Smoke Visualization Research Articles

Mimicking the Helix with a porous disc for wind tunnel testing

A promising method to reduce wake effects in offshore wind farms is the Helix approach, which increases the mixing of the wake with the surrounding flow by exciting the individual blade pitch. This increases the wind speed in the wake, resulting in a higher power output at a downstream turbine. Wind tunnel testing is crucial to gather further understanding of the governing mechanisms behind the Helix and its efficiency in larger wind farm arrays. However, model turbines are expensive and complex. Porous Discs (PD) have proven to supply a less expensive and less complex alternative for wake-focused wind tunnel studies. In this study we present a novel PD model to mimic the Helix. The fundamental idea is to mimic the non-uniform, unsteady energy extraction over the rotor plane as observed at a Helix-controlled turbine. For this purpose, we derive a non-uniform porosity distribution over the PD, based on Large Eddy Simulations of a three-bladed turbine controlled with the Helix approach, and the actuator disc theory. The resulting non-uniform PD rotates at the excitation frequency described by the Strouhal number to mimic the Helix. We verified the novel experimental setup with smoke visualisation techniques and thrust measurements at a second PD in the wake and observed the typical characteristics of the Helix wake of a model turbine: First, the wake was deformed into a helical shape, and second, the wake velocity increased depending on the excitation Strouhal number.

Experimental characterization of exhaled flow dynamics of human breathing and vocalization

During the onging pandemic of COVID-19, there are numerous asymptomatic patients who are infectious. The exhaled droplets from their daily respiratory activities like breathing or speaking can be the sources of airborne disease transmission of COVID-19. The understanding of the airflow dynamics of these respiratory activities may be helpful to develop effective measures to prevent and control the spread of the disease. In this study, the exhaled flows from human breathing and vocalization of specific syllables are characterized using particle image velocimetry (PIV) and smoke visualization. The exhaled flow generated by ten phonemes including vowels, fricatives, affricates, plosives and nasals as well as mouth breathing are studied. The dynamic developments of the airflow processes are described by a series of parameters, including peak velocity, peak velocity time, duration time, propagation velocity and distance. Results show that vocalization of affricates and plosives as well as mouth breathing tend to have higher peak velocities and propagation distances. The evolutions of exhaled flows generated from these respiratory processes are found to have different jet structures, which are related to the stroke ratio (L/D). The flow field of a small L/D only has a pair of dominant vortices. Whereas that of a large L/D presents both a pair of dominant vortex ring and a trailing jet. Certain phoneme (e.g., /t/) is found to display a two-stage jet similar to a cough with the starting jet and an interrupted jet. The characterization of human exhaled flow of this study may be helpful to provide basis of CFD simulations and a better understanding of the spread of airborne diseases from human breathing.

Design of Low Subsonic Wind Tunnel with Open Return System for Testing Wind Turbines at Low Airspeeds

The The Republic of Indonesia is rich in potential for renewable energy, including abundant wind energy. This study aims to design a subsonic open return wind tunnel for testing wind turbines at low airspeeds. The testing focus includes the evaluation of blade efficiency, bearing performance, and other aspects. Testing at low airspeeds (<5 m/s) is highly relevant to the wind conditions in Indonesia. The design process utilizes Computer Aided Design (CAD), while data collection and analysis are conducted through Computer Aided Engineering (CAE) simulations and theoretical calculations. The wind tunnel comprises components such as contraction, honeycomb, test section, diffuser, and support structure. Airflow over the turbine blades can be observed using smoke visualization in the test section. This research is expected to provide practical contributions to the development of low-speed wind turbines in Indonesia.

Optimization Study of Inert Gas Distribution for Multiple-Bay Fuel Tank

Inert gas distribution has a great influence on the inerting effect, especially for the multiple-bay fuel tank. In order to find out the optimal scheme, an optimization method based on the entropy-weight improvement TOPSIS method is proposed, and an experimental system of inert gas distribution is established to measure the speed index and uniformity index. The results show that the position of the inlet and outlet has a significant effect on the overall inerting effect. The inerting scheme designed by the entropy-weight improvement TOPSIS method can not only reduce the flow demand of inert gas but also make the oxygen distribution more uniform. The optimization inerting scheme of the Boeing 747 aircraft has improved the average speed index by 3.01% and the average uniformity index by 26.18%. The smoke visualization experiment also showed that the scheme designed by the entropy-weight improvement TOPSIS method has the denser white smoke, which means that the scheme has better performance.

