Small Angles Of Attack Research Articles

Aerodynamic Characteristics of Flow Past Naca 0008 Airfoil at Very Low Reynolds Numbers

The flow around NACA 0008 airfoil was investigated computationally at two very low values of Reynolds numbers 2000 and 6000 and for various angles of attack 0 to 8°. The flow being laminar it was seen to separate at very small angles of attack. Due to large viscous effects the pressure recovery at the trailing edge is poorer as compared to the high Reynolds number case leading to lower values of lift coefficient Cl, higher values of drag coefficient Cd, and lower values of (Cl/Cd). The wake flow exhibits similarity behaviour similar to that in high Reynolds number flat-plate wake. The quantitative validity of the boundary layer equations in a limited range of parameters is ascertained.

Математическая модель продольного движения космического аппарата на различных участках его полета

The article studies the spacecraft stability at small and large angles of attack under the influence of disturbances arising from aerodynamic drag, wind speed and linear thrust eccentricity caused by deviations (misalignment) of the installation angle of the booster engine of the spacecraft. Possible use of spacecraft with opening braking surfaces when moving in the atmosphere is described. A mathematical model of disturbed motion of the spacecraft at various stages of its flight to the destination planet has been developed. The resulting mathematical model was used in the studies of the launch of a spacecraft from an inclined, vertical launch facilities and descent for landing on the surface without taking into account the operation of braking engines. The parameters of the trajectories of a spacecraft with a booster in the pitch plane are calculated depending on the flight altitude and the inclination of the launch pad.

Transient analysis of thermo-sensitive cavitating flows over a hydrofoil in various angles of attack and temperatures

The present work aims to contrastively study the cavitation intensity, temperature drop, turbulence kinetic energy as well as the changed lift and drag forces for various free-stream temperatures and angle of attacks (T∞=68.64-88.54K,AOA=3.5o-12o) in a thermo-sensitive cavitating flow. The simulation results indicate that the higher temperature and larger angle of attack intensify the cavitation thermal effect and increase the temperature drop jointly. The difference lies in the large angle of attack promotes the cavitation development, but the high temperature weakens the cavitation intensity and reduces the vapour volume. The influential mechanisms exist in the cavity evolution, vorticity, turbulence kinetic energy as well as angle of attack are revealed. The high-level turbulence kinetic energy occurs in the unstable flow regions, include the cavity tail, the separation zone on the suction surface and cavitation vorticity region. For the attached cavity, the larger angle of attack suppresses the turbulence kinetic energy due to the enhanced cavity adhesion on hydrofoil surface. The turbulence kinetic energy caused by cavity shedding and cavity collapse becomes more obvious at a smaller angle of attack. The study on the force evolution show that increased temperature and angle of attack weaken the fluctuation amplitudes of lift and drag forces and reduce the occurrence frequencies of peak values, and then the averages of lift and drag forces are increased. The temperature rise increases the average of lift force and reduces the average of drag force, which is characterized by a increase of lift-drag ratio. Meanwhile, the quantitative results include transition of shedding period and temperature drop in cavity thickness direction are also analysed.

High-fidelity modelling of lift-based wave energy converters in a numerical wave tank

This work presents the development of a numerical model for the investigation of lift-based wave energy converter (lift-WEC) hydrodynamics. The developed model, based on the RANS equation, is validated regarding its ability to replicate foil-flow interaction in various conditions based on experimental references from literature. Limitations of the numerical model are discussed, such as the overprediction of stall angles at low Reynolds numbers and the possibility of artificial rotor ventilation near the free surface. Within the range of relevant operating conditions for a lift-WEC, the model is found to replicate experimental results with good accuracy. Subsequently, a particularity of the cyclorotor WEC concept is investigated: Load measurements in calm water showed positive tangential forces, apparently indicating the generation of thrust in the absence of external energy sources. Numerical simulations reveal that this is the result of a significant azimuthal shift of the hydrodynamic load centre for non-zero pitch and that the consideration of foil pitch moments is required in order to obtain accurate shaft torque predictions. The hydrodynamic coefficients of the cyclorotor foils are compared with coefficients obtained for symmetrical foils in straight flight. It is shown that the assumption of similarity, which is often applied when modelling these devices in semi-analytical tools, is only valid for small angles of attack.

