Air Wake Research Articles

An overall evaluation toolchain for assessing helicopter ship deck landings

AbstractA compact toolchain is implemented for the control system design for helicopter ship deck landings in a simulated environment which provides a cost-efficient and safe environment to test different scenarios. The evaluation of the ship deck landing manoeuvres is mostly performed for Ship Helicopter Operation Limits (SHOL) evaluation and performance of the helicopter. In most of the assessments, a subjective assessment is performed using different pilot rating scales which is not sufficient and the objective assessment of the simulation data for the mission success itself is seldom performed. The developed toolchain evaluates the successful operation of ship-based rotorcraft by considering both ship and helicopter dynamics, environmental conditions, and the pilot’s responses. The toolchain evaluates various touchdown conditions like helicopter position, velocity and attitude relative to the ship deck. It also displays additional evaluation parameters for the entire approach. A preliminary pilot study was performed in a simulator within a complete maritime scenario design. The maritime simulation environment includes a nonlinear helicopter flight model with a model-based flight control system, a ship dynamic model with the simulated turbulent air wake and realistic wave and water effects. For a subjective assessment of the ship deck landings, subjective pilot ratings using different rating scales were recorded during the pilot study. Simulation results show that the toolchain is convenient and well suited for assessing helicopter ship deck landing performance in a simulation environment as it provides a complete objective and subjective evaluation.

Fault-Tolerant Control for Carrier-Based Aircraft Based on Adaptive Fuzzy Sliding-Mode Method

Carrier-based aircraft landing involves complex system engineering characterised by strong nonlinearity, significant coupling and susceptibility to environmental disturbances, and autonomous landing of carrier-based aircraft under fault states is even more challenging and riskier. To address the control-system problems of loss of efficiency and performance due to actuator faults and performance degradation due to various unknown disturbances, presented here is fault-tolerant control for carrier-based aircraft based on adaptive fuzzy sliding-mode fault-tolerant control (AFSMFTC). First, three models are built (the carrier-based aircraft fault model, the carrier air wake model and the deck motion model), and the control framework of the autonomous landing control system is introduced. Next, a longitudinal and lateral flight channel controller comprising an adaptive fuzzy network, adaptive laws and a sliding-mode controller is designed using the AFSMFTC method. The adaptive fuzzy network implements fuzzy approximation for the sliding-mode switching terms to further offset errors induced by unknown disturbances, the adaptive laws compensate for actuator faults, and the sliding-mode controller ensures tracking of the overall flight path. Furthermore, the stability of the fault-tolerant method is demonstrated using the Lyapunov function. Finally, simulation and comparative experiments show that the proposed fault-tolerant method has outstanding control performance and strong fault-tolerant capability, thereby providing an effective and feasible solution for designing an autonomous landing system for carrier-based aircraft under fault states.

Capturing the bi-stable asymmetry exchanges in the turbulent air wake of a simplified frigate using Large Eddy Simulation

Turbulent air wake of a Simplified Frigate Ship (SFS2) model is simulated using wall-resolved Large Eddy Simulation. The wake behind the superstructure presents an asymmetric configuration for an otherwise symmetric free stream condition. Bi-stable characteristics of the wake have also been simulated as evidenced by asymmetric state switching events. The wake behind the stern also exhibits bi-stable switching and adapts an opposite asymmetric orientation to the wake behind the superstructure, as also observed experimentally. The resulting time-averaged flow fields for each bi-stable state reveal a differential flux formation around the upstream sides of the superstructure. The instantaneous wake fields also suggest a possible influence of flow structures developed around the upstream end of the superstructure to generate a particular asymmetric wake state downstream.

