Positive Angles Of Attack Research Articles

An experimental investigation of supersonic conical cooling films with angles of attack

While the flow mechanisms of two-dimensional supersonic cooling films have been studied in-depth, this paper used the nanoparticle planar laser scattering and particle image velocimetry techniques to investigate the flow of supersonic conical cooling films at different angles of attack (AOAs). The mainstream Mach number was Ma∞=3.8, and the supersonic conical cooling film was tangentially injected via a precisely calibrated Maj=2.8 annular nozzle. Initially, the streamwise boundary layer transition process without cooling film injection was analyzed. The boundary layer transition on the leeward side occurred prematurely, whereas on the windward side, the transition process was notably delayed. Subsequently, the supersonic conical cooling film flow was qualitatively and quantitatively evaluated from the perspectives of turbulent structures, and the time-averaged and statistical characteristics of the velocity field. On the windward side, as the ratio of static pressure decreased, the effective cooling length also decreased with an increase in AOA. On the leeward side, at a small positive AOA, the supersonic conical cooling film mixed with the low-energy fluid within the thickened inner layer of the mainstream boundary layer, which mitigated the growth rate of the mixing layer and ultimately enhanced the effective cooling length. With a further increase in AOA, the supersonic conical cooling film experienced the three-dimensional detrimental effects of crossflow-separation vortices and downwash mainstream on the leeward surface, resulting in a decrease in the effective cooling length.

Computational investigations of Ka-226T helicopter coaxial rotor aerodynamics in the vortex ring state

The vortex ring states are observed when the rotor is flowed at positive angles of attack. For the main rotor, these conditions are realized during a steep descent of the helicopter at low speeds. The rotor vortex ring states are affected by significant phenomena related to the behavior of its aerodynamics, including negative phenomena. The latter is, firstly, referred to decrease in rotor thrust, increase in the required power, pulsations of thrust and torque, unsteady flapping blade motion, etc. In terms of helicopter piloting, it means a sharp loss of altitude, increase in the control force, a high level of vibration, defocusing attenuation of rotor spinning cone, as well as controllability deterioration. All these factors determine the relevance of research on these modes and the importance of solving a problem of defining their boundaries. Recently, due to the rapid development of computer technologies and computational models, it has become practical to perform numerical research of the rotor aerodynamics in vortex ring states. The paper presents the study results of Ka-226T helicopter coaxial rotor aerodynamics in steep descent modes in the field of vortex ring states. The angles of rotor attack αВ = 90...30° and the range of vertical descent velocities Vу = 0...26 m/s are considered. The original nonlinear bladed vortex model of the rotor developed at the Moscow Aviation Institute (MAI) was used. The total and distributed aerodynamic rotor characteristics were calculated. The shapes of the vortex wake and the rotor flow patterns were analyzed. The boundaries of the vortex ring states in velocity coordinates “Vx − Vу” were constructed according to various criteria reflecting the known features of these states. The results obtained significantly complement the existing experience of experimental and numerical research in this field.

Effect of asymmetric leading edge on transition of suction side

Abstract Asymmetric leading edge offers several advantages over conventional leading edge. However, there are few related published research on compressor blades. This study proposes a blade parameterization method that superimposing the thickness distribution on the camber line at the middle and curve fitting at the leading and trailing edges. This parameterization method is applied to a stator in the final stage of an axial compressor, and numerical simulation is used to calculate the aerodynamic performance at various attack angles under the designed inlet Mach number. There is an optimal deflection angle regardless of which side of the leading edge deflects, and deflection toward the pressure side is conducive to the suppression of separation bubble near the leading edge at positive attack angle. When the maximum curvature of the pressure side is decreased, the operating range changed slightly, but the positive attack angle range grew from 5.166° to 5.466°.

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.

