Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session D18: Flow Instability: Multiphase Flow |
Hide Abstracts |
Chair: Khaled Sallam, Oklahoma State University Room: D135 |
Sunday, November 20, 2016 2:57PM - 3:10PM |
D18.00001: Supersonic Injection of Aerated Liquid Jet. Abhijit Choudhari, Khaled Sallam A computational study of the exit flow of an aerated two-dimensional jet from an under-expanded supersonic nozzle is presented. The liquid sheet is operating within the annular flow regime and the study is motivated by the application of supersonic nozzles in air-breathing propulsion systems, e.g. scramjet engines, ramjet engines and afterburners. The simulation was conducted using VOF model and SST k-$\omega $ turbulence model. The test conditions included: jet exit of 1 mm and mass flow rate of 1.8 kg/s. The results show that air reaches transonic condition at the injector exit due to the Fanno flow effects in the injector passage. The aerated liquid jet is alternately expanded by Prandtl-Meyer expansion fan and compressed by oblique shock waves due to the difference between the back (chamber) pressure and the flow pressure. The process then repeats itself and shock (Mach) diamonds are formed at downstream of injector exit similar to those typical of exhaust plumes of propulsion system. The present results, however, indicate that the flow field of supersonic aerated liquid jet is different from supersonic gas jets due to the effects of water evaporation from the liquid sheet. The contours of the Mach number, static pressure of both cases are compared to the theory of gas dynamics. [Preview Abstract] |
Sunday, November 20, 2016 3:10PM - 3:23PM |
D18.00002: Stability of two-layer Couette flow with application to drag reduction. Alireza Mohammadi, Alexander J. Smits We consider the linear stability of flows composed of two superposed fluids in Couette flow in order to improve our understanding of the longevity and performance of superhydrophobic surfaces (SHS) or liquid-infused surfaces (LIS) which are important for drag reduction. Here, we assume that the fluids are immiscible, incompressible, and Newtonian with constant properties. Single-fluid Couette flow is known to be linearly stable for any Reynolds number. However, inclusion of the second layer of fluid enriches the problem and introduces five new parameters: viscosity ratio, density ratio, thickness ratio, Froude number and Weber number. Two kinds of instability can appear: an unstable interfacial mode, and a Tollmein-Schlichting mode. In this work we parametrically study the flow stability with specific emphasis on the effects of viscosity ratio, interfacial tension, and thickness ratio. [Preview Abstract] |
Sunday, November 20, 2016 3:23PM - 3:36PM |
D18.00003: Mechanisms, role of vorticity, and time scales for planar liquid sheet breakup Arash Zandian, William Sirignano, Fazle Hussain The 3D, temporal instabilities on a planar liquid sheet are studied using DNS with level-set and VoF surface tracking methods. $\lambda_2$ contours relate the vorticity to the surface dynamics. The breakup character depends on the Ohnesorge number ($Oh$). At high $Oh$, hairpin vortices form on the braid and overlap with the lobe hairpins, thinning the lobes, which puncture creating holes and bridges. The bridges break, creating ligaments that stretch and break into droplets by capillary action. At low $Oh$, lobe stretching and thinning is hindered by high surface tension and splitting of the original Kelvin-Helmholtz vortex, preventing early hole formation. Corrugations form on the lobe edges, influenced by the split vortices, and stretch to form ligaments. Both mechanisms are present in a transition region that shifts in $Oh$ values based on the liquid/gas density ratio. Different characteristic times exist for the hole formation and the lobe and ligament stretching, related to surface tension and liquid viscosity, respectively. In the transition region, both times are of the same order. Streamwise vorticity triggers the 3D instabilities. Vorticity stretching and baroclinicity dominate, while the spanwise and cross-flow vorticity tilting are less important early in the breakup. [Preview Abstract] |
Sunday, November 20, 2016 3:36PM - 3:49PM |
D18.00004: Temporal length-scale cascade and expansion rate on planar liquid jet instability William Sirignano, Arash Zandian, Fazle Hussain Using the local radius of curvature of the surface and the local transverse dimension of the two-phase (i.e., spray) domain as length scales, we obtained two PDFs over a wide range of length-scales at different times and for different Reynolds and Weber ($We$) numbers. The PDFs were developed via post-processing of DNS Navier-Stokes results for a 3D planar liquid sheet segment with level-set and Volume-of-Fluid surface tracking, giving better statistical data for the length scales compared to the former methods. The radius PDF shows that, with increasing $We$, the average radius of curvature decreases, number of small droplets increases, and cascade occurs at a faster rate. In time, the mean of the radius PDF decreases while the rms increases. The other PDF represents the spray expansion in a more realistic and meaningful form, showing that the spray angle is larger at higher $We$ and density-ratios. Both the mean and the rms of the spray-size PDF increase with time. The PDFs also track the transitions between symmetric and anti-symmetric modes. [Preview Abstract] |
