Negative Angles Of Attack Research Articles

INVESTIGATION OF AERODYNAMIC PERFORMANCE OF A NACA 0021 AIRFOIL MODIFIED WITH CAVITY

In this paper, an experimental investigation was conducted to investigate the effect of cavity on the NACA 0021 airfoil at low Reynolds number. The force measurement and smoke-wire flow visualization experiments were conducted in a low-speed open suction wind tunnel with a test section of 57cm×57cm×100 cm at a Reynolds number of 5.0×104 and 1.0×105. Two different airfoil models produced for the experiments such as base and cavity models. The diameter and location of the cavity was chosen as 0.08C and 80% x/C, respectively. The maximum lift coefficient of the cavity modified airfoil was increased about 6% for both positive and negative angles of attack compared to base airfoil at Reynolds number of 1.0×105. The cavity modified airfoil showed higher lift coefficient values at negative attack angles when compared to base airfoil at Reynolds number of 5.0×104.

Flutter performance of a suspension bridge equipped with separated oblique stabilizers covering different span lengths

Vertical stabilizers have been proved to be effective for flutter suppression of long-span bridges, while their self-stability under strong winds should be taken seriously. This study takes a steel truss stiffening girder as an example, and effects of oblique stabilizers on the flutter performance are investigated. After discussing the wind environment above the bridge site, the flutter derivatives of girder extracted by numerical simulations are given to compute the critical flutter wind speed by a bimodal coupled flutter analysis method. Subsequently, with considering the lateral self-excited force, the flutter performance is accurately evaluated by performing complex modal characteristics based on a full-order modal flutter analysis method. After setting different types of stabilizers with different covering lengths, improvement of the flutter performance is discussed. The results show that the flutter performance is related to multiple modes. Temporary flutter instability of a modal branch may be caused due to the curve-veering phenomenon among similar modes. Thus, the critical flutter wind speed may be overestimated by bimodal-coupled flutter analyses. At negative angles of attack, the set of oblique stabilizers are flutter favorable by improving the temporary flutter instability. Setting partial stabilizers around the mid-span is more flutter favorable than covering the entire girder.

Benchmarking of Aerodynamic Models for Isolated Propellers Operating at Positive and Negative Thrust

Operating a conventional propeller at negative thrust results in the operation of positively cambered blade sections at negative angles of attack, leading to flow separation. Consequently, accurately simulating the aerodynamics of propellers operating at negative thrust poses a greater challenge than at positive thrust. This study offers a comprehensive assessment of the aerodynamic modeling capabilities of numerical methods, spanning low to high fidelity, for computing propeller performance across both positive and negative thrust regimes. Low-fidelity methods, namely, blade-element momentum and lifting line theories, effectively predict propeller performance trends at positive thrust. However, they fail to capture trends at negative thrust beyond the maximum power output point due to the neglect of three-dimensional flow effects. Both steady and unsteady Reynolds-averaged Navier–Stokes (RANS) simulations with y+<1 perform well across both positive and negative thrust conditions, with errors below 2% for both thrust and power magnitudes near the maximum power output point. Lattice-Boltzmann very-large-eddy simulations (LB-VLESs) with y+≤10 exhibit excellent agreement with experimental data with less than 1% error near the maximum power output point but with significant computational costs. Conversely, LB-VLESs with y+≥15 offer a more economical approach to capture general trends with the computational cost of the same order as unsteady RANS. However, wall models introduce errors in modeling flow separation, leading to a 16% overestimation of power magnitude near the maximum power output point. The results highlight the necessity of using tools with increased fidelity levels when considering propeller operation at negative thrust compared to the conventional positive thrust regime.

