Increasing Angle Of Attack Research Articles

A study on the cavitation vortex dynamics and loads of water-exiting axisymmetric projectile

This paper analyzes the cavitation vortex dynamics and load characteristics of projectiles launched underwater at different angles of attack. The results show that as the velocity of the projectile decreases during the underwater cruising stage and the cavitation number continuously increases, the cavity begins to shed. This phenomenon occurs because of the development of a re-entrant jet at the end of the cavity. As the angle of attack (0°–8°) increases, the cavity asymmetry is strengthened and the scale of the cavity shedding increases. The cavity prevents the generation of hairpin vortex packages. This phenomenon is more pronounced in cavities with a larger angle of attack. As the angle of attack increases (4°–8°), the turbulent vortex structures of the wall and free wake are further developed and enriched. The vortex legs of wall hairpin vortices are significantly elongated at large angles of attack, which may be the result of the combined effect of the cavity and transition area. During the water-exiting stage, the collapse of isolated vacuoles generated by the cavitation flow generates a significant collapse load. The collapse position undergoes at least two pressure pulsation peaks or increases, with the first peak or increase occurring during the initial underwater cruising stage.

Study on mixing and combustion characteristics of solid rocket ramjet with a large angle of attack

Solid rocket ramjet faces the problems of large angle maneuvering and stage transformation in the actual operation. In order to study the effect of the change of angle of attack and excess air coefficient on the performance of boron-containing solid rocket ramjet, the fluid field with different conditions were calculated. The applicability of the numerical model was verified by ground direct-connected experiments. The results showed that as the angle of attack increases, the mixing in the head reflux area and the initial air-gas contact area are enhanced. The combustion ratio of combustible components in the front section is greatly increased, which helps to improve the combustion efficiency of the motor. When the excess air coefficient increases from less than 1 to much greater than 1, the mixing combustion in the head region is also strengthened. However, the radial impact of excess air causes the gas to deflect to the upper side of chamber, and then most of the particles move close to the wall, resulting in a lower level of combustion. The general trend is as follows, under the same mixing-dominated pattern, the average mixing at the outlet decreases with increasing angle of attack, and the combustion efficiency is reversed. Under different mixing-dominated patterns, higher combustion efficiency can be obtained at lower mixing when the head region dominates.

Aeroelastic Response of Spinning Projectiles with Large Slenderness Ratio at Supersonic Speed

Obvious aeroelastic deformation occurs in spinning projectiles with large slenderness ratio, which seriously affects flight stability and maneuverability. This paper investigates the aeroelastic response of spinning projectiles with large slenderness ratio under supersonic speed. Based on a dynamic mesh method, an unsteady numerical simulation method is developed to study the aeroelasticity of spinning projectiles by coupling aerodynamics and structural dynamics. The numerical simulation method is well validated by the experimental results of AGARD 445.6 wing flutter. Then, the aeroelastic response of spinning projectiles with large slenderness ratio is numerically explored under different flight conditions. The aeroelastic response is obtained, revealing the presence of beat vibrations and variations in response frequency. Furthermore, the influence mechanism of flight conditions on the aeroelastic response is analyzed. The results suggest that the coupling of the first two modes of the projectile caused by the spinning motion leads to the occurrence of beat vibrations in the aeroelastic response; the coupling degree of the first two modes decreases as the angle of attack increases and it increases with the increase in spinning speed; and the time−averaged deformation caused by the time−averaged aerodynamic force is beneficial to the convergence of the aeroelastic response of spinning projectiles, while the rotation−induced Magnus effect is counterproductive.

Swallow-tailed separation bubble on a low-aspect-ratio trapezoidal plate: effects of near-wall spanwise flow

The three-dimensional flow over a low-aspect-ratio (low- $A\!R$ ) trapezoidal plate is investigated experimentally with a focus on how the tip effects impact the structure and dynamics of the separation bubble. The chord-based Reynolds number is $5800$ , and the angle of attack varies from $4^\circ$ to $10^\circ$ . Once the flow separates, the separation bubble emerges and features a swallow-tailed structure that shrinks near the midspan, which is first found for the flows over low- $A\!R$ plates. This structure develops into the conventional single-tailed structure as the angle of attack increases. Moreover, the vortex shedding within the swallow-tailed separation bubble is restored from multiple asynchronously measured local velocity fields. It is revealed that the leading-edge vortex undergoes the novel transformation from a C-shape vortex into an M-shape vortex. This vortex transformation stems from the mass transport of the near-wall spanwise flow, which affects the fluid motion on the windward side of the C-shape vortex head, strengthening and accelerating the vortex head. The strengthened vortex head facilitates the entrainment of high-momentum fluid from the outer flow. This is responsible for the formation of the swallow-tailed structure. These findings help to fill the gaps left by the downwash at low angles of attack for low- $A\!R$ wings, and are of value in improving the cruising and gliding performance of micro-air vehicles.

