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Measurement of Lagrangian Trajectories in a 3 L Stirred Tank Reactor using 4D Particle Tracking Velocimetry with Shake-the-Box
No full textStirred 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
Visualization and Quantification of the Cerebral Microcirculation using Contrast-enhanced Ultrasound Particle Tracking Velocimetry
Get PDFNoninvasive measurements of the regional microvascular perfusion might lead to sensitive biomarkers for the changes in intracranial hemodynamics that could guide timely surgical interventions for neonatal brain injuries. The current work utilizes a clinically available contrast enhanced ultrasound (CEUS) system and particle tracking velocimetry to perform ultrasound localization microscopy for measuring the microcirculation in piglets. A new deep learning method based on U-net is proposed for enhancing noisy raw CEUS images and detecting the microbubbles. Subsequently, the bubbles are tracked using a Kalman filter based method, which incorporates conditions of spatio-temporal consistency in flow direction and globally optimizes the assignment of bubbles to trajectories. Based on analysis of synthetic data, the U-net results demonstrate significant improvement in the processing speed and localization accuracy over a conventional blind deconvolution method. Visualization of the microvasculature is performed by superposing the bubble trajectories, enabling depiction of a complex micro-vessel network, where neighboring vessels separated by 40 µm can be distinguished. The corresponding perfusion map shows the velocity distribution in these vessels. Based on the current frame rate (44 fps), speeds in the 0.1 to 12 cm/s range can be well captured. These methods show promise as potential clinical tools for bedside measurement of cerebral microcirculation
Micro-PIV Measurements of Multiphase Flow in Deforming Porous Media Subject to Mineral Dissolution
Get PDFMineral dissolution is studied in novel calcite-based porous micromodels under single- and multiphase conditions, with a focus on the interactions of mineral dissolution with pore flow. Microscopic particle image velocimetry (PIV) was utilized to simultaneously characterize the local velocity field and the instantaneous shapes of the dissolving grains. The preliminary results provide a unique view of the coupled dynamics between pore flow and mineral dissolution
Hilbert transform revisited – Proper orthogonal decomposition applied to analytical signals of flow fields
Get PDFThe modes delivered by proper orthogonal decomposition (POD) are uncorrelated as per definition; but interestingly, they are not necessarily independent in terms of spatio-temporal flow-pattern dynamics. For instance, periodic structures that travel as waves through a series of snapshots often consist of pairs of modes with harmonic functions shifted 90 degree in phase and/or a spatial offset by a quarter of the spatial wave length of the convective flow pattern. Identification of such pairs, however, largely builds upon experience, visual inspection and/or the analysis of the reconstructed coefficients in cyclograms (Lissajous figures). This effort becomes even more challenging if measurement noise or other spurious information contaminates the raw data under consideration. One possibility to automatically pair corresponding patterns with common POD algorithms is the immediate application of the POD method to complex data (see Pfeffer et al., 1990). As outlined by Horel (1984), the Hilbert transform is a well-known and straight forward means to obtain the required extension of the original signal with an appropriate 90 degrees phase shift, which is independent of the fundamental frequencies. The complex extension of the original (real) signal Xi and its (discrete) Hilbert transform HT{Xi} as the imaginary part Xi +iHT{Xi} with the imaginary unit i is commonly known as the so-called analytical signal
Calibration correction of arbitrary optical distortions by non-parametric 3D disparity field for planar and volumetric PIV/LPT
Get PDFIn this study, a new image calibration approach is presented that corrects arbitrary optical distortions by utilizing non-parametric, 3D disparity fields. A calibration plate with a high spatial resolution (i.e., high density of calibration marks) was used to identify optical distortions that remain after the initial calibration, which were then used to create a correction field for the pinhole or polynomial mapping functions. Results from a pipe flow experiment with four cameras using volume self-calibration (VSC) and Shake-the-Box Lagrangian particle tracking (STB LPT) are presented and the impact of the improved calibration is discussed. Using the calibration marks with the correction field, distortions of initially more than 20 pixels are reduced below 1 pixel. Using VSC with the correction field allows further reduction of average calibration disparities below 0.02 pixels (maximum 0.5 pixels), whereas without a correction field the remaining average disparity is much higher at 1 pixel (maximum 5 pixels). STB analysis of the data shows a considerable higher spatial resolution at the pipe wall and a consistent spatial distribution of the number of detected particles in the measurement volume
Time-resolved flow field investigation in an industrial centrifugal compressor application involving TR-PIV synchronized with unsteady pressure measurements