Characteristics and optimization of multi-body separation in supersonic ground effects

The electromagnetic launch of aerospace vehicles is an option for reusable earth/orbit space transportation in the future. In contrast to open space, complex ground effects emerge in supersonic near-ground multi-body separations. Here, the separation processes between a supersonic aerospace vehicle (traveling at Mach 1.5) and an electromagnetic sled were studied with a focus on the characteristics of the flow evolution and aerodynamic interference. The results show that the evolution of the supersonic near-ground separation can be divided into three substages: choked flows in the narrow gap, multi-body shock interactions and independent ground effect interference. The aerodynamic loads of the vehicle strongly correspond to the separation stages. The high-pressure region at the leading edge of the electromagnetic sled continuously sweeps the vehicle abdomen and causes significant fluctuations in the aerodynamic lift and moment, which play a dominant role in the separation trajectory. The altitude of the vehicle that flies away from the electromagnetic sled is relatively stable when the preset attack angle is 0° Following this separation, the vehicle has a pitch angle of -0.25° and tends toward a negative angle of attack. A vertical distance of 4.2 m is insufficient for optimal separation. Thus, the separation characteristics were optimized by combining the preset rudder deflection angle of 0° and inclined track with a slope angle of 1.5° Following this separation, the pitch angle of the vehicle is 3.4°, and the vertical distance increases to 14.8 m, which significantly improves the separation safety.

Impact of dynamic stall model tailoring on wind turbine loads and performance prediction

The future of wind turbines will be characterised by long, slender blades subject to dynamic inflow and aeroelastic deflections. This makes the next generation of blades more prone to encounter dynamic stall effects, in which significant forces and loads fluctuations can be expected. Dynamic stall models can be tailored to suit the aerodynamics of different airfoils. Although different dynamic stall models exist, the impact of the choice of model, its implementation and calibration on the overall wind turbine performance remains to be assessed. In this work, we gathered an experimental dynamic dataset for a representative airfoil, the FFA-W3-211, to define the semi-empirical time constants for the Beddoes-Leishman dynamic stall model. An important differentiation is made between stall regions for positive and negative angles of attack, and the impact of tailored coefficients is assessed at airfoil scale. The difference between the tailored and untailored model is quantified for power performance and loads of the IEA 15 MW reference wind turbine. The results highlight a significant load over-prediction from the untailored Beddoes-Leishman model, whereas changes in power performance are negligible.

Experimental investigation of the occurrence of transonic flow effects on the FFA-W3-211 airfoil

For the largest wind turbines currently designed, when operating at rated power and at high wind speeds, the tip airfoils can experience large negative angles of attack. For these conditions and in combination with turbulence, the airfoils are at risk of reaching locally supersonic flow, even at low free-stream Mach numbers. The possibility of shock wave formation and its consequences endangers the lifetime of these largest rotating machines ever built. So far only numerical analyses of this challenge have been attempted with significant modelling uncertainty. Here, for the first time, a wind turbine airfoil (the FFA-W3-211, used at the blade tip of the IEA 15MW reference wind turbine) is studied under transonic conditions using experimental techniques. Schlieren visualization and Particle Image Velocimetry were employed for free-stream Mach numbers of 0.5 and 0.6 and various angles of attack. It was shown that calculations based on isentropic flow theory and compressibility corrections were able to predict the situations where supersonic flow occurred. However, they could not predict the frequency of occurrence and whether shock waves were formed. In conclusion, an unsteady characterization of such airfoil behavior in transonic flow seems to be warranted.

Analysis of Aerodynamics Load of a Flapping Wing by Vortex Lattice Method

The aim of this study is to investigate the effect of changing the angle of attack and flapping angle on the aerodynamic characteristics of an unsymmetrical wing with the NACA 2412 profile. This work uses the Vortex Lattice Method. The results of this study show that increasing the flapping angle causes the pitching moment to decrease while induced drag and moment coefficient increase, meanwhile, the lift coefficient increases at positive angles of attack and then decreases at negative angles of attack. The pitching moment decreases with increasing angle of attack at positive flap angles, but this value increases at negative flap angles. Meanwhile, the induced drag decreases from an angle of attack -15° to a minimum at an angle of attack -5° and then increases. The moment coefficient increases with increasing angle of attack at positive flapping angles, but decreases at negative flapping angles. Meanwhile, the lift coefficient increases with increasing angle of attack.

