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Quantification of Brownian motion under presence of flow using particle diffusometry
Get PDFParticle diffusometry (PD), a quantification method for the Brownian motion, is performed by recording temporally sequential images and using correlation analysis to obtain an ensemble diffusion coefficient for all particles captured in the imaging region (Clayton et al., 2017). PD is proven to be successful in the detection of the waterborne pathogen V. cholerae in environmental samples using different imaging techniques, including an inverted fluorescence microscope as well as a handheld hardware device operated with a smartphone (Clayton et al., 2019; Moehling et al., 2020). Although we intend to use PD to calculate diffusion coefficients in quiescent fluid, oftentimes unintentional fluid flows occur, creating measurement error when calculating the diffusion coefficient. In previous work, recordings under the presence of flow were discarded to avoid incorrect measurements of the sample. 
Spatially and temporally resolved measurements of turbulent Rayleigh-Bénard convection by Lagrangian particle tracking of long-lived helium-filled soap bubbles
Get PDFWe present spatially and temporally resolved velocity and acceleration measurements of turbulent RayleighBénard convection spanning the whole volume (~ 1 m³) of a cylindrical sample with aspect ratio one. With the "Shake-The-Box" (STB) Lagrangian particle tracking (LPT) algorithm, we were able to instantaneously track up to 560,000 particles, corresponding to mean inter-particle distances down to 6 - 8 Kolmogorov lengths. We used the data assimilation scheme ‘FlowFit’, which involves continuity and Navier-Stokesconstraints, to map the scattered velocity and acceleration data on cubic grids, herewith recovering the smallest flow scales. Lagrangian and Eulerian visualizations reveal the dynamics of the large-scale circulation and its interplay with small scale structures, such as thermal plumes and turbulent background fluctuations. As a result, the complex time-dependent behavior of the LSC comprising azimuthal rotations, torsional oscillation and sloshing can be extracted from the data. Further, we found more seldom dynamic events, such as spontaneous reorientations of the LSC in the data from long-term measurements
An Experimental Study of Unsteady Heat Transfer and Phase Changing Process Upon Impacting of Ice Crystals onto Heated Surfaces Pertinent to Aero-Engine Icing Phenomena
Get PDFIce accretion on exposed surfaces of aero-engine components has been widely recognized as a significant hazard to aviation safety in cold weathers. Icing process due to the impingement of the supercooled water droplets suspended in the cloud onto the cold surfaces of inlet components of aeroengines have been studied extensively for decades. Since ice particles were believed to simply bounce off from the exposed surfaces of aero-engine components, ice crystals in the clouds were initially considered not to pose a threat to aviation safety. Therefore, the ice accretion process due to the impacting of ice crystals onto the surfaces of hot engine componentes has not been studied until recently. It has been found recently that, tiny ice particles in the cloud may be partial/full melting upon impacting onto the hot surfaces of aero-engine components, such as heated Inlet Guide Vanes (IGV) and various probes. The partially/fully melted ice crystals were found to stick onto the hot surfaces and form thin water film, which would intercept more oncoming ice particles and lead to significant ice accretion over the surfaces of the hot engine components. The ice crystal induced ice accumulation on the critical aero-engine components has been found to cause significant engine performance loss and erroneous data being read from the probes.
In the present study, a series of experimental investigations were conducted to elucidate the underlying physics of the dynamic ice accretion process pertinent to ice crystal icing phenomena. A novel ice crystal icing test rig with the capacity of generating controllable amount of ice crystals and flying speed up to 100 m/s was developed in a temperature-controllable environment chamber for the ice crystal icing studies. By using a high-speed imaging system, a digital particle image velocimetry(PIV), and an Infrared (IR) thermal imaging system, a comprehensive experimental campaign was performed to characterize the transient impacting process of ice crystals, dynamic ice accretion and unsteady heat transfer process associated with the impacting of ice crystals onto heated surfaces, in comparison to those due to the impingement of supercooled water droplets. By using an ultra-sensitive force sensor and a high-speed image system, a comparative study is conducted to examine the differences in the transient impinging dynamics of single water droplets, supercooled water droplets, and ice crystals onto solid surfaces with different wettability and stiffness. By upgrading the unique Icing Research Tunnel of Iowa State University (i.e., ISU-IRT) with additional ice crystal icing capability, a set of explorative studies are also conducted to examine the characteristics of the dynamic ice accretion processes over the heated surfaces of an aero-engine Inlet Guide Vane (IGV) model under both ice crystal icing and supercooled droplet icing conditions. The anti-/de-icing performance of a novel hybrid strategy by integrating icephobic coatings and minimized surface heating are also evaluated under both supercooled water droplet icing and ice crystal icing conditions. The new findings derived from the present studies are very helpful to gain further insights into the ice crystal icing phenomena for the development of more effective and robust anti-/de-icing strategies to ensure safer and more efficient aircraft/aero-engine operations in cold weathers
