Tomographic X-ray particle tracking velocimetry

We investigate the feasibility of in-laboratory tomographic X-ray particle tracking velocimetry (TXPTV) and consider creeping flows with nearly density matched flow tracers. Specifically, in these proof-of-concept experiments we examined a Poiseuille flow, flow through porous media and a multiphase flow with a Taylor bubble. For a full 360$$^\circ$$ ∘ computed tomography (CT) scan we show that the specially selected 60 micron tracer particles could be imaged in less than 3 seconds with a signal-to-noise ratio between the tracers and the fluid of 2.5, sufficient to achieve proper volumetric segmentation at each time step. In the pipe flow, continuous Lagrangian particle trajectories were obtained, after which all the standard techniques used for PTV or PIV (taken at visible wave lengths) could also be employed for TXPTV data. And, with TXPTV we can examine flows inaccessible with visible wave lengths due to opaque media or numerous refractive interfaces. In the case of opaque porous media we were able to observe material accumulation and pore clogging, and for flow with Taylor bubble we can trace the particles and hence obtain velocities in the liquid film between the wall and bubble, with thickness of liquid film itself also simultaneously obtained from the volumetric reconstruction after segmentation. While improvements in scan speed are anticipated due to continuing improvements in CT system components, we show that for the flows examined even the presently available CT systems could yield quantitative flow data with the primary limitation being the quality of available flow tracers. Graphic abstract

Experimentelle Untersuchung von Staubteufeln in einem Laborexperiment

<p>Durch bodennahe Erwärmung der Luft bei starker Sonneneinstrahlung können sich aufsteigende Luftwirbel mit vertikaler Rotationsachse bilden, sogenannte Staubteufel. Die Struktur eines solchen Staubteufels wird dominiert von einem horizontalen Zufluss nahe der Erdoberfläche und einer vertikalen Aufwärtsströmung des Wirbels. Da die Staubteufel auf trockenem Untergrund entstehen, ist der im Zustrom mitgetragene Staub ursächlich für ihr Erscheinungsbild. Die Mechanismen ihrer Entstehung sowie ihre charakteristischen Eigenschaften sind bis heute kaum im Detail erforscht, da experimentelle Untersuchungen auf in-situ Messungen in der Atmosphäre beschränkt sind. Aufgrund der Nichtvorhersagbarkeit solch seltener Ereignisse sind Felduntersuchungen nur schwer möglich.</p> <p>Mithilfe von Laborexperimenten können Staubteufel jedoch gezielt erzeugt und ihre Entstehungsmechanismen untersucht werden. Hierfür wird die Versuchsanlage „Ilmenauer Fass“ verwendet, die im Wesentlichen aus einem luftgefüllten, zylindrischen Tank mit einem Innendurchmesser von 7,15 m und einer Höhe von 8 m besteht. Am Boden des Behälters befindet sich eine Heizplatte, deren Temperatur zwischen 20°C und 80°C variiert werden kann. Eine zweite freischwebende Kühlplatte kann in einer beliebigen Höhe zwischen 0,2 m und 6,3 m positioniert und in einem Temperaturbereich von 10°C bis 30°C geregelt werden. Die Seitenwand des Zylinders ist adiabatisch. Mithilfe dieses Rayleigh-Bénard-Experiments lassen sich die Randbedingungen in einer konvektiven, atmosphärischen Grenzschicht gut nachbilden und die Entstehung von Wirbelstrukturen unter kontrollierten Bedingungen detailliert untersuchen. Hierfür wurde das optische Messverfahren Particle Tracking Velocimetry (PTV) genutzt, mit dem das dreidimensionale Strömungsfeld innerhalb und außerhalb solcher Wirbel erfasst werden kann. Dabei werden der Strömung dichteneutrale Seifenblasen zugeführt und mit vier Kameras aus unterschiedlichen Positionen zeitaufgelöst detektiert. Über Triangulation werden die 3D-Trajektorien der einzelnen Partikel im Raum berechnet.</p> <p>Während bei der letzten Tagung noch das Messverfahren vorgestellt wurde, können nun die ersten Ergebnisse präsentiert werden. In Messreihen über mehr als insgesamt 20 Stunden wurden große Datenmengen gesammelt und vertikale Wirbelstrukturen identifiziert. Wie in numerischen Simulationen gezeigt werden konnte, nimmt die Häufigkeit solcher Strukturen mit zunehmender Größe stark ab. Deshalb wurde alternativ in der Mitte der Bodenplatte eine Heizmatte installiert, mit der eine lokale Temperaturüberhöhung bis 10 K über der Temperatur der Heizplatte realisiert werden kann. Mit dieser Modifikation des Laborexperiments kann nun auch die Wahrscheinlichkeit der Entstehung von Staubteufeln erhöht werden.</p>