Experimental Investigations on High-Speed Aerodynamic Heating over Inflatable Decelerators

Heat transfer measurements and high-speed schlieren flow visualization were performed over a cylinder with an aft-attached inflatable decelerator configuration at Mach 3.9. Back wall temperature measurements using thermocouples were used to derive the heat flux distribution over the decelerator surface. A strong nonlinear rise in peak heat flux with angle of attack is observed and windward–leeward effects are quantified. Using appropriate flow and geometric symmetries, heat maps are generated over the regions of the decelerator, which are critical for its thermal design. The effect of fore-end bluntness in terms of peak heat fluxes and integrated heat loads is investigated. The results indicate a possibility of flying at a small angle of attack to reduce the integrated heat loads while simultaneously generating some lift. Tests up to an angle of attack of 20 deg show that even though the peak heating rises strongly, the overall integrated heat loads on the surface of the decelerator remain manageable. A local minimum is seen in peak heat flux levels on the surface at angles of attack of 12–16 deg.

О влиянии винглетов на аэродинамические свойства тонкого крыла

The paper considers the problem of symmetrical airflow around a thin plate with winglets. There is studied the effect of the winglets on the lifting force of a thin plate. There are presented the data obtained by using a mathematical model, which is based on a linearized theory of thin wing and a method of small perturbations. In frames of this theory, the problem is reduced to a system of two dual integral equations, which is solved by a discrete vortex method. For comparison, we also present the data obtained in natural experiment with thin plates of various sizes with and without winglets. Then we perform a comparison between the analytical theory and the data of the natural experiments. Some conclusions are made about the effect of winglets on the lifting force. It is stressed that with small attack angles the experimental results confirm the theoretical conclusions. The general conclusion is that the winglets of the discussed type increase the lifting force up to 10 % that may lead to fuel economy with their usage.

Effect of Small Angles of Attack on Turbulence Generation in Supersonic Boundary Layers on Swept Wings

We present the new (for Mach numbers М = 3 and 3.5) and generalizing (for Mach numbers from 2 to 4) results of experimental investigations on the effect of small angles of attack on laminar-turbulent transition in the supersonic boundary layer on a swept wing with the leading-edge slip angle of 72°. The angle-of-attack variation has a strong effect on the transition Reynolds number. The transition Reynolds number decreases with increase in the Mach number. The measurements were carried out by means of a constant-temperature hot-wire anemometer using the proven procedure of determining the transition location. The eN method is used for the first time for numerically estimating the transition Reynolds numbers in the supersonic boundary layer on a swept wing with the leading-edge slip angle of 72°. The growth of the amplitudes of the steady and unsteady modes of the boundary layer crossflow are calculated in accordance with the linear stability theory, within the framework of the Lees–Lin system of equations. The numerical results indicate that, in accordance with the experimental results, laminar-turbulent transition in the boundary layer on the model swept wing is governed by the growth of stationary modes of the crossflow instability.

Evaluation of a modified URANS prediction of unsteady cavitating flow around a hydrofoil by comparing with LES results and experimental results

The research makes the analysis of cavitating flow field around hydrofoil NACA0009 through both experiments and numerical simulations. We previously proposed an interface effect-based correction model (IE model) for turbulent viscosity correction of cavitation flow, which performed well for hydrofoil cavitation simulation at small angles of attack. This research makes a further evaluation of the IE model. Firstly, the cavitation experiments of the NACA0009 blunt trailing edge hydrofoil with a 10-degree angle of attack are carried out on a closed test platform. The images of quasi-periodic shedding of partial cavitation are obtained. Secondly, the standard k-ε turbulence model, the IE model, and the large eddy simulation (LES model) are used to reproduce the cavitation flow around the hydrofoil. The results show that the standard k-ε model over-predicts the turbulent viscosity of the flow field, resulting in the cavitation region stably attached to the leading edge of the hydrofoil, which can't simulate the unsteady characteristics of the cavity bubbles. The LES model successfully reproduces the process of cavitation structure quasi-periodic shedding off. The IE model modifies the turbulent viscosity in the high shear flow region so that the reentrance flow can reach the leading edge of the hydrofoil and cut the attached cavity off. IE model can properly simulate the macro features of the cavity and the shedding frequency. In each stage of cavitation periodic shedding, the results of the IE model and the LES model on the pressure distribution and velocity field are consistent well. In addition, the calculation time cost of the IE model is basically the same as that of the standard k-ε model, only 0.14 times the LES model costs. The IE model is more suitable for cavitation evaluation in engineering applications.

