Flow Structure Research Articles

3D Lagrangian Particle Tracking In A Model Of The Human Airways

The three-dimensional flow field in the trachea of a physiologically correct model of the human respiratory tract from the nasal/oral cavities up to the 6th bronchi generation is investigated. The investigations comprise steady inflow conditions, i.e., steady inhalation and oscillatory flow combining inspiration and expiration to simulate calm breathing. A realistic breathing pattern is approximated by a sinusoidal waveform for two Womersley numbers of 3 and 4.5 and two Reynolds numbers of 400 and 1200. Due to the inherent three-dimensional and asymmetric nature of the flow field, 3D particle-tracking velocimetry (3D-PTV) measurements are performed using the Shake-The-Box (StB) algorithm to determine the influence of oral and nasal inhalation and exhalation on the flow field in the lower human airways. By using a refractive-index matched fluid consisting of water and glycerin, the complex flow structures inside the trachea are fully resolved. During oscillatory inhalation/exhalation, an integration window size of 6/40 pi was chosen as it can be considered short enough to assume a steady state. The PTV measurements confirm that the nasal and/or oral cavity must be taken into account when analyzing the flow field in the lower respiratory tract. Characteristic sets of velocity profiles in the sagittal and coronal plane of the trachea as well as contour plots evidence nearly identical flow structures for both oral and nasal inhalation, but also reveal that the presence of both cavities possesses a huge impact on the flow field compared to ideal, i.e., fully developed inflow conditions

Event Based Cameras: A Comparative Study For Fluid Event Detection

A feasibility study using an event-based camera for the detection of plane-wise motion of a 2D array of bio-inspired sensing pillars to detect generated flow events. EBCs show potential for future applications where PIV may have been classically used. They have the benefit of only generating data when an event happens and with comparatively cheaper setups, they hold great potential for flow monitoring. EBCs collect pixel x and y locations, as well as pixel contrast change data at each time there is an event and can be trained on regions of interest. Comparative experiments were carried out in the labs at City, University of London, to investigate the feasibility of using EBCs for the detection of generated large flow structures in both air and water. The disturbances were produced by a gust generator, an upstream cylinder and an air gun for a variety of controlled situations, such as disturbances caused by gusts, crossflows and wake downstream oscillations. Some results were also compared to known PIV results from our lab to see how EBCs can detect a known flow structure over the sensing pillars. The results presented demonstrate the capability of the pillars and EBC to detect large flow structures. These initial results show promise for future applications of flow monitoring, such as real-time flow monitoring via a machine learning algorithm that is trained to detect certain flow patterns over the sensing pillars. This would be an intermediate step into a fully automated flow control loop.

Flow Measurements Above A DBD Plasma Actuator Array By Means Of Defocusing PTV

Lagrangian defocusing particle tracking velocimetry (DPTV) measurements are conducted in a thin, wall-parallel volume above a plasma actuator array that is applied to mimic the effect of wall oscillations by inducing alternating, wall-parallel forcing in opposite directions into the air above the actuator surface. The aim of the experiments is to capture the plasma-induced flow structures in otherwise quiescent air in order to increase the understanding of different actuation parameters. For this purpose, high-speed particle image velocimetry equipment with one camera is used in a DPTV setup, where the out-of-plane particle coordinate is obtained through the diameter of a defocused particle image. On this basis, an approach for continuous particle tracking in several consecutive frames is presented, allowing to derive three component, three dimensional velocity and acceleration data. Light reflections that occur on the adjacent actuator surface give raise to particular challenges concerning the measurement uncertainty estimation as well as the calibration procedure for the evaluation of the wall-normal coordinate of tracer particles. To overcome the latter, a calibration approach is presented for which solid particles are applied to the actuator surface and their particle image diameter is captured at different camera positions in a separate measurement. The estimation of in-plane and out-of-plane displacement measurement uncertainties is conducted following a newly-developed procedure where the deviation of particle displacements from a straight track is evaluated for measurements in quasi-quiescent air. The obtained results show the suitability of DPTV measurement technique for the practical application of the characterization of flow structures above a plasma actuator array. The measurement accuracy is found to be limited due to the available illumination, which depends on the used components. The measured flow fields together with optical and electrical measurement data allow for a further analysis of the present forcing strategy. Particularly, by recording phase-resolved, three-dimensional flow velocity and acceleration fields in the vicinity of the wall, the spatio-temporal occurrence and homogeneity of the near-wall forcing effects can be analyzed in future investigations.

