Vortex identification schemes and its application to PIV data

The dynamics of vortical structures in turbulent flow are of key interest both in fundamental research and in engineering development. Contemporary vortex identification criteria are based on the evaluation of the three-dimensional velocity gradient tensor of the flow. Numerical simulations can provide the necessary time-resolved volumetric flow data. From experimental investigations using present optical measurement techniques, however, only planar data—sectional two- or three-componential velocity fields—is available. Then, two-dimensional representations of the three-dimensional identification criteria have to be applied. The different, commonly used criteria are shown to be fully equivalent when applied to sectional velocity data. However, future measurement systems will allow to obtain time-resolved volumetric velocity data. Then, three-dimensional vortex identification criteria can be employed to extract vortical structures, and thus, to assess their topology in full spatial coherence. This requires the proper implementation of identification schemes into PIV evaluation and flow analysis software

Towing tank PIV measurements on 2- and 4-vortex systems IGE

The present report is part of the WP 3 “Wake evolution near the ground” of the FarWake project. Experimental investigations were performed in the tow tank at Göttingen to describe spatial-temporal flow evolution of 2- and 4-vortex systems in ground effect (IGE). The F13 model consisting of one or two rectangular wings is used as vortex generator and is towed along a ground plate at specific heights h/b = 0.5, 0.25, 0.125. In case of the 4-vortex system a span width ratio of b2/b1 = 0.3 and circulation ratios of Γ2/Γ1 = ± 0.3 are considered; i.e. co- and counter-rotating vortex pairs. A Stereo Particle Image Velocimetry setup is employed to determine the flow velocity fields in a cross plane. The flow fields are analysed with respect to flow separation at the ground, the generation of secondary vortices, the spatial-temporal development of the vortex trajectories and vortex circulation

Influence of Renolds number on generation and decay of aircraft wakes: Experimental investigation using generic models, F13 and F13X

The development of multiple-vortex wake systems in the far field of a generic aircraft model is assessed by evaluation of experimental data obtained from MonoPIV measurements in the large towing tank facility of the Hamburg Ship Model Basin (HSVA). In the selected experimental configurations DLR's F13X vortex generator has been operated at two geometrical configurations for three different chord-based Reynolds numbers. From the tips of the rectangular main wing a 2-vortex system shed off, and adding a horizontal tail plane to the vortex generator produced a 4-vortex system of two unequal-strength counter-rotating vortex pairs. The 2-vortex systems showed a gradual decay, its characteristic parameters have been evaluated well into the wake far field up to vortex ages of 4.5 without providing any evidence for cooperative long-wavelength instabilities. Contrarily, for the same Rec the counter-rotating vortex pairs of the 4-vortex systems underwent short-wavelength instabilities in the extended near field leading to a strong vortex interaction and rapid threedimensional reorganization. After the 'catastrophic event' only a weak 2-vortex systemremained that gradually decayed. Comparing the development of different 2-vortex systems in the given range of Rec, the behavior in the far field is Rec invariant. Also for the 4-vortex systems no distinct indication has been found for a significant Rec dependance

Vortex characterization in multiple-vortex systems of wing model wakes

In the wake of an aircraft large-scale vortical structures organize in multiple vortex systems with their vortex axes aligned approximately parallel to the wake axis. Equal-strength counter-rotating vortex pairs of 2–vortex wakes are known to be stable for rather high vortex ages until growing long-wavelength instabilities may lead to mutual interaction and final disintegration. Contrarily, unequal-strength counter-rotating vortex pairs eventually will grow short-wavelength cooperative instabilities and will undergo a rapid three-dimensional reorganization of the vortex system, as has recently been shown from numerical simulation [2, °3] and evidenced experimentally. By enforcing the generation of vortices that are prone to severe vortex interactions the wake vortex system could be diffused significantly faster. Such optimal perturbation by passive control of the wake could contribute to minimize aircraft separation distances necessary for air safety reason

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Experimental investigations of the influence of axial jets on 2- and 4-vortex systems

The effects of jets on wake vortices were investigated experimentally in a towing tank. Considered are two cases of 4-vortex systems consisting of two counter-rotating vortex pairs. For a comparisons, the corresponding 2-vortex system is also considered, too. The used vortex generator consisting of one or two rectangular wings was equipped with two pressurized water jet nozzles. Two different jet strengths were used, resulting in parameter valuess of R (ratio between jet thrust and vortex strength) of 0.2 and 0.74. Cross-plane velocity fields were determined by means of Stereo-PIV. The jet/vortex interactions are discussed, as well as the influence of the jets on the occurrence of an interaction between inner and outer vortices in the near far field, which is presumed to lead to a more rapid decay of the wing vortice

Analysis of Flow Field Measurements Obtained in a Large Tow Tank Regarding the Decay of Wake Vortices in the Far-Field for Two- and Four-Vortex Systems

Stereo PIV measurements on a 2- and 4-vortex system have been performed in a large tow tank of SVA in Potsdam. The considered 4-vortex system consists of two counter-rotating vortex pairs generated by wings of a span width ratio of b2/b1 = 0.3 and a circulation ratio of Γ2/Γ1 = -0.3. As reference the corresponding 2-vortex system is also considered. A submersible Stereo PIV system attached to a vertical traversing system was employed. Cross-plane velocity fields are obtained of the descending wake vortex system to access the vortex dynamics well into the wake far field (x/b > 500, τ > 11). The spatio-temporal evolutions of the vortex systems are analyzed with respect to the vortex trajectories, circulations and core radii as well as the total kinetic energy contained in the measured flow field

Identification and visualization of parallel vortex pairs in aircraft wakes from PIV data

Large-scale laboratory investigations of multiple--vortex wake systems provide time-resolved flow velocity fields normal to the wake axis. Large vortices shed off the wings and flaps are identified employing the Q criterion. Their cooperative mechanisms of generation and decay are evidenced experimentally from iso-surfaces of Q and of the axial vorticity ωx