Vortex Ring Model of Tip Vortex Aperiodicity in a Hovering Helicopter Rotor

2014 ◽  
Vol 136 (7) ◽  
Author(s):  
Anand Karpatne ◽  
Jayant Sirohi ◽  
Swathi Mula ◽  
Charles Tinney
Keyword(s):  
Vortex Core ◽  
Distribution Functions ◽  
Tip Vortex ◽  
Helicopter Rotor ◽  
Time Step ◽  
Tip Vortices ◽  
Ring Model ◽  
Image Velocimetry ◽  
Blade Tip ◽  
Stereo Particle Image Velocimetry

The wandering motion of tip vortices trailed from a hovering helicopter rotor is described. This aperiodicity is known to cause errors in the determination of vortex properties that are crucial inputs for refined aerodynamic analyses of helicopter rotors. Measurements of blade tip vortices up to 260 deg vortex age using stereo particle-image velocimetry (PIV) indicate that this aperiodicity is anisotropic. We describe an analytical model that captures this anisotropic behavior. The analysis approximates the helical wake as a series of vortex rings that are allowed to interact with each other. The vorticity in the rings is a function of the blade loading. Vortex core growth is modeled by accounting for vortex filament strain and by using an empirical model for viscous diffusion. The sensitivity of the analysis to the choice of initial vortex core radius, viscosity parameter, time step, and number of rings shed is explored. Analytical predictions of the orientation of anisotropy correlated with experimental measurements within 10%. The analysis can be used as a computationally inexpensive method to generate probability distribution functions for vortex core positions that can then be used to correct for aperiodicity in measurements.

Investigation of Helicopter Rotor-Blade-Tip-Vortex Alleviation Using a Slotted Tip

AIAA Journal ◽  
2004 ◽  
Vol 42 (3) ◽  
pp. 524-535 ◽  
Author(s):  
Yong Oun Han ◽  
J. Gordon Leishman
Keyword(s):  
Rotor Blade ◽  
Tip Vortex ◽  
Helicopter Rotor ◽  
Helicopter Rotor Blade ◽  
Blade Tip

Numerical Experiments for Flow Around a Ducted Tip Hydrofoil

2002 ◽  
Author(s):  
Hildur Ingvarsdo´ttir ◽  
Carl Ollivier-Gooch ◽  
Sheldon I. Green
Keyword(s):  
Turbulence Model ◽  
Vortex Core ◽  
Vortex Flow ◽  
Computational Study ◽  
Near Field ◽  
Vortex Formation ◽  
Tip Vortex ◽  
Navier Stokes ◽  
Computational Studies ◽  
Tip Vortices

The performance and cavitation characteristics of marine propellers and hydrofoils are strongly affected by tip vortex behavior. A number of previous computational studies have been done on tip vortices, both in aerodynamic and marine applications. The focus, however, has primarily been on validating methods for prediction and advancing the understanding of tip-vortex formation in general, rather than showing effects of tip modifications on tip vortices. Studies of the most relevance to the current work include computational studies by Dacles-Mariani et al. (1995) and Hsiao and Pauley (1998, 1999). Daeles-Mariani et al. carried out interactively a computational and experimental study of the wingtip vortex in the near field using a full Navier-Stokes simulation, accompanied with the Baldwin-Barth turbulence model. Although they showed improvement over numerical results obtained by previous researchers, the tip vortex strength was underpredicted. Hsiao and Pauley (1998) studied the steady-state tip vortex flow over a finite-span hydrofoil, also using the Baldwin-Barth turbulence model. They were able to achieve good agreement in pressure distribution and oil flow pattern with experimental data and accurately predict vertical and axial velocities of the tip vortex core within the near-field region. Far downstream, however, the computed flow field was overly diffused within the tip vortex core. Hsiao and Pauley (1999) also carried out a computational study of the tip vortex flow generated by a marine propeller. The general characteristics of the flow were well predicted but the vortex core was again overly diffused.

scholarly journals Impact of Tip-Vortex Modeling Uncertainty on Helicopter Rotor Blade–Vortex Interaction Noise Prediction