Experimental and numerical study on the high-speed ship hydrodynamics influenced by an interceptor with varied angle of attack

Efforts to improve the hydrodynamic performance of high-speed ships have been underway for a long time. There are different approaches, one of which is to take advantage of an interceptor. Conventionally, the interceptor blades are mounted vertically on the ship's bottom transom, oriented at a zero-degree angle of attack (AoA). This study comprehensively explores high-speed ships' hulls with and without interceptor configurations, encompassing both negative and positive AoA of the interceptor, conducted through experimental and numerical methods using a fully captive model. The interceptors are strategically positioned and configured. Each configuration was examined under varying AoA settings, with uniform interceptor depths and systematic trim angle adjustments. The Computational Fluid Dynamics (CFD) approach simulates the local flow dynamics around the hull, thoroughly analyzing resistance, pressure distribution, lift force, wave profile, and trim moment. The results indicate that interceptor placement near the keel with AoA adjustments significantly reduces hydrodynamic resistance, while AoA changes have limited impact in other positions. Lift force analysis shows interceptors improve lift compared to the bare hull, but this improvement is not linear across positions. Furthermore, it is observed that adjustments in AoA influence lift, with a negative AoA generally being considered favorable. In summary, carefully considering placement, AoA, and height-to-length ratio is necessary to maximize interceptor advantages.

BEM modelling of a hydrodynamic lifting surface working in ground proximity

Present paper aims to assess the performance and limits of boundary element method for the reliable simulation of ground effect phenomena on hydrodynamic lifting surfaces. Benefitting from the ground effect, especially in extreme ground proximity, suggests a promising concept for lifting bodies. On the other hand, the more pronounced the ground effect, the more complicated flow physics it exhibits, making the problem harder to handle numerically. In this study, a potential-based boundary element method (BEM) is applied to the flow past a 3D rectangular wing within a wide range of ground clearances corresponding to out of ground effect (OGE), in ground effect (GE) and extreme ground effect (EGE) conditions. Both positive and negative angles of attack are tested to examine the lift and downforce producing cases. The potential flow around the airfoil is computed using the mixed constant-strength source and constant-strength dipole based panel method. The results show that BEM successfully predicts the lift force variation for a wing working in ground proximity (GE). However, approaching the bounds of extreme ground effect is seen to introduce a progressive decrease of the fidelity of the method. A similar tendency is observed in the downforce generating conditions, which is also an important scenario for ground vehicle aerodynamics. BEM remarkably overestimates the magnitude of the downforce when the ground clearance reaches up to the EGE conditions, still, it produces quite satisfactory results for the rest of the tested configurations. The pressure distributions are also provided to examine the flow field in the aforementioned cases. Overall, the results lead us to conclude that BEM stands as a robust and reliable prediction tool for ground effect on 3D wings but the extreme ground effect condition represents the limit of its accuracy range.

Performance Study of a Humpback Whale Fluke Turbine on Foil Shape Variation Based on Double Multiple Streamtube Model

Exploration of ocean current energy allows for the development of turbines as the primary conversion device. Turbine technologies have been developed in various types, including bio-inspired turbines, such as the humpback whale fluke turbine. In this study, the achievement of a humpback whale fluke turbine is investigated by applying various forms of foil, both symmetric and asymmetric, to obtain the appropriate foil profile. Symmetric foils were represented by NACA 0012, NACA 0018, and NACA 0021, while asymmetric foils were represented by NACA 4312, NACA 4512, and NACA 4712 foils. Simulations were performed using QBlade software, which was developed based on the DMST theory. In general, symmetric foils have a more stable performance than asymmetric foils because they produce a better performance at positive and negative angles of attack. This result is also supported by a review of efficiency and self-starting capability where symmetric foils have significantly higher CP values and positive CQ along the azimuth angle than asymmetric foils. Finally, NACA 0021 foil is recommended for a humpback whale fluke turbine based on its efficiency and self-starting capability.

