Variation Of Aerodynamic Coefficients Research Articles

Fixed-time angle of attack constrained control for aircraft considering dynamic icing process

Aircraft icing deteriorates aerodynamic performance and reduces stall angle of attack, the fast convergence rate of tracking error is required to stabilize the aircraft when aircraft icing occurs. The state-of-the-art control methods for icing aircraft mostly assume that the icing of aircraft is instantaneous. Aiming at these issues, a fixed-time angle of attack-constrained control strategy is designed considering dynamic icing process. In order to explore the variation of aerodynamic coefficients in the process of dynamic icing, an ice wind tunnel experiment is implemented, and the relationship between lift coefficient, drag coefficient and pitching moment coefficient with angle of attack and icing intensity is obtained by fitting method. In order to prevent the stalling problem caused by the decrease of the stalling angle of attack in the process of dynamic icing, a method to determine the stalling angle of attack based on deep neural network is proposed. Considering the asymmetric and time-varying angle of attack constraint, a fixed-time convergent angle of attack-constrained robust control method is designed. The ice wind tunnel experiment shows the process of dynamic icing of the airfoil, and the simulation results verify the effectiveness of the proposed control method.

Aerodynamic center of a wing at low Reynolds number

The location of the aerodynamic center, with respect to the center of gravity, determines the longitudinal static stability of an aerial vehicle. While most studies in the past focused on high Reynolds number (Re) flows, we consider flows at low Re (100≤Re≤6000) past an end-to-end wing and a finite wing of various semi-aspect ratio (0.25≤sAR≤5). It is shown that two-dimensional computations, even for an end-to-end wing, are inadequate for accurate prediction of aerodynamic coefficients. The chordwise pressure distribution as well as the variation of aerodynamic coefficients with angle of attack (α) at low Re are significantly different from that at high Re. Beyond a certain α, the flow is associated with a large recirculation zone, resulting in a non-linear variation of the aerodynamic coefficients with α. The conventional definition of the aerodynamic center is too restrictive at low Re. Owing to the non-linear variation of the pitching moment with change in α, it is not possible to determine one single location where it is invariant over a large range of α. The aerodynamic center is, therefore, determined over various sub-regimes of α. It varies significantly over the sub-regimes. The wing-tip vortex significantly affects the vortex shedding, flow structures, and pressure distribution on a finite wing. The locations of the aerodynamic center and center of pressure move fore, toward the leading edge, with a decrease in aspect ratio compared to that for an end-to-end wing.

Integrated guidance and control design by active disturbance rejection method for high-velocity target interceptor with DCS thruster

The present paper proposes a novel integrated guidance and control (IGC) method for engaging with high-speed targets such as ballistic projectiles. considering an extreme short period of terminal engagement due to high relative velocity between target and interceptor, it is particularly important for IGC law to show desirable performance in the presence of various uncertainties (e.g. variation in aerodynamic coefficients) and disturbances (e.g. target maneuver and drag). This article extends the ICG law for mismatched and feedback form equations based on the Active Disturbance Rejection Control (ADRC) method using the back-stepping technique and the Reduced-order Extended State Observer (RESO). The primary consideration is the application of thrusters on the center of mass as the Divert Control System (DCS), along with the daisy-chain technique for control allocation between the fins and thruster commands. Contrary to previous research, the filter and angle measurement error are modeled for the seeker as a crucial parameter to highlight the significance of the thruster. The simulation results indicate the efficiency of the developed method for near-miss or hit-to-kill engagement with tactical ballistic targets. It is shown that the thruster plays a significant role in high-altitude engagements, specifically in the presence of non-ideal seeker. Finally, using the Monte Carlo simulation, it is proved that adding inner loops to the developed technique will not remove the IGC’s advantage over the conventional approach and Non-singular Terminal Sliding Mode (NTSM) guidance law.

