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Flow visualization is necessary in fields such as engineering, since it allows us to know what is happening around the element being studied by means of a preliminary method, although it is relatively close to future research and computation methodology. The present project studies the interference at the anemometers of the Mars rover Perseverance, caused by the mast holding one of its cameras. After obtaining the model, manufactured by a 3D printer, it was placed inside a hydrodynamic towing tank, and red dye was added for a visual observation of the interference during the experiment. A comparison was made between the results achieved and those seen in a wind tunnel, realizing the high correlation they have. Finally, this paper promotes the use of the hydrodynamic towing tank in preliminary studies due to its low costs, considering the adequate comparison with other higher precision methodologies.

An experimental investigation of the shock-buffet phenomenon subject to unsteady pitching supercritical airfoil around its quarter chord has been conducted in a transonic wind tunnel. The model was equipped with pressure taps connected to the fast response pressure-transducers. Measurements were conducted at different free-stream Mach number (M∞) from 0.61 to 0.76. The principle goal of this investigation was to experimentally discuss the shock-buffet criterion over a SC(2)-0410 supercritical pitching related to the hysteresis loops of total drag and trailing edge pressure, the behaviour of the shock wave foot location, the pressure distribution over the upper surface, and by implementing the wavelet analysis of the normal force. To ensure capturing the buffet phenomenon by utilizing these criteria, a pressure port has been drilled exactly at the trailing edge of the airfoil where its output was used to detect the buffet phenomenon for different conditions. Visual representation of the flow using the shadowgraph flow visualization technique for different test cases is further used to illustrate the unsteady shock wave motion. A comparative analysis of experimental measurements shows that the conducted criteria confirm each other when the buffet phenomenon occurs at the position of the oscillating cycle.

This work is dedicated to the systematic investigation of wind turbine wakes under the effect of pressure gradients. Wind tunnel experiments are carried out with a wind turbine positioned on straight ramps of increasing angle such that it experiences an approximately linear flow speed-up/slow-down from the induction region into the far wake. Fifteen ramp angles are studied: 7 favorable (FPG), 7 adverse (APG), and 1 zero pressure gradient. The wake center is shown to follow the base flow streamline originating from a virtual turbine hub height. A quasi-linear relationship between the pressure gradient and near wake length is demonstrated. Far wake characteristics, such as the recovery of the wake center velocity deficit and wake growth rate, are observed to systematically vary with the pressure gradient. The wake recovery rate increases (decreases) with the increase in the FPG (APG), and the wake growth rate shows a linear increase from most favorable to most adverse pressure gradient. The turbine power coefficient decreases significantly with increasing APG to a greater degree than the increase in power coefficient under FPG. The engineering approach of superposing the wake deficit predicted by the standard Gaussian model on the modified base flow is shown to work for very moderate pressure gradients. In light of this, a threshold in terms of flow speed-up/slow-down along the wake trajectory is established, below which the engineering approach can be reasonably employed. Finally, a physics-based model for wakes under the pressure gradient is tested. A new theoretical relation for near wake length under the pressure gradient is proposed. Using the theoretical near wake length, the pressure gradient model predicts the turbine wakes for all cases with good accuracy and shows a significant improvement from the engineering approach.

The coaxial mixers enhance the suspension of concentrated slurries in an agitated reactor. In this research work, the complex slurry suspension and dissemination behavior in a coaxial slurry mixing system (comprised of a close clearance anchor rotating with a low speed and an inner axial impeller rotating with a high speed) was analyzed employing ERT (electrical resistance tomography, a non-intrusive flow visualization technique), and computational fluid dynamics (CFD). The numerical models were validated by comparing the axial solid concentration profiles generated using the ERT data and the CFD simulation results. The influences of various important parameters such as the diameter of the inner axial impeller, the inner impeller type, and the inner impeller spacing on the hydrodynamic characteristics of the slurry suspensions in a coaxial mixing vessel were thoroughly analyzed. The radial and axial velocity profiles of solid particles were generated using the validated mathematical models. The assessment of energy loss due to the solid–solid collisions, the particle–fluid frictions, and the particle–vessel wall collisions was conducted. The evaluation of optimum inner impeller clearance and inner impeller diameter is essential to attain a high degree of solids suspension and dissemination in a coaxial slurry mixing system.

This paper presents two-dimensional heat transfer results for the first time in an expansion tube, using infrared thermography at a Mars entry condition. Experiments were performed in the X2 expans...

Two-phase pressure drop in the debris has been studied by many researchers concerning the debris cooling characteristics during a severe accident in a nuclear reactor. However, its flow regime transition of the two-phase flow in the debris has not been well understood, which strongly affects the interfacial drag and the pressure drop. Conventional models for gas-liquid two-phase flow pressure drop have not been established to evaluate interfacial drag accurately. In this study, high-speed imaging of a two-dimensional network model was performed to clarify the effect of flow patterns on interfacial drag and pressure drop. Usually, it would not be easy to visualize such two-phase flow behavior in a randomly packed bed due to the reflection/refraction of light and/or overlapping bubbles, even if the test section is made of transparent materials. Therefore, in this study, a test section, which simulates a two-dimensional network of porous structures, was fabricated to avoid overlapping bubbles. The two-phase flow pattern in the porous structure has been identified by high-speed imaging of the two-dimensional network model. The flow regime map based on the flow pattern visualization results is applied to the pressure drop evaluation and it could reduce the overestimation of experimental values. The experimental results suggested that the interfacial drag term should be modified in the gas-liquid two-phase flow pressure drop model.