THE DESIGN AND HYDRODYNAMIC ASSESSMENT OF A SUBMARINE CONCEPT WITH OFF-AXIS RESOLVING THE IMPACT OF SHIP’S DESIGN ON DECK AND WELL-DECK LAUNCH AND RECOVERY OPERATIONS BY APPLYING OF QUIESCENT PERIOD PREDICTION (QPP)

One of the most challenging tasks in the deployment of ship-based manned and unmanned VTOL aviation is the launch and recovery and deck handling operational performance on smaller ship platforms. The same may be said of ship Well-deck-based surface boats, like Rhib and LCACs. Awareness and prediction of the ship’s position and movement in the ocean is key to ship-based air operations by targeting the right time to deploy and recover. The ships above waterline design coupled by the propulsion systems, affects ship’s motion and ship motion induced air wake behaviour. MoD/University of Exeter’s Quiescent Period Prediction (QPP) Program Aims to deliver technologies which support launch and recovery from Naval assets, particularly in higher sea states. The Quiescent Period Prediction (QPP) system achieves this by using a wave sensor system to measure the sea surface several hundred meters in advance of the ship. From the measured sea surface, a short-term deterministic wave model can be constructed allowing the wave system to be propagated to the ship's location. The ship's response to the wave spectrum is calculated to determine the level of quiescence at the time of encounter. The results are mapped dynamically on speed-polar visual graphics providing the ship’s best course to steer to ensure encountering the quiescent zone in a timely manner. The ability to fully define a ship’s motion as a function of a recovery device, greatly enhances the overall system operational capability whilst fundamentally reducing the inherent risks associated with platform manipulation in high sea conditions. The use of simulation, routinely considered in the UK, as an alternative to traditional approaches such as scale model testing and fullscale sea trials, to predict the safety and performance envelopes of platforms and systems, is discussed. Operating over deck whilst the ship is experiencing a quiescent period normally has the secondary effect of reduced air wake confusion. This is owing to fewer ship structure excursions into and out of the air flow. To better define off deck airflow around the ship a future QPP improvement will be the development and installation of a Doppler LiDAR federate. The objective of this work is to define ship-helicopter operational deck limits (SHOL), and then to safely expand ship operating deck limits. This article discusses the background to aircraft/ship operations in relation to QPP and goes on to consider both short term implementations (the Landing Period Designator System) and longer horizon deterministically predictive forms of QPP. This article describes the development of the QPP System concentrating on test procedures, timely seaway mapping and ship motion characteristics to complete specific motion sensitive task. An overview of the theory and application is provided concerning dynamic interface analysis with the focus on encountered forces and vehicle to platform response stability.

Simulation of simplified frigate using embedded LES turbulence model: Part-I validation and turbulence scales

Understanding and characterization of turbulence over the flight deck of large marine vehicle is important to assess its influence on the helicopter pilot workload while performing launch and recovery maneuvers. This research is regarding understanding the turbulent flow field over the flight deck of simplified frigate model (SFS2) using an Embedded Large Eddy Simulation turbulence model in which a portion of flow field that is of interest is resolved using LES model, thereby reducing the overall computational cost. The influence of placement of RANS-LES interface is explored and detailed validation of the numerical scheme suggests that it can capture the turbulent flow structure, especially bi-stable air wake over the flight deck of SFS2 at the zero-yaw flow condition, with better accuracy. The development of downstream air wake into the far field is also discussed. Once the numerical scheme is validated, the characterization of integral time and length scales of turbulent vortical structures in the superstructure air wake is carried out using single point correlations. It is observed that flow field at flight deck center is influenced by vortices shed from top of superstructure and funnel whereas its edges towards port and starboard sides are predominantly affected by superstructure edge vortices.

Model Predictive Control for Automatic Carrier Landing with Time Delay

This paper focuses on the problem of automatic carrier landing control with time delay, and an antidelay model predictive control (AD-MPC) scheme for carrier landing based on the symplectic pseudospectral (SP) method and a prediction error method with particle swarm optimization (PE-PSO) is designed. Firstly, the mathematical model for carrier landing control with time delay is given, and based on the Padé approximation (PA) principle, the model with time delay is transformed into an equivalent nondelay one. Furthermore, a guidance trajectory based on the predicted trajectory shape and position deviation is designed in the MPC framework to eliminate the influence of carrier deck motion and real-time error. At the same time, a rolling optimal control block is designed based on the SP algorithm, in which the steady-state carrier air wake compensation is introduced to suppress the interference of the air wake. On this basis, the PE-PSO delay estimation algorithm is proposed to estimate the unknown delay parameter in the equivalent control model. The simulation results show that the delay estimation error of the PE-PSO algorithm is smaller than 2 ms, and the AD-MPC algorithm proposed in this paper can limit the landing height error within ±0.14 m under the condition of multiple disturbances and system input delay. The control accuracy of AD-MPC is much higher than that of the traditional pole assignment algorithm, and its computational efficiency meets the requirement of real-time online tracking.

