Free-wake Analysis Research Articles

A parallel, object-oriented framework for unsteady free-wake analysis of multi-rotor/wing systems

The development, validation, and applications of an object-oriented free-wake solver for multi-rotor and fixed-wing systems are outlined here. Advantages of utilizing the object-oriented philosophy for modeling the multi-rotor/wing free-wake problem are described. To explore the feasibility of utilizing conventional desktop workstations, the vortex lattice methodology’s time complexity is examined as an evolving n-body problem and the advantage gained from employing wake roll up models is demonstrated. Modifications in typical formulae to leverage features of multi-core systems and improvements in computational performance obtained from these modifications are illustrated using a roofline analysis. The free-wake solver incorporating these optimizations is then employed—after extensive validation—to simulate a distributed propulsion aircraft and a multi-rotor urban mobility vehicle. The paper also proposes a novel lattice skew parameter for monitoring instabilities in wake solutions commonly observed in rotorcraft free-wake analysis.

Improved Mars Helicopter Aerodynamic Rotor Model for Comprehensive Analyses

The Mars Helicopter is part of the NASA Mars 2020 rover mission scheduled to launch in July of 2020. Its goal is to demonstrate the viability and potential of heavier-than-air vehicles in the Martian atmosphere. The low density of the Martian atmosphere and the relatively small-scale rotor result in flows with very low Reynolds number, reducing the lifting force and lifting efficiency, respectively. This paper describes the generation of the improved Mars Helicopter aerodynamic rotor model. The goal is to generate a performance model for the Mars Helicopter rotor using a free wake analysis in CAMRADII. The improvements in the analysis are twofold and are expanded on from two prior publications. First, the fidelity of the simulations is increased by performing higher-order time-accurate OVERFLOW simulations allowing for higher-accuracy aerodynamic coefficients and a better understanding of the boundary-layer behavior. Second, a model is generated for the testing conditions in the 25-ft-diam Space Simulator at the Jet Propulsion Laboratory, allowing for better correlation of rotor performance figures. The higher temperatures in the experiment are expected to give conservative performance estimates, as they give rise to an increase in speed of sound and decrease in observed Reynolds numbers.

A novel hybrid free-wake model for wind turbine performance and wake evolution

Abstract A new free-wake analysis for wind turbine aerodynamics is developed to accurately predict turbine performance and downstream wake evolution. A key feature is the Constant Circulation Contour Method (CCCM) which is a novel free-wake model for wind turbine wakes. This method characterizes a turbine wake by a number of resultant vortex filaments, avoiding the numeric artifact of a vortex lattice comprised of trailing and shedding vortices. The natural capture of wake roll-up and distortion using CCCM is illustrated and its computational complexity is demonstrated to be lower than Vortex Lattice Method (VLM). For accurate blade downwash calculation, a hybrid free-wake model is developed by combining CCCM with a VLM wake immediately behind turbine blades, which transitions to CCCM further downstream. Important properties of this hybrid wake are discussed and optimized to enhance model accuracy and efficiency. Blade static stall and unsteady effects are included. The hybrid model is validated through comparison to the wind tunnel experiments UAE Phase VI and MEXICO. Simulation examples are presented showing the utilization of this free-wake analysis to investigate wake steering, demonstrating the potential application of this method to wind farm scale wake simulations.

Generalized Aerodynamic Modeling of Dynamic Wake Curvature for Open Rotors With Slender Blades

