bluuebunny/crossref_metadata_2025 huggingface dataset:
Genetically-Based Active Flow Control And PIV Measurements Of A Circular Cylinder Wake
Authors: Scala Alessandro , Paolillo Gerardo , Greco Carlo Salvatore , Astarita Tommaso , Cardone Gennaro
Published: July 2024

In the field of flow manipulation, considerable attention is devoted to controlling the wake of bluff bodies, which are characterized by the phenomenon of vortex shedding. Specifically, the control of the wake behind a single circular cylinder has garnered substantial research interest due to its engineering applications, such as reducing aerodynamic drag in the automotive and aerospace industries. Therefore, in the current work, through the application of the planar PIV technique, the fluid dynamic behavior of a circular cylinder wake controlled via synthetic jets (SJ) and Machine Learning (ML) methods is analyzed and discussed. ML algorithms are used to determine the optimal voltage signal for the loudspeaker, which effectively minimizes the aerodynamic drag of the cylinder. This approach overcomes the limitations associated with the a priori assumption of a parametric waveshape. In particular, the gradient-enriched machine learning control (gMLC) algorithm is chosen as optimization tool to search the best open-loop control policy for aerodynamic drag reduction. The cost function for this problem has been defined considering both the drag reduction contribution and the electrical power spent for the actuation. Initial optimization tests are performed by minimizing a cost function considering only the reduction in aerodynamic drag. Subsequently, a term related to the electrical power consumption for actuation is incorporated into the cost function. Once obtained the optimal control policies for these cases (hereinafter renamed CC1 and CC2, respectively), the physical mechanism responsible for reducing the aerodynamic drag is investigated more closely by analyzing the effect of the control laws on the turbulent flow statistics. Figure 1 reports a comparative assessment of the CC1 and CC2 with the baseline configuration and the configuration controlled by a sinusoidal law (CCS). The baseline configuration is characterized by an extended region of localized turbulent fluctuations in the far wake, where the time-averaged signature of von Kármán vortices is found. In contrast, all the controlled configurations show a global reduction in wake turbulence, with a peak of turbulent energy at the rear stagnation point due to the momentum injection from the SJ actuator. Notably, the CCS and CC2 configurations share similar flow field morphologies, while the CC1 configuration shows a greater reduction in turbulent fluctuations compared to the other controlled cases. This control law also achieves the largest drag reduction among all configurations studied reporting a 9.7% of drag reduction, compared to CCS and CC2, which exhibit similar reductions of 8.4% and 8.5%, respectively. Therefore, the optimization procedure has proven the capability of finding control policies able to outperform the traditional sinusoidal control widely studied in literature.
Effects Of The Nozzle Exit Section Shape On The Velocity Field And Heat Transfer Of Impinging Synthetic Jets
Authors: Longobardo Giosuè , Paolillo Gerardo , Astarita Tommaso , Cardone Gennaro , Greco Carlo Salvatore
Published: July 2024

"The versatility and effectiveness of synthetic impinging jets for cooling applications have pushed the scientific community to optimize the design of such devices investigating the effects of all the relevant flow parameters, such as the stroke length, the Reynolds number and the shape of the driving wave. The current work focuses on the influence of different nozzle exit section shapes on the flow field evolution and heat transfer behavior of synthetic jet. In work presented herein, the actuator consists of a loudspeaker oscillating inside a cavity provided with a nozzle that has a circular entry section and a variable-shape exit section with a fixed hydraulic diameter (equal to 20 mm). The exit section shapes investigated in the current work are the circular, the triangular, the square and rectangular one with aspect ratio equal to 2. Simultaneous synchronized two component particle image velocimetry and infrared thermography measurements are performed. Consistent results are found both in terms of flow fields as well as Nusselt patterns: small nozzle-to-plate distances exhibit less spreading of the jet combined with a less spreaded, although more intense, Nu pattern. Smaller nozzle-to-plate distances also reveal the shape of the nozzle in the thermal pattern: this behavior can be seen for every nozzle, and it is associated with the presence of a Nu peak in correspondence of every corner of the shape, where the three-lobe footprint of the triangular nozzle exit section shape is clearly visible. Larger nozzle-to-plate distances are associated with a higher spreading in the jet and a lower vertical velocity in the vicinity of the impinged plate.
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Move With The Flow To Track The Evolving Turbulent/Non-Turbulent Interface
Authors: Khojasteh Ali Rahimi , Been Coen , Dalen Lyke Van , Water Willem Van De , Westerweel Jerry
Published: July 2024

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.
Novel plenoptic Imager for 3D Particle Tracking Velocimetry for Fluid Structure Interactions
Authors: Yamakaitis Mark , Rezaiguia Anis , Capanna Roberto , Bardet Philippe
Published: July 2024