Experimental and numerical simulations of simple frigate with suction flow control over the deck

The air wake generated behind a frigate superstructure is a very complex and unsteady three-dimensional flow characterized by highly turbulent flow structures with great velocity gradients that result in flow separation over the flight deck where helicopter operations take a significant role. Naturally, this turbulent flow should be removed or, at least, reduced as maximum as possible to avoid accidents during the interaction between frigates and helicopters. This paper involves experimental and numerical analysis on a simple frigate model (SFS2) to understand and simulate the adverse effect of the turbulent flow of the air wake over the deck and tries to minimize it as with active flow control. The experimental study has been performed in the Low Speed Wind Tunnel nº1 of Instituto Nacional de Técnica Aeroespacial ‘‘Esteban Terradas'’ (INTA) with Particle Image Velocimetry (PIV) to obtain the velocity field over the flight deck. The frigate has been tested for the headwind condition of 0° (wind aligned with the flight deck) with a velocity of 10 m/s. The active flow control consists on generating suction on the flight deck by a mesh of holes. Several configurations of this suction mesh have been studied according to the diameter and location of holes and suction intensity. The suction mesh configuration with the highest diameter and flow rate presents the greatest aerodynamic improvement. The low-speed area over the deck is reduced from 36% to a minimum of 3%.

Development of PIV measurement technique in turbulent flow laden with binary-size particle groups

Abstract Particle image velocimetry (PIV) is an instantaneous whole-field measuring diagnostic that makes it feasible to measure the microscale spatial information of the interphase dynamics for good understanding of two-phase flow. However, application of PIV to the two-phase flow measurement is still a state of the art so far. A double-discriminating process in terms of gray level and size of image patterns together with the median mask technique is developed. The test flow is a turbulent air wake laden with a binary group of particles with the mean size of 2.7 μm (representing the carrier phase) and 55 μm (representing the dispersed phase). It is demonstrated that the velocity measurements of both phases can be successfully performed through the combined PIV/PTV (particle tracking velocimetry) scheme associated with the developed phase discrimination method. It is noted that the discriminating capability of the size ratio between the large- and small-particle groups in the study is around 20 together with the mean size of O(100 μm) for small particles, which is the commonly required size for the seedings used in the PIV measurements of airflows, as compared to the size of O(101 μm) adopted in the current two-phase PIV measurement methods.

SIMULATION STUDY ON VORTEX-INDUCED VIBRATION AIR WAKE ENERGY FOR AIRPORT RUNAWAY APPLICATION: A PRELIMINARY ANALYSIS

The objective of this research paper is to conduct a preliminary analysis of the effects of various cylindrical cross-sectional shapes on vortex-induced vibration for airport runaway air wake energy generation. In the case of the airport runway, vortices are generated from the aeroplane bodies that exit and enter the runway during take-off and landing operations. These oscillations can be utilized to generate power due to the large fluctuations produced by the vortices. Vortexinduced vibration works on the principle of Kármán vortices where a cylindrical or bluff-body shaped object oscillates due to the alternate vortex formation on the boundary layers by adverse fluid pressure. The oscillation depends on the unsteady lift force generated. This mechanical oscillation is later converted to electrical energy. Five cylindrical crosssection cases are investigated by computational fluid dynamics (CFD) k-ω turbulence model to identify the best case that will provide the largest lift force; hence the maximum power output generation. These cases are simulated using typical take-off air speeds for jetliners in order to finalize the overall performance. The observed results show that the elliptical cross-section with 3” height and 2” length provides the best cross-section in producing the greatest amount of energy. The total power generated from a single-cylinder was 707.1 Watts. The system can be optimized for larger aeroplanes and operation frequencies as well as could be expanded for larger energy output.Keywords: airport runaway, lift force, oscillation, renewable, turbulence, vortex-induced vibration

