Fluid dynamic and aeroacoustic investigations of shrouded jets

The preliminary design calculations of an adiabatic shrouded nozzle were performed by solving iteratively the one-dimensional fluid dynamic equations governing the compressible flow. A two-dimensional flow model, consisting of second order partial differential equations of a parabolic type, was used to find the effect of shroud length on the ejector performance. This model consists of the conservation laws with thin shear layer assumptions incorporating the Prandtl's mixing-length hypothesis for turbulence closure. The numerical integration method developed by Patankar and Spalding was used to solve the governing fluid dynamic equations of motion. The aeroacoustic characteristics of the shrouded jet were analyzed based on the Lighthill’s V⁸ law.

In the free field test facility, the experiments were carried out on a slot nozzle of an aspect ratio (slot width/ slot height) of 27. The inlet area of the hard wall shroud was three times the exit area of the primary slot nozzle. The effects of geometric and operating parameters on the ejector performance were analyzed employing both hot and cold primary jet flows.

The predicted fluid dynamic characteristics of the thrust augmentation, mass flow ratio, velocity profile, etc., agree very well with the experimental results. For the cold flow through the 15 inch shrouded nozzle, the thrust augmentation of 33% was achieved theoretically and experimentally. A noise reduction of 7 dB was measured instead of the predicted value of 11 dB due to the internal noise generated in the mixing region underneath the hard wall shroud. For an increase in stagnation temperature of the primary jet to 1210°R, the predicted and experimental results show that the thrust augmentation increased to 27%. A noise reduction of 10 dB was obtained instead of the calculated value of 16 dB.

The experimental result shows that if the shroud exit area is increased by increasing the shroud angle, the noise radiation increases in the far field. This result contradicts the classical jet noise theory in which noise radiation decreases with the increase of the shroud exit area. This discrepancy is due to the combination of the noise generated from the high velocity primary jet and low velocity jet at the shroud exit. The directivity patterns obtained in the free field test facility were analyzed.

In general, the theoretical and experimental results show the shrouded nozzle has a great potential of thrust augmentation and jet noise reduction. The mathematical models used in this investigation give systematic design details for the shrouded nozzle.