Rip currents are like rivers of fast-moving water that can quickly carry the unwary out to sea. They are not easy to recognize, especially to the untrained observer. Other than in-situ current measurements, there exists a number of methods that analyzes images and videos to detect rip currents. Most of these techniques base their detection on the appearance of rip currents such as foamy pattern, discoloration of water, and locations of breaking waves. Leading methods use either image processing or machine learning of images and/or video input. In this paper, we analyze the behavior of water movement rather than simply its appearance to detect rip currents. Specifically, we investigated several flow visualization methods and tune them to detect rip currents. Based on our study, we recommend two methods that allowed us to detect rip currents where other methods have failed. And because the methods originated as visualization techniques, any presence of rip currents are automatically highlighted. We also evaluated these two methods against previously annotated results by rip current experts, and found that our detections were sufficiently sensitive that some expert annotations were relabeled.

Experiments were performed on the motion of isolated air bubbles and drops of Fluorinert FC-75 moving in a Dow-Corning silicone oil under the action of an applied temperature gradient in a reduced gravity environment aboard the Space Shuttle in orbit. The disturbance of the imposed temperature field due to the motion of the objects was studied optically using a shearing interferometer with a Wollaston prism and the results of a typical bubble run were compared with theoretical predictions. Also, the liquid velocity field surrounding the bubbles and drops has been qualitatively investigated in a few runs by the observation of tracer particles dispersed in the continuous phase fluid. The measurement techniques are described, and the results for the temperature and flow fields are presented and discussed.

Wind tunnel flow visualization tests were conducted to analyse the efflux velocity impacts and the yaw angle on the smoke dispersion of the exhaust for a generic frigate. An analytical study was also implemented to obtain the exhaust plume trajectories. The 1/100 scale generic frigate, having a platform for helicopters on the aft of the ship, was built and employed during the experimental study. The forward and astern cruises of the frigate were considered. It is found that the plume height and the exhaust gases momentum increase with the velocity ratio. The problem of smoke nuisance was observed for the ratios with low velocity such as K=0.2. The plume was also directed towards the helicopter platform when the yaw angles are higher than 10°. The experimental results are compared with the analytical solutions for three different velocity ratios. The compliance between the experimental and analytical results is found to be consistent.

In the immersive flow visualization based on virtual reality, how to meet the needs of complex professional flow visualization analysis by natural human–computer interaction is a pressing problem. In order to achieve the natural and efficient human–computer interaction, we analyze the interaction requirements of flow visualization and study the characteristics of four human–computer interaction channels: hand, head, eye and voice. We give out some multimodal interaction design suggestions and then propose three multimodal interaction methods: head & hand, head & hand & eye and head & hand & eye & voice. The freedom of gestures, the stability of the head, the convenience of eyes and the rapid retrieval of voices are used to improve the accuracy and efficiency of interaction. The interaction load is balanced by multimodal interaction to reduce fatigue. The evaluation shows that our multimodal interaction has higher accuracy, faster time efficiency and much lower fatigue than the traditional joystick interaction.

Human milk extraction from the breast is affected by the infant's oral activities. Natural suckling by the infant includes both intraoral vacuum and peripheral oral compression during breastfeeding. However, the contribution of each of these motions to milk extraction at the outlet and at the duct bifurcations is unclear. In this work, we investigated the flow field in a lactating breast model considering bifurcated milk ducts and multiphase breast-infant interactions. A bio-inspired breastfeeding simulator device was utilized to mimic an infant's oral feeding mechanism during breastfeeding and extract the human milk-mimicking Fluid from the transparent and elastic lactating breast phantom during experiments. Using a particle image velocimetry system, we found that the oscillatory flow under vacuum pressure provides a higher velocity field at the outlet compared to that when an infant applies both vacuum and oral compression pressures. Additionally, the intraoral vacuum coordinated with the oral peripheral compression causes stronger vorticities and secondary flows at the adjunction of the bifurcated ducts than the vacuum-only case. Vacuum-only extraction yields an increase in flow velocity at the outlet and could be one of the reasons for nipple pain, whereas infant's oral activities on the breast generated more vortices in the milk duct adjunctions and might cause milk duct clogs. This phenomenon is rationalized based on the validation of a previous in vivo clinical study of milk production compared between commercial pumps and infant suckling. The fact that milk consumption of vacuum-only extraction is less than that of vacuum plus oral compression further explains the effectiveness of applying a natural suckling pattern in human lactation.

Additive alignment in polymer composites can enhance the physical properties of the constitutive material. Researchers have attempted to understand and predict the additive orientation during the fabrication process, such as injection molding. In this study, the rotation of carbon fibers embedded in uncured polydimethylsiloxane flowing in an orifice channel is analyzed via a flow visualization experiment and image processing of obtained images. The angular velocity of additives is correlated with the shear rates in the flow field calculated from the computational fluid dynamics simulation with the assumption of a single-phase and a non-Newtonian flow. The effective shear rate is found to have a larger effect on the rotation of fiber compared with the aspect ratio of the fiber and the initial alignment angle to the flow direction. A correlation between the effective shear rate and angular velocity of the fibers is developed and verified experimentally. This correlation can be used to predict the fiber rotation during polymer composite fabrication within an orifice channel and hence provide useful information regarding the anisotropic physical properties of the final product.

