TITLE:
A Numerical Study on Jet Structure and Noise Emission of Underexpanded Radial Jet
AUTHORS:
Takumi Nojima, Yoko Sakakibara, Masaki Endo, Hiromasa Suzuki
KEYWORDS:
Compressible Flow, Screech, Oscillation, Underexpanded Jet
JOURNAL NAME:
Open Journal of Fluid Dynamics,
Vol.9 No.1,
March
14,
2019
ABSTRACT: In this study, an underexpanded radial jet issuing from a small gap between
two circular tubes facing each other is investigated numerically. Radial jet is
formed, for example, downstream of high-pressure valves in piping system
and of poppet valves in engines, and causes many industrial problems such
as the noise generation and the fatigue failure of structure. In this study, the
jet issuing from a small gap between two tubes with same diameter is numerically
simulated. The flow field is assumed to be axisymmetric against
the central axis of tubes and to be symmetric against the intermediate plane
between the exits of two tubes. The axisymmetric Euler equations are solved
using symmetric TVD (Total Variation Diminishing) scheme. The effects of
nozzle pressure ratio and of diameter of circular tubes on the structure and
the behavior of jets are examined. Typical cell structure of underexpanded
jet appears in radial jet and the length of cell becomes smaller in downstream
region because the jet spreads radially like a disc. The length and
width of first cell are larger with higher nozzle pressure ratio. Many vortices
are generated one after another near the jet boundary and move downstream,
which cause the oscillation of jet. Outside of jet, two types of density
waves are observed. One of them propagates toward the nozzle (toward the
upstream region) and the other propagates in opposite direction. Focusing
on the pressure change caused by the former waves, which is related to
well-known screech, dominant frequency obtained by FFT analysis was
found to become lower with higher pressure ratio and smaller diameter of
tube.