Experimental and numerical investigation on the wake flow and vortex shedding of a rotating circular cylinder

When fluid passes through a still cylinder, alternate shedding vortices are formed on the two sides of the cylinder in the wake. Regarding a rotating circular cylinder, the rotation can affect the wake flow and vortex shedding pattern. To investigate the wake flow and surface pressure characteristics of a rotating cylinder at different rotational speeds, wind tunnel tests and numerical simulation methods through Fluent were used. The dimensionless rotational speed was discussed for its impact on the vortex shedding intensity and pattern. Additionally, the correlation between the cylinder surface wind pressure and the vortex shedding pattern was analyzed. The results of this study provide useful insights into the mechanisms underlying the vortex shedding phenomenon and the effects of rotational speed on the wake flow and surface pressure of a rotating cylinder. The results show that an increase in the dimensionless rotational speed will change the characteristics of the wind pressure distribution, leading to the variation in aerodynamic coefficients. On the other hand, the vortex shedding characteristics of the wake flow will also be affected, with changes in the vortex shedding pattern and direction, thereby changing the characteristics of the wake deviation angle and correlation. Based on the analysis of wake flow speed power spectrum characteristics and the Reynolds number effect, the mechanism of the vortex shedding change caused by flow transitions is speculated and verified by numerical simulation of the vorticity field.

Identification and Validation of the Cessna Citation X Longitudinal Aerodynamic Coefficients in Stall Conditions using Multi-Layer Perceptrons and Recurrent Neural Networks

The increased number of accidents in general aviation due to loss of aircraft control has necessitated the development of accurate aerodynamic airplane models. These models should indicate the linear variations of aerodynamic coefficients in steady flight and the highly nonlinear variations of the aerodynamic coefficients due to stall and post-stall conditions. This paper presents a detailed methodology to model the lift, drag, and pitching moment aerodynamic coefficients in the stall regime, using Neural Networks (NN). A system identification technique was used to develop aerodynamic coefficients models from flight data. These data were gathered from a level-D Research Aircraft Flight Simulator (RAFS) that was used to execute the stall maneuvers. Multilayer Perceptrons and Recurrent Neural Networks were used to learn from flight data and find correlations between aerodynamic coefficients and flight parameters. This methodology is employed in here to optimize neural network structures and find ideal hyperparameters: training algorithms and activation functions used to learn the data. The developed stall aerodynamic models were successfully validated by comparing the lift, drag, and pitching moment aerodynamic coefficients predicted for given pilot inputs with experimental data obtained from the Cessna Citation X RAFS for the same pilot inputs.

Modeling and switching adaptive control for nonlinear morphing aircraft considering actuator dynamics

This paper investigates the problems of modeling and control of the morphing aircraft. Henceforth, a nonlinear dynamic model of a wing-sweep morphing aircraft is first established in this work. This model is suitable for larger envelopes by elaborating the variation of aerodynamic coefficients, air density, mass distribution at different altitudes, Mach numbers, and sweep angles. Considering the alterations in the dynamic characteristics of the morphing aircraft owing to the various flight conditions, we design the switching adaptive backstepping controllers for the altitude subsystem and velocity subsystem. The actuator dynamics have been explicitly included in the process of the controller design to alleviate the chattering problem caused by the switching. Furthermore, the closed-loop stability is rigorously proved via the Lyapunov stability theory and the technique of compact set. Comparative results from the simulations finally validate the effectiveness of the proposed scheme.

Forecasting of aerodynamic coefficients of tri-axially symmetrical Y plan shaped tall building based on CFD data trained ANN

From Burj Khalifa to Jeddah Tower, the use of a triaxially symmetrical plan is very common for skyscrapers. Corner modification is inevitable to decrease the wind-induced load, motion, and vibration, at least on the upper floors. This study focuses on forecasting force, moment and torsional coefficients of triaxially symmetrical Y plan shaped tall building. Initially, the Computational Fluid Dynamics (CFD) study of building models is done on the RANS k −ε turbulence model. The variation of aerodynamic coefficients with the corner cut percentage and angle of attack (AOA) is discussed. The flow patterns are also utilised for analysing the variation in the coefficient values. The design parameters and results are used for predicting some rational parametric equations of the aerodynamic coefficients. Outcomes from CFD are further utilised for the training of Artificial Neural Networks (ANN). Comparison of the results from CFD, ANN and rational parametric equations are made, and wind tunnel results are used for the validation purposes of the surrogate modelling. The maximum observed error for ANN modelling is less than 4% which suggests very good predictability of the networks.

