<p>Nanosecond spark plasma actuators provide an opportunity to
reduce pollutants by promoting efficient combustion in engines or provide
targeted, tunable, flow control over vehicles, due to their ability to
influence flow and combustion through multiple mechanisms. The plasma actuators
can be physically unobtrusive, can be turned on and off and their low duty
cycle, large bandwidth, and light weight make them more appealing than other
control approaches. One method by which these plasma actuators interact with
the environment is by inducing a complex local flow field and in order, to
design scalable, high frequency actuators effectively, it is necessary to first
understand the flow induced by a single spark discharge. Most experimental
analysis on the flow induced by spark discharges has been restricted to qualitative
descriptions of the flow field, primarily due to the difficulties associated
with measuring such a transient and highly complex flow with sufficient
spatiotemporal resolution. Quantitative, experimental characterization of the
flow induced by a spark discharge remains lacking. </p><p> </p><p>A spark discharge produces a shock wave and a hot gas kernel
with a complex flow field following the shock. In this work, combined experimental
and theoretical characterization of the spark induced flow is performed through
a series of high spatiotemporal resolution measurements of the density and velocity
fields and reduced-order modeling. The work investigates the mechanisms driving
the cooling and vorticity generation in spark induced flow and the 3D nature of
the flow field. Planar (2D-3C) and volumetric (3D-3C) velocity measurements are
taken using stereoscopic particle image velocimetry (SPIV) and tomographic PIV,
respectively. Density measurements are taken using background oriented
schlieren (BOS) and high speed schlieren imaging is used to capture the shock
wave induced by the spark.</p><p> </p><p>The work shows that spark plasma discharges induce vortex
rings whose vorticity is likely generated due to baroclinic torque arising from
the non-uniform strength of the induced shock wave. The hot gas kernel cools in
two stages: an initially fast cooling regime, followed by a slower cooling
process. Reduced order analytical models are developed to describe the cooling
observed in the fast regime and the role of the vortex rings in the entrainment
of cold ambient gas and the cooling of the hot gas kernel. The results show
that the vortex rings entrain ambient gas and drive cooling in the fast, convective
regime, cooling approximately 50% of the hot gas within the first millisecond
of the induced flow. An increase in the electrical energy deposited in the
spark gap increases the shock strength and curvature and increases the vortex
ring strength, thereby increasing the cooling rate and expansion of the hot gas
kernel. The volumetric velocity measurements capture one of the two induced vortex
rings and provide a framework for the improvements needed in future tomographic
PIV experiments of the spark induced flow field, necessary in assessing the 3D
nature of the induced vortex rings.</p><p> </p><p>
The results of this work provide the first set of
quantitative, experimental data on flow induced by nanosecond spark discharges
that can be used for validation of computational fluid dynamics (CFD) simulations.
The results demonstrate that spark plasmas induce vortex ring-driven mixing
flows and the results on mixing and cooling of the hot gas kernel can be
extended to any passive scalars present in the flow field as well as inform
pulsation frequencies and actuator designs for flow and combustion control. The
results from the reduced order modeling can inform future studies and
applications of nanosecond spark discharges and can be extended to a variety of
other types of plasma discharges like laser sparks, long duration sparks and
surface discharges with similar induced flow fields.<br></p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/14403392 |
| Date | 07 May 2021 |
| Creators | Bhavini Singh (10586768) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Spark_induced_flow_in_quiescent_air/14403392 |
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