National Advisory Committee For Aeronautics Research Articles

Aerodynamic characteristics and flow topology of tapered symmetrical airfoil equipped with Clark-Y shaped vortex generators

The aerodynamic characteristics and surface flow topology of tapered swept-back National Advisory Committee for Aeronautics 0020 airfoil equipped with Clark-Y shaped vortex generators (CsVGs) are researched. The force and surface oil flow visualization for the tapered swept-back wing having a swept angle of 25° and a taper ratio of 0.4 is performed at a Reynolds number (Re) of 1.2×105 via CsVGs at x/c = 0.2, 0.3, 0.4, and 0.5. The airfoil equipped with CsVGs at x/c = 0.2 [T2 at an angle of attack (AoA) of 13°] showed a 15.53% improvement in the maximum CL and a delaying stall by 2° comparison with the baseline [at AoA of 11°]. According to oil flow visualization, the T2 model enhanced the post stall characteristics via CsVGs by increasing the attached flow region from the root to the tip as compared to the baseline. While the straight laminar separation bubble (LSB) for the baseline was observed, a wavy LSB for T2 occurred due to the interaction of LSB with CsVGs located at 0.2c. Primary attach flow regions on the downstream of each CsVG, attached flow regions on the downstream of CsVG pairs, and interaction line due to the interaction of primary and attached flow region were identified via oil flow visualization.

Time-resolved deep reinforcement learning for control of the flow past an airfoil

The current work proposes a method for the active control of flow over a National Advisory Committee of Aeronautics 0012 airfoil under turbulent condition based on time-resolved deep reinforcement learning (DRL). To leverage the coherent structures in the temporal evolution of the flow field, we integrate the long short-term memory (LSTM) network with the proximal policy optimization (PPO) method. Based on this LSTM-PPO method, the model obtained an improved strategy for controlling the mass flow rates of the three jets located on the upper surface of the airfoil to control the flow and increase the lift-to-drag ratio. The LSTM-PPO method is first compared with the traditional PPO method at Re = 2500, achieving a remarkable 160.9% enhancement of the lift-to-drag ratio. Then, the trained LSTM-PPO model is tested under several operation conditions, manifesting its adaptability. Dynamic mode decomposition is also used to study the change in the dynamics with and without the active flow control (AFC) based on the LSTM-PPO method. This study highlights the potential of recurrent neural networks to cooperate with DRL algorithms, paving the way for real-world applications of AFC.

Reynolds-averaged Navier–Stokes assisted direct numerical simulation of low Reynolds number flow over airfoils

The simulation of insect flight, like that of dragonflies operating at low Reynolds numbers, has numerical challenges due to the complex morphological structure. The corrugated airfoils trap vortices, and in these recirculation zones, turbulence models may be inadequate to resolve the near-wall flow features well. Hence, accurately capturing the laminar–turbulence transition and identifying the point of separation and reattachment requires resorting to direct numerical simulations (DNS) over a large domain, that is computationally expensive. We propose conducting DNS over a truncated subdomain constrained by Reynolds-averaged Navier–Stokes (RANS) solution to reduce the computational domain size and costs. A precomputed RANS simulation over a large domain is used to prescribe a velocity boundary condition (BC) at the truncated domain of the DNS; a convective BC is imposed as outflow. We validate the proposed RANS-assisted DNS (DNSR) by simulating subdomains of varying sizes and comparing the mean and fluctuating velocity fields, and aerodynamic characteristics with the DNS with free-slip BC. This technique reduces domain size and computational cost significantly (by at least half). A criterion for the ideal subdomain size is determined by satisfying the condition at the location where the non-dimensional mixing length is approximately 60. We validated our criterion by simulating flow over the NACA (National Advisory Committee for Aeronautics) 0012 airfoil at angles of attack α≤12°, corroborating with established literature. Finally, we study a three-dimensional corrugated airfoil with our approach, to capture the transition from two- to three-dimensional structures in the wake as the angle of attack increases.