Effect of a bioinspired upstream extended surface profile on flow characteristics and a drag coefficient of a circular cylinder

<abstract> <p>In the current work, the passive drag reduction of a circular cylinder for the subcritical Reynolds number range of 5.67×10<sup>4</sup> to 1.79×10<sup>5</sup> was computationally and experimentally investigated. First, inspired by nature, the aerodynamic drag coefficient of a whole Peregrine Falcon was measured in a subsonic wind tunnel for various angles of attack and Reynolds numbers (<italic>Re</italic>) and compared with the bare cylinder. At a 20° angle of attack and <italic>Re</italic> = 5.67×10<sup>4</sup>, the whole falcon model had a 75% lower drag coefficient than the bare cylinder. Later, with the moderate Falcon model, in which the falcon's beak and neck were linked to the cylinder as an extended surface, the drag coefficient decreased up to 72% in the subcritical Reynolds number zone. Finally, the extended surface with a falcon beak profile was connected to the cylinder with a stem and investigated both numerically and experimentally for various stem lengths, angles of attack, and Reynolds numbers. It was found that at low <italic>Re</italic>, the drag coefficient can be reduced by up to 47% for the stem length of 80 mm (<italic>L</italic>/<italic>D</italic> = 1.20) with an angle of attack 10°. The computational investigation yielded precise flow characteristics, and it was discovered that the stem length and the <italic>Re</italic> had a substantial influence on vortex generation and turbulent kinetic energy between the beak and cylinder, as well as downstream of the cylinder. Investigation revealed that percentile drag reduction was much lower for the whole Falcon model over a wide range of Reynolds numbers and positive angles of attack, which exist in nature. Similarly, when compared to the other stem lengths, the 60 mm stem length (<italic>L</italic>/<italic>D</italic> = 0.97) produced similar results to the whole Falcon model. The numerical results were well validated with the experimental results.</p> </abstract>

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.

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.

Performance Investigation of a Large Turning Blade for Marine Gas Turbine Based on Experimental and Numerical Methods

Replacing the last-stage stator and the outlet guide vane of the low-pressure compressor of the marine gas turbine with a large turning blade can reduce the number of compressor blades and reduce the size and weight of marine gas turbines. At present, there are few studies on similar profiles, and it is necessary to verify the feasibility of this type of compressor profile with a large turning angle. The performance of this profile is investigated by combining experimental measurement with numerical simulation calculation. The analysis of the experimental and numerical results reveals that this profile has a large flow turning angle, a wide operating range, and low overall total pressure losses. The loss of the profile only suddenly increases at some large positive angles of attack due to the large separation of the suction surface. The results show that this profile can compress air and increase the turning ability at a low loss, and can play the role of both the original last-stage stator and the outlet guide vane. This research provides a reference for the design and analysis of marine gas turbines and guidance for the application of the blade to gas turbines in other fields.

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.

Corner separation control in compressor stators with jets from the blade leading edge

Corner separation occurs easily in highly loaded compressor stators. This paper proposes a novel slot inside the stator blade, with the slot inlet located at the blade leading edge and the slot outlet located at the suction surface, by using the momentum of the incoming flow to form a high velocity jet to control the development of the boundary layer on the blade suction surface. For 3D compressor cascades, the influence of the key geometrical slot parameters on the flow fields in the slot and cascade passage and the aerodynamic performance of the cascade were investigated with a numerical method. Then, the stator blades in a highly loaded compressor stage were slotted to improve the flow in the stator blade passages. The main results can be summarized as follows. The use of slotted blades in the cascade effectively suppresses corner separation. When the cascade inlet velocity is high or the inlet flow is at a positive attack angle, the separation control effect improves. In the highly loaded compressor stage, slotted stator blades considerably reduced the flow loss at the stator corner, and the stage efficiency increased over the entire flow rate range. Furthermore, two cascades with unslotted and slotted blades were tested. The experimental and numerical results were consistent, demonstrating that the total pressure loss coefficient of a cascade can be effectively reduced by slotting the blades in the cascade.

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.

