Unsteady Aerodynamics Research Articles

Adaptive Fuzzy Sliding Mode Controller Design Using a High-Gain Observer for a Nonlinear Aeroelastic System under Unsteady Aerodynamics

Adaptive Fuzzy Sliding Mode Controller Design Using a High-Gain Observer for a Nonlinear Aeroelastic System under Unsteady Aerodynamics

Unsteady aerodynamic modeling and flight trajectory simulation of dual-spin projectile based on DNN and transfer-learning

To evaluate flight performance and aerodynamic characteristics of a dual-spin projectile, the coupled computational fluid dynamics and rigid body dynamics (CFD/RBD) method is commonly used, which can simultaneously solve the flight mechanics and flow field. However, the efficiency is compromised by the large number of CFD calculations required. This paper develops an unsteady aerodynamic modeling method that combines deep neural networks and transfer learning, which can accurately predict unsteady aerodynamics of dual-spin projectiles under varying initial conditions. Considering the influence of flight state and aerodynamic data from short-term historical moments, we integrate them as input features of the aerodynamic model to reduce the impact of long-term historical data. To enhance the model generalization under varying initial conditions, we fine-tune the built aerodynamic model using small amounts of data under new conditions by transfer learning. The proposed method is validated through interpolated and extrapolated prediction cases, respectively. The results indicate that the proposed method can achieve better accuracy and generalizability than long short-term memory neural networks and autoregressive moving average method in unsteady aerodynamic modeling of the dual-spin projectile. By coupling the flight dynamics equations with the aerodynamic model in the time domain, the flight simulation only takes a few seconds, which can reduce computing time by three orders of magnitude compared to the coupled CFD/RBD method.

Research on nonlinear instability of blended-wing-body aircraft based on experimental bifurcation analysis

Focusing on the nonlinear instability problem of the blended-wing-body aircraft, first, numerical bifurcation analysis is adopted to obtain catastrophe points and branches where the elevator is taken as a continuous parameter. Second, based on virtual flight tests and bifurcation theory, an experimental bifurcation analysis method is developed. Open-loop and closed-loop experiments are carried out, and the corresponding experimental bifurcation diagrams are obtained. The comparative analysis shows that the experimental bifurcation analysis enables the aircraft to make a response that is close to reality under the influence of nonlinear and unsteady aerodynamics and has a more intuitive embodiment of instability characteristics, providing an accurate prediction of departure. The open-loop experiment predicts a sudden longitudinal nose-up at an angle of attack of 7.4°. The closed-loop experiment can transform the open-loop unstable branch into a stable branch. However, roll instability still occurs at a critical angle of attack of 16.0°. These research results provide important dynamic information for the stable flight and the design of the controller.

Dynamic Formulation of the Double Multiple Stream Tube Model of Offshore Vertical Axis Wind Turbines

Abstract The paper proposes a novel algorithm to simulate Vertical Axis Wind Turbines in floating motion based on a low-fidelity approach. The conventional Double Multiple Stream Tube Model is formulated as a steady state model to deal with the inherent unsteady aerodynamics of VAWTs. The Double Multiple Stream Tube Model makes use of an average contribution of the blade passages over a revolution to solve the Blade Element Momentum equation. The model reduces the dependence of the solution (the induction field) to a space variable. Floating Vertical Axis Wind Turbines undergoes a variation of aerodynamic quantities according to wave periods, also different from the rotor period. This paper provides a new formulation of the Double Multiple Stream Tube Model to solve the turbine rotation in time and space, introducing the variation of the platform velocity and a revised approach to the downstream actuator disc. The most impactful parameter is found to be the wave frequency: wave periods at full-scale are lower than the ones of the turbine rotor, featuring optimal operating points at low tip speed ratios. The increase of the wave frequency highlights the differences of the average torque prediction for a single blade up to 10% between the unsteady and the standard formulation. In this context, the development of fast low-fidelity computational tools play a key role in the assessment of the preliminary designs of Floating Offshore Vertical Axis Wind Turbines and it will be used to optimise the aerodynamic design of the laboratory scaled model under surge motion.

