Airflow Separation Research Articles

Application of magnus effect on wings

As an important part of the aircraft, the wing provides enough lift for the aircraft, but also creates additional drag. An effective wing design guarantees sufficient lift while minimizing the impact of drag on the aircraft. Aerodynamic analysis is an interdisciplinary research in the course of wing design. In this paper, the design of Magnus type is discussed by means of computational fluid dynamics. First, the airfoil with Magnus rollers was designed. Then, the key parameters such as rotational speed and Angle of attack are studied. Through the analysis of the line chart and calculation domain image of the calculation results, the results show that the installation of Magnus roller on the wing can effectively increase the lift and reduce the drag. With the increase of the rotational speed of Magnus Roller, the lift generated is larger. In the process of increasing the rotational speed, the reflux area behind the airfoil keeps decreasing until it disappears. It is proved that the Magnus airfoil can effectively avoid the phenomenon of airflow separation and avoid the stall of the aircraft. The conclusion can provide theoretical basis for the optimization design of the new generation aircraft airfoil.

The multi-objective optimisation design of outlet guide vanes of diagonal flow fan based on sobol sensitivity analysis

Diagonal flow fans offer substantial energy-saving potential and find broad application across various sectors. Their performance relies heavily on factors like outlet guide vanes and spacing relative to moving blades. However, research into enhancing fan performance through optimized guide vanes and spacing remains limited. In this study, we focus on improving the accuracy of predicting the internal flow field of diagonal flow fans. This paper incorporate rotation and curvature effects using the Large Eddy Simulation (LES) model and introduce stress terms with helicity constraints to create a non-linear subgrid-scale model. This refined model enables more precise numerical simulations. By employing accurate simulations, we optimize the outlet guide vane configuration and conduct sensitivity analysis. We utilize a Radial Basis Function (RBF) model coupled with the Sobol method for this purpose. The optimized guide vane design exhibits enhanced resistance to airflow separation compared to the original, resulting in notable reductions in flow losses within the grille channel. Experimental tests are performed on the diagonal flow fan both before and after optimization. At the specified operating point, the second guide vane optimization leads to a 1.28 m3/min increase in fan flow, a 4.33% rise in total pressure efficiency, and a 2.2 dB noise reduction. These findings underscore the accuracy of the helicity correction model in predicting diagonal flow fan behavior. The multi-objective optimization approach, combining the RBF proxy model with the Sobol method, proves highly reliable. It offers valuable design insights for similar fans and establishes a credible design methodology.

Wave spectrum characterization in turbulent stratified air-water flows in a large diameter pipe

This study presents an analysis of interfacial waves in turbulent stratified air-water pipe flow in a 10 cm diameter pipe at atmospheric conditions. Interface elevation measurements are performed by means of conductance probes at a high sampling rate. In particular, the frequency spectra are characterized and the wave structure is assessed through the analysis of the power spectral density (PSD) and its important features, such as dominant frequencies, peak densities and dissipation region. The influence of both gas and liquid superficial velocities is assessed. For the range of superficial velocities investigated (USL = 0.08-0.15 m/s, USG = 1.0-4.5 m/s), dominant frequencies of approximately 7-8 and 3-4 Hz are observed in the wave growth and saturation regimes, respectively, defined by Ayati & Carneiro (2018). A transition from wave growth to wave saturation regime was observed to occur in the range of USG = 1.0-2.0 m/s, for the current conditions studied. In the saturation regime, a secondary peak is observed in the frequency spectra at approximately 7-8 Hz. A scaling analysis is performed to support the interpretation of results and parameterization of spectral parameters, as an indication of the wave structures. In the saturation regime, the high frequency region of the frequency spectra (spectral tail) displays a nearly monotonic decay, and the data supports in general a power law of the form F−n, n ∼ 4-4.5, as evidenced by the non-dimensional analysis covering the full range of conditions tested in this work. This is in accordance with the power law expression derived by Phillips (1985) and supports recent interpretation of air-flow separation and micro-breaking as wave dissipation mechanisms (Ayati, 2018). Power-laws of the form S(F0) ∼(F0) -5 for the peak densities in the PSDs are also found to represent well the current data set for the envelope curve of the spectral peaks. Particularly for the wave saturation regime. For the waves analyzed in the current work, a direct relation between the dominant frequencies and the mean liquid depth is shown to hold.

