Nominal Angle Of Attack Research Articles

Stability analysis of two-dimensional flat solar trackers using aerodynamic derivatives at different heights above ground

In this paper the aerodynamic stability of a flat-plate solar tracker under two-dimensional conditions is studied for different conditions of incident wind speed, U∞, nominal angle of attack, αn∈−40∘,40∘, and shaft height - chord ratio H/B=0.3,0.4,0.5,0.6,1and2. The stability is studied through the analysis of the aerodynamic derivatives A2∗ and A3∗ whose behaviours as a function of U∞ are presented for the H/B range studied. Two methods of obtaining the derivatives (one in the time domain and the other one in the frequency domain) are presented. A method for calculating the effective damping coefficient of the solar tracker, ξeff, as a function of the incident wind speed, U∞, is presented. This method can be used for any solar tracker structural characteristics (Jmech, Cmech and Kmech). With this method, the critical speed, Ucrit, can be determined by imposing the condition ξeff=0. The method was validated experimentally using the results of two different experimental set-ups.

A New Method of Determination of the Angle of Attack on Rotating Wind Turbine Blades

The angle of attack (AoA) is the key parameter when extracting the aerodynamic polar from the rotating blade sections of a wind turbine. However, the determination of AoA is not straightforward using computational fluid dynamics (CFD) or measurement. Since the incoming streamlines are bent because of the complex inductions of the rotor, discrepancies exist between various existing determination methods, especially in the tip region. In the present study, flow characteristics in the region near wind turbine blades are analyzed in detail using CFD results of flows past the NREL UAE Phase VI rotor. It is found that the local flow determining AOA changes rapidly in the vicinity of the blade. Based on this finding, the concepts of effective AoA as well as nominal AoA are introduced, leading to a new method of AOA determination. The new method has 5 steps: (1) Find the distributed vortices on the blade surface; (2) select two monitoring points per cross-section close to the aerodynamic center on both pressure and suction sides with an equal distance from the rotor plane; (3) subtract the blade self-induction from the velocity at each monitoring point; (4) average the velocity of the two monitoring points obtained in Step 3; (5) determine the AoA using the velocity obtained in Step 4. Since the monitoring points for the first time can be set very close to the aerodynamic center, leading to an excellent estimation of AoA. The aerodynamic polar extracted through determination of the effective AoA exhibits a consistent regularity for both the mid-board and tip sections, which has never been obtained by the existing determination methods.

Unsteady aerodynamics of a pitching-flapping-perturbed revolving wing at low Reynolds number

Due to adverse viscous effects, revolving wings suffer universally from low efficiency at low Reynolds number (Re). By reciprocating wing revolving motion, natural flyers flying at low Re successfully exploit unsteady effects to augment force production and efficiency. Here we investigate the aerodynamics of an alternative, i.e., a revolving wing with concomitant unsteady pitching and vertical flapping perturbations (a pitching-flapping-perturbed revolving wing). The current work builds upon a previous study on flapping-perturbed revolving wings (FP-RWs) and focuses on combined effects of pitching-flapping perturbation on force generation and vortex behaviors. The results show that, compared with a FR-RW, pitching motion further (1) reduces the external driving torque for rotating at 0° angle of attack (α0) and (2) enhances lift and leads to a self-rotating equilibrium at α0 = 20°. The power loading of a revolving wing at α0 = 20° can be improved using pitching-flapping perturbations with large pitching amplitude but small Strouhal number. Additionally, an advanced pitching improves the reduction of external driving torque, whereas a delayed pitching weakens both the lift enhancement and the reduction of external driving torque. Further analysis shows that pitching effects can be mainly decomposed into the Leading-Edge-Vortex (LEV)-mediated pressure component and geometric projection component, together they determine the force performance. LEV circulation is found to be determined by the instantaneous effective angle of attack but could be affected asymmetrically between upstroke and downstroke depending on the nominal angle of attack. Pitching-flapping perturbation thus can potentially inspire novel mechanisms to improve the aerodynamic performance of rotary wing micro air vehicles.

