Flow Visualization Research Articles

Two-Field Excitation for Contactless Inductive Flow Tomography

Contactless inductive flow tomography (CIFT) is a flow measurement technique allowing for visualization of the global flow in electrically conducting fluids. The method is based on the principle of induction by motion: very weak induced magnetic fields arise from the fluid motion under the influence of a primary excitation magnetic field and can be measured precisely outside of the fluid volume. The structure of the causative flow field can be reconstructed from the induced magnetic field values by solving the according linear inverse problem using appropriate regularization methods. The concurrent use of more than one excitation magnetic field is necessary to fully reconstruct three-dimensional liquid metal flows. In our laboratory demonstrator experiment, we impose two excitation magnetic fields perpendicular to each other to a mechanically driven flow of the liquid metal alloy GaInSn. In the first approach, the excitation fields are multiplexed. Here, the temporal resolution of the measurement needs to be kept as high as possible. Consecutive application by multiplexing enables determining the flow structure in the liquid with a temporal resolution down to 3 s with the existing equipment. In another approach, we concurrently apply two sinusoidal excitation fields with different frequencies. The signals are disentangled on the basis of the lock-in principle, enabling a successful reconstruction of the liquid metal flow.

Boosting Cardiac Color Doppler Frame Rates with Deep Learning.

Color Doppler echocardiography enables visualization of blood flow within the heart. However, the limited frame rate impedes the quantitative assessment of blood velocity throughout the cardiac cycle, thereby compromising a comprehensive analysis of ventricular filling. Concurrently, deep learning is demonstrating promising outcomes in post-processing of echocardiographic data for various applications. This work explores the use of deep learning models for intracardiac Doppler velocity estimation from a reduced number of filtered I/Q signals. We used a supervised learning approach by simulating patient-based cardiac color Doppler acquisitions and proposed data augmentation strategies to enlarge the training dataset. We implemented architectures based on convolutional neural networks. In particular, we focused on comparing the U-Net model and the recent ConvNeXt models, alongside assessing real-valued versus complex-valued representations. We found that both models outperformed the state-of-the-art autocorrelator method, effectively mitigating aliasing and noise. We did not observe significant differences between the use of real and complex data. Finally, we validated the models on in vitro and in vivo experiments. All models produced quantitatively comparable results to the baseline and were more robust to noise. ConvNeXt emerged as the sole model to achieve high-quality results on in vivo aliased samples. These results demonstrate the interest of supervised deep learning methods for Doppler velocity estimation from a reduced number of acquisitions.

Implementation Of Optical Diagnostics For Study Of A Non-Canonical Hypersonic Geometry

The study of shock/boundary-layer interactions on a non-canonical hypersonic geometry involves physical complexities not found on the canonical investigations that dominate the literature. The present case examines a cone-slice-ramp, which combines an upstream asymmetric flow expansion with a downstream three-dimensional compression ramp. Optical diagnostics of the off-body flowfield are a necessary complement to surface instrumentation including high-frequency pressure sensors and Temperature Sensitive Paint (TSP). Focused Laser Differential Interferometry (FLDI) measured the flowfield distribution of second-mode instability waves responsible for transition. Filtered Rayleigh Scattering (FRS) was used for flow visualization of the separation region and unsteady shear layer. Velocimetry through the separation shear layer was provided by Femtosecond Laser Electronic Excitation Tagging (FLEET) whereas Coherent Anti-Stokes Raman Scattering (CARS) measured the thermal profiles through multiple flow features generated by the interaction. These flowfield diagnostics detail the alterations in the shock/boundary-layer interaction as the flow state moves from laminar to transitional to turbulent conditions, revealing behaviors absent from canonical flows.

Study Of Mixing Enhancement Of The 3-D Acoustofluidic Mixer Using Digital In-Line Holographic Micro Particle Tracking Velocimetry

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.