Effect of Small Angles of Attack on Turbulence Generation in Supersonic Boundary Layers on Swept Wings

We present the new (for Mach numbers М = 3 and 3.5) and generalizing (for Mach numbers from 2 to 4) results of experimental investigations on the effect of small angles of attack on laminar-turbulent transition in the supersonic boundary layer on a swept wing with the leading-edge slip angle of 72°. The angle-of-attack variation has a strong effect on the transition Reynolds number. The transition Reynolds number decreases with increase in the Mach number. The measurements were carried out by means of a constant-temperature hot-wire anemometer using the proven procedure of determining the transition location. The eN method is used for the first time for numerically estimating the transition Reynolds numbers in the supersonic boundary layer on a swept wing with the leading-edge slip angle of 72°. The growth of the amplitudes of the steady and unsteady modes of the boundary layer crossflow are calculated in accordance with the linear stability theory, within the framework of the Lees–Lin system of equations. The numerical results indicate that, in accordance with the experimental results, laminar-turbulent transition in the boundary layer on the model swept wing is governed by the growth of stationary modes of the crossflow instability.

Hyplane: a single-stage suborbital aerospaceplane.

This paper has the aim to report the status of the HYPLANE project to date. HYPLANE is a horizontal take-off and landing Mach 4.5 bizjet-size aerospaceplane conceived by Trans-Tech and University Federico II of Naples and under study within the industrial-academic ecosystem of the Campania Aerospace District (DAC). HYPLANE has the aim to offer very fast suborbital flight for space tourism, microgravity experimentation and training, and also shortening time to connect two distant airports within a door-to-door scenario. The concept is based on the access to stratospheric altitudes (30km) for either point-to-point stratospheric or suborbital flights as safe as today's commercial air transportation, by integrating enhanced state-of-the-art aeronautical and space technologies. Essentially, HYPLANE is mostly based on already relatively high TRL technologies which guarantees a sufficiently short time to market. The low wing loading configuration and designed ability to manoeuvre along the flight trajectories at small angles of attack, allow HYPLANE to guarantee accelerations and load factors of the same order as those characterizing the present civil aviation aircraft (FAA/EASA specifications). Thanks to its technical features, it may operate from/to more than 5000 airports all over the world needing short runways to take-off and land, which for point-to-point business aviation is paramount. Furthermore, characteristics such as small dimension, configuration and high cruising altitude determine reduced noise in the airports surrounding and low sonic boom impact on ground. These conditions will further facilitate not only the development of the commercial use of such kind of transportation mean, but also its social acceptability.

Stability of fixed points in an approximate solution of the spring-mass running model

Abstract We consider a classical spring-mass model of human running which is built upon an inverted elastic pendulum. Based on previous results concerning asymptotic solutions for large spring constant (or small angle of attack), we introduce an analytical approximation of a reduced mapping. Although approximate solutions already exist in the literature, our results have some benefits over them. They give us an advantage of being able to explicitly control the error of the approximation in terms of the small parameter, which has a physical meaning—the inverse of the square-root of the spring constant. Our approximation also shows how the solutions are asymptotically related to the magnitude of attack angle $\alpha $. The model itself consists of two sets of differential equations—one set describes the motion of the centre of mass of a runner in contact with the ground (support phase), and the second set describes the phase of no contact with the ground (flight phase). By appropriately concatenating asymptotic solutions for the two phases we are able to reduce the dynamics to a one-dimensional apex to apex return map. We find sufficient conditions for this map to have a unique stable fixed point. By numerical continuation of fixed points with respect to energy, we find a transcritical bifurcation in our model system.

