Flow Structure on a Delta Wing of Low Sweep Angle

The instantaneous and averaged flow structure past a delta wing of low sweep angle is investigated using a technique of high-image-density particle image velocimetry. Emphasis is on crossflow planes, where vortex breakdown and stall occur, and the identification of buffeting mechanisms in these regions. At all values of angle of attack up to the fully stalled condition, the averaged vorticity layer exhibits an elongated form; the classical (single) large-scale concentration of vorticity within the leading-edge vortex of a highly swept wing is not present. At low angle of attack ?, this elongated, averaged layer can exhibit, however, well-defined concentrations of vorticity. These elongated vorticity layers are accompanied by narrow recirculation zones adjacent to the wing surface. Furthermore, the averaged streamline topology exhibits, at lower ?, a saddle point located slightly outboard of the leading edge, in contrast to a saddle point located on the plane of the symmetry of a highly swept wing. Patterns of velocity fluctuation and Reynolds stress show peaks that are generally coincident with large values of averaged vorticity, which indicates that they arise from unsteady events in regions of high shear. Well-defined concentrations of instantaneous vorticity can be identified at all values of angle of attack ?. At low ? individual concentrations retain their identity, but at moderate and high ? larger-scale clusters of instantaneous vorticity occur. In turn, these patterns of vorticity are in accord with the time-averaged spectra of the fluctuating velocity; the predominant peaks of such spectra take on lower values in regions where larger-scale clusters of vorticity appear. Control in the form of a small amplitude perturbation of the wing, at a frequency corresponding to the subharmonic of the spectral component in the initial region of development of the separated layer, can restabilize the time-averaged patterns of streamline topology and vorticity, such that they resemble those occurring at lower angle of attack ?

Computational and experimental investigations of the vortical flow structures in the near wake region downstream of the Ahmed vehicle model

The present study aims to investigate flow characteristics downstream of the Ahmed vehicle model using both experimental and computational methods. Ahmed vehicle model having ¼ scale and 25° slant angle is employed at Reynolds number of ReH=1.48×104. Investigations are conducted in two parts. In the first part, Large Eddy Simulation (LES) method is used to resolve the flow structures downstream of the Ahmed model, computationally. In the second part, the technique of the particle image velocimetry (PIV) is employed to obtain the flow fields downstream of the Ahmed model. The PIV and LES investigations provides time-averaged and instantaneous velocity field results, such as vorticity contours, streamline topology, velocity profiles and spectral analysis of the flow velocity. Flow features that have been predicted by computational study are in a good harmony with the results predicted by experimental studies both on the slanted surface and in the near wake region downstream of the Ahmed model. Results present that characteristics of flow features that exist on the rear slanted surface and in the near wake region of the Ahmed model exhibit great variations in a very short distance in both stream-wise and vertical direction of the flow. © 2016 Elsevier LtdFirat University Scientific Research Projects Management Unit: AAP20025The authors acknowledge the funding of the office of Scientific Research Projects of Cukurova University under contract no: AAP20025

Numerical Flow Investigation of Morphing Leading Edges for the Enhancement of Maneuverability of Unmanned Combat Air Vehicles

Observing the progress in the technology of unmanned combat aerial vehicles (UCAVs) than it can be foreseen that in the future the role of manned combat aircraft will be taken over more and more by unmanned systems. The abandonment of pilots allows for more freedom in the aerodynamic design of the vehicle in regard to weight and acceleration, however, new stealth constraints have a severe impact. The design of UCAV configurations is driven by the special requirements of upcoming missions, as for instance the capabilities of long endurance fl ights joined with a low observability. The new demands have a crucial impact on the aerodynamic shape, and, hence, require new solutions for maneuver con- trol in respect to integration of engine in- and outlets, actuators and other devices. Additionally the new capabilities of long endurance ights has to be joined with low observability. In particular UCAVs are suited to the exploitation of non-conventional control technologies, such as aerodynamic morphing, flow control, or thrust vectoring. This paper presents a numerical investigation of innovative morphing technologies for aircraft applications and explores the feasibility for such technologies to enhance the maneuverability of unmanned combat aerial vehicles. In Aerodynamics the morphing is understood as a smooth, continuous change in geometry of the outer surface, e.g. the twist of complete wing can be changed in order control role moments. In the considered case morphing is used to provide additional ow control mechanisms taking into account the constraint of fllow radar signature. In particular morphing can be used to change the geometry of the airfoil leading edge to control the ow around the wings in order to generate additional lift or induce role moments. This paper is concerned with the investigation of the feasibility and effectiveness of morphing devices for the aerodynamic control of UCAVs. Focal point of this work is the morphing of leading edges in order to generate additional lift or role moments for UCAVs flying in high angle of attack mode. Therefore, the objective of the numerical investigations is the evaluation of the potential of morphing leading edges especially for the enhancement of the maneuverability of Delta- and Lambda-wing configurations with their vortex-dominated ow field. Of special interest is ow control at high angles of attack (AoA) by targeting an minimized shape adaptation in order to generate a needed additional lift, to induce role moments or to reduce the risk of the potential deleterious effects of vortex breakdown. The morphing of the leading edge must not confused with classical Kruger leading edge flaps. Although very effective in regard to the increase of lift such fl aps can't be used for UCAVs since gaps between fl aps and the wing have to be avoided in order to fulll the hard conditions of radar camouflage . The presented work is part of the DLR Project FaUSST which is the successor of the DLRProject UCAV-2010. The UCAV-2010 project was set up to identify and assess UCAV relevant technologies. The investigations have covered the pre-design process with fast, low fidelity methods as well as the detailed examination using high fidelity methods like URANS simulations. The verification of the promissing technologies and tools have been done by virtual and experimental prototypes. The combined research has lead to a Lambda wing type UCAV configuration with a medium sweep of the leading edge. The outcoming DLR-F17 conguration has been mainly derived from the so called Saccon geometry developed by EADS-MAS for the RTO/AVT-161 task group. Since the capability of medium to high AoA maneuverability will be investigated the angle of attack has been varied from 6 to 16 degrees. After presentation of the technical part, the results of leading edge manipulation are discussed. One focal point is the comparison of the impact of differently morphed leading edge shapes on the fl ight performance at specific flight situations. The aerodynamical coefficients, in particular Cl, Cd and Cm, are related to morphing parameters of the changed shape of the leading edges. Another key aspect is the varied effective angle of attack of the middle part of the wings, see Figure 2. In case of a basic high angle of attack the effective AoA (due to morphing) can change the vortical flow above the deformed wings enormously, even when the geometrical angle of the airfoil has been morphed only slightly. This work discusses the effectiveness of counter-rotating morphing of leading edges, since the requirements in regard to radar signature do not allow large geometry changes. This point reduces the degree of freedom of any shape morphing drastically. Thereto, only proper combinations of left and associated right wing deformations have been analyzed and discussed. After the discussion of the most interesting deformations an evaluation of the presented test cases in regard to the maneuverability enhancement of UCAVs concludes the presentation