AN AERODYNAMIC INVESTIGATION OF A HIGH-PRESSURE TURBINE USING ROTOR CASING STATIC PRESSURE MEASUREMENTS AT ENGINE REPRESENTATIVE CONDITIONS WITH DIFFERENT TIP DESIGNS, TIP GAPS AND INLET TEMPERATURE PROFILES

Abstract A rotating High-Pressure (HP) turbine, in modern aircraft engines, is subjected to high cyclic thermal and mechanical loads. Shroudless turbines experience high heat loads on the rotor tip and casing, which makes them one of the life-limiting components for an engine. If the rotor tip starts to erode, then the tip clearance increases which increases the over-tip leakage flow, resulting in reduced stage efficiency and ultimately, the engine lifetime. The Oxford Turbine Research Facility (OTRF) is a high- speed rotating transient test facility that allows unsteady aerodynamics and heat transfer measurements, at engine representative conditions. This paper presents an experimental investigation, of the HP turbine rotor casing aerodynamics, performed in the OTRF. Steady and unsteady casing static pressure measurements were acquired, using pneumatic lines and high-frequency response (∼ 100 kHz) Kulite pressure transducers respectively. The single-stage HP turbine consisted of cooled vanes (featuring film and trailing edge slot cooling), and uncooled rotor blades. Three different tip designs (two squealers and one flat) were tested, at two tip gaps, and with two inlet temperature profiles that included a spatially uniform total temperature profile and an engine representative HP turbine inlet total temperature profile. In parallel, a numerical investigation of the experimental test cases is presented using unsteady Computational Fluid Dynamics (CFD) predictions. The experimental results from this study provide a unique dataset to validate numerical models.

Development on Unsteady Aerodynamic Modeling Technology at High Angles of Attack

Development on Unsteady Aerodynamic Modeling Technology at High Angles of Attack

Investigation of wind turbine unsteady aerodynamics and trailing-edge noise characteristics under wake steering

In wake steering strategy, the shaft of an upstream wind turbine is yawed to deflect the wake and decrease the wake-swept area of the downstream wind turbine rotor. This intentional yaw misalignment increases the output power of the downstream wind turbine at the expense of some loss of upstream wind turbine power, consequently increasing the total wind farm power. However, the asymmetric non-uniform inflow into the downstream wind turbine due to the deflected wake causes unsteady aerodynamic loads and affects the wind turbine noise characteristics. In this study, the unsteady vortex lattice method coupled with the curled wake model is used to predict the unsteady aerodynamics of a downstream wind turbine in wake steering. Based on the unsteady aerodynamic results, the trailing-edge noise characteristics are predicted by a semi-empirical method. The impact of wake steering on trailing-edge noise level and amplitude modulation is evaluated. The results show that the asymmetric non-uniform inflow due to wake steering increases the amplitude modulation of the downstream wind turbine.

Blade Surface Pressure Measurements in the Field and Their Usage for Aerodynamic Model Validation

ABSTRACTThis study presents results from a long‐term measurement campaign on a research wind turbine in the field. Pressure measurements are conducted at 25% blade radius over several months. Together with inflow measurements provided by a LiDAR system, they form an extensive dataset, which is used in the validation of numerical aerodynamic models. The model validation is conducted based on both ten‐minute average data as well as time‐resolved unsteady data. Initially, it is investigated how representative ten‐minute average pressure measurements are of the underlying unsteady aerodynamics. Binned ten‐minute average pressure distributions are then analysed together with their numerical counterpart, consisting of a combination of rotor and airfoil level aerodynamic/aeroelastic simulation results using average environmental and operating conditions as input. Finally, time‐resolved measurements and simulation results are compared, validating the aeroelastic tools' capability to reproduce unsteady aerodynamics.Overall, reasonable agreement is found between numerical simulations and field experiment data showcasing two aspects: Numerical tools based on blade element momentum theory and panel methods with viscous‐inviscid interaction remain relevant for simulating modern multi‐megawatt wind turbines, and long‐term pressure measurements provide invaluable means for validating such tools.