Evaluating High-Frequency Fluctuations And Its Effect On Hemodynamics At Varying Heart Rates In Intracranial Aneurysms Via 4D PTV

Hemodynamics have been shown to affect the growth and rupture of intracranial aneurysms (IA). Studies have increasingly reported the presence of high frequency velocity fluctuations in aneurysms. However, it remains undetermined how transient physiological factors like heart rate and inflow blood waveform amplitude affect the hemodynamic parameters and high frequency fluctuations in and around an IA, as well as the risk of IA progression. In this work, we address this question using a volumetric particle tracking velocimetry (PTV) study where a patient-specific right middle cerebral artery (R MCA) aneurysm model was subjected to two heart rate conditions (60 bpm and 137 bpm). The input waveform amplitude for the 137 bpm case was increased by a factor of 1.6 to be physiologically consistent. Captured PTV images were processed using the Shake-the-Box (STB) workflow. Flow parameters including velocity, vorticity, and turbulent kinetic energy (TKE) were evaluated. To evaluate the presence of high frequency fluctuations, a direct numerical simulation (DNS) using the R MCA geometry and same two heart rate cases was conducted and a method for use with STB particle tracks was proposed. As compared to the 60 bpm case, the 137 bpm case had a more concentrated inflow jet that extended further into the aneurysmal sac and a stronger and larger vortical flow structure in the aneurysmal sac. The 60 bpm case maintained a region of slightly elevated TKE at the main junction point where the aneurysmal sac and all vessels merge. In the 137 bpm case, TKE in this region was dramatically higher. Using probe points, the DNS data revealed high frequency velocity fluctuations in an outflow vessel during the deceleration phase of the cycle. Notably, these fluctuations were only observed for the 137 bpm case. For both the DNS and STB data, high frequency components were more prevalent and stronger outside of the aneurysmal sac as opposed to inside the aneurysmal sac. Overall, these results suggest that changes in heart rate induce changes to the flow structure and can induce significant increases in TKE and high frequency fluctuations. Based on our findings, a higher heart rate would be expected to yield a flow field more akin to IA progression. The specific correlation between the high frequency components computed using our proposed STB-based method and the high frequency fluctuations in the flow field could not be well understood. Thus, additional analysis and development of this method is needed.

3D Lagrangian Particle Tracking In A Model Of The Human Airways

The three-dimensional flow field in the trachea of a physiologically correct model of the human respiratory tract from the nasal/oral cavities up to the 6th bronchi generation is investigated. The investigations comprise steady inflow conditions, i.e., steady inhalation and oscillatory flow combining inspiration and expiration to simulate calm breathing. A realistic breathing pattern is approximated by a sinusoidal waveform for two Womersley numbers of 3 and 4.5 and two Reynolds numbers of 400 and 1200. Due to the inherent three-dimensional and asymmetric nature of the flow field, 3D particle-tracking velocimetry (3D-PTV) measurements are performed using the Shake-The-Box (StB) algorithm to determine the influence of oral and nasal inhalation and exhalation on the flow field in the lower human airways. By using a refractive-index matched fluid consisting of water and glycerin, the complex flow structures inside the trachea are fully resolved. During oscillatory inhalation/exhalation, an integration window size of 6/40 pi was chosen as it can be considered short enough to assume a steady state. The PTV measurements confirm that the nasal and/or oral cavity must be taken into account when analyzing the flow field in the lower respiratory tract. Characteristic sets of velocity profiles in the sagittal and coronal plane of the trachea as well as contour plots evidence nearly identical flow structures for both oral and nasal inhalation, but also reveal that the presence of both cavities possesses a huge impact on the flow field compared to ideal, i.e., fully developed inflow conditions

On The Determination Of Conditional Statistics Along Gradient Trajectories At The Turbulent/Non-Turbulent Interface Of Jet Flows.