Combined experimental and computational investigation of the influence of micro vortex generator on incipient cavitation mode

The objective of this paper is to investigate the effect of a passive control method on the incipient cavitation mode around a hydrofoil. Two micro vortex generators (mVGs) with different positions are installed on the leading edge (LE) of the NACA (National Advisory Committee for Aeronautics) 66 hydrofoil. The mVG-1 has the same structural parameters as the mVG-2, but it is closer to the LE of hydrofoil. A high-speed camera is used to record the transient behavior of the cavitating flow. The large eddy simulation combined with a mass transport model is applied to analyze the influence mechanism of mVG on the incipient cavitation mode. The results show that the three typical incipient cavitation modes are observed on the baseline hydrofoil, i.e., spot cavity, patch cavity, and finger cavity. The mVG induces the generation of vortex shaped like thin strips of fingers at its trailing edge, called the fingerlike vortex cavitation. The neighboring fingerlike vortices constitute a pair of counter-rotating vortices with equal sizes and opposite directions. It influences the near-wall flow state upstream and downstream of mVG and, thus, the incipient cavitation structure. For the mVG-1 hydrofoil, the fingerlike vortex cavitation is a unique form of the incipient cavitation mode, making the cavitation onset position fixed at the mVG tail. For the mVG-2 hydrofoil, the mVG has a significant effect on the incipient cavitation structure at small attack angles, changing both the incipient cavitation mode and position.

Effect of morphed trailing edge on the acoustic and flow characteristics of National Advisory Committee for Aeronautics (NACA) 4418 airfoil

Trailing-edge profile has an important impact on the aeroacoustic behavior of an airfoil, particularly the tonal noise in the low-medium Reynolds number range. Three profiles, NACA 4418 defined by the National Advisory Committee for Aeronautics (NACA) and its morphed variants with the trailing-edge deflections at ±8°, are investigated by wind tunnel experiments to reveal the acoustic and flow characteristics at the chord-based Reynolds number ranging from 1.2×105 to 3.1×105. The consistent observation across all profiles is the dominant feedback loop located on the pressure surface. The aeroacoustic findings show that the morphed trailing edge alters the tonal frequency spacing, mainly due to the change of the vortex convection velocity in the feedback loop. Furthermore, the trailing-edge morphing is observed to modulate the mode switching of the dominant tones. The local flow results reveal that the laminar separation bubble on the pressure side amplifies the flow oscillation. Notably, the profile with the trailing-edge upward deflection, characterized by a laminar separation bubble on the pressure surface, is particularly susceptible to the low-frequency broadband oscillation. The joint analysis of the acoustic and flow fields suggests that the laminar separation bubble heightens the sensitivity of the dominant tone mode switching to the incoming flow velocity.

Efficient aerodynamic shape optimization using transfer learning based multi-fidelity deep neural network

Computational efficiency and precision pose a classic contradiction in aerodynamic shape optimization. To address this challenge, this study introduces an effective optimization framework based on multi-fidelity fully connected neural network (MFFCN). The framework utilizes transfer learning (TL) to train a multi-fidelity surrogate model that establishes direct mappings between geometric configuration parameters and aerodynamic performance by adaptively capturing linear or nonlinear relationships concealed between high-fidelity (HF) and low-fidelity (LF) information. The HF and LF data are derived from fine and coarse grids, respectively, evaluated using the same computational fluid dynamics (CFD) model. The MFFCN-TL framework is applied to optimize the National Advisory Committee for Aeronautics 0012 (NACA0012) airfoil (12 design variables) and the Office National d'Études et de Recherches Aérospatiales M6 (ONERA M6) wing (50 design variables). Simulation results demonstrate that the NACA0012 airfoil achieves a 69.47% enhancement in lift–drag ratio in 1.069 s, compared to a 12.76% gain over 24.8 h in single-fidelity CFD-based optimization. The ONERA M6 wing achieves a 24.66% reduction in drag coefficient in 694 ms compared to 18.37% over 237.3 h in the CFD model. Statistical results show that the MFFCN-TL framework can reduce optimization cost by more than 90% compared to the single-fidelity CFD-based model. These findings suggest that the MFFCN-TL framework significantly enhances optimization efficiency and provides superior feasible solutions over single-fidelity methods.