Enhanced heat transfer and flow topology of hydrogen regenerative-cooling channels with novel X-shape ribs

One of the essential issues for spacecraft is maintaining the combustion chamber wall from exceeding acceptable temperatures. The cooling channels' thermal resistance and flow topology can be augmented by adding ribs to the heated surface. The present work proposed a Novel X-shape rib to enhance the thermal efficiency of hydrogen regenerative-cooling channels. The numerical investigation was carried out for the cooling system with rectangular and trapezoidal X-shape ribs at various attack angles (30°,45°, and 60°). The investigation findings demonstrated that both X-shape rib constructions optimized the cooling system's thermal efficiency. Since the trapezoidal X-shape ribs' chamfering angle reduces the resistance to coolant flow, the pressure loss of these ribs is less than rectangular ribs. Additionally, the heat transfer rate is maximum for 30° X-shape ribs, decreasing gradually with the increase of the X-shape rib's attack angle. Furthermore, compared to a smooth channel, the rectangular 30° X-shape ribs enhance the thermal efficiency by 44% while reducing the wall temperature by 24%.

First Passage Dynamic Reliability Analysis of Non-Stationary and Non-Gaussian Buffeting Random Dynamic Responses of Long-Span Suspension Bridge

In the complex mountain wind environment, especially in the strong wind, fluctuating wind speed is a non-stationary and non-Gaussian random process. With the increase of suspension bridges’ span, they are more sensitive to wind-induced vibration response. In this paper, a non-stationary and non-Gaussian buffeting reliability analysis method for long-span bridges is presented. Firstly, the non-stationary wind speed is simulated by the modulation function based on the stationary wind speed model. Secondly, the load is obtained by simulated wind speed, the obtained loads are loaded on the finite element model by ANSYS and the whole bridge time-domain analysis is performed, the time history of displacement response is obtained. Thirdly, based on the first excursion failure criterion of random vibration, the samples of the non-stationary and non-Gaussian displacement time history are transformed into a standard Gaussian process through a modified Fleishman approximation method, and then the non-stationary Poisson distribution method is used for structural reliability analysis. Finally, a mountainous long-span suspension bridge is used as the engineering background, and the proposed reliability analysis method is applied to analyze the reliability of the bridge. The influence of different wind attack angles and modulation functions on the dynamic reliability of the bridge is further studied. The results indicate that the displacement response obtained by the transformation of non-stationary and non-Gaussian random process is less reliable than that obtained by traditional analysis method. Its dynamic reliability is maximum at 1[Formula: see text] wind attack angle, the reliability of negative wind attack angle is lower than that of positive wind attack angle, and decreases with the increase of wind attack angle. The reliability obtained by using different modulation functions is lower than that of traditional method. If the traditional analysis method is still used for reliability analysis, it will produce unsafe consequences and reduce the engineering safety reserve.

Investigation on the inflow distortion of a propulsive fuselage aircraft and its impact on the propulsion fan performance

Boundary layer ingesting (BLI) propulsion represents a promising concept that can achieve a great fuel burn reduction compared to conventional aircraft. This study numerically investigates the BLI inflow distortion and its impact on propulsion fan performance using commercial software CFX. First, Reynolds-averaged Navier–Stokes (RANS) simulations were performed to obtain the inflow distortion of a propulsive fuselage aircraft with various geometries and flight conditions. Then, full-annulus URANS simulations were carried out to assess the effect of various inflow distortions on the performance of NASA rotor 67. The results show that the fuselage sweep dominates the BLI inflow distortion, while the effects of the wing and vertical tail are local. As the angle of attack increases, the BLI inflow distortion is alleviated due to the freestream being ingested from the bottom of the propulsor. With the increase in the angle of sideslip, the redistributing and impinging flow significantly aggravates the total pressure and swirl distortion. With regard to the fan performance, various BLI inflow distortions share a similar static pressure distribution upstream of the rotor, as a result of the comparable total pressure distribution, so the cases without swirl distortion show a similar distribution of migration-induced swirl angle. At the peak efficiency condition, a minimum reduction in efficiency of approximately 4.5% and a total pressure ratio reduction of over 2.9% are observed for the investigated inflow distortions, in comparison with the Clean case. Moreover, the findings reveal that the effect of swirl distortion on fan performance is minor for the cruise case, but substantial for the two sideslip cases. These results offer useful insights into BLI propulsion.