Conditional sampling and averaging are valuable techniques for discerning and quantifying notable regions within turbulent flows. These methods have been particularly prevalent in experimental and numerical investigations of turbulent shear flows. Conditional statistics effectively analyze the dynamics and characteristics of turbulent flows, offering insights into transitional behaviors and distinct features within these regimes. This approach is instrumental in studying turbulent shear flows, such as jets or mixing layers, where a well-defined zone separates the turbulent core from the non-turbulent surrounding fluid. This zone, known as the Turbulent/Non-Turbulent Interface (TNTI), plays a crucial role in various phenomenological changes, including the transfer of mass, momentum, and scalar fluxes. While statistics in jet flows are often reported along the radial direction for a given downstream position, this method can obscure the underlying physical processes. Scalar dissipation rate, transport, or turbulent kinetic energy involve velocity or concentration gradients at the flow interface. To date, the impact of sampling direction on conditional statistics within jet flows has not yet been thoroughly discussed. This study focuses on determining gradient trajectories at the interface of turbulent flows. Gradient trajectories have been used to explore structures in turbulent flows, proving effective in analyzing homogeneous shear turbulence. This study introduces a methodology for sampling conditional data by computing gradient trajectories intersecting the TNTI. The primary objective is to verify whether sampling statistics along the gradient of scalar concentration provide a more detailed understanding of the processes occurring near the TNTI. The methodology involves computing a field of gradient trajectories from concentration fields obtained through Planar Laser-Induced Fluorescence (PLIF) in a N2/N2 jet with Re = 2900. Conditional statistics at the TNTI of a jet flow along the gradient trajectory for a given downstream position are calculated and reported. The paper aims to compare conditional statistics along gradient trajectories with those obtained along radial and normal directions at the TNTI. It is expected that gradient trajectory statistics align more closely with normal direction statistics near the interface and diverge as it gets into the jet or coflow fluid.

Toward A Micrometeorological Assessment Of Agrivoltaic Farms: A Feature Tracking Velocimetry-Based Analysis

We investigate the flow dynamics around a model of an array of photovoltaic panels via laboratory experiments conducted in a water channel. The model farm consists of transparent polycarbonate panels with varying tilt angles, simulating different configurations typical of agrivoltaic systems. Velocities are measured using a feature tracking velocimetry technique. Results reveal distinct flow structures depending on the tilt angle of the panels and the distance between them, with varying numbers of recirculation regions observed. An increase in the distance between panels stretches the recirculation regions along the streamwise direction. Turbulent kinetic energy is more intense with negative tilt angles, and upstream turbulence penetrates more between panels. These findings suggest different transport and heat transfer properties, particularly relevant for tracker PV arrays that adjust tilt angles throughout the day. Understanding these dynamics is crucial for optimizing the design and operation of agrivoltaic farms to maximize energy production while minimizing impacts on agricultural productivity and the surrounding environment.

Global POD Modes From PIV Asynchronous Patches

A method is proposed to obtain full-domain spatial modes based on Proper Orthogonal Decomposition (POD) of Particle Image Velocimetry (PIV) measurements performed at different (overlapping) spatial locations. When performing robotic volumetric Particle Image Velocimetry (PIV) (Jux et al., 2018), the large-scale mean velocity fields are estimated merging several measurements performed at different (adjacent) locations covered by the robot sequence. The proposed methodology leverages the definition of POD modes as eigenvectors of the spatial correlation matrix to obtain also large-scale modes. Performing measurements over overlapping (50-75%) regions allows to approximate the correlation matrix in the two adjacent domains. When applied over a sequence of views, this method has the potential to deliver full-domain POD modes spanning the volume covered by the robot sequence, even if different regions are covered asynchronously. This methodology is particularly well-suited for applications that seek to investigate large-scale flow structures, whenever the dynamic spatial range (DSR) of the measurement system does not allow to capture the whole domain at once. The methodology is validated using a 2D experimental dataset of a turbulent boundary layer, where patches are artificially created from splitting the PIV measurements, later used as ground truth to assess the results. Furthermore, we apply the technique to a 3D robotic volumetric PIV experiment of the flow around a wall-mounted cube.