Numerical and experimental investigation of Darrieus vertical axis wind turbines to enhance self-starting at low wind speeds

Wind energy, being renewable, cost-effective, and environmentally friendly, has attracted global attention. However, due to suboptimal performance and limited research, vertical axis wind turbines (VAWTs) lag behind horizontal axis wind turbines (HAWTs) in commercial applications, particularly for large-scale installations. This study aims to improve the self-starting capability of the Darrieus VAWT. While some parameters, such as the number of blades (N) and solidity (σ), have been studied extensively, the airfoil shape has not received as much attention. This study compares the performance of National Advisory Committee for Aeronautics (NACA) airfoils and Selig airfoils at a Reynolds number (Re) of 40,673. The investigation revealed that the NACA0015 airfoil exhibited the highest peak power coefficient (Cp). Further analysis utilizing an advanced double multiple stream tube (DMST) code in MATLAB increased the peak Cp by adjusting the thickness-to-camber ratio (t/c) of the NACA0015 airfoil, resulting in a 12.50 % increase in the maximum achievable Cpat a Re of 40,673. This study compared four modes of VAWT operation, utilizing the NACA0015 airfoil and a modified NACA0015 airfoil for both straight-bladed and embossed-bladed VAWTs. The results showed that the modified NACA0015 airfoil for embossed-bladed VAWTs exhibited the best self-starting capability and rotation at wind velocities of 1 to 9 m s-1. Additionally, the self-starting force required by embossed-bladed VAWTs was lower than that needed by straight-bladed VAWTs due to the ability of the embossed material to enhance airflow attachment to the VAWT and suppress turbulence.

Large-eddy simulation of tip clearance cavitating flow around a hydrofoil

Large-eddy simulations of tip clearance cavitating flow between a National Advisory Committee for Aeronautics 0012 hydrofoil and the endwall have been performed to investigate the effects of cavitation on vortical structures, turbulence statistics and hydrofoil performance within the tip clearance region. The wall-adapting local eddy-viscosity model is employed to compute the subgrid-scale stress, and the Zwart-Gerber-Belamri cavitation model is used to predict the cavitating flow. The simulation results show that cavitation has insignificant effects on the tip clearance flow structures and the wandering motion of the tip leakage vortex (TLV). Detailed analysis of turbulence statistics indicates that the turbulent kinetic energy and pressure fluctuation inside the TLV core are significantly increased under the effect of cavitation. In addition, it is found that cavitation enhances the unstable wall-normal motion of the TLV, leading to the enhancement of turbulence near the TLV core. The hydrofoil performance affected by cavitation is also investigated, showing that the lift and drag forces acting on the hydrofoil is substantially reduced due to the cavitation.

Blade height impact on self-starting torque for Darrieus vertical axis wind turbines

Self-starting torque (TSelf-starting) presents a significant challenge for Darrieus vertical axis wind turbines (DVAWTs), often necessitating external assistance to initiate rotation. This study addresses the issue by optimizing airfoil design, employing embossed blades (EBs), and adjusting blade height (H) to reduce TSelf-starting. From an analysis of 43 rotors at a chord-based Reynolds number (Rec) of 45,192, national advisory committee for aeronautics (NACA) 0015, NACA4412, and NACA4415 rotors were selected for their superior power coefficients (Cp). These rotors were optimized using double-multiple streamtube theory (DMST) and particle swarm optimization (PSO), focusing on the thickness-to-chord ratio (TCR). Among them, the NACA0015-Opt rotor achieved the highest Cp, demonstrating its effectiveness in enhancing DVAWT efficiency. This study also investigates the effect of H on the performance of EBs, comparing H of 35 cm and 75 cm. Experimental findings reveal that combining airfoil optimization with EBs, along with an increased H, leads to a substantial decrease in TSelf-starting. Specifically, higher H enhance the aerodynamic performance of EBs by improving airflow over the blade surface, further reducing drag and contributing to a significant reduction in TSelf-starting. At a H of 75 cm, the embossed blade Darrieus vertical axis wind turbine (EB-DVAWT) equipped with the optimized NACA0015-Opt rotor required 15.92 %, 17.04 %, 18.12 %, 21.23 %, 52.06 %, 49.23 %, 51.25 %, 35.20 %, 14.12 %, and 9.09 % less TSelf-starting at wind velocities (U∞) of 1, 2, 3, 4, 5, 6, 7, 8, 9, and 9.5 m/s, respectively, compared to the baseline smooth blade Darrieus vertical axis wind turbine (SB-DVAWT) with the original NACA0015 rotor.