Dynamics and spectral character of unsteady pressure field on afterbody of generic space launcher: Transonic flows

The unsteady pressure field over an axisymmetric backward-facing step was investigated experimentally at transonic freestream Mach numbers of 1.05, 1.2, and 1.4. The study was aimed at examining the influence of transonic freestream Mach numbers on the spatio-temporal character of the unsteady pressure field and on the dominant modes/mechanisms driving it. Surface flow visualization, schlieren, and unsteady pressure measurements were carried out as part of the experimental investigation. From oil flow visualization and schlieren, the reattachment region was identified, and consequently, the mean reattachment length was estimated. The mean reattachment length shows an increase with the increase in freestream Mach number. The coefficient of mean pressure along the rearbody imitates a classical backward-facing step flow profile and can be divided into three distinct regions. The peak values of the coefficient of mean pressure and the coefficient of root mean square of the fluctuation are seen to decrease with an increase in the freestream Mach number. Conventional spectral analysis reveals that as the freestream Mach number increases, the dominant peak in the spectra shifts to lower frequencies. From the spectra, three dominant fluid dynamic mechanisms depending on the freestream Mach number have been identified. Proper Orthogonal Decomposition (POD) analysis shows that 79–84 % of the total energy contribution comes from the first six modes. The temporal dynamics of the POD modes indicate three prominent mechanisms are responsible for the unsteady pressure field. Spectral analysis of POD modes indicates that the spectra are primarily driven by the first three POD modes for freestream Mach number of 1.05 and the first two modes for freestream Mach numbers of 1.2 and 1.4. Moreover, it reveals the presence of three dominant modes, and the freestream Mach number strongly dictates the dominant mode that is driving the pressure field.

Modeling and mitigation of vortex formation in ejector deep hole drilling with smoothed particle hydrodynamics

AbstractEjector deep hole drilling achieves high-quality boreholes in production processes. High feed rates are applied to ensure a high productivity level, requiring reliable chip removal from the cutting zone for a stable process. Therefore, a constant metalworking fluid flow under high volume flow rates or high pressure is required. Experimental results show a vortex formation at the outer cutting edge. This vortex can lead to delayed chip removal from the cutting zone, and ultimately, it can lead to chip clogging and result in drill breakage due to increased torque. This paper investigates modified drill head designs using the smoothed particle hydrodynamics method. The investigated modifications include various designs of the chip mouth covering. Besides graphical analysis based on flow visualizations, flow meters are placed at the tool’s head to evaluate the impact of the modifications on the flow rate and possible increased resistance and relocation of the fluid flow from the outer cutting edge to other parts of the tool. The simulation results for the reference design show the experimentally observed vortex formation, validating the simulation model. By adding the tool’s rotation in the SPH simulation, which is not included in the experiments for observation reasons, the vortex formation is positively influenced. In addition, some designs show promising results to further mitigate the vortex formation while maintaining a sufficient fluid flow around the cutting edges.

Development of Dehydrogenation System for Liquid Organic Hydrogen Carrier with Enhanced Reaction Rate

Owing to the massive expansion and intermittent nature of renewable power, green hydrogen production, storage, and transportation technologies with improved economic returns need to be developed. Moreover, the slowness of the dehydrogenation reaction is a primary barrier to the commercialization of liquid organic hydrogen carrier (LOHC) technology. The present study focused on increasing the speed of dehydrogenation, resulting in the proposal of a triple-loop dehydrogenation system comprising reaction, heating, and chilling loops. The reactor has a rotating cage containing a packed bed of catalyst pellets, which is designed to enhance both heat and mass transfer by helping to detach precipitated hydrogen bubbles from the catalyst surface. In addition, the centrifugal force aids in isolating the gas phase from the LOHC liquid. A dehydrogenation experiment was conducted using the reaction and chilling loops, which revealed that the average hydrogen production rate during the first hour was 52.6 LPM (liter per minute) from 26.3 L of perhydro-dibenzyl-toluene with 1.5 kg of 0.5 wt% Pt/Al2O3 catalyst. This was approximately 48% more than the value predicted with the reaction kinetics measured with a small-scale plug flow dehydrogenation reactor with less than 1.0 g of 5.0 wt% Pt/Al2O3 catalyst. The concept, construction methods, and results of the preliminary gas infiltration, flow visualization, and reactor pumping experiments are also described in this paper.