2D3C Measurement Of Velocity, Pressure And Temperature Fields In A Intake Flow Of An Air Turbine By Filtered Rayleigh Sattering (FRS) And Validation With LDV And PIV

A Filtered Rayleigh Scattering Technique is implemented in two different experimental setups and compared to the established velocity measurement techniques Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV). The Frequency Scanning Filtered Rayleigh Scattering Method employed uses an imagefiber bundle which allows for the simultaneous observation of the flow situation from six independent perspectives, utilizing only one sCMOS camera. A testrig with a nominal diameter of 80 mm was implemented by ILA R&D GmbH. Here measurements with straight pipe flow and a swirl generator were realised, as well as comparisions with LDA. A second experiment utilized Cranfields University’s Complex Intake Facility (CCITF), enabling the simulation of the flow field for an engine intake as observed behind an S-Duct diffuser. The diameter in the measuring plane was 160 mm. Measurements up to a mach number of 0.4 were performed and compared with HighSpeed Stereo-PIV (S-PIV) measurements. Good agreement was achieved in respect to both the absolute magnitude of the velocity measurements as well as to the resolution of complex flow structures. The developed FRS multi-view Setup is able to simultaneously determine the 3D velocity components, the pressure and the temperature on a measurement plane with high resolution and without seeding. After calibration the FRS system yields the pressure and temperature within 3 percent respectively 0.8 percent of the reference values. The measured velocity was within 1-2 m/s of the reference.

Towards Time-Resolved Multi-Property Measurements By Single-Frequency FRS

The filtered Rayleigh scattering technique (FRS), extended by the method of frequency scanning, has historically been limited to time-averaged multi-property flow measurements. In our recently published work, we present a concept that potentially enables the combined measurement of time-resolved pressure, temperature and three-component (3C) velocity fields. It is based on the observation of the region of interest from six perspectives and a single excitation frequency. This work summarizes and expands on a follow-up publication that experimentally verifies this concept on an aspirated circular duct flow. For this purpose, the results obtained from single-frequency data processing are compared with reference pressures, temperatures and corresponding LDA velocity measurements. Overall, a very good agreement is found for all operating points with accuracies of 3.4% in pressure, 1.3% in temperature and ±2 m/s in axial velocity. Concerning precision, a newly developed multistage evaluation procedure enables values for pressure, temperature and velocity as low as 3 hPa, 2.2 K and 1.7 m/s. In a second flow configuration, an axial swirler is introduced into the duct. The resulting secondary flow structure and deformation of the axial velocity field caused by swirler geometry and support are very well captured with the single-frequency analysis. A closing discussion on the implementation challenges of a single-frequency multi-property FRS instrument with pulsed laser radiation reveals significant obstacles to overcome. Due the considerable optimization potential identified, chances are high that true time-resolved multi-property measurements by FRS will become a reality.

2D3C Measurement Of Velocity, Pressure And Temperature Fields In A Intake Flow Of An Air Turbine By Filtered Rayleigh Sattering (FRS) And Validation With LDV And PIV

A Filtered Rayleigh Scattering Technique is implemented in two different experimental setups and compared to the established velocity measurement techniques Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV). The Frequency Scanning Filtered Rayleigh Scattering Method employed uses an imagefiber bundle which allows for the simultaneous observation of the flow situation from six independent perspectives, utilizing only one sCMOS camera. A testrig with a nominal diameter of 80 mm was implemented by ILA R&D GmbH. Here measurements with straight pipe flow and a swirl generator were realised, as well as comparisions with LDA. A second experiment utilized Cranfields University’s Complex Intake Facility (CCITF), enabling the simulation of the flow field for an engine intake as observed behind an S-Duct diffuser. The diameter in the measuring plane was 160 mm. Measurements up to a mach number of 0.4 were performed and compared with HighSpeed Stereo-PIV (S-PIV) measurements. Good agreement was achieved in respect to both the absolute magnitude of the velocity measurements as well as to the resolution of complex flow structures. The developed FRS multi-view Setup is able to simultaneously determine the 3D velocity components, the pressure and the temperature on a measurement plane with high resolution and without seeding. After calibration the FRS system yields the pressure and temperature within 3 percent respectively 0.8 percent of the reference values. The measured velocity was within 1-2 m/s of the reference.

