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Transition and flow development in a separation bubble formed on an airfoil are studied experimentally. The effects of tonal and broadband acoustic excitation are considered since such acoustic emissions commonly result from airfoil self-noise and can influence flow development via a feedback loop. This interdependence is decoupled, and the problem is studied in a controlled manner through the use of an external acoustic source. The flow field is assessed using time-resolved, two-component particle image velocimetry, the results of which show that, for equivalent energy input levels, tonal and broadband excitation can produce equivalent changes in the mean separation bubble topology. These changes in topology result from the influence of excitation on transition and the subsequent development of coherent structures in the bubble. Both tonal and broadband excitation lead to earlier shear layer roll-up and thus reduce the bubble size and advance mean reattachment upstream, while the shed vortices tend to persist farther downstream of mean reattachment in the case of tonal excitation. Through a cross-examination of linear stability theory (LST) predictions and measured disturbance characteristics, nonlinear disturbance interactions are shown to play a crucial role in the transition process, leading to significantly different disturbance development for the tonal and broadband excited flows. Specifically, tonal excitation results in transition being dominated by the excited mode, which grows in strong accordance with linear theory and damps the growth of all other disturbances. On the other hand, disturbance amplitudes are more moderate for the natural and broadband excited flows, and so all unstable disturbances initially grow in accordance with LST. For all cases, a rapid redistribution of perturbation energy to a broad range of frequencies follows, with the phenomenon occurring earliest for the broadband excitation case. By taking nonlinear effects into consideration, important ramifications are made clear in regards to comparing LST predictions and experimental or numerical results, thus explaining the trends reported in recent investigations. These findings offer new insights into the influence of tonal and broadband noise emissions, resulting from airfoil self-noise or otherwise, on transition and flow development within a separation bubble.

The ongoing COVID-19 pandemic has highlighted the importance of aerosol dispersion in disease transmission in indoor environments. The present study experimentally investigates the dispersion and build-up of an exhaled aerosol modeled with polydisperse microscopic particles (approximately 1 μ m mean diameter) by a seated manikin in a relatively large indoor environment. The aims are to offer quantitative insight into the effect of common face masks and ventilation/air purification, and to provide relevant experimental metrics for modeling and risk assessment. Measurements demonstrate that all tested masks provide protection in the immediate vicinity of the host primarily through the redirection and reduction of expiratory momentum. However, leakages are observed to result in notable decreases in mask efficiency relative to the ideal filtration efficiency of the mask material, even in the case of high-efficiency masks, such as the R95 or KN95. Tests conducted in the far field ( distance from the subject) capture significant aerosol build-up in the indoor space over a long duration ( ). A quantitative measure of apparent exhalation filtration efficiency is provided based on experimental data assimilation to a simplified model. The results demonstrate that the apparent exhalation filtration efficiency is significantly lower than the ideal filtration efficiency of the mask material. Nevertheless, high-efficiency masks, such as the KN95, still offer substantially higher apparent filtration efficiencies (60% and 46% for R95 and KN95 masks, respectively) than the more commonly used cloth (10%) and surgical masks (12%), and therefore are still the recommended choice in mitigating airborne disease transmission indoors. The results also suggest that, while higher ventilation capacities are required to fully mitigate aerosol build-up, even relatively low air-change rates ( ) lead to lower aerosol build-up compared to the best performing mask in an unventilated space.

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Vortex merging in a laminar separation bubble (LSB) is studied experimentally. The bubble is formed on the suction side of a NACA 0018 airfoil at an angle of attack of 4°and a Reynolds number of 125 000. The merging process in the bubble is manipulated through acoustic forcing applied as a tone at either the fundamental vortex shedding frequency or at the first subharmonic of this frequency. The flow field is assessed using time-resolved, two-component particle image velocimetry. A method for detecting merged structures using wavelet analysis is introduced, allowing for reliable quantification of merging events. The results show that vortex merging occurs naturally in the separation bubble, while forcing at the subharmonic and fundamental frequencies promotes and inhibits merging, respectively. While these trends are similar to those observed for free shear layers, the subharmonic forcing of an LSB is found to directly promote disturbance development at the subharmonic frequency. For all cases, the majority of merging events take place in the aft portion of the bubble, i.e., downstream from the maximum bubble height location and upstream of mean reattachment, with subharmonic forcing causing merging to shift upstream. The merged structures are found to be the most energetic flow features, however, promoting vortex merging through subharmonic forcing does not lead to significant changes in the mean bubble topology. The spanwise behavior of the vortex merging process is studied, revealing that structures merge in a spanwise non-uniform manner, with localized merging occurring away from where forward or rearward streamwise bugles develop in the vortex filaments. Statistical characterization reveals that merging tends to occur more often over some specific spanwise segments, with the number of primary structures that merge varying by as much as 50% between spanwise locations. These findings offer insight into vortex merging in laminar separation bubbles and the attendant influence of forcing, while also highlighting the need to consider spanwise aspects of flow development.

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