Ignition overpressure (IOP) is a principal dynamic load experienced by a launch vehicle during liftoff. The IOP arises due to the sudden ignition of the solid rocket boosters (SRBs) of a launch veh...

The sound barrier for bullet trains remains a challenge due to the piston effect causing compression waves at the entry and exit of the tunnel. The air ahead of the train nose is compressed, and the wave propagates through the tunnel at the speed of sound and exits with the generation of micro pressure waves. It gives rise to a complex wave pattern comprising compression at the train nose & expansion at the train tail leading to the positive pressure around the nose and suction around the tail. This is intended to provide exhaustive input for the proper design of a futuristic tunnel. The cross-sectional shapes of the tunnel, whether square, rectangular, circular, or semi-circular, will experience pressure compression wave generated by high-speed train but will influence the flow pattern and hence the compression wave. This paper presents the pressure load on the walls of long and short tunnels for subsonic compressible and transonic flows. The experimental investigation is carried out only for length parameters to study short and long tunnels. Further, flow visualization is also provided after the formation of the sonic boom. The results of this investigation can be an essential data source for optimum design of high-speed tunnels so as to suppress or break the sound barriers, thus, resulting in a safer high-speed train network.

An experimental study on inclined slot jets with negative buoyancy is presented. Flow visualization experiments for negatively buoyant jets inclined at 15o, 30o, 45o, 50o, 60o, 75o, 80o, 85o and 90o to the horizontal were conducted in a laboratory apparatus. A thin non-confined slot of 0.20 m × 0.002 m was used to discharge colored saltwater in a tank with stationary tap water. The flow was turbulent with the densimetric Froude number ranging between 15 and 56. Flow images are presented and visual observations for the flow behavior are reported. The initial (maximum) terminal height of rise, the final (steady-state) terminal height of rise and its horizontal location, as well as the location where the upper boundary of the jet returns to the slot level were determined from flow images. These geometric characteristics, normalized by the slot width w, are linearly related to Fo4/3 (Fo being the densimetric Froude number) for each discharge angle, confirming dimensional analysis for two-dimensional (plane) jets. The constants of proportionality, i.e., the geometric characteristics normalized by the length scale $$\mathrm{w}{\mathrm{F}}_{\mathrm{o}}^{4/3}$$ , are computed from the experimental data. The variation of the dimensionless geometric characteristics with the discharge angle is studied and functions are obtained from data fitting. The dimensionless initial and final terminal heights of rise increase almost linearly with discharge angle up to their maximum occurring at 85o and 80o, respectively. The dimensionless horizontal distances follow approximately second degree polynomial curves with maximum at 45o.

It is established that in the territory of the Stryi river basin the natural landscapes are maximally preserved in comparison with other regions of Western Ukraine, but in modern conditions we observe an increase of negative anthropogenic impact on the quality of natural waters. Important for the efficiency of self-purification processes are hydrodynamic barriers or hydrodynamically active areas (HAA) of mountain rivers, where there is a significant oxygen saturation of the water flow, which activates the processes of biochemical and biological self-purification of surface waters. To verify the results of theoretical and field research, an existing experimental setup was designed and improved, which provided conditions for modeling the flow of mountain rivers in accordance with the laws of similarity theory and the principles of hydrodynamic modeling. Based on the results of field observations in the foothills of rivers, as well as regulatory and design documentation, the limits of the main operating factors were determined, namely the Freud number, flow depth and flow rate, which are decisive in studying the impact of HAA on flow self-cleaning processes. By changing the length of the tray section, the gullies and the waterfall niche of the real hydrodynamically active section were simulated, on which field experiments on the Tyshivnytsia River were conducted. In the studies, measurements were performed at different flow regimes, which simulated different hydrological parameters. To analyze the impact of HAA on self-cleaning processes, the flow was visualized using photography. The change of structure of a stream of natural waters at passage of HAA is established. The high oxygen saturation of the river waters of the Stryi river basin is caused by the influence of HAA, which contribute to the purification of polluted waters from biological pollutants and other man-made pollutants and form the high quality of water resources in the region. A method of modeling and visualization of mountain rivers in the laboratory has been developed.