Natural convection in vertical solar heat collector nanotextured with reduced graphene oxide and silver nanowires

In this study, a vertical shaft is coated using silver nanowires (AgNWs) together with reduced graphene oxide (rGO) via the supersonic spray-coating technique to enhance the absorption of solar radiative heat for use in solar air heaters. The rGO/AgNW-coated surface induces multiple light reflections, similar to that in a perfect black body inside a Helmholtz jar, thus enhancing the collection of solar radiation. Air fed into the vertical heated shaft ascends owing to buoyancy. The air temperature difference, ΔT, between the outlet and inlet is measured for various mass flow rates to quantify the heat transferred to the air from the solar-heated vertical shaft. The longitudinal air velocity and temperature distributions inside the shaft are numerically simulated using the fire dynamics simulator. Both two- and three-dimensional simulations are performed, and the results are compared with experimental data for various mass flow rates. The results confirm that the trends observed in both the experiments and simulations agree well for all cases. Multiple metal meshes are installed inside the vertical shaft to induce turbulence, which enhances the heat transfer intensity. This turbulence enhancement is confirmed via smoke visualization and infrared images.

A visualization study of vacuum enhancement on vapor flow and yield in tubular solar still

Operating solar still under vacuum is known to significantly boost the freshwater yield, and the bonus yield could be mainly attributed to the enhanced vapor flow. Smoke visualization technique can characterize the vapor flow field within a distillation chamber, and help to reveal structural/operational shortcomings and achieve an optimal design. To quantitatively explain the mechanism of vacuum enhancement on vapor flow within a tubular solar still (TSS), a visualization experiment using laser sheet and smoke tracer was conducted to capture the flow pattern and diffusion process of vapor. The accelerations of the vapor flow with decreasing pop and elevating Tw were captured. At Tw = 60 °C, average velocity of vapor flow (vavg) under 40 kPa was 66.6 mm/s, increased by 153% compared with that under 95 kPa. An empirical correlation was then presented with an accuracy of −12% ∼ 22%. Based on the correlation, a novel model for predicting the yield of solar still was proposed, and validated through independent data, showing an average deviation of 5.11%. Furthermore, three flow regimes (laminar, transition, turbulent) were classified with different Grashof number, where laminar flow was observed at Gr < 3.0 × 104 and turbulence was occurred when Gr exceeded 5.6 × 104. This work visualized the acceleration of vapor transportation under vacuum, and supplied guidance for further optimal design in solar stills.

Effect of Partial Ground and Partial Ceiling on Propeller Performance

Finite extent flat obstacles behind a propeller (“partial ground”) and ahead of the propeller (“partial ceiling”) were examined experimentally for a small-scale fixed-pitch propeller in hover. Cases included circular and annular plates, which can be summed up or superimposed. Near-surface smoke and flow visualization show a projected time-averaged streamline on the ground plate, dividing a swirling flow inboard and a radial flow outboard. The flow pattern was juxtaposed with pressure data on the ground plate. Changes in propeller thrust and power due to ground/ceiling effect were measured by a force balance and correlated with flow visualization. For a plate of diameter equal to that of the propeller, ground effect on thrust and power was comparable to that of an “infinite” plate, while the flowfield also resembles that of the infinite plate. For a plate of less than half of the propeller diameter, ground or ceiling effect was found to be negligible. Thrust and power effects for circular and respective annular plates were superimposed to determine whether the increase in thrust or decrease in power required is itself additive to the respective effects seen with the constituent obstacles. For ceiling effect, this holds but less so for ground effect. A curve fit to thrust and power data resulted in a predictive model accurate to within 6%.

