Stereo-PIV measurements of spatio-temporal turbulence correlations in an axisymmetric jet

Abstract

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 $1~\text{m}~\text{s}^{-1}$, giving $\mathit{Re}=4\times 10^{4}$) 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 ($R_{ij,kl}$) – 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).

Effect of non-parallel mean flow on the acoustic spectrum of heated supersonic jets: Explanation of “jet quietening”

“…While the increase is modest, our choice of c ⊥ used in Fig. 8 remains more consistent with turbulence measurements 48,50 since c ⊥ c 1 as Refs. 22, 48, and 50 have found.…”

Measurements of the single-point and joint turbulence statistics of high subsonic jets using hot-wire anemometry

“…The main two disadvantages are that the probe intrudes into the flow and that point-wise data acquisition is time-consuming. Laser Doppler velocimetry (Kerhervé et al 2006) and particle image velocimetry techniques (Bridges and Podboy 1999;Pokora and McGuirk 2015;Dahl 2015) have extended the number of reference probe locations; however, the maximum discernible frequency is limited (e.g. Strouhal number St ≤ 1 ).…”

Transitional and Turbulent Flow in a Bed of Spheres as Measured with Stereoscopic Particle Image Velocimetry

“…This is especially important for flow within a bed of spheres since the flow is fully three dimensional. Stereoscopic PIV is a new technique for flow in porous media, but it is frequently used for a number of other applications (Willert 1997;Pokora and McGuirk 2015). In what follows the experimental set-up for measurement of relatively high Re flow through a bed a spheres with stereoscopic PIV is described.…”