National Advisory Committee For Aeronautics Research Articles

Investigation of transient sheet/cloud cavitating flow dynamics from multiscale perspective

Sheet/cloud cavitation usually leads to a wide range of length scales in both turbulence and phase distribution from microbubbles to cavity advection. In the present work, the Eulerian–Lagrangian multiscale cavitation model with two-way coupling is utilized to simulate the cavitating flow around a (National Advisory Committee for Aeronautics) NACA66 hydrofoil at an incidence angle of 8° and a cavitation number of σ = 1.4. The model can simultaneously capture the large-scale cavities and the microscale bubbles. The cavitating flow features are in good agreement with the experimental observations containing not only the periodical formation, growth, detachment, and advection of large-scale cavities, but also thousands of microbubbles around the large-scale cavities. The results show that the overall evolution frequency in the flow is about 45 Hz. Meanwhile, the dynamic mode decomposition method is utilized to identify the large-scale coherent spatial and temporal features of the sheet/cloud cavitating flow, which indicates that complex vortices in various scales dominate the evolution of cavities in the corresponding scale, and the evolution frequency of large-scale vapor structure decreases with increasing the length scale of cavities. Under the effect of turbulence, the large-scale cavities break into microbubbles, causing the size and number of discrete bubbles to increase rapidly in the re-entrant jet and cloud shedding regions. Additionally, the bubble-size spectrum of the time-averaged distribution of a period in sheet/cloud cavitating flow has two size regimes. For larger bubbles, the bubble density is proportional to the bubble radius to the power of −10/3. The bubble size spectrum of smaller microbubbles exhibits a −4/3 power-law scaling.

Numerical thermal–hydraulic analysis and multiobjective design optimization of a printed circuit heat exchanger with airfoil overlap fin channels

AbstractIn this study, we numerically investigated the performance of a printed circuit heat exchanger (PCHE) featuring National Advisory Committee for Aeronautics (NACA) 0020 airfoil overlap fins integrated into its straight flow channels. Numerical simulations were performed by solving Navier–Stokes and energy equations to analyze liquid water at temperatures ranging from 30 to 90°C, which is possible for use with a geothermal heating system, for example. The optimization of the PCHE airfoil fin geometry is performed using nondominated sorting genetic algorithm II (NSGA‐II) with three design features: front, back, and overlap lengths. The effectiveness and pressure drop of the PCHE are evaluated as two objective functions. The present study shows that, when a Reynolds number falls between 1088 and 5000, the overlapping airfoils exhibit lower pressure drops and higher convective heat flux relative to the separate airfoils, and an increase in overlap length always decreases the pressure drop positively. It is also found that using overlapping airfoils of front length of 0.1 mm, back length of 0.119 mm, and overlap length of 0.203 mm leads to almost the same effectiveness and a 22% decrease in pressure drop (which is generally believed to preferably maintain the high heat transfer performance and minimize pressure drop, as a favorable design), compared to a base design of the airfoils with front and back lengths of 0.2 mm and an overlap length of 0.1 mm.

Wavelet analysis of the flow field around an oscillating airfoil undergoing pure pitching motion at low Reynolds number

This paper investigates the flow field around a NACA (National Advisory Committee for Aeronautics) 0012 airfoil undergoing pure pitching motion using continuous wavelet transform. Wind tunnel experiments were performed with a test-stand that provides a wide range of oscillation frequencies (f = 0–10 Hz). Sinusoidal pure pitching motion was considered with respect to the quarter chord for five reduced frequencies (K = 0.05, 0.1, 0.15, 0.2, and 0.3) at a Reynolds number of Re = 6 × 104. Mean angle of attack and pitch amplitude for all the cases were considered 0° and 6°, respectively. Unsteady surface pressure measurement was conducted, and the lift coefficient was calculated based on the phase-averaged surface pressure coefficient. The unsteady velocity distributions in the airfoil wake have been measured employing a pressure rake. The results indicate that the maximum value of the lift coefficient decreases by increasing the reduced frequency due to the “apparent mass” effects. For K = 0.05, close to the quasi-steady regime, the cl-α loop approximately follows the trend of the static case. Wavelet transform was used as a tool to examine the surface and wake pressure time series. Surface pressure wavelet transform plots indicate the presence of oscillation frequency and its superharmonics. Moreover, surface pressure wavelet analysis shows that the third and higher superharmonic frequencies are sensitive to the airfoil pitch angle during the oscillation cycle. Wavelet transform on wake reveals that the effective wake width gets smaller by increasing the reduced frequency. Furthermore, the trailing edge vortices get weaker by increasing the reduced frequency.