Fluid structure interactions of a pitching wing at high angle of attack
Get PDFThis paper deals with the study of the flow around a pitching wing at high angle of attack. Different pitching amplitudes and frequencies are studied using DIC and LPT measurements. The Fluid Structure Interactions are shown and exhibit that the vortex shedding could be reduced by actuating the wing at specific frequencies
An Infrared Quantitative Imaging Technique (IR-QIV) for Remote Sensing of River Flows
Get PDFStage and discharge are some of the oldest measurements in environmental fluid mechanics and are vital in forecasting water supply and flood safety. These measurements are traditionally manpower intensive, hence expensive, and dangerous under high flow conditions. Considering climate change and the planet’s increasing population there is a critical need for better, more accurate, and frequent, in space and time, data for model and forecast guidance. This need spans monitoring small-scale turbulent processes to calibrating and nudging continental scale river dynamics models. Driven by applications from river gaging networks to fish behavior modeling to flood and erosion forecasting, and more generally, the near-shore environment of lakes, estuaries and the coasts, remote sensing with quantitative imaging tools is a rapidly expanding field. Such tools can be deployed from fixed platforms, drones, planes and satellites with valuable information contained within the visible to infrared spectral bands
Characterization of interactive force acting on colloidal particles near an electrode in presence of a high-frequency (>10 kHz) AC electric field using particle diffusometry
No full textColloidal particles like polystyrene beads and metallic micro and nanoparticles are known to assemble in crystal-like structures near an electrode surface under both DC and AC electric fields. Various studies have shown that this self-assembly is governed by a balance between an attractive electrohydrodynamic (EHD) force and an induced dipole-dipole repulsion (Trau et al., 1997). The EHD force originates from electrolyte flow caused by interaction between the electric field and the polarized double layers of both the particles and the electrode surface. The particles are found to either aggregate or repel from each other on application of electric field depending on the mobility of the ions in the electrolyte (Woehl et al., 2014). The particle motion in the electrode plane is studied well under various conditions however, not as many references are available in the literature that discuss the effects of the AC electric field on their out-of-plane motion, especially at high frequencies (>10 kHz). Haughey and Earnshaw (1998), and Fagan et al. (2005) have studied the particle motion perpendicular to the electrode plane and their average height from the electrode mostly in presence of DC or low frequency AC (<1 kHz) electric field. However, these studies do not provide enough insight towards the effects of high frequency (>10 kHz) electric field on the particles’ motion perpendicular to the electrode plane.
Highly accurate optical flow method based on volumetric segmentation for 3D PIV
Get PDFIn this study, we present a new three-dimensional optical flow method based on volumetric segmentation for the velocity estimation of fluid flow. The proposed method uses a segmented smoothness term that is designed on the assumption that the particle velocity varies continuously in each segmented volume and discontinuously on the surfaces of the segmented volumes. Subsequently, the data term is proposed on the basis of the segmented volumes and the fluid mass conservation equation, which is derived from the Reynolds transport equation. In addition, the robust local level-set method is applied to segment the particle volume according to the velocity distribution of fluid flow. The proposed method is evaluated quantitatively on synthetic data and qualitatively on experimental data, and the velocity results are compared to the advanced 3D velocity estimation methods. The results indicate that the proposed method can obtain velocity fields with greater measurement accuracy for Tomo-PIV
On the challenges of precise velocity measurements in vertical convective wall jets
Get PDFFor the transition of our energy supply towards a higher share of renewables, thermal energy storage (TES) systems are, besides electric batteries and chemical energy storage systems, one promising solution to overcome the volatile nature of renewable energy sources. For the most efficient operation, the liquid storage material in the tank should be stratified by its temperature-dependent density. As a result, the cold fluid remains at the bottom, and the heated fluid rises to the top (Alva et al. (2018)). Typically steel tanks are used for TES, and thus, the wall material has a thermal diffusivity that is one to two orders of magnitudehigher than that of the storage fluid. Consequently, the tank’s sidewalls work as a thermal bridge between the stratified layers. In recent studies, the authors have shown that the resulting heat flux induces two counterdirected, convective wall jets near the sidewalls of the tank, which increase mixing of the stratification and thus lowers the exergy content and the storage efficiency (Otto et al. (2019, 2020)). Using a model experiment of a TES, the entire vertical extent of the detected wall jets is investigated. Hence, the typical flow structures of vertical, natural convection under the influence of non-zero temperature gradients in the ambient fluid can be analyzed, which can help to improve storage tanks in the future.
