Showing papers on "Pitching moment published in 2020"

Experimental study of inertia-based passive flexibility of a heaving and pitching airfoil operating in the energy harvesting regime

Abstract: The effects of passive, inertia-induced surface flexibility at the leading and trailing edges of an oscillating airfoil energy harvester are investigated experimentally at reduced frequencies of k = fc/U∞ = 0.10, 0.14, and 0.18. Wind tunnel experiments are conducted using phase-resolved, two-component particle image velocimetry to understand the underlying flow physics, as well as to obtain force and pitching moment estimates using the vortex-impulse theory. Results are obtained for leading and trailing edge flexibility separately. It is shown that both forms of flexibility may alter the leading edge vortex inception and detachment time scales, as well as the growth rate of the circulation. In addition, surface flexibility may also trigger the generation of secondary vortical structures and suppress the formation of trailing edge vortices. The total energy harvesting efficiency is decomposed into contributions of heaving and pitching motions. Relative to the rigid airfoil, the flexible leading and trailing edge segments are shown to increase the energy harvesting efficiency by approximately 17% and 25%, respectively. However, both the flexible leading and trailing edge airfoils operate most efficiently at k = 0.18, whereas the peak efficiency of the rigid airfoil occurs at k = 0.14. It is shown that the flexible leading and trailing edge airfoils enhance the heaving contribution to the total efficiency at k = 0.18 and the negative contribution of the pitching motion at high reduced frequencies can be alleviated by using a flexible trailing edge.

Reduced-order modeling of dynamic stall using neuro-fuzzy inference system and orthogonal functions

Abstract: To consider stall flutter in the design procedure of a blade, accurate models of flow loading are needed. This paper first presents a numerical simulation of an airfoil undergoing a deep dynamic stall employing a computational fluid dynamics code. Overset and polyhedral grid techniques are adopted to accurately simulate the flow field at high angles of attack. Having validated the simulation, the occurrence of stall flutter over a pitching airfoil with an increase in amplitude and frequency of oscillations is examined. The results express that the amplitude of the lift and pitching moment depends on the amplitude of the forced oscillation and there are higher harmonics of the pitching moment compared to the forced oscillation frequency content, both indicating the nonlinearity of aerodynamic lift and pitching moment. Subsequently, a nonlinear reduced model of the dynamic stall is derived using a fuzzy inference system (FIS) and the adaptive network-based FIS (ANFIS). Due to the unsatisfactory results of modeling, especially at post-stall angles of attack, the Gram–Schmidt orthogonalization technique is used to construct a more complex structure of the input variables. The new higher-order input variables have been re-employed by FIS and ANFIS. The results show that excellent modeling is achieved by ANFIS between the new structure of the inputs and the corresponding aerodynamic coefficients using only 10% of input–output data. Having found an appropriate relation, the proposed reduced-order model could properly predict the aerodynamic response of the pitching airfoil at two reduced frequencies.

Experimental Aerodynamic Analysis of a 4.6%-Scale Flying-V Subsonic Transport

Abstract: The experimental investigation into the aerodynamic characteristics of a 4.6%-scale half-model of a flying-wing transport aircraft is detailed. The study is performed in an open-jet wind tunnel where forces and moments are recorded using a 6-axis balance. Two control surfaces in the outboard wing are deflected to measure their effect in terms of lift, drag, and pitching moment up to a Reynolds number of 1 million. The results are subsequently combined with the estimated thrust force using a flight mechanics model of the aircraft in order to predict the most forward and most aft center-of-gravity locations for which the aircraft can be balanced with the control surfaces, while still being statically stable. The results show that the aircraft can attain an untrimmed maximum lift coefficient of 1.02 at an angle of attack of 35 degrees. Furthermore, the pitching moment around the leading-edge of the mean geometric chord is negatively correlated to the angle of attack up to 19 degrees, after which a strong pitch-break is observed, making the aircraft statically unstable. This is associated with a forward shift of the aerodynamic center to a longitudinal position 35%c ahead of the moment reference point which is caused by the formation of strong vortices over the wing surface. The effectiveness of the control surfaces hardly deteriorates with angle of attack and all three control surfaces are shown to be effective up to the maximum lift coefficient. Analysis shows that the center-of-gravity location should reside between [-7.5; 0.5] %c, in power-off conditions, and between [-6; 1] %c, in power-on conditions, from the leading edge of the mean geometric chord to ensure an ultimate static stability margin of 4.4% as well and a minimum landing speed of 20 m/s. Within these ranges, trimmed maximum lift coefficient values of 0.68 and 0.66 can be achieved respectively in power-off and power-on conditions.

