Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session G36: Jets I |
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Chair: Tobias Rossmann, Lafayette College Room: Alcove A |
Monday, November 24, 2014 8:00AM - 8:13AM |
G36.00001: The influence of upstream boundary conditions on swirling flows undergoing vortex breakdown Lothar Rukes, Moritz Sieber, Kilian Oberleithner, Oliver Paschereit Swirling jets undergoing vortex breakdown are common in research and technology. In part this is because swirling jets are widely used to anchor the flame position in gas turbines. Recently, the benefit in terms of flashback safety of axial air injection via a center body in the upstream mixing tube of a simplified premixed burner was demonstrated, Reichel (ASME Turbo Expo 2014). However, the presence of a center body alone alters the upstream boundary conditions for the downstream swirling flow. This study investigates how different upstream conditions modify the downstream swirling jet in a more generic setup. A swirling jet facility is used, consisting of a swirler, a pipe, a nozzle and an unconfined part. The focus lies on two large-scale flow features: the precessing vortex core (PVC) and the recirculation bubble. The flow field is measured with Particle Image Velocimetry and proper orthogonal decomposition is conducted to extract the dominant coherent structures. Additionally, a feature tracking approach is used to track the instantaneous shape and position of the recirculation bubble. We find that different center bodies modify the inflow profiles of the unconfined part of the flow in a specific way. This leads to significant differences in the large scale dynamics. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G36.00002: Experimental investigation of the influence of temperature differences on the precessing vortex core in swirling jets Moritz Sieber, Lothar Rukes, Kilian Oberleithner, C. Oliver Paschereit Swirling jets undergoing vortex breakdown are commonly used in gas turbine combustors. The vortex breakdown is accompanied by a meandering motion of the vortex core around the jet axis. This is referred to as the precessing vortex core, or short PVC. Extensive research has been done on the occurrence of the PVC in isothermal swirling jets. It was demonstrated that the PVC is a global instability mode. Measurements of the isothermal flow in gas turbine combustors usually show the presence of the PVC. However, recent investigations at our institute revealed that the PVC may be supressed in the reacting flow, depending on the flame position. This feature of non-isothermal swirling jets is of particular interest, because the PVC is known to be a robust structure that is hard to suppress in general. A subsequent theoretical investigation of the flow showed that the suppression of the PVC is related to a change of the hydrodynamic stability. This is again related to the temperature distribution within the flow. In the presented work this phenomenon is experimentally investigated in a swirling jet, where temperature differences are generated by electric heating. Therefore, the influence of the temperature can be investigated separately from the combustion. The experimental investigations consistently show that the PVC is strongly reduced by imposing temperature differences on the flow field. These characteristics are obtained by particle image velocimetry and proper orthogonal decomposition. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G36.00003: Axial Reynolds Stress Budget for Turbulent Swirling and Non-Swirling Jets Sara Toutiaei, Jonathan Naughton The terms of the axial Reynolds stress budget were studied for turbulent swirling and non-swirling jets. Laser Doppler anemometry (LDA) was used for acquiring measurements at locations where the two jets were at similar stages of development. A large number of data ($\sim$50,000 samples) was obtained at each measurement point in order to achieve high accuracy for the third order moments. The mean velocity and Reynolds stress results were consistent with previous studies. Production, convection and turbulent transport terms of Axial Reynolds stress equation were used as a means to investigate the differences in the two jets. Higher production in the swirling jet contributes to the higher Reynolds stress magnitude compared with the non swirling jet. In general, production exhibits higher values compared with the turbulence transport for both jet cases. Near the center of the jet where production has near zero values, turbulence transport has higher magnitudes. The turbulence transport is thus moving turbulence away from where it is largely produced toward the center of the jet. The faster development of turbulence transport term coupled with higher production in the swirling jet is found to be responsible for its faster growth compared to the non-swirling jet. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G36.00004: Acoustic response of an isothermal coaxial swirling jet Saptarshi Basu, Santhosh Rudrasetty This experimental study concerns acoustic response of internal recirculation zone (IRZ) in an unconfined isothermal coaxial swirling jet (geometric swirl number: 2-3). Two IRZ modes with characteristic modified Rossby number \textit{Ro}$_{m}$ \textgreater 1 and \textit{Ro}$_{m} \le $ 1 are considered. It is observed that as the amplitude of excitation is increased till a critical magnitude the IRZ topology (time-mean streamline plot obtained from PIV) with \textit{Ro}$_{m} \le $ 1 \textit{fans-out}/widens. At super-critical amplitude the IRZ transits back and finally undergoes a transverse shrinkage. However, \textit{fanning-out} is absent in \textit{Ro}$_{m}$ \textgreater 1 IRZ mode as this flow is dominated by pressure deficit due to entrainment (at the interface between central and co-annular jet) effect when compared to radial pressure gradient due to rotational influence. Thus the central jet with high kinetic pressure exists in \textit{Ro}$_{m}$ \textgreater 1mode (swirl fails to penetrate till the central axis and impart recirculation) but not in \textit{Ro}$_{m} \le $ 1. The imparted acoustics fails to disrupt IRZ in \textit{Ro}$_{m}$ \textgreater 1against the high kinetic pressure of the central stream, failing to impart a \textit{fan-out}. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G36.00005: Instability wavepackets in optimally forced subsonic jets Onofrio Semeraro, Lutz Lesshafft Jets are known to be very receptive to ambient perturbations, due to their strong convective instability. Coherent wavepackets are formed as a result, which may, as recent experiments suggest, represent the dominant source of jet noise. We model these wavepackets as the linear flow response to a harmonic forcing input that yields the highest amplification in a fully non-parallel setting. Axisymmetric turbulent jets are considered. Mean flows are taken from numerical simulations as well as from experiments, characterized by high subsonic Mach numbers ($Ma=0.84$ and $Ma=0.9$) and high Reynolds numbers. The formalism relies on singular mode decomposition of the linear resolvent operator, based on the fully compressible Navier-Stokes equations. Two different objectives are used for the optimization: the maximum energy of the near-field wavepacket and the maximum radiated acoustic power. The effects of turbulence are modeled through a turbulent viscosity formulation. The predicted acoustic radiation will be compared against simulation and experiment, and the influence of the chosen turbulent viscosity model will be discussed. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G36.00006: Analysis of a steady laminar stagnation flow and its self-similarity properties Gianfranco Scribano, Fabrizio Bisetti The velocity field in a steady laminar stagnation flow is analyzed experimentally and numerically. The flow configuration is characterized by a stagnation plane formed between two streams flowing from opposite directions. This configuration is used in the study of flames and condensing aerosols. The flow is characterized geometrically by the nozzle diameter D and the separation H between the nozzles. Together with the bulk velocity U, the separation H is used to define the Reynolds number. Particle Image Velocimetry is used to measure the velocity field and simulations are conducted to further characterize the flow. For this analysis, we explore values of H/D equal to 0.5, 1, 1.5, and 2, and values of the Reynolds number equal to 300, 600, 900, and 1200. The analysis is repeated for four nozzles having identical shape and diameters D equal to 7.5, 15, 30, and 35 mm. Our results show that the non-dimensional velocity fields are parametrized well by Re and H/D for all the diameters and that the simulations agree with the PIV data very well. The non-dimensional fields depend mostly on H/D, while the influence of Re is negligible for Re $>$ 300, in accordance with theoretical results. The parameter H/D plays an important role in influencing the flow inside and outside the nozzle. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G36.00007: Global stability analysis of electrified jets Javier Rivero-Rodriguez, Miguel P\'erez-Saborid Electrospinning is a common process used to produce micro and nano polymeric fibers. In this technique, the whipping mode of a very thin electrified jet generated in an electrospray device is nhanced in order to increase its elongation. In this work, we use a theoretical Eulerian model that describes the kinematics and dynamics of the midline of the jet, its radius and convective velocity. The model equations result from balances of mass, linear and angular momentum applied to any differential slice of the jet together with constitutive laws for viscous forces and moments, as well as appropriate expressions for capillary and electrical forces. As a first step towards computing the complete nonlinear, transient dynamics of the electrified jet, we have performed a global stability analysis of the forementioned equations and compared the results with experimental data obtained by Guillaume et al [2011] and Guerrero-Mill\'an et al [2014]. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G36.00008: Study on a liquid jet with cavitation bubbles Akihito Kiyama, Yoshiyuki Tagawa A focused liquid jet is important in medical applications such as needle-free drug injection systems. A method for generating a liquid jet by laser-induced shock wave is proposed. However, there are some problems. Hence we examine another method for generating a focused liquid jet. We drop a liquid filled test tube on the rigid plate, leading to the emergence of a jet. Within certain experimental conditions, the jet velocity in our experiment agrees well with the semiempirical relation proposed by Tagawa, et al., (2012, Phys. Rev. X) and Peters, et al. (2013, J. Fluid Mech.). In other conditions, we find that the jet velocity remarkably increases. In order to understand the jet velocity increment, we use two high-speed cameras: One records motion of a jet. Another films cavitation bubbles inside a liquid bath. We categorize jets into three types based on their shape and the existence of cavitation bubbles. We find that the jet with cavitation bubbles is much faster than that without cavitation bubbles. For elucidating the mechanism of jet velocity increment, we discuss the effect of pressure wave, which propagates in a liquid bath. We propose a model for describing these phenomena and verify it experimentally. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G36.00009: Characterization of far-field jet flows from complex nozzles via Particle Tracking Velocimetry Jin-Tae Kim, Alex Liberzon, Carlo Zuniga Zamalloa, Leonardo P. Chamorro Understanding the advection, diffusion and mixing of turbulence and scalars of jet flows under various geometric configurations and Reynolds numbers is of high relevance in environmental and engineering applications. In this experimental study, we characterize the far-field turbulence of jet flows in the proximity of twelve rotor diameters downstream of a series of complex nozzle geometries. The jet flows are released into a still body of water from a series of nozzles of different cross sections but with common hydraulic diameter dh$=$ 0.01 m at a Reynolds number Re$=$ U0*dh/$\nu $ approx. 7000, where U0 is the flow velocity at the outlet of the jet and $\nu $ is the kinematic viscosity of the flow. The system is closed-loop and seeded with particles of 100 $\mu $m diameter. Results are analyzed from Lagrangian and Eulerian frames of references via 3D particle tracking velocimetry (OpenPTV, www.openptv.net). Lagrangian features of the particles are characterized in terms of the nozzle geometries and high-order turbulence statistics are obtained at various planes within the interrogation volume, which mimics 3D PIV. [Preview Abstract] |
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