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|Document Type: ||Thesis or dissertation|
|Title: ||Parametric investigation of synthetic jet and its boundary layer control|
|Authors: ||Kim, Y-H.|
|Supervisors: ||Garry, Kevin P.|
|Issue Date: ||2005|
potential for active control of low Reynolds number boundary layers using synthetic jet
generators (SJG) has been established. The results from a four stage experimental study are
presented in which the operational and geometric parameters of a rectangular slot choice SJG are
optimised. A time-dependent, analysis of the SJG velocity profile is carried out in quiescent
prior to application of the SJG in (i) a nominally zero pressure gradient at plate
boundary layer (l.54><l05<ReX<2.86><l05), and (ii) a simulated longitudinal pressure gradient, in
order to asses the
impact of the device on boundary layer development downstream
piezoelectric driven synthetic jet is used in this study in which the resonance characteristics of an
cavity are used to establish a fluid jet. Two characteristic resonance frequencies are
identified; the mechanical resonance frequency of the diaphragm (FD), and the acoustic resonance
frequency of the cavity (FC). The latter is shown to be the most energy efficient excitation
frequency in terms of creating the highest synthetic jet velocity. Since the optimum operational and
geometrical parameters are difficult to predict theoretically, a parametric study is presented in
which, for a constant slot width (H=0.28mm) and resonance cavity geometry, an optimum slot
length (L/H=l7.86~2l.43), which creates the highest synthetic jet velocity, can be established.
Similarly, for a constant resonance cavity diameter, a narrower resonance cavity results in a higher
synthetic jet velocity. Although not a experimental parameter, diaphragm clamping force is also
seem to be
significant in terms of synthetic jet velocity.
When studied in
conjunction with a zero-pressure gradient boundary layer - such that the jet exits
normal to the surface -the orientation of the
rectangular slot relative to the freestream (ß) is seen to
result in a flow downstream of the slot which changes from a counter rotating vortex pair (when
ß=0°) to a single streamwise vortex (when ß=20°). Increasing the slot angle beyond 20° diminishes
the induced vortex structure such that, at ß=90°, the effect is similar to a surface obstruction.
dependent analysis of the synthetic jet velocity profile shows that the jet has a structured
fluctuation which, in the longitudinal sense, is strongest in the middle of slot (X/H=O, Y/I-I=0) and
plane close to the slot (Z/H=3). The suction stroke of the synthetic jet is seen to induce a flow
surrounding air which is identified by measurements at various lateral locations with a single
component HWA probe.
When a dual
diaphragm is used for a SJG, synchronisation of the diaphragm displacement is seen to
important. If the velocity peaks corresponding to each diaphragm occur at a different phase in
cycle, the performance increase attributed to the dual diaphragm operation is reduced.
Studies of SJG effectiveness within a 2D boundary layer subjected to a controlled longitudinal
pressure gradient show that activation of the SJG can both trigger laminar-turbulent transition and
eliminate a laminar
separation bubble. The potential for low Reynolds number flow control by
aeroshaping is therefore demonstrated.|
|Appears in Collections:||PhD and Masters by research theses (School of Engineering)|
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