Balance measurements on a frigate type ship model

Balance measurements performed by testing sub-scaled ship models determine the global forces and moments acting on the ship, which allows knowing the power required for the ship's movement and provides insight to be applied in the design of the control systems used to steer the ship and to avoid instabilities while sailing.The ship superstructure may produce large separated regions and high air wake turbulence levels resulting in a set of fluctuations of the flow parameters usually determined by measuring velocity or pressure. This paper presents the balance measurement of the aerodynamic forces acting on the ship hull. Aerodynamic forces and moments produced on the ship can be interpreted as an integration of the flow parameters (velocity and pressure distributions) over the ship surface wetted by the air. Balance method provides averaged values and fluctuations of forces coefficients.Aerodynamic environment in the vicinity of a ship is influenced by a large number of factors (atmospheric wind, sea state, ship superstructure, masts, stacks, antennas …) affecting helicopter operations on board ships and their safety during the take-off and landing manoeuvres.

Automatic flight control design considering objective and subjective risks during carrier landing

In this article, a design scheme of automatic carrier landing system control law based on combination of the objective risk and the subjective risk is proposed, in order to improve the safety and flying quality of the landing. The nonlinear longitudinal mathematical model is constructed in the air wake turbulence condition during carrier landing, which is transformed into a linear perturbed model by the state-space equations with deviation state variables. The concepts of the objective risk and the subjective risk in the recovery of an aircraft aboard a carrier are addressed. A principle of predicting the future states based on the current ones is put forward so that a mathematic model for the objective risk is established, synthetically considering the current and future landing state deviations. For the other risk, the corresponding model is obtained by the subjective experiences of the pilots in the flight simulation tests. Furthermore, a novel model predictive control algorithm, which contains the additional subjective risk and the time-varying weights of the state terms, is proposed. Automatic carrier landing control law is built by introducing the objective risk, the subjective risk, and the effect of carrier air wake disturbance. In the rolling optimization progress, these time-varying weights are dynamically tuned according to the constantly changing objective risk to control the state deviations and suppress this risk, while the subjective risk is handled by the additional risk terms. Besides, the action of carrier air wake disturbance is considered and compensated in the derivation of the linear matrix inequalities. Test results based on a semi-physical simulation platform indicate that the new automatic carrier landing system control algorithm proposed by this article brings about an excellent carrier landing performance as well as an improved flying quality.

Design of Automatic Carrier-Landing Controller Based on Compensating States and Dynamic Inversion

In this paper, a design scheme of automatic carrier landing system (ACLS) control law based on compensating states and dynamic inversion is proposed, in order to eliminate landing risk and air wake disturbance and improve flight quality during landing. First of all, the mathematical model for an aircraft during carrier landing is established and transformed into a low-order linear perturbed model with the involved state variables, which denotes the reference model. Second, a high-dimension field of the landing risk is addressed, which reflects the current and the potential risk degrees due to the subjective prediction of the pilots. The coefficients of the landing risk field are established based on a pilot behavioral model. Third, a concept of compensating states is proposed by this paper. On one hand, the compensating states of the air wake disturbance are estimated from the nonlinear and reference models. On the other hand, the compensating states of the landing risk are composed of the high-dimension risk field. Fourth, automatic carrier landing control law is built with the compensating states of air wake and landing risk, nonlinear dynamic inversion, and feedback signals of the flight states. In the rolling optimization progress, the landing risk, air wake, and nonlinear factors of flight states are dynamically introduced to control the state deviations and suppress landing risk. Test results based on a semi-physical simulation platform indicate that the proposed algorithm brings about an excellent landing performance and an ability to eliminate landing risk and air wake.

Numerical Simulation and Characteristic Analysis of Ship's Air Flow Field

The air flow field of ship was simulated by using computational fluid dynamics technology to analyze its prime characteristics with reasonable accuracy. The different results of Reynolds-Averaged Navier-Stokes (RANS) method and Detached Eddy Simulation (DES) were compared, and the calculation traits of these methods were discussed. The results show that the air flow field of ship is unsteady. The accuracy of RANS simulation is insufficient for capturing this unsteady phenomenon. However, DES can catch this with better accuracy and expresses a comparatively great conformity with experimental data. Then, the aircraft carrier's flow field was calculated by DES. The characteristics of vortexes and velocity fluctuation on the ideal landing track were discussed in different wind directions. Those simulations indicate that there are complicated vortexes produced by blunt edges of the island and deck in the flow field. Those vortexes interact and mainly exist in the rear of flight deck and its adjacent air wake. Moreover, they cause a conspicuous and periodical velocity fluctuation on the ideal landing track as time goes on.

