Vortex Encounters Research Articles

EFFECT OF TURBULENCE ON SOUND RADIATION BY VORTEX–BODY INTERACTION

This study examines the sound generated by the interaction between turbulent vortices and solid bodies, and its propagation in a nonuniform flow. Single vortex encounters with a flat plate and a nonrotating cylinder are considered. The solutions show that as the turbulence intensity increases, the sound radiated by the vortex–body interaction is strengthened while the effect of the mean flow speed on the sound waves weakens. The sound profile and sound directivity do not change with the Reynolds number. Neglecting turbulence in vortices will not affect the prediction of the fundamental properties of the radiated sound waves; however, it will underestimate the magnitude of the produced sound.

Wake vortex encounter severity for rotorcraft in final approach

The Advisory Council for Aeronautics Research in Europe (ACARE) predicts that European air traffic may nearly triple by 2020. The growth in air traffic is already an increasing problem with capacity at some airports becoming limited due to congestion. This could be alleviated by providing additional passenger capacity at hubs through the introduction of rotorcraft using new IFR procedures and operating simultaneously but independently of the fixed-wing traffic. These Simultaneous Non-Interfering Operations (SNIOps) will be enabled by a ‘reconfiguration’ of the airspace, taking advantage of new navigational and air traffic management systems. SNIOp's raise critical safety questions for rotorcraft wake vortex encounters (WVE's) and will require consideration of the longitudinal and lateral aircraft separation and the locations of the rotorcraft FATO's (Final Approach and Take-Off areas). This paper presents analysis from work carried out as part of the Framework 6 project ‘OPTIMAL’ including the development of predictive methodology and analysis for rotorcraft WVE's, using a severity rating scale. In particular, scenarios are considered where the rotorcraft is following precision glideslopes of up to 12° in both good and degraded visual conditions. Handling qualities criteria have already been found to be well suited to investigating severity of an encounter. Within this framework, draft boundaries are proposed for assessing the severity of an encounter. Furthermore, the results have shown a pilot may be able to recover from an encounter, but the question of whether the required navigational precision would be compromised and a go-around required is also addressed.

Wake Vortex Model for Real-Time Flight Simulation Based on Large Eddy Simulation

with asix-degree-of-freedom real-time flight simulator. The papercombines the use of largeeddy simulation dataas the basis of a wake vortex encounter model with methods to access the dataset in real time. The second half of this paper describes the data compression scheme implemented to reduce the size of the time-varying dataset. Additionally, rapid data access and issues regarding the real-time data management aspects are discussed. Analysis of the performance of the model is made and preliminary comparisons with a traditional analytical wake vortex model are discussed.

A Severity Analysis for Rotorcraft Encounters with Vortex Wakes

Our paper concerns the severity of the response of a rotorcraft encountering the vortex of a fixed-wing aircraft. One of the key questions is whether a rotorcraft designed to meet handling performance standards will have sufficient control margin for the pilot to overcome the effects of a vortex encounter. This question is addressed through an analytical study supported by preliminary piloted simulation tests. The handling criteria are found to be well suited to establishing the severity and associated hazard category of typical encounters. Cases are illustrated where insufficient control margin is available to overcome the effects of the encounter. The pilot intervention time is critical as expected. In risk assessment parlance, an intervention time of 3 seconds leads to hazardous encounters (> Level 3 HQs) while a reduced intervention time of 1.5 seconds is more likely to be major (Level 3 HQs).

Critical wake vortex encounter scenarios

A simulation-based method was developed to investigate the severity of Wake Vortex Encounters (WVEs). This paper describes an important part of this method: the determination of worst-case WVE conditions, which is referred to as Worst-Case Search (WCS). The WCS results permit to reduce time and costs of WVE severity related piloted simulator tests and allow the comparison of the hazard that vortices of different generator aircraft exert on a follower aircraft. The WCS is based on a high fidelity, offline simulation of the follower aircraft that includes the interacting wake vortex, a hazard criterion that rates the severity of each WVE, and a pilot model. It can be formulated as an optimisation problem that is solved with the optimisation tool MOPS (Multi Objective Parameter Synthesis). MOPS varies the encounter geometry until the simulation yields maximum values of the WVE hazard criterion. Worst-case encounter conditions for different parameters were investigated and sensitivity studies performed. The influence of the height above ground, the core radius, and the models for the wake vortex velocity profiles on the severity of a WVE was examined. To show the capability of the method, a comparison of the severity of a WVE behind two different heavy transport aircraft for a 20 t aircraft was made. The WCS method demonstrated its applicability and delivered the worst-case encounter geometry.