Accuracy and Computational Cost Comparison of Numerical Flight Dynamics Reduced-Order Models

This paper compares several approaches to model the unsteady and nonlinear longitudinal aerodynamics of a generic unmanned combat air vehicle configuration based on computational fluid dynamics (CFD). Three different reduced-order models (ROMs) are implemented through unsteady simulations to predict the evolution of forces and moments during prescribed trajectories carried out at M=0.15. The first model investigates a linear quasi-steady model based on a first-order Taylor series expansion of the aerodynamic coefficients. The second one picks up the mathematical formulation of the first model but adds a dynamic coefficient in order to take into account the unsteady effect of rotation rate variations. Finally, the third model is built on indicial functions computed from positive step changes in the angle of attack or pitch rate. The unknown parameters of the first two models and the unknown indicial functions of the third one are obtained, respectively, from the study of forced oscillations and step motions. These prescribed trajectories are performed using unsteady Reynolds-averaged Navier–Stokes simulations coupled with a grid displacement tool. Two different methods are tested and compared to carry out the CFD mesh motions: an overset method and a full moving grid (FMG) method. The indicial functions obtained using the FMG method give results equivalent to those found in the literature in half the time of the overset method. To reduce the computational costs to set up the ROMs, the FMG method is preferred to the overset method. ROMs are then applied, and their predictions are compared with CFD simulations for prescribed trajectories with different angular rates and angles of attack. A study of instantaneous and overall accuracy associated with the implementation cost of the ROMs has allowed to show their strengths and their weaknesses. Similar results are observed at low angles of attack for the ROMs studied, while a major modeling advantage of the indicial method has been identified for higher angles of attack in the nonlinear domain.

Unsteady Aerodynamic Forces of Tandem Flapping Wings with Different Forewing Kinematics.

Dragonflies can independently control the movement of their forewing and hindwing to achieve the desired flight. In comparison with previous studies that mostly considered the same kinematics of the fore- and hindwings, this paper focuses on the aerodynamic interference of three-dimensional tandem flapping wings when the forewing kinematics is different from that of the hindwing. The effects of flapping amplitude (Φ1), flapping mean angle (ϕ1¯), and pitch rotation duration (Δtr1) of the forewing, together with wing spacing (L) are examined numerically. The results show that Φ1 and ϕ1¯ have a significant effect on the aerodynamic forces of the individual and tandem systems, but Δtr1 has little effect. At a small L, a smaller Φ1, or larger ϕ1¯ of the forewing can increase the overall aerodynamic force, but at a large L, smaller Φ1 or larger ϕ1¯ can actually decrease the force. The flow field analysis shows that Φ1 and ϕ1¯ primarily alter the extent of the impact of the previously revealed narrow channel effect, downwash effect, and wake capture effect, thereby affecting force generation. These findings may provide a direction for designing the performance of tandem flapping wing micro-air vehicles by controlling forewing kinematics.

Validation of a Methodology To Assess The Flutter Limit Cycle Oscillation Amplitude of Low-Pressure Turbine Bladed-Disks - Part II: Rotational Speed Effects

Abstract The effect of the operating conditions on the vibration amplitude trends of an isolated low-pressure turbine rotor is described. The study utilizes an analytical model correlating the aerodynamic and dry-friction work introduced in Part I of the paper. In this Part II, the analysis has been extended to incorporate the influence of rotational speed. The force distribution and the penetration length of the fir-tree contact surfaces are key parameters within the heuristic micro-slip model used to characterize the friction forces. These parameters change with rotational speed, consequently influencing the dry-friction work involved in the process. The model is closed with numerical simulations to compute the aerodynamic damping, and it is compared against experimental data gathered from the experimental campaign detailed in Part I. The results demonstrate a significant impact of the shaft speed on flutter vibration amplitude. The vibration amplitude has been observed to reach a maximum near the on-design conditions. The analytical model can correctly capture this trend indicating that the essential physics is retained in it. Nonlinear friction, mistuning and 3D unsteady aerodynamics have shown to play a predominant role to explain the change of vibration amplitude with the shaft speed.