We show a novel plenoptic camera architecture and demonstrate its ability to perform three-dimensional three-component velocimetry using standard multi camera processing techniques. The novel architecture needs only a custom lens assembly with no modification to the camera body. This allows the use of any camera with an appropriate sensor size. The architecture directly creates multiple views of a scene side by side on the camera sensor which can be separated and treated as if they originated from independent cameras. Standard calibration techniques for creating 3D to 2D correspondence on the images can then be implemented to reconstruct 3D scene information. For this case we focus on performing 3D PTV using the commercial software DaVis and the "shake-the-box" algorithm. We look at the flow induced by an oscillating rod and compare the results with the analytical solution to validate the results.
Planar Concentration Field Measurement In A Transient Jet Using Mie Scattering
Authors: Hartz Benjamin A. K. , Meyer Knud Erik
Published: July 2024

An experimental method for time resolved concentration measurements in the near field of a transient starting turbulent air jet is presented. Planar Laser Mie Scattering (PLMS) using DEHS droplets produced by a Laskin nozzle as tracers of concentration is applied on an air jet with Reynolds number of 62.000 and is recorded with 10.000 frames per second. A jet penetration of about 20 nozzle diameters into the ambient environment is resolved with more than 100 frames. A linear response between grey scale values and concentration is found for mixtures up to 13% of the air-particle flow from the Laskin nozzle mixed with clean air. The calibration is based on images with no particles and images with a homogeneous particle concentration. A correction for light scattered by particles outside the laser sheet is proposed. The experimental results agree well with mean concentration measured with other techniques.
Instantaneous Pressure Reconstruction From PIV In Compressible Launcher Environments
Authors: D'Aguanno Alessandro , Schrijer Ferry , Oudheusden Bas Van
Published: July 2024

Modern launcher configurations, often characterized by a larger diameter in the payload fairing than the rest of the launch vehicle (hammerhead configuration), face significant challenges during transonic operations, given their susceptibility to flow separation and intense pressure fluctuations. This experimental study aims to reconstruct pressure from PIV under transonic conditions using Taylor's hypothesis. The focus is on the ESA VEGA-E hammerhead launcher model, investigated in the transonic regime at Ma=0.8 and at α=0°. Initially, the methodology of the pressure reconstruction algorithm is described and the validity of Taylor's hypothesis is established through validation with numerical data. Subsequently, the experimental data are first used to provide a general characterization of the flow phenomenon, highlighting key features such as an oscillating shockwave, separated and reattached flows. Later, the reconstruction of pressure from velocity data is compared to unsteady pressure transducer data, for instantaneous, average and standard deviation values, obtaining a very good agreement (ΔCpAvg~0.01-0.02 and ΔCpStd~0.02). The experimental set-up is also designed to showcase the impact of neglecting the out-of-plane velocity component on pressure reconstruction, showing comparable data for CpAvg, while larger discrepancies in terms of CpStd in particular in the separated area region.
Plenoptic Molecular Tagging Velocimetry For In-Situ Deployment In Water
Authors: Williams Steven W. , Fort Charles , Sawalhi Yazan W. , Capanna Roberto , Bardet Philippe Matthieu
Published: July 2024

We introduce a compact plenoptic imager to study wall-bounded turbulence along the surface of a 6:1 prolate spheroid, at a Reynolds number of 12 × 10 6 with molecular tagging velocimetry (MTV). This paper discusses the design of the imager and the technical solutions necessary to deploy the system in a large specimen. This includes using optical fiber laser delivery for UV and visible lasers. The end goal is the measurement of three-dimensional velocity fields near the wall, which is imperative for detailed turbulence analysis. We also stress the importance of understanding wall shear stress, a key factor in studying boundary layer dynamics in high-speed flows. Overcoming these challenges and refining our methods will result in an improved understanding of flow dynamics in high-turbulence regimes.
An Experimental Investigation Of Local Thermo-Fluidic Heat Transfer In A U-Shaped Mini-Channel Sink
Authors: Moscato Giorgio , Romano Giovanni Paolo
Published: July 2024

