Ship Airwake Research Articles

Coupled Simulations of Atmospheric Turbulence-Modified Ship Airwakes and Helicopter Flight Dynamics

The influence of a neutral atmospheric boundary layer on a one-way coupled dynamic interface simulation is evaluated. The effort is performed within the context of time-accurate large-eddy simulations of Simple Frigate Shape 2, which is immersed into a turbulence-resolving atmospheric inflow that emulates the dynamics of a realistic atmospheric boundary layer. Several baseline cases are used to represent a traditional approach of modeling the incoming wind as a constant, uniform inflow. When comparing the atmospheric boundary layer-resolving to the baseline simulations, it is observed that the velocities in a ship airwake are considerably different when formed from an atmospheric boundary layer. The unsteady computational fluid dynamics solution is then saved as a database and used as external disturbances for flight dynamics analysis. Such an approach is common in piloted flight simulation. The pilot workload is evaluated using a helicopter approach scenario and multiple hover locations. Such cases are evaluated, and it is found that the large scales present in the atmospheric boundary layer notably disturb the helicopter. The disturbances result in increased controls input and aircraft movement. Frequency-domain analysis of the increased stick movement suggest that the atmospheric boundary layer impacts the 0.1–2 Hz stick controls, which comprise the frequency range known to affect pilot workload.

Effect Analysis of the Hangar Rear Edge Curvature on the Ship Airwake

In order to examine the effects of hangar rear edge curvature on ship airwake, the SFS2 ship configuration is modified by changing rear vertical surface of hangar into curved surface. Airwake with different configurations subject to free-stream of 25.7m/s at headwind are simulated using commercial CFD solver ANSYS FLUENT, and the traditional Reynolds- average Navier-Stokes k-ε two equation turbulence model is applied. A very good agreement of experiment results in wind tunnel with predictions from the CFD demonstrate that both the FLUENT code and the meshes generated by ICEM perform well in simulating ship airwake. The comparison of different hangar rear edge curvatures shows that the curvature modifications improved airwake over the flight deck region of SFS2 by reducing mean downwash and turbulence intensity. 90° is the best angle corresponding to SFS2 hangar rear edge curved arc, which will reduce pilot workload during launch and recovery operations.

Combined numerical and experimental simulations of unsteady ship airwakes

To aid pilot training for shipboard helicopter operations, computational fluid dynamics (CFD) is increasingly being performed to model ship airwakes. The calculated velocity field data are exported to the flight simulator as look-up tables. In the Canadian context, work to expand ship airwake simulation capabilities for future use in flight simulators is currently being done using the open-source OpenFOAM. The current paper reports on the progress of this work using the simple frigate shape 2 (SFS2), which is a highly simplified ship geometry, to validate the method for low-sea-state (also referenced to as static) cases. By employing delayed detached eddy simulations (DDES), OpenFOAM was able to compute the unsteady ship airwakes well compared to experimental data and other references. After validation, OpenFOAM was applied to the Canadian Patrol Frigate (CPF), a more representative example of a naval vessel. Hybrid structured and unstructured grids were used because of the complexity of the CPF geometry. The agreement between the computed and the experimental results for the static CPF was reasonable, which built a solid foundation supporting further development of simulation for the CPF in motion.

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.

An Evaluation of the Effects of Resolved Shear-Driven Atmospheric Turbulence on Ship Airwakes

An Evaluation of the Effects of Resolved Shear-Driven Atmospheric Turbulence on Ship Airwakes

Impact Anlysis of Bow Flap on LHA-1 Ship Airwake

The sharp 90°corner of bow found on American Tarawa class general amphibious assault ship LHA-1 will produce large separated region in the airwake over the ship, and the turbulence in the separation region seriously affects the operation safety of the helicopter. In order to reduce the separation in bow region of the LHA-1, and optimize the helicopter operating environment, the numerical simulation method is used to study the influence of the bow flap on the airwake of LHA-1. The study results show that: the ANSYS k-ε two equation model based on the hybrid grid can be used to predict the steady-state characteristics of the ship airwake; the bow flap can improve the flow field downwind of the flap installation position, and the impact range can cover two spots; in headwind, the optimal installation angle of the bow flap is 15°~25°. The research results of this paper can provide modification and improvement reference for the amphibious assault ship in-service and under construction, so as to achieve the purpose of optimizing the ship airwake, thereby improving the safety of helicopter landing process and extending the service life.

