Flow Visualization Data Research Articles

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.

Thermal-hydraulic phenomena and heat removal performance of a passive containment cooling system according to exit loss coefficient

The natural circulation system has been widely studied for use in various applications because of its inherent advantage. However, it has a key weakness called flow instability that makes the system unstable. Through massive previous research, the mechanisms of flow instability were analyzed, but there was an ambiguous aspect related to the effect of experimental parameters on the phenomenon. Particularly, there has been no report on the heat transfer performance of the system when flow instability phenomena were present. In this study, thermal-hydraulic phenomena of a two-phase natural circulation system that functions as a passive containment cooling system (PCCS) was investigated according to experimental parameters, namely, the temperature boundary (120–158 °C) and exit loss coefficient (0–34.5) under atmospheric pressure conditions. The experimental results showed five different flow types in the loop. The flow modes that occurred by the interaction between flashing and boiling were classified by referring to the mass flow rate, void fraction, and visualization data. The system was more unstable when the temperature boundary conditions increased, but it was more stable when the exit loss coefficient increased. These results have only been confirmed in our research. The reason for the results is that the flow conditions are located on the boundary between Density Wave Oscillation I and the stable flow region, and that boundary does not have clear criteria. In addition, comparing the heat transfer performance of a system by heat rate can confirm the effect of flow instability on the thermal performance of the passive cooling system. As a result, the high exit loss coefficient stabilizes the system better than the low case and has similar heat removal performance.

Supersonic Wind Tunnel Flow Characterization Using Planar Laser CO2 Rayleigh Scattering

A complementary experimental and computational study was performed to characterize the test section flowfield of a variable-Mach-number supersonic wind tunnel. A planar laser Rayleigh scattering technique using CO2 nanocrystals was used to capture cross-sectional flow visualizations at various test section streamwise locations. Reynolds-averaged Navier–Stokes simulations of the wind tunnel flow were performed using a k-ω shear stress transport turbulence model to compare against the flow visualizations. Mach 3.20 and 3.55 flow conditions were investigated in the wind tunnel with an empty test section. The Rayleigh signal profile near the wind tunnel walls was compared against boundary-layer thickness data from shadowgraph visualizations, pitot survey measurements, and numerical simulation predictions to guide the interpretation of the scattering results. A brief discussion regarding the nature and potential source of certain unexpected scattering patterns in the images was provided. The most distinguishable flowfield characteristics observed in the Rayleigh scattering visualizations were the asymmetric thickness of the boundary layers along the test section walls and the appearance of two floor lobes near the flow-path corners. The results of the numerical simulations showed favorable agreement with the experimental data and were used to provide insight into the mechanism that caused the boundary-layer features observed. This study demonstrated the usefulness of CO2 Rayleigh scattering to obtain flow visualization data of great value to better understand flow quality in supersonic wind tunnels with two-dimensional nozzles.

The effect of cant angle on the sound production of mutually inclined rectangular supersonic jets

Rectangular nozzle geometries offer a number of benefits over their axisymmetric counterparts, particularly for military aviation. Among these benefits are altered thermal and noise signatures, ease of airframe integration, and simpler thrust vectoring. Many next-generation tactical aircraft concepts employ two rectangular jets in close proximity. These jets can couple together, producing high-amplitude acoustic tones that are deleterious to aircraft structure. In this work, we take a combined experimental and theoretical approach to examine the effect of cant angle (i.e., mutual inclination angle) on the jets' proclivity to couple. We employ modal decomposition of flow visualization data alongside resonance predictions based on linear stability theory to explore the degree to which mutual inclination can suppress resonance.

Physics-Informed Neural Network for Flow Prediction Based on Flow Visualization in Bridge Engineering

Wind loads can endanger the safety and stability of bridges, especially long-span cable-supported bridges. Therefore, it is important to evaluate the potential wind loads during the bridge design stage. Traditionally, wind load evaluation is performed by wind tunnel testing, which is relatively expensive. With the development of computational fluid dynamics and high-performance computing, numerical simulations are becoming more accessible for designers. However, the costs required for accurate numerical results are still high, especially for high-fidelity simulations. Under this condition, searching for a more efficient method to evaluate the wind loads in bridge wind engineering has become a new goal. It seems that flow visualization is a good entry point. Although flow visualization techniques have been developed in recent years, it remains difficult to extract velocity and pressure fields from images. To address this problem, physics-informed neural networks (PINNs) have been developed and validated. This study establishes a PINN to investigate the two-dimensional viscous incompressible fluid flow passing a generic bridge deck section. Two cases with different Reynolds numbers are tested. After careful training, it is found that the PINN can accurately extract the velocity and pressure fields from the concentration field and predict the drag and lift coefficients. The results demonstrate that PINNs are a promising method for extracting useful flow information from flow visualization data in engineering applications.

