The unsteady force response of an accelerating flat plate with controlled spanwise bending

The unsteady force response of an accelerating flat plate, subjected to controlled spanwise bending, is investigated experimentally. The flat plate was held normal to the flow (at an angle of attack of $90^{\circ }$ ), and it was dynamically bent along the spanwise direction with the help of internal actuation. Two bending directions were tested. In one case, part of the plate (denoted by flexion ratio) was bent into the incoming flow (the bend-down configuration). In another case, the plate was bent away from the flow (the bend-up configuration). We used two different aspect ratio ( $AR$ ) plates, namely $AR = 2$ and 3. Three acceleration numbers, namely $A_c = 0.57$ , 1.6 and 3.2 (corresponding to dimensional acceleration of 0.036, 0.1 and 0.2 m s $^{-2}$ , respectively) were tested with a fixed terminal Reynolds number (Re) of 18 000. For each acceleration number, three bending durations, namely 1.2, 2.4 and 3.6 s were implemented. The results indicate that the highest impulse was imparted by the highest bending rate (duration 1.2 s) during all three accelerations tested. We show that controlled spanwise bending can significantly change the unsteady force response by manipulating the inertial forces during a start-up manoeuvre. The unsteady forces depend on the vector sum of the forward acceleration and the bending acceleration of the plate. The unsteady drag was augmented when the plate was bent towards the incoming flow. The initial force peaks were significantly reduced when the bending direction was reversed. The development of the edge vortices from the flat plate was measured with the help of particle image velocimetry (PIV) at the 70 % and the 90 % span locations. The PIV measurements were also carried out at the midchord plane closer to the tip region to capture the growth of the tip vortex. The vorticity field calculated from these PIV measurements revealed that controlled bending contributed to a variation in the circulation growth of the edge vortices. During the bend-down case, the circulation growth was faster and the tip vortices stayed closer to the plate. This resulted in increased interaction with the edge vortex at the 90 % span. This interaction was more severe for $AR = 2$ . During the bend-up case, the growth of the edge vortex was delayed, but the vortex grew for a longer time compared with the bend-down case. Finally, a mathematical model is presented which correctly captured the trend of the force histories measured experimentally during both the bend-up and bend-down cases.

Outlier detection for PIV statistics based on turbulence transport

The occurrence of data outliers in PIV measurements remains nowadays a problematic issue; their effective detection is relevant to the reliability of PIV experiments. This study proposes a novel approach to outliers detection from time-averaged three-dimensional PIV data. The principle is based on the agreement of the measured data to the turbulent kinetic energy (TKE) transport equation. The ratio between the local advection and production terms of the TKE along the streamline determines the admissibility of the inquired datapoint. Planar and 3D PIV experimental datasets are used to demonstrate that in the presence of outliers, the turbulent transport (TT) criterion yields a large separation between correct and erroneous vectors. The comparison between the TT criterion and the state-of-the-art universal outlier detection from Westerweel and Scarano (Exp Fluids 39:1096–1100, 2005) shows that the proposed criterion yields a larger percentage of detected outliers along with a lower fraction of false positives for a wider range of possible values chosen for the threshold. Graphical abstract

The flow topology transition of liquid–liquid Taylor flows in square microchannels

This work investigates the change of the flow topology of Taylor flow and qualitatively relates it to the excess velocity. Ensemble-averaged 3D2C-$$\upmu$$ μ PIV measurements simultaneously resolve the flow field inside and outside the droplets of a liquid–liquid Taylor flow that moves through a rectangular horizontal microchannel. While maintaining a constant Capillary number Ca = 0.005, the Reynolds number ($$0.52 \le {\text{Re}} \le 2.14$$ 0.52 ≤ Re ≤ 2.14 ), the viscosity ratio ($$0.24 \le \lambda \le 2.67$$ 0.24 ≤ λ ≤ 2.67 ) and surfactant concentrations of sodium dodecyl sulfate (0–3 CMC) are varied. We experimentally identified the product of the Reynolds number Re and the viscosity ratio $$\lambda$$ λ to indicate the momentum transport from the continuous phase (slugs) into the droplets (plugs). The position and size of the droplet’s main vortex core as well as the flow topology in the cross section of this vortex core changed with increased momentum transfer. Further, we found that the relative velocity of the Taylor droplet correlates negatively with the evoked topology change. A correlation is proposed to describe the effect quantitatively. Graphical abstract