This paper elaborates on the theoretical development of an analytical approach, capable of modeling the effect of dynamic wake curvature on the aeroelastic response of open rotors with slender blades. The classical solution of incompressible, potential flow derived for a curved vortex tube of uniform vorticity strength is employed. The previously developed curved vortex tube analysis is mathematically generalized to account for arbitrary radial and circumferential variations of circulatory disk loading. An orthogonality analysis is carried out to obtain a finite set of inflow perturbation coefficients that describe the aerodynamic effect of wake curvature in a generalized manner. The end result is a set of integral expressions that provide the interharmonic coupling between the inflow perturbations on the rotor disk due to a curved trailing wake and the corresponding variations of disk loading. The obtained perturbation coefficients are subsequently superimposed upon an existing finite-state induced flow model that assumes a skewed, noncurved cylindrical wake. The developed mathematical approach for fluid mechanics is coupled with an unsteady blade element aerodynamics model, a rotor blade structural mechanics model, and a nonlinear rotor dynamics model. The combined formulation is implemented in an existing helicopter flight mechanics code. The overall method is initially employed to assess the effect of wake curvature on the dynamic response of a small-scale articulated rotor with a flap frequency ratio equal to unity. Subsequently, the integrated model is deployed to investigate the influence of wake curvature and inflow modeling fidelity on the predicted oscillatory blade loads and transient control response of a full-scale helicopter rotor. Comparisons are carried out with flight test measurements as well as with complex free-wake analysis methods. It is shown that including the effect of wake curvature is essential for predicting the transient control response of the investigated rotor. Good agreement is demonstrated between the proposed analytical model and nonlinear predictions carried out by resolving the complex wake geometry. The developed fluid mechanics formulation is a time-accurate method derived from first-principles and is applicable to both axial and nonaxial flow conditions.

A coupled free-wake/panel method for rotor/fuselage/empennage aerodynamic interaction and helicopter trims

A coupled free-wake/panel method to predict the rotor downwash aerodynamic interaction and helicopter trims is presented. Free-wake method was developed to analyze aerodynamics interference of rotor wake, based on lifting-surface theory and vortex method, which relates the core radius, span station, and circulation of initial tip vortex with the blade-bound circulation distribution, and eliminates the empirical parameters during rotor free wake analysis. Helicopter fuselage with empennage was discretized into source panels. The vortex line mirror method was adopted to account for wake acceleration phenomenon that resulted from the close interaction between rotor wake and fuselage surface. The rotor wake geometry and downwash simulations were investigated including comparisons with the available measured data. Combined with the helicopter flight dynamics model and embedded in the trim procedure, the free-wake/panel coupled method was applied to calculate the rotor wake and its interference on other components of a full-scale UH-60A rotorcraft in level flight. Comparisons among predictions, referenced results, and experimental results are made for rotor wake geometries, blade-bound vortex, blade tip vortex, rotor downwash velocity, and control stick positions, and encouraging results were obtained. To validate the present method, this paper discusses the fundamental formulation, the numerical algorithms, and the simulation results.

Compound Helicopter: Insight and Optimization

The paper presents an insight into the complex task of design and optimizing a compound helicopter configuration. It introduces the “drag versus power chart” (DP chart) as a tool for separating the rotors, thrusters, wings, and fuselage contributions and understanding their optimal combination in a generic compound configuration. The analysis shows the dependency of the optimal configuration on the efficiencies of the rotors, the thrusters, and the wings, and a way to carefully examine the effect of many design parameters. As such, the analysis may be applied to various configurations including single and coaxial rotor systems. Among other conclusions, it is shown when and why a thruster is absolutely essential for high speed and clarifies the role of the wing in such cases. The paper also supplies a unique optimization process, which is based on a comprehensive and detailed nonlinear free-wake analysis of a compound configuration that includes a thruster and fixed wings. The optimization process is twofold: First, for a global search, a variety of randomly selected configurations are analyzed to determine an initial hover-forward flight Pareto frontier. Then, various types of local analyses are carried out to improve the above frontier. Such successive frontier refinements lead to an improved, detailed, and continuous frontier that may be exploited for a variety of missions. The configurations on the resulting Pareto frontier show design trade-offs between configurations that are more efficient in hover and those that are more efficient in high-speed forward flight.

