Blade Vortex Interaction Phenomena Research Articles

COMPUTATIONAL STUDIES OF HELICOPTER AERODYNAMICS

The present research concerns the helicopter aerodynamics and the blade-vortex interaction phenomenon. The computational studies are carried out using the large-eddy simulation approach for subsonic incompressible flow of Reynolds number Re=1.3x106. The helicopter aerodynamics is dominated by the blade-vortex interaction (BVI) phenomenon which is responsible for noise and vibrations. During the helicopter flight, a tip-vortex filament is formed and its interaction with the advancing blade causes the blade-vortex interaction phenomenon. The study shows that the blade-vortex interaction causes oscillations of the aerodynamic coefficients. Due to the turbulence phenomenon, the oscillations exhibit a non-linear behaviour.

Dynamic inflow model for a floating horizontal axis wind turbine in surge motion

Abstract. Floating offshore wind turbines may experience large surge motions, which can cause blade–vortex interaction if they are similar to or faster than the local wind speed. Previous research hypothesized that this blade–vortex interaction phenomenon represented a turbulent wake state or even a vortex ring state, rendering the actuator disc momentum theory and the blade element momentum theory invalid. This hypothesis is challenged, and we show that the actuator disc momentum theory is valid and accurate in predicting the induction at the actuator in surge, even for large and fast motions. To accomplish this, we develop a dynamic inflow model that simulates the vorticity–velocity system and the effect of motion. The model's predictions are compared to other authors' results, a semi-free-wake vortex ring model, other dynamic inflow models, and CFD simulations of an actuator disc in surge. The results show that surge motion and rotor–wake interaction do not result in a turbulent wake or vortex ring state and that the application of actuator disc momentum theory and blade element momentum theory is valid and accurate when applied correctly in an inertial reference frame. In all cases, the results show excellent agreement with the higher-fidelity simulations. The proposed dynamic inflow model includes a modified Glauert correction for highly loaded streamtubes and is accurate and simple enough to be easily implemented in most blade element momentum models.

Numerical Investigation of Inter-Rotor Spacing Effects on Wake Dynamics of Coaxial Rotors

In this Paper, the wake dynamics of coaxial rotors are studied using the high-wake-resolution method. The high-wake-resolution method is a novel combination of the truncated vortex tube model for initial and boundary conditions and the wave-number-extended finite-volume interpolation scheme. The wake resolution of this method is verified by comparison with other general computational fluid dynamics techniques. By using this method, this Paper analyses the wake dynamics of coaxial rotors with different inter-rotor spacing. The aerodynamic characteristics of coaxial rotors are compared with those of single-rotor systems to identify the effect of wake interaction on coaxial rotor blades. Variations of thrust and sectional thrust coefficients indicate that the lower rotor is continuously influenced by the wake of the upper rotor. In addition, the blade/vortex interaction and the wake instability phenomena are observed near the lower rotor. Aerodynamics and wake dynamics are strongly correlated with inter-rotor spacing. Details of important features such as the wake trajectory, miss distance of each rotor, blade-vortex interaction phenomenon, and wake instability at different inter-rotor spacing are discussed.

A fully-coupled CFD/CSD computational approach for aeroelastic studies of helicopter blade-vortex interaction

Blade-vortex interaction (BVI) is one of the main sources of noise and vibrations in helicopters. The aeroelastic response of the blade, to BVI phenomenon, is one of the main factors affecting the helicopter's aerodynamic performance and stability. In the present research we developed and implemented a novel CFD-based, strong (two-way) time-domain coupling, mathematical model, for the numerical prediction of the aeroelastic response of the helicopter blade to BVI. Another novelty, of the fully coupled CFD/CSD approach, is that the fluid flow is solved using the large-eddy simulation (LES) approach. In the present research, the aeroelastic response of a symmetric NACA0012 airfoil to BVI is computed using a two-degree of freedom (2-DOF) mass/spring model. The numerical studies show that the blade-vortex offset distance, (h), plays a key role in the mechanism of BVI and therefore, it influences the aerodynamic coefficients. The computational studies show that the BVI phenomenon diminishes with the increase of the offset distance (h). Also, it was observed that the elastic nature of the airfoil acts as a damper and it diminishes the effect of BVI on the aerodynamic coefficients. Therefore, for a flexible airfoil, the lift coefficient exhibits a decay, at the instant of blade-vortex interaction, when compared with rigid airfoil.