Mode Transition Of A Flexible Film Motion In The Circular Cylinder Wake

The fluid-structure interaction between a flexible film and the wake of a circular cylinder is an abstract of a series of natural phenomena like flag flapping and fish swimming. Diverse patterns of both solid deformation and fluid dynamics arise in this coupling system as the nondimensional parameters are varied. In our recent experiments, we find that beyond the extensively studied periodic coupling process, irregular processes also emerge in this system under specific conditions. However, due to the complexity inherent in the irregular coupling processes, no in-depth investigation into this topic is available yet. In this work, we delve into the irregular aperiodic flutter of a flexible film behind a circular cylinder in the uniform flow as well as the related irregular evolution of flow structures. Two representative cases of film streamwise length (L/D) pertinent to the irregular process, specifically L/D=3 and L/D=4, have been investigated experimentally in the wind tunnel with time-resolved synchronized measurements of both film deformation and flow field. Continuous wavelet transform and virtual dye visualization are employed to extract the time-frequency characteristics of film motion and Lagrangian coherent structures in the flow fields during the film’s irregular flutter, respectively. It is determined that the irregular flutter is aperiodic in terms of the overall view, but it also encompasses transient quasi-periodic stages as its motion modes alter. Hence, we term this irregular flutter as the “hybrid flutter” state. In the quasi-periodic stages of the hybrid flutter, the large-scale vortex structures are periodically formed on the surfaces of the film and evolve in the wake as a Kármán vortex street. The periodic formation of large-scale vortices exerts periodic alternating force on the film, which, therefore, leads to the quasi-periodic flutter of the film with the second-order mode. By comparison, the aperiodic stage entails the significant extension of the separated shear layers in such a way that the whole film is enveloped by the shear layers; then, large-scale vortices are formed downstream of the film’s trailing edge. In this scenario, no periodic force is acted on the film; as a result, the film flutters aperiodically with the first-order mode.

Move With The Flow To Track The Evolving Turbulent/Non-Turbulent Interface

This study investigates the turbulent/non-turbulent interface (TNTI) in self-similar turbulent axisymmetric jet flows, focusing on a novel approach named ’move with the flow’ where the image acquisition is space-based rather than time-based. Experiments were conducted at three different Reynolds numbers (9000, 12000, and 31000), utilizing particle image velocimetry (PIV) and laser-induced fluorescence (LIF) techniques. The core of this research was developing a traverse system specifically designed to follow the evolving flow structures at the TNTI synchronously. We succeeded in tracking large-scale events in TNTI from creation to dissolution.

Multi-Scale Flow Estimation Within Stereoscopic PIV Processing Based On Deep Learning

Classical methods of particle-image velocimetry (PIV) based on state-of-the-art cross-correlation algorithms constitute a spatial filtering operation that leads to a loss of spatial resolution of the estimated velocity fields. This is associated with numerous limitations, e.g., when it comes to aligning high-fidelity numerical simulations, since the detection of small turbulent scales is often precluded. An optical flow neural network adapted for PIV processing, called RAFT-PIV, can overcome the drawback of reduced spatial resolution while maintaining the reliability and robustness of the classical approach. The present work aims to extend the application of the aforementioned deep learning based method by integrating it within a stereoscopic PIV processing workflow. Experimental data from an optically accessible single-cylinder combustion engine the flow field of which is characterized by complex, three-dimensional flow structures and a broad range turbulent scales are processed using RAFT-PIV. The 2D-3C vector fields obtained from stereoscopic reconstruction are compared against a classical cross-correlation based PIV evaluation, providing insight into instantaneous quantities and turbulent structures. A qualitative comparison of the instantaneous in-plane and out-of-plane velocity components reveals that the reconstructed flow obtained from the deep learning based estimation includes a wide range of resolved turbulent length scales. It outperforms the classical method. Furthermore, the findings of the present study demonstrate that the pixel accuracy has an effect on both physical quantities and turbulence characteristics. In particular, an increase in turbulent kinetic energy is observed across the engine cycles of the investigated engine flow states. Moreover, a higher fraction of three-dimensional isotropic turbulent structures is derived from RAFT-PIV compared to the results from the cross-correlation based technique.

