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

Wake Structures and Surface Patterns of the DrivAer Notchback Car Model under Side Wind Conditions

The flow field topology of passenger cars considerably changes under side wind conditions. This changes the surface pressure, aerodynamic force, and drag and performance of a vehicle. In this study, the flow field of a generic passenger vehicle is investigated based on three different side wind angles. The study aimed to identify vortical structures causing changes in the rear pressure distribution. The notchback section of the DrivAer model is evaluated on a scale of 1:4. The wind tunnel tests are conducted in a closed section with a splitter plate at a Reynolds number of 3 million. The side wind angles are 0∘ , 5∘ , and 10∘ . The three-dimensional and time-averaged flow field downstream direction of the model is captured by a stereoscopic particle image velocimetry system performed at several measurement planes. These flow field data are complemented by surface flow visualizations performed on the entire model. The combined approaches provide a comprehensive insight into the flow field at the frontal and side wind inflows. The flow without side wind is almost symmetrical. Longitudinal vortices are evident along the downstream direction of the A-pillar, the C-pillars, the middle part of the rear window, and the base surface. In addition, there is a ring vortex downstream of the vehicle base. The side wind completely changes the flow field. The asymmetric topology is dominated by the windward C-pillar vortex, the leeward A-pillar vortex, and other base vortices. Based on the location of the vortices and the pressure distributions measured in earlier studies, it can be concluded that the vortices identified in the wake are responsible for the local minima of pressure, increasing the vehicle drag

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

Phase-Averaging Methods for the Natural Flowfield of a Fluidic Oscillator

The presented study examines various methods for phase averaging the naturally oscillating flowfield of a scaled-up fluidic oscillator. No external trigger is employed to control the oscillation of the flow. Mathematical and signal conditioning approaches for phase averaging the data are categorized and described. The results of these methods are evaluated for their accuracy in capturing the natural flowfield. The respective criteria are based on the minimum fluctuation in oscillation period length, the conservation of velocity amplitudes, and the number of snapshots per phase-averaging window. Although all methods produce reasonable qualitative results, only two methods are identified to provide the desired quantitative accuracy and suitability for the investigated flowfield. The first method is based on conditioning a time-resolved pressure signal from the feedback channels in the oscillator. An autocorrelation applied to the reference signal improves the period identification. The second method employs a mathematical approach by means of proper orthogonal decomposition. Because the conventional use of proper orthogonal decomposition reveals shortcomings in quantitative accuracy, it is modified by imposing an even distribution of snapshots per phase angle window. The results demonstrate the feasibility and improved accuracy of the modified proper orthogonal decomposition. Therefore, accurate phase averaging can be conducted without the need for a time-resolved reference signal

Experimental investigations of mixing characteristics in model rotating detonation engine geometries

This work examines the mechanisms of reactant mixing in a model Rotating Detonation Engine (RDE) geometry. RDEs are emerging as one of the highest potential applications for achieving Pressure Gain Combustion (PGC). Reactant mixing has been identified as a crucial component of efficient RDE operation. Therefore, a scaled model of a typical RDE engine geometry was examined in a water tunnel using Planar Laser Induced Fluorescence (PLIF) to observe the influence of fuel injection position, confinement geometry, and blowing ratio on the mixing characteristics and quality of mixing