Numerical study identifies the interaction between two adjacent dry cooling towers on fluid flow and heat transfer performances of the radiators at different points of each tower

A reliable thermal power plant in arid area usually has twin power-generating units equipped with two dry cooling towers. For a twin-tower configuration, a three-dimensional numerical model was built to capture thermo-hydraulic characteristics of both towers and distribution patterns of heat transfer performances of their radiators. Results showed that the interaction between two adjacent towers is a combination of negative blocking effect and positive wind-breaking effect of one tower to another, and its influences on radiators of different orientations are distinct and dependent on wind direction and speed. As wind attack angle increases, the overall cooling performance of the upstream tower improves and its sensibility to wind speed declines, while performance variation of the downstream tower shows a decrease followed by an increase. The downstream tower performs worse than the upstream tower in most crosswind cases, except for the wind situation where the downstream tower is almost entirely blocked by the upstream one. For a given wind speed of 12 m/s, cooling efficiency of the upstream (downstream) tower rises from 69.4% to 73.7% (91.7%) as wind attack angle varies from 0° to 90°, causing condenser pressure of the associated power unit to reduce from 23.5 kPa to 20.5 kPa (14.4 kPa). As the tower spacing increases, the tower interaction mitigates and cooling performance difference between the two towers narrows. The decrease of ambient temperature barely impacts cooling efficiency of the tower, but does cause a drop in condenser pressure of the unit without changing the sensitivity of condenser pressure to crosswind. The increase in condenser heat load helps to improve cooling tower performance and reduces its sensitivity to crosswind. Whereas, the growth in condenser heat load dominates the increases in condenser pressure and its sensitivity to crosswind.

Experimental Research on the Effect of Particle Parameters on Dynamic Stall Characteristics of the Wind Turbine Airfoil

The frequent appearance of sandy and dusty weather in Northwest China impacts the wind turbine. Meanwhile, the non-constant phenomena, dynamic stall speed during the wind turbine operation, will lead to large load fluctuations and unsafe operation. However, few studies have been conducted at home and abroad on the effect of particle parameters on the dynamic stall of airfoils. This paper investigates the impact of particle parameters on the airfoil dynamic stall through numerical simulation of the coupling between the continuous phase and discrete phase by using the SST k-ω turbulence model for a two-dimensional NACA 0012 airfoil. The effect of particle parameters on the airfoil dynamic stall aerodynamic performance, the impact of the flow field around the airfoil, and the particles motion were studied, respectively. The investigation shows a reduction in the aerodynamic performance of the airfoil, due to the addition of particles. The effect is more prominent under a large angle of attack and less under a small angle of attack. When the angle of attack increases, the loss rate of lift coefficient in the windy and sandy environment gradually decreases, while irregular fluctuations emerge when the angle of attack decreases, and the overall rate of change increases more significantly, compared to the stage of the increasing angle of attack. For the particle diameter under 50 μm, the larger the particle diameter, the more significant the change of lift coefficient becomes, as well as the larger the vortex volume near the airfoil’s leading edge, and a large number of particles gather at the suction surface of the airfoil. For the particle diameter of 50 μm, the lift coefficient decreases at any angle of attack of the airfoil movement to the oscillation cycle, the vortex volume decreases, and a large number of particles gather at the pressure surface of the airfoil. However, for particle diameters above 50 μm, the lift coefficient gets reduced, followed by a decrease in the vortex volume near the airfoil leading edge with the increase of particle diameter, so that plenty of particles gather on the pressure surface of that airfoil. At the stage of increasing the airfoil angle of attack, with the increase of particle concentration, there is a gradual decrease of the peak lift coefficient and stall angle of attack of the airfoil, as well as a corresponding decrease of the drag coefficient divergence angle of attack and peak value. In contrast, when the airfoil angle of attack is decreased, the airflow reattachment process obviously lags behind that of the clean air. As the particle concentration increases, the airfoil separation point occurs earlier, and the higher the concentration, the earlier the separation point. The erosion maximum airfoil erosion rate increases with the particle concentration.

