Experiments with synthetic jets: Performance and crossflow interactions

Abstract

Synthetic jet actuators operate by discharging momentary slugs of fluid, which emanate
from the periodic oscillations of a diaphragm. The interaction of these trains of vortices
produces an unsteady jet with the ability to impart momentum to a fluid. An understanding of the behaviour of such actuators in contrasting environments is a key facet
in honing flow control applications. An initial parametric study identifies the primary
features which influence synthetic jet performance and establishes the internal geometry as a possible area of improvement. A novel concept, called the orifice lip ratio, is
introduced and its influence on the synthetic jet flowfield is quantified. Further, various
applications rely on the interaction of a synthetic jet with an incoming flow. The ability
to predict the trajectory is important as it allows targeted and efficient use of the actuator. Although dependency of jet trajectory on parameters such as the Strouhal number
and the velocity ratio have been accounted for, the influence of the orifice dimensions
are yet to be determined. Thus, the characteristics of the time-averaged jet trajectory
and its sensitivity on non-dimensional parameters such as the orifice aspect ratio, the
momentum ratio and Strouhal number is established through a large parametric study.
Often, large-scale applications require the implementation of an array of synthetic jet
actuators. Under such configurations, the synthetic jet not only interacts with the crossflow but it also experiences the influence of adjacent actuators. Understanding the rich
and complex structures emanating from the interaction of a turbulent boundary layer
with twin jets, ejected from actuators having identical geometrical and fluidic properties,
is achieved by varying two main parameters. By altering the physical spacing between
the two orifices and the phase difference between the actuation signals, it becomes not
only possible to interpret the behaviour and trajectory of the twin jets, but also enables
the sensitivity of Reynolds shear stresses to be analysed.

Finally, a common technique used for data acquisition is particle image velocimetry
(PIV), both in the planar and stereoscopic configurations. Pixel-locking is a common
error occuring in stereoscopic PIV (SPIV), which is often overlooked. The acquired data
showed that its existence influences the turbulent fluctuation statistics. Defocussing the
camera lens slightly can conveniently tackle the problem. However, with the evolution
of PIV, superior cameras and more powerful lasers have enabled the study of large field
of views. Under such circumstances, defocussing is not an option and due to the small
nature of the particle diameter, pixel-locking is inherent. Hence, by means of synthetic
images and experimental data, the analysis has demonstrated the possibility to quantify
and mitigate the effects of pixel-locking under a SPIV configuration.

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Identifiers

ORCID for Girish Kumarsing Jankee: orcid.org/0000-0002-9178-1070

ORCID for Bharathram Ganapathisubramani: orcid.org/0000-0001-9817-0486

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