Towards the noise reduction of piezoelectrical-driven synthetic jet actuators

This work details an experimental investigation aimed at reducing the noise output of piezoelectrical-driven synthetic jet actuators while minimising peak jet velocity reduction. The study considers double-chamber actuator for anti-phase noise suppression and lobed orifice as a method to enhance jet turbulent mixing to suppress jet noise. The study involved the design, manufacture and bench test of interchangeable actuator hardware. Hot-wire anemometry and microphone recordings were employed to acquire velocity and sound pressure level measurements respectively across a range of excitation frequencies for a fixed diaphragm clamping and input voltage. The data analysis indicated a 26% noise reduction (16 dB) from operating a single-chamber, round orifice actuator to a double-chamber, lobed orifice one at the synthetic jet resonant frequency. Results also showed there was a small reduction in peak jet velocity of 7% (∼3 m/s) between these two cases based on orifices of the same discharge area. The electrical-to-fluidic power conversion efficiency of the double-chamber actuator was found to be 15% for both orifice types at the resonant frequency; approximately double the efficiency of a single-chamber actuator

The resurgence of trypanosomosis in Botswana : short communication

No sleeping sickness or nagana cases have been reported in Botswana since 1985. In view of several confirmed clinical cases of nagana and reports of heavy bovine mortality, a parasitological survey was conducted to determine the prevalence of trypanosome infection in cattle in Maun and Shakawe areas of Ngamiland district. Wet blood films, buffy coat and Giemsa-stained thick and thin blood smears were used to detect trypanosomes in animals. Overall,trypanosome infection rate was 15.98%, with 5.94% and 27.29% in Maun and Shakawe respectively. The urgent need to combat trypanosomosis in Ngamiland, particularly in the Shakawe area, is highlighted, and a 3-phase integrated tsetse control strategy for this disease problem is discussed

Numerical study on the flow field generated by a double-orifice synthetic jet device

In the last few years Synthetic Jet (SJ) actuators have shown their full potential in controlling and manipulate an incoming crossflow. Indeed, these devices have been able to control separated flows over aerodynamic bodies, delay or anticipate transition to turbulence, suppress or enhance turbulence and control liquid jets and sprays. In many applications arrays of SJ actuators or multiple-orifice (or multi-slot) devices are preferred to single-orifice/single-slot actuators. Multi-orifice strategies have been often employed for the control of separated flows, in order to cover the entire spanwise length of the flow to be controlled or to introduce a spanwise modulation of the control. Moreover, such devices are also employed in cooling applications, since multiple-orifice devices exhibit a larger heat dissipation with respect to a single, centred orifice one. Despite this fact, a great part of the studies concerning the design of a SJ actuator have been based on single-slot or single-orifice configurations. As a consequence, the present work is focused on the interaction between the jets generated by a multiple-orifice actuator. In particular, the external flow field generated by a double-orifice SJ actuator is investigated. The analyzed actuator is sealed at one side by an elastic diaphragm, which is composed of a piezoelectric disk and a flexible shim, and connected to the external environment via two circular orifices. The numerical setup matches the flow parameters of the experiments and the preliminary numerical simulations reported in [1]. A series of numerical simulations are carried out, varying the distance between the orifices. The computational domain includes the entire cavity, the orifices, and the external environment. Differently from [1], the investigation focuses on the development of the external flow field, rather than on the vortex motion near the exit plane and within the cavity. The instantaneous flow field is characterized by the presence of two, in-phase, zero-net-mass-flux jets. These jets become turbulent, converge towards each other and merge. The characteristics of the flow are strongly dependent on the distance between the orifice centers and their momentum [2]. It is important to find a scaling law for the merging point streamwise position as a function of these parameters, since jet merging is responsible for circulation cancellation and could be detrimental for flow control applications. Time-averaged flow fields are obtained, and their features are compared with the (time-averaged) characteristics of three-dimensional, continuous twinjets [3]. Moreover, time and phase-averaged velocity fields and fluctuations are compared with those of single-orifice actuators. Finally, spectral analysis of probes (located along the jets trajectories) and dynamic mode decomposition (DMD) are used to investigate the inner shear-layer interactions and recognize shifts in the dominant frequency along the streamwise direction. The latter analyses are useful to detect the vortical motions which are responsible of the jet convergence, of the entrainment of external fluid and of the far-field behaviour of the jet