Interaction of heavy aircraft wakes

transactions of the institute of aviation no. 4 (245), pp. 309-320, Warsaw 2016 Doi: 10.5604/05096669.1229485 ISSN 0509-6669 eISSN 2300-5408 interaction of heavY aircraft WaKes Pamela Bugała, adam dzIuBIńSkI Center of New Technologies, Institute of aviation, al. krakowska 110/114, 02-256 Warsaw, Poland , abstract In the next few years the problem of heavy aircraft wakes may increase on the account of continuous air transport growth. However, it can be noticed that even today the number of accidents resulting from an interaction with wakes is increasing. That is the reason why methods of wake vortex description should be searched for. The aim of this study is to analyze interaction of example aircraft wakes. In this paper the characteristics of vortex wake behind three-dimensional wing are presented. It shows how a separation between aircraft affects the decay of vortex. Two- and three-dimensional calculations were performed using commercial RaNS code. The following cases have been taken into consideration: flow past a full commercial aircraft, three-dimensional flow over the simplified wing and a two-dimensional analysis of vortex decay caused by the landing aircraft, including the separation effect. For all these cases a CFd simulation of the aircraft wakes was conducted. One of the main outcome of this work is a confirmation that the interaction between wakes consists of spreading out and lifting wakes. The achieved results show that the two-dimensional simulation is a sufficient tool for a preliminary analysis of wake vortices. Conclusions from this analysis can be used by the managements of busy international airports to enhance safety. keywords: CFd, vortex decay, vortex interaction. 1. introDuction Wingtip vortices are always generated by aircraft in flight. differences in pressure between the upper and lower surface of wing causes formation of a vortex. The strength of vortex wake depends on the aircraft’s design, gross mass or configuration corresponding to a flight phase, flight altitude and speed. One of the first studies of vortices, concerning those formed by the wingtip, were published in 1923. at that time, it was noted that vortices occur in a real flow past other bodies [1]. Full scale flight tests for research of wake vortex encounters started in the 1950s [2] and were continued for the next years [3-6]. On the other hand, NaSa initiated tunnel tests to study the wake-vortex encounter [7-8]. all those works enabled the development of an aircraft Vortex Spacing System (aVOSS) concept, which will provide dynamic, weather dependent wake vortex spacing requirements for an advanced automated air-traffic control system [9]. In order to prevent accidents due to wake turbulence, the International Civil aviation Organization (ICaO) introduced separations in air traffic [10]. depending on maximum Certificated Takeoff 310 Pamela Buagała, adam dzIuBIńSkI Weight (mCTOW) of the leading and following aircraft, a separation distance is determined. Taking into consideration the close inter-dependence between aircraft’s design and the strength of vortex wakes, aircraft are divided into the following categories of mCTOW: heavy, medium and light (Tab.1). ICaO’s aircraft separation distances to avoid wake vortex encounter are shown in Table 2. Tab. 1. Weight categories [10] Today, modern calculation tools and optical methods of flow visualization are available. Those methods allow to describe more strictly the wake vortices problem. Predictions of aircraft wakes using numerical methods are shown in [11-14]. In [11] a wake interaction between aircraft on closely spaced parallel paths, obtained using large eddy Simulation (leS) method have been shown. The paper [12] covers methods for modelling vortex wakes behind aircraft at a low altitude and close to the ground during takeoff and landing operations. References [13-14] concern a potential risk for rotorcraft encountering wake vortices of the fixed-wing aircraft. The Boeing 777, a type chosen for the analysis, is a jet airliner developed and manufactured by Boeing Commercial airplanes. It is long-range wide-body twin-engine airplane and has a typical seating capacity for 314 to 451 passengers, with a range of 9,695 to 17,594 km. Table 3 provides information on specification of the Boeing 777-200, a version used in the analysis. So far, the Boeing company has delivered about 1,283 of such airplanes to 42 customers worldwide [15]. Tab. 2. ICaO aircraft separation distances to avoid wake vortex encounter [10] Tab. 3. Specification of example airplane [15] The aim of this study is to analyze the interaction between wakes of two aircraft following each other on approach. In this paper, the characteristics of the vortex wake behind the 3d wing is presented. This study shows how separation between aircraft affects the vortex decay. Two- and threedimensional calculations were performed using commercial RaNS code. On the basis of calculations made for three cases: flow past a full commercial aircraft, 3d vortex analysis of the wing and 2d INTeRaCTION OF HeaVy aIRCRaFT WakeS 311 analysis of vortex decay behind the landing aircraft with separations, the analysis of aircraft wakes has been conducted. The achieved results show that 2d analysis is sufficient to make a preliminary assessment of the wake vortices behaviour. 2. MethoD The flow simulations were computed with the use of Reynolds averaged Navier Stokes (RaNS) method of solving flow equations, using one equation turbulence model (Spalart–allmaras) [16]. The software code, FlueNT, is based on the finite volume method [17]. The Spalart-allmaras turbulence model was developed mainly for aerodynamic flows in scales used in the simulation. This model is a transport equation for the eddy viscosity. The model of the 3d geometry of the Boeing 777 is based on the NaSa Common Research model (CRm, dPW-6) [18]. It is a Standard Research model agreed between american research facilities to validate results from different aerodynamic wind tunnels. The model is freely available in the form of a Cad drawing. Since the model is based on the B 777 airframe, in this work it has been re-scaled (an original is in a wind tunnel model scale) to the dimensions of a real plane. Figure 1 presents CRm/dPW-6 model geometry. The Boeing 777 is a low wing monoplane in a classic configuration. Its tapered wing has a high sweep angle, similar for the horizontal stabilizer. The model is not equipped with a vertical stabilizer as usually a mounting device for aerodynamic balance is attached there. Fig.1. CRm/dPW-6 model geometry [dziubiński, 2016] Both wing and horizontal stabilizer have a positive dihedral. an airfoil distribution on the wing is rather complicated, and also wing deformation caused by the flow is introduced, so it is fully reasonable to distribute such geometry in a digital form. additionally, the model is equipped with empty duct mock-ups for the engine nacelles, mounte (...truncated)