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The removal of droplets on surfaces by an (air-) flow is relevant, e.g., for cleaning processes or to prevent corrosion or damage of electronic devices. Still the condition for droplet movement is not fully understood. Droplets start to move downstream at a critical (air-) flow velocity vcrit. For increasing flow velocity, this process is related to a strong oscillation of the droplet. This oscillation is supposed to be a key mechanism for the onset of droplet movement in conjunction with the flow field around the droplet. We report on measurements in the wake of the adhering droplet by means of laser-Doppler velocity profile sensor and hot wire anemometry. Thanks to the excellent spatial and temporal resolution of laser-Doppler velocity profile sensor and its capability to measure bidirectional flows, a backflow region can be detected in the wake of the droplet. Therefore, it can be concluded that this backflow structure is the driving mechanism for the strong flow movement inside the droplet against channel flow direction found in previous work. Analyzing the frequency spectra of the flow velocity, it was found that the flow is also oscillating; frequency peaks are in the same range as for the contour oscillation. Based on frequency, diameter and flow velocity, a Strouhal number can be calculated. This Strouhal number is almost constant in the investigated regime of droplet volumes and is between 0.015 and 0.03. Therefore, it can be assumed that an aeroelastic self-excitation effect may be present that eventually leads to droplet movement. Graphic abstract

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The expected close coupling between engine and fuselage of future aero-engine architectures will lead to highly distorted inflows at the engine face, presenting a major design risk for efficient and reliable engine operation. In particular, the increase in flow unsteadiness is perceived as a significant challenge. In this context, the Cranfield Complex Intake Test Facility (CCITF) is currently being installed at Cranfield University to reproduce the anticipated level of total pressure and swirl distortion arising from novel, closely coupled airframe-engine configurations. To address the expected demand for much more comprehensive flow field data, it is intended to establish the filtered Rayleigh scattering (FRS) technique for non-intrusive testing of aero-engine intake flows. Unlike the previously used particle image velocimetry (PIV) or Doppler global velocimetry (DGV), which are limited to the measurement of a single flow quantity, FRS can be used for the combined planar measurement of velocity and scalar fields without the need to add a flow tracer. In this study, an FRS concept with the ability to simultaneously measure high-accuracy time-averaged and time-resolved three-component velocity, static pressure and temperature fields is verified on a simplified mock-up of the CCITF facility. Time-averaged results show excellent agreement with benchmark laser Doppler anemometry (LDA) velocities, static pressure probe measurements and analytical temperature calculations. Moreover, it is shown that the developed concept can be used to determine multiple flow variables from a single-frequency measurement, opening the path towards time-resolved multi-parameter measurements by FRS.

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