Smoke visualization for the invasion of pollutants during door-opening process in pharmaceutical cleanrooms: Effects of initial pressure differential and airlock barriers

Little research has been conducted on containing pollution invasion when the doors of pharmaceutical cleanrooms are opened. In this study, a clean pharmaceutical laboratory was built, and smoke visualization experiments were conducted. The transient effects of the initial pressure differential between rooms, airlock barrier, and time delay of airlock doors on the containment of disturbance by a normal door cycle of 4.5 s and by personnel entry were investigated. The initial pressure differentials were set from 5 to 25 Pa, and the delay time of the airlock doors was set from 6 to 36 s. The results indicated that the door operation and personnel entry caused smoke to invade the cleanroom. An increase in the pressure differential improved the containment of smoke migration. The amount of smoke invasion was lower, and the smoke was almost invisible by 18 s after the door opening at differential pressures of 20 and 25 Pa. An airlock exhibited an effective buffer effect even if the pressure differential across each airlock door was less than 5 Pa. Increasing the time delay was beneficial for eliminating migrated polluted air in the airlock. A delay time of 16 s significantly reduced the pollution for an initial pressure differential of 5 Pa across the airlock door. These findings can provide guidance for the containment of short-term disturbances in pharmaceutical cleanrooms and other controlled spaces.

Boundary conditions for exhaled airflow from a cough with a surgical or N95 mask.

Wearing surgical or N95 masks is effective in reducing the infection risks of airborne infectious diseases. However, in the literature there are no detailed boundary conditions for airflow from a cough when a surgical or N95 mask is worn. These boundary conditions are essential for accurate prediction of exhaled particle dispersion by computational fluid dynamics (CFD). This study first constructed a coughing manikin with an exhalation system to simulate a cough from a person. The smoke visualization method was used to measure the airflow profile from a cough. To validate the setup of the coughing manikin, the results were compared with measured data from subject tests reported in the literature. The validated coughing manikin was then used to measure the airflow boundary conditions for a cough when a surgical mask was worn and when an N95 mask was worn, respectively. Finally, this study applied the developed airflow boundary conditions to calculate person‐to‐person particle transport from a cough when masks are worn. The calculated exhaled particle patterns agreed well with the smoke pattern in the visualization experiments. Furthermore, the calculated results indicated that, when the index person wore a surgical and a N95 mask, the total exposure of the receptor was reduced by 93.0% and 98.8%, respectively.

Power augmentation of ducted wind turbines for urban structures: Experimental, numerical, and economic approaches

AbstractRecent development in using wind turbines for urban areas results in inserting turbines inside buildings. As buildings' walls may act as a duct for the turbine, this study focuses on a ducted wind turbine with a fixed duct geometry. A method is organized for achieving the improved generated power and the wind speed augmentation with fixed geometry of duct regardless of the type of the turbine, which is the aim of building designers. Using a porous disc (PD) instead of a wind turbine rotor makes the study cost and time effective. PDs within a duct help estimate any given duct's maximum available power extraction capability. In addition, experimental and numerical tests examine the effect of PDs solidity on the performance of diffuser augmented wind turbines and the corresponding economic analysis. Both experimental and numerical results agree that the power coefficient highly depends on the solidities of the PD. The power coefficient of a ducted PD with a solidity of 0.3 is augmented by up to 30%. Nevertheless, in some cases, employing a duct can contribute to the power reduction if the solidity exceeds a critical value. A smoke visualization technique helps vortex study. Economic assessment of a ducted turbine for three scenarios belonging to Germany and Italy shows a 15.3% decline in cost per electricity production. The payback period decreases by 3.42 years, 7.68 months, and 6.36 months for Scenarios 1, 2, and 3.

