Flow measurements in the wake of an adhering and oscillating droplet using laser-Doppler velocity profile sensor

Abstract

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

CFD simulation of water droplet adhesion on the GDL of a low temperature PEM FC in air cross-flow conditions

“…Experimental investigations confirmed that the droplets featured two oscillation modes, forwardbackward and upward-downward [11] when the airflow velocity was lower than the critical detachment value. Vertical oscillations were analysed in time ranges given by a minimal variation of the average height (<4%) but sufficiently wide to furnish statistically adequate samples (>100ms).…”

Measurements in a turbulent channel flow bymeans of an LDV profile sensor

“…In the present work, a recently introduced commercial ILA R&D LDV-PS system with an integrated 5 MHz Bragg shift is employed, which was recently shown to allow measurements of all three velocity components based on trigonometric reconstruction in a laminar Couette-like oil flow in a clutch test rig (Leister et al, 2022). The commercial measurement system has furthermore been successfully applied to measure the turbulent flow in the wake of a droplet with a spatial resolution of 10 µm (Burgmann et al, 2021) and the flow topologies appearing in rotary pumps used for blood (Strauch et al, 2022). The two above-mentioned studies, therefore, present the LDV-PS as a promising diagnostics tool for non-intrusive measurements in turbulent flows and, especially, for the determination of the velocity gradient in the viscous sublayer, which would in turn offer the desired possibility of direct local wall shear stress measurements.…”

Toward droplet dynamics simulation in polymer electrolyte membrane fuel cells: Three-dimensional numerical modeling of confined water droplets with dynamic contact angle and hysteresis

“…It must be noted that an important physical phenomenon here is the occurrence of oscillations, which are rooted in the concurrent effect of inertia and surface tension 8,66,67 . In order to prevent strong droplet oscillations in this section, first, the gravity is linearly increased from zero to g = 9.81m/s 2 with a slope of g/τ r while the tilting angle is kept zero.…”