Unsteady Aerodynamic Characteristics of Antenna Rotating in Different Elevation Angles

The aerodynamic characteristics of radar antennas should be considered in computing their wind resistance and designing pedestal servo systems. In this paper, the aerodynamic characteristics of a flat plate antenna with azimuthal rotation are explored using a wind tunnel, and the effects of the antenna elevation angle and reduced frequency on the aerodynamic coefficients are analyzed. The corresponding results of numerical simulation are given to compare with the experimental results. The variation of aerodynamic coefficients with respect to the azimuth angle is found to depend on the reduced frequency and the antenna elevation angle. When the increase in antenna elevation angle is slight, the mean and root mean square values of the aerodynamic coefficients are not monotonic with respect to increases in elevation angle and may increase at individual elevation angles. When the elevation angle increases significantly, the mean, maximum, and root mean square values of the aerodynamic coefficients all significantly decrease. The simulation results are in good agreement with the experimental results, which verify the feasibility of using unsteady numerical simulations to obtain the flow field structure when the antenna is rotating. This approach allows the influence mechanism of the elevation angle change on the aerodynamic characteristics of the rotating antenna to be identified.

Sensitivity studies of ANFIS based force recovery technique towards prediction of aerodynamic load

Accelerometer force balance (AFB), in conjunction with various force prediction methodologies, has been used in hypersonic flow regime. Present studies focus on its ability to capture small and large alterations in the force and moment coefficients. For this objective, shock tunnel experiments are conducted on a bi-conic model without and with hemispherical or sharp aerospike. A three component AFB is equipped to measure acceleration responses during the experiments. Adaptive Neuro Fuzzy Inference System (ANFIS) is employed herein for the force recovery. Aerodynamic coefficients obtained from ANFIS are noticed to have encouraging match with the AFB theory. It has been shown here that, AFB and recently developed ANFIS based force recovery methodology can precisely predict as small as 6.3% and as large as 38% change in the force along with its temporal variation. Hence, this study strengthens the use of a soft computing technique like ANFIS for predicting steady state and temporal variations of aerodynamic coefficients during shock tunnel testing of hypersonic configurations.

Effects of infrastructure on the aerodynamic performance of a high-speed train

The aerodynamic characteristics of typical high-speed train can be affected by the operating infrastructure, which will affect the flow structure around train body. Five different infrastructure scenarios, including no infrastructure, flat ground, embankment, viaduct and truss bridge, are systemically studied. The purpose is to examine the uncertainties of aerodynamic coefficients caused by the infrastructure. Attention is drawn to variations of aerodynamic coefficients at certain yaw angles caused by the changes in crosswind and train speed. The middle car is chosen for quantifying the effects of five infrastructures by using wind tunnel test and numerical simulations, then followed by a detailed study on aerodynamic characteristics of three cars of train running on viaduct. Pressure distributions are also drawn for a better interpretation. Result shows that the uncertainties in aerodynamic coefficients becomes more obvious as the infrastructure gets complex and yaw angles get bigger. The aerodynamic coefficients of three cars with the viaduct scenario show the similar uncertainties, which are mostly affected by the change in crosswind rather than the train speed.