Flow boiling provides an effective means of heat removal but can suffer from thermal and hydrodynamic transients that compromise heat transfer performance and trigger device failure. In this study, the transient flow boiling characteristics in two thermally isolated, hydrodynamically coupled parallel microchannels are investigated experimentally. High-speed flow visualization is synchronized to high-frequency heat flux, wall temperature, pressure drop, and mass flux measurements to provide time-resolved characterization. Two constant and two transient heating conditions are presented. For a constant heat flux of 63 kW/m2 into each channel, boiling occurs continuously in both channels and the parallel channel instability is observed to occur at 15 Hz. Time-periodic oscillations in the pressure drop and average mass flux are observed, but corresponding oscillations in the wall temperatures are virtually nonexistent at this condition. At a slightly lower constant heat flux of 60 kW/m2, boiling remains continuous in one of the channels, but the other channel experiences time-periodic flow regime oscillations between single-phase and two-phase flow. At this condition, extreme time-periodic wall temperature oscillations are observed in both channels with a long period (~7 s) due to oscillations in the severity of the flow maldistribution. For the transient heating conditions, square-wave heating profiles oscillating between different heat flux levels are applied to the channels. Because of their relatively high frequency, the heating transients are attenuated by the microchannel walls, resulting in effectively constant heating conditions and flow boiling characteristics like that of the aforementioned constant heating conditions. This study illustrates the susceptibility of parallel two-phase heat sinks to flow maldistribution, particularly when undergoing transient flow regime oscillations.

Reducing the consumption of secondary flow is of significance to future development of fluidic thrust vectoring (FTV) technology. A new coflow FTV control method based on sweeping jet (SWJ) excitat...

The effect of doping distilled water with MgSO4 on the boiling flow regime and heat transfer on a vertical surface at low pressures was experimentally determined. The experiments were performed in a sealed 20 mm x 20 mm square cross section, 300 mm long vertical pipe. Two opposing walls were made of copper and the other two walls were made of polycarbonate for flow visualization. The tube was filled to 100 mm and heat was applied at the lower end of one copper wall and cooled at the upper end. The boiling dynamics on the heated copper vertical wall and the resulting two phase flow was visualized using a high speed camera for pure distilled water and 0.1 M, 0.2 M and 0.4 M concentrations of MgSO4. The presence of MgSO4 significantly changed the boiling dynamics, with a suppression of bubble coalescence and a promotion of nucleate boiling. The two phase flow exiting the heated section changed from slug to bubbly with the addition of the MgSO4. There was a significant increase in the heat transfer coefficient with the addition of the MgSO4.

Delivering pesticide droplets accurately to a target has always been a challenge for crop spraying. Pesticide spray that drifts or runs-off the crop are major sources of soil and groundwater pollution and encourage pesticide overuse. This study optimised an agricultural air-induction nozzle to overcome these difficulties. Various configurations were designed and manufactured to investigate the geometrical parameters. A range of experiments were designed and carried out to simulate field conditions and examine spray structure, stability, droplet size range, and the probability of microbubbles to determine correlations between droplet size and spray drift and droplet rebound potential with or without entrained microbubbles. The performance of the designed nozzle was compared to that of commercial nozzles. Optical diagnostic methods, including high-speed photography, laser diffraction-based droplet sizing, and microscopic flow visualisation, were employed to perform accurate investigations of droplet size range and microbubble presence potential. In addition, the spatial distribution of spray influenced by lateral wind was investigated in an open-type wind tunnel and measured using a 2-dimensional patternator and water-sensitive papers. The rebound potential of the droplets was investigated using high-speed photography for artificial leaves. The results demonstrate the significance of geometrical design factors on the performance of nozzles and the effectiveness of induced air on the reduction of drift and rebound potential. Droplets including microbubbles were found to distribute across the surface of the target leaves on impact and exhibit increased rebound resistance.

The separation circuitry has been proven in the past to improve the performance of microchannel condensers. In the vertical second header of the condenser, liquid separates from vapor mainly due to gravity. However, separation is usually not perfect, expressed through the separation efficiency. This study presents the phase separation result in the second header calculated by the Euler-Euler method of Computational Fluid Dynamics (CFD). Simulations are conducted for two-phase refrigerant R134a flow in the second header with 21 microchannel tubes in the 1st pass. The inlet mass flux to the second header (through the microchannels of the 1st pass) in the simulation is 166 kg m − 2 s − 1, 207 kg m − 2 s − 1, and 311 kg m − 2 s − 1. The inlet quality is 0.13 to 0.21. The results agree well with the experimental results with flow visualization and the results of a simpler 1-D numerical model. Results show that the liquid separation efficiency decreases as the vapor separation efficiency increases, following a linear trend in the experimental range. The void fraction result shows liquid mainly flows in the half of the header without microchannel tube intrusions. The velocity profile in the header is presented and reverse flow is identified on the exit planes of the inlet section connecting to the 2nd-upper pass and the 2nd-lower pass. The pressure profile in the header is also revealed and it indicates that the 1-D pressure assumption may still apply to two-phase flow in a header.

The major purpose of this investigation was aimed at assessing the mixing capability of a coaxial system comprised of a central pitched blade impeller with a high rotating speed and a wall scrapping outer anchor with a low rotating speed in suspending highly concentrated slurries using flow visualization (electrical resistance tomography) and numerical (computational fluid dynamics) techniques. The 2D images acquired by tomography and simulation methodologies were utilized in creating solid concentration profiles. Different mixing indexes were employed to analyze the impacts of solids concentration gradients on the solids distribution. The effects of different factors namely solid concentration, average particle size, speed ratio, anchor speed, rotating mode, and solid specific gravity on the suspension and dispersion quality was analyzed. The results clearly revealed that the coaxial mixer provided better suspension quality at lower power utilization compared to that achieved by a single central impeller in dense suspension systems.

