Pitch Oscillations Research Articles

Dynamic Stall on High-Lift Airfoil 30P30N in Ground Proximity

Computational prediction of stall aerodynamics in free air and in close proximity to the ground considering the 30P30N three-element high-lift configuration is carried out based on CFD simulations using the OpenFOAM code and Fluent software. Both the attached and separated flow regimes are simulated using the Reynolds Averaged Navier-Stokes (RANS) equations closed with the Spalart-Allamaras (SA) turbulence model for static conditions and pitch oscillations at Reynolds number, Re = 5 x 106 and Mach number, M = 0.2. The effects of closeness to the ground and dynamic stall are investigated and the reduction in the lift force in close proximity to the ground is discussed.

Evaluation of Dynamic Coupling Intensity and Passive Attitude Control of Underwater Vehicle-Manipulator Systems

In the future, underwater manipulation tasks are intended to be performed by moving base robots, namely Underwater Vehicle-Manipulator Systems (UVMSs). However, the manipulation task induces reaction forces and torques on the underwater vehicle, affecting its positioning. Therefore, it is necessary to compensate for these disturbances to perform the coordinated control of the UVMS or the station keeping of the underwater vehicle. For this sake, this paper presents a new methodology to evaluate the intensity of the dynamic coupling as a function of the working frequency of the manipulator. With such information will be possible to predict the frequency range in which the dynamic coupling is significant. Also, this approach could be used on the task planning level to produce dynamically feasible reference trajectories, improving the control performance. Beyond the identification of a safe frequency range, the obtained results also provide interesting insights about the dynamic coupling influence for the attitude Degrees of Freedom (DoFs) of the underwater vehicle, which can be related to results reported in previous works. Moreover, motivated by the results obtained here and the originality of this application, a passive control is synthesized for the attitude of UVMSs. The simulation results show good attenuation for the pitch oscillations with the proposed control.

Attitude dynamics of a rigid body in Keplerian motion

This paper studies the attitude dynamics of a rigid body in a Keplerian orbit. We show that the use of Classical Rodrigues Parameters for the attitude motion of the rigid body subject to gravity-gradient torques enables us to characterize the equilibria associated with the rotational motion about its mass center. A parametric study of the stability of equilibria is conducted to show that large oscillations are induced due to the energy exchange between the pitch and roll–yaw motions, specifically near the 2:1 resonant commensurability regions. A visualization tool is developed to study these pitch oscillations and gain insight into the rigid body motion near internal resonance conditions. A measure of coupling between the pitching and roll–yaw motions is developed to quantify the energy exchange utilizing information from the state transition matrix.

Parallel Efficient Mesh Deformation Method Based On Support Vector Regression

Mesh deformation method is widely used in unsteady numerical simulations involving moving boundaries. This kind of method redistributes the position of grid points in accordance with the movement of the computational domain without changing their connectivity relations. In this paper, we present a parallel mesh deformation method based on the support vector machine regression (SVR). In each time step, the proposed method first trains three SVRs by the coordinates of the boundary points and their known displacements in each direction, and then predicts the displacements of the internal points using the SVRs. After deforming the mesh, the dual-time step flow solver is used to solve the governing equations. To ensure the consistency of the method running in parallel, the training part of the method is executed with all global boundary points in each decomposed domain. Therefore, each CPU needs to maintain a copy of the entire boundary points via a point-to-point communication. The internal evaluation of the method is predicted separately in each decomposed domain without any data dependency. An oscillatory and transient pitching airfoil case is simulated to demonstrate the applicability of the proposed mesh deformation method, and its parallel efficiency is over 60% with 64 cores.

Effect of Waves on the Behavior of Emergent Buoyantly Rising Submarines Using CFD

Emergent buoyantly rising submarines encounter excess roll problems, partially owing to waves that significantly affect their behavior. This study predicts the behavior of a submarine, including when it rises in static water, beam sea, head wave, following wave, 30∘ bow wave, 60∘ bow wave, 30∘ quartering wave, and 60∘ quartering wave, using the computational fluid dynamics method. The beam sea has a slight effect on pitch prior to the submarine rising to the water surface, but the maximum roll angle in the beam sea is 4.43 times that in static water. After a submarine submerges in water, the pitching oscillation does not decay quickly owing to the yaw angle. The head wave and the following wave have a continuous significant effect on the pitch; the submarine sail remains under the water surface after it submerges from the highest position. The head wave and the following wave have a slight effect on the roll and yaw before the submarine rises to the water surface; however, the roll angle suddenly increases after the submarine submerges from the highest position. As the initial angle between the submarine centerline and wave direction increases, the effect of waves on the longitudinal motion decreases. The amplitude of the pitching oscillation decreases with an increase in the initial angle between the submarine centerline and wave direction, and the waterline when the submarine oscillates on the water surface decreases. The difference in the maximum roll angle between when a submarine rises in an oblique wave and when it rises in beam sea is below 6.3∘. Submarines should try to avoid rising in a head wave and the following wave.