Experimental investigation of the wake of tandem cylinders using pivoted flapping mechanism for piezoelectric flag

In this paper, the current of ocean at a depth with the velocity of 0.1–0.3 m/s has been studied. The effect of the mounting mechanism of the piezoelectric flag on the energy efficiency of the energy harvester is investigated. The flag is mounted uniquely, and the results are analyzed to explain the underlying mechanism associated with variations in power output. The experiments described extend our previous work and it is the experimental investigation of the baseline case for our previous work. It was found that using a pivoted mounting mechanism caused a significant improvement in flapping amplitude (51%) and dominant frequency (23%). The variation in the flag's distance from the bluff body (Gd) is also investigated. The maximum output for a passively mounted flag was obtained at Gd=2D due to the existence of a vortex core position at the same point where the flag's head was positioned. The same mechanism is implemented for tandem cylinder arrangement. The study found that increasing the distance between the bluff bodies resulted in the suppression of vortex shedding of the upstream cylinder by the rear cylinder. However, this behavior was found to be random and was thoroughly explored in this study. The obtained results were also verified through the flow visualization technique. A comparative analysis is made to show the optimal performance of the harvester by fine-tuning parameters such as the gap between cylinders and piezo flag, flow rate, and flag-mounting technique. The comparative analysis with benchmark cases demonstrated a rise in the A/L ratio by 30.25% and in the flapping frequency by 19.09% in the case of 120° cut C-shaped inverted cylinder upstream, indicating a higher percentage gain of 66% more energy harvesting. An increase of 51.18% in the A/L ratio and 22.79% in flapping frequency with the use of a pivot-mounted piezoelectric flag was observed, indicating higher energy harvesting performance compared to the baseline case (circular cylinder with fixed flag head).

Wind Tunnel Tests of a Thick Wind Turbine Airfoil

ABSTRACTThis article reports about a wind‐tunnel experiment carried out in the ONERA F2 low‐speed wind tunnel on a model of the DU 97‐W‐300Mod airfoil designed for wind turbine application. The wind tunnel, the airfoil model, and experimental techniques used are presented, with special emphasis on the data processing and corrections required to derive airfoil forces and pressure distribution. To better document the flow physics at play, the results are illustrated by infrared thermography and surface oil flow visualization. The test allowed investigating Reynolds number effects between 1 and 3.8 millions. To ameliorate the understanding of the benefits and limitations of such airfoil testing, one section is devoted to the comparison of present results with previous experiments in other wind tunnels. Some of the difficulties arising in airfoil testing are evidenced and discussed to contribute to the improvement of test methods.

Characteristics of the jet shear layer of a round elevated jet in crossflow

The present research aims at an experimental study to investigate the effect of velocity ratio (R) on the evolution of elevated round jet issued from a stack having an aspect ratio (AR) of 9.0 in a uniform crossflow at a Reynolds number of 2000. The experiments are conducted in a low-speed, recirculating water tunnel. Laser Doppler Velocimetry (LDV) is employed to characterize the water tunnel. Particle Image Velocimetry (PIV) is used to measure the flow field, and the dye visualization technique is utilized to capture the accompanying instantaneous flow structures. Depending on the velocity ratio (R=0.16––1.5), distinct jet shear layer (JSL) structures are observed in the symmetric plane (XY), confirmed by both the flow visualization and PIV data. These structures include clockwise vortices, anticlockwise vortices, swing-induced mushroom vortices, and jet-like vortices. For R<0.5, entrainment of a substantial amount of jet fluid into the stack-wake region (downwash) has been observed. At R=0.5, the streak image captured in the wall parallel plane reveals the ring-like vortices that appear to wrap around the jet core while upright vortices are detected downstream on the lee side of the jet. Additionally, variations in average velocity and turbulent fluctuations in the near field, contingent on different velocity ratios, are discussed.