Reservoir computing reduced-order model based on particle image velocimetry data of post-stall flow

The present study proposes a reservoir computing reduced-order model (RCROM) of the post-stall flow around the National Advisory Committee for Aeronautics 0015 airfoil based on the time series velocity field, and the estimation accuracy of the RCROM is evaluated compared to that of a linear reduced-order model (LROM). The data were experimentally obtained by particle image velocimetry at a chord Reynolds number of 6.4 × 104 and an angle of attack of 18°. The low-dimensional description of the velocity field can be obtained by decomposing the velocity field with a proper orthogonal decomposition (POD) technique and by employing the leading POD mode coefficients as temporal variables of the data instead of the velocity field. Reservoir computing (RC) is adopted as a nonlinear function that predicts several steps ahead of the leading POD mode coefficients. The hyperparameters of RC are tuned by Bayesian optimization, and the optimized RCROM outperforms the LROM in terms of estimation accuracy. The estimation accuracy of the RCROM can be investigated under different numbers of the predicted dominant POD modes and prediction step conditions. As a result, the RCROM shows higher estimation accuracy than the LROM.

Occupational Accidents Among Search and Rescue Providers During Mountain Rescue Operations and Training Events.

We analyzed occupational accidents reported among Corpo Nazionale Soccorso Alpino e Speleologico (CNSAS) providers during mountain search and rescue operations and training events in Italy (1999 to 2019). We extracted anonymized data from the CNSAS accident database for all cases of injured mountain search and rescue providers that activated CNSAS insurance (1999 to 2019). We report epidemiological characteristics, mechanisms, type, and severity of injury or illness, clinical outcome, and recovery time. A total of 784 cases of injuries in CNSAS mountain search and rescue providers were recorded. Forty-one percent of the cases occurred during rescue operations and 59% during training events. Overall, trauma was the main cause of injury (96%), whereas only 4% of the cases were classified as medical or environmental illnesses. Moderate injury (National Advisory Committee for Aeronautics II to III) occurred in 80% of the reported accidents. Recovery time differed based on the degree of accident severity. Fatalities occurred in 2% of the cases reported and occurred during rescue operations only. In this long-term retrospective analysis, we showed that accidents occurred among mountain search and rescue providers both during rescue operations and training events. Given the high prevalence and associated costs, it is of pivotal importance to understand the epidemiology and characteristics of occupational injury and illness among this out-of-hospital workforce to better inform future prevention strategies.

Numerical investigation and aerodynamic simulation of Darrieus H-rotor wind turbine at low Reynolds numbers

