Unsteady Aerodynamic Effects Research Articles

Transient dynamical effects induced by single-pulse fluidic actuation over an airfoil

Flow control experiments on a NACA 0015 airfoil using pulsed vortex generator jets (PVGJs) are carried out at a chord-based Reynolds number of $$4.6 \times 10^5$$ . Configurations of partially separated flow over the airfoil are studied with a special focus on the transient effects induced by actuation. The main objective is the improvement of the rate of variation of the aerodynamic loads. The response to a single-pulse actuation, of duration smaller than the convective time over the airfoil, is compared with the response to the onset of steady blowing. We observe that the single-pulse actuation, if correctly tuned, induces a higher rate of loads variation (up to 50 $$\%$$ for the lift). By using Lagrangian analyzing tools (finite-time Lyapunov exponent—FTLE), we show that this improvement is due to dynamical effects at the scale of the airfoil, associated with the formation of a closed separation region during the initial transient phase after the end of actuation. Such load control could be effective in situations where a fast time response is needed to compensate unsteady aerodynamic effects, such as in gusting flows or during rapid maneuvers.

Nonlinear modeling of electro-aeroelastic dynamics of composite beams with piezoelectric coupling

Abstract Structural and aerodynamic non-linearities can lead to persistent oscillations in aeroelastic systems, which allows the conversion of mechanical energy into electric power. Flexible beams represent an example of structures that can be used as energy harvesters. This work aims to model and analyze the non-linearities induced by the flow-structure interaction of an energy harvester consisting of a laminated beam integrated with a piezoelectric sensor. The cantilevered beam and the piezoelectric lamina are modeled using a nonlinear finite element approach, while unsteady aerodynamic effects are described by a state-space model that allows for arbitrary nonlinear lift characteristics. Wind tunnel tests for a fluttering beam in a broad range of flow speeds and preset angles of incidence were used to validate the electro-aeroelastic model. The matching of experimental and computational results reveals the importance of appropriately modeling structural and aerodynamic non-linearities for reproducing the physical electro-aeroelastic behavior of the system. These findings are of practical and theoretical relevance, and are further supported by the model’s complete inability to reproduce experimental results when either of the non-linearities are “switched off”.

A new deformation control approach for flexible wings using moving masses

Abstract This paper presents a new deformation control concept based on moving mass control technique for the flexible wing of the HALE vehicles. Such mass actuated wings can actively adjust its aeroelastic deformation to avoid the unexpected large deflection and also function as a “seamless morphing wing” to improve flight performance. This investigation focuses on the modeling, analysis, and primary controller design of a high-aspect-ratio wing controlled by an internal moving mass. The equations of motion are derived first by considering the couplings among the structural dynamics, the unsteady aerodynamics, and the transient motion of the mass. The variations in flutter speed and steady-state deformation with the locations of the mass are presented. The time-domain simulations are presented under various travel profiles for the moving mass. The feasibility of using moving mass for deformation control of the flexible wing is primarily proved. Placing the moving mass in front of the elastic axis is preferable to obtain higher flutter speed and larger deformation control authority. Continuous changes in aerodynamic forces can be realized following the structural deformation. The unsteady aerodynamic effect is beneficial for obtaining the moderate deformation control performance without oscillations and also eliminates the complex dynamic nonlinear coupling effects. Finally, a proof-of-concept control system integrated with incremental and positional PD controllers is designed to achieve the desired configuration of the wing.

Efficient Prediction of Aerodynamic Control Surface Responses Using the Linear Frequency Domain

In modern aircraft design the accurate prediction of dynamic control surface deflections is crucial for the evaluation and application of new load alleviation techniques. Time-marching approaches, such as the unsteady Reynolds-averaged Navier–Stokes equations, do provide a complete modeling of the aerodynamic flowfield; however, they are extremely time-consuming and still too expensive for design applications. In this paper a model is presented, which enables the fast and accurate prediction of aerodynamic responses for arbitrary control surface deflections. The model hereby reflects the time signal of the control surface deflection as a superposition of frequency components and computes the dynamic response behavior of the control surface using the linear frequency domain. The frequency responses are precomputed in a surrogate model for a wide parameter space of Mach number, Reynolds number, angle of attack and flap chord ratio, so that a frequency response for a new flight condition can be computed by mere interpolation. After building the surrogate model, the method achieves reduction in computational time of up to six orders of magnitude in comparison to time-marching simulations, while still covering the viscous and unsteady aerodynamic effects in the flow.