Cross-correlation-based convolutional neural network with velocity regularization for high-resolution velocimetry of particle images

Particle image velocimetry (PIV) stands as a pivotal experimental technique in fluid dynamics, enabling the visualization and analysis of fluid flows. Traditional methods for extracting velocity fields from particle images often rely on window-cross correlation PIV or, more recently, optical flow techniques rooted in intensity conservation principles. However, the former approach suffers from low resolution, whereas the latter is hampered by computational inefficiency and a high susceptibility to noise. Recent studies have demonstrated the effectiveness of convolutional neural networks (CNNs) in processing particle images to obtain high-resolution and high-accuracy velocity fields, though traditional CNN architectures are still not satisfying in accuracy. The present study introduces an enhanced network, En-FlowNetC, based on the cross correlation-based CNN FlowNetC, specifically designed to process PIV particle images and achieve high-accuracy, high-resolution velocity fields. It incorporates a velocity regularization and is trained and validated on canonical datasets. The results indicate that En-FlowNetC surpasses traditional CNN networks in accuracy and markedly outperforms the classic Horn–Schunck optical flow method in both complex and simple flow scenarios. Furthermore, this study confirms the beneficial impact of velocity regularization, when judiciously applied, on network accuracy. The proposed modifications compared to the original FlowNetC are also examined in the ablation experiments. Overall, En-FlowNetC provides an effective deep-learning solution for PIV analysis, paving the way for future endeavors aimed at achieving highly accurate and resolved velocimetry.

Cross-correlation-based convolutional neural network with velocity regularization for high-resolution velocimetry of particle images

Particle image velocimetry (PIV) stands as a pivotal experimental technique in fluid dynamics, enabling the visualization and analysis of fluid flows. Traditional methods for extracting velocity fields from particle images often rely on window-cross correlation PIV or, more recently, optical flow techniques rooted in intensity conservation principles. However, the former approach suffers from low resolution, whereas the latter is hampered by computational inefficiency and a high susceptibility to noise. Recent studies have demonstrated the effectiveness of convolutional neural networks (CNNs) in processing particle images to obtain high-resolution and high-accuracy velocity fields, though traditional CNN architectures are still not satisfying in accuracy. The present study introduces an enhanced network, En-FlowNetC, based on the cross correlation-based CNN FlowNetC, specifically designed to process PIV particle images and achieve high-accuracy, high-resolution velocity fields. It incorporates a velocity regularization and is trained and validated on canonical datasets. The results indicate that En-FlowNetC surpasses traditional CNN networks in accuracy and markedly outperforms the classic Horn–Schunck optical flow method in both complex and simple flow scenarios. Furthermore, this study confirms the beneficial impact of velocity regularization, when judiciously applied, on network accuracy. The proposed modifications compared to the original FlowNetC are also examined in the ablation experiments. Overall, En-FlowNetC provides an effective deep-learning solution for PIV analysis, paving the way for future endeavors aimed at achieving highly accurate and resolved velocimetry.

Marangoni flows triggered by cationic-anionic surfactant complexation

HypothesisThe gradients in surfactant distribution at a fluid-fluid interface can induce fluid flow known as the Marangoni flow. Fluid interfaces found in biological and environmental systems are seldom clean, where mixtures of various surfactants are present. The presence of multi-component surfactant mixtures introduces the possibility of interactions among constituents, which may impact Marangoni flows and alter flow dynamics. ExperimentsWe employed flow visualization, surface tension and reaction kinetic measurements, and numerical simulations to quantitatively investigate the Marangoni flows induced by the reacting surfactant mixtures. Different binary surfactant mixtures were utilized for comparative analysis. FindingsThe impact of surfactant interactions on Marangoni flows is confirmed through the observation of diverse complex flow patterns that result from the combination of oppositely charged surfactants in varying composition ratios and concentrations. Unique flow patterns originate from the composition-dependent interfacial phenomena upon mixing surfactants. Our findings provide vital insights that could be used to guide the development of effective oil remediation or the spreading of waterborne pathogens in contaminated regions.