A Study of the Relationship between Dipole Noise Sources and the Flow Field Parameters around the Rearview Mirror of Passenger Cars

When air flows through the mirrors, A-pillars, and other surfaces of a passenger car, airflow separation occurs to generate vorticity and generates aerodynamic noise. The dipole noise source is the main source of aerodynamic noise when the passenger car is travelling at high speed. In order to reveal the relationship between the physical quantities in the flow field and the intensity of the dipole noise source, this paper equates each dipole noise source to a sphere radiating noise to the outside. The acoustic wave fluctuation equations expressed as spherical coordinates are combined with the acoustic boundary conditions and the equations of motion of an ideal fluid medium to obtain the relationship between the intensity of the dipole noise source and the flow velocity and vorticity in the flow field. Then, through flow field simulation, we establish a method of identifying the dipole noise source and its distribution area, and analyze the generation mechanism of the dipole noise source. Through the analysis, it is concluded that the dipole noise sources are mainly concentrated in the airflow separation areas, which easily generate vortices. The size of the vorticity is the key factor affecting the intensity of the dipole noise source. When the intensity of the dipole noise source reaches its peak, the direction of the flow vorticity is perpendicular to the direction of the flow velocity and the peak of the flow velocity occurs before the peak of the dipole noise source, which indicates that the angle between the flow vorticity and the flow velocity, as well as the flow velocity, also has a certain effect on the generation of the dipole noise source.

Novel wall-attached multidirectional jets: Mathematical formulation, CFD modeling and experimental validation

Achieving a healthy and comfortable indoor air environment has long been an objective of human endeavors. For a novel ventilation system, the wall-attached multidirectional jet pattern, the mathematical model describing the jet trajectory, temperature and velocity changes was first proposed to predict airflow attachment and separation moments from walls, covering a densimetric Froude number range from 0.5 to 4.0. Subsequently, this mathematical model was solved using a comprehensive numerical methodology and validated by full-scale experiments, which demonstrated that this established model could accurately depict the wall-attached multidirectional jet flow, with controllable errors — 16.4% for jet trajectory and 8.2% errors for jet axis temperature — when maintaining a supply velocity of 0.2 m/s. Also, the flow characteristics within a room featuring internal heat sources were analyzed, varying air supply velocities from 0.1 to 0.8 m/s and jet angles from 0° to 60°. Finally, this flow pattern of the wall-attached multidirectional jet was further confirmed and reproduced through CFD modelling which illustrated temperature stratification ranging from 18.1 °C to 25.2 °C, similar to the pattern observed in displacement ventilation. Present research could be valuable for the development of wall-attached multidirectional jet and the establishment of a healthy indoor air environment in practical applications.

Study on the transport and deposition of micron-sized particles with different densities in the coal mine environment in miners

To address the health problems caused by the long-term intrusion of high-concentration dust generated in coal mining operations into the human respiratory tract, this study reconstructed a 3D digital model of the upper airway based on fine industrial CT scanning images, and established the micron particle migration calculation model by Euler-Lagrange method. The transport and deposition mechanisms of dust with different densities under different inspiratory intensities were simulated and analyzed. The simulated results were compared with the results of dust exposure experiments in the upper airway combined with 3D printing technology, and the relative errors obtained were in the range of 2.9% to 6.3%. The results showed that the airflow volume peaked in the nasopharynx, oropharynx, epiglottis, and larynx, leading to the airflow separation and formation of circulation that tend to cause dust deposition in these regions. As the dust density and inspiratory airflow volume increased, the deposition fraction of small dust particles (<1μm) in the upper airway slightly changed, and most of them escaped to the lung. For 7.07 μm dust particles, as the dust density increased from 1210 kg/m3 to 2750 kg/m3, the escape rate decreased from 31.43% to 0.48%, and this trend can be seen in the fitting function of escape rate and inspiratory airflow volume Y7.07μm=(80–1.37ν + 0.0064ν2) × 100%. When the dust particle diameter exceeded 20 μm, the impact of dust density was weakened by the large size of the dust, and the escape rate was close to 0% when the inspiratory airflow volume was 50–110 L/min. This study can provide theoretical support for the research and development of respiratory dust sensors and targeted dust prevention technologies.