Identification of the two-sources at the aircraft slat/fuselage juncture based on phased array

Slat/fuselage juncture noise or slat horn noise has been identified as a major airframe noise component and is commonly assumed to be similar to the flap side-edge noise. However, whether the slat/fuselage juncture noise has similar characteristics to flap side-edge noise has not been confirmed. In this study, the slat/fuselage juncture noise sources in realistic aircraft configurations are investigated based on phased microphone arrays. Two different source mapping methods of L1/2-DAMAS and CLEAN-SC are applied to the airframe noise benchmark test DLR1 for cross validation. It is shown that both methods obtained identical noise characteristics and source strength distributions. The slat/fuselage juncture noise is revealed to be completely different from the flap side-edge noise. The spectrum of the slat/fuselage juncture noise is Helmholtz-dependent, rather than Strouhal-dependent, as the flow Mach number increases. The broad peak of the spectrum shifts towards a higher frequency as the angle of attack increases. Furthermore, both the two source mapping methods all identified two noise sources existing at the aircraft slat/fuselage juncture. The first noise source is the inboard slat side-edge vortex scattering and the second noise source is the horseshoe vortex on the fuselage. It is demonstrated that the side-edge vortex scattering dominates the spectrum, especially in the middle- and high-frequency domains. The horseshoe vortex is a major noise contributor in the low-frequency domain. These findings provide improved insight into the underlying noise generation mechanisms and can help develop prediction methods for slat/fuselage juncture flow noise.

Numerical simulation of the effect of atmospheric condition on the lightning strike for wind turbine

The risk of lightning exposure increases as wind turbine size increases, and lightning accidents have grown up to be a severe threat to wind turbines. The present paper focuses on the influence of the changes in atmospheric conditions around the rotating blade for the upward leader initiation. A 2D computational fluid dynamics model was established to obtain the air pressure distribution around the blades, and the simplified inception model was used to determine the initiation of the upward leader mechanism. In this paper, two significant factors of velocity and attack angle were studied. The results show that the trigger height is about an 11.2% difference for 120 m/s with the peak current of return stroke at 30 kA; the difference has reached about 28% for the attack angle of 10°. The research indicates that the area with higher air pressure is exposed to a greater risk of lightning strikes, and the probability of lightning strikes will increase as the blade attack angle increases.

Gusts Encountered by Flying Vehicles in Proximity to Buildings

There is a growing desire to operate Uncrewed Air Vehicles (UAVs) in urban environments for parcel delivery, and passenger-carrying air taxis for Advanced Air Mobility (AAM). The turbulent flows and gusts around buildings and other urban infrastructure can affect the steadiness and stability of such air vehicles by generating a highly transient relative flow field. Our aim is to review existing gust models, then consider gust encounters in the vicinity of buildings as experienced by flight trajectories over the roof of a nominally cuboid building in a suburban atmospheric boundary layer. Simplified models of fixed- and rotary-wing aircraft are used to illustrate the changes in lift and thrust experienced by flight around the building. The analysis showed that fixed-wing aircraft experienced a substantial increase in angle of attack over a relatively short period of time (<1 s) as they fly through the shear layer at a representative forward velocity, which can be well above typical stall angles. Due to the slow flight speeds required for landing and take-off, significant control authority of rotor systems is required to ensure safe operation due to the high disturbance effects caused by localized gusts from buildings and protruding structures. Currently there appears to be negligible certification or regulation for AAM systems to ensure safe operations when traversing building flow fields under windy conditions and it is hoped that the insights provided in this paper will assist with future certification and regulation.