High angle-of-attack aerodynamics of a straight wing with finite span using a discrete vortex method

Abstract: The leading-edge-suction-parameter modulated discrete vortex method is extended to a wing with a finite span and no sweep, in order to get the development of aerodynamic coefficients with an angle-of-attack, from attached to completely detached flow conditions. A first case considering the unsteady pitching motion of a flat plate is compared with published experimental and numerical results. Then, dependence of lift, drag, and pitching moment coefficients with the angle-of-attack is discussed for a wing built on an SD7003 airfoil at a constant angle-of-attack. The three-dimensional effects on the lift coefficient curve for a completely detached wing are established.

The Effect of Engine Location on the Aerodynamic Efficiency of a Flying-V Aircraft

Abstract: The Flying-V is a novel flying wing concept where the main lifting surface has been fully integrated with the passenger cabin. This study focuses on the effect of engine positioning on aerodynamic interference under regulatory and structural constraints. An initial benchmark for the lift-to-drag ratio is obtained from a baseline Flying-V configuration, and the influence of the x, y and z position, as well as engine orientation are subsequently analysed. An Euler solver on a three-dimensional, unstructured grid is used to model the flow at cruise condition: M = 0.85, h = 13, 000 m, α = 2.9 ◦, and a thrust per engine of 50 kN. The viscous drag contribution is computed using an empirical method. A total of forty different engine locations are tested under these conditions to build a surrogate model that predicts the aircraft’s lift-to-drag ratio based on the position of the engine. The results obtained show that misplacing the engine can lead to significant lift-to-drag ratio losses going as high as 55% when compared against the ideal integration configuration. A region behind the airframe’s trailing edge is identified where the interference losses due to the installation are minimized. At this location, engine installation causes a 10% penalty in aerodynamic efficiency, a minimum one-engine-inoperative yawing moment and a small thrust-induced pitching moment.

Experimental study on free-surface deformation and forces on a finite submerged plate induced by a solitary wave

Abstract: Physical experiments are conducted to study the interaction between a solitary wave and a finite horizontal plate submerged at a depth equal to 1/4 of the water depth. The spatial and temporal variation of the three-dimensional (3D) surface deformation is measured using a multi-lens stereo reconstruction system. The hydrodynamic loads are measured by underwater load cells. The plate-induced shoaling causes 3D wave focusing, leading to an increased maximum elevation along the streamwise centerline of the plate. The detailed wave focusing process and the influence of wave amplitude on focusing are presented based on the results obtained through image processing. The characteristics of the horizontal forces, vertical forces, and pitching moments are discussed. A 6-stage loading process based on the maxima of vertical wave force and pitching moment is proposed. It is coupled with the synchronous surface deformation to reveal the loading mechanism. It proves that the vertical wave force on the plate reduces apparently compared with the results from 2D experiments. The surface elevation and wave-induced load data provide an excellent benchmark for further studies on the 3D nonlinear interaction between a solitary wave and a submerged plate.

Experimental Investigation of Propeller Induced Flow on Flying Wing Micro Aerial Vehicle for Improved 6DOF Modeling

Abstract: In this research effect of propeller induced flow on aerodynamic characteristics of low aspect ratio flying wing micro aerial vehicle has been investigated experimentally in subsonic wind tunnel. Left turning tendencies of right-handed propellers have been discussed in literature, but not much work has been done to quantify them. In this research, we have quantified these tendencies as a change in aerodynamic coefficient with a change in advance ratio at a longitudinal trim angle of attack using subsonic wind tunnel. For experimental testing, three fixed pitch propeller diameters (5 inch, 6 inch and 7 inch), three propeller rotational speeds (7800, 10800 and 12300 RPMs) and three wind tunnel speeds (10, 15 and 20 m/s) have been considered to form up 27 advance ratios. Additionally, wind tunnel tests of 9 wind mill cases were conducted and considered as baseline. Experimental uncertainty assessment for measurement of forces and moments was carried out before conduct of wind tunnel tests. Large variation in lift, drag, yawing moment and rolling moment was captured at low advance ratios, which indicated their significance at high propeller rotational speeds and large propeller diameters. Side force and pitching moment did not reflect any significant change. $\frac {L}{D}$ at trim point was found a nonlinear function of propeller diameter to wingspan ratio $\frac {D}{b}$ , and propeller rotational speed. Rate and control derivatives were obtained using unsteady vortex lattice method with propeller induced flow effect modeled by Helical Vortex Modeling approach. In this research, we have proposed improved 6-DOF equations of motion, with a contribution of advance ratio $J$ . It is concluded, that propeller induced flow effects have a significant contribution in flight dynamic modeling for vehicles with large propeller diameter to wingspan ratio, $\frac {D}{b}$ of 22% or more.