Experimental characterization of H-VAWT turbine for development of a digital twin

A digital twin can be described as a digital replica of a physical asset. The use of such models is key to understanding complex loading phenomena experienced during testing of vertical axis wind turbines. Unsteady aerodynamic and structural effects such as dynamic stall and dynamically changing thrust and blade loading are difficult to predict with certainty. This leads to inefficient turbine designs or worse yet premature failures. Many of these phenomena can be better understood through scaled wind tunnel testing. The analysis of these test results is greatly improved by having a well calibrated digital twin model of the turbine. This paper discusses the methodologies used in the development of the model for a H style vertical axis wind turbine. This includes physical measurements of the as built system, updates to the models based upon experimental testing and a final correlation between test and model on a component by component as well as fully assembled system.

Investigations of unsteady aerodynamic effects generated by heave and pitch motion in different vehicle body shapes with model excitation tests

Investigations of unsteady aerodynamic effects generated by heave and pitch motion in different vehicle body shapes with model excitation tests

Deep neural network for unsteady aerodynamic and aeroelastic modeling across multiple Mach numbers

Aerodynamic reduced-order model (ROM) is a useful tool to predict nonlinear unsteady aerodynamics with reasonable accuracy and very low computational cost. The efficacy of this method has been validated by many recent studies. However, the generalization capability of aerodynamic ROMs with respect to different flow conditions and different aeroelastic parameters should be further improved. In order to enhance the predicting capability of ROM for varying operating conditions, this paper presents an unsteady aerodynamic model based on long short-term memory (LSTM) network from deep learning theory for large training dataset and sampling space. This type of network has attractive potential in modeling temporal sequence data, which is well suited for capturing the time-delayed effects of unsteady aerodynamics. Different from traditional reduced-order models, the current model based on LSTM network does not require the selection of delay orders. The performance of the proposed model is evaluated by a NACA 64A010 airfoil pitching and plunging in the transonic flow across multiple Mach numbers. It is demonstrated that the model can accurately capture the dynamic characteristics of aerodynamic and aeroelastic systems for varying flow and structural parameters.

Effects of blade aspect ratio and taper ratio on hovering performance of cycloidal rotor with large blade pitching amplitude

Abstract In recent years, a lot of research work has been carried out on the cycloidal rotors. However, it lacks thorough understanding about the effects of the blade platform shape on the hover efficiency of the cycloidal rotor, and the knowledge of how to design the platform shape of the blades. This paper presents a numerical simulation model based on Unsteady Reynolds-Averaged Navier–Stokes equations (URANSs), which is further validated by the experimental results. The effects of blade aspect ratio and taper ratio are analyzed, which shows that the cycloidal rotors with the same chord length have quite similar performance even though the blade aspect ratio varies from a very small value to a large one. By comparing the cycloidal rotors with different taper ratios, it is found that the rotors with large blade taper ratio outperform those with small taper ratio. This is due to the fact that the blade with larger taper ratio has longer chord and hence better efficiency. The analysis results show that the unsteady aerodynamic effects due to blade pitching motion play a more important role in the efficiency than the blade platform shape. Therefore we should pay more attention to the blade airfoil and pitching motion than the blade platform shape. The main contributions of this paper include: the analysis of the effects of aspect ratio and taper ratio on the hover efficiency of cycloidal rotor based on both the experimental and numerical simulation results; the finding of the main influencing factors on the hover efficiency; the qualitative guidance on how to design the blade platform shape for cycloidal rotors.

Experımental Investıgatıon of Performance of a Small Scale Horızontal Axıs Wınd Turbıne Rotor Blade

Experimental testing of a scaled-down rotor blade for a 2kW small scale HAWT is the forefront of the present research work. To facilitate the objectives, a rotor blade for a 2kW HAWT has been analytically designed employing the iterative Blade Element Momentum Theory (BEMT). For the ease of manufacturing, the linearized blade obtained through the BEM theory is fabricated and subsequently, its performance is evaluated at wind velocities V=1-20m/s and TSR=1-20. The performance parameter defined by coefficient of power C p is considered for appraising and comparing the performance of the analytical and the experimental scaled-down versions of the blade. At the design conditions, the coefficient of power C p for the blade was calculated to be C p =0.40. Further, appropriate scaling laws have been incorporated to scale down the original blade to retrofit to an existing wind turbine generator. Performance of the blade was examined at the prescribed wind velocities V and TSR λ. The experimental outcomes depicted a C p trend for the scaled-down model to be similar to the actual model. At design conditions, the experimental results rendered a maximum C P =0.380 for the scaled-down model with an error of around 6%. However, the investigation showed that the C P for the scaled-down model was aligned with the actual model. The error in C p of the analytical and scaled-down models are deemed due to the ignorance of hub losses, 3-Dimensional flow effects and unsteady aerodynamic effects in the study.

