Browse Topic: Mathematical analysis
The current effort presents novel investigations of rotor-wake–surface interactions for the Dragonfly lander, NASA's rotorcraft lander to explore Titan. The numerical framework couples unsteady RANS with blade-element and virtual disk rotor models and a coupled Lagrangian particle tracking method to examine rotor–ground interactions and brownout. Simulations span a range of complexity, from isolated rotor benchmarks and rotor pairs to full eight-rotor configurations without a fuselage and the eight-rotor configuration with a simplified Dragonfly fuselage. To quantify model fidelity and near-ground shear, blade-resolved simulations of the isolated rotor are performed using Spalart–Allmaras and Reynolds Stress turbulence models with vorticity confinement, demonstrating that virtual blade models under-predict tip-vortex strength and local inflow distortion but reproduce wall shear reasonably well, whereas blade-resolved RSM solutions yield higher peak shear levels relevant to brownout prediction. These findings improve understanding of planetary rotorcraft aeromechanics and sediment transport in ground-effect while supporting ongoing efforts to assess environmental risks for Dragonfly operations and inform multi-rotor VTOL design for terrestrial applications.
This paper investigates an output-based approach for tiltrotor whirl flutter bifurcation analysis. The approach uses free decay output data for a quantity of interest at various forward speeds to estimate the system's recovery rate to equilibrium while capturing its variation with amplitude. The recovery rate is then extrapolated to predict the bifurcation diagram, which gives the limit-cycle oscillation amplitude for the quantity of interest as a function of the forward speed. The approach is demonstrated using output data from transient simulations of a notional tiltrotor model with polynomial structural nonlinearities. The approach accurately predicts the tiltrotor whirl flutter speed and limitcycle oscillation amplitudes while only requiring two free decays. This approach can facilitate whirl flutter bifurcation analyses of tiltrotor systems exhibiting nonlinear dynamics.
ABSTRACT In this work, iced rotors are studied to develop insight in the potential of acoustics-based ice detection. Based on the HMB CFD solver, approximate iced shapes are used and results are analyzed using the FW-H method. Several candidate monitoring positions are assessed for acoustic sensors to be placed on the helicopter fuselage. The influence of ice on the aero-acoustic characteristics of a rotor is calculated, and parameters such as the ice amount and the icing position on the blade are quantified.
The engineering model determining the onset of Vortex Ring State (VRS) was applied to eVTOL aircraft, and the effect of different landing trajectories and aircraft drag was investigated. Next, the new model to compare the VRS susceptibility according to the different blade geometries and trajectories is proposed by extending Ahlin & Brown's model to incorporate the two-dimensional thrust and inflow distribution on the rotor disc. For validation, two different trajectories crossing the boundary of the onset of the VRS were simulated, and the results were compared with the Vorticity Transport Method (VTM). Furthermore, the disturbance distribution of moderately and highly twisted blades are compared. The extended model can capture the physical phenomena by the distribution of the disturbances and reflect the effect of blade geometries and trajectories. It is essential to investigate the model further through a correlation analysis using experiments or numerical analysis.
This paper investigates a sliding-window matrix pencil method for predicting flutter points and limit-cycle oscillation amplitudes of nonlinear aeroelastic systems that experience whirl flutter. The approach applies the matrix pencil method to a short time window that slides along the free decay of a quantity of interest, quantifying the variation in the system's recovery rate to equilibrium with amplitude. The recovery rates at each amplitude and various forward speeds are extrapolated to predict the critical forward speed of zero recovery rate at those amplitudes. This process yields a set of limit-cycle oscillation solutions that can be visualized as a bifurcation diagram. The approach is demonstrated using output data from transient simulations of a propeller-nacelle test case with hardening structural nonlinearities. The impact of each parameter in the sliding-window matrix pencil method is first characterized via sensitivity analyses. Next, the bifurcation diagram is predicted using the recovery rates for the optimal parameter values. The results are compared with direct time marching and with the extrapolation of recovery rates estimated from envelope functions. The proposed method accurately captures the bifurcation diagram using two pre-flutter transient simulations with no need for envelope functions. This approach shows promise for output-based bifurcation analysis of nonlinear aeroelastic systems exhibiting limit-cycle oscillations associated with whirl flutter.
