14th International Symposium on Particle Image Velocimetry – ISPIV 2021
August 1–5, 2021

Investigation of the shear-layer instabilities in
supersonic impinging jets using dual-time velocity

measurements
T. Sikroria1∗, J. Soria2, S. Karami2, R. Sandberg1, A. Ooi1

1 Department of Mechanical Engineering, University of Melbourne, Parkville Campus, Melbourne,
Victoria - 3010, Australia.

2 Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC), Department of Mechanical
and Aerospace Engineering, Monash University, Clayton Campus, Melbourne, Victoria - 3800, Australia.

∗ tsikroria@student.unimelb.edu.au

1 Introduction
Motivated by applications in the propulsion industry, the fundamental study of phase-locked shear-layer
instabilities in supersonic impinging jets has been of research interest for long time. While such flows
have been experimentally investigated in various research studies using time-unresolved particle image ve-
locimetry (PIV) techniques, the understanding of the shear-layer dynamics is limited, due to the absence of
temporal information. Time-resolved PIV measurements for high-speed flows require a large bandwidth,
which is challenging to achieve with the current state of technology. An alternate approach using time-
unresolved double-PIV measurements is presented in the current study, which provides multiple samples of
dual-time data, depicted in figure 1. Such data can be obtained using two co-visual PIV systems, triggered
at a user-selectable time-offset, ∆t. As shown by Sikroria et al. (2020), the application of techniques like dy-
namic mode decomposition (DMD) on time-unresolved dual-time data provides valuable information about
the flow structures governing the shear-layer instabilities. The experimental setup for such measurements in
supersonic impinging jets, followed by the determination of the relevant dynamical flow structures from the
data, will be presented in the conference.

2 Experimental measurements
The experiments were conducted using the sub- and super-sonic jet facility in the Laboratory for Turbulence
Research in Aerospace and Combustion (LTRAC), Monash University, whose details have been reported
in Mitchell et al. (2013), Weightman et al. (2019). The main challenge in double-PIV experimental mea-
surements was to ensure that the camera from the first PIV system did not capture the illumination by the
laser from the second PIV system and vice versa. The principle of the difference in polarization between
two laser systems was used for the measurements in the current study, which has been used previously in
the work of Christensen and Adrian (2002). The optical setup used for the experiments is shown in figure
2. A high-power and high-frequency Innolas SpitLight DPSS EVO IV laser, which was a composite of two
laser systems, the MASTER and the SLAVE, respectively, was used for the double-PIV experiments. The
MASTER and the SLAVE lasers had opposite polarization and this difference in the polarization formed
the basis of the dual-time data acquisition. The images were captured using two cameras, mounted on a
T-shaped plate. Both the cameras were connected to a polarizing beam splitter, attached to the lens. The
arrangement ensured that each of the two cameras could see the laser light corresponding to a particular
polarization only. The timing was synchronized by programming the BBB (Beagel Bone Black) controller
(Fedrizzi and Soria (2015)) which provided the platform to specify a user-selectable time-shift, ∆t, between
the two PIV systems. Multi-grid cross-correlation algorithm proposed by Soria (1996) was used for the
processing of images.

Following the work of Sikroria et al. (2020), the double-PIV measurements were carried out for mod-
erately under-expanded jet at a nozzle pressure ratio (NPR) of 3.4 and impinging distance of 5 nozzle
diameters. Acoustic measurements were additionally conducted for the selected test-case, for validation of



Figure 1: Sampling of velocity fields in double-PIV measurements showing ensemble of pairs of velocity
fields.

Figure 2: Schematic of the optical setup for double-PIV measurements.

the spectral information obtained from the double-PIV data. G.R.A.S. 46BE 1/4” CCP free-field standard
microphone, having a frequency range of 4 Hz - 80 kHz, was used for the acoustic measurements. The
impingement tones were determined from the peaks observed in the spectrum obtained using the pressure
signals from the microphone.

Acknowledgements
The research is being funded by a Discovery Project Grant from the Australian Research Council (ARC).

References
Christensen K and Adrian R (2002) Measurement of instantaneous eulerian acceleration fields by particle

image accelerometry: method and accuracy. Experiments in Fluids 33:759–769

Fedrizzi M and Soria J (2015) Application of a single-board computer as a low-cost pulse generator. Mea-
surement Science and Technology 26:095302

Mitchell DM, Honnery DR, and Soria J (2013) Near-field structure of underexpanded elliptic jets. Experi-
ments in fluids 54:1578

Sikroria T, Soria J, Karami S, Sandberg RD, and Ooi A (2020) Measurement and analysis of the shear layer
instabilities in supersonic impinging jets. in AIAA AVIATION 2020 FORUM. page 3070

Soria J (1996) An investigation of the near wake of a circular cylinder using a video-based digital cross-
correlation particle image velocimetry technique. Experimental Thermal and Fluid Science 12:221–233

Weightman JL, Amili O, Honnery D, Edgington-Mitchell D, and Soria J (2019) Nozzle external geometry
as a boundary condition for the azimuthal mode selection in an impinging underexpanded jet. Journal of
Fluid Mechanics 862:421–448


	Introduction
	Experimental measurements