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Vortex shedding lock in due to pitching oscillation of a wind turbine blade section at high angles of attack

Vortex shedding lock in due to pitching oscillation of a wind turbine blade section at high angles of attack

The unsteady fl ow around a pitching two-dimensional airfoil section (NREL S809) has been simulated using unsteady RANS with the transition SST turbulence model. This geometry is chosen to represent a wind turbine blade in a standstill con fi guration. The Reynolds number is Re = 10 6 based on a chord length of 1 m. A prescribed sinusoidal pitching motion has been applied at a fixed amplitude of 7 ° for a range of high angles of attack 30 ° < α < 150 ° . At these incidences, the airfoil will behave more like a blu ff body and may experience periodic vortex shedding. It is well known that, in blu ff body fl ows, oscillations can lead to a lock-in (lock-in) of the vortex shedding frequency, f v , with the body ’ s motion frequency, f p . In order to investigate the susceptibility of airfoil to lock-in, the frequency ratio r ( r = f p / f v0 ) has been varied around r = 1. The lock-in region boundaries have been proposed, and an analysis of the e ff ect of the oscillation amplitude has been conducted. The lock-in map obtained suggests that, for the vibration amplitude considered, the risk of vortex-induced vibration is more signi fi cant in the regions of α ≈ 40 ° and α ≈ 140 ° , i.e., for shallower characteristic lengths. Finally, a lumped parameter wake oscillator model has been proposed for pitching airfoils. This simple model is in qualitative agreement with the CFD results.
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Vortex shedding from a wind turbine blade section at high angles of attack

Vortex shedding from a wind turbine blade section at high angles of attack

circular cylinder body due to its simple geometric shape. For other shapes, such as rectangular prisms, long-span bridge sections or blade section, there is limited knowledge about vortex shedding, even for many problems of prac- tical importance. Some studies have been conducted on inclined flat plates (e.g. Fage and Johansen, 1927; Perry and Steiner, 1987) and elliptic bodies (e.g. Nair and Sengupta, 1996, 1997), the cross-flow behaviour of which, at high angles of attack, can give a good idea of what could happen in the case of a blade section.

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Wing inertia as a cause of aerodynamically uneconomical flight with high angles of attack in hovering insects

Wing inertia as a cause of aerodynamically uneconomical flight with high angles of attack in hovering insects

Flying insects can maintain maneuverability in the air by flapping their wings, and, to save energy, the wings should operate following optimal kinematics. However, unlike conventional rotary wings, insects operate their wings at aerodynamically uneconomical and high angles of attack (AoA). Although insects have continuously received attention from biologists and aerodynamicists, the high AoA operation in insect flight has not been clearly explained. Here, we used a theoretical blade-element model to examine the impact of wing inertia on the power requirement and flapping AoA, based on 3D free-hovering flight wing kinematics of a horned beetle, Allomyrina dichotoma. The relative simplicity of the model allowed us to search for the best AoA distributed along the wingspan, which generate the highest vertical force per unit power. We show that, although elastic elements may be involved in flight muscles to store and save energy, the insect still has to use substantial power to accelerate its wings, because inertial energy stores should be used to overcome aerodynamic drag before being stored elastically. At the same flapping speed, a wing operating at a higher AoA requires lower inertial torque, and therefore lower inertial power output, at stroke reversals than a wing operating at an aerodynamically optimal low AoA. An interactive aerodynamic-inertial effect thereby enables the wing to flap at sufficiently high AoA, which causes an aerodynamically uneconomical flight in an effort to minimize the net flight energy.
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Unsteady Double Wake Model for the  Simulation of Stalled Airfoils

Unsteady Double Wake Model for the Simulation of Stalled Airfoils

In the present work, the recent developed Unsteady Double Wake Model, USDWM, is used to si- mulate separated flows past a wind turbine airfoil at high angles of attack. The solver is basically an unsteady two-dimensional panel method which uses the unsteady double wake technique to model flow separation and its dynamics. In this paper, the calculated integral forces have been successfully validated against wind tunnel measurements for the FFA-W3-211 airfoil. Further- more, the computed highly unsteady flow field is analyzed in detail for a set of angles of attack ranging from light to deep stall conditions.
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Improved Stall Prediction for Swept Wings Using Low-Order Aerodynamics.