Wind tunnel tests on the aerodynamic characteristics of vehicles on highway bridges

Accurately quantifying the aerodynamic forces acting on vehicles and long-span bridges is critical for assessing the safety of moving vehicles on bridges which are subjected to strong wind. It is necessary to consider the aerodynamic interference between vehicles and the bridge, especially for this with the bluff body section and wind barriers. However, very few investigations have been carried out to find aerodynamic coefficients of vehicles on a bridge with the bluff body section and considering the effect of wind barrier. This article therefore carried out wind tunnel tests to determine aerodynamic coefficients of container truck on a bridge with a π-cross section and wind barriers. The influence of vehicle position in different road lanes of the bridge deck and the aerodynamic interference between vehicles on the aerodynamic characteristics of the vehicle and the bridge are investigated. Different heights and ventilation ratios of wind barrier are taken into consideration to examine variations of aerodynamic coefficients with different wind barriers. Furthermore, the change mechanism in the aerodynamic coefficients of the vehicles is observed by analyzing the wind pressure distribution on the surface of the vehicles. The test results show that the different lane locations of the vehicle affect the aerodynamic coefficients significantly, as well as the aerodynamic interference between vehicles with transverse arrangement or longitudinal arrangement, especially for the side force coefficient. The existence of wind barrier reduces the side force coefficients of the vehicle remarkably. Such effects also vary with the ventilation ratio and height of wind barrier.

Low Reynolds number effects on aerodynamic loads of a small scale wind turbine

Small-scale or scaled-down wind turbines for model experiments mostly operate in low-Reynolds-number flow. The nonlinear variations of aerodynamic coefficients with respect to the angle of attack caused by viscous effects and laminar boundary layer separation affect the wind turbine performance under these conditions. Although the vortex lattice method (VLM) is an efficient way to predict rotor performance, it tends to suffer from numerical error because nonlinear aerodynamic characteristics cannot be considered. In this study, the nonlinear vortex lattice method (NVLM) is adopted to compute the aerodynamic loads of two small-scale wind turbines. This method involves a sectional airfoil look-up table and vortex strength correction and can be applied to a wide range of operating conditions. The simulations of TU Delft and NTNU wind turbines are conducted to validate the prediction capability of numerical models by comparing predictions with the measurements. It was found that the overall results from the NVLM simulation are more accurate than the VLM results, which implies that the nonlinear aerodynamic characteristics associated with low-Reynolds-number flow should be considered to accurately assess the aerodynamic performance of small-sized wind-turbines, particularly at the low tip speed ratio at which the rotor blade may experience flow separation and dynamic stall.

Designing adaptive PID controller non-sensitive to changes in aerodynamic characteristics of an unmanned aerial vehicle

The method of implementing an adaptive PID controller using a reference model of an unmanned aerial vehicle is presented. The unmanned aerial vehicle has a nonlinear characteristic and high sensitivity to external influences. The operation of a standard controller in a nonlinear model in the event of disturbing influences does not meet the specified quality criteria. Problems that affect the flight time of the unmanned aerial vehicle are represented by variations in aerodynamic coefficients in the known ranges. Herewith, aerodynamic parameters change, and the system becomes unstable. To eliminate unwanted deviations, an adaptive PID controller loop is introduced into the aerial vehicle control system. Using the reference model of the control object, the adaptation comparator provides the necessary PID controller settings. The introduction of such a correction control signal allows countering various failures and disturbances that lead to uncontrolled control. It was found that this method of control of the unmanned aerial vehicle is very effective, since the obtained result is closer to the experimental one. The study of failures was carried out through the observation of changes in aerodynamic coefficients. The study of changes in aerodynamic coefficients allows failure-free determination of the nominal values of the object coefficients. Such an approach to modeling the unmanned aerial vehicle also makes it possible to solve the economic side of the problem – to conduct experiments in the ANSYS-CFX aerodynamic application without costs for restoring vehicles and structures lost as a result of experimental testing

