Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session Q5: Jets: GeneralMultiphase Shear layer
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Chair: Gioacchino Cafiero, Imperial College London Room: 405 |
Tuesday, November 21, 2017 12:50PM - 1:03PM |
Q5.00001: Insights into the instability mechanisms of radially lobed nozzles Noushin Amini, Aarthi Sekaran The enhanced mixing capability of radially lobed nozzles (in comparison to circular nozzles) has promoted their use in varied applications -- the precise mechanisms however are yet to be fully understood. The present study was inspired by previous experimental ((Hu et al, 2000) and numerical studies (Cooper et al, 2005) of a six -- lobed nozzle. We have also carried out some preliminary hot-wire anemometry (Amini et al, 2012) and numerical studies (Amini and Sekaran, 2015, 2016) in the past, in order to qualitatively study the flow downstream from the nozzle and obtain a three-dimensional data set from Large Eddy Simulations. The current study employs a wavelet based analysis of our experimental data in order to isolate specific instability mechanisms and identifies these modes in our well resolved numerical studies. The analysis also `follows' the formation and the transport of coherent structures (from within the nozzle to within the free jet) and confirms these mechanisms. [Preview Abstract] |
Tuesday, November 21, 2017 1:03PM - 1:16PM |
Q5.00002: Modal Decomposition of Single- and Double-Pulsed Unsteady Jets Zheng Zhang, Dhuree Seth, Sravan Artham, Gordon Leishman, Ebenezer Gnanamanickam This study focused on the differences in the evolution of turbulent puffs, where two puffs separated by a short time interval (double-pulsed jet) are ejected from a circular nozzle into quiescent air. The $Re$ based on the jet exit velocity ($V_j$) and nozzle diameter ($d$) was 1,500. The interval between the puffs varied from $\delta t V_j/d =$ 225 to 787. The evolution of near jet exit flow field ($\leq 17d$) was measured using a time-resolved PIV system. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) were used to analysis the unsteady flow modes, as well as their spatial and temporal evolution. There were considerable differences in the coherent structure of the starting vortex for the second ejection because of wakened shear layer in the first ejection wake. These differences were observed from the POD energy distribution. The Ritz value of the dynamic modes indicated that the double-pulsed jet system is more stable than the single-pulsed jet. The stability of the double-pulsed jet increased with decreasing time interval between the ejections. A combination of the increase in momentum flux and the turbulent wake of the first ejection was found to stabilize the flow field of the second ejection. [Preview Abstract] |
Tuesday, November 21, 2017 1:16PM - 1:29PM |
Q5.00003: Identification of coherent wavy motion in round turbulent jets Rustam Mullyadzhanov, Richard Sandberg, Sergey Abdurakipov, William George, Kemal Hanjalic Large-scale coherent vortical structures are at the heart of free-shear turbulent flows, such as wakes, mixing layers and jets. These structures are involved in intensive mixing, entrainment and generation of aeroacoustic noise. We analyze direct numerical simulation data of a turbulent jet, performed with an in-house high-order finite-difference/pseudo-spectral code that solves the compressible Navier-Stokes equations. Using appropriate statistical tools we show that the jet dynamics can be represented as a superposition of propagating helical waves. We apply a snapshot-based Proper Orthogonal Decomposition to the azimuthally Fourier decomposed velocity fields for five cylindrical subdomains chosen at different downstream positions over a sufficiently long ensemble. We note that the main eigenfunctions with low non-zero azimuthal amplitudes come in pairs of (virtually) equal amounts of energy, taking the shape of helical vortices. Further decomposition of complex-valued temporal coefficients provides a convenient framework to analyze wavy motion downstream of the identified helical vortices and corresponding phase speed of these structures. The results show good agreement with the linear local stability analysis. [Preview Abstract] |
Tuesday, November 21, 2017 1:29PM - 1:42PM |