ABSTRACT In order to select an airfoil suitable for the wind tunnel testing on a three-bladed Darrieus H-rotor wind turbine, several symmetrical National Advisory Committee for Aeronautics (NACA) 4-digit, NACA 4-digit, NACA 5-digit, and Selig airfoils with chord Reynolds numbers ( R e c ) of 17,890, 35,780, and 53,670 were studied. To achieve higher aerodynamic performance at Re of 17,890 to 53,670, the NACA0015 airfoil was selected. The lift coefficient ( R e c ) of the NACA0015 airfoil after shape modification has reached values of 0.788 to 0.936 at Re of 35,780 and 0.925 to 1.027 at Re of 53,670 with the same stall angle ( A o A s t a l l ) in both investigated Re, according to the results of modifying the airfoil’s thickness and chord. Additionally, the NACA0015 modified airfoil’s maximum lift-to-drag ratio coefficient ( C L / C D ) increased by 34.01% and 17.94% at Re of 35780 and 53,670, respectively. Likewise, the analysis showed that the NACA0015 modified airfoil’s maximum performance coefficient ( C P ) increased from 0.129, 0.242, 0.285, 0.308, and 0.33 to 0.152, 0.272, 0.307, 0.331, and 0.354 at Re of 100,000, 150000, 200000, 250000, and 300,000, respectively. As well as the findings demonstrated that the XFOIL code provides the most accurate overall prediction results at low Re.

Investigation of cavitation noise using Eulerian-Lagrangian multiscale modeling

We have employed the large eddy simulation (LES) approach to investigate the cavitation noise characteristics of an unsteady cavitating flow around a NACA66 (National Advisory Committee for Aeronautics) hydrofoil by employing an Eulerian-Lagrangian based multiscale cavitation model. A volume of fluid (VOF) method simulates the large cavity, whereas a Lagrangian discrete bubble model (DBM) tracks the small bubbles. Meanwhile, noise is determined using the Ffowcs Williams-Hawkings equation (FW-H). Eulerian-Lagrangian analysis has shown that, in comparison to VOF, it is more effective in revealing microscopic characteristics of unsteady cavitating flows, including microscale bubbles, that are unresolvable around the cloud cavity, and their impact on the flow field. It is also evident that its evolution of cavitation features on the hydrofoil is more consistent with the experimental observations. The frequency of the maximum sound pressure level corresponds to the frequency of the main cavity shedding for the noise characteristics. Using the Eulerian-Lagrangian method to predict the noise signal, results show that the cavitation noise, generated by discrete bubbles due to their collapse, is mainly composed of high-frequency signals. In addition, the frequency of cavitation noise induced by discrete microbubbles is around 10 kHz. A typical characteristic of cavitation noise, including two intense pulses during the collapsing of the cloud cavity, is described, as well as the mechanisms that underlie these phenomena. The findings of this work provide for a fundamental understanding of cavitation and serve as a valuable reference for the design and intensification of hydrodynamic cavitation reactors.

A proper orthogonal decomposition analysis upon aerodynamic structures under clean and rough conditions

A transitional flow regime is known to induce complex flow structures upon aerodynamic geometries such as airfoils, and the dynamics of laminar separation bubbles (LSBs) pose a relevant research field. In addition to being affected by the Reynolds value itself, LSBs are shown to be equally sensitive to the surface roughness of the airfoils. The study analyses wind tunnel-derived surface-pressure distribution datasets obtained for a particular airfoil of the standard family developed by the National Advisory Committee for Aeronautics (NACA), namely, the NACA0021, subjected to the range 0.8×104<Re<1.6×104 at different angles of attack under two flow configurations that correspond to a clean and roughened surface. The analysis is undertaken via the proper orthogonal decomposition (POD) technique. The results show that the decomposition of the temporal series of surface-pressure data and the processing of the most energetic POD modes recovers the position of the LSBs, properly capturing the closure point of the separation bubbles and, hence, the turbulence transition location. Some of the most energetic POD modes observed are closely related, in terms of shape, to the POD modes present at the reattachment point on a 5:1 rectangular cylinder. This could indicate there is a recognizable pattern in coherent structures of pressure fluctuations when it comes to a reattached flow. Therefore, a principal component analysis such as the POD presented in this study can be used to determine the reattachment position of the flow or the transition point in the presence of a LSB.