Influences of geometric parameters on serpentine nozzles for turbofan

Serpentine nozzles possess excellent advantages in improving the stealth ability of aero-engine exhaust. For a real turbofan, the flow characteristics of the serpentine nozzle are evidently affected by the geometric parameters and the complex exhaust mixer. This is an issue that needs to be seriously considered. The aim of this paper is to investigate the influences of the critical geometric parameters on the serpentine nozzles for turbofan. The detailed flow field of the serpentine nozzle for a small-scale model is precisely measured. The influence of the ratio of axial length to inlet diameter on the flow field and on the aerodynamic performance of the serpentine nozzle is numerically explored. Results show that the pressure on the upper wall first drops rapidly and then ascends with the local minimum occurring at the first inflection. The relative error between the numerical prediction and the experimental data is less than 2%. The calculated distributions of the expansion-shock waves and the jet shear layers are highly consistent with the schlieren flow visualization data. Thus, the reliability of the numerical method is effectively confirmed. The internal flow and the external jet features of the serpentine nozzle are extremely non-uniform. Such poor uniformity is reflected in the phenomena including the existence of the local high-shear-stress regions and the high-velocity regions, the multiple bending of the limiting streamlines, as well as the velocity fluctuation of plume core. The aerodynamic performance of the serpentine nozzle first enhances greatly and then stays unchanged as the value of L/D rises. The vortex loss and the shock loss both reduce evidently owing to the flow separation and the disappearance of the shock wave during the rise of L/D from 2.2 to 2.4. The value of the discharge coefficient and the thrust coefficient increases by 1.5% and 2.1%, respectively. The length of the plume core is effectively shortened due to the disappearance of the shock wave inside the nozzle.

RANS-Based CFD Simulation of Jet-Induced Mixing and Jet Impingement on Large Bubble in Microgravity

For National Aeronautics and Space Administration’s space mission planning, tons of cryogenic propellants need to be stored under microgravity conditions. Because of heat leaks into cryogenic propellant tanks, thermal stratification develops from lack of natural convection leading to boil-off of precious propellants. A thermodynamic vent system operates with a jet mixer to reduce thermal gradients within the fluid and control pressure inside the tank. In this work, a Reynolds-averaged Navier-Stokes–based computational fluid dynamics model was developed to study the fluid dynamics of jet-induced mixing and jet impingement on the large ullage bubble in the Tank Pressure Control Experiment (TPCE) under microgravity conditions. First, the computational model was benchmarked against existing experimental flow visualization data on the jet impingement. The jet mixing was then compared quantitatively with correlations for the jet radius to analyze the volumetric flow rate of the jet due to entrainment in the near field of the nozzle. The findings show that the confinement of the jet due to the ullage and the walls contributes positively to the jet entrainment rate, thus increasing the jet volumetric flow rate. In addition, the turbulence parameters are plotted to study the flow development for the TPCE case where the jet does not penetrate the ullage. Last, the model was used to determine the jet Weber number for penetration on the ullage bubble by varying jet inlet velocities. Numerical results show that the jet can penetrate the ullage when the jet Weber number is greater than 1.3.

Detailed numerical investigation of the CO[formula omitted] two-phase ejector 3-D CFD model based on the flow visualisation experiments

3-D CFD-based simulations of CO2 flow inside two-phase ejectors are commonly used with various turbulence models. However, none of the previously published studies examined the influence of a local turbulence phenomenon on the simulation results. Therefore, in this study, a detailed numerical analysis of CO2 local flow behaviour was compared to experimental flow visualisation data as well as to turbulence approach validation. Furthermore, the kinetic behaviour of mixed streams in the boundary layer was examined under transcritical operating conditions for motive nozzle pressures above 80.0 bar. The mixing process mapped by different turbulence models showed differences in recapturing the angle of suction flow in the mixing chamber. Furthermore, the different flow shapes of the simulated motive expanded flow were observed. The best prediction of the mixed two-phase flows was obtained using the Reynolds stress model with linear pressure-strain approach with the suction mass flow rate discrepancy of 2%. The higher pressure lift caused the suction flow to be more perpendicular towards the motive flow, elongating the suction flow course in the mixing chamber and intensifying the mixing process. A further increase in the pressure lift eventually leads to the possible occurrence of suction reverse flow.