An experimental study on boundary layer transition detection over a pitching supercritical airfoil using hot-film sensors

In the present work, experimental tests are conducted to study boundary layer transition over a supercritical airfoil undergoing pitch oscillations using hot-film sensors. Tests have been undertaken at an incompressible flow. Three reduced frequencies of oscillations and two mean angles of attack are studied and the influences of those parameters on transition location are discussed. Different algorithms are examined on the hot-film signals to detect the transition point. Results show the formation of a laminar separation bubble near the leading edge and at relatively higher angles of attack which leads to the transition of the boundary layer. However, at lower angles of attack, the amplification of the peaks in voltage signal indicate the emergence of the vortical structures within the boundary layer, introducing a different transition mechanism. Moreover, an increase in reduced frequency leads to a delay in transition onset, postponing it to a higher angle of attack, which widens the hysteresis between the upstroke and downstroke motions. Rising the reduced frequency yields in weakening or omission of vortical disturbances ensuing the removal of spikes in the signals. Of the other important results observed, is faster movement of the relaminarization point in the higher mean angle of attack. Finally, a time–frequency analysis of the hot-film signals is performed to investigate evolution of spectral features of the transition due to the pitching motion. An asymmetry is clearly observed in frequency pattern of the signals far from the bubble zone towards the trailing edge; this may reflect the difference between the transition and relaminarization physics. Also, various ranges of frequency were obtained for different transition mechanisms.

Six-Degree of Freedom Mathematical Dynamic Model of a Light-Sport Aircraft

The paper considers the applications of six-degree of freedom mathematical model of a light-sport aircraft and analytically reveals aircraft dynamic response both longitudinal and lateral-directional stability. The model composes of both the kinematics equations of motion and the kinetics equations of aerodynamic forces and moments, and it is known as the aerodynamic model equations. Simulation results show responses of the perturbed dynamic system at the trim condition, and indicate the dynamic stability of both short-period pitching oscillation and phugoid in longitudinal axes. The spiral, roll subsidence, and dutch roll modes in lateral-directional axes are dynamical stable as well. These are essential to understand and evaluate the dynamic behavior, stability, safety and other aspects of the designed aircraft through mathematical model before conducting operational flight test.

Aeroelastic response of an airfoil to gusts: Prediction and control strategies from computed energy maps

A method to predict the aeroelastic pitch response of an airfoil to gusts is presented. The prediction is based on energy maps generated by high-fidelity fluid dynamic simulations of the airfoil with prescribed pitch oscillations. The energy maps quantify the exchange of energy between the pitching airfoil and the flow, and serve as manifolds over which the dynamical states of aeroelastic airfoil system grow, decay and attain stationary states. This method allows us to study the full nonlinear response of the system to large gusts, and predict the growth and saturation of aeroelastic pitch instabilities. We also show that the manifold topology in these maps can be used to make informed modifications to the system parameters in order to control the response to gusts.

Simulation on the dynamic stability derivatives of battle-structure-damaged aircrafts

Accurately evaluating the aerodynamic performance of a battle-structure-damaged aircraft is essential to enable the pilot to optimize the flight control strategy. Based on CFD and rigid dynamic mesh techniques, a numerical method is developed to calculate the longitudinal and longitudinal-lateral coupling forces and moments with small amplitude sinusoidal pitch oscillation, and the corresponding dynamic derivatives of two fragment-structure-damaged and two continuous-rod-damaged models modified from the SACCON UAV. The results indicate that, at the reference point set in this paper, additional positive damping is generated in fragment-damaged configurations; thus, the absolute values of the negative pitch dynamic derivative increase. The missing wingtip induces negative pitch damping on the aircraft and decreases the value of the pitch dynamic derivative. The missing middle wing causes a noticeable increase in the absolute value of the pitch dynamic derivative; the missing parts on the right wing cause the aircraft to roll to the right side in the dynamic process, and the pitch-roll coupling cross dynamic derivatives are positive. Moreover, the values of these derivatives increase as the damaged area on the right wing increases, and an optimal case with the smallest cross dynamic derivative can be found to help improve the survivability of damaged aircraft.