Turbulent jet flows at the Mach number equal to 0.25 from an industrial nozzle are studied using the large eddy simulation (LES). The feasibility of controlling the jet flow and noise by using chevrons is explored. A comparative study is carried out between the baseline and chevrons nozzles with the focus paid to the influences of the chevrons on the flow mixing and noise. The results show that the chevrons can significantly change the flow field. Especially the jet potential core length is reduced. In addition, the contours of velocity in the cross sections of the jet change from circular into daisy shaped structures by the chevrons. Flow visualization using the vorticity and the Q criterion shows that large vortical structures in the shear layer are fragmented into small-scale eddies. Meanwhile, the sound pressure level of the jet noises is weakened by chevrons.

The intraventricular blood flow changed by blood pump flow dynamics may correlate with thrombosis and ventricular suction. The flow velocity, distribution of streamlines, vorticity, and standard deviation of velocity inside a left ventricle failing to different extents throughout the cardiac cycle when supported by an axial blood pump were measured by particle image velocimetry (PIV) in this study. The results show slower and static flow velocities existed in the central region of the left ventricle near the mitral valve and aortic valve and that were not sensitive to left ventricular (LV) failure degree or LV pressure. Strong vorticity located near the inner LV wall around the LV apex and the blood pump inlet was not sensitive to LV failure degree or LV pressure. Higher standard deviation of the blood velocity at the blood pump inlet decreased with increasing LV failure degree, whereas the standard deviation of the velocity near the atrium increased with increasing intraventricular pressure. The experimental results demonstrated that the risk of thrombosis inside the failing left ventricle is not related to heart failure degree. The “washout” performance of the strong vorticity near the inner LV wall could reduce the thrombotic potential inside the left ventricle and was not related to heart failure degree. The vorticity near the aortic valve was sensitive to LV failure degree but not to LV pressure. We concluded that the risk of blood damage caused by adverse flow inside the left ventricle decreased with increasing LV pressure.

This study was conducted to investigate the flow and heat transfer characteristics of a flat plate located between two symmetrical cylinders. Flow visualization experiments were conducted based on ...

An experimental study on the traveling crossflow instability over a 60 $$^\circ $$ swept flat plate was conducted. The Mach number is 6, the angle of attack of the model is 5 $$^\circ $$ . The traveling crossflow waves and the secondary instability of the traveling crossflow waves were visualized by nano-tracer-based planar laser scattering (NPLS) technique. In the spanwise NPLS images, the traveling crossflow waves appeared as regular strikes, and the secondary instability appeared as small eddies attached to strikes. The wavelet transform was used to study the wavelength contents of the traveling crossflow waves. The most amplified wavelength is stable before the secondary instability happening, which is around 12 mm at $$Re_\infty $$ = 3.45 $$\times $$ 10 $$^6$$ m $$^{-1}$$ . Besides, the Reynolds number effects on the boundary layer transition and traveling crossflow instability were discussed.

Experiments were carried out to assess the effect of rear face geometrical modifications of a wall mounted rectangular cavity on pressure fluctuations at supersonic speed. Initially, tests were conducted on a base cavity of length to depth ratio of 3.6, in a 50 × 100 mm at a Mach number of 2, and a Reynolds number of 2 × 106 based on cavity length. Geometrical modifications to the rear face of the base cavity in the form of single ramp, double ramp and partial ramp circle were done to study their acoustic emission characteristics. Oil flow images revealed the complex nature of the flow inside the cavities. Time resolved flow visualization revealed attenuation of receptive interaction at the cavity leading edge, and minimized mass flux into the cavity by adopting these modifications. Unsteady measurements indicated reduction in the peak tones, broadband levels, and coherence. Among the modified geometries, double ramp configuration significantly reduced the overall sound pressure levels and the fluctuating pressures of the order of 10 dB and 50% respectively.

Efforts were made to examine the effect of textile roughness for the reduction of aerodynamic drag experienced on a cyclist model. First of all, flow visualization experiments were conducted in a w...

Flow visualization experiments were conducted in a water channel and a low-speed wind tunnel at Reynolds number of 1.54 $$\times$$ × 104 to 1.2 $$\times$$ × 105 for a leading-edge extension model, which is referred to as a NASA TP-1803 model in this study. In addition, particle image velocimetry velocity measurements were taken in the water channel to obtain the quantitative information about the three-dimensional velocity field over the strake and wing surfaces. The results obtained at low, medium and high angles of attacks represent three distinct cases of interaction between the strake and wing vortices. Namely, at α = 5o and 10° the strake and wing vortices were developed over the wing surface without significant interaction noticed; at α = 20°, the strake vortex strongly interacted with the wing vortex in an intertwining manner, which was sensitive to Reynolds number; at α = 30°, the breakdown of the strake vortex took place close to the junction of the strake and the wing; thus, the interaction of the strake and the wing vortices appeared to be less significant than the case of α = 20°. Graphic abstract

This research focused on furthering the current understanding of the flowfield inside a cavity under typical flight conditions. The flight envelope included altitudes of up to 40,000 ft and maximum...