On the role of wake-capture and resonance in spanwise-flexible flapping wings in tandem

Numerical simulations of the flow around spanwise-flexible flapping wings in tandem are reported, focusing on a thrust-generating configuration. Wings of aspect ratio 2 and 4 in forward flight undergo heaving and pitching motion following optimal 2D kinematics. The Reynolds number of the simulations is Re=1000. The effect of flexibility is explored by varying the effective stiffness of the wings, while the effective inertia is kept constant. The aerodynamic performance of the tandem system results from a combination of unsteady aerodynamics mechanisms, fluid–structure resonance, vortex–wing interactions (denoted wake capture in this study) and aerodynamic tailoring. It is found that the aerodynamic performance and structural behavior of forewings are dominated by a fluid–structural resonance. The maximum mean thrust for the forewings is obtained when the driving frequency approaches the first natural frequency of the structure, ωn,f/ω≈1, similarly to what is observed in isolated wings undergoing the same kinematics. On the other hand, hindwings show optimal performance in a broad region near ωn,f/ω≈2, and their aerodynamic performance seems to be dominated by wake–capture and aerodynamic–tailoring effects. The aerodynamic performance of the hindwings is dependent on the flexibility of the forewing, which impacts the intensity of the vortices shed into the wake and the resulting effective angle of attack (i.e., wake capture). The timing between the effective angle of attack and the pitching motion of the hindwing controls the generation of thrust (or drag) of each spanwise section of the hindwing (i.e., aerodynamic tayloring). A proof of concept study on the aerodynamic performance of systems made of wings with different flexibility suggests that they could outperform tandem systems with equally flexible wings. Thus, the optimal mixed–flexibility tandem system is composed by a resonant forewing, which maximizes the thrust generation of the forewing and the intensity of the vortices shed into the wake, and a hindwing whose flexibility must be tuned to maximize wake capture effects.

Unsteady aerodynamic flow control at low Reynolds numbers via internal acoustic excitation

Aerodynamic flow control using internal acoustic excitation holds promise as it combines the simplicity of passive flow control techniques (in terms of added weight and operational complexity) with the control authority of active flow control methods. While previous studies have analyzed the effects of acoustic excitation on steady-wing aerodynamics, the effect of excitation on the unsteady aerodynamics is not known, which is the aim of the current effort. Internally mounted speakers on a symmetric National Advisory Committee for Aeronautics (NACA) 0012 wing are used to excite the unsteady boundary layer at the wing's leading edge as it executes linear pitch motions ranging from quasi-steady (trailing-edge driven stall) to vortex-dominated (mixed leading- and trailing-edge driven stall) motions at freestream Reynolds numbers (Re) of 120 000 and 180 000. Experimental results show that, although acoustic excitation delays stall for quasi-steady motions, it enhances lift in the linear region and increases leading-edge vortex strength for vortex-dominated motions. The degree of change was observed to be a function of the excitation frequency, with lower frequencies (≤ 250 Hz) leading to an increase in aerodynamic efficiency and higher frequencies having a negligible effect. The current work establishes the effects of acoustic flow excitation in unsteady, low-Re wing aerodynamics and provides insights on the path forward to effectively implement the method for active flow control.