Spatial coherence of low-frequency unsteadiness associated with a normal shock wave

The present data illustrate a unique analysis approach to explore the physics of shock-wave-flow interactions, based upon correlation and spectral analysis of digitized shadowgraph visualization time-sequence data. As such, demonstration is provided of the use of high resolution, flow visualization data for quantitative correlation and spectral analysis. A shadowgraph system is employed to visualize time-varying, shock wave flow features within the test section of one leg of the transonic/supersonic wind tunnel (which is also referred to as the SS/TS/WT or SuperSonic/TranSonic/WindTunnel), located within the Propulsion Research Center of the University of Alabama in Huntsville. Of interest is a flow field with a well-defined normal shock wave, lambda foot, and separated turbulent boundary layer near the entrance of the lower flow passage (which is part of the test section), produced with a test section inlet Mach number of 1.54. Shadowgraph flow visualization images are processed from different pixel locations to compute frequency spectra. Data associated with two separate pixel region locations, each from a different flow region, are employed to determine magnitude squared coherence values, and associated time lag magnitudes. The data associated with lower Strouhal numbers in vicinity of 0.0013 and 0.0039 (which correspond to respective frequencies of 5 Hz and 15 Hz) illustrate important and pronounced interactions between the normal shock wave and the boundary layer separation zone. As this zone breathes and oscillates, additional flow locations are affected in a subsequent and significant manner. With Strouhal numbers in the vicinity of 0.0091, 0.0104, and 0.0260 (which correspond to respective frequencies of 35, 40, and 100 Hz), the most pronounced and significant interactions between the normal shock wave occur with respect to the downstream boundary layer. When Str equals 0.0091, spatially-varying time lag data show events with significant coherence propagate from a range of locations within the downstream boundary layer to the normal shock wave location. As such, events which originate within the downstream boundary layer at these experimental conditions are far more important (compared to events which originate with the upstream boundary layer) in regard to their effects on shock wave unsteadiness and associated flow motions.

Reconstructive Mapping from Sparsely-Sampled Groundwater Data Using Compressive Sensing

Compressive sensing is a powerful method for reconstruction of sparsely-sampled data, based on statistical optimization. It can be applied to a range of flow measurement and visualization data, and in this work we show the usage in groundwater mapping. Due to scarcity of water in many regions of the world, including southwestern United States, monitoring and management of groundwater is of utmost importance. A complete mapping of groundwater is difficult since the monitored sites are far from one another, and thus the data sets are considered extremely “sparse”. To overcome this difficulty in complete mapping of groundwater, compressive sensing is an ideal tool, as it bypasses the classical Nyquist criterion. We show that compressive sensing can effectively be used for reconstructions of groundwater level maps, by validating against data. This approach can have an impact on geographical sensing and information, as effective monitoring and management are enabled without constructing numerous or expensive measurement sites for groundwater.

The generation of screech tones by shock leakage

Abstract

Quantitative Analysis of Forced and Unforced Turbulent Multiphase Coaxial Jets

Abstract This study explores the structure of liquid/gas coaxial jets under forced and unforced conditions. The forcing is in the form of a transverse acoustic resonance within the confined space where the mixing occurs. The studied flows are relevant to combustion instabilities which involve an interaction between acoustic waves and reactant mixing. A variety of local and global signal processing methods were applied to digital flow visualization data to identify spatial and temporal features. The unforced case is in particular chaotic and influenced by a broad range of spatial and temporal phenomena. Proper orthogonal decomposition (POD) was able to extract flapping and convecting features, and spectral content of these behaviors is presented. The forced case results in organized structures that emerge above the background turbulence, including harmonics of the forcing frequency and nonlinear interactions between specific frequencies. The dynamic mode decomposition (DMD) performs the best in the forced case, clearly isolating all of these features. Wavelet analysis showed that forcing tended to reorganize energy from longer to shorter time scales. Bicoherence analysis of the data showed that the forcing causes a much different energy exchange in the outer and inner shear layers. The outer-to-inner jet coupling during forced conditions appears to be limited to an axial extent of about one to three inner jet diameters downstream of the jet exit. The recirculation zone between the inner and outer jet, extending about one inner jet diameter downstream, appears to disrupt the influence of forcing on the inner jet.