Influence of attack angle on ventilated supercavitating flow at high Froude number condition

To study the influence of attack angle on ventilated supercavitating flow, a numerical model is established to simulate the supercavitation flow by using the inhomogeneous multiphase flow model and the SST k-ω turbulence model. And then, the model is verified and validated by comparing with the experimental results. The effect of attack angle on the profile of ventilated supercavity was numerically investigated over high Froude number. Results show that the attack angle causes the linear deviation of the ventilated supercavity axis, the deviation angle is in direct proportion to the attack angle, the deviation direction is opposite to the lift of the cavitator, and the deformation of the supercavity contour increases along the flow direction. The attack angle changes the gas flow structure inside the ventilated supercavity, causing the different deformation of the upper profile and lower profile. Under the condition of positive attack angle, the deformation of the lower profile of the supercavity generated by the disk cavitator is significantly greater than that of the upper profile.

Research on Soft Flutter of 420m-Span Pedestrian Suspension Bridge and Its Aerodynamic Measures

In order to study the flutter of long-span pedestrian suspension bridge and its aerodynamic control, a 420m-span pedestrian suspension bridge is used as an engineering example, the wind-induced vibration of seven particular aerodynamic sections is studied by wind tunnel tests, and the soft flutter phenomenon of two kinds of aerodynamic sections is identified. The results show that the wind fairing and the wind-retaining plate measures are not necessarily effective measures to improve the wind-induced stability of long-span pedestrian suspension bridge, as these two measures may reduce the flutter stability: the wind fairing section in the positive angle of attack is prone to torsion-based soft flutter phenomenon, in which the vertical vibration spectrum contains multiple vibration frequencies, so the conventional formulation of the linearized self-excited forces is no longer satisfied; the wind-retaining plate section in the negative angle of attack is prone to soft flutter dominated by vertical vibration, and the beat vibration phenomenon is found in the torsional vibration time history of the wind-retaining section. Slotting in the center of the girder section can significantly change the flow state of the section, which is an effective measure to improve the flutter stability of the pedestrian suspension bridge.

Numerical study of the energy loss in the bulb tubular pump system focusing on the off-design conditions based on combined energy analysis methods

The unsteady flow characteristics of the low-head bulb tubular pump highly affects the operation efficiency and stability of the whole system. The purpose of this paper is to investigate the dominating loss area in a typical tubular pump using the combined energy analysis methods. The transient multi-condition flow through the pump system is simulated based on N–S equation integrating SST k-ω model. The power loss characteristics of each flow passage component are analyzed in terms of total pressure variation, entropy production and energy balance equation. The turbulent entropy generation power (PSD') in the entropy production and the turbulent kinetic energy production (PL3) of the energy balance equation are the primary source of power loss, indicating that the fully developed turbulent flow tends to generate considerable loss in the pump system, followed by the power loss of solid wall (PSW and PLW). The distribution of power loss in the impeller and diffuser was comprehensively exhibited and it was found that the undesirable flow patterns are mainly caused by blades and walls. The rim region in impeller domain with volume fraction of 15% induces the energy loss of approximately 45% due to the presence of solid wall and tip leakage vortex (TLV). Furthermore, the individual grid volume is considered to better locate and visualize regions with our great interest, and a number of typical vortex structures are identified under several flow rates showing that the positive attack angle is more likely to caused flow instability and energy loss at low flow rate condition.