A Novel Control Allocation Method for Yaw Control of Tailless Aircraft

Abstract: Tailless aircraft without vertical stabilisers typically use drag effectors in the form of spoilers or split flaps to generate control moments in yaw. This paper introduces a novel control allocation method by which full three-axis control authority can be achieved by the use of conventional lift effectors only, which reduces system complexity and control deflection required to achieve a given yawing moment. The proposed method is based on synthesis of control allocation modes that generate asymmetric profile and lift induced drag whilst maintaining the lift, pitching moment and rolling moment at the trim state. The method uses low order models for aerodynamic behaviour characterisation based on thin aerofoil theory, lifting surface methodology and ESDU datasheets and is applied to trapezoidal wings of varying sweep and taper. Control allocation modes are derived using the zero-sets of surrogate models for the characterised aerodynamic behaviours. Results are presented in the form of control allocations for a range of trimmed sideslip angles up to 10 degrees optimised for either maximum aerodynamic efficiency (minimum drag for a specific yawing moment) or minimum aggregate control deflection (as a surrogate observability metric). Outcomes for the two optimisation objectives are correlated in that minimum deflection solutions are always consistent with efficient ones. A configuration with conventional drag effector is used as a reference baseline. It is shown that, through appropriate allocation of lift based control effectors, a given yawing moment can be produced with up to a factor of eight less aggregate control deflection and up to 30% less overall drag compared to use of a conventional drag effector.

Aerodynamic characterization of space debris in the VKI Longshot hypersonic tunnel using a free-flight measurement technique

Abstract: Hypersonic experiments are performed in the VKI Longshot wind tunnel on hemispherical and annular geometries representative of space debris, aiming at determining their lift, drag and pitching moment coefficients. A free-flight technique is used for this purpose where the model is automatically released in the hypersonic flow and safely recovered by the end of the experiment. A schlieren flow visualization technique coupled with a high-speed camera follows the motion of the object as it is exposed to the high-velocity flow. The instantaneous attitude of the free-flying model and its trajectory are determined from image analysis techniques. Correction terms are included to account for the gravity and for the relative motion of the object within the flow. Experimental results are compared with both numerics and theory.

Flow theory in the side chambers of the radial pumps: A review

Abstract: With continuing demand for high and stable operational reliability of hydraulic pumps, it has become vital to take into account the effects of leakage flows in the side chambers in-between the rotating impeller and the stationary casing. Leakage flows have the potential to produce unsteady flow behavior that inherently leads to substantial vibration, undesirable noise, energy losses, and fatigue of pump components. Thus, the purpose of the present study is to discuss and review the various aspects of these harmful unsteady flow behaviors resulting from leakage flows. The first part deals with the theoretical studies on the boundary layers, core swirl, moment coefficient, and pressure and velocity distribution of rotor–stator flows. Then, a simplified model of the prediction of through-flow on moment coefficient Cm and thrust coefficient CF with good correctness has been extensively discussed. Finally, a summary of the experimental and numerical studies on rotor–stator cavities is presented in the second part of this study. This review concludes with a discussion of the calculation of axial thrust and moment coefficient during the design process of radial pumps in a more precise manner.

Vortex moment map for unsteady incompressible viscous flows

Abstract: In this paper, a vortex moment map (VMM) method is proposed to predict the pitching moment on a body from the vorticity field. VMM is designed to identify the moment contribution of each given vortex in the flow field. Implementing this VMM approach in starting flows of a NACA0012 airfoil, it is found that, due to the rolling up of leading-edge vortices (LEVs) and trailing-edge vortices (TEVs), the unsteady nose-down moment about the quarter chord is higher than the steady-state value. The time variation of the unsteady moment is closely related to the LEVs and TEVs near the body and the VMM gives an intuitive understanding of how each part of the vorticity field contributes to the pitching moment on the body.

Novel Approach to Leading-Edge Vortex Suppression

Abstract: A novel approach to reduce the peak lift and pitching moment on a plunging airfoil is investigated through force, moment, and velocity measurements. This approach, unlike previous investigations of...