Effects of vehicle dynamics on small UAS-gust encounters

This study investigates the relative impacts of different gusts on representative small Uncrewed Air Systems through the use of a simple analytical model. The model uses basic vehicle characteristics, including planform area and mass, to account for the altitude gained during gust interactions. This is a missing component from most current fundamental gust studies, both experimental and numerical. The results show that the induced negative effective angle of attack created by the vertical motion of the vehicle has a significant countering effect on the gust and suggests that this effective angle of attack should be accounted for in future gust studies. Efforts are undertaken to determine the relative importance of accounting for this motion. This is done by investigating the impact of the induced negative angle of attack on the altitude trajectory. An equation is found using gust and vehicle characteristics to determine the critical gust ratio at a given gust development rate, which is the minimum gust ratio which will lead to vehicle stall. Lastly, an empirical relationship is derived to allow for the effective gust ratio conversion between fixed-model experiments and flying-vehicle gust interactions.

Numerical simulation study on hydrodynamic time-delay characteristics of supercavitating vehicle

Due to the existence of the time-delay effect of the supercavity, the fluid dynamics of the supercavitating vehicle in the maneuvering will exhibit extremely strong unsteady characteristics, which will bring great challenges to the hydrodynamic acquisition, motion performance analysis and control scheme design and evaluation of the vehicle. Therefore, it is of great significance to study the hydrodynamic characteristics and formation mechanism of the vehicle in supercavitating state. In this paper, a three-dimensional numerical model for continuous motion of the supercavitating vehicle at varying angles of attack is established by using the dynamic mesh technique. The accuracy of the model is verified by using the water tunnel test results. Through simulation, the unsteady cavitation development and hydrodynamic characteristics of the supercavitating vehicle at different speeds, swing frequencies and preset rudder angles are obtained. The results show that the hydrodynamics of the supercavitating vehicle under unsteady motion has time-delay characteristics, and the prediction results of the conventional hydrodynamic calculation method of the vehicle have large errors. The cavitation time-delay effect and the dynamic development of the attached cavitation are the fundamental reasons for the formation of the unsteady hydrodynamic time-delay characteristics of the supercavitating vehicle. The hydrodynamic time-delay characteristics of the supercavitating vehicle is significantly enhanced with the increasing of velocity and weakened with the increasing of swing frequency. The preset rudder angle of the cavitator results in different time-delay characteristics when the vehicle swings at positive and negative angles of attack.

Influence of cavity floor injection strategy on mixing improvement study of a splitter plate-assisted supersonic combustor

In the current research, an effort has been made to investigate the splitter plate-based scramjet combustor by inserting a cavity in the lower wall and observing how the different geometric parameters of the combustor, namely the cavity's aft wall angle, the bottom wall's divergence angle, and the splitter plate's angle of attack, affect the mixing performance of the supersonic combustor. Initially, experimental data from the published literature for a splitter plate is used to validate the implemented computational approach. Next, different aft wall angles of the cavity are compared, and the optimal value is found to be 120° The current investigation demonstrates that geometric modification significantly affects the performance of a splitter plate-assisted supersonic combustor. An increase in the cavity's aft wall angle improves the combustion efficiency. A further study is conducted on the splitter plate + cavity at various divergence angles of the bottom wall. According to our study, the separation region on the top wall increases as the divergence angle increases, resulting in a reduction in combustion performance. Finally, while simulating different angles of attack of splitter plate, it has been noticed that, for a negative angle of attack (-1°), the cavity provides a recirculation region and a sufficient shear mixing layer. This allows adequate air-fuel mixing to sustain combustion.

Effect of Blowing Ratio on Turbine Blade Air Film Cooling Under Different Engine Conditions

Under different working conditions of the aeroengine, the rotating speed of the turbine blades is diverse, and this causes the high-temperature gas mainstream to impact the turbine blades at disparate angles of attack. In order to explore the film cooling mechanism of the pressure surface and suction surface of aeroengine turbine blade at unusual speed, a 3D model of the turbine blades and internal runners is constructed, which refers to Pratt &amp; Whitney PW4084 primary HPT blade. In this model, the high-temperature gas mainstream is set to attack the turbine blades through three distinct angles, the turbine blade air film cooling model is established, and the numerical simulation is conducted at the different blowing ratios. The results showed that the angle of impact (rotational speed) is the key factor affecting the cooling efficiency of the blade. The cooling effect of the suction surface is the best under the positive attack, however, the cooling effect of the pressure surface under the negative attack angle is the first-rate. With the increase of the speed, the surface temperature of the top and tail of the blade pressure surface will gradually decrease, and as the speed reduces, the surface temperature of the lower part of the suction surface of the blade will slightly increase. Finally, under three different attack angles, the cooling efficiency of the air film on the surface of the blade will augment with the increase of the blowing ratio.