Modelling and analysis of two-dimensional static and dynamic aeroelasticity of Fish Bone Active Camber morphing aerofoils

As a continuous and smooth morphing concept for aerofoils, the Fish Bone Active Camber (FishBAC) concept has demonstrated significant aerodynamic efficiency improvements over traditional hinged flaps. In this paper, to investigate the static and dynamic aeroelasticity of FishBAC aerofoils, an unsteady two dimensional coupled fluid-structure interaction model is developed, which includes the structural response of the FishBAC spine, skin, stringers, tendons and actuator, coupled to an unsteady aerodynamics model. The structural dynamic model is Timoshenko beam-theory-based, while the aerodynamic model is based on Peters’ unsteady model. The static and dynamic aeroelasticity is studied after the model is validated. Results show that the increase in pulley rotational angle reduces the zero-lift angle of attack, while keeping the slope between the lift coefficient and angle of attack the same. A shorter morphing region closer to the trailing edge is beneficial for generating larger lift coefficient with the same tendon moment and angle of attack. Flutter occurs with the increase of the air speed. When the morphing end position is fixed at 0.9 chord, increasing the morphing length reduces the critical flutter speed significantly, with the second bending mode tending to drive instability. With the same morphing length, moving the morphing region closer to the leading edge increases the critical flutter speed, and the unstable mode changes from the second mode into the first mode.

Effect of Hopf-Hopf bifurcation on the post-flutter behavior of a three-degree-of-freedom airfoil

The post-flutter dynamics of a three-degree-of-freedom nonlinear airfoil with unsteady aerodynamics are investigated based on the Hopf-Hopf bifurcation theory. Many prior works have relied on Hopf bifurcation theory to predict flutter behavior in airfoil systems. Although this approach facilitates flutter prediction and characterization of post-flutter behavior in numerous scenarios, it may be invalid in specific instances, such as when the Hopf bifurcation of the system degenerates. Therefore, this study focuses on a classical degenerate case of Hopf bifurcation in airfoil systems, specifically the Hopf-Hopf bifurcation. We show that the system undergoes various Hopf-Hopf bifurcations under specific parameter conditions as the center of gravity shifts. The local dynamics near the Hopf-Hopf bifurcation points are represented, including quasiperiodic oscillations on a three-dimensional torus. The airfoil begins to oscillate quasiperiodically after the airflow speed crosses specific Hopf-Hopf bifurcation points. The study also uncovers complex quasiperiodic crises and quasiperiodic hysteresis loops, which have not been reported in previous studies of aeroelastic systems. Then, many singularities and bifurcation curves are obtained near the Hopf-Hopf bifurcation point by semiglobal unfolding. Furthermore, the influence of stall effects on the bifurcation structure of the system is represented. It is shown that the types of Hopf-Hopf bifurcations may vary with the changes of stall effects, influencing the system's semiglobal bifurcation structures consequently. For all Hopf-Hopf bifurcation scenarios, stall effects affect one of the Neimark-Sacker bifurcation curve structures unfolded from the Hopf-Hopf bifurcation point significantly, while the other Neimark-Sacker bifurcation curve experiences minimal impact from stall effects. Moreover, a large nonlinear stall coefficient will postpone the onset of quasiperiodic/chaotic oscillations.

Application of Time-Variant Systems Theory to the Unsteady Aerodynamics of Rotary Wings

This work employs the principles of time-variant systems theory to investigate the unsteady aerodynamics of rotary-wing configurations under periodic equilibrium conditions. Their application enables an extension of the pulse technique for system identification, as well as the adaptation of the linear-frequency-domain formulation commonly utilized in fixed-wing to rotary-wing scenarios. These methodologies effectively incorporate the aerodynamic nonlinearities associated with the equilibrium state into an efficient time-variant linearized representation of the unsteady aerodynamics. To promote its application in the context of rotary-wing aeroelasticity, a state-space realization based on a periodic autoregressive model with exogenous input is subsequently employed. Upon transformation from discrete to continuous time, the resulting aerodynamic model adopts a linear continuous-time periodic state-space formulation, offering compatibility for its coupling with a wide range of structural models. The proposed aerodynamic framework tailored to rotary-wing aeroelasticity holds applicability across a spectrum of aerodynamic models of arbitrary complexity, spanning from incompressible potential flow approximations to potentially more sophisticated methods. Showcasing the potential of this framework, the widely studied lossy Mathieu equation and the aerodynamic response to a flap perturbation about the periodic equilibrium condition of a prototypical rotor blade section, incorporating nonlinearities through an analytical dynamic stall model, are considered.