Hazard criteria for wake vortex encounters during approach

Airbus has prepared a methodology to assess the severity of a Wake Vortex Encounter (WVE). This methodology is called Vortex Encounter Severity Assessment (VESA). VESA makes it possible to compare the effect of wake vortices generated by different aircraft on an aircraft that encounters such a vortex. Important elements of VESA are the hazard criteria that rate particular aircraft reactions during WVEs. Hazard criteria are necessary to compare WVEs in batch simulations. This paper describes how piloted flight simulations for WVE investigations during landing approach were prepared and how the results were used for hazard criteria development. This work was part of the European S-WAKE program.

Aerodynamic Modeling and System Identification from Flight Data-Recent Applications at DLR

This paper provides an overview of a few selected system identification activities carried out during the last decade at DLR, the German Aerospace Center. After a brief account of parameter estimation methods and of consistency checking of recorded flight data, an emphasis is placed on: i) development of global, high-fidelity aerodynamic databases for training simulators, ii) models for in-flight simulator iii) modeling of stall hysteresis, iv) estimation of aerodynamic flow field characteristics and validation of aircraft reaction models from wake vortex encounter flights, and v) a parametric wake distortion model to improve helicopter coupling off-axis response. These examples cover a variety of flight vehicles with widely different characteristics, namely the military transport aircraft Transall C-160, commuter aircraft Dornier 328, in-flight simulator VFW-614 ATTAS, high altitude research aircraft G850 Strato 2C, Airbus type aircraft, fly-by-wire helicopter BO-105, and fly-by-light helicopter EC-135. Besides parameter estimation aspects, those related to model validation are discussed. Through succinct examples, it is brought out that system identification has become an indispensable tool to support not only research but also industry activities in various key areas such as model validation, handling qualities evaluation, control law design, and flight vehicle design and certification. Thus, it contributes significantly to risk and cost reduction.

Effect of Leading-Edge Cross-Sectional Shape on Nonslender Wing Rock

Conclusions Based on the convection of thin vortex sheets, it has been shown that aircraft vortices can temporarily intensify in the presence of crosswind shear. An aircraft following another one may, thus, be subjected to a stronger vortex than that expected in the wake of the leading aircraft in a quiescent atmosphere. Although wake turbulence measurements may soon become a part of the aircraft airworthiness process, it is believed that still greater safety of air travel is achievable by minimizing the vortex intensiŽ cation in wind shear and by avoidingwake vortex encounters as much as possible. This may involve guiding the aircraft to turn into crosswind only when sufŽ ciently distant from the airports and equipping them, individually,with wind shear detectors, wake vortex sensors, and other suitable warning devices. References 1Simpson, R. W., and Ausrotas, R., “A Critical Review of the FAA Safety Rulemaking on Wake Turbulence,” Air Transport Association of America, Sept. 1997. 2Spalart, P. R., “Airplane Trailing Vortices,” Annual Review of Fluid Mechanics, Vol. 30, 1998, pp. 107–138. 3Sarpkaya, T., “Decay ofWake Vortices of LargeAircraft,”AIAA Journal, Vol. 36, No. 9, 1998, pp. 1671–1679. 4Chorin, A. J., Vorticity and Turbulence, Springer-Verlag, New York, 1994, pp. 84–89. 5Mokry, M., “Numerical Simulation of Aircraft Trailing Vortices Interacting with Ambient Shear or Ground,” Journal of Aircraft, Vol. 38, No. 4, 2001, pp. 636–643. 6Schecter, D. A., and Dubin, D. H. E., “Theory and Simulations of TwoDimensional Vortex Motion Driven by a Background Vorticity Gradient,” Physics of Fluids, Vol. 13, No. 6, 2001, pp. 1704–1723. 7Proctor, F. H., “The NASA-Langley Wake Vortex Modelling Effort in Support of an Operational Aircraft Spacing,” AIAA Paper 98-0589, Jan. 1998. 8Zheng, Z. C., and Baek, K., “Inviscid Interactions Between Wake Vortices and Shear Layers,” Journal of Aircraft, Vol. 36, No. 2, 1999, pp. 477– 480. 9Mokry, M., “Hazard of Wake Vortex Encounters in Crosswind Shear,” AIAA Paper 2003-0378, Jan. 2003.