Numerical Research on the Jet Mixing Mechanism of the De-Swirling Lobed Mixer Integrated with OGV

The outlet guide vane (OGV) is integrated with the lobed mixer to improve the exhaust system’s performance with a high core inlet swirl. The best location for integrating the OGV is along the central line of the lobe’s trough and near the exit plane of the lobed mixer. Two types of lobed mixers (the scalloped reference lobed mixer and the scalloped de-swirling lobed mixer) integrating with/without OGVs, are numerically researched under eight inlet swirl conditions ranging from 0° to 35°. The simulation used the Reynolds-Averaged Navier-Stokes (RANS) method with Shear Stress Transport (SST) model based on an unstructured mesh of 30 million cells. The reserved outlet flow angle of the de-swirling lobed mixer is beneficial for enhancing the strength of downstream streamwise vortices and accelerating the jet mixing. After integrating with OGV: it can significantly suppress the leakage vortex between the lobe trough and the central body and the backflow downstream of the central body; on the other hand, it can further increase the strength and scale of streamwise vortices by expanding the radial range of inner secondary flow, thereby accelerating mixing and reducing total pressure loss &amp; thrust loss. Under the design condition, the integrated de-swirling lobed mixer can increase thrust by 3.18% and reduce the mixing loss by 31.17% compared with the reference lobed mixer. Even under non-design conditions, the integrated de-swirling lobed mixer can still use upstream inlet swirl to enhance the streamwise vortices and accelerate the jet mixing within the conditions studied in this paper. The outlet jet uniformity of the integrated de-swirling lobed mixer is better than that of the integrated reference lobed mixer for the case with the same core inlet swirl. Compared with the latter, the former also has better tolerance to the attack angle, especially for the negative attack angle conditions. Under the condition with a core inlet swirl of 35°, the thrust loss of the integrated de-swirling lobed mixer is 2.15% lower than that of the integrated reference lobed mixer.

Mach and Reynolds number effects on transonic buffet on the XRF-1 transport aircraft wing at flight Reynolds number

This work provides an overview of aerodynamic data acquired in the European Transonic Windtunnel using an XRF-1 transport aircraft configuration both at cruise conditions and at the edges of the flight envelope. The goals and design of the wind tunnel test were described, highlighting the use of the cryogenic wind tunnel’s capability to isolate the effects of M_{infty }, {text{Re}}_{infty } and the dynamic pressure q/E. The resulting dataset includes an aerodynamic baseline characterization of the full span model with vertical and horizontal tailplanes and without engine nacelles. The effects of different inflow conditions were studied using data from continuous polars, evaluating the changes in aeroelastic deformation which are proportional to q/E and the influence of M_{infty } and {text{Re}}_{infty } on the shock position. Off-design data was analyzed at the lowest and highest measured Mach numbers of 0.84 and 0.90, respectively. Wing lower surface flow and underside shock motion was analyzed at negative angles of attack using {{c}_{text{p}}} distribution and unsteady pressure transducer fluctuation data, identifying significant upstream displacement of the shock close to the leading edge. Wing upper-side flow and the shock motion near buffet onset and beyond was analyzed using unsteady pressure data from point transducers and unsteady pressure-sensitive paint (PSP) measurements. Buffet occurs at lower angles of attack at high Mach number, and without clearly defined lift break. Spectral contents at the acquired data points in the buffet range suggest broadband fluctuations at Strouhal numbers between 0.2 and 0.6, which is consistent with recent literature. The spanwise shock propagation velocities were determined independently via analysis of unsteady PSP and pressure transducers to be in the range between u_{text{s}} / u_{infty } = 0.24 and 0.32, which is similarly in line with published datasets using other swept wing aircraft models.