Get PDFWe report on combined velocity and unsteady pressure measurements obtained on an radial compressor with vaneless diffuser and asymmetric volute. Time-resolved PIV recordings were acquired at 26 kHz both upstream of the impeller as well as within the vaneless diffusor at several rotation speeds at clean conditions and prior to the onset of instabilities within the rotor. The velocity data was acquired with a high-repetition rate, double-pulse laser system consisting of two combined DPSS lasers and a high-speed CMOS camera that was synchronized with multi-point unsteady pressure measurements. Details on the facility, the utilized instrumentation and data processing are provided with particular focus on the spectral and coherence analysis. Power spectra obtained from time records of the inlet velocity and unsteady pressure reveal an increase of low-frequency fluctuations below the blade passing frequency and the occurrence of a mode-locked behaviour indicating the presence of rotating instabilities. High levels of correlation between velocity and unsteady pressure signals not only confirm the temporal coherence of the acquired data but also reveal a direct coupling between flow field and pressure signature that is more prominent upstream of the rotor rather than in the diffusor
Adjustable window for 2D PIV estimation based on local Lagrangian coherency
No full textWe present a novel approach to adjust shapes of the interrogation windows (IW) in Particle Image Velocimetry (PIV) measurements as a function of temporal and spatial local coherent motions. Lagrangian Coherent Structures (LCS) has been widely utilized to determine local flow boundaries. We propose using Finite-Time Lyapunov Exponent (FTLE) to quantify LCS separatrix boundaries (i.e. ridges) and adjust the interrogation window. We integrated the proposed method with a local optical flow PIV algorithm. The evaluation was performed using synthetic particle images of 2D homogeneous isotropic turbulence obtained from Direct Numerical Simulation (DNS). The results showed significant improvements in regions with complex flow behaviours, particularly shear, vortex and hyperbolic motions. We studied improvements of the velocity estimation in a real experiment of the wake flow behind a cylinder at Reynolds number equal to 3900. It was found that optical flow featured by coherency based interrogation window (coherent optical flow) reveals detailed vector field estimations in regions with complex behaviours inside the wake flow
Flow characteristics analysis for flow past the porous spheres: wake structures and drag force coefficients
No full textFlow past a permeable sphere is different from that of a solid sphere due to the penetration of the fluid within porous structures, which can arise a change of flow fields. In this work, flow past porous spheres with Darcy numbers (Da) ranging from [0,10−3 ] were measured using planar Particle Image Velocimetry (PIV). The whole flow fields, including both leading edge and trailing edge, were captured at six different Reynolds numbers (Re) varying from 400 to 1400. Time-average flow fields were calculated based on instantaneous flow fields within fully-developed stages. Local minimum method was used to search for stagnation point positions. The results show positions of stagnation points are nearly proportional to the logarithm of Re. For most porous spheres, positions of stagnation points are extended to farther downstream positions than that of a solid sphere. However, at some certain Darcy numbers, ranging from 5 ∗ 10−6 to 2 ∗ 10−5, positions of stagnation points are closer to the sphere centers than that of an impermeable one
DPTV-based analysis of the flow-structure/wall-shear interplay in open wet clutches
Get PDFThe trend to lower energy consumption in the automotive industry still offers potential in various fields of application. One powerful saving strategy is described by the idling behavior of wet clutches, where the speed difference between drive and output, and the cooling oil in combination with a sub-millimeter spacing leads to significant amounts of wall shear stress (WSS) and accordingly drag torque. Minimization of this adverse effect has been found to be possible by means of grooved clutch-disk geometries, which have been demonstrated to correlate with the drag torque (see e.g. Neupert et al., 2018). The main interplay between torque and fluid flow in open wet clutches has been analyzed by Leister et al. (2020) in a dimensionless way. Today, a detailed investigation of a clutch flow, however, is missing for a larger variety of groove patterns and the cause-effect relations remain yet to be fully understood. Especially, the clear identification of the so-called foot print of a particular groove geometry in the flow field and corresponding WSS – thus drag-torque predictions – still requires further research efforts
Development of micro soap bubble generator for PIV tracer using home stereolithography 3D printer
Get PDFA micro soap bubble generator for tracers for PIV measurement was developed using a home stereolithography 3D printer. The nozzle has a coaxial triple pipe structure, and an orifice cap is attached to the nozzle head. The inner diameter of the central pipe is 0.7 mm, and the wall thickness of the central pipe is 0.7 mm. From the comparison of the smoke wire visualization result of the flow around the cylinder placed under the mainstream flow velocity of 3 m/s and the PIV measurement result, it was confirmed that the generated micro soap bubbles have good followability to the flow. Generated bubbles’ particle size was estimated to be Φ0.2 mm at the minimum and Φ6.3 mm at the maximum. The most common was Φ0.9 mm ± 0.1 mm, accounting for more than 50% of the total