Evidence of preferential sweeping during snow settling in atmospheric turbulence

We present a field study of snow settling dynamics based on simultaneous measurements of the atmospheric flow field and snow particle trajectories. Specifically, a super-large-scale particle image velocimetry (SLPIV) system using natural snow particles as tracers is deployed to quantify the velocity field and identify vortex structures in a 22 m $\times$ 39 m field of view centred 18 m above the ground. Simultaneously, we track individual snow particles in a 3 m $\times$ 5 m sample area within the SLPIV using particle tracking velocimetry. The results reveal the direct linkage among vortex structures in atmospheric turbulence, the spatial distribution of snow particle concentration and their settling dynamics. In particular, with snow turbulence interaction at near-critical Stokes number, the settling velocity enhancement of snow particles is multifold, and larger than what has been observed in previous field studies. Super-large-scale particle image velocimetry measurements show a higher concentration of snow particles preferentially located on the downward side of the vortices identified in the atmospheric flow field. Particle tracking velocimetry, performed on high resolution images around the reconstructed vortices, confirms the latter trend and provides statistical evidence of the acceleration of snow particles, as they move toward the downward side of vortices. Overall, the simultaneous multi-scale particle imaging presented here enables us to directly quantify the salient features of preferential sweeping, supporting it as an underlying mechanism of snow settling enhancement in the atmospheric surface layer.

In vitro estimation of the energy loss through turbulence in a porcine aortic valve stenosis model and silicone ascending aorta phantom using backlight particle tracking velocimetry

Introduction Patients suffering from low-flow, low-gradient aortic stenosis present a decreased stroke volume due to decreased contraction or relaxation function of the left ventricle. As a low stroke volume tends to cause a low transvalvular flow, transvalvular pressure gradient (TVPG) and effective orifice area, the clinician cannot rely on those parameters with confidence for the evaluation of aortic stenosis severity. Hence new diagnostic parameters have to be developed. Energy loss through turbulence associated with aortic stenosis represented the wasted left ventricle work. Currently, echocardiographic measurement of the turbulence intensity is not validated for clinical evaluations of aortic stenosis. Methods Two porcine aortic valves were harvested and inserted in a flow loop that replicates the pulsatile flow of the heart. A stiffening of the valves was achieved by treating them with formaldehyde. The stiffening was externally confirmed by a custom-made force-displacement device quantifying the rigidity of the leaflet yielding two stiffness grades per valve. Each valve was tested under three different mean flow rates (1, 2.5, and 4 l/min) at each of the two stiffness grades. Moreover the pressure in the left ventricle chamber and in the aortic chamber was recorded to calculate the TVPG. Particle tracking velocimetry measurements into the transparent silicone ascending aorta phantom allowed the computation of the turbulent kinetic energy (TKE), to evaluate the energy loss due to turbulence. Results We could confirm the enhanced rigidity of the valve leaflets with our custom device (data not shown) and measure a consistent increase in TVPG across all mean flow rates between the two stiffness grades. Moreover, an explicit increase of the TKE in the aortic phantom could be measured after the stiffening process (73.1% under 1 l/min, and 43% under both 2.5 and 4 l/min). In addition, a good correlation (R = 0.86) between the mean TVPG and the TKE was found. Conclusions This project demonstrated the possibility of quantifying the energy loss attributed to turbulence for porcine valves in vitro for native and added stiffness grade. This project lays the foundation for the development of a new diagnostic tool for the assessment of stenosis severity in patients with low-flow, low-gradient aortic stenosis in cardiac imaging tool such as echocardiography. FUNDunding Acknowledgement Type of funding sources: None. TVPG and its correlation with TKE Intensity graphs of the TKE