Void fraction is one of the most important parameters that affect two-phase flow heat transfer and pressure drop. In this paper, a commercial gamma densitometer and a high-speed X-ray radiography system developed at the University of Michigan (UM) are used to measure the void fraction in two-phase boiling flows, with water as the working fluid, in a tubular test section. The test section is made of Incoloy 800H/HT with a total length of 1.589 m, an inner diameter of 12.95 mm, and a wall thickness of 3.05 mm. These two instrumentation systems are installed on a traversing platform that travels along the vertical test section to perform measurements at multiple elevations. Subcooled flow boiling and natural convection boiling experiments are performed to measure the void fraction in the test section. Flow visualization images are obtained for bubbly and slug flows from the X-ray radiography system. The wall temperature of the test section is measured at 17 elevations by thermocouples. In addition to the experiments, a multiphase computational fluid dynamics (MCFD) model is developed using ansysfluent to simulate the subcooled flow boiling. The measured wall temperature and void fraction from the experiments are compared with the MCFD simulation results. The root-mean-square (RMS) relative deviations are 3.6% and 16.1% for the wall temperature and void fraction, respectively, between the experimental data and MCFD simulations.

Smoke videos by layered concentration isosurfaces on a LES or URANS flow simulation

The drag reduction performance of a half airfoil attached to a trailer and the roof fairing at different positions was experimentally investigated in a wind tunnel. Half airfoils have NACA 0009 and NACA 0012 cross-section and the roof fairing position is changed from −4.5 mm to +4.5 mm. Smoke wire and surface oil flow visualization were also performed to elucidate the flow characteristic around the truck-trailer with drag reduction devices. While the drag reduction rate of the truck trailer with half airfoil is up to 10 % higher than that of alone truck-trailer at all Reynolds number ranges, drag reduction of the truck trailer with the half airfoil and the roof fairing was exceeded 30 % as compared to the alone truck trailer. Drag coefficient results indicated that the half NACA 0009 and NACA 0012 airfoil has an almost similar trend at the zero-roof fairing position. Laminar separation bubble formation was observed on the surface of roof fairing with the help of surface oil flow visualization.

Approaches for the experimental and numerical study of the distinctive features of fluid flow through media with double porosity that are based on micromodels of porous media and microfluidic technologies are proposed. The optic microscopy and tracer flow visualization methods are used to study experimentally fluid flow through model media. The numerical approach is based on the three-dimensional boundary element method accelerated with the use of the fast multipole method on heterogeneous computational architectures. The effect of structure parameters of micromodels on the flow distribution between the domains with different porosities is studied. Variation in the longitudinal and transvers velocity components in small and large pore channels is considered as functions of their geometric parameters and volumetric flow rate.

In this work, two-dimensional isothermal characteristics of material flow around the cylindrical tool pin during the friction stir welding (FSW) process are investigated by using both experimental and numerical approaches for the wide range of traverse speed (50–110 mm/min) and rotational speed (75–425 rpm) of the tool pin. The experimental results, obtained using particle image velocimetry (PIV) flow visualization technique, are supplemented with the computational fluid dynamics (CFD) modelling based on COMSOL Multiphysics. The rheological behaviour of experimental material with a rheometer yielded as non-Newtonian shear-thinning nature under the broader shearing (10−3–103 s−1) at the room temperature 25 °C. Both experimental and computational characteristics of material flow in terms of local velocity and strain rate profiles complement each other with the best level of accuracy and show an increase in both with increasing rotational speed. During FSW process, two flow zones, namely, rotational and transitional zones, are formed around the tool pin. The rotational zone is symmetric and has 1 mm width around the tool pin at 300 rpm and 50 mm/min. Finally, both PIV and CFD analyses have replicated an industrial process in an efficient manner. PIV technique has been successfully demonstrated for in-situ flow visualization of FSW process and overcoming the issues with earlier tracer-based techniques, but selecting a transparent material is a minor concern. The consistency established between the experimental and CFD results further provides that the CFD approach can efficiently and reliably be used to gain the precise mechanisms of the FSW process and to select the appropriate design and engineering parameters.

Hydrodynamic cavitating flows usually consist of 3-D intense vortical flows that are detached from solid boundaries. Detached vortical flows normally generate heaps of cavitating flow structures, which, in turn, govern the location of cavitation erosion before collapse. Thus, this study introduces a new numerical approach based on the improved delayed detached eddy simulation (IDDES) turbulence modeling for predicting cavitating flows. Then, the solution of compressible Eulerian-Eulerian two-phase flow and the IDDES turbulence model was linked to the microjet hypothesis and unsteady behavior of pressure and vapor volume to predict the corresponding erosion of cavitating flows. The method for cavitation erosion prediction, a modified version taken from previous studies, was applied as a post-processing tool. The validation of cavitating flow predictions was performed for the first time on the Grenoble axisymmetric nozzle by comparing them with 21 photos of cavitation from the previous experimental study. The results showed that the present numerical approach estimated various features of hydrodynamic cavitation well, including shedding processes and the length, shape, and collapse of cavitating structures. Using the numerical analysis, three main stages were detected for the present cavitating flow, and the vorticity-cavitation interactions were investigated by the vorticity transport equation. The streak-like and tube-like cavitating (STLIC and TULIC) structures were introduced in the second stage, initiated by flow instability, and entirely governed by corresponding turbulent flow structures. The collapse of these cavitating structures is one of the primary sources of cavitation erosion on lower and upper walls. The results of the numerical erosion predictions were compared with those of the previous erosion tests on the Grenoble axisymmetric nozzle. Satisfactory numerical performance was achieved in predicting the location and intensity of cavitation erosion.