Falling-Film evaporation over horizontal rectangular tubes: Part II—Modeling

Abstract A two-part study of on falling-film evaporation on horizontal rectangular tubes, the external geometry of extruded flat, microchannel tubes, was conducted. In the companion paper, Part I, heat transfer experiments on this geometry were reported. In this paper, Part II, three models were developed to predict the flow regime, wetted tube area, and heat transfer coefficients for this configuration. The flow regime model is based on a thermodynamic analysis, while the wetted tube area is found with a hydrodynamic model based on idealized flow assumptions and flow visualization data. The heat transfer model uses the classic Nusselt (1916) film theory as the starting point. These models demonstrated good agreement with the experimental data and trends in the literature.

Leading-Edge Vortex as a High-Lift Mechanism for Large-Aspect-Ratio Wings

Enabling a leading-edge vortex (LEV) is a possible mechanism to significantly increase the lift on wings. This is well known for slender delta wings, for which various analytical models were developed and show accurate predictions of the lift enhancement. This paper shows that, when suitably designing the wing planform, LEVs can also exist on high-aspect-ratio (high-AR) wings. In this study, a quasi-three-dimensional flow model is presented for solving the stationary LEV phenomenon over a high-AR swept back wing, with sweep increasing toward the wingtip. Our model captures the three-dimensional phenomenon by satisfying conservation of mass and vorticity within the LEV, and using a combination of strip theory and the lifting-line theory. Our model predictions are compared with flow visualization data on a parabolic swept back wing. Results confirm that suitable sweep wing geometry can fix an LEV steadily over the upper wing surface, resulting in significant lift enhancement of up to 70%.

Thermal performance and flow characteristics of two-phase loop thermosyphons

A detailed 2-D computational fluid dynamics (CFD) model simulating two-phase flow and heat transfer in a two-phase loop thermosyphon (TPLTS) is presented. The CFD simulation was built to represent two-phase flow and heat transfer phenomena during the transient and steady-state operation analysis of a TPLTS under various operating conditions. The two-phase flow was modeled using the volume of fluid (VOF) model, and the Lee model was utilized for evaporation and condensation. Simulation results were compared with experimental temperature, pressure, and flow visualization data.

Analysis of shock wave unsteadiness using space and time correlations applied to shadowgraph flow visualization data

Unsteady flow characteristics of a normal shock wave, a lambda foot, and a separated turbulent boundary layer are investigated within a unique test section with supersonic inlet flow. The supersonic wind tunnel facility, containing this test section, provides a Mach number of approximately 1.54 at the test section entrance. Digitized shadowgraph flow visualization data are employed to visualize shock wave structure within the test section. These data are analyzed to determine shock wave unsteadiness characteristics, including grayscale spectral energy variations with frequency, as well as time and space correlations, which give coherence and time lag properties associated with perturbations associated with different flow regions. Results illustrate the complexity and unsteadiness of shock-wave-boundary-layer-interactions, including event frequencies from grayscale spectral energy distributions determined using a Lagrangian approach applied to shock wave location, and by grayscale spectral energy distributions determined using ensemble-averaging applied to multiple closely-located stationary pixel locations. Auto-correlation function results and two-point correlation functions (in the form of magnitude squared coherence) quantify the time-scales of periodic events, as well as the coherence of flow perturbations associated with different locations, over a range of frequencies. Associated time lag data provide information on the originating location of perturbation events, as well as the propagation direction and event sequence associated with different flow locations. Additional insight into spatial variations of time lag and flow coherence is provided by application of magnitude squared coherence analysis to multiple locations, relative to a single location associated with the normal shock wave.

Experimental and theoretical studies on oscillation frequencies of liquid slugs in micro pulsating heat pipes

The purpose of this study is to experimentally examine the oscillation frequencies of liquid slugs in multi-turn micro pulsating heat pipes (MPHPs) and to perform a theoretical study for better understanding of the experimental results. A series of experiments on silicon-based MPHPs with different overall lengths of 40, 50, and 60 mm are performed at different heat inputs in a bottom-heating mode. Ethanol is used as a working fluid at a filling ratio of 55%. From a spectral analysis on flow visualization data, dominant frequencies of the MPHPs are identified for each experimental condition. To theoretically estimate the dominant frequencies, a ‘non-adiabatic’ vapor spring-liquid mass model is proposed: The spring action of a vapor plug is linked not only to a volume variation but also to a mass variation of a non-adiabatic vapor plug via phase change processes. A distinguishing feature of this model is that it is capable of handling the mass variation of a vapor plug due to phase change processes. Based on the model, a set of parameters related to the oscillation frequency is explicitly determined: the number of turns, channel length, filling ratio, liquid density, vapor pressure and specific heat ratio. A closed-form correlation of the oscillation frequency is proposed and found to be accurate in predicting the experimental data to within ±15%. It is also shown that the oscillation frequency is over-predicted by more than 100% of the experimental data when the spring-mass model is used without including the mass variation due to phase change processes.

