4 research outputs found
Stereo-PIV measurements of spatio-temporal turbulence correlations in an axisymmetric jet
Stereoscopic three-component particle image velocimetry (3C-PIV) measurements
have been made in a turbulent round jet to investigate the spatio-temporal correlations
that are the origin of aerodynamic noise. Restricting attention to subsonic, isothermal
jets, measurements were taken in a water flow experiment where, for the same
Reynolds number and nozzle size, the shortest time scale of the dynamically important
turbulent structures is more than an order of magnitude greater that in equivalent
airflow experiments, greatly facilitating time-resolved PIV measurements. Results
obtained (for a jet nozzle diameter and velocity of 40 mm and 1 m s\u1000001, giving
Re D 4 104) show that, on the basis of both single-point statistics and two-point
quantities (correlation functions, integral length scales) the present incompressible
flow data are in excellent agreement with published compressible, subsonic airflow
measurements. The 3C-PIV data are first compared to higher-spatial-resolution 2C-PIV
data and observed to be in good agreement, although some deterioration in quality
for higher-order correlations caused by high-frequency noise in the 3C-PIV data is
noted. A filter method to correct for this is proposed, based on proper orthogonal
decomposition (POD) of the 3C-PIV data. The corrected data are then used to
construct correlation maps at the second- and fourth-order level for all velocity
components. The present data are in accordance with existing hot-wire measurements,
but provide significantly more detailed information on correlation components than has
previously been available. The measured relative magnitudes of various components of
the two-point fourth-order turbulence correlation coefficient (Rij;kl) – the fundamental
building block for free shear flow aerodynamic noise sources – are presented and
represent a valuable source of validation data for acoustic source modelling. The
relationship between fourth-order and second-order velocity correlations is also
examined, based on an assumption of a quasi-Gaussian nearly normal p.d.f. for
the velocity fluctuations. The present results indicate that this approximation shows
reasonable agreement for the measured relative magnitudes of several correlation
components; however, areas of discrepancy are identified, indicating the need for
work on alternative models such as the shell turbulence concept of Afsar (Eur. J.
Mech. (B/Fluids), vol. 31, 2012, pp. 129–139)