Flow Visualization Measurements Research Articles

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.

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.

Influence of dispersed phase flow-rate pulsations on the liquid–liquid parallel flow in a T-junction microchannel

Active flow control in microfluidic devices to broaden the range of desired conditions is a significant practical concern. In this study, we experimentally investigate liquid–liquid parallel flow under the dispersed phase flow-rate pulsations in a T-junction microchannel. Sinusoidal flow rate disturbances with variations in amplitude and period were applied to dispersed phases of different viscosities. The study involves flow visualization and velocity field measurements using the micro-PIV technique for both free and excited flow conditions. The analysis of flow pattern maps, velocity fields, velocity gradients, and phase-averaged velocity profiles in a T-junction region and far downstream provided insights into the evolution of disturbances, which is different in low- and high-viscosity liquids. In the less viscous liquid, transverse waves amplify longitudinal ones due to relaxation of the liquid–liquid interface, while in the high-viscosity dispersed phase, longitudinal waves are damped significantly, with the transverse wave amplitude remaining almost constant. As a result, we revealed two distinct mechanisms of disturbance wave propagation and the loss of parallel flow stability, which are both dependent on the Ohnesorge number of the dispersed phase. Destabilization of the parallel flow led to the formation of plugs with a narrow length distribution. Single-mode, double-mode, triple-mode, and multi-mode plug formation regimes were identified. The results of the study can potentially be used to extend the range of segmented flow regimes and generate plugs of a desired length.

Design of a Compressor Test Rig for Immobilization of the Stall Cell

Capturing internal flow experimentally presents significant challenges due to the asymmetric propagation and strong unsteadiness of a stall cell in the circumferential direction. In this study, a low-speed counter-rotating axial compressor test rig was designed based on a counter-rotating compressor to immobilize a stall cell and measure its internal flow characteristics. Determining an appropriate speed ratio that is capable of stabilizing the circumferential position of the stall cell in the counter-rotating compressor enabled visualization measurements to be conducted successfully. The preliminary results demonstrated the successful immobilization of the stall cell using an appropriate rotating speed ratio. Furthermore, the oil flow visualization measurements confirmed the presence of a distinct stall cell structure on the casing wall. This work represents an innovative approach towards immobilizing stall cells in axial compression systems.

Synergetic benefits of microfluidics using artificial cilia and ZnO/SnFe2O4 for the degradation of pollutants

Water pollution due to the discharge of organic dye pollutants is a significant environmental concern. To mitigate this environmental threat, microfluidic devices are employed as a promising platform that provides innovative methods and perspectives in achieving effective environmental remediation. Meanwhile, the hydrodynamics of the fluid flow inside such miniaturized systems can also play a crucial role in the pollutant eradication process that can further establish a reliable framework for accomplishing the most promising treatment strategies. Herein, the microfluidic device comprised of ZnO/SnFe2O4 nanoparticles and artificial cilia was proposed to incorporate two essential functions in adequately adsorbing the cationic dye (methylene blue, MB) suspended in the aqueous medium. Amongst these, the employed ZnO/SnFe2O4 nanoparticles acted as an active adsorbent site. Eventually, the cationic MB dye was found to be adsorbed by these nanoparticles due to the presence of a negative surface charge potential. Further, to enhance this adsorption process, the employed artificial cilia were magnetically controlled to exhibit a tilted conical asymmetric motion. Consequently, such dynamic motion of the cilia was observed to enhance the hydrodynamics aspect of the dye-aqueous solution. Essentially, the rate of mass transfer of the dye-aqueous solution toward the absorbent site was improved. Thus, the dye-adsorption efficiency achieved through the proposed synergetic platform was estimated to be 90.2% at the end of 30 min. In comparison with the conventional motions of the artificial cilia, this obtained result with the actuated dynamic motion was found to be enhanced by 12.1%. Further, to aid an efficient adsorption process through the hydrodynamic principles, a flow visualization measurement was performed to identify the optimal design parameters of artificial cilia. Eventually, the spatial representative parameters of artificial cilia were observed to significantly influence the dye-adsorption behavior. The attained expertise from this study will be potentially advantageous for future dye treatment applications in designing several other distinct synergetic components for effective pollutant removal.