Hydrodynamic performance of flapping wings for augmenting ship propulsion in waves

The present work deals with the hydrodynamic analysis of flapping wings located beneath the hull of the ship and operating in random waves, while travelling at constant forward speed. The system is investigated as an unsteady thrust production mechanism, augmenting the overall ship propulsion. The main arrangement consists of a horizontal wing in vertical motion induced by ship heave and pitch, while pitching about its own pivot axis that is actively set. A vertical oscillating wing-keel is also considered in transverse oscillatory motion, which is induced by ship rolling and swaying. Ship flow hydrodynamics are modeled in the framework of linear theory and ship responses are calculated taking into account the additional forces and moments due to the above unsteady propulsion systems. Subsequently, a non-linear 3D panel method including free wake analysis is applied to obtain the detailed characteristics of the unsteady flow around the flapping wing. Results presented illustrate significant thrust production, reduction of ship responses and generation of anti-rolling moment for ship stabilization, over a range of motion parameters. Present method can serve as a useful tool for assessment, preliminary design and control of the examined thrust-augmenting devices, enhancing the overall performance of a ship in waves.

Aeroelastic Analysis of a Hingeless Rotor Using a Dynamic Wake Model

Aeroelastic Analysis of a Hingeless Rotor Using a Dynamic Wake Model

Helicopter Rotor Noise in the Merged Tip-Vortex and Blade Interaction Condition

BVI (blade vortex interaction) phenomena cause unsteady loading and noise in the helicopter rotor. It is difficult to accurately predict BVI aerodynamics and noise, because rotor wake and tip-vortices are numerically dissipated. In the present study, CFD and a time-marching free-wake coupling analysis were used to describe inflow and outflow without wake dissipation. Rotor noise was also computed using the Farassat formula, derived from the Ffowcs-Williams Hawkings equation. To validate aerodynamics and the noise prediction solver, a two-blade rotor was simulated and the results were compared with experimental data. To investigate the vortex interaction of a multi-blade rotor, a four-blade rotor was also simulated. In a slow descent flight condition, tip vortices are merged at the advancing side. The merged tip-vortex and blade interaction were simulated and are discussed in this paper.

Visualisation of the brownout phenomenon, integration and test on a helicopter flight simulator

AbstractFlying under degraded visual environments is a challenging task in helicopter operations. The brownout phenomenon, caused by the rotor downwash on a dusty or sandy underground has caused several critical incidences and accidents in the past. In contrast to an extensive free wake analysis of the rotor vortices, this paper presents a computer graphical solution to simulate brownout conditions. The animation approach builds upon a particle system with a basic aerodynamic assumption of a permanent ground vortex and additional turbulence. The created brownout effect module is a part of the RealTime Image Generator software, used in DLR’s ground based helicopter flight simulator. The object oriented structure of the underlying application programming interface OpenSceneGraph facilitates the development of a generic effect interface for future improvements. An overview of the developed module, the particle system engine and the physical dependencies is provided. To overcome the challenge of capturing the visual realism, a pilot test course with brownout typical tasks is presented. Criteria focussing on the visual and physical parameters are defined to build up a questionnaire. It is used to improve the visual standard and to meet the future pilot’s needs.

Far-Field Boundary Condition Effects of CFD and Free-Wake Coupling Analysis for Helicopter Rotor

Rotor aerodynamics is governed by wake geometry and strength. However, rotor wake characteristics computed by the rotor computational fluid dynamics are not clearly described due to numerical dissipation. To overcome this numerical problem, free-wake is used for wake simulation. The present free-wake describes the inboard vortices as well as the tip vortices of the blade. At each time step, the free-wake provides inflow and outflow conditions of the boundary of the Eulerian domain, and the Euler solver is used for solving the flow field near the rotor blade. Finally, the coupled method is compared with the conventional method and the experimental results.

