laws. The Mach number was measured at different vertical station downstream the nozzle exit (x/Dex=2, 5 and 7) and long the longitudinal axis by mean a miniaturised static Pitot probe. Analogous the total temperature was measured on the same zone by means of a dedicated miniaturised probe. Detailed flow field measurements were carried out by using PIV techniques on the jet symmetry plane. The free jet evolution was captured up to x/Dex=16.5. Two components ensemble average velocity field was obtained on a sample of 150 instantaneous velocity fields. The velocity fields clearly indicate the behaviour of the shear layer for the different conditions (Figure 1), the case characterised by under-expanded jet presented the larger growth of the mixing layer followed by the adapted jet and by the under-expanded jet. The axial and radial velocity investigation showed that the over-expanded jet and the adapted jet respected the self similarity conditions for x/Dex>11. The measured jet spreading rate value s and the velocity decay constant B were found in accordance with the data measured by Lumely (1993) and Hussein et al (1994), i.e s=0.1 and B=6 and the assumption that these values in the self similar region are independent by the Reynolds number was confirmed. Derivate quantities, as vorticity, shear strain, normal strain, enstrophy, Reynolds stress components and the turbulent kinetic energy were investigated. The expansion and compression waves have been clearly detected inside the jet flow and analysed.