Bayesian uncertainty quantification analysis of the SST model for transonic flow around airfoils simulation

Transonic flow simulation is a common but complex task in engineering, and poses a great challenge to computational fluid dynamics using the Reynolds-averaged Navier–Stokes approach. One of the important factors influencing the complexity of this task is the uncertainty introduced by turbulence models. In this paper, to improve the performance of the shear stress transport model, a Bayesian uncertainty quantification analysis of turbulence model parameters is carried out on transonic flow around the RAE2822 airfoil and the ONERA M6 wing. First, the Sobol indices are obtained for sensitivity analysis, the results of which show that the pressure coefficients are mainly sensitive to four parameters a1, κ, β⁎, and β1. A posterior uncertainty analysis is then performed based on the pressure coefficients of two-dimensional sections. The maximum a posteriori estimates from the two examples indicate opposite trends for the predicted shock wave positions. In the predictions for RAE2822, the shock wave position is advanced, while in those for ONERA M6, it is delayed. The estimates from RAE2822 enable a better prediction capability at a small angle of attack, while those from ONERA M6 enables an ability to simulate flow with separation at a large angle of attack, mainly because of the increase in a1. In practical engineering applications, the choice between the two sets of calibrated parameters to achieve the best simulation results may be facilitated by reference to experimental data.

Identification and Modeling Method of Longitudinal Stall Aerodynamic Parameters of Civil Aircraft Based on Improved Kirchhoff Stall Aerodynamic Model

The stall aerodynamic model based on Kirchhoff’s theory of flow separation is widely used in the identification and modeling of stall aerodynamic parameters. However, it has two defects. First, its model structure is significantly different from the pre-stall model used for a small attack angle, meaning the identification results cannot be combined with the pre-stall model to form the full flight envelope model. Second, the pitching moment model, which is used in conjunction with the Kirchhoff lift model, cannot accurately describe the aircraft stall pitching moment characteristics. To ensure the compatibility of the two models, this paper proposes a method to determine some unknown parameters in the stall model. The mechanism for the pitching moment generation of the aircraft stall is analyzed, and two high-order correction terms are added to the pitching moment model to better describe the longitudinal stall aerodynamic characteristics. Based on the identification of aerodynamic parameters, a longitudinal stall aerodynamic modeling method used for the aircraft stall process is developed. The identification and simulation validation results based on the quasi-steady stall flight data of a civil aircraft show that the improved stall model can accurately describe the quasi-steady stall pitching moment. The established stall aerodynamic model can accurately characterize the longitudinal quasi-steady stall aerodynamic characteristics of aircraft under different stall degrees.

Refining Kutta’s Flow over a Flat Plate: Necessary Conditions for Lift

In this paper, we present a variational theory of lift that, unlike Kutta’s theory, is derived from first principles in mechanics: Hertz’s principle of least curvature. In this theory, the unique value of circulation is determined by minimizing the Appellian of the flowfield. Interestingly, it recovers the Kutta condition in the special case of an airfoil with a sharp trailing edge. In this paper, we apply such a theory to the classical problem of the flow over a flat plate. The resulting ideal flow does not match Kutta’s solution in this case; it results in a nonlifting solution for any uncambered, fore-aft symmetric shape, confirming experimental findings in superfluids. This result provides necessary conditions for lift generation in an ideal fluid. For a real fluid over a flat plate with a sharp leading edge, viscosity plays an important role, leading to a flow separation at the leading edge, even at small angles of attack. This separation bubble creates asymmetry in the outer inviscid flowfield (outside the bubble), which enables lift. This problem is discussed in the light of the developed variational theory of lift and some historical details about the development of Kutta’s theory.

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.

Basic static stability and control characteristics of hypersonic vehicles with dorsal and ventral intake

The airframe/engine integrated configurations with dorsal intake can effectively decouple the inlet from the lifting surface, leading to an excellent aerodynamic performance at a small angle of attack at hypersonic speeds. For a comprehensive and systematic investigation of the dorsal intake configuration, it is necessary to compare it with a ventral intake configuration of equal scale. However, most of the present assessments for dorsal intake configurations focus on the fundamental aerodynamic characteristics with a lack of stability and control characteristics. Therefore, the computational fluid dynamics numerical simulation based on multi-GPU parallel computing is introduced for comparing the basic static stability and control characteristics between the dorsal and ventral intake configurations. The results show that the dorsal intake configuration has superior three-channel static stability at hypersonic speeds than the ventral intake configuration. However, this also results in worse control characteristics, which may be remedied by relaxing the static stability margin or installing canards. Moreover, the selection of intake configuration only interferes with the coupling of the pitch channel and does not affect the coupling of the yaw and roll channels.