A fully coupled analysis of unsteady aerodynamics impact on vehicle dynamics during braking

This study investigates the impact of unsteady aerodynamic loads on the behavior of car dynamics during braking maneuvers. In the analysis, two independent solvers were coupled to solve an existing fluid structure interaction (FSI) problem. Transient CFD analysis applying Fluent software was used to obtain realistic aerodynamic loads. A full car, nonlinear dynamic model with elastic suspension system was built in a dedicated multibody dynamic system MSC.Adams/Car. Both physics were integrated into one ecosystem via a block diagram environment for multidomain simulation (Matlab/Simulink). Methodology elements were validated against experimental results for a rectangular beam in cross-flow with vortex-induced vibrations. The final results were validated against a full-scale experiment on a real car. The solution of an untypical flow problem (decreasing car speed during a braking maneuver) required an untypical reference frame. The classic case, with an observer at rest (stationary model and moving air), was replaced with a moving observer. The whole domain was translocated during the analysis with a velocity which varied with time. The problem was further complicated by the presence of movable aerodynamic elements. The overset mesh technique was used to allow movement of the car body and active aerodynamic surfaces. The results obtained show the importance of aerodynamic effects on the braking process. It was shown that in the analyzed scenario, movable aerodynamic features can reduce the distance to stop by 6%. The complex interaction between the active aerodynamic surface and the car body with respect to load split is discussed as well. The methodology proposed to determine the behavior car dynamics with unsteady aerodynamic effects taken into account appears to be a significant improvement when compared with existing methodologies used for such an assessment.

Unsteady aerodynamic characteristics of long-span roofs under forced excitation

Abstract Wind-induced vibration is significant for light and flexible structures, and aerodynamic effect has to be considered in the estimation of wind load and dynamic response of these structures. The unsteady aerodynamic characteristics of long-span roofs are investigated numerically in this paper using Large Eddy Simulation. Roofs under forced excitation are simulated, and the distributions of aerodynamic coefficients on rigid and vibrating roofs are estimated. The computed coefficients show good agreement with the experimental results. The influences of inflow turbulence, shape of roof and other parameters on the aerodynamic coefficients are also analyzed. It is found that the mean wind pressure coefficients mainly depend on the vortex shedding and reattachment positions. These coefficients in the cases studied remain almost the same with increasing excitation frequency, while the fluctuating pressure coefficients and unsteady aerodynamic coefficients change with frequency regularly. The probability density functions of wind pressure on the vibrating roofs show obvious non-Gaussian characteristics, and the wind pressure spectra exhibit peaks at the excitation frequencies. The dynamic response of long-span roof is then predicted using the mechanical admittance function. Results show that the displacement response of the structure will be under-estimated without considering the unsteady aerodynamic effect especially for flexible structures.

Aeroelastic Investigation of a Transonic Compressor Rotor with Multi-Row Effects

According to the modern turbomachinery design trend, blades are getting more and more flexible and loaded, and therefore prone to vibration issues due to forced response and flutter phenomena. For this reason, the flutter stability assessment has become a key aspect to avoid high cycle fatigue (HCF) blade failures. The aim of this paper is to numerically evaluate the influence of unsteady aerodynamic effects, due to upstream and downstream rows, on flutter predictions using an uncoupled method. Both CFD aeroelastic and modal analyses have been carried out for a one and half compressor stage in transonic conditions. Flutter results are reported for the classic single-row approach and then for the multi-row model which includes rotor/stator unsteady interactions: a good agreement with measured data is shown for both cases and also a no negligible impact of adjacent rows can be observed.