On the Structure and Attenuation of an Aircraft Wake

The results of an experimental investigation of the wake-vortex structure behind a swept and tapered wing with slats and e aps arepresented. The wind-tunnel test is part of a project funded by the German Research Association to investigate the e ow structure of wings with different e ap extensions. The investigation aims at studying the characteristics of lift-generated vortices in order to e nd ways to attenuate dangerous wake vortex encounters for following aircraft during takeoff and landing such that an increase in airport capacity can be achieved. The wake structureismeasuredfordifferente ap settingsbymeansofhot-wireanemometry atdifferent streamwisepositions in the near e eld behind the wing. The measured data are evaluated to examine vortex parameters such as core radius, maximum tangential velocities, circulation distributions, turbulence levels, and maximum induced rolling moments on a following aircraft. In addition, for one e ap setting means of alleviation with a wing e n at different angles of incidence mounted at the outboard e ap edge region are examined. The e n causes a large increase of the vortex core and a reduction of the maximum induced rolling moment within the measured region.

Personnel Airdrop Simulation

We develop an object-oriented simulation that models the airdrop mission of a U.S. transport aircraft, the C-1 7 Globemaster III. The simulation, written in MODSIM III, is based on three basic object types: an aircraft, a wake vortex, and a paratrooper. The aircraft object provides the required aerodynamic constants for simulating the wake vortices generated off each wingtip; the vortex object includes velocity field and decay models as well as a position algorithm; and the paratrooper object implements a 6-degree-of-freedom trajectory model. Using the simulation, we generate vortex encounter information for various formations and wind conditions, and quantify the associated risk of paratrooper/vortex encounters in a potential en counter rate. Time over the drop zone is a primary concern in any airdrop operation, and the results of this analysis form a tool that allows the ground com mander to assess the risk involved in decreasing that time.

Influence of different flap settings on the wake-vortex structure of a rectangular wing with flaps and means of alleviation with wing fins

This paper presents the results of an experimental investigation of the wake-vortex structure behind a flapped rectangular wing. The wind-tunnel test is part of a project funded by the German Research Association (DFG) to investigate the flow structure in the wake of wings with different flap extensions. The purpose of this investigation is to study the main features of lift generated vortices in order to find ways to alleviate hazardous wake vortex encounters for following airplanes during start and approach such that an increase in airport capacity can be achieved. First, the wake structure at different flap settings is investigated by measuring the velocity field at different positions in the near field behind the wing. The measured data are evaluated in terms of vortex parameters such as core radius, maximum tangential velocities, circulation distributions, turbulence levels and maximum induced rolling moments on a following aircraft. Then additionally, for one flap setting, means of alleviation are examined with a wing fin mounted at different positions on the wing. Alleviation is judged by the decrease of the maximum induced rolling moment. A significant reduction is achieved for a wing fin placed near the outboard edge of the flap.

Development of the C-17 Formation Airdrop Element Geometry

The analysis and tests that determined the C-17 three ship element geometry suitable for formation airdrop operations at night are discussed. The objective is to avoid jumpers encountering vortices generated from aircraft upstream. A measure of merit is developed based on the closest predicted lateral proximity between a vortex and a jumper. The geometry tested underwent two changes during the course of the test, due to the introduction of tolerance boxes for aircraft position and vortex encounters which occurred during early testing. An echelon geometry 6000 ft long and 1500 ft wide was ultimately adopted. In 101 passes over the drop zone, 1349 mannequins and 1251 personnel were dropped from this formation geometry without vortex encounter. NOMENCLATURE b Wing span DM Formation design margin, eq. (1) DZ Drop zone ti Parachute opening time V Aircraft velocity, ft/sec X Downstream distance between aircraft, ft Y Lateral distance between aircraft, ft, positive to starboard