2021 ◽  
Vol 66 (1) ◽  
pp. 1-13
Author(s):  
Stavros Vouros ◽  
Ioannis Goulos ◽  
Calum Scullion ◽  
Devaiah Nalianda ◽  
Vassilios Pachidis
Keyword(s):  
Vortex Core ◽  
Numerical Procedure ◽  
Tip Vortex ◽  
Analytical Models ◽  
Aerodynamic Noise ◽  
Helicopter Rotor ◽  
Vortex Interaction ◽  
Blade Vortex Interaction ◽  
Free Wake ◽  
The Impact

Free-wake models are routinely used in aeroacoustic analysis of helicopter rotors; however, their semiempiricism is accompanied with uncertainty related to the modeling of physical wake parameters. In some cases, analysts have to resort to empirical adaption of these parameters based on previous experimental evidence. This paper investigates the impact of inherent uncertainty in wake aerodynamic modeling on the robustness of helicopter rotor aeroacoustic analysis. A free-wake aeroelastic rotor model is employed to predict high-resolution unsteady airloads, including blade–vortex interactions. A rotor aeroacoustics model, based on integral solutions of the Ffowcs Williams–Hawkings equation, is utilized to calculate aerodynamic noise in the time domain. The individual analytical models are incorporated into an uncertainty analysis numerical procedure, implemented through nonintrusive Polynomial Chaos expansion. The potential sources of uncertainty in wake tip-vortex core growth modeling are identified and their impact on noise predictions is systematically quantified. When experimental data to adjust the tip-vortex core model are not available the uncertainty in acoustic pressure and noise impact at observers dominated by blade–vortex interaction noise can reach up to 25% and 3.50 dB, respectively. A set of generalized uncertainty maps is derived, for use as modeling guidelines for aeroacoustic analysis in the absence of the robust evidence necessary for calibration of semiempirical vortex core models.

scholarly journals Blade Tip-Vortices of a Four-Bladed Rotor with Axial Inflow

2020 ◽  
Vol 65 (4) ◽  
pp. 1-13
Author(s):  
Andreas Goerttler ◽  
Johannes N. Braukmann ◽  
C. Christian Wolf ◽  
Anthony D. Gardner ◽  
Markus Raffel
Keyword(s):  
Particle Image ◽  
Test Facility ◽  
Vortex System ◽  
Tip Vortices ◽  
Image Velocimetry ◽  
Swirl Velocity ◽  
Blade Tip ◽  
Velocity Circulation ◽  
Bladed Rotor ◽  
German Aerospace

The vortex system of four rotating and pitching DSA-9A blades was examined numerically and experimentally. Numerical computations were performed using German Aerospace Center (DLR)'s finite-volume solver TAU and were validated against experimental data gathered using particle image velocimetry carried out at the rotor test facility (RTG) in Göttingen. Algorithms deriving the vortex position, swirl velocity, circulation, and core radius were implemented. Hover-like conditions with a fixed blade pitch were analyzed giving further physical insights of the static vortex system. These results are used to understand the vortex development for the unsteady pitching conditions, which can be described as a superpositioning of static vortex states. The use of a zonal detached-eddy simulations approach improved physical modeling of the vortex development by resolving finer scales than URANS. Trimmed cases agree well with differences less than 0.5% in the circulation and swirl velocity.

Effect of Blade Tip Modifications for Unducted Propulsors on Tip Vortex-Rotor Interaction Noise

2014 ◽  
Author(s):  
Florian Danner ◽  
Christofer Kendall-Torry
Keyword(s):  
Rotor Blade ◽  
Stokes Equations ◽  
Acoustic Emissions ◽  
Tip Vortex ◽  
Navier Stokes ◽  
Navier Stokes Equations ◽  
Tip Vortices ◽  
Blade Tip ◽  
Blade Loads ◽  
Blade Tips

Front rotor tip vortices impinging on a downstream blade row of an unducted propulsor induce distinct unsteadiness to blade loads with associated sound emissions. Since the region of unsteadiness is concentrated near the blade tips, reducing the rear rotor tip diameter represents a potential means for minimising interaction noise. A survey on the aeroacoustic effects resulting from a cropped rear rotor in combination with a front rotor blade tip modification is therefore presented. Analyses are based on data from computational fluid dynamics solutions with the Reynolds-averaged Navier-Stokes equations and direct acoustic predictions. The evaluation of polar directivities, blade surface pressure disturbances and details of the unsteady flow field provide insight into the underlying phenomena. Results show that an arbitrary reduction of the rear rotor tip diameter does not necessarily decrease noise radiation and that winglet-like structures applied to the front rotor blade tips are capable of reducing acoustic emissions due to tip vortex-rotor interactions.