Boundary layer suction timing and location effects on unstarting flows in a model scramjet

The effects of the suction timing and location on inlet unstart were investigated to determine the necessary response time and appropriate location of a flow control device. Inlet unstart is one of the causes for scramjet-powered engine failure, and boundary layer suction could suppress or delay the phenomenon. A model scramjet having a rectangular cross-section was tested in a Mach 4.5 freestream via nitrogen-jet-injection-induced inlet unstart. High-speed schlieren imaging and pressure measurements were performed to study the characteristic behaviors of the unstarting and unstarted flows. Suction extracted approximately 1–2% of the freestream flow depending on the test conditions. Boundary-layer suction successfully prevented inlet unstart with the most upstream suction location for the jet momentum ratio near the unstart threshold condition but lost the control authority as the suction delay time increased. Suction was not able to prevent unstart but was delayed with downstream suction locations, and a similar deterioration in the suction control performance was observed an increasing suction delay time. For a jet momentum 10% higher than the threshold, suction could not prevent unstart but suppressed the propagation speed of the unstart shockwave. Fluctuations in the wall pressure were significantly reduced with suction. For all tested cases, better control performance was achieved with a faster suction trigger timing. The results from the flow visualization and pressure measurements indicated that suction must be applied before the unstart shockwave passes through the suction location to achieve effective flow control.

A New Method to Develop Homogeneous and Heterogeneous Porous Micromodels Applicable to Enhanced Oil Recovery and Flow Visualization Experiments

AbstractIn this paper, a new method to fabricate micromodels having homogeneous and heterogeneous porous structures is reported to gain fundamental insight into the flow through porous media. The technique of microparticle image velocimetry (PIV) is used to map the pore-scale velocity field inside the micromodels. A thin perforated metal sheet composed of uniformly distributed circular holes is used as the master pattern, and the replica of the negative of this perforated sheet is transferred to a polydimethylsiloxane (PDMS) substrate using a method similar to the soft lithography. This method allows an efficient fabrication of micromodels having different porosity by adjusting and selecting the perforated sheets of different hole sizes. The prepared micromodels were tested for its applicability and reliability by carrying out the measurements of pore-scale velocity distribution using the micro-PIV technique. The experiments with micromodels with high porosity but different grain arrangements showed qualitative as well as quantitative differences in the velocity field. The pressure drop across the two ends of micromodel is also measured. The variation of pressure difference with the flowrate is found to be nonlinear with a significant effect on the patterns of micropillars. However, at low porosity, the variation of pressure difference with the flowrate is found linear and there is almost no influence of the micropillar patterns. The flow visualization measurements are also conducted with the dual porosity micromodels, and the flow patterns were examined by analyzing the velocity vector maps.

Friction dynamics of elasto-inertial turbulence in Taylor-Couette flow of viscoelastic fluids.

Dynamic properties of elasto-inertial turbulence (EIT) are studied in a Taylor-Couette geometry. EIT is a chaotic flow state that develops upon both non-negligible inertia and viscoelasticity. A combination of direct flow visualization and torque measurement allows to verify the earlier onset of EIT compared with purely inertial instabilities (and inertial turbulence). The scaling of the pseudo-Nusselt number with inertia and elasticity is discussed here for the first time. Variations in the friction coefficient, temporal frequency spectra and spatial power density spectra highlight that EIT undergoes an intermediate behaviour before transitioning to its fully developed chaotic state that requires both high inertia and elasticity. During this transition, the contribution of secondary flows to the overall friction dynamics is limited. This is expected to be of great interest in the aim of achieving efficiency mixing at low drag and low but finite Reynolds number. This article is part of the theme issue "Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical transactions paper (Part 2)".

Thermo-fluid dynamics and synergistic enhancement of heat transfer by interaction between Taylor-Couette flow and heat convection.

This study experimentally and numerically investigated the thermo-fluid dynamics of Taylor-Couette flow with an axial temperature gradient from the chemical engineering perspective. A Taylor-Couette apparatus with a jacket vertically divided into two parts was used in the experiments. Based on the flow visualization and temperature measurement for glycerol aqueous solutions with various concentrations, the flow pattern was classified into six modes: heat convection dominant mode (Case I), heat convection-Taylor vortex flow alternate mode (Case II), Taylor vortex flow dominant mode (Case III), fluctuation maintaining Taylor cell structure mode (Case IV), segregation between Couette flow and Taylor vortex flow mode (Case V) and upward motion mode (Case VI). These flow modes weremapped in terms of the Reynolds and Grashof numbers. Cases II, IV, V and VI are regarded as transition flow patterns between Case I and Case III, depending on the concentration. In addition, numerical simulations showed that in Case II, heat transfer was enhanced when the Taylor-Couette flow was altered by heat convection. Moreover, the average Nusselt number with the alternate flow was higher than that with the stable Taylor vortex flow. Thus, the interaction between heat convection and Taylor-Couette flow is an effective tool to enhance heat transfer. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (Part 2)'.

