Smoke Visualization Research Articles

Smoke visualization of the effect of freestream turbulence on a laminar separation bubble over an airfoil at low Reynolds numbers

Smoke visualization of the effect of freestream turbulence on a laminar separation bubble over an airfoil at low Reynolds numbers

Propagation characteristics of stable detonation waves in curved channels with the lateral expansion effect

When a stable detonation wave propagates in a curved channel, it will be influenced by the curvature of the channel, which has been investigated in previous studies. Actually, propagation of a stable detonation wave in a curved channel becomes more unstable if the lateral expansion effect is introduced. However, this is an objective phenomenon in practical rotating detonation chambers. In order to clarify the impacts of the channel curvature and the lateral expansion on the propagation of stable detonation waves, this study has been carried out using a mixture of ethylene, oxygen, and nitrogen. The effects of the channel curvature, the detonable mixture height, and the equivalence ratio on the propagation characteristics in both confined and semi-confined curved channels have been focused in this experimental work. The propagation process was recorded by high-speed photography, and the cell width was obtained using the smoke visualization method, while the peak pressures near the outer wall were also measured. The results imply that the deficits of wave velocity and peak pressure are magnified when introducing the lateral expansion phenomenon, especially under fuel-lean and fuel-rich conditions. Meanwhile, the channel curvature and the mixture height also have a great impact on the detonation propagation. Five propagation modes have been observed after the stable detonation waves enter the curved channels with a semi-confined configuration, i.e., a stable mode, a critical mode, a weakly unstable mode, a highly unstable mode, and a deflagration mode. In addition, for the two observed unstable modes, decoupling occurs periodically near the inner wall. Finally, the critical conditions of the stable detonation mode in the semi-confined channel have been clarified. The critical inner radius should be 13.10–15.24 times of the average cell width, and meanwhile, the minimum detonable mixture height is equivalent to 7.03–8.12 times of the average cell width.

Kinematics and Flow Field Analysis of Allomyrina dichotoma Flight.

In recent years, bioinspired insect flight has become a prominent research area, with a particular focus on beetle-inspired aerial vehicles. Studying the unique flight mechanisms and structural characteristics of beetles has significant implications for the optimization of biomimetic flying devices. Among beetles, Allomyrina dichotoma (rhinoceros beetle) exhibits a distinct wing deployment-flight-retraction sequence, whereby the interaction between the hindwings and protective elytra contributes to lift generation and maintenance. This study investigates A. dichotoma's wing deployment, flight, and retraction behaviors through motion analysis, uncovering the critical role of the elytra in wing folding. We capture the kinematic parameters throughout the entire flight process and develop an accurate kinematic model of A. dichotoma flight. Using smoke visualization, we analyze the flow field generated during flight, revealing the formation of enhanced leading-edge vortices and attached vortices during both upstroke and downstroke phases. These findings uncover the high-lift mechanism underlying A. dichotoma's flight dynamics, offering valuable insights for optimizing beetle-inspired micro aerial vehicles.

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.

Flow pattern around traditional Malay house using enhanced smoke wire technique in a boundary layer wind tunnel

Traditional Malay houses are well-known for their excellent natural ventilation. The claim was supported in a study which compared the temperature measurements in traditional Malay houses with temperature measurements in modern homes. However, few flow pattern studies have proven this fact. This paper aims to understand this phenomenon qualitatively by utilising smoke lines produced by the smoke wire technique to observe the flow pattern around a single model traditional Malay house mounted over a smooth plate in a small-scaled quasi-atmospheric boundary layer wind tunnel. Observation of the conventional model house has highlighted several common and unique flow patterns from the high-speed footage. The smoke visualisation technique will provide flow visualisation and capture using a high-speed camera at 1000 fps. In addition, the paper also focuses on developing improvements for the smoke-wire technique system by upgrading the controlled drip valve and adding a tensioner system. Limitations and the ideal conditions have been recorded for the smoke-wire technique, which improved the overall quality of the observation. The models were used did not have any opening or only a single opening has started to show good natural ventilation. However, having only inlets for the airflow has failed to show observable good natural ventilation.

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.