In this work, we investigate how concepts from medical flow visualization can be applied to enhance stroke prevention diagnostics. Our focus lies on carotid stenoses, i.e., local narrowings of the major brain-supplying arteries, which are a frequent cause of stroke. Carotid surgery can reduce the stroke risk associated with stenoses, however, the procedure entails risks itself. Therefore, a thorough assessment of each case is necessary. In routine diagnostics, the morphology and hemodynamics of an afflicted vessel are separately analyzed using angiography and sonography, respectively. Blood flow simulations based on computational fluid dynamics could enable the visual integration of hemodynamic and morphological information and provide a higher resolution on relevant parameters. We identify and abstract the tasks involved in the assessment of stenoses and investigate how clinicians could derive relevant insights from carotid blood flow simulations. We adapt and refine a combination of techniques to facilitate this purpose, integrating spatiotemporal navigation, dimensional reduction, and contextual embedding. We evaluated and discussed our approach with an interdisciplinary group of medical practitioners, fluid simulation and flow visualization researchers. Our initial findings indicate that visualization techniques could promote usage of carotid blood flow simulations in practice.

A novel model is proposed to estimate the optimal propulsive efficiency and the optimal dimensionless heaving velocity of a flapping foil based on a lift–drag ratio under the assumption of negligib...

No abstract available.

Purpose This paper aims to present new analytical model for the filling times prediction in flip-chip underfill encapsulation process that is based on the surface energetic for post-bump flow. Design/methodology/approach The current model was formulated based on the modified regional segregation approach that consists of bump and post-bump regions. Both the expansion flow and the subsequent bumpless flow as integrated in the post-bump region were modelled considering the surface energy–work balance. Findings Upon validated with the past underfill experiment, the current model has the lowest root mean square deviation of 4.94 s and maximum individual deviation of 26.07%, upon compared to the six other past analytical models. Additionally, the current analytically predicted flow isolines at post-bump region are in line with the experimental observation. Furthermore, the current analytical filling times in post-bump region are in better consensus with the experimental times as compared to the previous model. Therefore, this model is regarded as an improvised version of the past filling time models. Practical implications The proposed analytical model enables the filling time determination for flip-chip underfill process at higher accuracy, while providing more precise and realistic post-bump flow visualization. This model could benefit the future underfill process enhancement and package design optimization works, to resolve the productivity issue of prolonged filling process. Originality/value The analytical underfill studies are scarce, with only seven independent analytical filling time models being developed to date. In particular, the expansion flow of detachment jump was being considered in only two previous works. Nonetheless, to the best of the authors’ knowledge, there is no analytical model that considered the surface energies during the underfill flow or based on its energy–work balance. Instead, the previous modelling on post-bump flow was based on either kinematic or geometrical that is coupled with major assumptions.

Complementary heat transfer and flow visualization experiments were performed lo investigate the effect of yaw on tube. Wind tunnel experiments encompassing seven angles of yaw between ᶱ= o( crossflow ) and 28° yielded Nusselt numbers extending over the range of free stream Reynolds numbers from 9000 to 25000. It was found that as the cylinder was yawed with respect to the crossflow orientation, the Nusselt number at first increased, attained a maximum at a yaw angle o 5o and then decreased to a broad minimum which occurred in the range of yaw angles between 20 and 25o and then start to somewhat increase at yaw angle equal 28o. As a further supplement to the heat transfer experiments, the pattern of fluid flow adjacent to the cylinder surface was visualized by means of the oil lampblack technique. The results were compared with the available experimental data. The values of the average Nusselt numbers were fitted in a correlation. This correlation indicates that the Nusselt number is a significant function of Reynolds number and angle of yaw.

To predict the boundary layer transition on swept wings, experimental and numerical researches were conducted to obtained the accurate boundary layer transition front and to evaluate the performance of two transition predicting models. Flow visualization and temperature measurement over a 65° swept wing were carried out under M = 6 with nano-tracer-based planar laser scattering system and temperature sensitive paints. Meanwhile, the kT–kL–ω and γ–Reθt turbulence models were used to predict the boundary layer transition. Experiment results reflect that the transition front on the wing consists of three parts, including the transition near the wing root induced by separation, the transition dominated by stationary crossflow vortices and the transition in the connected region. The comparison between the simulation and experiment shows that the kT–kL–ω model performs better than γ–Reθt model in predicting the shape and position of the transition front, as well as the limiting streamlines. Thus, the kT–kL–ω model is more suitable in predicting the boundary layer transition for this type of swept wing.

The gas flow hydrodynamics in a vortex mixer was investigated using flow visualization and particle image velocimetry (PIV) techniques for several Reynolds numbers ranging from Re = 20 to 280 and equal momentum of the jets. The flow regime transition of two methane gas streams coming from two tangential jets was analyzed by the detection of the critical Reynolds number of the flow. This critical point was determined from time-averaged and instantaneous flow images, velocity fluctuations, and hence turbulence intensity values of the vortex mixer. Results show that the flow gained the swirl motion at Re = 70 and the instabilities in the flow started at around Re = 230, where the gas flow hydrodynamics was diffusion controlled. Beyond Re = 230, the flow was unsteady and chaotic flow overruled the molecular diffusion and the ensemble mixing of the gases dependent on the convection was achieved starting from this point. This study sheds light on the gas flow hydrodynamics of a vortex mixer and points out the flow regime transitions.