An experimental study of convective flow in a bottom heated cavity with ventilation duct - Laboratory modeling of convective flow in under coal gasification cavity

Convective flow in a bottom heated cavity with a ventilation duct is experimentally investigated using flow visualization and temperature measurement. Flow visualization reveals that convective flow in the cavity is dominated by natural convection on the bottom wall or mixed convection of the air jet from the duct and the plume on the bottom wall. Convective flow by natural convection does have a transition from symmetrical steady to unsteady states with an increase of the Rayleigh number (Ra), and even becomes chaotic if the Rayleigh number is sufficiently large for e.g., Ra = 3 × 106. Additionally, convective flow by mixed convection is characterized under different flow regimes, which are determined by the Rayleigh number and Reynolds number (Re). The dependence of the horizontal velocity above the bottom wall on the Rayleigh number and Reynolds number is quantified. Further, heat transfer by natural convection and mixed convection has been described in which the relationship between Nu and Ra is obtained.

Evaluation of Arterial Signal Coordination with Commercial Connected Vehicle Data: Empirical Traffic Flow Visualization and Performance Measurement

Emerging connected vehicle (CV) data sets have recently become commercially available. This paper presents several tools using CV data to evaluate traffic progression quality along a signalized corridor. These include both performance measures for high-level analysis as well as visualizations to examine details of the coordinated operation. With the use of CV data, it is possible to assess not only the movement of traffic on the corridor but also to consider its origin-destination (OD) path through the corridor. Results for the real-world operation of an eight-intersection signalized arterial are presented. A series of high-level performance measures are used to evaluate overall performance by time of day, with differing results by metric. Next, the details of the operation are examined with the use of two visualization tools: a cyclic time-space diagram (TSD) and an empirical platoon progression diagram (PPD). Comparing flow visualizations developed with different included OD paths reveals several features. In addition, speed heat maps are generated, providing both speed performance along the corridor. The proposed visualization tools portray the corridor performance holistically instead of combining individual signal performance metrics. The techniques exhibited in this study are compelling for identifying locations where engineering solutions are required. The recent progress in infrastructure-free sensing technology has significantly increased the scope of CV data-based traffic management systems. The study demonstrates the utility of CV trajectory data for obtaining high-level details of the corridor performance and drilling down into the minute specifics.

Experimental study on pressure characteristics and flow coefficient of butterfly valve

Air entrainment and self-aeration flow are phenomena that can generate fluctuation in pressure and induce disadvantages in piping systems. In this study, the mechanism of air bubble formation in Partially Filled Pipe Flow (PFPF) is investigated through a series of experiments on a butterfly valve with flow visualization and pressure measurement. This paper provides a description of the formation of bubbles induced by a butterfly valve and analyzes the characteristics of the pressure and flow coefficient of the valve. The pressures at various locations on the pipeline adjacent to the valve were measured to compare with the positions recommended by the International Society of Automation standard. The results indicated that in PFPF, the differences in the pressure between measured points downstream are noticeable, notably at valve openings less than 50°. The bubbles created at the valve, which lead to the highly-disordered aerated flow downstream, might induce the differences. The pressure drop and the flow rate across the butterfly valve in 100% water fraction flow are respectively 3–7.5 and 2–9 times higher than those in 90% WF. The flow coefficient of the 3-inch butterfly valve was found to increase linearly with the Reynolds number before leveling off when the Reynolds number exceeds 6×104. Besides, the flow coefficients in 100% water fraction flow are double those in 90% water fraction flow.