Numerical study of indoor flow field and ventilation performance of the power room of a diesel locomotive under crosswind

Power room is the core part of the railway internal combustion locomotives, houses many high-power equipment within a limited space. The ventilation performance of a power room are the key factors that ensure the efficient operation of locomotives. Numerical simulations were conducted to study the indoor and outdoor flow fields of the power room, with focus primarily on the indoor flow field of the locomotive. The effects of locomotive and crosswind speeds on the ventilation performance and indoor flow field of a power room were studied. Furthermore, the effects of fan performance on the power room ventilation were compared and analyzed. The numerical methods used were verified using experimental data. Results showed that as locomotive speed increased, the power room ventilation performance decreased, while the airflow rate of certain power room air intakes increased, with an increase in locomotive speed. Crosswinds significantly increased the power room ventilation quantity, especially when the intakes faced the crosswind. When the crosswind speed surpassed a certain threshold, the intakes on the leeward side of the power room entered an air-leakage state. The indoor flow field distribution of the power room was unaffected by the fan performance. Though the effect of fan performance on the amount of airflow entering the power room varied, the fan performance positively correlated with the overall ventilation performance of the power room. Considering the strong airflow separation caused by the back-step corner of the driver cab, attention should be paid to the placement and intake position of the power room in the design stage.

Atmospheric Sea Spray Modeling in the North‐East Atlantic Ocean Using Tunnel‐Derived Generation Functions and the SUMOS Cruise Data Set

AbstractThis study contributes to the communal effort to improve understanding of sea spray generation and transport. For the first time, laboratory‐derived sea spray generation functions (SSGFs) are parameterized in the Meso‐NH mesoscale atmospheric model and are field tested. Formulated from the MATE19 laboratory experiments (Bruch et al., 2021, https://doi.org/10.1007/s10546-021-00636-y) the two SSGFs are driven by the upwind component of the wave‐slope variance (herein B21A), or both and the wind friction velocity cubed (herein B21B). In this first attempt to incorporate the SSGFs in Meso‐NH, the simulations are run without a wave model, and the wave‐wind SSGFs are assumed wind‐dependent. Model evaluation is achieved with a new set of sea spray and meteorological measurements acquired over the 0.1–22.75 μm radius range and U10 1–20 m s−1 wind speeds onboard R/V Atalante during the 25 day SUMOS field campaign in the Bay of Biscay. The B21B SSGF offers particularly good sensitivity to a wide range of environmental conditions over the size range, with an average overestimation by a factor 1.5 compared with measurements, well below the deviations reported elsewhere. B21A also performs well for larger droplets at wind speeds above 15 m s−1. Associated with airflow separation and wave breaking, wave‐slope variance allows to represent multiple wave scales and to scale sea spray generation in the laboratory and the field. Using Meso‐NH simulations we find that sea spray may be transported inland and to altitudes well above the marine atmospheric boundary layer.

Study on Aerodynamic Design of the Front Auxiliary Inlet

Submerged inlets have been widely used in advanced aircraft due to their excellent stealth characteristics, but they also suffer from poor aerodynamic performance. To improve the aerodynamic efficiency while maintaining stealth capabilities, this paper proposes a design scheme for a front auxiliary inlet with an inlet grille. The front auxiliary inlet is connected to the main inlet to form a composite inlet system. The low-energy upstream airflow that accumulates at the inlet is guided by the front auxiliary inlet to flow into the mainstream, resulting in a stable and high-quality airflow. A certain type of cruise missile was used as the research subject, and intake systems with and without front auxiliary inlets were constructed to compare the inlet performance of the two configurations using the CFD method. Additionally, a sensitivity analysis of the main design parameters of the front auxiliary inlet was carried out. The study reveals that a reasonable design of the front auxiliary inlet can prevent low-energy airflow, which accumulates on the missile body surface, from directly entering the inlet. Moreover, the front auxiliary inlet can inject additional mechanical energy into the low-energy airflow, inhibit airflow separation, and improve the uniformity of the flow field. Under cruise conditions, the total pressure recovery coefficient of the front auxiliary inlet configuration increased by 12.39% compared to the model without a front auxiliary inlet configuration. Furthermore, the total pressure distortion index was reduced by 47.24%.