Experimental Study on Pulsed Suction under Different Momentum Coefficients in Variable Compressor Cascade with Annular Gaps

Variable Stator Vanes (VSVs) with penny cavities are crucial in the compressors of variable cycle engines, where complex interactions between annular and radial gaps can adversely affect aerodynamic performance. Unsteady active flow control techniques have significant potential for reducing secondary flow losses in compressors. This research investigates the internal flow characteristics of a variable compressor cascade with penny cavities using low-speed wind tunnel experiments and flow visualization techniques. This study is the first to experimentally explore the relationship between loss reduction and momentum coefficient using pulsed suction (PS) compared to steady continuous suction (SCS), evaluating the effects of various excitation frequencies. The findings show that at a momentum coefficient of 0.11 and an excitation frequency of 100 Hz, PS achieves optimal aerodynamic performance, reducing total pressure losses by 11.59%, marking a 92.59% improvement over SCS. As the momentum coefficient increases, the effectiveness of PS control significantly improves, whereas the changes in SCS effectiveness are less pronounced. The control effects of PS under varying excitation frequencies are complex and influenced by multiple factors. At higher momentum coefficients, the loss reduction effectiveness of PS is strongly correlated with excitation frequency, while at lower coefficients, performance changes due to frequency variations are minimal. This study provides valuable guidance for the design of modern multistage variable compressors.

Vortical structures and passive scalar transport in starting process of annular purging jet

The evolution of vortical structures and passive scalar transport in the starting process of annular purging jets are numerically investigated by large eddy simulation. Three flow configurations with different nozzle-to-plate distances at a fixed radius ratio of 0.71 and the Reynolds number of 13 750 are simulated. The numerical results are validated against documented experimental data. Three stages during the evolution are proposed based on instantaneous flow visualizations and assessed by calculating the circulation changes of the annular jets and vortex rings. The vortical structures are identified to understand the three-dimensional characteristics. The entrainment process is analyzed focusing on the passive scalar transport in the flow fields and is correlated with the cleaning performance of annular purging jets. The flow structures dominate the process of scalar mixing, especially the inner and outer vortex rings. The large-scale motions of trailing jets cause the intermittent events of scalar transport. During the starting process, the cleaning performance is better with a smaller nozzle-to-plate distance, while the cleaning efficiency may reach the optimum at a moderate distance. The cleaning process is limited by the scalar diffusion and entrainment process. These findings highlight the significance of flow structures for effective cleanness of temperature and contaminations in the purging systems.

Heat transfer measurement near injection hole of supersonic nozzle with a sonic jet injection

Heat transfer measurements were conducted to investigate surface heat transfer characteristics resulting from the interaction between a sonic jet and supersonic nozzle crossflow. A sonic jet was injected into the nozzle where the average Mach number is 2.88. Experiments were carried out for three different momentum flux ratios (J=0.5, 1.0, 1.5). A half-nozzle was introduced to measure heat transfer on the inner walls of the nozzle using Infrared thermography. Pressure measurements using PSP and oil flow visualization were also conducted to understand the heat transfer. From the oil flow visualization and heat transfer results, it was found that the area around the hole influenced by recirculation vortexes exhibited high heat transfer coefficients, showing up to 5 times higher values compared to the freestream. Furthermore, as the momentum flux of the jet increased, both the heat transfer augmentation area and the heat transfer coefficient also increased. To analyze the heat transfer characteristics between the nozzle and flat surface, comparisons were made using dimensionless heat transfer coefficients. The nozzle exhibited higher heat transfer in a smaller interaction area compared to that of a flat surface, but the average heat transfer coefficient near the hole was lower. Accordingly, distinct thermal treatment strategies should be employed for nozzles and flat surfaces, especially near injection holes. These insights could be valuable for the thermal design of rockets and other supersonic/hypersonic mobility systems.