The effect of gurney flap on flow characteristics of vertical axis wind turbine

Abstract: Recently, Gurney Flap (GF) has been used to improve the performance of Horizontal Axis Wind Turbine (HAWT) by enhancing its lift coefficient. Compared to HAWT, the research on GF application for Vertical Axis Wind Turbine (VAWT) is very limited. Moreover, most work studied a GF geometry attached to the trailing-edge of a stationary airfoil, without considering the rotating effect experienced by VAWT. For this reason, a three-straight-bladed VAWT rotating blades with GF is studied by transient RANS simulation together with a stress-blended eddy simulation turbulence model to investigate the GF height effect and the flow characteristics near the blade trailing-edge. Results have shown that by introducing the blade rotating, an optimum GF height is found to be 3% of the blade chord, slightly higher than 2% chord in a stationary airfoil case. In addition, the presence of GF can suppress deep stall of VAWT blades, thus eliminating negative instantaneous moment coefficient and improving the turbine performance.

Parallel vortex body interaction enabled by active flow control

Abstract: An experimental investigation has been conducted to demonstrate the utility of active flow control as a disturbance generator for vortex body interaction studies. The technique is used to explore the flow physics of parallel vortex body interaction between two NACA 0012 airfoils in series. Experiments were carried out at a chord-based Reynolds number of 740,000 relative to the first airfoil. Active flow control in the form of nanosecond pulse-driven dielectric barrier discharge plasma actuation, originating close to the leading edge, was used to produce vortex shedding from the upstream (disturbance) airfoil at various frequencies ($$0.038 \le F^+ \le 0.762$$). These vortices were characterized, showing reduced circulation and diameter with increasing frequency, before examining the downstream wake-airfoil interactions. Time-resolved pressure and phase-locked PIV measurements were taken on the downstream (target) airfoil for multiple angles of attack. For $$F^+ \le 0.5$$, the target airfoil is subject to strong oscillations from the wake of the disturbance airfoil that lead to large fluctuations in lift and pitching moment. However, a further increase in $$F^+$$ reattaches the flow over the disturbance airfoil and no major vortex body interactions are observed on the target. Governing parameters for this type of vortex body interaction are explored, and differences between isolated and non-isolated encounters as well as the presence of a viscous response are examined. Finally, means to alleviate loads caused by the incident vortex are explored.

Prediction and Analysis of the Aerodynamic Characteristics of a Spinning Projectile Based on Computational Fluid Dynamics

Abstract: Numerical simulations of a spinning projectile with a diameter of 120 mm were conducted to predict the aerodynamic coefficients, and the CFD results were compared with the semiempirical method, PRODAS. Six coefficients or coefficient derivatives, including zero and the quadratic drag coefficient, lift force coefficient derivative, Magnus force coefficient derivative, overturning moment coefficient, and spinning damping moment coefficient, which are important parameters for solving the equations of motion of the spinning projectile, were investigated. Additionally, the nonlinear behavior of these coefficients and coefficient derivatives were analyzed through the predicted flow fields. The considered Mach number ranges from 0.14 to 1.2, and the nondimensional spinning rate (PD/2V) is set to 0.186. To calculate the coefficient derivative of the corresponding force or moment, additional simulations were conducted at the angle of attack of 2.5 degrees. The simulation results were able to predict nonlinear behavior, the especially abrupt change of the predicted coefficients and derivatives at the transonic Mach number, 0.95. The simulation results, including the skin friction, pressure, and velocity field, allow the characterization of the nonlinear behavior of the aerodynamic coefficients, thus, enabling better predictions of projectile trajectories.

Stability and sensitivity analysis of bird flapping flight

Abstract: This paper investigates stability analysis of flapping flight. Due to time-varying aerodynamic forces, such systems do not display fixed points of equilibrium. The problem is therefore approached via a limit cycle analysis based on Floquet theory. Stability is assessed from the eigenvalues of the Jacobian matrix associated to the limit cycle, also known as the Floquet multipliers. We developed this framework to analyze the flapping flight equations of motion of a bird in the longitudinal plane. Such a system is known to be not only non-linear and time-dependent, but also driven by state-dependent forcing aerodynamic forces. A model accounting for wing morphing under prescribed kinematics is developed for generating realistic state-dependent aerodynamic forces. The morphing wing geometry results from the envelope of continuously articulated rigid bodies, modeling bones and feather rachises, and capturing biologically relevant degrees of freedom. A sensitivity analysis is carried out which allows studying several flight configurations in trimmed state. Our numerical results show that in such a system one instability mode is ubiquitous, thus suggesting the importance of sensory feedback to achieve steady-state flapping flight in birds. The effect of wingbeat amplitude, governed by the shoulder joint, is found to be crucial in tuning the gait towards level flight, but marginally affects stability. In contrast, the relative position between the wing and the center of mass is found to significantly affect the values of Floquet multipliers, suggesting that the distribution of pitching moment plays a very important role in flapping flight stability.