Q5.00004: Non-equilibrium turbulence scalings in turbulent planar jets Gioacchino Cafiero, John Christos Vassilicos A revised version of the Townsend George theory, as proposed by Dairay et al 2015, is applied to the study of turbulent planar jets (Cafiero and Vassilicos 2017). Requiring the self-similarity of only few quantities along with the non-equilibrium dissipation scaling law (Vassilicos 2015), it implies new mean flow and jet width scalings. In particular, the ratio of characteristic cross-stream to centreline streamwise velocities decays as the -1/3 power of streamwise distance in the region where the non-equilibrium dissipation scaling holds. In the definition of $C_\varepsilon$ both in Dairay et al 2015 and in Cafiero and Vassilicos 2017 the local Reynolds number is based on the local flow width rather than on the integral lengthscale. We verify that the ratio of the integral lengthscale to the flow width is constant, thus enabling the use of the integral flow width in place of the integral lengthscale for defining $C_\varepsilon$. The importance of this result is twofold: firstly it further strengthens the scalings obtained in the works of Dairay et al 2015 and Cafiero and Vassilicos 2017; secondly the flow width is immediately accessible by any mean flow measurement, whereas the estimation of the integral lengthscale often requires an additional hypothesis. [Preview Abstract] |
Tuesday, November 21, 2017 1:42PM - 1:55PM |
Q5.00005: The role of turbulence in the development of round jets in cross-flow Graham Freedland, Raul Bayoan Cal, Stephen Solovitz, Larry Mastin The behavior of wind-bent plumes is important to a wide variety of fields, including volcanic plume models. By understanding the fundamental physics involved in the mixing of a round jet in cross-flow, models predicting the concentration of volcanic ash can be improved by identifying the role of turbulence and wind speed on the rate of entrainment within the plume. Using laboratory experiments, jets of air are ejected orthogonally into a closed-loop wind tunnel with several cross-flow velocities. Mean flow statistics are collected using particle image velocimetry (PIV) to describe the production of turbulence within the jet. Mapping the trajectory of the jets and evaluating the Reynolds stresses produce a traceable shear layer that allows for control volume analysis of the jet. This is used to create an energy balance relating the inflow conditions to the entrainment of air. Further analysis provides relationships between the inflow conditions and the transfer of energy within the bending region of a jet. This is expanded through incorporation of an active grid system of winglets rotating within a predefined range of speeds to produce turbulence within the wind tunnel to identify the role of different turbulence properties on the development of jets in cross-flow. [Preview Abstract] |
Tuesday, November 21, 2017 1:55PM - 2:08PM |
Q5.00006: Free-stream turbulence influence on jets in cross-flow Raul Bayoan Cal, Graham Freedland, James McNeal, Larry Mastin, Stephen Solovitz A wind tunnel experiment is performed to produce a jet in cross-flow. Levels of background/inflow turbulence are varied to observe the effects on the development of the plume. The background turbulence is varied by means of a grid operated passively or actively; three levels are employed, two active and a passive. Flow fields are acquired using particle image velocimetry, thus providing access to computing first and second order statistics. The development of the jet is assessed in response to the free-stream turbulence. The mean flow as well as the Reynolds stresses prove to be susceptible to this effect. The findings have implications in description and modeling of volcanic plumes. [Preview Abstract] |
Tuesday, November 21, 2017 2:08PM - 2:21PM |
Q5.00007: 3D Velocity and Temperature Measurements of Freestream Turbulence Effects on a Pitched Jet in Crossflow David S. Ching, Haosen H.A. Xu, Christopher J. Elkins, John K. Eaton The effect of freestream turbulence on a heated round jet pitched 30$^{\circ}$ to a crossflow is studied with 3D velocity and temperature measurements. The jet and mainstream have the same velocity, corresponding to a jet Reynolds number of 3000. Two different turbulence grids give freestream turbulence levels of 5\% and 8\% at the hole location. The 8\% grid creates weak secondary flows in the mainstream, while the 5\% turbulence grid does not generate any measurable secondary flows. Magnetic Resonance Velocimetry and Magnetic Resonance Thermometry are used to acquire full-full 3D mean velocity and temperature fields. The results are compared to previous studies with no turbulence generators. The 5\% turbulence case has the most rapid thermal spreading and the lowest adiabatic wall temperatures averaged over the entire domain. However, the 8\% turbulence case has higher adiabatic wall temperatures. The mainstream secondary flows in the 8\% turbulence case push the injected fluid closer to the wall, raising the wall temperature and overwhelming the effect of turbulence. The results show that freestream turbulence increases thermal spreading, but that even small secondary flows overwhelm freestream turbulence effects. [Preview Abstract] |