Observations of pumping and vortex dynamics due to a cylinder oscillating normal to a plane wall

Understanding the fluid dynamics associated with a circular cylinder oscillating normal to a plane wall is important for safe design of offshore infrastructure, such as power cables and pipeline risers. This paper investigates the fluid dynamics of an oscillating cylinder with no imposed incident current experimentally using flow visualisation and force measurements where the ratio of the cylinder Reynolds number ( $Re$ ) to Keulegan–Carpenter number ( $KC$ ) is $\beta =500$ and $KC$ varies between 2 and 12. The minimum distance between the cylinder and wall was between 12.5 % and $50\,\%$ of the diameter. Across this parameter space three primary vortex flow regimes were observed: (i) for $KC\leq 5$ , the flow field is approximately symmetric about the cylinder centreline and the velocity field between the cylinder and the wall resembled a pumping flow in phase with cylinder motion, which is well predicted by potential theory for most of the cycle; (ii) for $5< KC<8$ , the flow field is increasingly asymmetric but with frequent switching of the side associated with vortex shedding; and (iii) for $KC\geq 8$ , the flow field is consistently asymmetric due to vortex shedding. The in-line force increases when the cylinder is near the wall due to dynamic pressures associated with pumping. This increase can be estimated using potential theory superimposed onto the force time history for an isolated cylinder at the same $KC$ and $Re$ . This study complements recent numerical modelling focused on low Reynolds number conditions and provides important insights into the fluid mechanics associated with trenching beneath cable and pipeline risers.

Vortex flow affected by disk rotation in an air jet impinging onto a confined heated horizontal disk

An experiment combining flow visualization and temperature measurement is carried out to explore the effects of disk rotation on the vortex flows induced in a round jet of air impinging onto a heated horizontal disk confined in a vertical cylindrical chamber. The experiment is conducted for the jet-disk separation distance at 20.0 mm and 40.0 mm. The jet flow rate is varied from 0 to 5.4 slpm (standard liter per minute) for the jet Reynolds number Rej ranging from 0 to 730. The disk rotation speed varies from 0 to 35.0 rpm for the rotational Reynolds number ranging from 0 to 2725. The temperature difference between the disk and the air injected into the chamber is varied from 0 to 25.0°C for the Rayleigh number ranging from 0 to 150,325. The data from the present study for the ratio of jet disk-disk separation distance to the jet diameter (denoted as HDj) are compared. The results indicate that at the disk rotation rate Ω ≥ 10.0 rpm the secondary inertia-driven roll is completely wiped out for HDj = 4. For HDj = 2 at a higher disk rotation rate of Ω ≥ 20.0 rpm, the buoyancy-driven roll can be substantially squeezed by the disk rotation to become smaller and weaker. Meanwhile, the primary inertia-driven roll is stretched out to become slender and weaker. But for HDj = 4, due to a stronger buoyancy-driven vortex flow at a higher jet-disk separation distance, a higher Ω is needed to stabilize the buoyancy-driven roll.

Subsonic jet noise source location as a function of nozzle exit boundary layer

Over the years, there have been numerous studies on determining subsonic jet noise source locations, typically plotted as Strouhal number as a function of distance from the nozzle exit. A comparison of the results of various studies yields a spread of about two nozzle diameters in measured source location. This work examines how boundary layer thickness, which can vary from nozzle to nozzle, could be the cause of observed differences in different studies in subsonic jet noise source location. Source location measurements of unheated jets from ASME nozzles, which have comparably thinner nozzle exit boundary layers, and conical nozzles, which have comparably thicker nozzle exit boundary layers, are compared. These results are substantiated with the use of schlieren flow visualization and velocity profile measurements. It is found that the nozzles with thinner nozzle exit boundary layers have noise source distributions that are 0.25–2 diameters upstream of those with thicker nozzle exit boundary layers. Thinner nozzle exit boundary layers result in higher growth rates of instability waves, increasing mixing and thereby moving noise sources upstream.

Active flow control in an S-shaped duct at Mach 0.4 using sweeping jet actuators

An experimental study was conducted on the flow control capability of sweeping jet actuators (SJAs) in an aggressive offset S-shaped duct with an inlet Mach number of 0.4. The SJAs were placed in two different spacing arrangements to suppress flow separation in the duct and flow distortion at the aerodynamic interface plane (AIP). Multiple measurements were taken, including surface oil flow visualization, steady pressure on the wall surface, five-hole probe pressure, and velocity at the AIP. For the baseline case without control, the surface oil flow visualization and pressure measurement results clearly reflected the three-dimensional flow separation characteristics in the duct. In the controlled cases, the streamwise surface pressure coefficient distribution showed that SJAs can effectively suppress the flow separation. At a spacing ratio of 8.13 and an excitation pressure of 250 kPa, the inflection point of the pressure coefficient Cp was completely eliminated. As a result, the absolute pressure recovery at the upper region of the AIP increased by nearly 5%, and the mean value and RMSE of the cross-flow velocity declined by 54.9% and 48.1%, respectively.