This study investigates experimentally a heated U-shaped mini-channel heat sink using Infrared Thermography and Particle Image Velocimetry for a water coolant flow of Reynolds numbers of 280, 650, and 1350 (based on the hydraulic channel diameter). The choice of this cell geometry is based on its role as a simplified unit of a serpentine heat exchanger, which is proved to be one of the most promising for cooling processes. The use of the infrared camera allows the detection of temperature fields on top of the external surface of the cell. Therefore, aiming to derive the temperature distribution on the channel roof, a dedicated transfer function is implemented. Moreover, we employed the lumped capacitance model for thermal analysis on both infrared measurements and thermocouple data. The latter are recorded to capture the cooling process of the aluminium base and water temperature at the end of the outlet tube. As a result, thermal transient rate, cooling magnitude and equilibrium temperature are obtained. These parameters indicate that higher Reynolds numbers correspond to increased thermal transient rates, enhanced cooling effects, and lower equilibrium temperatures. A non-uniform distribution of heat transfer along the channel is reported, with the most efficient cooling area localized close to the first 90-degree corner. These findings are consistent with numerical simulations and previous experimental observations. PIV results reveal the presence of two fluid acceleration zones following both 90-degree corners, which contribute to improve the water cooling ability in their respective regions. Additionally, the formation of two recirculating bubbles is reported at the inner wall from corners vertices, whose intensity is dependent on Reynolds number, pushing the main flow towards the outer channel wall and reducing the local heat transfer. Turbulent kinetic energy distributions are also investigated, pointing out the presence of intense areas that better match with regions of minimum equilibrium temperature than maximum velocity zones. This suggests the presence of local turbulent unsteadiness and three-dimensional phenomena, which contributes significantly to cooling enhancement.
Assessing The Potential Of PIV Data To Resolve Hidden Frequency Scales
Authors: Vocke Maegan , Kapulla Ralf , Morton Chris
Published: July 2024

The present work investigates the performance of an advection-based flow reconstruction model to increase the temporal resolution of PIV measurements recorded in a turbulent, planar jet. Using a semi-Lagrangian technique in combination with Rapid Distortion Theory, a modified trajectory tracking procedure is implemented. The method introduces a specification for a two-dimensional convective velocity based on the least squares minimization of the linearized advection equation, in contrast to the previously introduced one-dimensional mean velocity profile outlined in Vocke et al. (2023). The new implementation is based on local flow measurements, making it well-suited towards streamwise heterogeneity and spatially developing flows. With this, the flow at some unknown time can be estimated from the know flow measurements at the forward and backward time. Spectral analysis illustrates the model's proficiency in recovering spatiotemporal information far exceeding the Nyquist frequency, with spectral reconstruction errors of less than 5% for the most extreme case. It is demonstrated that the spectral content can be estimated at least two orders of magnitude beyond the sampling frequency of the original recording. Improved performance compared to alternative methods is demonstrated with only minor impact on computational time. This indicates the approach may be used as a tool for experimental researchers a) having no access to a high-speed PIV or b) with high-speed PIV to increase the spectral resolution even further.
Study Of Mixing Enhancement Of The 3-D Acoustofluidic Mixer Using Digital In-Line Holographic Micro Particle Tracking Velocimetry
Authors: Tarigan Olivia Christy , Li Chen-Chia , Chang Chia-Kai , Tien Wei-Hsin
Published: July 2024

In this study, the 3-D flow patterns and mixing performances are investigated for an acoustofluidic Y-junction micromixer using Digitial In-line Holographic Micro-Particle Tracking Velocimetry (DIHμPTV) and epi-fluorescence flow visualization. Two parallel longitudinal spines were placed in the channel to induce sustaining solenoidal 3-D acoustic streaming vortices, which work as the secondary flow in the mixing channel to reduce the diffusion mixing length and in turn enhance the mixing performances. The two spines have a triangular cross-section, with a base width of 0.2mm and a height of 0.3mm, placed 1.2mm apart from each other in a 2.8mm width and 1mm height channel. The Y-junction has two configurations, one has the spines directly connected to the side wall at the Y-junction, and the other microchannel has a gap between the two spines and a flat junction shape. The driving mechanism is provided by two piezoelectric disks, operating at 12 kHz oscillation frequency and 20V peak-to-peak amplitude. The 3-D flow patterns were recorded by the DIHμPTV in two measurement volumes to fully cover the test section’s height. The holographic reconstruction scheme is based on the 1st Rayleigh-Sommerfeld solution with an extra deconvolution step to improve the accuracy of the reconstruction of the depth position. The deconvolution step utilizes a simulated hologram of the Point-Spread Function (PSF) and reduces the depth uncertainty during the deconvolution of the raw data. The mixing performances of the acoustofluidic Y-junction micro-mixers were evaluated by the epi-fluorescence experiments, and the mixing indices were calculated and compared to verify the improvements compared to the Yjunction micromixers without the acoustic streaming vortices. The results confirmed the improvement of the mixing and the sustaining 3-D flow patterns even with a high flow rate, which could not be achieved with 2-D sharp-edge obstructions tested previously. The novel configuration may have the potential to inspire designs of high-throughput and low-pressure loss active micro-mixers.
Simultaneous PIV/LIF Measurements Of Velocity And Oxygen In An Advecting Air-Water Channel Flow
Authors: Shankaran Adharsh , Hearst R Jason
Published: July 2024

