Performance and compensation of a synthetic jet actuator

The actuator used to control the cavity flow resonance is a two-dimensional synthetic-jet type issuing from a high aspect-ratio converging nozzle embedded in the cavity leading edge as shown in Fig. 1. Actuation is provided by the movement of the titanium diaphragm of a Selenium D3300Ti compression driver attached to the contoured nozzle that exhausts at an angle of 30 degrees with respect to the main flow through a 1-mm high slot spanning the whole width of the cavity.

 actuator cutout
Figure 1: Cutout of the cavity-flow facility showing
the compression driver and the converging nozzle of
the synthetic-jet actuator.

When used as designed, i.e. for producing sound connecting it to an acoustic diffuser, the compression driver has a relatively flat frequency response. In the rather unconventional arrangement above, the compression driver exhibits a highly frequency-dependent behavior where some frequencies are reinforced while others are reduced. Measurements of the synthetic-jet velocity at its exit slot obtained with a miniature hot wire, Fig. 2, evidence this behavior. For instance Fig. 3 presents the variation of the positive-velocity peak at the actuator exit slot with the frequency of sinusoidal excitation for various voltages in absence of main flow. Conversely, at all frequencies the amplitude response is approximately a linear function of the input voltage level, as can be judged from the same figure.

 hot wire  actuator velocity
Figure 2: Miniature hot-wire for velocity measurements
at the actuator exit slot.
Figure 3: Positive-velocity peak at the actuator exit
slot with the frequency of sinusoidal excitation for
various rms voltages.

The highly frequency-dependent behavior of the actuator is also revealed in Fig. 4 presenting the acoustic transfer function of the actuator (blue) in response to white noise excitation (black).

 actuator wn response
Figure 4: Noise (blue) measured at the actuator exit slot with
white noise excitation (black).

The results above compare well with those observed by other researchers using high aspect ratio rectangular synthetic jets.

A highly frequency-dependent behavior is not a desirable characteristic for an actuator. With the colleagues of CCCS I am developing some feedback based compensation techniques to alleviate this behavior by flattening the actuator response.

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© Copyright 2005 Marco Debiasi
Last modified on: 21 February 2011