Aeroacoustic Simulation of a Complete H145 Helicopter in Descent Flight

In the past years, the aeroacoustic noise emission of a helicopter became one of the most important, but also challenging issues in helicopter development. The blade vortex interaction phenomenon is one of the dominant phenomena characterizing the helicopter's aeroacoustic footprint, which is insufficiently predicted by low fidelity computational methods. For a high fidelity noise prediction of a helicopter configuration, a multidisciplinary CFDCSD-CAA tool chain has been established at the Institute of Aerodynamics and Gas Dynamics of the University of Stuttgart. With higher order CFD computed noise generation at the near field and the noise convection using a Ffowcs-Williams Hawkings based CAA code, very good agreement to measured aeroacoustic noise in wind tunnel as well as free flight experiments of helicopters is achieved. However, the simulations had been limited to the main rotor's geometry up to now, where some residual deviations to the experiment still exist. In this paper, we present a high fidelity aeroacoustic simulation of a complete helicopter configuration and the benefit compared to an isolated rotor simulation in predicting its aeroacoustic noise emission. Shading and reflection effects are clearly resolved, influencing the behaviour of the helicopter's noise radiation. The simulated aeroacoustic noise emission of the helicopter lies within the experimental variation and shows therefore highly promising results for the next generation of aeroacoustic noise prediction.

Parallel Blade-Vortex Interaction Modelling for Helicopter Rotor Noise Control Synthesis

The present work deals with a methodology for the alleviation of loads and corresponding emitted noise generated by blade-vortex interaction (BVI) phenomena occurring on helicopter main rotors in low-speed, descent flight. It consists of a multi-cyclic, optimal control approach driving higher harmonic blade pitch actuation. First, for the specific flight condition to be examined, the BVI phenomena are simulated as equivalent two-dimensional, multi-vortex, parallel BVI problems and then a local controller methodology is applied for the efficient identification of the closed-loop control algorithm. In order to assess the capability of the proposed control approach to alleviate rotor blade loads and emitted noise, the results of a numerical investigation concerning a realistic helicopter main rotor in descent flight are presented and discussed.

Application of an active device for helicopter noise reduction in JAXA

Important issues in noise problems for current helicopters are described. An active tab (AT) was developed as a new active device for noise/vibration reduction under research cooperation between Japan Aerospace Exploration Agency (JAXA) and Kawada Industries, Inc. The wind tunnel test was conducted in order to investigate the effectiveness of the AT on the aeroacoustic characteristics of a helicopter. From the wind tunnel test, the capability of reducing blade vortex interaction (BVI) noise by an AT was verified. A new control law using instantaneous pressure change on a blade during BVI phenomena was introduced and applied to the wind tunnel testing. This new control law shows reasonable controllability for helicopter noise reduction. Furthermore, in order to analyze noise characteristics, the advanced computational fluid dynamics (CFD) code named JAXA_ov3d was developed in JAXA and extended to include CFD–CSD (computational structure dynamics) coupling by using the beam theory for blade deformation.

Experimental Study of Active Techniques for Blade/Vortex Interaction Noise Reduction

This paper presents the experimental results of the effect of Higher Harmonic Control (HHC) and Active Flap on the Blade/Vortex Interaction (BVI) noise. Wind tunnel tests were performed with a 1-bladed rotor system to evaluate the simplified BVI phenomenon avoiding the complicated aerodynamic interference which is characteristically and inevitably caused by a multi-bladed rotor. Another merit to use this 1-bladed rotor system is that the several objective active techniques can be evaluated under the same condition installed in the same rotor system. The effects of the active techniques on the BVI noise reduction were evaluated comprehensively by the sound pressure, the blade/vortex miss distance obtained by Laser light Sheet (LLS), the blade surface pressure distribution and the tip vortex structure by Particle Image Velocimetry (PIV). The correlation among these quantities to describe the effect of the active techniques on the BVI conditions is well obtained. The experiments show that the blade/vortex miss distance is more dominant for BVI noise than the other two BVI governing factors, such as blade lift and vortex strength at the moment of BVI.