Numerical analysis of pitch-break and all moving wingtip aileron of lambda wing configuration

Lambda wing, as a commonly used configuration for modern combat aircraft, tend to reduce stability margin known as Pitch-Break before the wing completely stalls. This phenomenon greatly affected the safety of aircraft at high angles of attack. In this paper, the characteristics of Pitch-Break are clarified and it is found that the initial airflow separation position of the outer wing is the principal reason for generating nonlinear moment of lambda wing. The airflow separation position is generally affected by leading edge vortex and the trailing edge spanwise airflow. Furthermore, the following conclusions are obtained: 1. The distribution of the absolute value of the leading edge radius is a decisive parameter of the initial position of leading edge vortex at the beginning of Pitch-Break zone; 2. The difference of trailing edge pressure gradient brings difference of trailing edge spanwise airflow at the end of Pitch-Break zone. To solve the aerodynamic problem of Pitch-Break, this paper adopts an all moving wingtip (AMT) as auxiliary aileron which has minor fluctuations during the Pitch-Break zone compared with conventional aileron. The result shows the defect of the conventional aileron pitch control at high angle of attack and presents the engineering feasibility of the all moving wingtip auxiliary aileron.

Investigation on aerodynamic noise for leading edge erosion of wind turbine blade

Wind turbine blade is extremely prone to leading edge erosion under complicated operation conditions. This decreases the aerodynamic performance and power generation efficiency of wind turbine. Currently acoustic-based erosion detection of the blade still faces a significant challenge. Consequently, this paper presents aerodynamic noise mechanism of the eroded blade using the computational fluid dynamics method. The Zonal Detached Delay Eddy Simulation turbulence model is applied to obtain the three-dimensional instantaneous turbulent flow field. Then the far-field aerodynamic noise is carried out using the Ffowcs Williams and Hawkings approach. Simulations are performed for the erosion condition that determined by the length and depth at leading edge of 5-MW National Renewable Laboratory wind turbine. Results indicate that erosion leads to unsteady pressure pulsation near the leading edge, decrease of surface pressure difference in tip area, increase of airflow separation region and forward movement of separation point. Furthermore, the eroded blade achieves higher acoustic level than that of the normal one. From the noise directivity of wind turbine blade, erosion causes obvious asymmetry of noise pattern in the upwind and downwind directions.

Aerodynamic analysis of an ultra–long and ultra–wide industrial building under wind loading: Insights into flow dynamics and pressure distribution characteristics

Large ultra–long and ultra–wide industrial facilities have a huge windward surface, resulting in severe wind suction on the roof, back, and side enclosures. Wind–induced damage on structures may occur due to the turbulence effect caused by airflow over the irregular shape of the roof, which results in complex wind pressure distribution characteristics on the roof. Therefore, it is critical to elucidate the pressure distribution of ultra–long and ultra–wide structures to ensure the normal functioning of the structure and predict wind–induced dynamic disasters. In this paper, the wind–induced pressure field characteristics of ultra–long and ultra–wide roof structures are investigated by Computational Fluid Dynamics (CFD) analysis. A series of qualitative and quantitative analyses were performed to assess the pressure coefficient distribution on the roof as a result of wind direction angle, terrain roughness, and size effect. Meanwhile, the complicated variations in the wind field of the structure generated by the flow around bluff bodies are studied to establish a relationship between flow dynamics and pressure distribution characteristics. The results showed that the roof is mainly exposed to wind suction under the influence of wind attack angle, and the region with severe negative pressure gradient change is controlled by the large–scale eddies generated from the separated airflow. Moreover, the terrain roughness and size effect intricately modify the pressure distribution pattern on the local roof, and wind pressure intensity may be estimated using the information on eddy diffusion and velocity distribution.

Aerodynamic Characteristics Analysis of Rectifier Drum of High-Speed Train Environmental Monitoring Devices

To study the aerodynamic characteristics of the convex structure of a surface-monitoring device on a high-speed train and to evaluate its impact on the aerodynamic performance of the high-speed train, numerical simulation research was conducted on three different layouts of the monitoring device. The computational fluid dynamics (CFD) method was used for the simulation study, and the unsteady compressible NS equation was used as the control equation. Hexagonal grid technology was used to reduce the demand for the grid quantity. The rationality of the grid size and layout was verified through grid independence research. To increase the accuracy of the numerical simulation, the γ-Reθ transition model and improved delayed detached eddy simulation (IDDES) method were coupled for the simulation research. The aerodynamic characteristics of the different operation directions and configurations were compared and analyzed. The research results showed that the windward side of the single pantograph detection device experienced positive pressure, and the sideline and leeward sides experienced negative pressure. Increasing the fillet radius of the sideline could appropriately reduce the aerodynamic resistance. When the speed was about 110 m/s, the drag force coefficient of the detection device was 210~410 N, and the lateral force was small, which means that it had little impact on the overall aerodynamic force of the train. According to the results of the unsteady analysis of the layout with a large space, the resistance during forward travel was greater than that during negative travel. The streamlined upwind surface was conducive to reducing the scope of the leeward separation zone and the amplitude of the pressure fluctuation in the leeward zone, and it thus reduced the resistance. For the running trains, a vortex was formed on their leeward surface. The pressure monitoring results showed that the separated airflow had no dominant frequency or energy peak. The possibility of the following train top and other components experiencing resonance damage is low.