Variation in aerodynamic coefficients with altitude

Precise aerodynamics performance prediction plays key role for a flying vehicle to get its mission completed within desired accuracy. Aerodynamic coefficients for same Mach number can be different at different altitude due to difference in Reynolds number. Prediction of these aerodynamics coefficients can be made through experiments, analytical solution or Computational Fluid Dynamics (CFD). Advancements in computational power have generated the concept of using CFD as a virtual Wind Tunnel (WT), hence aerodynamic performance prediction in present study is based upon CFD (numerical test rig). Simulations at different altitudes for a range of Mach numbers with zero angle of attack are performed to predict axial force coefficient behavior with altitude (Reynolds number). Similar simulations for a fixed Mach number ‘3’ and a range of angle of attacks are also carried out to envisage the variation in normal force and pitching moment coefficients with altitude (Reynolds number). Results clearly depict that the axial force coefficient is a function of altitude (Reynolds number) and increase as altitude increases, especially for subsonic region. Variation in axial force coefficient with altitude (Reynolds number) slightly increases for larger values of angle of attacks. Normal force and pitching moment coefficients do not depend on altitude (Reynolds number) at smaller values of angle of attacks but show slight decrease as altitude increases. Present study suggests that variation of normal force and pitching moment coefficients with altitude can be neglected but the variation of axial force coefficient with altitude should be considered for vehicle fly in dense atmosphere. It is recommended to continue this study to more complex configurations for various Mach numbers with side slip and real gas effects.

Design of Power and Load Reduction Controller for a Medium-Capacity Wind Turbine

A control algorithm for a 100 kW wind turbine is designed in this study. The wind turbine is operating as a variable speed variable pitch (VSVP) status. Also, this wind turbine is a permanent magnet synchronous generator (PMSG) Type. For the medium capacity wind turbine considered in this study, it was found that the optimum tip speed ratios to achieve the maximum power coefficients varied with wind speeds. Therefore a commercial blade element momentum theory and multi-body dynamics based program was implemented to consider the variation of aerodynamic coefficients with respect to Reynolds numbers and to find out the power and thrust coefficients with respect tip speed ratio and blade pitch angles. In the end a basic power controller was designed for below rated, transition and above rated regions, and a load reduction algorithm was designed to reduce tower vibration by the nacelle motion. As a result, damage equivalent Load (DEL) of tower fore-aft has been reduced by 32%. From dynamic simulations in the commercial program, the controller was found to work properly as designed. Experimental validation of the control algorithm will be done in the future.

Design and optimisation of an aerofoil with active continuous trailing-edge flap

ABSTRACTThis paper presents the design and optimisation of an aerofoil with active continuous trailing-edge flap (CTEF) investigated as a potential rotorcraft active control device. Several structural cross-section models are developed: high-fidelity NASA STRucture ANalysis (NASTRAN) and University of Michigan/Variational Asymptotic Beam Section Code (UM/VABS) models and a reduced-order analysis model. The validation of the reduced-order model is established by comparing its predictions of CTEF deformations with those of NASTRAN and UM/VABS analyses, which both show good agreement. The 2D aerodynamic characteristics of the CTEF aerofoil are evaluated using XFOIL and Computational Fluid Dynamics (CFD) analyses: FUN3D and Overset Transonic Unsteady Rotor Navier-Stokes (OVERTURNS). XFOIL, coupled with the reduced-order structure model, is adopted for optimisation study. The accuracy of XFOIL in predicting the aerodynamic pressure of the CTEF aerofoil is verified using CFD simulations, which shows sufficient fidelity. The predicted variations of aerodynamic coefficients with a CTEF angle are compared among the aerodynamic analyses. The optimisation process is developed and applied to two bimorph bender configurations: a Macro-Fibre Composite (MFC) solid bender and an MFC stack bender. The solid bender is used to confirm the functioning of the optimisation procedure and to use its optimal layout as a reference to the stack design, the primary design object. A linear tapered shape is found to be the optimum for a MFC solid bender, which generates an average of 63% more CTEF angles than those of an optimal rectangular bender. An optimised MFC stack bender is shown to resemble the shape of the solid bender. A four-ply bimorph is considered the best choice among the stack layouts because of its large output of CTEF angles and relatively less plies required. The CTEF angle produced by the four-ply optimal layout ranges from 7.6° to 5.3° with speeds from 0 to 200m/s at an angle of attack (AoA) of 6°. The reduction in the CTEF angle with AoA is less steep than that with speed, ranging from 6.5° to 5.8° with AoA from 0 to 8° at speed of 166m/s. An average of 14% increase in CTEF angles is achieved through optimisation for the four-ply bimorph.