We investigate oxygen transfer across an air-water interface using an integrated Particle Image Velocimetry (PIV) and Laser-Induced Fluorescence (LIF) system. This setup allows for the simultaneous measurement of velocity and dissolved oxygen (DO) concentration fields, alongside the visualization of the air-water interface. The imaging process begins with zero DO concentration and continues until oxygen saturation in the water is achieved. Calibration of the oxygen LIF intensity field is performed using these images, benchmarked against measurements from an optical oxygen point probe, to ensure accurate conversion to a physically relevant unit. The air channel generates an air flow with a center-line mean velocity of 8.84 m/s, corresponding to a Taylor Reynolds number of 100. This high-velocity air flow results in the formation of wavy structures on the water interface creating situations similar to those found in nature (such as rivers and oceans). The wavy structures create reflections from the air-water interface; these reflections are mitigated during post-processing using a Wavelet-Fast Fourier Transform (W-FFT) method to remove high intensity noise. Additionally, the wind shear generated by the air flow induces high turbulence intensity and vortices near the air-water interface, significantly enhancing the dissolution of oxygen into the water. This turbulence and the associated vortical structures play a crucial role in the mixing process and the overall oxygen transfer efficiency. The study presents instantaneous dissolved oxygen structures overlapped with velocity structures in a flow with deformable interface, providing some insights into the oxygen transfer and mixing processes.
Neural Networks For Particle Diffusometry In The Presence Of Flow And With Defocused Particles
Authors: Sardana Pranshul , Wereley Steve
Published: July 2024

Diffusion is a natural phenomenon in fluids. Its measurement can be done optically by seeding an otherwise featureless fluid with tracer particles and observing their motion. However, existing algorithms for particle-based diffusion coefficient measurement have multiple failure modes especially when the fluid has a flow or the particles are defocused. We present a method based on Convolutional Neural Networks (CNNs) for predicting diffusion coefficients in the presence of these real-world effects. The CNNs were trained, validated, and tested on crops from four-frame temporally averaged images. The study includes three simulated datasets: Gaussian-shaped particles under no fluid flow condition, Gaussian-shaped particles under an arbitrary flow condition, and defocused particles under no fluid flow condition. The results show that the CNNs have a low Mean Absolute Error (MAE) of 0.12 µm 2 /s between the true and predicted diffusion coefficient values for the dataset with Gaussian-shaped particles under no fluid flow condition. The CNNs have a slightly higher MAE of 0.19 µm^2 /s for Gaussian-shaped particles under arbitrary flow and defocused particles under no fluid flow conditions. The performance of the CNNs was also benchmarked against four conventional algorithms on these simulated datasets. The results show that the conventional algorithms perform better than the CNNs when the particles are assumed to be Gaussian and the fluid has no flow. However, the CNNs outperform conventional methods in the presence of flow or if the particles are defocused. Finally, the outputs of CNNs were compared against the outputs of conventional algorithms on experimental datasets. This provides uncertainty in the range of 0.19 µm^2 /s - 0.47 µm^2 /s, which is slightly larger than the errors obtained from simulated datasets. Hence, the study shows that CNNs can be used to reliably predict diffusion coefficients from complex particle datasets with as little as four frames.
On The Preparation Of Training Data For 3D Tracking Of Position And Orientation Of Microswimmers Using Defocusing And Deep Learning
Authors: Youshanlouei Mohammad Mehdizadeh , Miano Federica , Rossi Massimiliano
Published: July 2024

The use of image analysis algorithms based on deep convolutional neural networks (CNNs) is opening new possibilities in experimental techniques for fluid dynamics. In the field of microscopic 3D particle tracking based on defocusing, it has been shown on synthetic images that CNNs can be used to determine not only the depth position but also the orientation of non-spherical microscopic particles or specimens. However, the translation of such approaches to experimental setups is challenging, especially due to the difficulty of obtaining reliable labeled images to train the neural networks. In this work, we propose a method to obtain labeled images of microswimmers from conventional microscopy images, exploiting the properties of the swimming motion at low Reynolds numbers, where inertial effects are negligible. This method allows us to obtain ground truth values of orientation and depth position of a microorganism swimming at a regular pace that can be used as training data for CNNs. The proposed methodology was used to obtain labeled images from experimental videos of the micro-organism Euplotes Vannus. The labeled images were used to train a VGG16 network, providing an estimated uncertainty in the determination of the orientation angles psi, theta, phi, and the depth position Z of 2.4%, 1.4%, 2.3%, and 3.5%, respectively.
Volumetric PTV For Fermenting Bread Dough Visualized By Micro X-Ray Computed Tomography
Authors: Saito Mana , Kikuchi Kenji , Numayama-Tsuruta Keiko , Ishikawa Takuji
Published: July 2024