Wind turbine optimization through optimal blade shape design for low wind speeds

Current electricity needs continue to rely on depleting fossil fuels such as fossil fuel and coal. The current government effort is to find non-fossil fuel alternative energy sources. Wind energy is one of the efforts to use ecologically friendly renewable alternative energy. Studies on the development and use of new and renewable energy are now undertaken. One of them is a series of research on the utilization of wind in numerous places of Indonesia through the construction of wind turbines. The blade that explicitly makes contact with the wind is one of the most critical sections of a wind turbine. The shape of the airfoil determines whether or not the blade is used. The focus of this research was to determine the optimal type of airfoil by comparing the coefficient of power (Cp), maximum power, and lowest power produced by various NACA (National Advisory Committee for Aeronautics) airfoils. NACA 4410, 4412, and 4415 airfoils were employed in this study.

Active control of airfoil turbulent boundary layer noise with trailing-edge blowing

Large Eddy Simulation (LES) and Ffowcs Williams-Hawkings acoustic analogy are performed to study the effect of trailing-edge blowing on airfoil self-noise. Simulations were conducted using a National Advisory Committee for Aeronautics 0012 airfoil at zero angle of attack and a chord-based Reynolds number of 4 × 10 5. The aerodynamic and aeroacoustic characteristics of the baseline airfoil were thoroughly verified by comparison with previous numerical and experimental data. The noise reduction effects of continuous and local blowing with different blowing ratios and blowing momentum coefficients were compared. A maximum noise reduction of 20 dB was achieved via trailing-edge blowing and the noise reduction mechanisms of the two blowing methods were discussed. The LES results show a pair of recirculation bubbles in the airfoil wake which are suppressed by trailing-edge blowing. As the blowing vortices convect into the wake, they stretch and stabilize the shear flows from airfoil surfaces. Instantaneous vorticity and root mean square velocity fluctuations are also weakened. There is a decrease in the spanwise coherence and an increase in the phase difference, which contribute to noise reduction. It is concluded that the suppression of turbulence fluctuations in the near wake is the main mechanism of noise reduction for airfoil trailing-edge blowing.

Airfoil-based convolutional autoencoder and long short-term memory neural network for predicting coherent structures evolution around an airfoil

Airfoil stall induced by unsteady flow separation is a major inhibitor that constrains the aircraft design. Hence, it is critical to have a better understanding of the flow dynamics around the airfoil near stall conditions. Recently, machine learning (ML) has been widely used to predict the spatiotemporal evolutions of coherent structures over the airfoil. Nonetheless, due to the existence of strong nonlinearity introduced by turbulence, most of the existing approaches focus on prediction of the flow fields in the laminar flow regime. In this work, we seek to examine the applicability of deep neural network (DNN) to prediction of turbulent flows around a NACA (National Advisory Committee for Aeronautics) 4412 airfoil. Large eddy simulations (LES) are performed to prepare the training dataset. Both proper orthogonal decomposition (POD) and DNN-based reduced-order models (ROM) are employed for reconstructing the flow fields. A convolutional autoencoder (CAE) is applied to map the high-dimensional flow dynamics into the low-dimensional nonlinear manifold. The geometric features are considered in the input matrix of the CAE network by setting physical quantities inside the airfoil to zero to reflect the physics. Then, the multi-head attention-strengthened long short-term memory (MH-LSTM) is utilized to predict the evolutions of latent vectors. Finally, the high-dimensional flow dynamics can be reconstructed by utilizing the decoder part of CAE. We believe that the MH-LSTM methodology provides a promising path towards predicting the unsteady flow field over the airfoil by capitalizing on the capability of multi-head attention allowing for attending to distinct parts of input sequence.