Improved Stall Prediction for Swept Wings Using Low-Order Aerodynamics.

Modeling the lift characteristics of swept wings using this method yields poor results. The geometry of swept wings induces pressure gradients in the spanwise direction, especially at the high-loading conditions encountered at high inflow angles. These spanwise pressure gradients move the boundary layer from the root to the tip on aft-swept. The separated flow encountered at high angles of attack is moved tipwards, causing a non-uniform change in the shape of the effective body across the span. On aft-swept wings, it is observed that the root sections encounter an extended range of attached flow, while the tip sections encounter separation at significantly lower inflow angles than on similar wings without sweep [9]. The change in separation behavior of the sections invalidates the assumption that the airfoil C l vs. α curve is representative of
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Linear Parameter-Varying Control of an F-16 Aircraft at High Angle of Attack

Linear Parameter-Varying Control of an F-16 Aircraft at High Angle of Attack

The LPV antiwindup control scheme was applied to the F-16 longitudinal model without thrust vectoring augmentation. The flight envelope was chosen at high angles of attack, and the aerodynamic control surfaces would be saturated. The correspond- ing LPV model has unstable poles at one of the gridding points. A nominal LPV controller was first designed to stabilize the LPV system and provide nominal control. The proposed LPV antiwindup compensator design was then added to compensate for actuator saturation. Numerical simulations were carried out for the nonlinear F-16 longitudinal model. The results showed that the LPV antiwindup compensator could quickly overcome the saturation and greatly improve the aircraft performance. The thrust vectoring control scheme was also implemented to compare the satura- tion control effect. Under the same condition, the simulation results showed that the antiwindup control achieved better performance.
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Surface Pressure Contour Prediction Using a GRNN Algorithm

Surface Pressure Contour Prediction Using a GRNN Algorithm

Many theoretical researches were performed to study the role of the lee side vortices on the aerodynamic behavior of the bodies of revolution in low speed subsonic flow at high angles of attack. It has been shown [12] that the lee side vortices on the body are strongly affected by both the wing downwash and the wing sweep angle. Impact of body on the tail flow pattern is depicted in Figure 6. There, the vortex development for zero tail deflection is shown as the body angle of attack increases. From Figure 6, it is evident that the pressure distribution over the tail varies only with angle of attack and is independent of the flow structure over the body for small to moderate body or tail angles up to about 10º. Within the small to moderate deflection angle range means that the surface pressure distribution over the tail remains unaffected, if either the body is set to α ≤ 10º at zero tail deflection or the tail is deflected up to 10º at zero body angle of attack. Thus, within small to moderate angle range, no significant viscous effect exists in flow behavior and the flowfiled
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Experimental Study of Owl Like Airfoil Aerodynamics at Low Reynolds Numbers

Experimental Study of Owl Like Airfoil Aerodynamics at Low Reynolds Numbers

This study experimentally investigates aerodynamic characteristics and flow fields of a smooth owl-like airfoil without serrations and velvet structures. This biologically inspired airfoil design is intended to serve as the main-wing for low-Reynolds-number aircrafts such as micro air vehicles. Reynolds num- ber dependency on aerodynamics is also evaluated at low Reynolds numbers. The results of the study show that the owl-like airfoil has high lift perfor- mance with a nonlinear lift increase due to the presence of a separation bubble on the suction side. A distinctive flow feature of the owl airfoil is a separation bubble on the pressure side at low angles of attack. The separation bubble switches location from the pressure side to the suction side as the angle of at- tack increases and is continuously present on the surface within a wide range of angles of attack. The Reynolds number dependency on the lift curves is in- significant, although differences in the drag curves are especially pronounced at high angles of attack. Eventually, we obtain the geometric feature of the owl-like airfoil to increase aerodynamic performance at low Reynolds num- bers.
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The aerodynamics of revolving wings I  Model hawkmoth wings