To elucidate the relationship between the formation of internal structures and changes in viscoelastic properties for improving the quality stability of porous and heterogeneous materials, we focused on bread dough that forms an internal bubble structure through yeast metabolism. Specifically, we conducted a time-series measurement of the internal structure and viscoelastic properties during fermentation, particularly in the heterogeneous condition when additives are mixed into the homogeneous dough. We performed a time-lapse imaging using a digital camera to investigate the inflation volume and rate of fermenting bread dough. We used X-ray CT to visualize the internal bubble structure during the fermenting process and evaluated the changes in viscoelastic properties using a dynamic mechanical analysis. The results indicated that the addition of raisins to the bread dough negatively affected its inflation, made the bubble structure of the dough near the raisins larger and coarsened, and altered the viscoelastic properties.
An Assessment Of Event-Based Imaging Velocimetry For Dimensionality Reduction In Turbulent Flows
Authors: Franceschelli Luca , Raiola Marco , Willert Christian , Discetti Stefano
Published: July 2024

"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."
Bubble Induced Mixing In A Bubble Column With Counter-Current Liquid Flow
Authors: Kováts Péter , Zähringer Katharina
Published: July 2024

Bubble columns have been widely investigated over the last few decades, concentrating on the bubble parameters, and gas/liquid motion. However, mixing and often the resulting interaction between chemical reaction and hydrodynamics in the column are rarely analysed. For this reason, experiments with Laser Induced Fluorescence (LIF) applying Sulforhodamine G as fluorescent dye, and shadow imaging were performed in a square laboratory-scale counter-current flow bubble column at 21 different flow conditions and three different dye inlet positions. In these experiments the mixing efficiency was investigated by varying the bubble size, the gas and the liquid flow rates, to obtain the necessary data for a further understanding of mixing processes in bubbly flows. The strong influence of the bubble presence, their size and the counter-current liquid flow rate on mixing in the column is obvious from these results. Bubble induced mixing leads to a good homogenisation inside the column, compared to a flow without bubbles. Also, the larger the bubbles the higher the bubble induced upward velocity, which leads to a better mixing. The highest counter-current liquid flow rate led to a more concentrated dye jet, which was less dispersed than at lower liquid flow rates, while the moderate counter-current liquid flow rate (11.1 l· min-1) has been found optimal for mixing in the investigated cases. The combination of large bubbles generated with the 3.6 mm capillaries, and a moderate counter-current liquid flow rate led to the best mixing performance in the bubble column.
Tomographic Reconstruction Of Combustion Flow Fields From Density Measurements Based On Physics-Informed Neural Networks
Authors: Gürtler Johannes , Tasmany Sami , Woisetschläger Jakob , Kuschmierz Robert , Czarske Jürgen
Published: July 2024

Achieving stable, low-emission combustion with green hydrogen is crucial for climate-neutral ground-based power generation in turbomachinery. Lean combustion modes with green hydrogen reduce fuel consumption but increase unsteadiness. Thus, multimodal detection techniques for parameters like density and flow velocity are essential to understand the interconnected behavior of combustion, advection velocity and noise production. We previously introduced a high-speed camera-based laser interferometric vibrometer system to detect thermoacoustic oscillations and record advection velocities, using interferometric detection of density fluctuations and correlation-based velocity estimation. However, this method only estimates integral velocity fields, necessitating the solution of the inverse problem. This is relying on iterative optimization, which is computationally expensive and struggles with high dimensional, noisy or limited data. Physics-Informed Neural Networks offer an innovative and efficient approach to these problems, combining neural network flexibility with physical law constraints to unravel intricate cause-effect relationships. Here an approach for reconstructing local velocity fields from a single projection velocity calculated from integral density data is presented. Using a U-net and model assumptions for coupling local and integral velocity fields, the training minimizes errors between measured integral input fields and the predicted local field integrals. The comparison of measured local velocity and the network prediction achieved a relative mean squared error of 3 %.
Development Of An Endoscopic, Absorption Corrected And Calibrated OH Laser Induced Fluorescence Probe System for High Pressure Combustion Diagnostics
Authors: Stockhausen Guido , Beversdorff Manfred , Burow Eike , Heinze Johannes , Klinner Joachim , Weiss Armin , Willert Chris
Published: July 2024