Effect of angle of attack on the performance of the supersonic intake for High Mach Integrated Control Experiment (HIMICO)

Experimental, numerical and theoretical investigations are carried out to grasp the effect of angle of attack (AoA) on the performance of the supersonic intake for High-Mach Integrated Control Experiment, HIMICO. The results of theoretical analysis using simple shock wave (TFD) suggests that mass capture ratio (MCR) and total pressure recovery (TPR) are increased with positive angle of attack. In fact, comparing the result of 2.5 deg AoA with that of −2.5 deg AoA gained by CFD, MCR and TPR become 21pt and 5pt larger when the nozzle throat height (HNT) is 12.5 mm. The main factors of these phenomena are the increased projected area of the intake entrance and the change of the oblique shock system. On the other hand, TFD incorrectly estimates HNT and TPR in critical state because the pseudo-shock wave is generated instead of the terminal normal shock wave considered in TFD. So the characteristics of the pseudo-shock wave are investigated by CFD. As a result, it was found that the total pressure loss due to the pseudo-shock wave is larger than that of the normal shock wave, and that the total pressure loss becomes smaller with positive AoA due to the decrease in the Mach number at the intake throat. Using the CFD results, we created a model to estimate TPR and HNT in the critical state from the relational equation between the intake throat Mach number and the total pressure loss due to the pseudo-shock wave.

Numerical Investigation of Aerodynamic Characteristics of Supercritical RAE2822 Airfoil with Gurney Flap

Gurney flap (GF) is well-known as one of the most attractive plain flaps because of the simple configuration and effectiveness in improving the lift of the airfoil. Many studies were conducted, but the effects of GF on the various airfoil types need to be further investigated. This study aimed to clarify the effect of GF in the case of the supercritical airfoil RAE2822. This research includes a steady, two-dimensional computational investigation carried out on the supercritical airfoil type RAE-2822 to analyze Gurney flap (GF) effects on the aerodynamic characteristics of this type of airfoil utilizing the Spalart-Allmaras turbulence model within the commercial software Fluent. The airfoil with the Gurney flap was analyzed for three different height values 1%c, 2%c, and 3%c, and five mounting angles (30°,45°,60°,75°, and 90°) with the axial chord for angles of attack (-1°,-2°,-3°,0°,1°,2°,3°). The calculations showed that when GF height is increased, the maximum suction pressure on the upper surface increases by 25.4%, 36.5%, and 68.83% when the height of the Gurney flap is 1%c, 2%c, and 3%c, respectively, compared with that on the airfoil without GF. The lift coefficient was also increased, and the shock waves moved downward by increasing GF height. As Gurney flap heights increase, the drag coefficient increases gradually for positive angles of attack but for negative angles of attack. The drag coefficient also decreases with increasing the GF heights. As long as the angle of the mounting is between 45o and 90o, the lift coefficient does not differ on a large scale. For mounting angles less than 45o, the lift coefficient drops quite fast. As a result, reducing the Gurney flap’s lift enhancement and the drag coefficient increases gradually for positive angles of attack, but for negative angles, it can be noted that the drag coefficient decreases with increasing the mounting angles of GF. The calculated values of the lift and drag coefficients with an attack angle and pressure coefficient compared with the experimental values, and a good agreement was noticed.

Effect of the angle of attack on flow around an elliptic cylinder near a moving wall

The present work investigates the combined effect of the angle of attack (AOA) (the angle between the semi-major axis of the cylinder and the vertical axis), and the gap ratio G/D (where G represents the distance between the cylinder center and the moving wall and D denotes the semi-major axis of the elliptic cylinder) on the flow around an elliptic cylinder near a moving wall. Here, AOA covers ±15°,±30°, and ±45° with G/D ranging from 0.6 to 2.5. The Reynolds number (based on the free-stream velocity and the semi-major axis) is fixed to 150. The resulting Kármán vortex street, the two-layered wake, and the secondary vortex street have been investigated and visualized. The resulting patterns have been classified and mapped out in the (G/D, AOA)-space. At small gap ratios, a clockwise rotation of the cylinder (negative AOA) leads to a stronger suppression effect between the moving wall and the backside of the cylinder. This results in more transitions between the different wake patterns than for the counterclockwise rotated cylinder (positive AOA). As the cylinder is rotated more, either clockwise or counterclockwise (i.e., as |AOA| increases), the crest value of the drag coefficient decreases while the crest value of the lift coefficient first increases and then decreases for G/D≥1.0. For a given G/D, the time-averaged drag coefficient decreases with the increased rotation of the cylinder, except for an increase observed as AOA increases from 0° to 15° for G/D≤0.9. Moreover, the lift force is directed upward and downward for positive and negative AOA, respectively, and its magnitude decreases with the increase in G/D when the cylinder is counterclockwise rotated, while it increases when the cylinder is clockwise rotated.