A Study of Recirculating Flow Fields Downstream of a Diverse Range of Axisymmetric Bluff Body Geometries Suitable for Flame Stabilization

This work investigates the non-reacting time averaged and fluctuating flow field characteristics downstream of a variety of axisymmetric baffles, operating in combination with an upstream double-cavity premixer arrangement. The study aims to broaden knowledge with respect to the impact of different bluff body shapes, leading and trailing edge flow contours, blockage ratios and incoming flow profiles impinging on the bluff body, on the development and properties of the downstream recirculating wake. Particle Image Velocimetry (PIV) measurements have been employed to obtain the mean and turbulent velocity fields throughout the centrally located recirculation zone and the adjacent developing toroidal shear layer. The results are helpful in demarcating the cold flow structure variations in the near wake of the examined baffles which support and, to some extent, determine the flame anchoring performance and heat release disposition in counterpart reacting configurations. Additionally, such results could also assist in the selection of the most suitable flame stabilization configuration for fuels possessing challenging combustion behavior such as multi-component heavier hydrocarbons, biofuels, or hydrogen blends.

Characterizing the surface texture of a dense suspension undergoing dynamic jamming

Measurements of the surface velocity and surface texture of a freely propagating shear jamming front in a dense suspension are compared. The velocity fields are captured with particle image velocimetry (PIV), while the surface texture is captured in a separated experiment by observing a direct reflection on the suspension surface with high-speed cameras. A method for quantifying the surface features and their orientation is presented based on the fast Fourier transform of localized windows. The region that exhibits strong surface features corresponds to the the solid-like jammed region identified via the PIV measurements. Moreover, the surface features within the jammed region are predominantly oriented in the same direction as the eigenvectors of the strain tensor. Thus, from images of the free surface, our analysis is able to show that the surface texture contains information on the principle strain directions and the propagation of the jamming front. Graphic Abstract

Particle Image Velocimetry for in vitro characterization of Paravalvular Leakage of TAVR-sealing concepts

Paravalvular leakage (PVL), defined as the leakage between the aortic annulus and a transcatheter aortic valve replacement (TAVR), is verifiably associated with short- and long-term clinical outcome, especially with increased mortality. Therefore, with the ambition to reduce or even prevent PVL of next generation TAVR, it is necessary to extend the hemodynamic understanding of PVL. This study presents an in vitro flow measurement method to localize PVL during hydrodynamic characterization of TAVR and furthermore presents different design features, socalled outer skirt, to reduce PVL. Particle image velocimetry (PIV) measurements were performed for flow field assessment during hydrodynamic characterization of TAVR. Additionally, two different sealing concepts were developed to reduce PVL. The skirts were manufactured from polymeric-nonwoven and sued to pericardium-based TAVR-prototype. The prepared TAVR-prototypes were then deployed in a pathophysiological model of the aortic root with a calcification nodule of 2 mm according to ISO 5840:2021. To assess PVL, the flow field and the regurgitation volume was measured. The PIV measurements showed a clearly visible leakage jet between the TAVR-prototypes without skirt and the pathophysiological aortic annulus model. Jet velocities of up to 0.5 m/s were measured depending on presence or configuration of a PVL-preventing skirt. When implanted in the physiological annulus model without calcification nodule, PVL was hardly recognizable. The regurgitation volume of a TAVR-prototype without skirt at 5 l/min was 36.26±1.89 ml (n = 10). The developed and manufactured polymeric-nonwoven skirts reduced PVL from 37.67±1.17 ml to 18.36±1.8 ml (n = 10, TAVR-skirt-design1) and from 46.97±1.07 ml to 17.85±1.29 ml (n = 10, TAVR-skirt-design2) at 5 l/min. The localization of PVL during hydrodynamic characterization by means of PIV was successful. The sealing concepts developed in this work were very effective and led to a PVL-reduction of the tested TAVR prototypes of about 50% to 70%.