Thermal Protection Mechanism of a Novel Adjustable Non-Ablative Thermal Protection System for Hypersonic Vehicles

In order to improve the thermal protection performance of the active thermal protection system (TPS) based on the spike and jet, an adjustable non-ablative thermal protection system, of which the spike can be rotated in the direction of the free stream, is proposed in this paper. The thermal protection mechanism and the optimal installation angle are analyzed by adopting the numerical method. The results show that the angle of attack has great influence on the peak heat flux of hypersonic vehicles, the dangerous point is on the windward side of the vehicles at the non-zero angle of attack. With the increase in angle of attack, the heat flux of the windward side of the vehicles rises rapidly, leading to the decrease in the global thermal protection efficiency. The adjustable non-ablative TPS in this paper greatly reduces the aeroheating of the windward side through the installation angle between the spike and nose cone, thus improving the global thermal protection efficiency. The optimal installation angle can be obtained by numerical or experimental methods in engineering design, and the difference between the angle of attack and the optimal installation angle is about 2.4° for the proposed model. Therefore, the installation angle can be automatically adjusted based on the angle of attack to achieve the highest thermal protection efficiency.

CFD Simulations of the Effect of Equalizing Duct Configurations on Cavitating Flow around a Propeller

This study presented the results of a computational study of cavitating flows of a marine propeller with energy saving equalizing ducts. The main purpose of the study was to estimate the cavitating flows around a propeller with a duct, and to investigate the interaction between a duct and a propeller in cavitating flows. The INSEAN E779A propeller was used as a baseline model. Validation studies were conducted for non-cavitating and cavitating flows around a hydrofoil and a propeller. A comparison with the experimental data showed good agreement in terms of sheet cavity patterns and propulsion performances of the propeller. Various duct configurations have been presented, and it was found that a duct in front of the propeller had effects on the propeller’s cavitation and propulsion performance. Higher angles of attack of the duct showed a significant effect on the propeller’s cavitation behavior, especially with a small duct. The small duct lowered the cavitation inception radius with increase in angle of attack of the duct, while the large duct had more effect on the tip cavitation. The propeller with large duct gave higher thrust, however, the higher torque loading affected the propeller efficiency. Overall, it was found that the propeller with small duct provided a higher propeller efficiency

Numerical investigation of the wake transition and aerodynamic efficiency of the two-dimensional propulsive wing

The propulsive wing is a new concept wing of automatic propulsion with high lift coefficients and has great application value in plant protection and forest fire control. The propulsive wing wake is a reverse Bénard–von Kármán (RBvK) vortex street, which is considered a thrust-generating wake. The wake structure will change greatly at high angles of attack and lead to changes in the aerodynamic performance of the propulsive wing. To explore the optimal working range and the wake characteristics of the propulsive wing, the wake transition and aerodynamic efficiency of the propulsive wing in cruise are numerically studied. The results indicate that there are three types of structures for the propulsive wing wake. When α ≤ 20°, the wake transits from the RBvK vortex street to the critical state with the increase in cruise speed, and the Strouhal number approaches 1.9. The critical wake region decreases gradually with the increase in the angle of attack. The maximum propulsive efficiency is 0.17 at a cruise speed of 15 m/s. When α > 20°, the wake transits directly from the RBvK vortex street to the Bénard–von Kármán (BvK) vortex street and the Strouhal number approaches 0.34. The maximum propulsive efficiency appears at a cruise speed of 10 m/s, which is close to the BvK vortex street boundary. Before entering the stall state, the lift efficiency of the propulsive wing increases with the increase in cruise speed and angle of attack, up to 3–5.

Aerodynamic Characteristics of a Ducted Fan Hovering and Transition in Ground Effect

Ducted fans installed on vertical takeoff and landing vehicles experience significant ground effect during takeoff and landing. The aerodynamic characteristics of a ducted fan hovering and transitioning in the ground effect are studied using numerical simulations in this paper. The flowfields are obtained by solving Reynolds Averaged Navier–Stokes equations with the Multiple Reference Frame approach. When a ducted fan hovers in the ground effect, the blade thrust increases due to the combined effect of the increase in the effective angle of attack of the blade and the increase in ambient pressure; the duct thrust decreases due to the combined effect of the decrease in the effective angle of attack of the duct and the increase in ambient pressure. Stall occurs at a certain advance ratio and angle of attack when transitioning in the ground effect. The ground effect delays the occurrence of stall at some advance ratios. The ground effect is hardly detectable at angles of attack less than 30° even if the height drops to 0.5 times the duct exit diameter. At this height and high angles of attack, the different positions and influence regions of the ground vortex at different advance ratios contribute to the different variation trends in the ducted fan performance.