Flow Visualization and Particle Dispersion Measurements Inside an Ambulance Rear Saloon while Stationary and in Motion

Flow Visualization and Particle Dispersion Measurements Inside an Ambulance Rear Saloon while Stationary and in Motion

Flow visualization and aerosols in performance

The COVID-19 pandemic has created a great deal of fear and uncertainty around whether aerosols from singers and musical instruments can transmit the virus. The World Health Organization has recognized that SARS-COV-2 is transmissible through aerosols. Outbreaks from choir performances, such as the Skagit Valley Choir, showed that singing brings potential risk of COVID-19 infection. Additionally, outbreaks from singing have resulted in superspreader events leading to many infections and deaths. There is less known about the risks of airborne infection from other musical performance, such as playing wind instruments or performing theater. Hence, aerosol generation in performance (playing brass and woodwind instruments, singing, and theater speech delivery) should be quantified, monitored, and mitigated. To tackle this issue our research questions were as follows: (i) What is the aerosol generation rate? (ii) How does air flow from the performer’s mouth/instrument and how does it disseminate into the environment? and (iii) What control methods can be employed to mitigate risk? In this study, we used a variety of methods, including flow visualization, aerosol and CO2 measurements, and computational fluid dynamics (CFD) modeling to understand the different components that can lead to transmission risk from musical performance and risk mitigation.

Flow Field Study of Effect of Canard Location on Aircraft Aerodynamic Performance

AbstractThe present study investigates the flow behavior due to three different vertical canard locations on the scaled‐down model aircraft by using tuft flow visualization and boundary layer measurements. The aircraft is fabricated with wing airfoil S1223 modified and canard with N22 airfoil using laser cutting technique. Three different color tufts are pasted in wing and canard at 30%, 60%, and 90%. The assembled model is placed inside the subsonic wind tunnel with flow chord Reynolds number 3.8*106, which corresponds to 35 m s−1 of the flow velocity and zero‐degree angle of attack. The boundary layers are measured at 70% of the wing chord with 00 incidence angle at three different span locations, i.e., 30%, 60%, and 90% of the wingspan with varying vertical canard positions. The results are compared with wing alone and it is found that the high wing and high canard configuration are outperformed by providing minimum velocity gradient and hence minimum skin friction drag. The tuft flow visualization also shows the identical results as the boundary layer measurements results. The aerodynamic performance of the high wing high canard configuration is found to be promising, as the skin friction drag coefficient is found to be minimum.

Torque scaling at primary and secondary bifurcations in a Taylor–Couette flow of suspensions

The Taylor–Couette flow of non-colloidal, neutrally buoyant spherical particle suspensions in the $\phi =0\,\%-28\,\%$ concentration range and (17–250) Reynolds number ( $\mathcal {R})$ range is studied using synchronized flow visualization and torque measurements. Both methods are applied in ramp-up/down (acceleration/deceleration of the inner cylinder) experiments to detect the various flow structure states and bifurcation natures, their critical conditions and their lifetime in $\mathcal {R}$ range. Torque measurement allows us to discuss the evolution of the (pseudo) Nusselt number, $\mathcal {N}$ , and friction coefficient with $\mathcal {R}$ or alternatively the Taylor number, Ta. Flow visualization brings additional information on the unsteady dynamics of flow states. For concentrations higher than $\phi =6\,\%$ , two unsteady (spiral vortex flow, wavy vortex flow) and one steady (Taylor vortex flow) flow state are observed in both ramp-up/down experiments. Hysteretic behaviour is reported for some primary, secondary and tertiary bifurcations, which are thus found to be subcritical. A critical concentration is observed at $\phi =15\,\%$ for the range of $\mathcal {R}$ at which spiral vortex flow is encountered. Characteristic frequencies of unsteady flow state (spiral vortex flow, wavy vortex flow) for different suspension concentrations are evaluated. Finally, three hydrodynamic concentration subregimes are identified for the first time, with their distinct sets of concentration-dependent critical conditions, torque scaling exponents and friction coefficients.