An experimental study is performed towards identifying cross-talk effects between DBD plasma actuators and external flow. An actuator is positioned in a boundary layer operated in a range of free stream velocities from 0 to 60 m/s, and tested both in counter-flow and co-flow forcing configurations. Electrical measurements are used for estimating the power consumption and the discharge formation is visualized using a CCD camera. The actuator's force is measured using a sensitive load cell. Results show the power consumption is constant for different flow velocities and actuator configurations. The plasma light emission is constant for co-flow forcing but shows a trend of increasing intensity with counter-flow forcing for increasing free stream velocities. The measured force is constant for free stream velocities larger than 20 m/s, with same magnitude and opposite direction for the counter-flow and co-flow configurations. In quiescent conditions, the measured force is smaller due to the change in wall shear force by the induced wall-jet. An analytical model is presented to estimate the influence of external flow on the actuator force. It is based on conservation of momentum through the ion-neutral collisional process while including the contribution of the wall shear force. Satisfactory agreement is found between the prediction of the model and experimental data at different external flow velocities.

SUMMARYWe tested the hypothesis that females of a small vocal marine fish with exclusive paternal care, the painted goby, prefer high parental-quality mates such as large or high-condition males. We tested the effect of male body size and male visual and acoustic courtship behaviour (playback experiments) on female mating preferences by measuring time spent near one of a two-choice stimuli. Females did not show preference for male size but preferred males that showed higher levels of courtship, a trait known to advertise condition (fat reserves). Also, time spent near the preferred male depended on male courtship effort. Playback experiments showed that when sound was combined with visual stimuli (a male confined in a small aquarium placed near each speaker), females spent more time near the male associated with courtship sound than with the control male (associated with white noise or silence). Although male visual courtship effort also affected female preference in the pre-playback period, this effect decreased during playback and disappeared in the post-playback period. Courtship sound stimuli alone did not elicit female preference in relation to a control. Taken together, the results suggest that visual and mainly acoustic courtship displays are subject to mate preference and may advertise parental quality in this species. Our results indicate that visual and acoustic signals interplay in a complex fashion and highlight the need to examine how different sensory modalities affect mating preferences in fish and other vertebrates.

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Animals often vocalize during territorial challenges as acoustic signals may indicate motivation and fighting ability and contribute to reduce aggressive escalation. Here, we tested the function of agonistic sounds in territorial defence in the painted goby. Pomatoschistus pictus, a small vocal marine fish that defends nests during the breeding season. We first measured the number of times a male approached, avoided, explored, entered and exited two unattended nests associated with either conspecific agonistic sounds or a control: silence or white noise. Acoustic stimuli were played back when the male approached a nest. In a second experimental set, we added visual stimuli, consisting of a conspecific male in a small confinement aquarium near each nest. Even though we found no effect of the visual stimuli, the sound playbacks induced similar effects in both experimental conditions. In the sound vs. silence treatment, we found that when males approached a nest, the playback of conspecific sounds usually triggered avoidance. However, this behaviour did not last as in longer periods males visited nests associated with agonistic sounds more often than silent ones. When the control was white noise, we found no significant effect of the playback treatment in male behaviour. Although we cannot exclude the possibility that other sounds may dissuade nest occupation, our results suggest that agonistic sounds act as territorial intrusion deterrents but are insufficient to prevent nest intrusion on their own. Further studies are needed to test the significance of sound production rate, spectral content and temporal patterns to deter territorial intrusion in fish.

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