Hybrid Modeling for Solutions of Sediment Deposition in a Low-Land Reservoir with Multigate Sluice Structure

At the multigate sluice structure on a fluvial river, undesired sediment deposition affects the normal operation of the reservoir in question. Physical and numerical models are hybridized to help explore flow and sedimentation patterns. Field and laboratory investigations show that the deposition is attributable to the formation of large recirculation zones at low and medium discharges. As a potential countermeasure, an array of guide vanes is recommended to cope with the concern. Their attack angle with the flow is a dominant parameter that needs to be evaluated. Tests in the fixed-bed model demonstrate that the vanes bend the reservoir flow towards the sluice and suppress the circulation zones along both banks. The favorable range of attack angle is 15–20°. With the examination of sedimentation of both bed and suspended loads, the numerical modeling indicates that the sediment-removal efficiency increases with an increase in attack angle. By weighing the flushing efficiency and the risk of local scouring at the vanes, the 15° vane layout is recommended. This study is expected to provide a reference for guide-vane design in similar situations.

Investigation on Forced Convective Heat Transfer at the Building Façade with Louver Blinds under Different Wind Directions

Exterior louver blinds are one of the most common shading devices attached to the building façade, which will not only screen solar radiation but also affect the flow around the building and change the convective heat exchange at the façade. To better understand how different wind directions affect the convective heat properties of louvered building façades, this paper conducted a CFD simulation with the validation of previous wind-tunnel experiments. A total of 63 shading cases and 7 unshaded cases were both investigated by simulations. It was found that with the increase in wind attack angle θ, the surface-averaged convective heat transfer coefficient (CHTC) at the louvered surface will first increase by about 40~50% (θ from 0° to 60°) and then be cut down by 60% (θ from 60° to 90°). In addition, the convective heat exchange rate at the louver slats will gradually decrease with the growth of θ. Finally, modifications for previous correlations were proposed to quantify the effect of wind directions on the CHTCavg at the blinds and façade, of which the maximum deviation for convective heat exchange rate is less than 18%.

Effect of the trailing-edge flap on tones due to self-excited oscillation within the leading-edge slat cove

To study the tonal noise characteristics of the leading-edge slat of high-lift configurations with and without a deployed trailing-edge flap, experiments are conducted in the D5 aero-acoustic wind tunnel at Beihang University. The numerical simulation method is used to obtain the necessary flow information. The experimental results show that low to mid frequency tonal noise generated by self-excited oscillation within the slat cove is dominant and its corresponding frequencies are basically unchanged whether the flap is deployed or not. However, the primary mode of self-excited oscillation within the slat cove switches to a higher one when the flap is deployed. Further analysis results demonstrate that variation of the primary mode is found to be closely related to the flow characteristics in the self-excited oscillation feedback loop. The number of the primary mode is generally proportional to the ratio between the vortex shedding frequency and the self-excited oscillation frequency. The flap being deployed results in an increase in the effective angle of attack of both the main wing and slat, which leads to a thinner separating boundary layer, thus increasing further the vortex shedding frequency and this ratio.

Simplified analytical models for hypersonic lateral-directional stability

The balance of lateral and directional static stability plays an important role in the lateral-directional dynamics and vehicle design. The theory of geometric influence on the lateral and directional static stability is mature for conventional aircrafts. However, the distinct configuration characteristics and flow physics for hypersonic vehicles may lead to different design theory, which is absent now. In order to solve this problem, simplified analytical models of lateral-directional static derivatives are proposed based on the simplified geometry model. It's found that the lateral and directional stability both increase as the deflection angle, dihedral angle, or angle of attack increases, whereas the influence of sweep angle can be neglected. The difference lies in that the lateral stability is proportional to dihedral angle while the directional stability is proportional to the square of dihedral angle. The reasonable accuracy is validated in comparison with the inviscid numerical solutions. Furthermore, the influence mechanism of viscous effects on the lateral and directional stability is investigated in detail based on the numerical results. It's found that the strong viscous interaction can improve the lateral stability evidently, and both the strong viscous interaction and the shear stress can lead to the increase of the directional stability. In addition, the lateral stability is slightly reduced by the chemical nonequilibrium effects.