Coupled flight dynamics and CFD – demonstration for helicopters in shipborne environment

ABSTRACTThe development of high-performance computing and computational fluid dynamics methods have evolved to the point where it is possible to simulate complete helicopter configurations with good accuracy. Computational fluid dynamics methods have also been applied to problems such as rotor/fuselage and main/tail rotor interactions, performance studies in hover and forward flight, rotor design, and so on. The GOAHEAD project is a good example of a coordinated effort to validate computational fluid dynamics for complex helicopter configurations. Nevertheless, current efforts are limited to steady flight and focus mainly on expanding the edges of the flight envelope. The present work tackles the problem of simulating manoeuvring flight in a computational fluid dynamics environment by integrating a moving grid method and the helicopter flight mechanics solver with computational fluid dynamics. After a discussion of previous works carried out on the subject and a description of the methods used, validation of the computational fluid dynamics for ship airwake flow and rotorcraft flight at low advance ratio are presented. Finally, the results obtained for manoeuvring flight cases are presented and discussed.

Rotor-State Feedback Control to Alleviate Pilot Workload for Helicopter Shipboard Operations

This paper presents an integrated flight control system with rotor-state feedback for helicopter approaching to the ship deck. First, a high-order nonlinear flight dynamics model coupled with the reconstructed ship airwake turbulence is developed. Then, a rotor-state feedback control law is developed and integrated into the baseline explicit model-following control system to form an integrated control system. The feedback gains of body and rotor states are designed together to improve the disturbance rejection over the ship airwake frequencies (1 to ). A feedforward compensator is introduced to improve aircraft response bandwidth. Furthermore, a method to adjust the command model is developed for further improvement of the response bandwidth. Finally, the effectiveness of the integrated control system is demonstrated with linear analysis, and pilot workload is evaluated with nonlinear simulation. In comparison to the baseline control system, the integrated control system improves both the disturbance rejection over the ship airwake frequencies and attitude response bandwidths of rolling and pitching motions. The pilot workload for the roll and pitch attitude regulation in ship airwake turbulence is reduced by 45 and 26%, respectively.

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.

Rotorcraft downwash impact on ship airwake: statistics, modelling, and simulation

ABSTRACTHelicopter downwash impact on ship airwake is addressed from a three-pronged approach: (1) Analysis of one-point statistics of autospectrum and two-point statistics of cross-spectrum and coherence from a Computational Fluid Dynamics database of flow velocities effected by helicopter downwash and shipboard airwake; (2) Development of a mathematical framework for extracting interpretive turbulence models in closed form from these autospectral statistics; and (3) Simulation through white-noise-driven filters for the extracted models. The framework begins with an earlier-exercised perturbation-type series expansion of the autocorrelation for all three velocity components, where the first term of the series has a form of the von Karman longitudinal or lateral correlation function. After transformation into equivalent series of autospectrum, the coefficients in the series are evaluated by satisfying theoretical constraints and fitting a curve on a set of selected autospectral data points generated from the database. The framework represents a sensible combination of series expansion, exploitation of a database, and theoretical constraints to provide a foothold on airwake-downwash phenomenon for engineering analysis. It ensures that the extracted model and the autospectral data points have the same time scale, mean square value, and asymptotic decay according to the Kolmogorov –5/3 Law. The framework's strengths and weaknesses, and its major advancement over the earlier series-expansion schemes are also addressed. Finally it is shown that downwash increases airwake energy (mean square value) by one order of magnitude, and almost all of this airwake-downwash energy is concentrated within the bandwidth (0.16 <f(Hz) < 1.6) that affects flight mechanics.

Wireless telemetry system for real-time estimation of ship air wakes with UAVs

This paper presents a wireless instrumentation system developed for real-time estimation of air turbulence patterns arising from the interaction of wind with any structure under consideration, which is an important study in the aerospace industry. In particular, this paper focuses on the application of the proposed system in a naval research problem for off-board measurement of ship air wake patterns using an instrumented radio controlled (RC) helicopter. We propose the use of an Inertial Measurement Unit (IMU) as a sensor to measure air wake in the form of induced vibrations on the helicopter while it maneuvers through regions of active air wake. The proposed system makes use of Back Propagation Neural Networks to compensate for the vibrational noise contributed by pilot inputs. The instrumentation system was integrated and tested on a modified training vessel in the Chesapeake Bay, which provided a wide range of wind conditions.

Model Predictive Control for Helicopter Shipboard Operations in the Ship Airwakes

This study investigates the model predictive control design for helicopter shipboard operations in the presence of ship airwakes and rough seas. A control oriented helicopter model that captures the essential dynamics of a generic helicopter, including fuselage dynamics, blade flapping and lead-lagging, and main rotor inflow in the implicit nonlinear ordinary differential equations form, is used for model predictive control design and evaluation. The control equivalent turbulence inputs model and the computational fluid dynamics data of the airwake are incorporated into the nonlinear helicopter model to simulate the airwake’s effects on the helicopter. The feasibility of the model predictive control design is investigated using two case studies: automatic deck landing on a moving ship in sea state 5 with the control equivalent turbulence inputs airwake model, and lateral reposition toward the ship in different wind-over-deck conditions extracted from the computational fluid dynamics airwake data. To improve the overall model predictive control performance, an updating scheme for the internal model of the model predictive control is proposed for helicopter shipboard operations using linearization around operating points. Nonlinear simulations are also performed to explore the capabilities of the model predictive control in handling system constraints, flight deck tracking, and disturbance rejection.