Dynamics of transitional jets emanating from a non-circular nozzle

The paper presents the results of an experimental study comparing free jets issuing from either a non-circular nozzle (square, hexagonal or triangular) or a circular one included as reference case. The measurements are performed at two Reynolds numbers equal to Re=5000 and 10000. The experimental set-up is characterized by an extremely low level of fluctuation intensity (Tu≤0.1%) and a thin shear layer at the nozzle exit. These conditions induce the development of large scale vortices clearly visible both in the velocity measurements and in smoke visualizations. The dynamics of the jets issuing from the circular, hexagonal and square nozzles is found to be quite similar. In these cases the influence of the nozzle shape is quite weak. The results obtained for the triangular jet turned out to be qualitatively different. Only weak vortices form in the vicinity of the nozzle, which are found to break up immediately downstream. Compared to the other three jets this implies a much faster spreading rate and faster axial velocity drop along the centreline. An interesting phenomenon was observed at Re=10000 for all nozzle shapes, namely the occurrence of two distinct peaks in the velocity spectra, not present at Re=5000. This suggests that in this case, the Kelvin–Helmholtz instability coexists with other instability mechanism leading to slightly different frequencies of the flow oscillations.

On the interactions of the induced flow field of heat exchangers with axial fans

In a large number of applications, heat exchanger modules consist of a combination of a heat exchanger and a turbomachinery. An axial fan is often used as the turbomachine in these scenarios. These modules are part of the daily life of people and are located in the direct vicinity of them. The interaction of the flow field with the heat exchanger generates disturbed inflow conditions upstream of the axial fan, which are characterized by increased turbulence properties. These disturbed inflow conditions often lead to increased sound radiation from the fan. In this study, the flow field of eight different heat exchangers was characterized and the effects on the sound emissions of the axial fan were analyzed. The main focus was on the geometry of the heat exchanger. By means of smoke visualization, the formation of significant turbulence points in the flow field of heat exchangers could be demonstrated. The spots with increased turbulence intensities are induced by the geometry transition from rectangular to round between the heat exchanger and the duct of the axial fan. It was found that a round geometry of the heat exchanger leads to a more homogeneous flow field and thereby reduces the sound emissions of the axial fan. Also a increase of the flow through area of the heat exchanger brings benefits for the acoustics of the axial fan. Especially the tonal components of the blade passing frequency of the fan were strongly dependent on the chosen heat exchanger. The tonal components could be reduced if the homogeneity of the flow field was increased by a round heat exchanger geometry.

Wake Propagation and Characteristics of a Multi-Rotor Unmanned Vehicle in Forward Flight

In this study, experimental investigations are used to explore the wake propagation and characteristics of a multi-rotor unmanned air vehicle (UAV) in a forward flight mission. Qualitative smoke visualization is used first to gain a qualitative understanding of wake characteristics above and below the body of the multi-rotor UAV which is used as guidance for quantitative particle image velocimetry (PIV) experiments which better resolve the region in the vicinity of the multi-rotor UAV body. The experimental results over a wide range of advance ratios show that as the advance ratio increases, achieved by either lower rotor speeds or higher flight speeds, the distance by which the wake propagates below the UAV is reduced. While above the UAV, the flow returns to the freestream flow closer to the body as the advance ratio increases. Therefore, this study concludes that proximity effects are reduced as the advance ratio increases. Findings from this study can be used to inform in situ sensor placement so that sensor readings are minimally affected by the wake from the multi-rotor UAV. Velocity measurement corrections are provided for sensors mounted above the UAV which can be used to improve sensor data reliability in forward flight. These results can advance autonomous sensing and increase the utility of multi-rotor UAV observations while providing designers and users further guidance to avoid proximity effects.

Numerical and experimental study on flow separation control of airfoils with various leading-edge tubercles

In recent years, leading-edge tubercles of humpback whales have received increasing attention as a passive stall control mechanism. The control mechanism and flow field structure of a reference (ref) and two modified airfoils (mod-1 and mod-2) are investigated numerically and experimentally at a Reynolds number of Re = 5 × 105 in this paper. The results demonstrate that the lift coefficient of the modified airfoils is enhanced to a certain extent after these airfoils stall. In addition, the stall processes of mod-2 are smooth and stable over the observed sudden decrease in ref. Counter-rotation vortex pairs (CRVPs), which improves momentum exchange in the trailing edge boundary layer on the suction side, according to streamline slices and iso-surfaces of the vorticity. Moreover, interaction between streamwise vortices is the main reason for the improved hydrodynamic characteristics at high angles of attack (AOAs). Furthermore, when smoke visualization is carried out in a low-Re wind tunnel, mod-1 and mod-2 exhibit similar characteristics, vortex pairs are periodic and symmetric along the wingspan direction at α = 5°, and vortex pairs begin to interact and influence flow separation at α = 15°. Finally, numerical analysis of the characteristics of tubercle turbines is conducted. The findings reveal that the energy efficiency can be enhanced within a narrow range of tip speed ratios (TSRs).