Rapid generation of entry trajectory with multiple no-fly zone constraints

An entry trajectory planning algorithm that generates flyable trajectories satisfying multiple no-fly zones and other path and terminal constraints is presented. The algorithm divides the entry trajectory into initial and glide phases. In the initial phase, the maximum value of heating rate is controlled accurately as the altitude of the quasi-equilibrium glide condition (QEGC) transition point is adjusted with a nominal angle of attack and a parameterized bank angle. In the glide phase, a geometry based planning algorithm that relies on the center positions and radius of the multiple no-fly zones, is proposed to calculate the waypoints of the virtual flight path. The magnitude and reversal point of the bank angle in each sub-phase are searched based on a reduced-order lateral system with the predictor-corrector method. The altitude is designed as an analytical function of energy, and the QEGC is employed to analytically solve the remaining state variables. Finally, a linear quadratic regulator is used to test the realization of the 3D trajectory. The algorithm is tested using the common aero vehicle-H model. The results demonstrate that the algorithm can rapidly generate the entry trajectory with multiple no-fly zone constraints and achieve complex flight missions satisfying all flight constraints.

Highly constrained entry trajectory generation

An entry trajectory planning algorithm that generates flyable trajectories satisfying waypoints, no-fly zones, and other path and terminal constraints is presented. The algorithm tactically divides the entry trajectory into the initial and glide phases. In the initial phase, a nominal angle of attack and a constant bank angle are used to generate the 3-D trajectory. In the glide phase, a planner is developed based on the evolved acceleration guidance logic for entry (EAGLE). The planner is divided into a longitudinal sub-planner and a lateral sub-planner. For longitudinal planning, the drag-energy profile is represented as five piecewise linear functions of the normalized non-conventional energy to make it consistent with both the desired trajectory length and the lateral maneuverability required to meet waypoint and no-fly zone constraints. The longitudinal sub-planner determines the magnitude of the bank angle, whereas the lateral sub-planner determines the appropriate sign of the bank angle for passing waypoints, avoiding no-fly zones, and minimizing the final heading error. The longitudinal and lateral sub-planners are iteratively employed until all path and terminal constraints are satisfied. Then, a tracker is employed to follow both the reference drag acceleration and the heading angle profiles to generate a feasible closed-loop entry trajectory. The approach is tested using the Common Aero Vehicle model. Simulations demonstrate that the generated trajectories can pass the predetermined waypoints, avoid no-fly zones, and achieve the desired target conditions within allowable tolerances.

Computational/Experimental Aeroelastic Study for a Horizontal-Tail Model with Free Play

A computational code has been developed for aeroelastic analysis for an all-movable horizontal tail, and a companion wind-tunnel test program has been conducted. The structural bending and torsional stiffness are based upon an experimental tail model that has served as the basis for free-play design criteria. A linear three-dimensional time-domain vortex-lattice aerodynamic model is used to study the flutter and also nonlinear limit-cycle oscillations induced by a free-play gap in the actuating mechanism at the root of the tail as well as the effects of the root rotation angle (nominal angle of attack) on limit-cycle-oscillation response and gravity loads. It is found that the root rotation angle, gravity loads, and the initial conditions all significantly affect the limit-cycle-oscillation behavior. The computed aeroelastic responses are compared with the experimental data.

Numerical Study of Flexible Flapping Wing Propulsion

In this study, a strong-coupling approach is applied to simulate highly flexible flapping wings interacting with fluid flows. Here, the fluid motion, solid motion, and their interaction are solved together by a single set of equations of motion on a fixed Eulerian mesh, with the elastic stress being solved on a Lagrangian mesh and projected back to the Eulerian mesh. To provide necessary flapping mechanism, control cells are implemented in solid area (i.e., the wing) as skeleton. The moving trajectory of the skeleton is therefore prescribed by a conventional direct-forcing type of immersed boundary method, while the rest of the wing moves passively through elasticity and fluid-structure interaction. This combined algorithm is then used to study the propulsion characteristics of flexible flapping wings with different elastic moduli and at different flapping frequencies and amplitudes. A two-dimensional NACA0012 airfoil is chosen as a model wing, and it is under active plunging defined by control cells and corresponding passive pitching motion. With different input parameters, very different wake structures can be observed. As a result, the coupled plunging-pitching motion can be either drag-producing or thrust-producing. Finally, passive pitching angle θ and nominal angle of attack α for flexible wings are defined to characterize the flapping motion. It is found that θ needs to be greater than 0.26 and α needs to be greater than 0.3 to generate thrust instead of drag for the flapping motion within the current parametric matrix.

Experimental Determination of Steady and Unsteady Loads on a Surface Piercing, Ventilated Hydrofoil

Measured steady and unsteady section lift and moment coefficients at two spanwise locations on a surface-piercing ventilated hydrofoil are presented. The foil, of wedge cross section, was supported vertically, and submerged one chord length from the tip. Excitation in rigid-body rolling and pitching modes to the cantilevered foil produced the unsteady loads. All tests were made at a nominal angle of attack of 12 deg.