Investigation on transition characteristics of hydrofoil boundary layer based on algebraic local-correlation-based transition modeling model

Hydrofoil shapes are used for the marine turbine blades to capture kinetic energy from water currents effectively. Predicting transitions is a critical concern when studying the hydrofoil boundary layer. This paper analyzed the transitional behavior of the boundary layer in the National Advisory Committee of Aeronautics (NACA) hydrofoil, NACA0009, with a blunt trailing edge using the Algebraic Local-Correlation-based Transition Modeling (Algebraic LCTM) model. First, through sensitivity analysis, the effects of the maximum y+ (the dimensionless distance y to the wall), grid expansion ratio, number of normal and streamlined grids, and timescale on transition prediction were studied. The results indicate that finer y+ value and appropriate grid expansion ratios can improve the accuracy of transition prediction, while the influence of timescale on the prediction results is relatively small within the range of Courant number theory values. Second, further analysis was conducted on the transition prediction performance under different Reynolds numbers. It was found that the model predictions were consistent with experimental values at low Reynolds numbers, but the predicted transition position was advanced at high Reynolds numbers, mainly because of the significant disparity in eddy viscosity coefficients within the free flow field. In the study of leading-edge roughness bands' impact on boundary layer transition for hydrofoil, the introduction of roughness significantly expedited the transition process. The Algebraic LCTM model outperformed the gamma (γ) transition model, reducing prediction errors by 5–40% for boundary layer parameters and maintaining errors between 0.005 and 4% for wake vortex shedding frequency, as opposed to the γ model's 0–23%. The results of this study provide a theoretical basis for hydrofoil design.

Noise reduction of an airfoil model covered by bio-inspired herringbone riblets

It is curious that whether the typical herringbone pattern of bird flight feathers plays a role in attenuating aeroacoustic noise. Being motivated by this, experiments are conducted to investigate noise reduction of the NACA0012 (National Advisory Committee for Aeronautics) airfoil-based model with one side surface covered by bio-inspired herringbone patterns of riblets. The herringbone-ribbed surface is defined by the divergent angle β (= 60°) of the riblets pattern, the spanwise wavelength λ (= 0.2c and 0.4c) of the pattern, and the riblet height h (= 0.6%c and 1.2%c), where c is the chord length of the airfoil. While far-field sound pressure fluctuations are measured via microphones in an anechoic wind tunnel, flow fields around the model are captured using particle image velocimetry (PIV) in a water tunnel. The effective angle of attack of the test models ranges from αeff = −11.1° to 11.1° and Reynolds number considered is from Re = 2.3 × 105 to 7.8 × 105. Compared with the baseline smooth models, the models with the riblet pattern on the pressure side are able to substantially suppress the tonal noise, associated with considerable reduction in the overall noise. The reduction in the overall sound pressure levels of the tonal noise and the overall noise are up to 21.3 dB and 20.5 dB, respectively, at αeff = −2.2° and Re = 3.6 × 105. The noise reduction is attributed to the transition of laminar to turbulent boundary layers over the herringbone-ribbed surface, particularly in the saddle location where the spanwise-repeated herringbone pattern converges. Behavior of shear layers separating from the trailing edge of the model is examined, corroborating the proposition that the acoustic feedback loop is impaired by the herringbone-ribbed surface.

Large eddy simulation of micro vortex generator-controlled cavitation across multiple stages