Automatic Carrier Landing System multilayer parameter design based on Cauchy Mutation Pigeon-Inspired Optimization

The parameter adjusting in Automatic Carrier Landing System (ACLS) is a time-consuming and tedious task. In order to improve the efficiency of the adjusting task and overcome the difficulties in the manual parameter adjustment, a multilayer optimization strategy, in which ACLS is clearly divided into four layers including inner loop, autopilot, guidance control and guidance compensation, is proposed in this study and adopted for the parameter design. Besides, a novel algorithm, named Cauchy Mutation Pigeon-Inspired Optimization (CMPIO) which is inspired by Cauchy distribution, is proposed to optimize ACLS parameters in each layer. Comparative simulations are conducted to verify the feasibility of the multilayer design strategy and the superiority of CMPIO. To enhance the authenticity of the simulations in the guidance compensation layer, some stochastic conditions are considered with different deck motion, air wake and radar noise turbulences alleviated by several rejection methods. The simulation results prove that the designed ACLS based on the multilayer design strategy satisfies the acknowledged criteria including the time and the frequency domain. Furthermore, the stability of the inner loop and the autopilot integrated with Approach Power Compensation System (APCS) are confirmed.

Novel wireless sensing platform for experimental mapping and validation of ship air wake

This paper presents the mechatronic design and analysis of a wireless sensing platform developed for the experimental mapping and validation of the air wakes generated by cruising naval vessels. The presented sensing system uses an RC helicopter as its carrier platform and uses the helicopter's dynamics for spatial 3D mapping of wind turbulence. In this paper, the proposed telemetry system models the dynamic response of the helicopter to pilot inputs under artificially generated wind conditions and then uses neural network based models to estimate the air wake distribution. The telemetry system uses a wireless sensor network comprising of sensors such as an Inertial Measurement Unit (IMU), optical trackers, and GPS to measure the dynamics of a flying RC helicopter. The system was trained and calibrated in a climate controlled indoor environment with artificially generated wind conditions. This paper focuses on both hardware and software aspects of the latest iteration of the telemetry system (version 3.0). The presented telemetry system is also tested with a modified YP676 naval training vessel in the Chesapeake Bay area, under a wide range of wind conditions and the results were compared against CFD simulations.

Sampling Optimal Control Based on Discrete Error Expanded System for Carrier Landing

The response ability and robust performance of carrier based aircraft landing control system are very high. The existing literature has not considered the discrete sampling of carrier landing control, but in fact the discrete sampling has a great influence on the control performance of the system. In this paper, the error state of the discrete linear system is expanded, and the controller is obtained from the error expanded system. This method is used to design the landing continuous dynamics optimal controller under sampling. The simulation results show that the system has good control effect on initial height deviation correction, ship air wake suppression and model perturbation suppression. And its steady-state accuracy, dynamic performance and robustness are much stronger than the carrier landing engineering PID control, even though PID control has no sampling in simulation. The backside area of landing drag curve and the direct force effect of the elevator will each cause two non-minimum phase zeros for the aircraft dynamics. The sampling optimal control method can eliminate the two zero points, while the PID control can only eliminate the former zero point.

Automatic carrier landing system based on active disturbance rejection control with a novel parameters optimizer

This paper proposes an active disturbance rejection control (ADRC) scheme for automatic carrier landing system (ACLS) in final approach. To track desired touchdown point (DTP) altitude in the presence of carrier air wake turbulence and deck motion, based on the estimation and compensation of internal dynamics and external disturbances by extended state observer (ESO), outer-loop altitude controller and inner-loop attitude controllers are designed via ADRC. Moreover, tracking differentiator-based deck motion compensation (TD-DMC) is designed to eliminate the inherent phase lag in ACLS. In addition, controller parameters are optimized via a novel Levy flight based whale optimization algorithm (LWOA). Finally, comparative simulations demonstrate that the proposed ADRC-based ACLS has better tracking performance and robustness compared to conventional ACLS.