Exploring Electrohydrodynamic Flows Inside Leaky Dielectric Drops: A Laser Velocimetry Approach

A novel experimental methodology based on laser velocimetry for the investigation of electrohydrodynamic (EHD) flows inside a neutrally buoyant leaky dielectric drop of 2.25 mm of radius is proposed. Utilizing fluorescent particles, 2D and 3D Lagrangian Particle Tracking (LPT) measurements of the EHD flows inside an initially spherical drop under a constant electric field are reported. Two leaky dielectric fluids, namely Silicone and Castor oils, were used as either the drop or the medium phases, depending on the case investigated, thus covering oblate and prolate drops both with small and larger deformations. 2D measurements of the trajectories and velocities of the tracer particles in a Lagrangian referential enabled a direct comparison with the leaky dielectric model (LDM), a well-stablished theory covering drops with smaller deformations, showing a good agreement with our measurements. The symmetry inside a drop with increased deformation is investigated from an analysis of the mean 3D velocity field, whose measurements converge to the two-dimensional analysis at the plane of symmetry of the drop, suggesting symmetrical flow structures inside the drop.

Experimental Study On Bubble Pairs And Induced Flow Fields Using Tomographic Particle Image Velocimetry

Gas-liquid two-phase flows, such as bubbly flows, are widely used across various engineering and military applications due to their distinctive fluid dynamics. This study utilizes advanced imaging techniques, including shadowgraphy and laser-induced fluorescence tomographic particle image velocimetry (LIF-TPIV), to provide a quantitative analysis of bubble motions and the resulting flow fields. We systematically investigate the effects of orifice distances (s) set at 10, 15, 20, and 25 mm to assess the impact of spacing on bubble behavior and interactions. Our findings indicate that at orifice distances of 20 mm or less, the proximity of bubbles facilitates interactions, promoting dynamic bubble behaviors. In contrast, at distances greater than 25 mm, interactions become markedly weaker, leading to increased separation between bubbles during their free oscillation stages. This weak interaction directly influences the characteristics of bubble-induced flow fields. As orifice spacing increases, there is a noticeable decrease in the velocity of the flow field between bubbles, particularly noticeable at spacings of 20 mm and above. At these larger spacings, a low-velocity zone develops between the bubbles at higher heights, which, according to Bernoulli's principle, corresponds to a high-pressure area. This high pressure contributes to a reduction in bubble volume with increasing orifice spacing. Furthermore, the intensity of wake vortices between two bubbles is observed to be highest at the smallest spacing of 10 mm, closely resembling the flow structure characteristic of a single rising bubble. For spacings of 15 and 20 mm, the induced flow fields of the bubbles continue to interact significantly. However, at a spacing of 25 mm, the flow fields appear to function independently, suggesting a threshold beyond which bubble interactions stop to significantly affect their surrounding fluid environments. These findings are helpful for comprehending the two-phase flow dynamics, particularly in multi-orifices bubbly flow.

An Assessment Of Event-Based Imaging Velocimetry For Dimensionality Reduction In Turbulent Flows

"This study investigates the feasibility of using neuromorphic event-based vision (EBV) camera for low-order modelling, a key enabler for real-time flow control. A synchronized experiment with simultaneous Event Based Image Velocimetry (EBIV) and Particle Image Velocimetry (PIV) is performed on a submerged water jet flow at Re = 2600. Flow statistics, spectral content, and reduced-order modeling capabilities using Proper Orthogonal Decomposition (POD) are assessed. Velocity fields are computed via standard PIV processing and compared after interpolation onto a common grid. The analysis reveals good agreement in jet flow statistics and spectra between EBIV and conventional PIV, with differences observed in the power spectral density (PSD) for high frequencies (St>1.5) due to higher noise levels in EBIV data. Nonetheless, most of the spectral content is correctly captured by the EBIV. EBIV successfully identifies dominant flow structures and spectral distribution of energy. The correct identification of temporal modes confirms EBIV's potential for flow control applications based on reduced order models. Despite minor discrepancies downstream of the jet in the Low Order Reconstruction (LOR) analysis, attributed to EBIV's challenge in accurately reconstructing broad-spectrum turbulent regions, the technology proves promising for real-time imaging-based flow control."

