Vortex Breakdown in a Swirling Jet with Axial Forcing

A swirling jet has been generated in water by passing the fluid through a rotating honeycomb and discharging it into a water tank. Experiments were conducted at 3000 < Re < 7000 and 1.15 < S < 1.5. In addition, axial periodic perturbations were applied in order to excite the shear layer by an axisymmetric mode m=0. The amplitudes of the forcing were in the range 4% < A < 44%. Quantitative measurements were carried out by using STEREO-PIV. Experiments show that at higher Reynolds numbers the vortex breakdown does not occur abruptly as often mentioned in the literature. Instead, all mean quantities which characterize the vortex breakdown show a continuous change with increasing swirl. The high turbulence level may explain the differences to former studies. Velocity contours of the cross-sectional view indicate azimuthal modes that decrease from m=4 close to the nozzle to m=2 near the vortex breakdown. Phase-locked data show that the location of vortex breakdown alternates with the forcing frequency without a significant mean displacement, whereas for a certain combination of frequency and amplitude it sheds downstream while losing its intensity

Spectral proper orthogonal decomposition

The identification of coherent structures from experimental or numerical data is an essential task when conducting research in fluid dynamics. This typically involves the construction of an empirical mode base that appropriately captures the dominant flow structures. The most prominent candidates are the energy-ranked proper orthogonal decomposition (POD) and the frequency ranked Fourier decomposition and dynamic mode decomposition (DMD). However, these methods fail when the relevant coherent structures occur at low energies or at multiple frequencies, which is often the case. To overcome the deficit of these "rigid" approaches, we propose a new method termed Spectral Proper Orthogonal Decomposition (SPOD). It is based on classical POD and it can be applied to spatially and temporally resolved data. The new method involves an additional temporal constraint that enables a clear separation of phenomena that occur at multiple frequencies and energies. SPOD allows for a continuous shifting from the energetically optimal POD to the spectrally pure Fourier decomposition by changing a single parameter. In this article, SPOD is motivated from phenomenological considerations of the POD autocorrelation matrix and justified from dynamical system theory. The new method is further applied to three sets of PIV measurements of flows from very different engineering problems. We consider the flow of a swirl-stabilized combustor, the wake of an airfoil with a Gurney flap, and the flow field of the sweeping jet behind a fluidic oscillator. For these examples, the commonly used methods fail to assign the relevant coherent structures to single modes. The SPOD, however, achieves a proper separation of spatially and temporally coherent structures, which are either hidden in stochastic turbulent fluctuations or spread over a wide frequency range

Top-down fabrication of ordered arrays of GaN nanowires by selective area sublimation

We demonstrate the top-down fabrication of ordered arrays of GaN nanowires by selective area sublimation of pre-patterned GaN(0001) layers grown by hydride vapor phase epitaxy on Al$_{2}$O$_{3}$. Arrays with nanowire diameters and spacings ranging from 50 to 90 nm and 0.1 to 0.7 $\mu$m, respectively, are simultaneously produced under identical conditions. The sublimation process, carried out under high vacuum conditions, is analyzed \emph{in situ} by reflection high-energy electron diffraction and line-of-sight quadrupole mass spectromety. During the sublimation process, the GaN(0001) surface vanishes, giving way to the formation of semi-polar $\lbrace1\bar{1}03\rbrace$ facets which decompose congruently following an Arrhenius temperature dependence with an activation energy of ($3.54 \pm 0.07$) eV and an exponential prefactor of $1.58\times10^{31}$ atoms cm$^{-2}$ s$^{-1}$. The analysis of the samples by low-temperature cathodoluminescence spectroscopy reveals that, in contrast to dry etching, the sublimation process does not introduce nonradiative recombination centers at the nanowire sidewalls. This technique is suitable for the top-down fabrication of a variety of ordered nanostructures, and could possibly be extended to other material systems with similar crystallographic properties such as ZnO.Comment: This is the accepted manuscript version of an article that appeared in Nanoscale Advances. The CC BY-NC 3.0 license applies, see http://creativecommons.org/licenses/by-nc/3.0

Informational Cascades: A Mirage?

Experimental research found contradictory results regarding the occurrence of informational cascades. Whereas Anderson and Holt (1997) confirmed the model of Banerjee (1992), and Bikhchandani et al. (1992) through lab tests, Huck and Oechssler (2000) came to contradictory results on crucial issues. This article presents experimental evidence supporting further doubts concerning "Bayesian" informational cascades: Just under two thirds of all decisions are characterized by an excessive orientation towards the private signal, and only a small number of the subjects (<6%) make rational decisions systematically and consistently

On the impact of swirl on the growth of coherent structures

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Spatial linear stability analysis is applied to the mean flow of a turbulent swirling jet at swirl intensities below the onset of vortex breakdown. The aim of this work is to predict the dominant coherent flow structure, their driving instabilities and how they are affected by swirl. At the nozzle exit, the swirling jet promotes shear instabilities and, less unstable, centrifugal instabilities. The latter stabilize shortly downstream of the nozzle, contributing very little to the formation of coherent structures. The shear mode remains unstable throughout generating coherent structures that scale with the axial shear-layer thickness. The most amplified mode in the nearfield is a co-winding double-helical mode rotating slowly in counter-direction to the swirl. This gives rise to the formation of slowly rotating and stationary large-scale coherent structures, which explains the asymmetries in the mean flows often encountered in swirling jet experiments. The co-winding single-helical mode at high rotation rate dominates the farfield of the swirling jet in replacement of the co- and counter-winding bending modes dominating the non-swirling jet. Moreover, swirl is found to significantly affect the streamwise phase velocity of the helical modes rendering this flow as highly dispersive and insensitive to intermodal interactions, which explains the absence of vortex pairing observed in previous investigations. The stability analysis is validated through hot-wire measurements of the flow excited at a single helical mode and of the flow perturbed by a time- and space-discrete pulse. The experimental results confirm the predicted mode selection and corresponding streamwise growth rates and phase velocities

Stability Analysis of Time-averaged Jet Flows: Fundamentals and Application

We report on experimental and theoretical investigations of shear flow instabilities in jet flows. Linear stability analysis is applied to the time-averaged flow taken from experiments, contrasting the ‘classic’ stability approach that is based on a stationary base flow. To some extend, mean flow stability eigenmodes may deal as a model for instability waves at their nonlinearly saturated state, which is typically encountered in experiments. The capability of mean flow stability models is first demonstrated on laminar oscillating jets where the primary interaction takes place between the mean flow and the instability wave. We then focus on turbulent swirling jets where additional interactions occur between the fine-scale turbulence and the instability waves. Swirling flows are widely used in combustion applications where the associated high turbulence levels and internal recirculation zones (vortex breakdown bubble) are exploited for flame stabilization. We demonstrate the application of mean flow stability analysis on the flow field of a industry- relevant swirl-stabilized flame. We show that the flame response to acoutstic perturbations is closely linked to the flow receptivity predicted from linear stability analysis, which suggests that the adopted theoretical framework is very useful for thermoacoustic modeling