Experimental analysis of the interaction of adhering droplets for different arrangement of droplets under the influence of flow and surface vibration
Final Report Abstract
Within the scope of the project, the interaction of an adhering pair of droplets under the influence of various forces was investigated. In addition to a pure air flow around the droplet pairs, the influence of a superimposed one-dimensional oscillatory excitation on the interaction of the droplets was examined. By means of hot-wire and PIV (Particle Image Velocimetry) measurements, a correlation between droplet oscillation itself and the fluctuation behavior of the wake flow was established. The frequency spectra calculated from the experimental data reveal the following relationship: in the frequency spectrum of a single droplet subjected to an air flow, only one frequency peak is detected, which coincides with the second eigenfrequency of the droplet. PIV recordings confirm that the signal measured in the frequency spectrum of the wake flow is due to periodic vortex shedding. An extended recirculation zone behind the droplet can be identified through temporally averaged flow field analysis. The combination of two droplets (V1 : upstream, V2 : downstream) results in a modification of the flow field due to their interaction. Hotwire measurements show a change in the frequency peaks in the frequency spectrum of the wake flow. It is evident that the measurable frequency peak depends on the volume ratio V1 /V2 . PIV recordings clearly demonstrate a change in the flow field, especially in the wake of the downstream droplet, where no recirculation zone is formed. Vortical formations can be observed in the wake of the downstream droplet, but the characteristic frequencies of the vortices and droplet oscillation seem to cause a dissipation-enhancing rather than an amplifying effect. The altered flow field around the droplets has a direct influence on the critical flow velocity (vcrit ) required for droplet detachment. The investigations reveal a dependence on the volume ratio V1 /V2 . Additionally, a dependence on the distance between the two droplets is observed. For volume ratios V1 /V2 < 1, the larger downstream droplet always sets into motion. For the downstream droplet V2 , when both droplets are closely positioned, a flow velocity up to 20 % higher is required compared to an undisturbed single droplet. As the distance between the droplets increases, the critical flow velocity approaches that of a single droplet. If the volume ratio V1 /V2 exceeds one, the upstream droplet V1 is always set into motion first, resulting in the subsequent merging of both droplets during the course of the experiment. The interaction of a pair of droplets is further altered by simultaneously superimposing a vertical oscillatory excitation. It is evident that the superimposed vertical oscillation excitation can force the two droplets to merge for certain frequency ranges, which would not be possible with pure air flow around the droplets. Conversely, prevention of droplet merging is possible in the inverse scenario.
Publications
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Droplet motion induced by superposition of shear flow and horizontal surface vibration. International Journal of Multiphase Flow, 155, 104163.
Rohde, Martin; Barwari, Beawer; Burgmann, Sebastian & Janoske, Uwe
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Droplet motion induced by superposition of shear flow and two-dimensional surface vibration. 14th European Fluid Mechanics Conference, Athen Greece, 13.-16. September 2022.
Rohde, M.; Burgmann, S. & Janoske, U.
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Inner and outer flow of an adhering droplet in shear flow. International Journal of Multiphase Flow, 153, 104140.
Burgmann, Sebastian; Krämer, Veronika; Rohde, Martin; Dues, Michael & Janoske, Uwe
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Tropfenverhalten bei Superposition einer eindimensionalen Vibrationsanregung und Scherströmung. Jahrestreffen der ProcessNet-Fachgruppen Mehrphasenströmungen, Mechanische Flüssigkeitsabtrennung sowie Zerkleinern und Klassieren, Paderborn, 21.–22. Februar 2022.
Rohde, M.; Burgmann, S. & Janoske, U.
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Messung der Nachlaufströmung eines haftenden und oszillierenden Tropfenpaares mittels Hitzdrahtanemometrie. Jahrestreffen der DECHEMA-Fachgruppen Aerosoltechnik, Mehrphasenströmungen, Partikelmesstechnik und Gasreinigung, Paderborn, 28.–30. März 2023.
Rohde, M.; Burgmann, S. & Janoske, U.
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The impact of a two-dimensional vibration excitation on the critical incident flow velocity of a sessile droplet. International Journal of Multiphase Flow, 171, 104663.
Rohde, Martin; Burgmann, Sebastian & Janoske, Uwe