The aerodynamics of revolving wings I Model hawkmoth wings

similarities between the leading-edge vortex over flapping wings and those found over swept and delta wings operating at high angles of incidence (Van den Berg and Ellington, 1997b) suggest that the detail of the leading edge may be of interest (Lowson and Riley, 1995): the sharpness of the leading edge of delta wings is critical in determining the relationship between force coefficients and angle of attack. Protuberances from the leading edge are used on swept-wing aircraft to delay or control the formation of leading-edge vortices (see Ashill et al., 1995; Barnard and Philpott, 1995). Similar protuberances at a variety of scales exist on biological wings, from the fine sawtooth leading-edge of dragonfly wings (Hertel, 1966) to the adapted digits of birds (the alula), bats (thumbs) and some, but not all, sea-turtles and pterosaurs. The effect of a highly disrupted leading edge is tested using a ‘sawtooth’ variation on the basic hawkmoth planform.
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Misconceptions and generalizations of the Den Hartog galloping criterion

Misconceptions and generalizations of the Den Hartog galloping criterion

Commenting on the individual plots in Fig.4, the first impression is that in most cases all the values follow roughly similar trends and predict close instability zones with respect to the angle of attack. Especially for sections being or resembling rectangles, including the square in Fig.4(d), the rectangle with side ratio 3:1 in Fig.4(g), and the rectangle with rounded ends in Fig.4(h), the ranges of angles of attack for instability from the three different criteria are almost indistinguishable, showing some insensitivity of the susceptibility to galloping for the different cases. The square and rectangle were actually chosen for this study for exhibiting different characteristics, with the square galloping for zero angle of attack and the 3:1 rectangular not (for a classical treatise on the instabilities of rectangular sections with different side ratios see Nakamura and Hirata (1964)), although the most severe zone of instability is for an angle of attack near 10 ◦ in both cases.
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CFD ANALYSIS OF 5KW HORIZONTAL  AXIS WIND TURBINE

CFD ANALYSIS OF 5KW HORIZONTAL AXIS WIND TURBINE

In this project, for analyzing the power performance over a 5KW horizontal axis wind turbine, the S809 airfoil for the desired wind turbine has been selected and designed. The modification factor and models were also combined into the BEM theory to predict the blade performance and there is a good comparison of radius ratio and various angles in each section between the improved BEM theory and numerical simulation. Performance analysis has been carried out for a ten different chord length of a selected airfoil by predicting co efficient of lift, drag, velocity and pressure. A modeling approach using the Spalart-Allmaras in an attempt to describe the flow behavior of the respective ten airfoils. The power performance from the respective airfoil is predicted for different angle of attack from 0-15 degree. The result indicates that the co efficient power at lower angle of attack is less compared to higher angle of attack.
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ANGLES OF ATTACK

ANGLES OF ATTACK

To use the entrenching tool against a rifle with fixed bayonet, the attacker lunges with a thrust to the stomach of the defender along a low No.5 angle of attack (Figure 7-31, Step 1).. [r]

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Experimental Study on Flow Characteristics Around Twin Wind Blades (RESEARCH NOTE)

Experimental Study on Flow Characteristics Around Twin Wind Blades (RESEARCH NOTE)

Rajabi et al. [11] represented a comprehensive computational fluid dynamic design and analysis over an axial fan and have shown that the blade angles and gap ratio have significant effects on the pressure distribution around fans and also affects on the efficiency of the device. Numerous relevant studies on flow analysis using computational fluid dynamics approach and experimental methods are reported in the literature [12-15].

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NUMERICAL PREDICTIONS ON FLOW AND HEAT TRANSFER IN HEAT EXCHANGER TUBE EQUIPPED WITH VARIOUS FLOW ATTACK ANGLES OF INCLINED-WAVY SURFACE

NUMERICAL PREDICTIONS ON FLOW AND HEAT TRANSFER IN HEAT EXCHANGER TUBE EQUIPPED WITH VARIOUS FLOW ATTACK ANGLES OF INCLINED-WAVY SURFACE

Figs. 13a, b, c and d show the turbulent kinetic energy (TKE) in transverse planes for the heat exchanger tube inserted with the inclined wavy surface for the flow attack angle of 15 o , 30 o , 45 o and 60 o ,respectively, at Re = 6000. The high TKE is detected at the core flow for all angles. The peak of the TKE is found in case of the 60 o inclined wavy surface, while the 15 o inclined wavy surface performs the reverse trend. This means that the flow attack angle of 60 o for the inclined wavy surface can create the greatest strength of the vortex flow.
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Presenting Three Design Methods for Axial Compressor Blade via Optimization