Dynamic lift characteristics of nonslender delta wing in large-amplitude-pitching

The unsteady aerodynamic characteristics of a 50° sweep delta wing performing pitching oscillations at angle of attack between 0° and 60° were tested in a water channel facility. Both force and velocity measurement results were analyzed and compared with the numerical simulation results for the pitching reduced frequency of 0.069 and 0.55. Both lift hysteresis and an unsteady phenomenon of leading-edge vortex (LEV) evolution were observed, which were significantly influenced by the pitch rate of the wing. As the wing pitch rate became sufficiently high, instead of being dissipated and convecting downstream, the LEV remained over the suction surface of the wing, which provide additional lift overshoot at a high angle of attack. In addition, as the wing pitched downstroke due to the negative induced camber effect, the lower surface of the wing turned from a pressure surface to a suction one and hence, there were significant lift losses.

In-phase and out-of-phase pitch and roll oscillations of model wind turbines within uniform arrays

We experimentally explored the impact of in-phase and complete out-of-phase pitch and roll oscillations on the mean power output and the structure of the power fluctuations of turbines within 3×2 uniform arrays. The flow past selected configurations was also characterized with hotwire anemometry. The parameter space included combinations of three amplitudes and two frequencies of the turbine oscillations, and two turbine layouts; measurements with fixed units are also included for comparison. Results show that the structure of the flow past the wind-turbine arrays was significantly affected by the frequency of the tower oscillations. The turbines under in-phase pitching induced a signature that extended in the intermediate wake, which was measured up to 7 rotor diameters downwind of the last row of turbines. The roll oscillations produced an increase of the combined power output independent of the amplitude and frequency of the oscillations, where the complete out-of-phase motions resulted in larger values with a maximum at the largest amplitude of oscillations (β=20°) of over than 10%. Opposite behavior occurred with the turbines under pitching, which led to negligible changes or decrease of the combined power with a maximum reduction of ~10%. The spacing of the turbines substantially affected the power from the roll motions; however, this was not the case for the pitching. Finally, an inspection of the cross-correlation of the instantaneous power output of the turbines in the first and second rows revealed the strong modulation of the induced motions on the structure of the power fluctuations.

An improved dynamic stall model and its effect on wind turbine fatigue load prediction

Due to the nature of the atmospheric boundary layer, large Horizontal-Axis Wind Turbines (HAWTs) generally operate in highly unstable environment, leading to nonstationary loads on HAWT structures. Therefore, it is important to accurately estimate the nonstationary loads to ensure appropriate design boundaries. In this paper, an improved dynamic stall model based on the Beddoes-Leishman (B-L) model is proposed for the estimation of nonstationary aerodynamic loads. The B-L model is modified to account for the characteristics of wind turbine aerofoils which operate at lower Mach numbers and have a larger thickness-to-chord ratio compared to aviation aerofoils. Validation of the model is performed extensively through simulations of the S809 aerofoil under pitch oscillation with different mean angles of attack, oscillating amplitudes and reduced frequencies. To understand the effects of the modifications introduced to the dynamic stall model on the aerodynamic fatigue loads over the lifetime of a wind turbine, a load analysis of a 2 MW HAWT is conducted. The load analysis uses the Blade-Element Momentum (BEM) theory in conjunction with the dynamic stall models. The presented modified dynamic stall model is applicable to wind turbine aerofoils with relative thicknesses greater than 15%.

Experimental analysis of the scaled DTU10MW TLP floating wind turbine with different control strategies

The experimental testing of a Tension Leg Platform (TLP) floating wind turbine at 1:60 scale in wind and waves with a pitch-regulated 10 MW wind turbine is presented. The floating wind turbine was tested with three different control configurations: two closed-loop controllers and one open-loop controller. The experimental setup and program is described in this paper, and system identification and the responses of the floater to hydrodynamic loading are analysed and compared for the different control strategies. It was observed that negative aerodynamic damping for the onshore controller resulted in high oscillations in blade pitch, yielding an increased response in surge for all wave types. It was also observed that the surge motion governed the mooring line tensions, thus the onshore controller yielded the highest tensions in the front mooring line. Further the shutdown cases of the offshore controller led to larger surge displacement when the shutdown was initialized right before the wave impact.