Effects of vortex generator on the hydrodynamic characteristics of hydrofoil and horizontal axis tidal turbine

Tidal turbine blades are prone to flow separation in the boundary layer under high speed or high angle of attack, which will reduce energy efficiency and even the stall damage of the blades. This paper proposes introducing the flow control theory of vortex generators (VGs) to tidal turbines and studying the influence of VGs on the hydrodynamic characteristics of the tidal turbine blades. First, a numerical study is performed to investigate the effects of VGS on the hydrodynamic performance of the National Advisory Committee for Aeronautics (NACA) 4418 hydrofoil. The impact of different parameters, such as VG arrangement, spacing, height, and length, on the hydrodynamic performance of hydrofoil is studied by the computational fluid dynamics method. The results show that VGs can effectively suppress the flow separation and improve the maximum lift coefficient of the hydrofoil. The influence of VGs on flow separation characteristics of horizontal axis tidal turbines is studied by the CFD method. The results show that the flow separation of turbine blades mainly occurs at the root part of the suction surface, and the flow separation region expands radially as the flow velocity increases. VGs can effectively reduce the flow separation area on the suction side of turbine blades by suppressing the flow separation effect. Compared with the turbine blades without VGs, the power coefficient of turbine blades with VGs is increased by up to 5%. The flume experiment verifies the accuracy of the simulation results.

Inflow turbulence distortion for airfoil leading-edge noise prediction for large turbulence length scales for zero-mean loading.

Turbulence distortion due to airfoil finite thickness is an important but not fully understood phenomenon that affects the airfoil radiated noise, resulting in inaccurate noise predictions. This study discusses the turbulence distortion in the leading edge (LE) region of an airfoil aiming to obtain more accurate LE noise predictions. Wind tunnel experiments were performed for National Advisory Committee for Aeronautics (NACA) 0008 and NACA 0012 airfoils at zero angle of attack subjected to large turbulence length scales (between 10 and 43 times the airfoil LE radius) generated by a grid and a rod. Hot-wire and surface pressure measurements were performed in the LE region. Results show that the root mean square of the velocity fluctuations urms and the turbulence integral length scale Λf at the stagnation line decrease considerably as the LE is approached. Rod-airfoil radiated noise was measured and compared with Amiet's model. The predicted noise overestimates the LE noise for high frequencies. However, the prediction agrees well with measurements when the turbulence spectrum based on the rapid distortion theory is used in Amiet's model, with as inputs the urms and Λf values measured close to the LE. This work's main contribution is to demonstrate that more accurate noise predictions are obtained when the inputs to the model consider the turbulence distortion effects.

Epidemiology of Emergency Medical Search and Rescue in the North Shore Mountains of Vancouver, Canada, from 1995 to 2020.

Little is known about the epidemiology of emergency medical search and rescue incidents globally. The purpose of this study was to describe the epidemiology of emergency medical search and rescue incidents in the North Shore Mountains of Vancouver, British Columbia, Canada. This was a retrospective review and descriptive analysis of search and rescue incident reports created by North Shore Rescue over a 25 y period from 1995 to 2019, inclusive. Incident reports were screened for inclusion against a priori criteria defining a medical callout. The National Advisory Committee of Aeronautics (NACA) severity score was used as a method to grade medical acuity of included subjects. We included 906 subjects. Their median age was 35 y (interquartile range, 24-53), and 65% of subjects were men. Forty-one percent (n=371) of subjects were classified as non-trauma and 54% (n=489) as trauma. The top 3 activities were hiking (53%), biking (10%), and snow sports (10%). Forty-nine percent of incidents were classified as having a NACA score of ≥3. For subjects with trauma, the top 3 body regions were lower limb (52%), head (18%), and torso (12%). For subjects with non-traumatic conditions, the top 3 causes were mental health crises (25%), exposure (25%), and cardiovascular incidents (11%). Half of the incidents were serious enough to require medical assessment at a hospital (NACA score ≥3). Given this medical acuity, there is a need for evidence-based guidelines and core training competencies for mountain medical search and rescue. Standardized core data sets and outcomes are needed to monitor quality of care over time.