We present the design and preliminary results of an endoscopic OH laser-induced fluorescence (LIF) system that is capable of acquiring 1D-line measurements in high pressure combustion processes within large-scale test facilities. The system can be exploited to acquire line-of-sight OH-chemiluminescence images and absorption corrected absolute OH-species concentration from which additionally temperature distribution can be extracted in case of lean mixtures in chemical equilibrium. Different critical components, such as light guides for high power UV laser pulses and flexible UV image guides, were first tested and characterized in a laboratory setup. Optimization regarding pulse energy and beam quality led to the selection of an 1m long hollow core fiber for laser light delivery as a central element for the experimental setup despite high losses caused by bending the fiber. The image transfer is accomplished by a 7m long flexible UV image bundle, which is preferred to the use of a rigid borescope in harsh environments. We designed the endoscopic UV optics using readily available parts and achieved an approximated image resolution of 21.6 line pairs/mm. Additionally, post-processing methods, such as flat-field correction, can improve the image quality substantially and remove the pixilation effect of the fiber bundle structure nearly completely. To validate the complete system, probe-based qualitative OH-LIF measurements were performed in a well-known laminar flame of a matrix burner, which serves as a mock-up for anticipated applications in an industrial test environment with a technology readiness level (TRL) greater than 4. The final step consists of the integration of the miniaturized optics into a cooled probe system which protects the sensitive optics against the harsh environment of an aero-engine sector combustor. For this purpose, two probe systems were designed to include, on the one hand, the endoscope optics together with the fiber image bundle end tip and, on the other hand, the laser emitting and receiving system for performing an absorption measurement needed to quantify the recorded LIF signal.
Non-Intrusive Frequency Measurements With Laser Shadowgraphy And Event-Based Schlieren Imaging Of Bluff Body Wakes In Compressible Flow
Authors: Nüßle Max , Ebertz Felix Luc , Essig Nicklas , Schulz Christof , Weigand Bernhard , Steinhausen Christoph
Published: July 2024

Event-based imaging is an emerging technology in thermodynamics and fluid dynamics. In this work, we are leveraging an event-based camera with a conventional schlieren set-up to investigate the vortex shedding dynamics behind a blunt central injector in a transonic nozzle flow. We offer different methods to evaluate the data stream of the event-based camera. Standard Fourier methods are not directly applicable to this data due to a non-constant time step. Therefore, the data needs to be treated with special care. Clusters of on- and off-events alternate and show the periodic nature of the wake. Using a clustering algorithm on the event groups also provides an estimation of vortex shedding frequency. These results are compared with laser shadowgraphy, a second novel method to measure the vortex shedding frequency in the wake. Both approaches to measuring the vortex shedding frequency are agreeing with each other within 1%. They each offer a simple set-up with comparatively low cost for their high temporal resolution.
Determination Of Two-Phase Flow Regimes Of A Condensing Water-Air Mixture In An Array Of Micro Channels
Authors: Blessing Sebastian , Lehle Moritz , Stroh Alexander , Kriegseis Jochen
Published: July 2024

Proton exchange membrane (PEM) full cells (FCs) are a promising alternative to combustion engines for mobile applications. Their signature low operating temperature leads to the possible existence of product water in its liquid form. Liquid water formation and transport are important phenomena to understand and model in order to facilitate the development of new, more efficient PEMFCs. This contribution investigates the two-phase flow phenomena separately by introducing a gaseous air-vapor mixture into an optically accessible fuel cell flow plate with 14 parallel micro channels (confinement number Co = 7.3) and a carbon paper inlay representing the FC's gas diffusion layer and bringing it to the point of onset of condensation. Special care is taken to provide a controlled and reproducible air-vapor mixture with low pressure pulsation. One key feature of the experiment is that the flow plate used is designed to be as similar to a regular research fuel cell flow plate as possible, retaining its channel surface properties and electrical parameters. It can also be heated to a given temperature to mimic the fuel cell operating conditions. Therefore, the same plate can also be used in in-situ experiments in an operated fuel cell. An air mass flow rate of 500 - 1000 mLn / min with relative humidities of 75% - 95% at 65°C and 75°C was investigated. The emerging flow patterns were captured using a high-speed camera at 250 Hz and categorized using canonical two-phase flow regime nomenclature. It is shown that the present setup can produce the relevant two-phase flow patterns that can be found in operated fuel cells as reported by previous studies and is furthermore equipped to investigate flow states not easily reached with an operated fuel cell (e.g. flooded channels).
Dynamic Wall Shear Stress Measurement Using Event-Based 3D Particle Tracking
Authors: Willert Christian , Klinner Joachim
Published: July 2024

We describe the implementation of a 3d Lagrangian particle tracking (LPT) system based on event-based vision (EBV) sensors and demonstrate its application for the near-wall characterization of a turbulent boundary layer (TBL) in air. The viscous sublayer of the TBL is illuminated by a thin light sheet that grazes the surface of a glass window inserted into the flat wall of the wind tunnel wall. The data simultaneously captured by three synchronized EBV-cameras is used to reconstruct the 3d particle tracks within 0.4 mm of the wall on a field of view of 12.0 mm by 7.5 mm. The velocity and position of particles within the viscous sub-layer permit the estimation of the local, unsteady wall shear stress vector under the assumption of linearity between particle velocity and wall shear stress. Thereby, 2d distributions and higher order statistics of the unsteady wall shear stress are obtained. The systematic underestimation of the spanwise shear stress fluctuation can be explained on the basis of DNS data; a methodology for its mitigation is proposed. The employed EBV hardware coupled with suited LPT algorithms provide data quality on par with currently used, considerably more expensive, high-speed framing cameras.
Integrated Single Camera µPTV And Florescence Imaging For Cell Tracking And Flow Investigation In Centrifugal Microfluidic Devices
Authors: Saffar Yeganeh , Nobes David S. , Sabbagh Reza
Published: July 2024