Helicopters in high-speed forward flight often generate High-Speed Impulse (HSI) noise, presenting a major challenge for noise control and narrowing the range of helicopter use. This paper proposes a novel method for active noise reduction by adjusting the rotor diameter length, effectively delaying HSI noise onset and reducing HSI noise impact. Utilizing the CLORNS solver and the Ffowcs Williams-Hawkings (FW-H) equation, this approach was tested on the AH-1G rotor through simulation analysis. The study simulated the rotor's dynamic diameter length changes, analyzing the effect of crucial parameters on the sound field. Results indicate that this method significantly controls the production of rotor high-speed pulse noise, achieving a noise reduction of up to 2dB at critical operational points. This research aids in formulating specific rotor noise control laws and expands the range of scenarios for helicopter usage.
ABSTRACT
Abstract Transient numerical simulations are conducted over a NACA 0012 airfoil with triangular protrusions at a Reynolds number (Re) of 100000 using the γ-Reθ transition Shear Stress Transport (SST) turbulence model. Protrusions of heights 0.5%c, 1%c, and 2%c are placed at one of the three locations, viz, the leading edge (LE), 5%c on the suction surface, and 5%c on the pressure surface, while the angle of attack (AOA) is varied between 0° and 20°. Results obtained from the time-averaged solution of the unsteady Navier-Stokes equation indicate that the smaller protrusion placed at 5%c on the suction surface improves the post-stall lift coefficient by up to 59%, without altering the pre-stall characteristics. The improvement in time-averaged lift coefficients comes with enhanced flow unsteadiness due to vigorous vortex shedding. For a given protrusion height, the vortex shedding frequency decreases as the AOA is increased, while the amplitude of fluctuations in lift coefficient increases as the protrusion height is increased or as the AOA is increased. Nevertheless, mitigation of static stall phenomena is observed for most configurations investigated, and this finding can be beneficial for the design of Unmanned Aerial Vehicles (UAVs) and Micro Aerial Vehicles (MAVs). The enhancement in the vortex shedding frequencies due to triangular protrusion can be utilized for Vortex-Induced Vibrations (VIVs)-based energy generators.
This paper presents an experimental-numerical investigation on the axial strain of highly-curved blades. Models of the blades were analyzed using a 2-D finite element code, SectionBuilder, coupled with a comprehensive analysis code, Dymore, and first validated using strain measurements under a static tip load. Frequency responses were obtained under an impulsive load for each of the blades with multiple boundary conditions and compared with the numerical models. Experimental measurements under centrifugal loading from 0 up to 3300 RPM in a vacuum showed the effect of curvature on the axial strain, with significant bending strains observed in the responses for the curved blades that were also well captured by the numerical analysis. The present analysis shows that even moderate levels of out-of-plane curvature significantly increases the strain magnitudes, while higher levels of in-plane curvature have a much smaller impact.
Project BloStEr (Block Structures - Mechanical joining innovations to replace conventional fasteners in aerostructures) is being realized within consortium between PZL Mielec and research partner Lublin University of Technology. In the project, a new approach for mechanical assembling of thin - walled structures is being realized by developing geometry of shaped connections for joining different materials. The scope of work cover 3 configurations of joints: metal + metal (aluminum alloys), metal + composite (hybrid) and composite + composite. The main idea is to eliminate or reduce connecting elements like rivets, screws, adhesives, or welding. The design of this approach required these mechanical connections to be analyzed using FEM (Finite Element Method) tools and verified numerical models in laboratory tests. In parallel with the design task the technology limitations have been studied for manufacturing of thin-wall integral elements made of carbon woven composites using autoclave technology, dedicated tooling and appropriate polymerization parameters. A critical element is the achieving the desired shape accuracy to get optimal connection properties.