Analysis of the Flow Structure inside an Oscillating Sessile Drops in a Shear Flow Using Double-Helix 3D Particle Tracking Velocimetry and Adaptive Optics

The motion and particularly the detachment mechanism of sessile drops in an air-shear-flow is highly relevant for many technical applications. For instance, water drops in fuel cells can block the oxygen transport and thus lower the overall efficiency. Hence, a deeper understanding of the drop detachment mechanism could facilitate efficiency optimisations. Nonetheless, until now only simplified models exist for predicting the critical velocity that do not take into account the mutual interaction of the air and the liquid via the non-rigid interface. One reason for this is the lack of knowledge of the flow inside the oscillating drop. In this contribution, we present the first three-dimensional flow measurements in oscillating sessile drops in shear flow. Time-averaged flow measurements were conducted for different Reynolds numbers. Furthermore, the phase-averaged flow field was measured. The measurements were conducted with a novel 3D-PTV method, which offers scanless three-dimensional flow measurements with a single optical access for a measurement volume that covers about 90 % of the drop volume. Moreover, we present a novel technique for conducting flow measurements through fluctuating gas-liquid interfaces based on adaptive optics. We demonstrate that this technique enables the correction of a systematic error corresponding to the magnitude of the first drop eigenfrequency, which seems to play an important role in the detachment mechanism of drops. The results give new insights about the fluid mechanics of sessile drops and thus may help in the optimisation of drop removal in e. g. fuel cells.

Towards Time-Resolved Multi-Property Measurements By Single-Frequency FRS

The filtered Rayleigh scattering technique (FRS), extended by the method of frequency scanning, has historically been limited to time-averaged multi-property flow measurements. In our recently published work, we present a concept that potentially enables the combined measurement of time-resolved pressure, temperature and three-component (3C) velocity fields. It is based on the observation of the region of interest from six perspectives and a single excitation frequency. This work summarizes and expands on a follow-up publication that experimentally verifies this concept on an aspirated circular duct flow. For this purpose, the results obtained from single-frequency data processing are compared with reference pressures, temperatures and corresponding LDA velocity measurements. Overall, a very good agreement is found for all operating points with accuracies of 3.4% in pressure, 1.3% in temperature and ±2 m/s in axial velocity. Concerning precision, a newly developed multistage evaluation procedure enables values for pressure, temperature and velocity as low as 3 hPa, 2.2 K and 1.7 m/s. In a second flow configuration, an axial swirler is introduced into the duct. The resulting secondary flow structure and deformation of the axial velocity field caused by swirler geometry and support are very well captured with the single-frequency analysis. A closing discussion on the implementation challenges of a single-frequency multi-property FRS instrument with pulsed laser radiation reveals significant obstacles to overcome. Due the considerable optimization potential identified, chances are high that true time-resolved multi-property measurements by FRS will become a reality.

Endoscopic Stereoscopic PIV Measurements Within An Enclosed And Tight Area Of A Hydraulic Turbine

The increasingly common utilization of hydraulic turbines in off design operating conditions in recent years have affected their lifetime due to damaging fluid-structure interactions. To decrease the downtime associated to more frequent maintenance of these turbines, a thorough understanding of the damaging flow phenomena is required. This may eventually lead to the development of mitigation strategies. However, measuring velocity at the entrance of the rotor of Francis turbines is difficult because of the confined spaces and limited optical access. This paper presents a stereoscopic PIV methodology enabling measurements at the inlet of a reduced-scale Francis turbine, along with multiple novelties developed to make accurate measurements possible. It elaborates on the solutions devised to address the challenges of performing optical measurements in highly confined spaces. The measured velocity fields cover the vaneless space and a large section of the interblade channels. In the phase-averaged velocity fields, three circulation zones are detected at the speed-no load operating point, representing the combined effect of numerous vortices and flow structures observed in the instantaneous velocity fields. The flow is characterized by a significant backflow within the runner, extending to the leading edge of the runner blades and overflowing into the vaneless space.