Geostrophic Regimes, Intermediate Solitary Vortices and Jovian Eddies

Abstract We examine the relevance to Jupiter's atmosphere of the solitary vortices favored at scales intermediate to those of the quasi-geostrophic (QG) and planetary-geostrophic motions. Horizontal divergence plays a crucial role in the intermediate-geostrophic (IG) dynamics and leads to asymmetries in vortex behavior; in partcular, anticyclonic vortices are generally more stable than cyclonic vortices when the mean flow is weak or westerly. The IG vortices always propagate westward at close to the planetary long-wave speed, regardless of the mean zonal flow. Meridional shear influences only secondary aspects of vortex behavior. Although governed by a form of the Korteweg-deVries (KdV) equation, vortex encounters produce coalescence not soliton behavior. Jupiter's Great Red Spot and Large Ovals appear to be in, or close to, an IG balance while the Small Ovals lie in a QG balance. The stability of anticyclonic IG vortices may explain why most of Jupiter's super-eddies prefer anticyclonic spin. Solutions t...

Comment on "Simplified Methods of Predicting Aircraft Rolling Moments Due to Vortex Encounters"

where subscripts R and / denote real and imaginary parts, respectively. For a particular aircraft the choice of the change in one of the model eigenvalues is based on its performance capabilities. Once AX is specified, the elements of the differential matrix AAm could be computed from Eq. (24). The model transition matrix that should be used in the expression for the optimal control vector, Eq. (16), is thus

Criteria Relating Wake Vortex Encounter Hazard to Aircraft Response

Piloted six-degree-of-freedom motion simulator investigations were conducted at the NASA Ames Research Center to determine criteria relating the hazard posed by a wake vortex encounter to the response of the encountering airplane. These investigations demonstrated that wake vortex encounters can be reproduced realistically on a simulator, established that the maximum bank angle due to the encounter provides the best correlation with the pilot's subjective assessment of the hazard, and determined hazard boundaries in terms of maximum bank angle for two classes of jet transport aircraft.

Simplified Methods of Predicting Aircraft Rolling Moments Due to Vortex Encounters

It is not necessary to use time-consuming lifting surface theories for the purpose of calculating global forces and moments during wake vortex encounters. This paper presents computational methods that are currently used to quickly and accurately predict these rolling moments of an aircraft encountering a wake vortex. Appropriate modifications to strip theory are developed that account for the effects of finite wingspan. The paper shows that, in the case of an elliptic wing, the aspect ratio correction to the lift curve slope should be based on the semispan and a recipricol theorem is used to relate the rolling moment on a wing in a downwash field to that on a wing in steady rolling motion. The paper also presents calculations for a wing encountering a vortex with a Betz velocity distribution.

Study of Control System Effectiveness in Alleviating Vortex Wake Upsets

The problem of an airplane being upset by encountering the vortex wake of a large transport on takeoff or landing is currently receiving considerable attention. This paper describes the technique and results of a study to assess the effectiveness of automatic control systems in alleviating vortex wake upsets. A six-degree-of-freedom nonlinear digital simulation was used for this purpose. The analysis included establishing the disturbance input due to penetrating a vortex wake from an arbitrary position and angle. Simulations were computed for both a general aviation airplane and a commercial jet transport. Dynamic responses were obtained for the penetrating aircraft with no augmentation and with various command augmentation systems. The results of this preliminary study indicate that it is feasible to use an automatic control system to alleviate vortex encounter upsets.

The Normal and Power Vortex in Meteorology

NUMEROUS attempts1 have been made to apply the simple plane vortex of hydrodynamical theory to the structure of symmetrical pressure centres on the synoptic weather chart. These studies, which are based on the so-called simple vortex, in which the tangential velocity at a distance r from the centre is given by vr = a constant, are inadequate, since the simple vortex encounters convergence difficulties both at r = 0 and as r0. Such a system of winds would have an infinite kinetic energy associated with it, while a consistent pressure field would have an infinite potential energy.