State space realisation and model reduction of potential-flow aerodynamics for HAWT applications

This paper presents a general state-space realisation of the unsteady vortex-lattice method and combines it with a novel model-order reduction strategy. The aim is to provide a computationally efficient aerodynamic description suitable for integration in horizontal axis wind turbine aeroelasticity. The consistent linearisation is in the 3D components of the vortex-lattice geometry. The resulting linear-time invariant system can, therefore, resolve all the component of forces, hence being suitable for linearisation around arbitrary wake shapes and blade geometries/deformations. The wake modelling captures unsteady aerodynamic effects but it results in large state-space models. Projection on low-dimensional degrees-of-freedom and balanced residualisation are, therefore, employed to reduce the model dimensionality. An iterative balancing algorithm based on Smith’s method is also developed so as to contain the computational cost of the process. The paper also presents an initial numerical investigation on aerofoils linearised around non-zero reference conditions, showing that this approach can reduce the size of the problem by several orders of magnitude and at a lower computational cost than standard direct methods for system balancing.

Unsteady aerodynamic effects in small-amplitude pitch oscillations of an airfoil

High-fidelity wall-resolved large-eddy simulations (LES) are utilized to investigate the flow-physics of small-amplitude pitch oscillations of an airfoil at Re = 100,000. The investigation of the unsteady phenomenon is done in the context of natural laminar flow airfoils, which can display sensitive dependence of the aerodynamic forces on the angle of attack in certain "off-design" conditions. The dynamic range of the pitch oscillations is chosen to be in this sensitive region. Large variations of the transition point on the suction-side of the airfoil are observed throughout the pitch cycle resulting in a dynamically rich flow response. Changes in the stability characteristics of a leading-edge laminar separation bubble has a dominating influence on the boundary layer dynamics and causes an abrupt change in the transition location over the airfoil. The LES procedure is based on a relaxation-term which models the dissipation of the smallest unresolved scales. The validation of the procedure is provided for channel flows and for a stationary wing at Re = 400,000.

Unsteady aerodynamic effects in landing operation of transport aircraft and controllability with fuzzy-logic dynamic inversion

Abstract Aircraft landing in strong wind has been a safety problem for all types of aircraft. The specific issues involve hard landing, roll oscillation and runway veer-off. These issues are related to atmospheric disturbances, and/or dynamic ground effect. As a result, the aerodynamics will be different from those in steady flow concept. In this paper, some of the pertinent stability and control derivatives based on a small-disturbance concept will be presented. How these local stability and control characteristics affect global controllability will be examined with Fuzzy-Logic Dynamic Inversion. Controllability is judged from whether necessary control deflections exceed the imposed limits. Specific examples involving a twin-jet transport with hard landing, rolling oscillation before touchdown and runway veer-off event due to varying crosswind after touchdown are illustrated.

Aeroelasticity Validation Study for a Three-Dimensional Membrane Wing

The objective of this effort is to study the aeroelastic characteristics of a flexible membrane wing at different Reynolds numbers using direct numerical simulation and to investigate techniques to reduce the adverse effects of flow separation and unsteady aerodynamics. For the aeroelastic predictions, a first-principles-based approach is undertaken, where the flow solver is fully coupled with the structural dynamics solver. This approach is able to accurately and efficiently predict the unsteady aerodynamics, boundary-layer separation, lift and drag, and the stress and structural deformation of the flexible membrane wing. The focus of this paper is on the systematic validation of this coupled solver for a specific membrane configuration, where extensive experimental measurements are available. Comparisons between the present simulations and experiments are made for the following quantities: 1) mean membrane shape; 2) dominant membrane oscillating frequencies under various conditions; 3) flowfield visualizations; 4) instantaneous velocity field; 5) membrane displacement response to instantaneous flowfield; 6) use of rigid and flexible membranes; 7) distribution of mean velocity field; and 8) distribution of turbulence intensity. Comparisons of these quantities show the favorable agreement of the present prediction against measurement, and hence build a sound validation study for the coupled fluid–structure solver.

Power extraction from stall-induced oscillations of an airfoil

Aerodynamic and structural nonlinearities of aeroelastic systems control different aspects of their limit cycle oscillations and bifurcations. One strong nonlinear unsteady aerodynamic effect that results in self-induced oscillations of airfoils is the dynamic stall. So, the concept of power extraction from stall-induced oscillations of a pitching airfoil is investigated. Experiments are performed to explore and enhance the conversion of the oscillations of a NACA0012 airfoil that is restrained elastically in pitching to electrical power. Wind tunnel tests are performed on an airfoil model connected to a DC electric motor to harvest the energy. The effects of varying the position of the axle, which defines the elastic axis, are investigated. Bifurcation diagrams as the air speed is increased and average power estimates for different experimental conditions are used to analyze the power extraction features. The results show an aerodynamic efficiency of about 40% indicating that an airfoil oscillating under the effects of dynamic stall is an adequate platform for energy harvesting.