Distinct Wing-in-Ground Effect of Airfoil in Proximity to Water Waves

The present study numerically explores the distinct aerodynamic characteristics of an airfoil flying in proximity to water waves. The water wave is treated by both two-phase-flow wavy water and rigid moving wavy ground, with a specific goal to clarify whether the water wave can be substituted by rigid wavy ground. Results show that for large ground clearance (), the aerodynamic forces and flow structures of the near-field airfoil are basically dominated by the passage shape between airfoil and wave surface where the wavy water can be replaced by means of using wavy ground. However, it is different for small ground clearance that the airfoil-induced airflow has a strong relative speed to the water flow and subsequently creates a strong shear force on the forthcoming wave to eject water spray from wave crest. The spray has a sufficient influence on the flow topology and consequently leads to a distinct aerodynamic behavior. Particularly, the strong upstream shear on the wave resulted from the stagnation region at a positive angle of attack induces upwind spray, which can be elevated to hit onto the airfoil with larger density and momentum than airflow causing extraordinarily high pressure on the lower surface as well as unstable aerodynamic forces.

Study on Aerodynamic Nonlinear Characteristics of Semiclosed Box Deck Based on Variation of Motion Parameters

In order to study the nonlinear characteristics of self-excited aerodynamic forces of bluff body bridge section with the change of motion parameters, a numerical wind tunnel is established by the dynamic mesh technique of computational fluid dynamics (CFD). A state-by-state forced vibration method is used to identify the self-excited aerodynamic forces of single degree-of-freedom (DOF) heaving and pitching motion. Fast Fourier transform (FFT) is adopted to obtain frequency-domain data for analysis. The reliability of the obtained aerodynamic results is verified by wind tunnel tests. The results show that the high-order harmonic components are found in the self-excited aerodynamic forces of semiclosed box deck section, which are more significant in aerodynamic lift than in aerodynamic moment. The proportion of aerodynamic nonlinear components increases with amplitude. The effect of amplitude on the nonlinear components of heaving motion is generally higher than that of pitching motion, and aerodynamic moment is highly sensitive to the increase of vertical amplitude. The variation of the nonlinear components of the deck section with frequency is not a simple monotonic relationship, and there is a stationary point at 10 Hz frequency. The existence of wind attack angle makes the proportion of nonlinear components reach more than 30% and greatly increases the proportion of second harmonic. In addition, the high-order harmonic components, which are not integer multiples, are found at large amplitude and positive angle of attack.

Flow and Mechanical Characteristics of a Modified Naca Wing Geometry

The flying ability is directly related to the structure of the wing geometry. The wing structure is designed differently according to each working conditions. In this study, a free-formed airfoil section was designed and the behaviour of the model under the influence of flow was investigated in terms of diving and takeoff angles. Computational fluid dynamics method was used in the analysis. The sensitivity of the method was checked by comparing the solution of a NACA airfoil section with the experimental results in the literature and its usability in the study was accepted. Also, in the study, the wing geometry was modelled as 3D and layered, and its mechanical properties were examined. The designed airfoil has more dominant flow structure in the lift direction. Non-symmetrical airfoil causes unsymmetrical Cl-Cd distribution. As a result of the wing structure being more dominant in lift, it was observed that the deformation and stress results of the positive angle of attack were higher than the negative results. Depending on the angle of attack, the pressure and flow effects on the wing caused a higher bending-torsion effect and increased the stresses in the fixation region of the wing. The lowest deformation and average stresses occurred at -4° angle of attack. The results are discussed as a result of flow and mechanical findings.