Tuesday, November 21, 2017 2:21PM - 2:34PM |
Q5.00008: High Fidelity Simulation of Transcritical Liquid Jet in Crossflow Xiaoyi Li, Marios Soteriou Transcritical injection of liquid fuel occurs in many practical applications such as diesel, rocket and gas turbine engines. In these applications, the liquid fuel, with a supercritical pressure and a subcritical temperature, is introduced into an environment where both the pressure and temperature exceeds the critical point of the fuel. The convoluted physics of the transition from subcritical to supercritical conditions poses great challenges for both experimental and numerical investigations. In this work, numerical simulation of a binary system of a subcritical liquid injecting into a supercritical gaseous crossflow is performed. The spatially varying fluid thermodynamic and transport properties are evaluated using established cubic equation of state and extended corresponding state principles with established mixing rules. To efficiently account for the large spatial gradients in property variations, an adaptive mesh refinement technique is employed. The transcritical simulation results are compared with the predictions from the traditional subcritical jet atomization simulations. [Preview Abstract] |
Tuesday, November 21, 2017 2:34PM - 2:47PM |
Q5.00009: Wall-resolved Large Eddy Simulations of turbulent heat transfer in a T-junction Michail Georgiou, Miltiadis V. Papalexandris In this talk we report on wall-resolved Large Eddy Simulations of turbulent heat transfer between a cold crossflow and a hot incoming jet in a T-junction. Due to their high efficiency in mixing and heat transfer, T-junctions are encountered in numerous industrial applications. Our study is motivated by the need to assess phenomena related to thermal fatigue that are often encountered at their walls. We first describe the important features of the flow with emphasis on the shear layers that are formed at the entry of the jet and the recirculation regions. We also show results for first- and second-order statistics of the flow and compare our predictions with previous experimental data. Lastly, we present results from the spectral analysis of the temperature signal that we performed in order to assess the oscillating mechanisms that dominate the flow and the risk of thermal fatigue at the walls of the T-junction. [Preview Abstract] |
Tuesday, November 21, 2017 2:47PM - 3:00PM |
Q5.00010: Spray Formation from a Charged Liquid Jet of a Dielectric Fluid William Doak, Victor De Bellis, Paul Chiarot Atomization of a dielectric micro-jet is achieved via an electrohydrodynamic charge injection process. The atomizer is comprised of a grounded nozzle housing (ground electrode) and an internal probe (high voltage electrode) that is concentric with the emitting orifice. The internal probe is held at electric potentials ranging from 1--10 kV. A pressurized reservoir drives a dielectric fluid at a desired flow rate through the 100-micrometer diameter orifice. The fluid fills the cavity between the electrodes as it passes through the atomizer, impeding the transport of electrons. This process injects charge into the flowing fluid. Upon exiting the orifice, the emitted jet is highly charged and it deforms via a bending instability that is qualitatively similar to the behavior observed in the electrospinning of fibers. We observed bulging regions, or nodes, of highly charged fluid forming along the bent, rotating jet. These nodes separate into highly charged droplets that emit satellite droplets. The remaining ligaments break up due to capillarity in a process that produces additional satellites. All of the droplets possess a normal (inertial) and radial (electrically-driven) momentum component. The radial component is responsible for the formation of a conical spray envelope. Our research focuses on the jet, its break up, and the droplet dynamics of this system. [Preview Abstract] |
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