Clara Marika Velte

Helical structure of longitudinal vortices embedded in turbulent wall-bounded flow

Embedded vortices in turbulent wall-bounded flow over a flat plate, generated by a passive rectangular vane-type vortex generator with variable angle β to the incoming flow in a low-Reynolds-number flow ( Re = 2600 based on the inlet grid mesh size L = 0.039 m and free stream velocity U ∞ = 1.0 ms −1 ), have been studied with respect to helical symmetry. The studies were carried out in a low-speed closed-circuit wind tunnel utilizing stereoscopic particle image velocimetry (SPIV). The vortices have been shown to possess helical symmetry, allowing the flow to be described in a simple fashion. Iso-contour maps of axial vorticity revealed a dominant primary vortex and a weaker secondary one for 20° ≤ β ≤ 40°. For angles outside this range, the helical symmetry was impaired due to the emergence of additional flow effects. A model describing the flow has been utilized, showing strong concurrence with the measurements, even though the model is decoupled from external flow processes that could perturb the helical symmetry. The pitch, the vortex core size, the circulation and the advection velocity of the vortex all vary linearly with the device angle β. This is important for flow control, since one thereby can determine the axial velocity induced by the helical vortex as well as the swirl redistributing the axial velocity component for a given device angle β. This also simplifies theoretical studies, e.g. to understand and predict the stability of the vortex and to model the flow numerically.

Flow analysis of vortex generators on wing sections by stereoscopic particle image velocimetry measurements

Stereoscopic particle image velocimetry measurements have been executed in a low speed wind tunnel in spanwise planes in the flow past a row of vortex generators, mounted on a bump in a fashion producing counter-rotating vortices. The measurement technique is a powerful tool which provides all three velocity components in the entire measurement plane. The objective of this study is to investigate the effect of vortex generators in a turbulent, separating, low Reynolds number (Re = 20 000) boundary layer over a geometry which is generating an adverse pressure gradient similar to the flow past a wind turbine blade. The low Reynolds number is chosen on the basis that this is a fundamental investigation of the structures of the flow induced by vortex generators and the fact that one obtains a thicker boundary layer and larger structures evoked by the actuating devices, which are easier to measure and resolve. The flow behaves as expected, in the sense that the vortices transport high momentum fluid into the boundary layer, making it thinner and more resistant to the adverse pressure gradient with respect to separation. The amount of reversed flow is significantly reduced when vortex generators are applied. The idea behind the experiments is that the results will be offered for validation of modeling of the effect of vortex generators using various numerical codes. Initial large eddy simulation (LES) computations have been performed that show the same qualitative behaviour as in the experiments.

Estimation of burst-mode LDA power spectra

AbstractThe estimation of power spectra from LDA data provides signal processing challenges for fluid dynamicists for several reasons: acquisition is dictated by randomly arriving particles, the registered particle velocities tend to be biased toward higher values, and the signal is highly intermittent. The signal can be interpreted correctly by applying residence time weighting to all statistics and using the residence time-weighted discrete Fourier transform to compute the Fourier transform. A new spectral algorithm using the latter is applied to two experiments: a cylinder wake and an axisymmetric turbulent jet. These are compared with corresponding hot-wire spectra as well as to alternative algorithms for LDA signals such as the time-slot correlation method, sample-and-hold and common weighting schemes.

Flow diagnostics downstream of a tribladed rotor model

This paper presents results of a study of vortex wake structures and measurements of instantaneous 3D velocity fields downstream of a triblade turbine model. Two operation modes of flow around the rotor with different tip speed ratios were tested. Initially the wake structures were visualized and subsequently quantitative data were recorded through velocity field restoration from particle tracks using a stereo PIV system.The study supplied flow diagnostics and recovered the instantaneous 3D velocity fields in the longitudinal cross section behind a tribladed rotor at different values of tip speed ratio. This set of data provided a basis for testing and validating assumptions and hypothesis regarding classical theories of rotors.

Vortex Generator Flow Model Based on Self-Similarity

The present work examines the downstream vortex evolution behind a cascade of vortex generators producing counter-rotating vortices in a boundary layer of negligible streamwise pressure gradient. For the current study, which is an extension of the model presented in [4], these parameters are all seen to vary linearly in the downstream direction. Based on the experimental observations triggered by a previous study, the vortices generated by vortex generators have been observed to be self-similar for both the axial (

Reduction of noise and bias in randomly sampled power spectra

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Experimental and numerical investigation of the performance of vortex generators on separation control

) and azimuthal (

The effect of dead time on randomly sampled power spectral estimates

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Alteration of helical vortex core without change in flow topology

) velocity profiles. The previous model, which is based merely on

Multiple vortex structures in the wake of a rectangular winglet in ground effect

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