Proper orthogonal decomposition analysis of the cavitating flow around a hydrofoil with an insight on the kinetic characteristics

In this research, the cavitating flow around a NACA0015 (National Advisory Committee for Aeronautics) hydrofoil obtained by the large-eddy simulation method is analyzed using the proper orthogonal decomposition (POD) theory. Various fundamental mechanisms have been investigated thoroughly, including the reentrant jet behavior, pressure gradient mechanism, vortex dynamics, and dynamic properties of the hydrofoil. The influence of the vortex dynamics, pressure mechanism, and temporal/spatial evolution is revealed. The POD decomposition indicates that the first four dominant POD modes occupy 97.4% of the entire energy. Based on the vortex force field extracted from the first four single POD modes, it is found that the lift-and-drag characteristics in the cavitating flow are determined by the specific spatial distribution of mode vortex structures. In addition, the coupling of velocity pulsations and pressure fluctuations is carried out to obtain the POD modal pressure gradient field, which reveals that the pressure gradient has a close connection with the cavity evolution. Furthermore, the vortex force and pressure gradient are reconstructed using the first four modes, 17 modes, and 160 modes, which indicates that the low-order POD modes without the impact of small-scale structures and noise can clearly capture the fundamental aspects of the flow field.

BASE Jumping in the Lauterbrunnen Valley: A Retrospective Cohort Study from 2007 to 2016.

BASE jumping, and especially BASE jumping with the help of wingsuits, is considered one of the most dangerous airborne sports. The valley of Lauterbrunnen in Switzerland has become infamous for the large number of BASE jumps and the high rate of accidents and fatalities. The aim of this study was to evaluate the morbidity and mortality of BASE jumping, to determine the severity of injuries and injury patterns of BASE jumping accidents and to compare preclinical assessment with clinical diagnoses to detect under- or overtriage. This retrospective, descriptive cohort study covers a period of 10 years (2007-2016). The evaluation covered all BASE jumping incidents in the valley of Lauterbrunnen that required either a helicopter mission by the local HEMS (Helicopter Emergency Medical Service) company of Lauterbrunnen, Air Glaciers, or medical care in the regional hospital, the level I trauma centre or the medical practice of the local general practitioner. Besides demographic data, experience in BASE jumping and skydiving as well as BASE jumping technique(s) and details about the rescue missions were collected. The medical data focused on the severity of injuries, as expressed by the National Advisory Committee of Aeronautics (NACA) score in the prehospital assessment as well as the Abbreviated Injury Scale (AIS) and Injury Severity Score (ISS) retrieved from the clinical records in the hospital or medical practice setting. The patients were predominantly young, experienced male BASE jumpers. Morbidity (injury risk) ranged from 0.05% to 0.2%, and fatality risk from 0.02% to 0.08%. Undertriage was low, with only two cases. Overtriage was significant, with 73.2% of all NACA 4-6 cases not qualifying for major trauma. BASE jumping remains a high-risk sport and is associated with significant rates of injuries and fatalities. Comparison with previous studies indicated that the injury rate may have decreased, but the fatality rate had not. In this known BASE jumping environment, prehospital assessment appears to be good, as we found a low undertriage rate. The high overtriage rate might be an expression of physicians' awareness of high-velocity trauma mechanisms and possible deceleration injuries.

Application and comparison of dynamic mode decomposition methods in the tip leakage cavitation of a hydrofoil case

The cavitation of the tip leakage vortex (TLV) induced by tip leakage has always been a difficult problem faced by turbomachinery, and its flow structure is complex and diverse. How to accurately extract the main structures that affect the cavitating flow of the TLV from the two-phase flow field is a key problem. In this study, the main mode extraction and low order mode reconstruction accuracy of the cavitation flow field of TLV downstream of National Advisory Committee for Aeronautics (NACA)0009 hydrofoil by two dynamic mode decomposition (DMD) methods are compared. The research shows that the main modes extracted by the standard DMD method contain a large number of noise modes, while the sparsity-promoting DMD eliminates the noise modes, showing obvious advantages in the reconstruction accuracy of the velocity field. The characteristics of cavitation signals are analyzed, and the cavitation signals are divided into four categories, which explains the reason why DMD methods have low reconstruction accuracy in cavitation. This study provides a theoretical basis and strong guarantee for the extraction of mode decomposition characteristics of the two-phase flow field. This is of great significance for accelerating the prediction of multiphase flow fields based on intelligent flow pattern learning in the future. Meanwhile, it also provides a new method and road for the introduction of artificial intelligence technology in future scientific research.