Designing and testing the applicability of a natural green ventilation building block in an Egyptian Climatic zone using waste materials

Natural ventilation is one of the ways used to enhance building spaces' indoor air quality, especially after the COVID-19 pandemic. It also saves energy by utilizing natural wind forces to provide internal building spaces with fresh air instead of energy-consuming mechanical systems. For a multiple-floor building, adding external wall ventilation blocks can improve natural cross ventilation by catching and directing fresh air from the external prevailing wind into its internal spaces. This research aimed to enhance the building's natural ventilation by using environmentally friendly blocks. This was done by designing a green block using marble and granite waste materials as a partial substitute for conventional materials in the block. Styrofoam beads were added to the block’s mixture to reduce its weight. The block’s shape was designed to direct fresh air through building side walls by the use of natural wind pressure. The block was developed from the initial shape of the standard hollow block to a block with a wind catcher. Ventilation block shape simulations were done using (ANSYS fluent) computational fluid dynamics (CFD) software to determine the best inlet angle, catcher angle, and catcher length. The study revealed that by increasing the block inlet angle more than 45°, the block outlet air velocity starts to decrease due to the flow separation in the front part of the block inlet. The catcher angle ranged from 65° to 85°, and the catcher’s ability to redirect the airflow is decreased at a catcher angle greater than 85°. The best nondimensional catcher length-to-block length ratio (height /length) h/L was 0.625. Substitution of 5% of cement by granite in the presence of 0.15 Kg of Styrofoam (C5G) produced a lightweight green ventilation block of weight 19.3 kg. Whereas, the substitution of 15% of sand for marble in the presence of 1 Kg of Styrofoam (S15M) produced a lightweight green ventilation block of weight 15.9 kg. The produced block was tested in a building located in New Cairo City (a region currently having a tremendous increase in the building construction sector) as a study sample representing one of the Egyptian Climatic zones. The research concluded that it is preferred to allocate the building corners towards the prevailing wind direction to prevent airflow separation and to magnify the quantities of air along the building sides.

Enhanced ocean wave modeling by including effect of breaking under both deep- and shallow-water conditions

Abstract. Accurate description of the wind energy input into ocean waves is crucial to ocean wave modeling and a physics-based consideration on the effect of wave breaking is absolutely necessary to obtain such an accurate description. This study evaluates the performance of an improved formula recently proposed by Xu and Yu (2020), who took into account not only the effect of breaking but also the effect of airflow separation on the leeside of steep wave crests in a reasonably consistent way. Numerical results are obtained through coupling an enhanced atmospheric wave boundary layer model with the ocean wave model WAVEWATCH III (v5.16). The coupled model has been extended to be valid in both deep and shallow waters. Duration-limited waves under controlled normal conditions and storm waves under practical hurricane conditions are studied in detail to verify the improved model. Both the representative wave parameters and the parameters characterizing the wave spectrum are discussed. It is shown that the improved source-term package for the wind energy input and the wave energy dissipation leads to more accurate results under all conditions. It performs evidently better than other standard source-term options of ST2, ST4 and ST6 embedded in WAVEWATCH III. It is also demonstrated that the improvement is particularly important for waves at their early development stage and waves in shallow waters.

Flow behavior and aerodynamic noise characteristics of ultra-high-speed elevator based on large eddy simulation

PurposeThis paper aims to guide the implementation of noise reduction measures in hoistway and reduce the aerodynamic noise generated by elevator operation, this paper aims to propose an aerodynamic noise analysis method that can solve the flow field in hoistway.Design/methodology/approachA turbulence-acoustic model solving the flow field in a hoistway and a numerical wind hoistway model of the ultra-high-speed elevator were established by using large eddy simulation (LES) and Curle acoustic theory.FindingsThe characteristics of pulsating flow field and aerodynamic noise around ultra-high-speed elevator are analyzed. The asymmetric characteristics of the flow field could be observed using the turbulent kinetic energy and the instantaneous vortexes in the wind hoistway model. Vortex shedding, air flow separation and recombination around the car were the key factors for aerodynamic noise generation. The sound pressure level was approximately linear to the logarithm of car speed. The increase of car deflection angle in a certain range would reduce the peak frequency of wake noise and increase the sound pressure level (SPL) value.Originality/valueThis paper provides important guidance for researches studying the aerodynamic noise in the hoistway and the technical personnel that look for the reduction measures, which greatly improves the shortcomings in the numerical simulation of the aerodynamic noise of the hoistway.