Unsteady Simulations of Bell-Agusta 609 Rotor Undergoing Higher Harmonic Oscillation

The unsteady flowfield of a Bell-Agusta 609 rotor is numerically investigated with an unsteady Reynolds-averaged Navier-Stokes flow solver based on unstructured grid topology. The geometry considered is a three-bladed rotor that is undergoing a higher harmonic blade-pitch oscillation. Methodology is presented to model the multiblade cyclic motion that is superimposed on the basic motion. The unsteady airloads and blade-vortex interaction phenomenon are investigated for hovering and forward flight conditions. Numerical simulations indicate a significant impact of the blade-pitch oscillation on the tip-vortex structure and associated blade-vortex interaction.

Comprehensive Study on Tip Vortex with Lateral Jet Flow

Comprehensive numerical and experimental investigations of tip vortical characteristics were conducted for lateral tip jet flow over a fixed wing as a step to reduce blade vortex interaction noise. The tip vortex of a NACA0012 blade was measured and visualized for the fundamental study of tip vortical flow, and the results were compared with numerical data as a validation of numerical solvers. Three-dimensional compressible Euler/Navier-Stokes codes were used to calculate the effect of jet flow from the tip of an OLS (modified BHT 540) fixed blade at various freestream velocities and jet conditions. The results show that the jet flowing from the wing tip can diffuse the tip vortex enlarging the core size of tip vortex and weakening its strength. When applied to the blade vortex interaction phenomena, this enlarged and weak vortex can produce a lower pressure gradient on the blade surface, which means that the jet flow can effectively reduce blade vortex interaction noise.

Optimal Control of Aeroacoustic Noise Generated by Cylinder Vortex Interaction

This paper presents an optimal control formulation and solution for an idealized Blade Vortex Interaction (BVI) problem. This problem consists of the interaction of an inviscid vortex pair with a circular cylinder in a steady Mach 0.3 uniform flow with wall-normal velocity used as control on the cylinder surface. This model problem captures the fundamental noise generation process of the BVI phenomena while mitigating many of the complexities of the full rotorcraft problem. The optimal control problem is solved using a gradient based method where gradient information is computed from a continuous adjoint analysis of the governing unsteady Euler equations. The BVI wave packet is targeted by defining an objective function that measures the square amplitude of pressure fluctuations in an observation region over a time interval of interest. When the observation region encloses the entire flow a 6 dB reduction in overall BVI noise is obtained. When the observation region is limited to the region of greatest BVI noise, a 13dB reduction is obtained. The optimal control, unlike most common mitigation methods, does not target the interaction directly — instead, the computed boundary control modulates the potential flow about the cylinder, producing a wave packet of the correct amplitude and phase to approximately cancel the BVI noise in the observation region.

UH-60A helicopter rotor airloads measured in flight

The flight test phase of the Nasa-Army UH-60A Airloads Program was completed in February 1994. The objective of the project was to measure the airloads in flight on an UH-60A Black Hawk using 221 blade pressure transducers installed in nine radial arrays. In addition, 21 transducers were installed along the blade leading edge to better identify blade-vortex interaction phenomena. This paper briefly describes the program's background and history, instrumentation, and data acquisition. Airloads data are presented for two test conditions with different levels of complexity: steady, level flight and a diving turn. These flight data provide useful information of relatively simple and complex nonlinear flow for the validation of theoretical analyses. Evidence of shock waves, reverse flow, dynamic stall and blade vortex interactions are presented.

Wavelet analysis of blade–vortex interaction noise

Fourier analysis techniques for analyzing helicopter noise signals have been shown to be an adequate, yet inefficient tool for the design of helicopter noise control systems. Impulsive sources of noise, such as blade–vortex interaction (BVI) noise cannot be localized in time without collecting relatively large amounts of data at high sampling rates. Attempts to minimize the BVI phenomenon through helicopter blade control schemes in the Fourier domain are inherently inefficient since the control matrices must account for the large number of Fourier components needed to represent the signal. Time-frequency representations, such as the wavelet representation, offer an efficient alternative to these Fourier domain methods. Wavelet analysis has been performed on an ensemble of helicopter noise signals. An interactive wavelet program was designed using quadrature mirror filters developed by S. Mallat. Using the bandpass representations of the signal, BVI noise was isolated and extracted from the overall helicopter noise signature. Future research will integrate the wavelet decomposition into an adaptive frequency domain controller for BVI noise minimization.a)With NASA Ames Res. Ctr., MS T-42, Moffett Field, CA 94035.