On the airflow in a cavity during water entry

The airflow physics in the cavity during the entry of a symmetric wedge into water are investigated experimentally using flow visualization techniques and particle image velocimetry (PIV) measurements. The spatial and temporal evolutions of the airflow velocity in the cavity are presented. Smoke visualization indicates some small-scale flow phenomena like secondary vortexes, free shear layers, and Kelvin–Helmholtz (K-H) instabilities within the water entry cavity. The information of airflow velocity in the cavity is obtained via PIV measurements. The results show that ambient gas is entrained into the transient air bubble at the wake of the wedge and the air flow rate is nearly equal to the cavity volume change rate. Thus, it is feasible to use the airflow rate to predict the cavity volume change rate. When the wedge descends, the air-water interface has a significant effect on the wake vortex behaviors. The vortex areas shrink gradually and wake vortex circulation decreases over time. Thus, the air-water interface not only suppresses vortex growth but also leads to vortex dissipation.

Pressure and velocity measurements of air flow past a proposed generic Aircraft carrier geometry

Research on the air flow disturbances in the aircraft carrier environment has gained prominence in recent times. However, there is presently no representative carrier model analogous to the Simplified Frigate Shape (SFS) which is generic naval frigate for air flow investigation. In the present study, a Generic Aircraft Carrier (GAC) model is proposed, as a simplified, benchmark model for aerodynamic research. With the motivation to provide validation data for future CFD studies, baseline experimental data is generated in the wind tunnel, in terms of pressure distribution over the deck, for two variants, namely, a complete flat deck configuration with no island and secondly, with the island in the baseline position of the GAC. Effect of the island in modifying the flow is discussed by a comparison between the two variants. Particle Image Velocimetry (PIV) is employed to record velocity and turbulence levels in the GAC environment, highlighting regions of velocity deficits, and unsteady flow which may hinder the landing procedure of an approaching pilot. Comprehensive database of experimental data is presented as baseline data for future work and for validation of numerical models. Traditional tuft and smoke visualization studies are also conducted to provide corroboratory qualitative insights.

Flow instabilities and heat transfer in a differentially heated cavity placed at varying inclination angles: Non-intrusive measurements

We report the non-intrusive investigation of the dependence of buoyancy-driven flow instabilities on the orientation angle of a differentially heated cavity of aspect ratio three. The cavity orientation angles considered are 60° and 30°. While moving from 60° to 30°, the cavity is inclined toward its stable configuration, wherein convection reduces. Flow instabilities have been captured through the spectral analysis of the transient history of temperature distribution recorded in a completely non-intrusive manner using a Mach–Zehnder interferometer. By virtue of the fact that in such configurations, corners of the cavity are the most active regions with regard to the interaction of buoyancy-driven fluid with the cavity walls, and the flow behavior is centrosymmetric (diagonal symmetry), the flow field in the top two corners of the cavity has been mapped. The spatio-temporally resolved interferometric measurements identified two distinct frequencies for cavity inclination angle (θ) of 60°. These two frequencies correspond to two different flow instabilities, namely, the Tollmien–Schlichting (TS) and gravity wave-induced instabilities. As the cavity is further inclined toward 30°, the instability in the boundary layer, i.e., the TS instability, ceases to exist, and only the gravity wave-induced instability is observed. The dependence of flow instabilities on cavity orientation angle is explained on the basis of interferometry-based measurements made in the form of interferograms and the corresponding whole field maps of temperature contours. The convective flow field in the differentially heated cavity has also been qualitatively captured using smoke visualization to provide direct support to interferometric measurements.