Digital mapping of surface turbulence status and aerodynamic stall on wings of a flying aircraft

Real-time monitoring of flow turbulence is very difficult but extremely important in fluid dynamics, which plays an important role in flight safety and control. Turbulence can cause airflow to detach at the end of the wings, potentially resulting in the aerodynamic stall of aircraft and causing flight accidents. Here, we developed a lightweight and conformable system on the wing surface of aircraft for stall sensing. Quantitative data about airflow turbulence and the degree of boundary layer separation are provided in situ using conjunct signals provided by both triboelectric and piezoelectric effects. Thus, the system can visualize and directly measure the airflow detaching process on the airfoil, and senses the degree of airflow separation during and after a stall for large aircraft and unmanned aerial vehicles.

Design of flap/spoiler follower mechanism for civil aircraft

With the gradual development of the lift device to a simple and efficient direction, the drooped spoiler has become an emerging lift device. The drooped spoiler can improve the aerodynamic performance of the wing by reducing the air flow separation and increase the lift coefficient, which requires the spoiler to deflect a certain angle following the downward deflection of the flap. Aiming at this requirement, this paper proposes a design method of spoiler follower mechanism based on the relative pole method. The applicability and correctness of the method are verified by kinematics simulation. Moreover, the kinematic characteristics of the simple four-bar linkage and the complex four-bar linkage are compared, and the results show that the complex four-bar linkage is more stable in the motion process.

Numerical study on the forced convection enhancement of flat-roof integrated photovoltaic by passive components

Photovoltaic (PV) modules installed on flat roof need to be cooled to improve the power conversion efficiency (PCE). The efficient use of natural wind for cooling is a low-cost alternative compared with the use of artificial cooling devices. In this study, a passive cooling scheme is proposed including components: curved eave and vortex generators (VGs) which is arranged on the roof to enhance the forced convective heat exchange and reduce temperature of rooftop PV module under prevailing wind conditions. The optimum arrangement of VGs was studied, temperature reduction and PCE of rooftop PV with the scheme were evaluated. The results show that the curved eaves installed on the flat-roof leading edge improved the convective heat exchange only in the airflow separation area. And the defect that surface convective heat exchange decreased along with the flow direction was improved when multi-row array VGs were arranged. The overall convective heat exchange on rooftop PV can be improved at a wide range of wind speed by scheme using curved eaves in combination with the symmetrically arranged VGs. After using the passive cooling scheme, the ratio of forced convective heat exchange on the rooftop PV upper surface to 60%, and the maximum temperature reduction is about 5.89 ℃, and PCE of PV is improved in summer. The low-cost passive cooling scheme proposed in this paper is expected to be applied to the new-built and reconstruction of flat-roof mounted PV system.

Fluorescent oil-film applied to measure skin friction of flat plate with leading edge separation bubble in low Reynolds number

Under low Reynolds number conditions, the airfoil is prone to a laminar separation bubble (LSB), which causes problems such as lift drop and drag increase. In this work, the fluorescent oil film was applied to study the leading edge LSB from the perspective of skin friction. The experimental study is carried out on a flat plate with the leading edge LSB under the Reynolds number of [Formula: see text] (the characteristic length of the plate chord). The camera captures the leading edge LSB that is formed by airflow separation and reattachment, and the flow in the bubble is opposite to the free flow. The position of separation and reattachment is determined by the distribution of skin friction, which is consistent with the visual image of the oil film. Applying the Poincare–Bendixson (P-B) index formula to test the topology of the separation bubble skin friction, the quantitative relations between the critical points and the boundary switching points in the closed region of several permeable boundary all confirm this topology law. The results show that the fluorescent oil film can quantitatively map the flow structure of the LSB, and further verify the advantage of this method in the visual measurement of the complex flow structure.