Comparison of Aerodynamic Forces and Moments Calculated by Three-dimensional Unsteady Blade Element Theory and Computational Fluid Dynamics

Abstract In previous work, we modified blade element theory by implementing three-dimensional wing kinematics and modeled the unsteady aerodynamic effects by adding the added mass and rotational forces. This method is referred to as Unsteady Blade Element Theory (UBET). A comparison between UBET and Computational Fluid Dynamics (CFD) for flapping wings with high flapping frequencies (>30 Hz) could not be found in literature survey. In this paper, UBET that considers the movement of pressure center in pitching-moment estimation was validated using the CFD method. We investigated three three-dimensional (3D) wing kinematics that produce negative, zero, and positive aerodynamic pitching moments. For all cases, the instantaneous aerodynamic forces and pitching moments estimated via UBET and CFD showed similar trends. The differences in average vertical forces and pitching moments about the center of gravity were about 10% and 12%, respectively. Therefore, UBET is proven to reasonably estimate the aerodynamic forces and pitching moment for flight dynamic study of FW-MAV. However, the differences in average wing drags and pitching moments about the feather axis were more than 20%. Since study of aerodynamic power requires reasonable estimation of wing drag and pitching moment about the feather axis, UBET needs further improvement for higher accuracy.

Analysis of the crash of a transport aircraft and assessment of fuzzy-logic stall recovery

Abstract A turboprop transport Flight Data Recorder data indicates that attempt to recover from the stalled conditions has failed, even though the pitch angle is continuously pushed nose-down in accordance with the published stall-recovery technique. The present study is to examine the reasons for the angle of attack being increased in the first place and not being reduced after stall, even though the longitudinal control is set to nose-down. Fuzzy Logic models are used to preserve nonlinear and unsteady aerodynamic effects. It is found that the increasing angle of attack is initially caused by the nose-up pitching moment due to inertial coupling. In the subsequent nose-down control attempt for stall recovery, the stall angles of attack are still increasingly higher because the pitch rate relative to the rotating axes stays mostly positive. The present simulation study is based on a new method to be called “Fuzzy-Logic Dynamic Inversion,” where desired dynamics with reduced Euler angles and angular rates are specified; while the required control inputs in elevator, aileron and rudder are determined, all through Fuzzy Logic models. Results in elevator, aileron and rudder controls are illustrated to demonstrate the possibility of stall recovery and accident prevention. It is shown that if at the first sign of stall in icing and crosswind flight conditions, and the landing is aborted, stall recovery is possible by applying proper control inputs simultaneously about three control axes to reduce the moments of inertial coupling and the roll and pitch angles.

Novel high-pressure turbine purge control features for increased stage efficiency

AbstractRim seals throttle flow and have shown to impact the aerodynamic performance of gas turbines. The results of an experimental investigation of a rim seal exit geometry variation and its impact on the high-pressure turbine flow field are presented. A one-and-a-half stage, unshrouded and highly loaded axial turbine configuration with 3-dimensionally shaped blades and non-axisymmetric end wall contouring has been tested in an axial turbine facility. The exit of the rotor upstream rim seal was equipped with novel geometrical features which are termed as purge control features (PCFs) and a baseline rim seal geometry for comparison. The time-averaged and unsteady aerodynamic effects at rotor inlet and exit have been measured with pneumatic probes and the fast-response aerodynamic probe (FRAP) for three rim seal purge flow injection rates. Measurements at rotor inlet and exit reveal the impact of the geometrical features on the rim seal exit and main annulus flow field, highlighting regions of reduced aerodynamic losses induced by the modified rim seal design. Measurements at the rotor exit with the PCFs installed show a benefit in the total-to-total stage efficiency up to 0.4% for nominal and high rim seal purge flow rates. The work shows the potential to improve the aerodynamic efficiency by means of a well-designed rim seal exit geometry without losing the potential to block hot gas ingestion from the main annulus.