David W. Hamilton

Wake Vortex Transport and Decay in Ground Effect: Vortex Linking with the Ground

Numerical simulations are carried out with a threedimensional Large-Eddy Simulation (LES) model to explore the sensitivity of vortex decay and transport in ground effect (IGE). The vortex decay rates are found to be strongly enhanced following maximum descent into ground effect. The nondimensional decay rate is f ound to be insensitive to the initial values of circulation, height, and vortex separation. The information gained from these simulations is used to construct a simple decay relationship. This relationship compares well with observed data from an IGE case study. Similarly, a relationship for lateral drift due to ground effect is constructed from the LES data. In the second part of this paper, vortex linking with the ground is investigated. Our numerical simulations of wake vortices for IGE show that a vortex may link with its image beneath the ground, if the intensity of the ambient turbulence is moderate to high. This linking with the ground (which is observed in real cases) gives the appearance of a vortex tube that bends to

Evaluation of Fast-Time Wake Vortex Prediction Models

*† Current fast-time wake models are reviewed and three basic types are defined. Predictions from several of the fast-time models are compared. Previous statistical evaluations of the APA-Sarpkaya and D2P fast-time models are discussed. Root Mean Square errors between fast-time model predictions and Lidar wake measurements are examined for a 24 hr period at Denver International Airport. Shortcomings in current methodology for evaluating wake errors are also discussed. I. Introduction ORTICITY is generated as a consequence of aerodynamic lift from the wings, flaps, tail and body of the aircraft. Complete roll-up of this vorticity into a pair of counter-rotating trailing vortices usually occurs within about 20 wing spans downstream of the aircraft 1 . The lateral spacing between the rolled-up vortices is initially about 78 % of the aircraft’s wing span, but may vary several percent due to non-elliptical wing loading and aircraft configuration. The initial strength for each of the rolled-up vortices (represented by its circulation) is proportional to the generating aircraft’s weight and inversely proportional to air density, airspeed, and wing span. Large aircraft tend to produce stronger vortices than smaller aircraft, since the heavier weight of large aircraft more than offsets their larger wingspan. Wake vortices descend downward due to mutual induction of the vortex pair, as they are transported with the ambient wind. Meteorology greatly affects the transport and decay of wake vortices. Interaction with the ground adds further complexity to the evolution of wake vortices. Wake vortices that descend into ground effect (IGE) begin to separate laterally and decay more quickly than those that remain out of ground effect (OGE). Lifetimes of wake vortices range from about 20 seconds to several minutes, depending upon the generating aircraft, proximity to the ground, and meteorological conditions. Aircraft encounters with wake vortices can be unsafe due to the potential for loss of control. Maintaining ample separation between aircraft can overcome this danger, but overly conservative separations may reduce capacity and increase delays, especially during periods of heavy traffic. Current aircraft separation standards are conservatively based on aircraft weight categories to avoid unsafe wake encounters. Additional system capacity may be gained, while maintaining the current levels of safety, by including factors that affect wake behavior. For example, the presence of significant crosswinds or turbulence may limit the residence of wake vortices within a flight corridor and create the potential for less restrictive aircraft separations. In order to realize this benefit, fast-time wake prediction models are needed that reliably predict wake characteristics based on atmospheric and aircraft inputs. Other applications of fast-time models include 1) guidance for wake separation standards for new aircraft, 2) optimizing separation standards for existing aircraft, 3) guidance for setting vertical separation standards during cruise, 4) accident reconstructions, and 5) testing of new concepts for safe increases in airport capacity for both single and multiple runway configurations. Fast-time prediction models also could be used in the cockpit -- with input from airborne sensors and traffic position broadcast -- to provide real-time wake hazard information to the pilot. 2,3,4 Fast-time models are semi-empirical, and are designed to predict wake vortex information behind a generating

TASS Driven Algorithms for Wake Prediction

The TASS Driven Algorithms for Wake Prediction (TDAWP) is a simple set of algorithms engineered from results of three-dimensional large eddy simulations of wake vortices. The model consists of two equations, one for the prediction of vortex transport and a second for wake vortex decay. The model is designed for real-time predictions of wake vortex position and strength, and is dependent upon, weather conditions. A detailed description of TDAWP, including validation with field data, is included.

Numerical Study of a Convective Turbulence Encounter

A numerical simulation of a convective turbulence event is investigated and compared with observational data. The specific case was encountered during one of NASAs flight tests and was characterized by severe turbulence. The event was associated with overshooting convective turrets that contained low to moderate radar reflectivity. Model comparisons with observations are quite favorable. Turbulence hazard metrics are proposed and applied to the numerical data set. Issues such as adequate grid size are examined.

An Aircraft Encounter with Turbulence in the Vicinity of a Thunderstorm

Large eddy simulations of three convective turbulence events are investigated and compared with observational data. Two events were characterized with severe turbulence and the other with moderate turbulence. Two of the events occurred during NASA’s turbulence flight experiments during the spring of 2002, and the third was an event identified by the Flight Operational Quality Assurance (FOQA) Program. Each event was associated with developing or ongoing convection and was characterized by regions of low to moderate radar reflectivity. Model comparisons with observations are favorable. The data sets from these simulations can be used to test turbulence detection sensors.

METEOROLOGY ASSOCIATED WITH TURBULENCE ENCOUNTERS DURING NASA'S FALL-2000 FLIGHT EXPERIMENTS

Initial flight experiments have been conducted to investigate convectively induced turbulence and to test technologies for its airborne detection. Turbulence encountered during the experiments is described with sources of data measured from in situ sensors, groundbased and airborne Doppler radars, and aircraft video. Turbulence measurements computed from the in situ system were quantified in terms of RMS normal loads (σ∆n), where 0.20 g ≤ σ∆n ≤ 0.30 g is considered moderate and σ∆n > 0.30 g is severe. During two flights, 18 significant turbulence encounters (σ∆n ≥ 0.20 g) occurred in the vicinity of deep convection; 14 moderate and 4 severe. In all cases, the encounters with turbulence occurred along the periphery of cumulus convection. These events were associated with relatively low values of radar reflectivity, i.e. RRF < 35 dBz, with most levels being below 20 dBz. The four cases of severe turbulence occurred in precipitation and were centered at the interface between a cumulus updraft turret and a downwind downdraft. Horizontal gradients of vertical velocity at this interface were found to be strongest on the downwind side of the cumulus turrets. Furthermore, the greatest loads to the aircraft occurred while flying along, not orthogonal to, the ambient environmental wind vector. During the two flights, no significant turbulence was encountered in the clear air (visual meteorological conditions), not even in the immediate vicinity of the deep convection.