Wind-tunnel studies on the characteristics of indoor/outdoor airflow and pollutant exchange in a building cluster

The diffusion of air pollutants in urban areas is essentially a multiscale problem, which has aroused increasing concerns. Wind-tunnel experiments were employed to explore the characteristics of indoor/outdoor airflow and pollutant exchange in a building cluster. Under two wind directions ( β ​= ​0°,45°), smoke visualization tests were firstly conducted to visualize the diffusion characteristics of air pollutants released from different pollution sources. Then the 3D hot-film anemometer and SF 6 quantitative detector were used to measure the turbulence parameters and pollutant distributions. The smoke visualization tests showed that under β ​= ​0°, the pollutants were mainly concentrated in the middle main street canyons (S-1 and S-2), spread downstream, and were gradually diluted. However, the influence area under β ​= ​45° was wider. The results confirmed that the indoor pollutant concentration along different floors was closely related to that around the outside target building. The window-opening mode had great influence on indoor pollutant concentration, but little influence on the other floors. Additionally, when the pollution source was located in the building leeward wake, regardless of the building height, it could reduce the indoor air quality of the leeward building. This study is helpful to understand the characteristics of flow and dispersion through different urban scales. The configurations of multiscale diffusion models. • Wind-tunnel tests are employed to study indoor/outdoor exchange in a building cluster. • Two wind directions and different window opening modes are considered. • The smoke visualization tests allow the visualization of pollutant diffusion. • Pollutants in the building wake can diffuse vertically to the top leeward rooms. • The characteristics of pollutant diffusion through different urban scales are understood.

Experimental study on flow characteristics and heat transfer of an oscillating jet in a cross flow

In the present paper, flow characteristics of an oscillating jet in a cross-flow (OJICF) emitted from a double feedback fluidic oscillator were investigated. The results were compared with a steady jet in a cross-flow (JICF) from a jet nozzle with length and diameter equal to those of the fluidic oscillator. Smoke visualization in a low-speed wind tunnel was used. The measurements were done in the symmetry plane with 3 cross sections for 4 different blowing ratios (BR) of 1 to 3.33. The OJICF shows high potential to enhance the effectiveness of film cooling since it penetrates to a lesser height and leads to considerable spanwise penetration. Then, heat transfer behaviors of an impinging OJICF (IOJICF) and impinging JICF (IJICF) were compared via thermographic phosphor thermometry (TPT). The cross-flow Reynolds number based on the hydraulic diameter of the jet exit and cross-flow velocity was varied between 510 and 2,700, and the spacing between the jet and target surface was 3 times the jet throat's hydraulic diameter. The results show that for the IOJICF, heat transfer is dominated by the cross flow, and changing the BR does not have a significant effect. However, the IJICF promotes heat transfer around 2 times more than the IOJICF, and the more the BR is, the greater the Nu number. Therefore, the OJICF could be applied for film cooling rather than impinging applications.

Experimental testing of exterior wall mounted mechanical ventilation exhaust air outlet devices

The purpose of the study was to experimentally test the performance of four types of wall-mounted mechanical ventilation exhaust air outlet devices. A full-scale mock-up of a segment of an external wall with an exhaust air outlet was constructed. The tested exhaust air devices include a gravity louver, fixed-blade louver, louver plate, and exhaust nozzle. The performance assessment included two types of experiments over the exhaust airflow rate range of 25–94 l/s at isothermal conditions with no influencing wind: (i) the particle tracer method with smoke to visualize the exhaust air jets from the outlets, and (ii) the tracer gas method to measure the dilution of CO2 concentration in the exhaust air jet. Furthermore, the aerodynamic performance was comparatively evaluated in terms of pressure drop and exhaust air face velocity at the outlet. The qualitative comparison of airflow patterns by smoke visualization showed notable differences between the tested device types. Concentration decrease evaluation indicated that the exhaust air pollutants are more efficiently transported away from the building wall by exhaust outlets that discharge at 0–45 degrees downwards from the horizontal plane. Discharge angles 60–90 degrees downwards produced a wall-attached jet and the pollutant tracer concentration remained relatively high in the vicinity of the wall.