A micro-hydrocyclone is investigated as a high throughput particle/cell sorting microfluidic device. A complex flow structure has been reported by limited numerical works that increases the importance of undertaking a comprehensive experimental investigation. In addition to the flow, utilizing damageable bio cells in a micro-hydrocyclone, requires a deep understanding of the interaction between the cells entering the device, the flow structure and its instabilities forming in different operational phases. In such cases the implementation of multiple measurement techniques can be challenging due to the small scale of devices, here < 5 mm diameter. Therefore, for this complex flow an experimental approach is introduced to capture the flow structure and the motion of cells. In this method, tracer particles and fluorescent stained cells are captured simultaneous by a single camera in the same flow field. A custom image processing scheme is used for partitioning cells and particles from the raw data. To determine velocity vectors, µPTV is employed on the segmented data sets to study the effect of flow on trajectory and velocity of each individual cell in the system. By a combined investigation in a vast range of Reynolds number (50<Re<800) the dynamics of cells are investigated in response to different flow structures and flow regimes.
LED-based PIV Of A Particle-Laden Turbulent Free-Surface
Authors: Anand Abhirath , Poelma Christian , Laskari Angeliki
Published: July 2024

An experimental method is proposed to study dispersed two-phase ﬂows at an airwater interface, a family of ﬂows of practical signiﬁcance in environmental and industrial settings. The applicability of this technique is demonstrated through the study of a lightly-deformed turbulent free-surface laden with ﬂoating particles (`ﬂoaters'). A low-mean turbulent ﬂow is generated in a turbulence box actuated by a 10×10 synthetic jet array. Using LEDs and a single camera, free-surface ﬂow measurements are carried out by Particle Image Velocimetry (PIV) simultaneously with Lagrangian tracking of the ﬂoaters, allowing the potential to characterise the coupling between the ﬂoater dynamics and the (sub)surface ﬂow. Discrimination of the dispersed and continuous phases is carried out based on size. Individual ﬂoaters and clusters of ﬂoaters are successfully tracked throughout the ﬁeld of view while they navigate through elongated and circular regions of high and low vorticity, characteristic features typically observed when a subsurface turbulent ﬂow interacts with a free surface. Preliminary results of the ﬂoater-ﬂuid interactions are presented to highlight the potential of this technique to better our understanding of ﬂoaterladen turbulent free surfaces.
Unsteady Standoff Shock Behaviour Associated With Supersonic Impinging Jets
Authors: New T. H. , Lim H. D. , Mariani R. , You R. G. Y. , Chan W. L.
Published: July 2024

It was observed in an earlier study [Lim, H. D., New, T. H., Mariani, R., & Cui, Y. D. (2019). Effects of bevelled nozzles on standoff shocks in supersonic impinging jets. Aerospace Science and Technology, 94, 105371.] that standoff shocks in M=1.45 supersonic impinging jets produced by non-bevelled and bevelled nozzles under small separation distances may undergo large and abrupt fluctuations in their positions above the impingement points. This occurs when the separation distance is h/d=1.5 and for nozzle-pressure-ratio (NPR) between NPR=4 and 5, where h is the distance between the nozzle exit and flat-wall and d is the jet diameter. It has to be highlighted that the observed behaviour is unlike the more minor positional fluctuations typically associated with supersonic impinging jets. To reveal further details on the unsteadiness of the standoff shock, color schlieren results will be presented here to shed more light on the flow phenomenon, while initial modelling efforts to reproduce these standoff shock behaviour will be elaborated. Present results reinforce the notion that the large standoff shock fluctuations are due to the standoff shock trying to achieve pressure equilibrium through abrupt displacements when variation in the NPR changes the location of the shock reflection point. Considering the small separation distance used, small NPR changes are able to trigger abrupt standoff shock unsteadiness.
A Full Meshless Algorithm For Super-Resolution In Image Velocimetry Based On KNN-PTV And Constrained RBFs
Authors: Tirelli Iacopo , Mendez Miguel Alfonso , Ianiro Andrea , Discetti Stefano
Published: July 2024