Onset of unsteadiness in the flow past a blade cascade

There has been little research on the system stability of the flow past a blade when the domain is finite in the transverse direction. In this study, we first explore the influence of the domain size and periodic boundary conditions in the transverse direction on the stability of the flow past a single blade in a finite domain. As the transverse width decreases, the base flow and instability characteristics change. Furthermore, the stability of the flow past a cascade including n blades is analyzed. There exist n perturbation modes corresponding to different temporal growth rates and frequencies with various staggered spatial distributions of the dominant region, embodied as the inter-blade phase angle in the frequency domain. The critical value and leading mode are related to the blade number. Therefore, when analyzing the stability of the flow past a cascade, it is important to extend the domain to the real blade number. The influence of the geometric cascade parameters on system stability is also studied. Finally, when the circumferential periodic flow is broken in a cascade including n blades, there exists only one eigenvalue near the stability boundary. The system stability deteriorates, and the critical Reynolds value drops sharply, even when several blades are restaggered to have a smaller angle of attack. From the distribution of the leading mode, the initial location of unsteadiness is associated with the region of maximum deficit in the velocity profile of the wake flow.

FLUID DYNAMIC PERFORMANCE AND PERFORMANCE ANALYSIS OF A NOVEL FLEXIBLE OSCILLATING WIND TURBINE

While the variable pitch angle design has been proven to significantly enhance the performance of vertical axis wind turbines, its implementation requires the addition of extra mechanical devices, such as a chain-and-sprocket system. These additional mechanisms not only increase system friction and produce extra vibrations but also cause vertical axis wind turbines to lose their omnidirectional characteristics. To address this issue, this study proposed a novel flexible oscillating wind turbine, wherein the function of changing the pitch angle is accomplished through the deformation of flexible blades. To reveal the internal flow field mechanism of this novel turbine, particle image velocimetry experiments were conducted. The results indicate that the flexible blade can alter its posture to form a suitable angle of attack under different azimuth angles. In the downstream region, the blades spread out to generate a substantial positive torque. In contrast, in the upstream region, the blades are in a diversion state, always maintaining a small angle of attack to avoid generating excessive negative torque. Wind tunnel experiments were carried out on the flexible oscillating wind turbine to explore the effects of wind speed and key geometric parameters on the turbine’s performance. The results show that the novel flexible oscillating wind Turbine maintains high efficiency even under low wind speed conditions.

FLUID DYNAMIC PERFORMANCE AND PERFORMANCE ANALYSIS OF A NOVEL FLEXIBLE OSCILLATING WIND TURBINE

While the variable pitch angle design has been proven to significantly enhance the performance of vertical axis wind turbines, its implementation requires the addition of extra mechanical devices, such as a chain-and-sprocket system. These additional mechanisms not only increase system friction and produce extra vibrations but also cause vertical axis wind turbines to lose their omnidirectional characteristics. To address this issue, this study proposed a novel flexible oscillating wind turbine, wherein the function of changing the pitch angle is accomplished through the deformation of flexible blades. To reveal the internal flow field mechanism of this novel turbine, particle image velocimetry experiments were conducted. The results indicate that the flexible blade can alter its posture to form a suitable angle of attack under different azimuth angles. In the downstream region, the blades spread out to generate a substantial positive torque. In contrast, in the upstream region, the blades are in a diversion state, always maintaining a small angle of attack to avoid generating excessive negative torque. Wind tunnel experiments were carried out on the flexible oscillating wind turbine to explore the effects of wind speed and key geometric parameters on the turbine’s performance. The results show that the novel flexible oscillating wind Turbine maintains high efficiency even under low wind speed conditions.

Unsteady Flow Transition Effects on a Pitching Transonic Laminar Airfoil

This paper presents a computational study of unsteady transonic effects on a pitching supercritical laminar airfoil using high-order implicit large-eddy simulation. A CAST10-2 airfoil operating at a chord-based Reynolds number of , a transonic freestream Mach number of , and a small nominal angle of attack of deg was subjected to forced pitching oscillations about the quarter-chord with a small amplitude of deg and reduced frequencies of , 0.23, and 0.285. Complex interactions between flow separation, the motion of multiple shocks, and excursions of transitional flow were elucidated and compared against inviscid simulations. The boundary layer was found to be highly sensitive to pitch frequency, due in large part to the effects of frequency on shock motion. was found to be indirectly affected by the impact of shock motion on the boundary-layer state. Aerodynamic damping diminished with increasing oscillation frequency until becoming negative at , indicating potential for aeroelastic instability.