Micro vortex generators (mVGs) control cavitation by altering the boundary layer flow structure. This study employs the wall-adapting local eddy-viscosity large eddy simulation (WALE-LES) turbulence model combined with the Zwart–Gerber–Belamri cavitation model to conduct transient numerical simulations on the National Advisory Committee for Aeronautics 0015 baseline hydrofoil and the hydrofoil equipped with mVGs under various cavitation numbers. The proper orthogonal decomposition method and experiments verify the accuracy and consistency of these simulations regarding cavity scale. The study elucidates mechanisms by which mVGs suppress cloud cavitation at low cavitation numbers and induce vortex cavitation at high cavitation numbers. Results indicate that mVGs maintain sheet cavitation characteristics at low cavitation numbers, reducing wall pressure fluctuations and enhancing flow stability. During cavitation inception, mVG-induced vortex cavitation leads to early cavitation formation. In the sheet cavitation phase, modal energy distribution is more dispersed, while in the inception phase, energy is concentrated with significant dominant modes. Moreover, the counter-rotating vortices generated by mVGs mitigate flow separation, enhance leading-edge flow attachment stability, and reduce high-frequency vibrations caused by bubble shedding. This study significantly advances the understanding of cavitation control by accurately simulating and revealing the cavitation control mechanisms of mVGs across different stages using the WALE-LES model. The findings demonstrate that mVGs can effectively stabilize cavity structures at low cavitation numbers, reducing flow instabilities and enhancing overall hydrofoil performance. These insights will have a significant impact on the design of hydrofoils and the development of cavitation control strategies.

Aeroacoustic and aerodynamic measurements at the rotor plane in the interaction of a small rotor with wings.

Current research on tiltrotor noise predominantly concentrates on configurations with high disk loading and Reynolds numbers, leaving smaller aircraft setups underexplored. This study investigates aeroacoustic and aerodynamic trends resulting from rotor-wing interaction at low disk loading (<100 N/m2) and Reynolds number (Re < 100 000). The experimental setup comprises an anechoic chamber housing a two-blade rotor, flat and National Advisory Committee for Aeronautics 0012 airfoil wings, an ATI mini 40 load cell for aerodynamic data acquisition, and microphones positioned at the rotor height and installed on a rotation stage for acoustic data capture and directivity check. Investigated factors encompass rotor height, rotation direction, revolutions per minute (RPM), and wing curvature. Contrary to expectation, wing curvature does not visibly impact rotor performance. However, the deflected rotor wake in rotor-wing interaction markedly amplifies low-frequency broadband noise and the overall sound pressure level for the tested scenarios. The presence and strength of the deflected rotor wake tend to obscure the primary tonal noise and mitigate the effect of rotor RPM at smaller rotor spacings. This study provides valuable insights into mitigating noise resulting from rotor wake impingement on the wing in smaller aircraft configurations, contributing to the ongoing evolution of urban air mobility design considerations.

Computational comparison of passive control for cavitation suppression on cambered hydrofoils in sheet, cloud, and supercavitation regimes

Cavitation is a transient, highly complex phenomenon found in numerous applications and can have a significant impact on the characteristics as well as the performance of the hydrofoils. This study compares the evolution of transient cavitating flow over a NACA4412(base) (NACA stands for National Advisory Committee for Aeronautics) cambered hydrofoil and over the same hydrofoil modified with a pimple and a finite (circular) trailing edge. The assessment covers sheet, cloud, and supercavitation regimes at an 8° angle of attack and the Reynolds number of 1×106, with cavitation numbers ranging from 0.9 to 0.2. The study aims to comprehensively understand the role of the rectangular pimple in controlling cavitation and its impact on hydrodynamic performance across these regimes. Numerical simulations were performed using a realizable model and the Zwart–Gerber–Belamri (ZGB) cavitation model to resolve turbulence and cavitation effects. The accuracy of the present numerical predictions has been verified both quantitatively and qualitatively with available experimental results. The present analysis includes the time evolution of cavities, temporal variation in total cavity volume, time-averaged total cavity volume, distributions of vapor volume fractions along the chord length, and their hydrodynamic performance parameters. Results demonstrate that rectangular pimples have significant impacts in the different cavitation regimes. In the sheet cavitation regime (σ=0.9), the NACA4412(pimpled) hydrofoil exhibits minimal cavity length and transient volume changes as compared to the NACA4412(base) hydrofoil. In the cloud cavitation regimes (σ=0.5), cavity initiation occurs differently, starting from the pimpled location for the NACA4412(pimpled) hydrofoil, unlike the initiation just downstream of the nose in the case of base hydrofoil. In the supercavitation regimes (σ=0.2), the cavity length remains comparable, but the NACA4412(pimpled) hydrofoil exhibits larger cavity volume evolution in both cloud and supercavitation regimes (σ=0.5 and σ=0.2) after initial fluctuations. Furthermore, hydrodynamic performance for the NACA4412(pimpled) hydrofoil shows 41%, 36%, and 17% lower lift coefficients, and 46%, 27%, and 9% lower drag coefficients in sheet, cloud, and supercavitation, respectively.