Unsteady nonlinear lifting line model for active gust load alleviation of airplanes

Active gust load alleviation is an important technology for designing future passenger airplanes to be lighter and thus more environmentally friendly. Unsteady Reynolds-averaged Navier–Stokes (URANS) simulations are typically used to accurately calculate gust loads, but because of their high computational cost, they can only be performed at a few selected operating points. In simpler potential theory models, stall is neglected, resulting in loss of accuracy. In this paper, a low-order unsteady aerodynamics wing model is presented, which is able to represent well compressible flow with stall. Furthermore, the model offers the possibility to modularly incorporate actuators, which allows the design and evaluation of active load alleviation systems. The model is based on a conventional unsteady 2D airfoil model including a dynamic stall model. The dynamic stall model requires viscous steady coefficients, e.g. from 2D steady RANS computations. This 2D airfoil model is coupled with a 3D steady-state lifting line model. The model is applied to the LEISA research airplane and extensively validated with URANS results. It performs well in calculating gust loads with and without simultaneous flap deflections, and provides significantly more accurate results in the case of stall than when stall is neglected.

On the unsteady aerodynamics of flapping wings under dynamic hovering kinematics

Hummingbirds and insects achieve outstanding flight performance by adapting their flapping motion to the flight requirements. Their wing kinematics can change from smooth flapping to highly dynamic waveforms, generating unsteady aerodynamic phenomena such as leading-edge vortices (LEV), rotational circulation, wing wake capture, and added mass. This article uncovers the interactions of these mechanisms in the case of a rigid semi-elliptical wing undergoing aggressive kinematics in the hovering regime at Re∼O(103). The flapping kinematics were parametrized using smoothed steps and triangular functions and the flow dynamics were simulated by combining the overset method with large eddy simulations. The analysis of the results identifies an initial acceleration phase and a cruising phase. During the former, the flow is mostly irrotational and governed by the added mass effect. The added mass was shown to be responsible for a lift first peak due to the strong flapping acceleration. The dynamic pitching and the wing wake interaction generate a second lift peak due to a downwash flow and a vortex system on the proximal and distal parts of the wing's pressure side. Conversely, aerodynamic forces in the cruising phase are mainly governed by the growth and the establishment of the LEV. Finally, the leading flow structures in each phase and their impact on the aerodynamic forces were isolated using the extended proper orthogonal decomposition.

Investigations on Trimming Strategy and Unsteady Aerodynamic Characteristics of Tiltrotor in Conversion Procedure

Investigations on Trimming Strategy and Unsteady Aerodynamic Characteristics of Tiltrotor in Conversion Procedure

Quantitative study on the flow control of a 1:2 rectangular cylinder based on the leading-edge suction

The present study employed the leading-edge suction flow control on a 1:2 rectangular cylinder to control its unsteady aerodynamics. To study the suppression on aerodynamic forces, the surface pressures were measured at the Reynolds number of 1.28 × 104. A dimensionless suction momentum coefficient (Cμ) was defined to quantify the suction flow rate. The detailed flow visualization was conducted on the wake flows as well as the leading-edge separated flows to reveal the control mechanism. The proper orthogonal decomposition (POD) analysis was conducted on the wake flows to study the modal transformations. Results show that the leading-edge suction flow control could significantly affect the flow separation-reattachment characteristics and the present control processing could be divided into five stages with increasing Cμ. The lift fluctuation could reach a maximum reduction of 66.4%. Meanwhile, the flow separation at the leading edge was gradually suppressed and eliminated with increasing Cμ. The wake flow was compressed in the transverse region and suppressed in the turbulent kinetic energy (TKE) as well as the Reynolds shear stress (RSS), which should be responsible for the suppression on the lift fluctuation.