"A novel full meshless super-resolution method for image velocimetry is proposed. This method builds upon the combination, presented in Tirelli et al. (2023a), of K-nearest neighbour Particle Tracking Velocimetry (Tirelli et al., 2023b, KNN-PTV) and constrained Radial Basis Functions regression (Sperotto et al., 2022, c-RBFs). The main novelty is that the algorithm is implemented here in a fully meshless version, i.e. it does not require the definition of a common Eulerian grid at any step of the process. KNN-PTV enhances the spatial resolution of vector fields by exploiting data
coherence in space and time, even for non-time-resolved measurements. On the other and, c-RBFs provide an analytical representation of the field from scattered data while allowing the introduction of physical constraints. In this new version, the dictionary to blend data from different snapshots is computed with a meshless POD approach developed by the authors and presented in another contribution to this conference. This approach removes the last constraint of Eulerian grids and paves the way for a fully meshless algorithm. The algorithm is validated on a challenging 2D
synthetic case, such as turbulent channel flow, and an experimental case involving the separation bubble around the frontal portion of a Ground Transportation System (GTS)."
Application Of Onboard PIV To Front Wheelhouse Wake Behavior Under Stationary Crosswind Conditions
Authors: Kuratani Naoshi
Published: July 2024

When driving a car, the driver and passengers may feel larger drag from a headwind, the steering wheel would be taken away by a crosswind or pushed by a tailwind. Anytime, anywhere, we will encounter various scenes with natural wind under real world conditions such as urban, suburb and highway. We will develop the appropriate vehicle by simulating these conditions to satisfy with various customer requirements. For vehicle aerodynamic research and development, we are simply running simulations in physical- and cyber-space, that is, wind tunnel facility and computational fluid dynamics to reproduce these real worlds. However, it is not easy to simulate these wind inlet conditions, such as the temporal change in crosswind, especially, in wind tunnel facility. The stationary yaw angle by rotating turntable would reproduce a crosswind, if there is no turbulent generator system like in Pininfarina and Swing in FKFS. Therefore, we focus on the unsteady and asymmetrical flow from the front wheelhouse of the vehicle with tire rotation. There is three-dimensional complicated flow that are interacted by the flow from various directions, such as the flow along the front bumper, the flow along the rotating tire and wheel, the blowing flow from the rotating wheel opening area and the flow from the tire house. These complicated flow produces the front wheelhouse wake along the body side that affects the vehicle aerodynamic performance. Furthermore, there are asymmetrical flows around the vehicle by different front wheelhouse wake in windward and leeward sides. The larger side wake increases the aerodynamic drag, and the fluctuating side wake may induce the instability of the vehicle handling stability. Here, the proposed onboard 2D-3C PIV measurement system in the previous study enables us to measure same measurement area along the vehicle body side without change in optical configuration effectively, that indicates to reduce the time by resetting the optical configuration. We will investigate what kind of flow field should be focused on and controlled to prevent the vehicle instability by investigating these asymmetrical flow field around the vehicle under crosswinds.
Multi-Scale Flow Estimation Within Stereoscopic PIV Processing Based On Deep Learning
Authors: Hristozova Lilia , Klaas Michael , Schröder Wolfgang
Published: July 2024

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.
Using Ultrasound Imaging To Identify Flow Reattachment In Shear-Thinning Suspensions
Authors: Barnes Moira , Rosi Giuseppe , Kaiser Frieder , Rival David
Published: July 2024

The current study explores ultrasound imaging velocimetry (UIV) as an alternative method for capturing flow fields in shear-thinning suspensions. In particular, UIV is used here together with pressure measurements to identify the reattachment point and to correlate the point's location with the pressure recovery downstream of an axisymmetric expansion. Four fluids are investigated: pure water, as well as three 1750ppm xanthum-gum-in-water solutions mixed with non-reactive mineral micro-particles at 0%, 15% and 30% volume fractions. Wall-pressure measurements were collected through taps located at 0 to 13 throat diameters downstream of the expansion with subsequent UIV measurements collected from 0.5 to 5 throat diameters downstream of the expansion. Pressure-tap measurements for the tested suspensions demonstrate 80% to 100% total pressure recovery achieved at approximately 4 to 5 throat diameters downstream from the throat. Furthermore, all four fluids exhibit total pressure recoveries within 10% of Borda-Carnot prediction. Despite similar pressure recoveries, UIV revealed different reattachment lengths for the tested fluids. Water exhibited a reattachment length of approximately four throat diameters, which is expected within the tested turbulent regime, while the xanthum-gum solution exhibited a delayed reattachment beyond the field-of-view when the effective Reynolds number is O(100). Pipe-wall velocity measurements suggest that the dense suspensions exhibit similar reattachment lengths to that of water despite expectations that the dense suspensions would behave similar to the pure xanthum-gum solution given: the similar Reynolds number between the pure xanthum-gum solution and the tested dense suspensions; and previous work done by Barnes et al. (2024) and others revealing shear-layer stabilization within dense suspensions. It is suspected that increased diffusion within the suspension mitigates the shear layer's ability to convect downstream, but improved measurements are required to state this conclusively.