59 research outputs found
Effects Of Pulsation Frequency On Trailing Edge Plasma Actuators For Flight Control
This thesis details the aerodynamic testing of a dielectric barrier discharge (DBD) plasma actuator operating over a separation step created at the trailing edge of a modified NACA 0012 aerofoil. The work focuses specifically on the use of pulsed or interrupted plasma actuation as opposed to continuously driven actuation, to increase the change in the lift produced by activating the system.
The behaviour of the actuation system is characterised in a lamina flow regime at a Reynolds number of 1.33 x 105 using force balance measurements. At zero incidence the actuator produced a peak change in CL of approximately 0.015. However, this result is sensitive to changes in the interruption frequency of the plasma, by changing the plasma drive waveform the system was able to produce both positive and negative changes in lift. A relationship was identified between the change in CL produced and the ratio of the plasma interruption frequency to the natural vortex shedding frequency. This effect was investigated using both time averaged particle image velocimetry (PIV) and instantaneous phase locked PIV images captured in sequence throughout the plasma interruption cycle. The phase locked images showed how variation in the pulsation frequency was able to produce bi-directional actuation by either constructively or destructively interfering with the vortex formation from the back of the separation step. This interference in turn altered the level of separation which was occurring, altering the degree of upwash in the wake and therefore the lift generated by the aerofoil. PIV images were also gathered for device operation at a Reynolds number of 2.3 x 104; this produced a much higher ratio of DBD jet energy to that of the freestream. These conditions showed modified actuator behaviour due to the increased authority over the flow. However, the data still showed a strong interdependence on the reinforcement or destruction of the vortex street by the actuator interruption.
Furthermore, work was undertaken to develop an actuator topology based on thin metallised films along with a dielectric which was hardened against the chemical and electrical stresses present in a functioning DBD device. The failure mechanisms of metallised film actuators were investigated, and actuators with lifetimes exceeding 8 hours were demonstrated. A manufacture method for a silicon polymer (PDMS) – Kapton® laminate is detailed; this is shown to be highly resistant to both electrical breakdown and chemical attack by the oxygen plasma
Exploitation of Super(de)wettability via Scalable Hierarchical Surface Texturing
The field of wettability is an age-old topic that has been
revitalized in the last two decades. Historically, the diverse
physical phenomena of wetting has influenced the development of
inventions that dates back to the paleolithic era (2,600,000 to
10,000 BC) in the form of charcoal and ochre -based cave
paintings, or the mesolithic (10,000 to 5,000 BC) and neolithic
(5,000 to 2,000 BC) periods as pottery and soaps. Since the end
of the Stone Age, human civilizations and scientific discoveries
have progressed by leaps and bounds. Despite the advances in
metallurgy, optics, chemistry, mechanics, mathematics and
electricity, our understanding of fluid-surface interactions
remained stagnant until 1804. Between 1804 and 1805, Thomas Young
described the concept of a wetting contact angle, which controls
the equilibrium shape of a fluid droplet on a surface, thus
making wettability a quantified branch of physics. The late entry
of this scientific field is astounding, considering the
ubiquitousness of water on Earth. Despite Young’s discoveries,
the area remained largely unexplored. Work on wettability was
intermittent, with Edward Washburn on capillary effects in 1921
and later on, Robert Wenzel and Cassie-Baxter in 1936 and 1944 on
the wetting of rough interfaces.
In 1997, almost exactly 20 years ago, the field was rejuvenated
by the corresponding discoveries of superhydrophilicity (water
droplets spread into a sheet) and superhydrophobicity (water
droplets ball up), by Wang et al. and Neinhuis et al.
respectively. Since their work into these distinct
super(de)wetting states, the field has grown exponentially.
Today, its revival can be attributed to biomimetics (engineering
mimicry / imitation of life) and a revolutionized understanding
behind super(de)wetting mechanisms that are found in nature. The
precise combination of hierarchical (multi-scale) texturing with
select surface chemical composition is vital towards fabricating
interfaces with specialized wetting properties. Knowledge behind
the careful control of surface texturing holds immense potential
for enabling a plethora of user-defined functional interfaces. As
of the time of writing, the field of wettability encompasses
multiple domains, such as superhydrophilicity (water-loving),[8]
slippery superhydrophobicity (water-fearing), adhesive
superhydrophobicity (an unintuitive love-fear relationship with
water), superoleophobicity (oil-fearing), superamphiphobicity
(water- and oil-fearing),[11] superomniphobicity (all-fearing) as
well as a range of other important intermediary,
cross-environment wetting states.
Methods employed for achieving super(de)wettability can be
broadly classified under 2 sub-classes. The first relies on
intricate top-down photolithography (-drawing with light) or
templating-based designs while the other uses the realms of
chaotic, but deterministic and scalable bottom-up self-assembly.
Both routes are promising for the development of unique
super(de)wetting states, albeit with considerable drawbacks on
both fronts. For instance, while lithography and templating have
demonstrated exemplary surface texturing precision and
super(de)wetting performance, these methods remain limited by
poor scalability, complexity and costs in instrumentation and
operation. Alternatively, scalable and cheap bottom-up
self-assembly methods can exist within complex electro-, hydro-,
aero-, thermal- or thermo-dynamically varied regimes.
Consequently, each system requires intense cross-optimization
research efforts in determining niche operating parameters.
In this work, we explore a series of highly promising
hierarchically structured material interfaces that were enabled
by understanding, taming and controlling scalable but chaotic
bottom-up methods. To this end, we demonstrate their potential
within the entire super(de)wetting spectrum, showcased through a
series of coatings and further exemplified by functional
micro(fluid)mechanical systems (M-F-MS)
2013 GREAT Day Program
SUNY Geneseo’s Seventh Annual GREAT Day.https://knightscholar.geneseo.edu/program-2007/1007/thumbnail.jp
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