Density predictions using a finite element/level set model of polyurethane foam expansion and polymerization

Polyurethane foams are used widely for encapsulation and structural purposes because they are inexpensive, straightforward to process, amenable to a wide range of density variations (1–50 lb/ft3), and able to fill complex molds quickly and effectively. Computational models of the filling and curing process are needed to reduce defects such as voids, out-of-specification density, density gradients, foam decomposition from high temperatures due to exotherms, and incomplete filling. This paper details the development of a computational fluid dynamics model of a moderate density PMDI structural foam, PMDI-10. PMDI is an isocyanate-based polyurethane foam, which is chemically blown with water. The polyol reacts with isocyanate to produce the polymer. PMDI-10 is catalyzed giving it a short pot life, meaning that the foam is liquid and flowable only for a short-time; it foams and polymerizes to a solid within 5 min during normal processing. To achieve a higher density, the foam is over-packed to twice or more of its free-rise density of 10 lb/ft3. The goal for modeling is to represent the expansion, filling of molds, and the polymerization of the foam. This model will be used to reduce defects, optimize the mold design, understand processing parameter sensitivities, and predict the final foam properties.A homogenized continuum model for foaming and curing was developed based on reaction kinetics, documented in a recent paper; it uses a simplified mathematical formalism that decouples these two reactions. The chemo-rheology of PMDI is measured experimentally and fit to a generalized-Newtonian viscosity model that is dependent on the extent of cure, gas volume fraction, and temperature. The conservation equations, including the equations of motion, an energy balance, and three rate equations are solved using a stabilized finite element method. The equations are combined with a level set method to determine the location of the foam-gas interface as it evolves to fill the mold. Understanding the thermal history and loads on the foam due to exothermicity and oven curing is very important to the results, since the kinetics, viscosity, and other material properties are all sensitive to temperature. Results from the model are compared to experimental flow visualization data to understand the interface evolution with time and post-test X-ray computed tomography (CT) data for the density prediction validation. Several geometries are investigated including two configurations of a mock structural part and a bar geometry to specifically test the density model. We have found that the model predicts both average density and filling profiles well. However, it under predicts density gradients, especially in the gravity direction. Further model improvements are also discussed for future work.

Flow hydrodynamics of immiscible liquids with low viscosity ratio in a rectangular microchannel with T-junction

We present an experimental study of the liquid-liquid system with extremely low viscosity ratio (10−3) in a T-shaped microchannel with 120 × 120 μm inlets and a 240 × 120 μm outlet channel. Six different flow patterns have been observed: plug, droplet, slug, throat-annular and parallel flow. A specific plug flow pattern was found where micron-sized droplets or even a jet breaks off from the rear meniscus of a plug. In addition to typical Taylor-shaped plugs the dumbbell-like plugs were observed. Flow visualization data were summarized in the flow pattern maps. Plug length and velocity were measured based on flow visualization results. It was found that the plug velocity can be fitted by power function rather than the linear function, which disagrees with experimental data at a low plug velocity. Front and tail plug surface curvature was scaled using dimensionless parameter, the flow rate ratio multiplied by Capillary number based on bulk velocity (Qd/Qc * Cabulk). Instantaneous velocity vector fields inside water plugs were measured by means of PTV technique. Different flow structures were found and discussed. As a result, it is proposed to use the values of Qd/Qc * Cabulk for distinguishing different plug shapes and circulation patterns inside the plugs.

Intermittent flow initiation in a horizontal tube: quantitative visualization and CFD analysis

© 2018, The Brazilian Society of Mechanical Sciences and Engineering. The onset of slugging in stratified gas–liquid flow in a 26.4-mm ID horizontal pipe was investigated experimentally using a high-speed digital video processing technique. Spatio-temporal displacement maps were produced to show the evolution of two-phase flow structures (waves, slug precursors and liquid slugs) in the flow entrance region of the air–water mixture flow. The two-phase distribution derived from the visualization technique was in good agreement with measurements performed using non-intrusive capacitance sensors. Numerical results from a hybrid CFD approach that combined the two-fluid model and volume-of-fluid method to track the position of the large-scale interface in time and space also agreed well with the quantitative flow visualization data for phase distribution, slug length and slug frequency. The relative errors between the flow structure velocities calculated by cross-correlating the numerical and experimental (image analysis) phase distribution signals were 6.8 and − 7.4% for the slug and plug flow cases, respectively.