The velocity in the region of the wall jets is measured via 2d particle-image velocimetry (PIV) in a rectangular model experiment of 750mm height on a base area of 375mm×375mm made from polycarbonate. The jets evolve on the surface of an aluminum plate simulating the storage tank’s sidewall. The measuring system consists of four cameras with a resolution of 2160×2560 pixels combined with objective lenses with 100mm focal length capturing the raw images in a plane perpendicular to the aluminum wall. A Nd:YAG laser with a wavelength of 532nm illuminates the measuring plane. Simultaneously using up to four cameras adjacent to each other and stitching their resulting vector fields, the vertical extent of the field of view increases from 38mm up to 140mm. Despite this, the field of view is still much smaller than the vertical extent of the model experiment, so that seven consecutive runs are performed to cover the entire height. Disturbing reflections of the laser light sheet on the aluminum wall are eliminated using optical filters for the cameras that are opaque for the green laser light in combination with fluorescently (Rhodamine B) dyed PMMA tracer particles with a diameter between 1–20μm. The particles emit light at a wavelength of 610nm (orange light) and can therefore be detected through the cameras’ filters. During four separate measuring periods, where each lasts for two minutes, double frame images are captured with a time difference of 19.981 ms (maximum possible value) at a measuring frequency of 7 Hz. Figure 1 shows a schematic of the camera setup next to the model experiment and the measurement and evaluation procedure to finally receive one time-averaged velocity field per measuring period of the full height of the experiment.
The raw data evaluation process starts with calculating the vector fields of all cameras used at a certain measuring position and stitching them to one flow field of this position. Since the wall jets’ horizontal extents are with 2–7mm relatively small and they show high velocity gradients, the raw images are evaluated in both single-frame and double-frame mode. With a velocity threshold that corresponds to a pixel displacement of 1/4 of the interrogation window size and the time difference of the single-frames, the resulting vector fields are masked and merged into one final vector field. This vector field consists of high velocities evaluated in double-frame mode and low velocities evaluated in single-frame mode (see Figure 2) thus minimizing the relative error. The algorithm used in this work is similar to the multi-frame PIV approach introduced by Hain and K¨ahler (2007). Figure 3 shows the time-averaged results of the first measuring period for each of the seven measuring positions in height
PIV measurement of turbulence over a streamwise preferential porous medium
Get PDFThis study examines the possibility of orthotropic porous medium whose streamwise permeability is larger than the wall-normal permeability to reduce turbulent friction inspired by recent numerical studies of Rosti et al. (2018); G´omez-de Segura and Garc´ıa-Mayoral (2019). Because G´omez-de Segura and Garc´ıa-Mayoral (2019) used Brinkman equation to approximate the flow in the porous media, it is uncertain that such porous media really reduce the friction. We make a layered porous medium, which satisfies the drag reducing condition suggested by G´omez-de Segura and Garc´ıa-Mayoral (2019), and carry out particle image velocimetry measurements of turbulent square duct flows over it and examine the drag reduction probability. From the analyses of the obtained data, it is found that the friction on the porous-wall is nearly the same as that of the smooth-wall at Reb < 10000 and tends to increase at Reb > 10000
Towards capturing a database of respiratory exhalations from flow visualisations
Get PDFOne of the most common modes of infection transmission is through pathogen laden droplets expelled during natural human respiratory exhalations such as speaking, coughing, and sneezing. Infection control guidelines for the prevention of respiratory infection make assumptions about two key parameters: the safe distance between an infected and healthy individual and the size of large and small droplets (Bahl et al., 2020). Studies in the past have utilised flow visualisation techniques to understand the dynamics of respiratory flows but most of them provide only qualitative data on respiratory droplets and do not provide sufficient detail to estimate accurate flow velocities (Bourouiba et al., 2014; Vansciver et al., 2011; Scharfman et al., 2016). One of the reasons this remains a demanding application is the vast range of droplet sizes that are expelled at various velocities. Here, we present an experimental framework using particle tracking to understand the flow dynamics of the expelled droplets. Three different illumination techniques were used to capture high-speed frames of different exhalations (see figure 1). The high density of droplets in case of sneezing lead to overlap of droplet trajectories with volume illumination approach, which was resolved using tailored optics to illuminate only a slice of sneeze flow. Thereafter, the image processing techniques required for precise PTV were refined to examine droplet dynamics of various exhalations (see figure 2). The techniques were applied to multiple cases of respiratory exhalations to understand subject to subject variability.
The results for sneezing revealed a mean droplet velocity of 2 m/s to 5.4 m/s across different subjects. Additionally, less than 1% of droplets were expelled at velocities greater than 10 m/s and almost 80% of were expelled at velocities less than 5 m/s. These values were substantially lower than the values usually assumed in studies modelling or replicating sneezes (Xie et al., 2007; Atkinson and Wein, 2008). The results also revealed a high variation in the droplet dynamics, even among the sneezes from the same subject. Flow direction, spread angle, and head movement were also quantified, and the results reveal substantial variation between the subjects. In the case of coughing, maximum droplet velocities observed were in the range of 10−15 m/s however, these high velocities were detected only during the initial 0.05 s.
This work addresses the critical gaps in the understanding of the respiratory transmission of infection by providing valuable data on the droplet dynamics of various exhalations, on which the experimental data was very limited in the existing literature. Furthermore, this data will aid in numerical modelling of respiratory flows, particularly for sneezes, as studies to date rely only on airflow data of the exhalations