In order to find out the effect mechanism of the cavitation on the hump characteristic of pump-turbines, three dimensional numerical simulations with and without cavitation were carried out using the SST (shear stress transport) k-ω turbulence model and ZGB (Zwart-Gerber-Belamri) cavitation model. Numerical results agreed well with the performance characteristic and flow visualization results. Under cavitation conditions, the simulations with cavitation obtained a more accuracy prediction on the hump characteristic. The relationship between the performance characteristics and the cavitation was analyzed in details. A cavitation condition (cavitation coefficient is 0.14), under which the cavitation just occurs, was selected to investigate the hump characteristic. The occurrence position of the cavitation and the variation law with the cavitation coefficient were obtained. The hump characteristic is related to the cavitation on suction surfaces close to the leading edges of the blades at the runner inlet. The action mechanism of the cavitation on the change of the hump characteristic was revealed. The sudden increase of the cavitation in the hump region leads to the decrease in the Euler head and the increase in the hydraulic loss, resulting in the drop of the head. Detailed analyses reveal that the occurrence of the cavitation will block the runner passages, which reduces the working ability of the runner and induces large size vortices in the adjacent passages.

A high-fidelity analysis is carried out in order to evaluate the effects of blade shape, airfoil cross-section. as well as twist angle distribution on the yielded torque and generated power of a horizontal axis Small-Scale Wind Turbine (SSWT). A computational modeling and an effective design for a small turbine with a blade length of 25 cm subject to a 4 m/s freestream velocity are presented, in which a segregated RANS solver is utilized. Four airfoil profiles are assessed, namely NACA0012, NACA0015, NACA4412, and NACA4415, and two blade shape configurations, rectangular and tapered, are evaluated. The flow around the rotating turbines is investigated along with blade stresses and performance output for each configuration. Subsequently, the impact of various linear and nonlinear twist distributions on SSWT efficiency is also examined. Results show that for the studied operating conditions corresponding to low-speed flows, the rectangular blade configuration outperforms the tapered blade shape from the generated torque and power perspectives, while the tapered shape configuration represents an attractive design choice from the yielded stresses point of view. Additionally, while the nonlinear twist configuration results in the best performance among the configurations studied, an SSWT blade design implementing a linear twist distribution can be highly competitive provided that a good slope is carefully selected.

In the gas turbine, gaps inevitably exist between the stationary and rotating parts. To improve the performance of gas turbines, tip leakage flow through the gaps must be minimized. The labyrinth seal increases the flow resistance by placing a number of teeth to minimize leakage flow. The labyrinth seal is still adopted in many fields because of its relatively high thermal resistance, simple structure, and wide pressure range. In this paper, the effects of geometric parameters (straight and stepped with solid land, tip clearance, number of teeth, and tooth front angle) on leakage flow were experimentally studied and the seal performance was described using the flow function. Also, flow visualization within the labyrinth seal was conducted using the Schlieren method. The dimensionless tip clearance (C/a) was adjusted from 0.8 to 3.3 (where a is tooth tip width) and the pressure ratio was adjusted from 1.1 to 3.0. The tooth front angle of the stepped seal was 90° and 60°. Results show that the flow function increased as the pressure ratio increased, however it tended to remain constant if the pressure ratio was higher than 2.0. At the same test condition, the flow function of the stepped labyrinth seal was smaller than that of the straight seal for all tested pressure ratios. Also, the seal performance was better for larger number of teeth, smaller tip clearance, and smaller tooth front angle cases. Based on the measured results, a new correlation for the labyrinth seals was suggested.

An experimental study is conducted on the qualitative visualization of the flow field in separation and reattachment flows induced by an incident shock interaction by several techniques including shear-sensitive liquid crystal coating (SSLCC), oil flow, schlieren, and numerical simulation. The incident shock wave is generated by a wedge in a Mach 2.7 duct flow, where the strength of the interaction is varied from weak to moderate by changing the angle of attack α of the wedge from 8° and 10° to 12°. The stagnation pressure upstream was set to approximately 607.9 kPa. The SSLCC technique was used to visualize the surface flow characteristics and analyze the surface shear stress fields induced by the initial incident shock wave over the bottom wall and sidewall experimentally which resolution is 3500 × 200 pixels, and the numerical simulation was also performed as the supplement for a clearer understanding to the flow field. As a result, surface shear stress over the bottom wall was visualized qualitatively by SSLCC images, and flow features such as separation/reattachment and the variations of position/size of separation bubble with wedge angle were successfully distinguished. Furthermore, analysis of shear stress trend over the bottom wall by a hue value curve indicated that the relative magnitude of shear stress increased significantly downstream of the separation bubble compared with that upstream. The variation trend of shear stress was consistent with the numerical simulation results, and the error of separation position was less than 2 mm. Finally, the three-dimensional schematic of incident shock-induced interaction has been achieved by qualitative summary by multiple techniques, including SSLCC, oil flow, schlieren, and numerical simulation.