Enhancing underwater unmanned vehicle efficiency through asymmetric dynamics in manta-like swimming

This study explores the hydrodynamic performance of a National Advisory Committee for Aeronautics 0012 foil using computational fluid dynamics with the unsteady Reynolds-averaged Navier–Stokes (URANS) method and shear stress transport k-ω model to assess the impact of asymmetric motion parameters in manta-like swimming. The angles of attack during the mid-upstroke (αmu), mid-downstroke (αmd), and stroke duration (S) are varied to understand their effect. At low Strouhal numbers (StA = 0.2–0.35), a smaller αmd compensates for thrust loss at the start of the upstroke due to a greater αmu. At high Strouhal numbers (StA = 0.5), a greater αmd reduces negative thrust and compensates for the smaller thrust generated by a small αmu during the upstroke. Shorter stroke durations increase asymmetry, leading to more significant positive thrust peaks during the downstroke. If both the angle of attack and S are large, the slower downward speed extends negative thrust, reducing thrust peaks and lowering average thrust. A smaller stroke duration combined with a large angle of attack enhances efficiency due to a greater thrust-to-power ratio, highlighting the interplay between these parameters. A smaller S and greater αmd and StA maximize thrust and efficiency, suggesting aquatic organisms increase thrust while ensuring propulsion efficiency by using a large angle of attack and an asymmetric stroke duration. This study demonstrates how asymmetric parameters interact, providing insights into designing biomimetic underwater vehicles. The findings suggest that asymmetric dynamics enhance propulsion efficiency.

Emergency Medical Care Provided by North Shore Rescue Advanced Medical Providers.

The North Shore Rescue (NSR) Advanced Medical Provider (AMP) program is composed of physicians and nurses based in North Vancouver who attend high acuity medical search and rescue (SAR) callouts in British Columbia, Canada. This study aimed to analyze the medical care provided by AMPs with appropriate comparisons to non-AMP callouts. A retrospective review of all NSR callouts from January 1, 2018, to December 31, 2022, was conducted. The analysis included AMP involvement, rescue logistics, subject demographics, activity, primary cause, provisional diagnosis, treatments, medical decision-making, and extraction means. National Advisory Committee for Aeronautics (NACA) scores were assigned by physicians to evaluate medical acuity as well as under-triage and over-triage. Of the 767 NSR callouts over the 5-year span, 283 (37%) were medical, and of these, 35% (n = 99) involved AMPs. Seventy-five percent of AMP rescues involved traumatic injuries, and 31% involved nontraumatic medical illnesses. The mean NACA score for AMP callouts was significantly higher than non-AMP callouts (3.1 ± 1.3 vs 1.9 ± 1.3, p < .00001). Medications were administered in 40% of AMP rescues, procedures were performed in 54%, and 37% involved advanced medical decision-making. Over-triage occurred in 33% of AMP callouts, with under-triage in 10%. The AMP program provides a useful service when advanced medical care in wilderness environments is needed. AMPs coordinate appropriate medical response and ensure safe, comfortable, and efficient transport to definitive care. The NSR AMP program may act as a model for the development of similar programs by other SAR teams.