Controlled axisymmetric body-wake coupling in a pitching motion

The coupling between a pitching axisymmetric bluff body and its wake is modified in wind tunnel experiments using controlled interactions between fluidic actuators and the cross-flow over its aft end. The model trajectory is controlled using eight servo-controlled support wires with inline force transducers that are operated in closed loop with feedback from a motion analysis system which effects control authority of the wind tunnel model in all six degrees of freedom. The model dynamics studied in the present work is constrained to pitch. Actuation is effected by two integrated aft-facing synthetic jet actuators in the plane of the body pitch oscillation such that each actuator effects a time-dependent segment of local flow attachment over the aft surface. The present investigation focuses on the reciprocal relation between the response of the near and far wake to the actuation and the associated changes in the induced aerodynamic loads when the body executes nearly time-harmonic pitch over a range of reduced oscillation frequencies (up to k = 0.26) for ReD up to 2.3 × 105. The response of the wake to stabilizing and destabilizing actuations that effect reduction or enhancement of the aerodynamic loads is investigated using particle image velocimetry (PIV) and hotwire anemometry in the near and far wake, respectively. It is shown that the studied flow control approach induces aerodynamic loads that are comparable to the loads on the base flow pitching motions and therefore may be suitable for in-flight stabilization. The global unsteady aerodynamic loads on a pitching axisymmetric bluff body are controlled by modification of the coupling between the flow over body and its near wake using two synthetic jet actuators in the plane of motion on the body’s aft end. This actuation results in a time-dependent partial flow attachment of the nominally axisymmetric separating shear layer of the base flow over the aft end of the body, and the asymmetric changes have a profound effect on the evolution of the wake. These changes in the wake dynamics feed back to alter the global aerodynamic forces and moments, leading to about a 50% increase or decrease in the dynamic lift force or pitching moment, depending on the flow control strategy.

Trunk pitch oscillations for energy trade-offs in bipedal running birds and robots

Bipedal animals have diverse morphologies and advanced locomotion abilities. Terrestrial birds, in particular, display agile, efficient, and robust running motion, in which they exploit the interplay between the body segment masses and moment of inertias. On the other hand, most legged robots are not able to generate such versatile and energy-efficient motion and often disregard trunk movements as a means to enhance their locomotion capabilities. Recent research investigated how trunk motions affect the gait characteristics of humans, but there is a lack of analysis across different bipedal morphologies. To address this issue, we analyze avian running based on a spring-loaded inverted pendulum model with a pronograde (horizontal) trunk. We use a virtual point based control scheme and modify the alignment of the ground reaction forces to assess how our control strategy influences the trunk pitch oscillations and energetics of the locomotion. We derive three potential key strategies to leverage trunk pitch motions that minimize either the energy fluctuations of the center of mass or the work performed by the hip and leg. We suggest how these strategies could be used in legged robotics.

Inflatable Leading Edge-Based Dynamic Stall Control considering Fluid-Structure Interaction

The inflatable leading edge (ILE) is explored as a dynamic stall control concept. A fluid-structure interaction (FSI) numerical method for the elastic membrane structure is constructed based on unsteady Reynolds-averaged Navier-Stokes (URANS) and a mass-spring-damper (MSD) structural dynamic model. Radial basis function- (RBF-) based mesh deformation algorithm and Laplacian and optimization-based mesh smoothing algorithm are adopted in flowfield simulations to achieve the pitching oscillation of the airfoil and to ensure the mesh quality. An airfoil is considered at a freestream Mach number of 0.3 and chord-based Reynolds number of 3.92×106. The airfoil is pitched about its quarter-chord axis at a sinusoidal motion. The numerical results indicate that the ILE can change the radius of curvature of the airfoil leading edge, which could reduce the streamwise adverse pressure gradient and suppress the formation of dynamic stall vortex (DSV). Although the maximum lift coefficient of the airfoil is slightly reduced during the control process, the maximum drag and pitching moment coefficients of the airfoil are greatly reduced by up to 66% and 75.2%, respectively. The relative position of the ILE has a significant influence on its control effect. The control laws of inflation and deflation also affect the control ability of the ILE.