Entropy-based detached-eddy simulation of the airwake over a simple frigate shape

The wind past the ship superstructure produces an unsteady turbulent airwake which has a significant effect on aircraft performance and consequently pilot workload during ship landing process. Computational fluid dynamics simulations of a generic simple frigate shape ship airwake have been performed using the entropy-based detached-eddy simulation method. The results were compared with the steady-state Reynolds-averaged Navier–Stokes calculations and the wind tunnel data, indicating the capability of entropy-based detached-eddy simulation to resolve the unsteady large-scale turbulent features. An analysis of the airwake flow topology at headwind condition highlights the vortex pairing process which dominates the flowfield above the deck. Additionally, the influence of hangar-door state (open, closed or half-open) on the airwake was also investigated in detail. Determination of this complex flow can assist in the definition of safe ship–helicopter operating limits and future ship design.

Design, Simulation, and Experimental Results for Flexible Rotors in a Ship Airwake

First, an overview of the design and construction of a th Froude-scaled flap-articulated rotor system that is immersed in a ship airwake flowfield is presented. Experimental challenges that were identified during and following the use of this system in an experimental study and their impact on the data are discussed. The experimental rotor system is presented as a useful testbed for theories and models of flexible multibody systems with gyrating beams undergoing large deformations. Second, the flap-articulated Froude-scaled experimental rotor system is tested in a scaled ship airwake environment using the National Research Council wind tunnel. Based on experimental and simulation data, conclusions are drawn regarding the influence of the aerodynamic environment and the rotor operation parameters on the rotor blade elastic deflections during the engagement/disengagement phase. It is concluded that the parameters of the engagement/disengagement profile play a minimal role in the occurrence of large elastic deflections, known as the blade sailing phenomenon. Increasing or decreasing the collective pitch setting along with increasing the wind speed and ship deck roll angle is found to amplify the blade sailing phenomenon. Additionally, it is further verified that, compared to an unsteady aerodynamic model, the nonlinear quasi-steady aerodynamic model with Mach number effects on the stall point is sufficient for blade sailing studies; however, the former is shown to be more accurate, particularly at higher wind speeds. The experimental data are also used to successfully validate developed multibody dynamics tools.

Exploratory Investigation into Rotorcraft Pilot Strategy and Visual Cueing Effects in the Shipboard Environment

The present work reports the results of a research investigation into the handling qualities of a helicopter conducting a ship approach and landing task. The investigation was performed via fixed-base, pilot-in-the-loop flight simulation and included six test pilots with extensive operational and test experience in the shipboard environment. Representative approach/landing tasks were flown with three different response types to a field landing zone and to multiple spots on an amphibious assault ship. The ship was stationary, and there was no simulated ship airwake, so the fundamental differences between the field and ship approaches were limited to visual cueing. Traditional subjective handling qualities ratings were supplemented with multiple other analysis techniques, including pilot eye-tracking analysis and pilot control activity analysis. The results show that there are fundamental differences in the pilot gaze patterns between field and ship approaches that are caused by the different cueing environments. Additionally, subjective assessments indicated that pilots preferred higher degrees of aircraft stabilization in the shipboard environment than in the field environment, though there were not significant differences between pilot control techniques in these environments for a given aircraft response type.

A spectral analysis of laser Doppler anemometry turbulent flow measurements in a ship air wake

Helicopters are one of the most important tactical elements in maritime operations. The necessity to improve the conditions in which the landing and take-off operations are carried out leads to the study of the flow that separates from the ship’s superstructure over the flight deck. To investigate this flow a series of wind tunnel experiments have been performed by testing a sub-scale model of a frigate. Measurements of the flow velocity have been taken by means of laser Doppler anemometry in five points that simulate the last path of the landing trajectory. The data obtained in these experiments is analyzed in the frequency domain where the corresponding spectra are calculated. Onboard measurements from an actual full-scale frigate are analyzed and compared with the wind tunnel results. Conclusions obtained consist of a series of illustrative values of turbulent energy frequency ranges, which are valuable for any study in this field. The comparison shows a similarity between both experiments, reasserting the role of the wind tunnel measurements in these kind of studies.