Tip Vortex Cavitation Suppression by Active Mass Injection

2011 ◽  
Vol 133 (11) ◽  
Author(s):  
Natasha Chang ◽  
Harish Ganesh ◽  
Ryo Yakushiji ◽  
Steven L Ceccio
Keyword(s):  
Vortex Core ◽  
Polymer Solutions ◽  
Tip Vortex ◽  
Substantial Decrease ◽  
Tip Vortex Cavitation ◽  
Flow Unsteadiness ◽  
The Core ◽  
Aqueous Polymer ◽  
Mass Injection ◽  
Stereo Particle Image Velocimetry

Injection of water and aqueous polymer solutions in to the core of a trailing vortex was found to delay the inception of tip vortex cavitation (TVC). Optimal levels of mass injection reduced the inception cavitation number from 3.5 to 1.9, or a reduction of 45%. At the optimal fluxes, injection of water alone produced a reduction of 35%, and the addition of polymer solution led to a reduction of 45%. Stereo particle image velocimetry was employed to examine the flow fields in the region of TVC inception and infer the average core pressure, and planar PIV was used to examine the flow unsteadiness in this region. The time-averaged pressure coefficients for the vortex core pressure were estimated and compared to the pressure needed for TVC inception and full development. Measurement of flow variability in the TVC inception region indicated that relatively low fluxes of mass injection in the TVC roll-up region led to a substantial decrease in flow unsteadiness in the core region near the observed location of inception, and this corresponded to a substantial decrease in the inception pressure. Increased injection of water or polymer solutions led to a modest increase in the average vortex core radius, which was discernable in the measured pressure needed for developed cavitation.

Tracking the vortex core from a surface-piercing flat plate by particle image velocimetry and numerical simulation

2018 ◽  
Vol 233 (3) ◽  
pp. 793-808
Author(s):  
Alexander Ashworth Briggs ◽  
Alan Fleming ◽  
Jonathan Duffy ◽  
Jonathan R Binns
Keyword(s):  
Shear Stress ◽  
Particle Image Velocimetry ◽  
Vortex Core ◽  
Particle Image ◽  
Tip Vortex ◽  
Adaptive Simulation ◽  
Shear Stress Transport ◽  
Image Velocimetry ◽  
Vortex Centre ◽  
Possible Cause

The wake flow around the tip of a surface piercing flat plate at an angle of incidence was studied using two-dimensional particle image velocimetry as part of benchmarking the particle image velocimetry technique on the moving carriage in the Australian Maritime College towing tank. Particle image velocimetry results were found to be in close agreement with those of the benchmarking work presented by the Hydro Testing Alliance, and a method of tracking the tip-vortex core near a free surface throughout numerical simulation has been demonstrated. Issues affecting signal to noise ratio, such as specula reflections from the free surface and model geometry were overcome through the use of fluorescing particles and a high-pass optical filter. Numerical simulations using the ANSYS CFX Solver with the volume of fluid method were validated against the experimental results, and a methodology was developed for tracking the location of the wandering vortex core experimentally and through simulation. The ability of the scale-adaptive simulation shear stress transport turbulence model and the shear stress transport model to simulate three-dimensional flow with high streamline curvature was compared. The scale-adaptive simulation shear stress transport turbulence model was found to provide a computationally less resource-intensive method of simulating a complex flow topology with large eddies, providing an insight into a possible cause of tip-vortex aperiodic wandering motion. At high angles of attack, vortex shedding from the leading edge separation of the test geometry is identified as a possible cause of the wandering phenomena. In this study, the vortex centre and point of extreme core velocity were found not to be co-located. The point of extreme stream wise velocity within the vortex core was found to be located within half the vortex radius of the vortex centre.

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