Is it worth trying to evaluate aircraft landings via passive seismic? The starting points and constraints

This paper discusses the difficulties related to aircraft landing evaluation as a result of pairing of data collected on board and the determination of plane-runway touches. Today’s on-board sensors provide data that is hard to connect with exact landing styles and resulting deformations. Passive seismic can provide valuable tools, because it has the ability to measure the runway vibrations that are proportional to the plane deformations. Even the style of the landing (symmetrical v. asymmetrical) can be caught by distributing more sensors along each side of the runway. Although it is an available and widely used method, it seems that passive seismic has only a small value for a plane landing evaluation today. The difficulties are resulting from the requirement on the runway homogeneity, a frequent runway calibration or noisy effects that will particularly affect the quality of soft landing seismograms. For passive seismic to become a valuable tool, five stages of its implementation are proposed. Aircraft landing is a critical part of every flight (Figure 1). A hard landing can lead to hidden damage that can put the people on board in danger in the following flights. The question is whether it would be useful to evaluate the quality of landings via runway vibrations instead of sensors on board and invest money in long-term tests. The use of seismic sensors in various branches of industry has been spreading over the past decades. They are used for measuring vibrations of bridges, highways or buildings. They are often used in vibroseis trucks for geophysical measurement. The shape of their sweep vibrations is controlled (Tellier et al., 2015). This paper deals with a reverse problem. The vibrations generated by aircraft landings cannot always be controlled. Thus the runway seismic properties have to be well known. There exist recent patents on real-time road or runway conditions monitoring (Friedlander and Kraemer, 2014; Hagelin et al., 2014). Other issues such as the use of residual vibration energy from air wake for production of electricity have been discussed (Agarwal and Ali, 2013). This paper is intended to provoke a discussion about the entry of real-time passive seismic monitoring to the field of airports.

Analysis of Rotor Start-Up and Shutdown on a Sea-Based Oil Rig

A combined rotor transient responses and analysis tools/computational fluid dynamics effort was completed to assess the rotor response in a complex flowfield. A generic helicopter on an oil-rig platform was chosen because the bluff nature of the rig presents unique challenges to rotorcraft operations. The rotor start-up and shutdown simulation model was validated with the test data. Comprehensive studies of rotor transient responses under various wind directions and wind speeds were conducted to establish the safe-operation envelope. The study found that horizontal wind velocities may be accelerated by the oil-rig building corner, and cause significant rotor flap motions. The upwash and downwash flow due to the blockage of the helicopter fuselage to the crosswind had great impact on rotor transient responses, and the peak blade flap responses can be 2.5 times higher than the simulation results when the effect of airframe on air wake was ignored. The influence of unsteady air wake on rotor transient responses was also investigated, and the results showed that it could increase the blade tip deflections by 70% if the unsteady-flow components were included. The sensitivity of the rotor-blade initial azimuth positions for rotor start-up operations was also examined.

Urban Wake-Field Generation Using Large-Eddy Simulation for Application to Quadrotor Flight

A method is presented for using large-eddy simulation to generate urban wake fields for use in studying the effects on the autonomous flight performance of a small quadrotor. The flowfield is solved around a single square building using OpenFOAM and stored in a database accessed by a MATLAB/Simulink flight simulator. Four flight missions are evaluated to compare the difference in performance between wake fields generated by Reynolds-averaged Navier–Stokes and large-eddy simulation solutions. The results of the holding position in a constant freestream wind show both methods produce similar results and can hold position in all three directions within approximately body lengths. When the quadrotor is in or on the boundary of the building wake, the maximum deviation volumes, as calculated when using a Reynolds-averaged Navier–Stokes or large-eddy simulation air wake, can differ by two orders of magnitude. Additionally, the large-eddy simulation air wakes can cause skewed deviations by as much as five to one in a given direction for both holding position and moving along a desired flight path. Since the turbulent large-eddy simulation wake field more accurately reflects the flow physics present within the wake of real-world structures, it is recommended that large-eddy-simulation-generated wake fields be used when designing and testing autonomous control algorithms for multirotor unmanned aerial vehicles on the order of 0.5 m in size and 2 kg in mass.