Dynamic stall is a highly complex phenomenon characterized by unsteady massive separated flow. It limits the flight envelope of helicopters by generating vibrations and large dynamic loads which can lead to fatigue and structural failure of blades. Dynamic stall involves several mechanisms which make the numerical prediction of stall difficult and the understanding of the phenomenon still incomplete. A loose coupling methodology between a Computational Fluid Dynamics and a Comprehensive Analysis codes is used to simulate the problem. Three stalled flight conditions have been selected in the wind tunnel 7A rotor test data to investigate the RPM effect on the dynamic stall onset and the related mechanisms. The lower the RPM, the more severe the stall is. A double stall has been observed on the lowest RPM case. The coupled simulations are in satisfactory agreement with experiment and are used to identify the mechanisms leading to stall. Simulations indicate that the blade-vortex interaction is an important factor in triggering the different stall events in these configurations.
Aerodynamic interactions between the rotor and the empennage can have a significant impact on steady and unsteady loads and often result in challenges in a rotorcraft design phase. In the present work, numerical analysis of rotor-empennage aerodynamic interactions were compared to full-scale flight test data with respect to steady and unsteady interactional aerodynamic effects. The flight tests provided loads for a low-empennage and a T-Tail configuration for various forward flight velocities. For the T-Tail configuration, additional pressure sensors provided validation data for steady and unsteady interaction effects. The numerical analysis was focused on an unsteady panel method, complemented by high-fidelity CFD/CSM-coupling results for a level flight state. Furthermore, a supplemental validation of the unsteady panel method was performed against an isolated wing-vortex interaction experiment. The flight test data revealed a strong asymmetry in mean empennage loads, which increases with forward flight velocity. The numerical analysis showed coherent results with a slight over-prediction in high-speed. The T-Tail configuration is furthermore subject to 3D effects between the vertical- and horizontal tail. These effects influence pressure and load-distributions on the T-Tail, which was captured by both numerical methods. The general characteristics of the unsteady pressure signatures were captured by both methods. The panel method showed slightly better representation of amplitude.
ABSTRACT Helicopter Emergency and Medical Service (HEMS) requires a specially designed cabin interior that can transport patients quickly to a full capacity hospital. During the transportation, a medical crew sustains the health condition of the patients using life-support equipments, hence the quality and safety of the service may depend on the vibratory level experienced by patients and crew. However, the bare dynamical response of the airframe can lead to erroneous evaluation of vibratory level and exposure. In fact crew, patients and medical equipments, ı.e. subjects of HEMS, dynamically interact with the helicopter through interfaces such as seats, handles, stretchers and flexible supports. For this reason, the design of a low vibration HEMS vehicle requires numerical analysis of the coupled helicopter-interface-subject system, and the capability to effectively and efficiently run the analysis for a large set of possible configurations to achieve optimal positioning. A viable tool should be able to formulate high-fidelity rotorcraft aeroservoelasticity, easily connect additional dynamical systems representing the dynamics of human and equipment and their interfaces, and calculate the vibration performance of the resulting models. This work presents an effective way of evaluating the vibratory performance of medical helicopters. The approach is illustrated on a medium weight helicopter by adding dynamical models of a human resting on a seat, a recumbent person lying on a stretcher, and medical equipment mounted on flexible supports at its ends.
ABSTRACT Helicopter reaches its flight domain limit at high-thrust forward flight or maneuver with high load factors because of dynamic stall. This phenomenon is due to complex unsteady three-dimensional flow separation mechanisms that occur on the retreating blade. These flow separations can be of different natures depending on the flight conditions. This paper proposes to investigate numerically two significantly different cases of dynamic stall on a helicopter rotor in forward flight. The results show that the numerical simulation can capture the variations of section pitching moment associated with dynamic stall, for each of these two flight cases. A deep analysis of the numerical results allows to identify distinct flow separation regions appearing on the rotor disk. Similarities and differences are highlighted between the dynamic stall characteristics of these two rotor flight conditions.
This technique for compensating the gravitational attraction experienced by a test-mass freely floating onboard a satellite is new, and solves an important problem that all gravitational wave missions face. Its application to the geostationary Laser Interferometer Space Antenna (gLISA) mission concept addresses and completely solves an important noise source: the gravity-gradient noise.
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