Computational study of transverse oblique injectors for improvement of fuel mixing in scramjet engine of supersonic vehicles

The efficient injection of fuel in the combustion chamber is significant for the advancement of the current supersonic vehicles. This study has focused on the role of non-alignment injector configuration on the fuel mixing of the hydrogen jet inside the combustion chamber of the scramjet engine. The fuel distribution and vortex generation behind both annular and coaxial jets are modeled and compared to disclose the mechanism of fuel diffusion in the combustion chamber. A three-dimensional model of two nozzle angles is produced to visualize the complicated flow interaction of four non-aligned circular jets at supersonic cross-flow. Our results indicate that the strength of the vortex pair produced by the core of the fuel jet is extended more when the angle of the nozzles is increased. The flow visualization of the jet also confirms that the flow becomes more complex in non-alignment jet configurations and fuel mixing is improved by the addition of the internal air jet in the suggested injection model.

Ship base vibration reduction design technology based on visualization of power flow and discrete optimization

The ship base is a structure that connects the equipment to the hull and may play a role in restraining and isolating the dynamic load. Adding damping on the base to improve the vibration isolation performance is an important measure to control ship vibration. In this research, the energy transfer route and vector cloud of the ship base were analyzed employing the power flow theory, and then the placement of the particle damper was determined. Through the discrete optimization of different particle parameters including the particle material, diameter and filling rate, the best vibration reduction effect was acquired. The simulation and experiment results show that the particle damping has obvious damping effect, and the steel particle has better damping effect than the lead particle and the aluminum particle. The change of particle filling rate influences the vibration characteristics, and the best effect is achieved when the filling rate is 82%. The vibration reduction performance relies strongly on particle diameters, and they all exert obvious vibration suppression effect at the peak acceleration admittance. The proposed discrete optimization strategy effectively saves experiment cost, and the presented particle damper may be traded as an optional scheme in vibration reduce treatment of ship base.

Towards Flow Visualization in the Region of Air Blowing through the Perforated Section of a Body of Revolution

Towards Flow Visualization in the Region of Air Blowing through the Perforated Section of a Body of Revolution

Weather-dependency of the thermographic flow visualization of the laminar-turbulent transition on wind turbines

Abstract Analyzing the airflow around wind turbines during operation requires an in-process-capable measurement approach that functions without modification of the rotor blade. Infrared–thermographic flow visualization is such a measurement approach. However, its measurement capabilities on wind turbines in operation are highly weather-dependent. Therefore, to understand the expected flow visualization quality in non-laboratory conditions, the dependence of the achievable contrast and contrast-to-noise ratio (CNR) of the laminar-turbulent transition on solar radiation and air temperature is studied, respectively. A linear dependence of the contrast on the absorbed solar radiation is derived as a first estimation from a theoretical study of the heat balance. While the air temperature variations are shown to have no effect under certain conditions. The slope of the linear dependence of about 0.025 m2K W−1 was validated by experiments. To further study the fundamental measurability limit, only the camera noise with constant variance is here applied to determine the achievable CNR, which is thus directly proportional to the contrast. As a result, the achievable contrast and CNR for visualizing the laminar-turbulent flow transition over the year, over the day, and for different yaw angles of the wind turbine are determined. For this investigation, a wind turbine location near in northern Germany, is assumed as an example, and a maximal achievable contrast and CNR of 4.2 K and 122, respectively, are estimated, which agree with previous measurements. The presented method applies to any other wind turbine location and thus enables planning thermographic flow measurements on any wind turbine in the world.