Application of hot-film anemometry to resolve the unsteady boundary layer transition of a laminar airfoil experiencing limit cycle oscillations

The unsteady boundary layer behavior of a supercritical laminar airfoil model, which undergoes limit cycle oscillations in pitch, is investigated by the application of hot-film anemometry. The data basis is a 2D flutter experiment under transonic flow conditions. The laminar airfoil model was elastically mounted with a single degree of freedom in pitch and performed self-excited limit cycle oscillations at a Mach number of {text {Ma}}=0.73 and a Reynolds number of {text {Re}}approx 2times 10^6. An analysis of the hot-film signals on the basis of the quasi-wall shear stress is carried out, with which the boundary layer state for steady and unsteady flow is resolved. An algorithm is presented which allows an automated detection of the transition position, so that a correlation between airfoil motion and transition location movement can be quantified. A sudden movement of the boundary layer transition is observed at the upper and lower reversal points of the limit cycle oscillation, while in parts of the up- and downstroke of the laminar airfoil a shock-induced transition occurs. During the limit cycle oscillation, a delayed response of the boundary layer occurs, resulting in a significant phase lag between the movement of the boundary layer transition and the motion of the laminar airfoil.

Dynamic mode decomposition based analysis of flow over a sinusoidally pitching airfoil

Dynamic mode decomposition (DMD) has proven to be a valuable tool for the analysis of complex flow-fields but the application of this technique to flows with moving boundaries is not straightforward. This is due to the difficulty in accounting in the DMD formulation, for a body of non-zero thickness moving through the field of interest. This work presents a method for decomposing the flow on or near a moving boundary by a change of reference frame, followed by a correction to the computed modes that is determined by the frequency spectrum of the motion. The correction serves to recover the modes of the underlying flow dynamics, while removing the effect of change in reference frame. This method is applied to flow over sinusoidally pitching airfoils, and the DMD analysis is used to derive useful insights regarding flow-induced pitch oscillations of these airfoils.

A robust sharp interface based immersed boundary framework for moving body problems with applications to laminar incompressible flows

This study presents a robust, sharp interface immersed boundary (IBM) framework for moving body problems. The in-house solver makes use of a density-based finite volume framework for solving unsteady, 3D Favre averaged Navier Stokes equation in a generalized curvilinear coordinate system. The immersed boundary formulation is capable of handling arbitrarily complex three-dimensional bodies. The sharp interface approach allows for the exact imposition of boundary conditions at the immersed surface by reconstructing the flow field along its local normal. The implemented reconstruction schemes maintain second-order accuracy. The study focuses on issues of mass conservation and spurious temporal oscillations (pressure as well as force) that the sharp interface IBM approach typically faces when encountering moving body problems. A ghost node-based field extension technique provides an efficient way to improve mass conservation and as a result, reduces not only spurious oscillations but also increases temporal accuracy. Flow past both bluff body (triangular, circular and spherical), as well as streamlined body (airfoil), is presented here as validation studies. The ability of the present formulation to deal with moving body problems in the laminar incompressible flow regime is demonstrated by presenting cases that involve motions such as pitching and in-line oscillation. The predictions are found to be in good agreement with the published results and measurements as well.

Influence of actuation parameters of multi-DBD plasma actuators on the static and dynamic behaviour of an airfoil in unsteady flow

The purpose of this paper is to perform a numerical study regarding the implementation of dielectric barrier discharge plasma micro-actuators for load alleviation on a NACA 23012 oscillating blade in a freestream flow.The work aims to evaluate the feasibility of using multiple dielectric barrier discharge (multi-DBD PAs) plasma actuators as a novel approach for load alleviation and stability control of airfoils in unsteady flow. A 6-actuators configuration, positioned at the trailing edge, is designed and tested. In this configuration, half of the actuators operate on the suction surface and the others on the pressure surface.In particular, the actuators located on the suction surface reduce the airfoil lift force if the induced body force is in a direction that is opposite of the main flow, because they lead to a slight and local increment of pressure on the surface. Instead, when actuators produce a body force aligned with the streamwise direction, they lead to a slight reduction of pressure, increasing lift. Contrarily, the actuators located on the pressure surface increase the lift, if the induced body force is in the direction opposite of the flow and reduce the lift in the opposite case. The effects of plasma actuators on the flow are incorporated into Navier–Stokes equations as a body force vector into the momentum equation.Different switching on/off laws of the actuators have been are compared, in order to reduce the loads amplitude of the airfoil, and to increase the stability of the blade response to the flow, and thus, alleviating fatigue phenomena on the blade and enhancing its aeroelastic stability. The effect of the phase of the plasma actuators switching law was evaluated for different pitching oscillation reduced frequencies, ranging from 0.1 to 0.5.The results underline the capability of DBD-PAs to control and reduce unsteady loads on an oscillating airfoil, improving also the airfoil stability if the phase of the actuation force is optimized for each pitching oscillation reduced frequency.