A simplified approach to assessment of mission success for helicopter landing on a ship

In this paper, operation of a helicopter near a ship is statistically investigated to provide an estimate of the probability of successfully recovering manned rotorcraft on the deck of a moving ship. To this end, pitch, roll, and heave motion of the ship are calculated according to sea states. In addition, effect on dynamics of the rotorcraft from ship airwake is also considered in a simplified way. By assuming that a helicopter can land on a ship if a pilot maintains relative position and attitude difference within the safe boundary for a given time, the operational limits are probabilistically determined in terms of pilot’s workload for a specified mission. The simulation environment consists of linearized helicopter dynamics, an optimal pilot model, effect of turbulent ship airwake, and ship motion. Control activities of the piloted helicopter for position holding with respect to the moving ship are transformed to generalized workload ratings. Simulation results show the proposed approach can evaluate and predict mission success rates for various operational combinations, allowing implementation into the overall effectiveness assessment of the ship and ship-helicopter combination.

Comparison of Various Spatial Discretization Schemes in Numerical Simulation for Ship Airwakes

For numerical simulation of ship airwake by CFD, based on the use of an unstructured grid, the k-ε turbulence model and SIMPLE algorithm, the characteristic features of complex fluid flows eg recirculation zones and strong vortex fields in the aircarft operating region of a generic 3D frigate model was presented. The accuracy of the predication was checked by performing calculations on different grid sizes and comparing with wind-tunnel flow visualization data. A comparison of several typical spatial discretization schemes was performed. y+ values were also tested. The general features of the flow predicted in this paper compare reasonably well with experimental data. However, CFD simulation produced a higher velocity in the vicinity of vortex zone when compared to experimental data. Obvious differences exist between results by first-order upwind scheme (power law scheme) and second-order upwind scheme (QUICK scheme, third-order MUSCL scheme). Second-order upwind scheme (QUICK scheme, third-order MUSCL scheme) are recommended for the CFD simulation of ship airwakes with a modest increase in computational cost. y+ values from o (10) to o (1000) can all be accepted for the CFD simulation of ships (e.g., SFS1 ) with Reynolds number 108 or more.

Numerical Study of Helicopter Rotors in a Ship Airwake

Operating helicopters in a naval environment is challenging because it imposes a pilot workload significantly higher than that during land-based operations. The aerodynamic interaction between the aircraft and the ship wake is known to play an important role in increasing the pilot workload, hence reducing the aircraft capability as a result of maintaining safety. As a further step toward numerical prediction of ship/helicopter operational limitations, computational-fluid-dynamics simulations are conducted for the Canadian Patrol Frigate. The effect of the rotor is included in the simulation, first using an actuator-disc method together with steady calculations, then using rotor blades and the unsteady Reynolds-averaged Navier–Stokes equations. Results using the actuator-disc method demonstrate the importance of coupling effects on the wake and rotor inflow when the rotor is operating close to the ship and therefore the invalidity of superposition methods. The case of a Sea King helicopter main rotor hovering above the deck just before touchdown is reproduced to overcome the limitations of steady calculations and the actuator-disc method. Predictions of rotor thrust compare well with the experimental data available and give confidence in the results. The findings highlight the differences in rotor loading between forward flight and near-deck operation. The possibility of using such simulations for determining safe flight envelopes is discussed as part of the future work.

Effects of visual and motion cues in flight simulation of ship-borne helicopter operations

Good visual cues are necessary in the flight simulation of ship-borne helicopter operations. Operating in a degraded visual environment cues has a negative impact on pilot workload and task performance. However, the need for motion cues in piloted flight simulation is still a widely debated issue. This paper describes a preliminary piloted flight simulation study into the effects of visual and motion cues on the operation of ship-borne helicopters and pilot workload. Unsteady CFD airwakes have been computed and integrated into the FLIGHTLAB modelling and simulation environment with a simulated rotorcraft model, configured to be representative of an SH-60B helicopter. A series of ship-deck landing and hover manoeuvres have been conducted using the University of Liverpool’s HELIFLIGHT-R motion-base flight simulator representing different visual and motion cues, for a range of ship airwakes and sea states (ship deck motions). The usable cue environment (UCE), handling quality and pilot workload ratings were assessed using visual cue ratings, handling quality rating and the Bedford workload rating scale and Deck Interface Pilot Effort Scale. This paper presents the results from simulation trials with two test pilots examining the effect of the simulation cueing on task performance and workload. Visual cues were found to have a significant impact both on the UCE ratings and pilot workload ratings. In degraded visual environments, the pilot’s ability to make corrections in attitude and translational rates was reduced. Pilots experienced higher workload in terms of compensatory control inputs to complete the same mission task as compared to operations in a good visual environment. Analysis of the pilots’ workload ratings and control activity show that motion cueing can cause differences in pilot perceived workload. For the simulation of ship-borne operations, the impacts of motion cueing are dependent on other simulation conditions, which include visual environments, airwake, sea states and ship deck motion. The effect of motion cueing on pilot workload and control activity was found to be more significant when the visual cueing was degraded. The variations in pilot workload ratings and control activities under different motion and visual cues indicate that the Ship Helicopter Operating Limits can be affected by the simulation cueing fidelity.