Variation of aerodynamic coefficients with air flow parameters on long span bridge deck

Wind load is critical for bridges with high strength to weight ratio and determined experimentally using wind tunnel study that involves huge expenditure and time. In this study, behaviour of bridge deck is analysed by changing air flow parameters considering the structure submerged in turbulent flow using k-e turbulence model with boundary conditions calibrated to simulate neutral equilibrium atmospheric boundary layer. A computational model was considered to determine aerodynamic behaviour of bridge deck for different angle of attack of wind and results were compared with wind tunnel experiments keeping boundary conditions same. The aerodynamic behaviour of different bridge deck cross section are analysed using the computational model for different air flow parameters. The variation of aerodynamic coefficients are found and presented for box girder, plate girder, trapezoidal section and section of Great Belt East Bridge and attempt was made to select the optimum section to minimise the aerodynamic forces.

A Numerical Investigation on Aerodynamic Coefficients of Solar Troughs Considering Terrain Effects and Vortex Shedding

Recently, increase in the cost of fossil fuels and taking into consideration the environmental effects of exploiting them, caused many researchers and governments to find some ways to make use of renewable energies more cost-effectively. Solar energy is a category of renewable energies which could be harvested via several technologies. One of the most practical methods is using parabolic troughs. In solar power plants, many parabolic troughs are set in parallel rows in order to concentrate the solar power onto a tube absorber. In designing and manufacturing parabolic troughs and their structures, it is essential to take into account the wind force. Any negligence in considering the wind force could be concluded in losing the accuracy and efficiency. In this article, the aerodynamic coefficients of parabolic solar troughs have been investigated using CFD methods. The variations of aerodynamic coefficients considering terrain effects, the angle of collectors and the gap between mirrors have been studied. Also, it will be demonstrated that in order to properly align trough collector in solar farms, it is essential to study the vortices shed created at the behind of parabolic troughs and its effects on collectors’ structures in the result of wind interaction. At the end and as an illustration, the drag and lift coefficients of collectors have been calculated in Yazd power plant.

Wind tunnel investigations of aerodynamic coefficients of road vehicles on bridge deck

To assess the safety of road vehicles running over a long span bridge in high wind, the knowledge of aerodynamic forces and moments on the vehicles is required. This could be obtained through wind tunnel investigations and expressed in terms of aerodynamic coefficients. However, very few investigations have been carried out to find aerodynamic coefficients of road vehicles on a bridge deck, compared with the aerodynamic coefficients of road vehicles on the ground. This paper therefore presents the results of wind tunnel tests carried out to determine aerodynamic coefficients of four types of road vehicles over a typical bridge deck. The aerodynamic coefficients of the same road vehicles on the ground are also presented to explore the effects of the bridge deck on aerodynamic coefficients. Different wind directions are taken into consideration to observe variations of aerodynamic coefficients with wind direction. Furthermore, the effects of vehicle position in different road lanes of the bridge deck are investigated. The test results show that the existence of bridge deck reduces side forces for all four types of vehicles remarkably but increases rolling moments to some extent. Such effects also vary with types of vehicles and lanes of the bridge deck where vehicles run.

Non-linear control of an uncertain hypersonic aircraft model using robust sum-of-squares method

In recent years, sum-of-squares (SOS) method has attracted increasing interest as a new approach for stability analysis and controller design of non-linear dynamic systems. In this study, the authors present a robust SOS/robust LMI method to design a non-linear controller for longitudinal dynamics of a hypersonic aircraft model with parametric uncertainties. To this end, the control design problem is first formulated as a robust SOS problem. Then, an LMI representation of the robust SOS problem is derived using a proposed algorithm and is solved via a stochastic ellipsoid method. As the simulation results show, the designed robust controller is capable of stabilising the aircraft and following pilot commands under up to 50% variation in aerodynamic coefficients.