Hover Performance Correlation for Full‐Scale and Model‐Scale Coaxial Rotors

The hover performance data of full-scale and model-scale coaxial rotors have been compared with CAMRAD II predictions having a free vortex wake analysis. Performance correlations of a coaxial rotor were made with a variation of key parameters including the rotor spacing and height. To understand aerodynamic behavior of the U.S. Army Aeroflightdynamics Directorate(AFDD)coaxialrotoroperatingoverarangeofReynoldsnumbersfrom36,000to180,000,theReynoldsnumber scaling effect was explored using an airfoil design code, MSES. It was found that the coaxial rotor spacing effect on hover performance was minimal for the rotor spacing larger than 20% of the rotor diameter. The measured performance data showed that more thrust was lost from the lower rotor of a coaxial than the upper rotor due to a larger rotor-to-rotor wake interference effect, and the lower rotor kept only an 81% of the single rotor OGE (out-of-ground effect) thrust whereas the upper rotor maintained a 90%. The lower rotor IGE (in-ground effect) thrust increased quickly by 26% as the rotor approached to the ground from the position of an 80% of the rotor diameter to 10%, and the corresponding IGE power increased by 17%. These thrust and power characteristics were well predicted. Overall, the performance prediction for the coaxial rotor was satisfactory when compared with the measured data.

Study on the Induced Velocity and Noise Characteristics of a Scissors Rotor

Aerodynamic and acoustic experiments were carried out to investigate the induced inflow distribution and noise characteristicsof ascissorsrotorinhover.Theaerodynamic experimentwasconductedonamodelrotorrigandthe induced inflows for two different scissors rotor configurations (the L configuration and U configuration) were measured. The experimental results indicate that the induced velocity near the lower blades is consistently larger than that near the upper blades. A free-wake analysis was used to predict the aerodynamic characteristics. The variation of the induced velocity with different scissors angles was simulated analytically and the results were compared with test data. The acoustic experiment was conducted in an anechoic aeroacoustic laboratory and the noise levels for both the scissors rotor configuration and a conventional configuration were measured. The effects of differentscissorsanglesonrotornoisewereinvestigated. Itwasshownthatthenoiselevel ofascissorsrotor depends on both the azimuthal and vertical spacing and is not necessarily less than that of a conventional configuration.

A full-span free-wake model using circular-arc vortex elements and incorporating rotor trim analysis

An analytical method incorporating a free-wake analysis with a rotor trim algorithm is developed for predicting rotor wake geometry and blade loading distribution for a helicopter in hover and forward flight. The wake model of this method is based on a multi-filament full-span free wake, using sophisticated circular-arc vortex elements. The blade model is based on the second-order lifting-line theory to improve the calculation of three-dimensional tip effects. A rotor trim method is given, based on the finite-difference approximation of the blade flapping motion and the trim Jacobian matrix. This is subsequently coupled into the free-wake model. In order to demonstrate the capability of the coupled method, comparisons between the calculated results and the available experimental data of the rotor blade flapping motion for a model rotor are presented covering conditions from hover through forward flight. The free-wake geometry and the blade loading distribution at a low advance ratio are calculated for the H-34 helicopter rotor, and the blade loading distribution is compared with both the measured data and calculated results on the basis of the undistorted wake.

자동차용 냉각홴의 설계와 시스템 개선을 통한 저소음화 연구

Axial fans are widely used for automotive engine cooling device due to their ability to produce high flow rate to keep engine cool. At the same time, the noise generated by these fans causes one of the most serious problems. Especially, engine cooling fan noise in idle condition of a car is noticeable. Therefore. the high efficient and low-noise fan is seriously needed. When a new fan system is designed, system resistance and non-uniform inflow are the key factors to get the high performance and low noise fan system. In this study, aerodynamic and acoustic calculations are carried out on the automotive cooling fan and system. Effects of various design parameters are studied through the free wake analysis and experiments. Better performance and noise characteristic are obtained for the new design fan using the methodology. Furthermore through the modification of the fan system geometry parameters, the fan system produce more flow rate and become less noisy.