Characteristics of flow passing over Hydrofoil Crested Stepped Spillway

A spillway comprises several sections, primarily the crest and chute, essential for safe water flow from an upstream reservoir to a downstream river. Incorporating a hydrofoil crest and stepped chute features creates an efficient Hydrofoil-Crested Stepped Spillway (HCSS). The hydraulic features of the 60 HCSS models, which were designed through the National Advisory Committee for Aeronautics (NACA) equation, are assessed through laboratory experiments under a skimming flow regime, where the five hydrofoil formation indexes (t), three numbers of steps (Ns), and four chute angles (θ) are of concern. The findings indicate that an escalation in the ratio of upstream depth (yup) to spillway height (P), correlates with a rise in the discharge coefficient (Cd) in HCSSs. Furthermore, augmenting the t effectively boosts the Cd value. On average, the Cd value of the crest at t = 1 surpasses that at t = 0.2 by 20 %. This increase in t accentuates the curvature of the spillway crest, thereby enhancing the curvature of streamlines and reducing the flow distance therein. The Energy Dissipation Ratio (EDR) of HCSS is influenced by factors such as t (positively correlated), Ns (positively correlated), and θ (negatively correlated). Optimal performance of the HCSS is observed at t = 1, where both the Cd and EDR reach their peak values.

Unsteady aerodynamic flow control at low Reynolds numbers via internal acoustic excitation

Aerodynamic flow control using internal acoustic excitation holds promise as it combines the simplicity of passive flow control techniques (in terms of added weight and operational complexity) with the control authority of active flow control methods. While previous studies have analyzed the effects of acoustic excitation on steady-wing aerodynamics, the effect of excitation on the unsteady aerodynamics is not known, which is the aim of the current effort. Internally mounted speakers on a symmetric National Advisory Committee for Aeronautics (NACA) 0012 wing are used to excite the unsteady boundary layer at the wing's leading edge as it executes linear pitch motions ranging from quasi-steady (trailing-edge driven stall) to vortex-dominated (mixed leading- and trailing-edge driven stall) motions at freestream Reynolds numbers (Re) of 120 000 and 180 000. Experimental results show that, although acoustic excitation delays stall for quasi-steady motions, it enhances lift in the linear region and increases leading-edge vortex strength for vortex-dominated motions. The degree of change was observed to be a function of the excitation frequency, with lower frequencies (≤ 250 Hz) leading to an increase in aerodynamic efficiency and higher frequencies having a negligible effect. The current work establishes the effects of acoustic flow excitation in unsteady, low-Re wing aerodynamics and provides insights on the path forward to effectively implement the method for active flow control.

An experimental investigation into the influence of the micro vortex generator on the leading stability of cloud cavities around a hydrofoil

The objective of this paper is to investigate the effect of a passive control method on the leading stability of a cloud cavity around a hydrofoil. Two differently positioned micro vortex generators (mVG) are installed on the leading edge (LE) of a National Advisory Committee for Aeronautics 66 hydrofoil. The structural parameters of mVG-1 are the same as those of mVG-2, but closer to the LE of the hydrofoil. A high-speed camera is employed to capture the transient evolution of cavitating flow. The results show that the cloud cavities on the baseline hydrofoil are divided into the hybrid cavity mode (α = 6°) and the fingerlike cavity mode (α = 8°–12°), relying on the cavity LE structure. The hybrid cavity consists of coupled traveling bubbles and fingerlike cavities, dominated by fingerlike cavities. The fingerlike cavity is attached to cavities with only a single form of LE. The hybrid cavity is replaced by fingerlike vortex cavitation on the mVG hydrofoil, leading to a fixed incipient position of the cavity. Fingerlike cavity structures on the three hydrofoils are generated by different mechanisms. The fingerlike vortex cavity of the mVG-1 hydrofoil is induced by the mVG, whereas the other two hydrofoils are induced by boundary layer separation and spanwise.

Cartesian mesh adaptation: Immersed boundary method based on high-order discontinuous Galerkin method

Immersed boundary method (IBM) can easily distinguish fluid and solid regions in the computational region, thereby the workload of complex grid generation can be reduced. To accurately characterize the solid geometry, a large number of cells are required near the solid surface. The h-adaptive algorithm is adopted to reduce the requirement for the number of cells. In addition, considering the inherent adaptability to the h-adaptive Cartesian grids of the discontinuous Galerkin method, a high-order discontinuous Galerkin solver with an IBM is developed. To validate the h-adaptive algorithm and the solver, three cases are tested, including the steady flow past the National Advisory Committee for Aeronautics 0012 airfoil, the steady flow past a cylinder, and the unsteady flow past a cylinder. Compared with the non-adaptive cases, the h-adaptive cases need smaller total number of cells, and the numerical accuracy is significantly improved with an increasing degree of mesh refinement.