Effect of a textured surface on the occurrence and development of cavitation on the hydrofoil

An experimental study of the cavitation flow around the National Advisory Committee for Aeronautics 0012 hydrofoil with different surface morphology was carried out in this work. The surface morphology was set by modern laser ablation technology. The average values and intensity of vapor–gas cavities were determined. It has been revealed that laser texturing delays the emerging cavitation and somewhat decreases its intensity at higher cavitation numbers. A decrease in the cavitation number leads to an increase in its intensity for a smooth hydrofoil in comparison with a rough one, which is also expressed in an increase in the frequency of cavities. The paper presents a comparison of the flow regime with equal cavitation numbers, which clearly describes the features of the development of a vapor–gas cavity on the suction side of the foil with different surface morphologies. The paper provides an explanation of the reasons for the influence of surface morphology on the development of cavities.

Large eddy simulation of dynamic stall over an airfoil and its control with plasma actuator

The purpose of this study is to investigate deep dynamic stall over a pitching National Advisory Committee for Aeronautics 0015 airfoil and its control with an alternating current dielectric barrier discharge (AC-DBD) plasma actuator by means of large eddy simulation. The airfoil oscillates at a reduced frequency of k = 0.08 with the angle of attack (AoA) varying between 5° and 25°. The chord-based Reynolds number is Re = 7.6 × 104. Rich fluid dynamics and physics involved in the dynamic stall are resolved by large eddy simulation. It is found that the dynamic stall under consideration initiates with the onset of a leading edge vortex (LEV). The deep dynamic stall is characterized and dominated by four successive LEVs at high angles of attack. The control of dynamic stall with AC-DBD plasma actuation is investigated in depth. At low AoAs, the plasma forcing can delay the laminar-to-turbulent transition of the attached flow over the suction surface of the airfoil. Although the formation of the first LEV is delayed by the plasma actuation, the first three LEVs cannot be effectively suppressed in high AoAs beyond 20°. The impact of the AC-DBD on the flow becomes increasingly pronounced in the downstroke phase, and at the intermediate angles of attack, the flow separation can be effectively controlled. Substantial gain from the flow control is obtained. The area of the lift hysteresis loop and drag force are appreciably reduced, and the nose-down pitch moment is alleviated notably. Finally, the influence of forcing frequency on flow control is examined.

Aeroacoustic control mechanism on near-wall-wing of Aero-train based on plasma jet

In this study, an aeroacoustic control mechanism of a plasma jet acting on a high-speed moving wing under a wing-in-ground effect is investigated. Moreover, a novel method is proposed to reduce the aeroacoustics of Aero-train wings. Numerical simulations of the aeroacoustics generated by flow around a National Advisory Committee for Aeronautics 4412 wing are performed under three different plasma excitation modes at four clearances with an incoming flow velocity of 0.3 Ma and an angle of attack of 5°. The results show that different plasma excitation modes interfere with the vortex generation and development in different ways to achieve aeroacoustic reduction. The UP excitation mode delays the airflow separation, delays the vortex generation and development, and reduces the vortex intensity. The BOTH excitation mode forces transverse vortices to transform into streamwise hairpin vortices and reduces the local pressure fluctuation intensity. Hence, plasma jets exhibit a good control effect on the peak aeroacoustics under different clearance conditions but result in the frequency shift effect of acoustic energy transfer to high frequencies. The modal analysis of the flow field of the three excitation conditions via a proper orthogonal decomposition method reveals that the trend of the modal change is similar for the three excitation conditions, and the change in each order of the modal corresponds to the energy decrease at the peak frequency and the energy increase at high frequencies.