Presenting Three Design Methods for Axial Compressor Blade via Optimization

and output angles, installation angle, and the proportion of the chord length to the distance among blades are considered to be equal to the primary blade. A double circular arc (DCA) blade is used as the primary guess in each of the three methods. The quality of the optimized blade is achieved by each method through comparing the performance of the blade with the initial blade. The results provided in Tables 2, 3, and 4 show that the greatest reduction in the design point loss coefficient and the greatest increase in the allowable range of the angles of attack are associated with the optimized blade using the first method. Reduction in the loss in the second method is more than the third method; however, increase in the range of allowable angles of attack is inconsiderable in these two methods.
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The Study of Damage Level of Tandem Breakwater

The Study of Damage Level of Tandem Breakwater

The comparison was made using quarry rock armour and cube armour layer with various angle of wave attack (θ) as shown in Figure 9 and 10. From both graphs, the highest damage is for 0° of wave attack. As the degree of wave angle is increasing, it can be seen that the damages for both armour layer is decreasing. As the wave angle is increasing, the wave attack is oblique to the breakwater. This will cause the reduction of wave height because of the refraction process hence the armour layer will be in the effective angle. Obliquely wave attack will reduce the damage compared with wave attack that is perpendicular to breakwater ([13], [14][15].
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Direct aeroacoustic simulation using compressible flow solver

Direct aeroacoustic simulation using compressible flow solver

High lift device can be analyzed with relative ease with the use of scale models. For the landing gear this is a lot harder. One reason for this is the fact that the landing gear is a complex system with mul- tiple sized objects 5 . The wheels are large, while the carrying construction has a diameter of about an order size difference, then there are the small systems and safety pins which are even smaller. These different sized objects also cause noise at different frequencies. A scale model will leave out small part of the construction and therefore also the noise at higher frequencies. Simulating the flow around the landing gear is also difficult because of these small parts and large total dimensions. Therefore a lot of acoustic research on landing gear is done in wind tunnels on full scale.
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Vol 5, No 4 (2017)

Vol 5, No 4 (2017)

The speed control of the DC motor is done by a knob attached to the rectifier. The experiment has been carried out in subsonic wind tunnel with a test section 300 mm high 300 mm wide and 1000 mm long. The airfoil has been fixed along the width of the test section. A clamp with holes has been made and fixed along the side wall of the test section for lifting the airfoil from center towards upper wall of the wind tunnel to investigate the wall effect on fluid flow characteristics. Measurement of free stream velocity is performed using a Pitot tube and with a linkage mechanism transducer to determine drag force. A protractor is attached to the airfoil and is fitted in the side wall of the wind tunnel to measure the angle of rotation of the airfoil.
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Dynamic Response of Dielectric Lenses Influenced By Radiation Pressure

Dynamic Response of Dielectric Lenses Influenced By Radiation Pressure

Through analytical modeling and numerical simulations the dynamic response and stability of dielectric lenses that are influenced by radiation pressure forces and torques is investi- gated. Radiation pressure forces and torques are applied to the system via momentum trans- fer between the laser beam light and lens. The 2D response of a rolling semi-cylindrical rod that is influenced by radiation pressure is simulated using constant and modulating light intensities. Stable oscillations and regions of stability in the motion of the semi-cylindrical rod are found for both a mirrored and non-mirrored rod. The results showed that at a crit- ical intensity of 1.72 × 10 6 W / m 2 and 12.81 × 10 6 W / m 2 the mirrored and non-mirrored rods motion bifurcates and begins to show neutrally stable oscillations around some higher angular orientation. Lastly, it was shown that by sinusoidally modulating the laser intensity that the motion showed stable oscillations around previously unstable equilibrium angles of attack for a constant intensity.
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Ransomeware : A High Profile Attack

Ransomeware : A High Profile Attack

3.1 Phishing Email – Phishing is one of the most popular and common method to deliver malware on victims machine. Attackers cleverly designed a legitimate email, send it to the victim, now a days attackers choose one of the best way to deliver malware via spear phishing attack, in this kind of attack, attackers gathered all kind of information about particular person or companies, the probability of getting successful chances is more rather than compare to other types of phishing attack.

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