Measuring binary fluidization of non-spherical and spherical particles using machine learning aided image processing

The binary fluidization of Geldart-D type non-spherical wood particles and spherical LDPE particles was investigated in a laboratory-scale bed. The experiment was performed for varying static bed height, wood particles count, as well as superficial gas velocity. The LDPE velocity field were quantified using Particle Image Velocimetry (PIV). The wood particles orientation and velocity are measured using Particle Tracking Velocimetry (PTV). A machine learning pixel-wise classification model was trained and applied to acquire wood and LDPE particle masks for PIV and PTV processing, respectively. The results show significant differences in the fluidization behavior between LDPE only case and binary fluidization case. The effects of wood particles on the slugging frequency, mean, and variation of bed height, and characteristics of the particle velocities/orientations were quantified and compared. This comprehensive experimental dataset serves as a benchmark for validating numerical models.

Pressure calculation for flows with moving surface boundaries from particle tracking velocimetry (PTV)

The examples of flow conditions, where an object of a fixed or deformable body moves in a fluid, or the interface between the flow phases instantaneously changes its topology, are numerous in industry and natural sciences. The advent of particle image velocimetry (PIV) [1] and particle tracking velocimetry (PTV) [2] enabled the measurement of the instantaneous velocity fields in these types of complicated flow fields. As a next step, several methodologies have been developed in the past decade to calculate the pressure fields from PIV or PTV data [3,4]. These methods were developed based on the assumption of a stationary flow domain, with surface boundaries that are fixed and independent of time. This makes the current pressure calculation methods inapplicable to a flow domain with deformable moving surface boundaries. Also, for most of the two-phase flows, the capillary forces are significant and the pressure drop over the two-phase interface must be considered. Therefore, the current pressure calculators require an improvement in the formulation of the algorithms to account for the deformable volume conditions and the effect of the surface tension force. For the calculation of pressure from sparse PTV velocity data, firstly, a tessellation method is required to interconnect the irregularly spaced vectors in the flow field using a highquality mesh grid. The mesh must be dynamic and adjust itself to the moving boundaries. This tessellation method has already been developed by the current authors [5]. As the next step, equations of motion for a deformable C.V. need to be coupled with the tessellation method to calculate the instantaneous pressures in a two-phase flow field, with a moving interface, which will be the ultimate goal of the current study.

Measurement of Lagrangian Trajectories in a 3 L Stirred Tank Reactor using 4D Particle Tracking Velocimetry with Shake-the-Box

Stirred tank reactors are widely used in the chemical industry and bioprocess engineering and, consequently, a large number of scientific publications deal with the characterization of those apparatuses. However, there is very little information about the flow conditions. This is mostly due to the fact that these apparatuses are generally made of stainless steel, which restricts optical access. Furthermore, three-dimensional flow field measurements are still not trivial and involve costly equipment, therefore, investigations often reduce to two-dimensional PIV measurements. Nevertheless, recent works (Rosseburg et al., 2018; Taghavi and Moghaddas, 2020; Kuschel et al., 2021) impressively show the formation of compartments which hinder and delay mixing. However, these measurements are based either on instantaneous concentration profiles by means of pLIF measurements or on a two-dimensional projection of the system and thus do not allow conclusions about the development of the three dimensional compartments and the exchange rates between the compartments. In this work, for the first time, instantaneous flow field measurements with high spatial and temporal resolution are performed in the entire volume of a 3L stirred tank reactor based on 4D particle tracking velocimetry. The highly resolved particle trajectories further allow detailed Lagrangian analysis of the mixing dynamics inside the reactor, data that was previously inaccessible.