Rotor Vibratory Load Prediction Based on Generalized Forces

Three rigid-body motion DOFs are introduced for the motion of the flap, lag hinge and pitch bearing. The rotor blade is discretized using a five-nodes, 15 DOFs beam finite element. The dynamic coupling effect between the rigid motion of the blade and the nonlinear elastic deflections is taken into account. Utilizing the constitutive law of the curvilinear coordinate system, the typical moderate deflection beam theory is reformulated. In addition, the Leishman and Beddoes unsteady and dynamic stall model is incorporated and the inflow is evaluated with the free wake analysis. The derived nonlinear ordinary differential equations with time-dependent coefficients of the rotor blade are given in the sense of the generalized forces. The sectional loads of the blade and the equations of motion are solved simultaneously in the physical space. The blade vibratory loads predicted by present analysis show generally fair agreement with the flight test data of the SA349/2 Gazelle helicopter.

Insight into the effects of rotor downwash on engine jet

An analytical method to predict the effects of rotor downwash on engine jet was proposed to investigate the flow pattern of helicopter engine jet. First, free wake analysis was carried out to calculate the rotor wake geometry and the downwash around the engine jet space. Then the engine jet path, cross section shape and physical variants were calculated and analyzed. Finally, an example was given to show the typical results for an insight into the influence of engine jet upon the temperature distribution around a helicopter. The method developed here can be used to predict rotor wake, engine jet and temperature distribution of a helicopter.

A PROCEDURE FOR THE EVALUATION OF INSTALLED PROPELLER NOISE

A method for the prediction of the acoustics of a propeller in the flow-field of a wing is presented. The method is used to study the noise generated by the unsteady loading induced on the propeller as it passes through the wing flow-field. Both the aerodynamic and acoustic methods are previously proven techniques, the aerodynamic method being based on a combination of free wake analysis and a three-dimensional boundary element method, while the acoustic calculation is a full-surface, moving medium form of the Ffowcs Williams–Hawkings equation. Calculations are presented for a reference case of a four-bladed low-speed propeller in forward flight. The acoustic predictions are supplemented with retarded time results which relate the radiated noise to the aerodynamic conditions on the blade at the time of noise emission. The noise data and the retarded time results are discussed and related to previous aerodynamic work.

Influence of blade-to-blade dissimilarity on alleviation of helicopter blade-vortex interactions

This paper numerically examines possible alleviation of parallel blade-vortex interaction (BVI), experienced by helicopters in low-speed descent, through the introduction of blade-to-blade dissimilarity. A four-bladed rotor with two sets of opposite blades is considered, and the radius of one set is reduced to 80% of the baseline radius. A free-wake analysis is developed for calculating the distorted wake geometry for such a rotor with dissimilar blades. Results indicate that with dissimilarity in blade length, the parallel BVI appear to be weaker due to factors such as increased miss-distance and increased angle (less parallel); so that the blade loading is generally less impulsive in nature. Increased miss-distances for the dissimilar rotor were attributed to the significantly different coning angle of the short and long blades, causing the tip vortices to be released from two different planes. The vortices from the longer blade are released higher, and tend to sail above the shorter blades with a low coning angle, and vice-versa. The acoustic code WOPWOP was used to calculate the BVI sound pressure level in a plane below the rotor disk. For the low-speed descent (severe BVI) flight condition examined, it was found that the peak BVI noise level was reduced by 4dB.

Prediction Measurement of a Low-Frequency Harmonic Noise of Hovering Model Helicopter Rotor

Far-field acoustic data for a model helicopter rotor have been gathered in a large open-jet, acoustically treated wind tunnel with the rotor operating in hover and out of ground-effect. The four-bladed Boeing 360 model rotor with advanced airfoils, planform, and tip shape was run over a range of conditions typical of today's modern helicopter main rotor. Near in-plane acoustic measurements were compared with two independent implementations of classical linear theory. Measured steady thrust and torque were used together with a free-wake analysis (to predict the thrust and drag distributions along the rotor radius) as input to this first-principles theoretical approach. Good agreement between theory and experiment was shown for both amplitude and phase for measurements made in those positions that minimized distortion of the radiated acoustic signature at low-frequencies.