Wire-driven parallel robot suspension system for SDM in a low-speed wind tunnel

A wire-driven parallel robot with eight wires (WDPR-8) acting as the suspension system for an aircraft model in a low-speed wind tunnel is introduced in this paper. A kinematics model for WDPR-8 is established here. A theoretical basis is also provided, through which movement control of the model is achieved by changing the wire length. A model motion control subsystem, a monocular vision measurement subsystem for the model position, and a data acquisition subsystem were also developed for WDPR-8. A prototype of WDPR-8 was built and mounted in a low-speed wind tunnel with an open test section. A six-component strain-gauge balance was installed inside the test model to acquire aerodynamic parameters. The aerodynamic coefficients of the static test show reasonable agreement with reference results. The difference between the test result of WDPR-8 and the results from reference is smaller in the positive angle of attack than the negative angle of attack, and the total difference between WDPR-8 and other facilities is under 10%. Pitch dynamic tests with different amplitudes and frequencies were used to verify the aerodynamic hysteresis phenomena. The research indicates it is successful that the solution of measuring aerodynamic parameters with strain-gauge balance set into the standard dynamic model suspended by WDPR. It demonstrates that WDPR-8 is effective and feasible as a model suspension system in a low-speed wind tunnel. Furthermore, test results without interference correction in accordance with the reference data proves that the interference of Wire-Driven support system to flow field during wind tunnel tests is negligible. Finally, WDPR suspension system introduced in this paper will remarkably improve the efficiency and accuracy in wind tunnel test.

Numerical Investigation on the Transition Flow around NLF Airfoil

A natural laminar flow (NLF) airfoil is designed to reduce drag by expanding laminar flow areas. In-depth knowledge of transition performance is essential for its aerodynamic design. The k-ω-γ-Reθ framework, which consists of the SST k-ω turbulence model and γ-Reθ transition model, is employed to simulate transitional flows around an NLF wing RAE5243 airfoil. The transition performances of the RAE5243 airfoil under various values of turbulent intensity, temperature, angle of attack, and Mach number are simulated and compared. The results show that the rise of inflow turbulent intensity will promote an earlier transition on both the suction and pressure sides. The influence of wall temperature on transition is limited. The rise of angle of attack will lead to an earlier transition on the pressure side but a later transition on the suction side. With the rise of Mach number, the transition happens earlier under a zero and positive angle of attack but later under a negative angle of attack. In addition, the correlation of transition onset locations with respect to turbulent intensity, surface temperature, angle of attack, and Mach number is established based on numerical results.

Experimental investigation of the acoustic emission of an electrically driven propeller in thrust and recuperation mode in a wind tunnel

This paper presents the results of an experimental performance study of an electrically driven propeller in thrust and power recuperation mode in a subsonic wind tunnel. Using the reverberation chamber method, acoustic measurements were performed to evaluate them as a reference in relation to the performance data obtained. The three approaches to recuperation were investigated: Fixed pitch propeller at speed reduction (case 1), controllable pitch propeller with negative angle of attack (case 2), and inverted, backward running propeller (case 3). Results show that in-flight recuperation can be optimized in terms of low noise emissions and generating shaft power from the flow efficiently by choosing the respective operating point. For the latter, case 3 shows the greatest potential for a two bladed propeller, while case 2 has the lowest noise emissions and might reach higher values in recuperated power by increasing the propeller blade number.

Моделирование взаимодействия конических ударников с подводными преградами при наличии у ударников угла атаки

In this paper, the effect of the angle of impact and the angle of attack on the interaction between truncated coneshaped impactors made of BHZh-95 alloy and underwater barriers made of aluminum alloy is studied using experimental and computational methods. When such an impactor enters water, a supercavern appears around the impactor and minimizes the fluid friction. During the motion, the impactor afterbody periodically experiences impermanent contact with the walls of the supercavern, which maintains the steady motion. Note that the center of mass of the impactor is located closer to its afterbody, which can induce the angle of attack appearance during the motion. The experimental study is conducted using a hydroballistic track with high-speed video recording. The speed range of 250-350 m/s is considered. Mathematical modeling is carried out using the EFES original software package. It is revealed that a positive angle of attack β ≤ 10° leads to the normalization of the impactor and a decrease in the effective thickness of the barrier with its perforation. In contrast, at a negative angle of attack, the impactor rebounds from the barrier.