Sunday, November 20, 2016 3:49PM - 4:02PM |
D18.00005: Stability of three-layered core-annular flow Dipin Pillai, Subramaniam Pushpavanam, T Sundararajan Stability of a three-layered core-annular flow is analyzed using the method of modal linear stability analysis. A temporal analysis shows that the flow becomes unstable to two modes of instability when inertial effects are negligible. An energy budget analysis reveals that these two modes correspond to capillary instability associated with each fluid-fluid interface. With an increase in Reynolds number, the system exhibits additional Reynolds stress modes of instabilities. These modes correspond to the Tollmien-Schlichting type of waves associated with high Reynolds number shear flows, and are considered precursor to transition to turbulence. An investigation of the parameter space reveals that the system may simultaneously show up to 5 distinct modes of instability, viz., the two capillary modes at each interface and three Reynolds stress modes in the bulk of each phase. In addition, a spatio-temporal analysis shows that the Reynolds stress modes are always convectively unstable whereas the capillary modes may undergo a transition from convective to absolute instability with decrease in Weber number. To obtain encapsulated droplets in experiments, the operating parameters must be chosen such that the system lies in the regime of convective instability. [Preview Abstract] |
Sunday, November 20, 2016 4:02PM - 4:15PM |
D18.00006: A 3D Computational Study on the Air-Blast Atomization of a Planar Liquid Layer Robert Chiodi, Olivier Desjardins The air-blast atomization of a planar liquid layer is a complex fluid phenomenon involving the destabilization of a low speed liquid layer by a high speed gas coflow. While progress has been made in recent years on understanding the instability of the liquid surface, it remains difficult to accurately predict using stability analysis and requires special expertise and equipment to perform thorough experiments. Simulations provide an excellent way to conduct parametric studies to determine the effect of splitter plate geometry and momentum flux ratio on the frequency and wavelengths of instability, however, they are extremely difficult due to the high density ratio and large range of length and time scales present in the flow. Using an accurate conservative level set method in conjunction with a newly reformulated reinitialization equation, we perform 3D simulations of the air-blast atomization of a planar liquid layer and compare them to experiments. We then go on to explore the role momentum flux ratio plays in the longitudinal and transverse wavelengths of instability. [Preview Abstract] |
Sunday, November 20, 2016 4:15PM - 4:28PM |
D18.00007: Taylor-Couette flow instabilities in neutrally-buoyant suspensions Madhu Majji, Sanjoy Banerjee, Jeffrey F. Morris Experimentally-determined instabilities and flow states of a neutrally-buoyant suspension are described. The flow is studied in a concentric-cylinder device with inner-to-outer cylinder ratio of 0.877 with inner cylinder rotating and outer stationary. The cylinder length to annular gap ratio is 20, while the gap to particle size ratio is approximately 30, for spherical particles of 250 $\mu$m diameter. Using a slowly increasing or decreasing $Re$ ramp, the flow agrees with all expectations for the pure fluid, while a slowly decreasing (quasi-static) ramp is used for the suspension flow, which is found to be unstable at lower Reynolds number $Re$ (based on the effective viscosity) than pure fluid, and exhibits spiraling and ribbon states not found for a pure fluid with only inner cylinder rotating. Strikingly, the suspension at solid fraction $\phi\ge0.05$ goes unstable first to a nonaxisymetric state rather than axisymmetric Taylor vortices. At $0.1\le \phi \le 0,2$, the flow exhibits numerous states during quais-static ramping of $Re$, while for $\phi = 0.3$, the base state Couette flow gives way to wavy spirals (WS) at $Re \approx 80$ and exhibits only the WS state up to $Re = 150$. Transient behavior on sudden change of $Re$ and particle tracking will also be presented. [Preview Abstract] |
Sunday, November 20, 2016 4:28PM - 4:41PM |
D18.00008: Liquid-liquid flow past a bluff body Kyeong H. Park, M.I.I. Zainal Abidin, Panagiota Angeli, Lyes Kahouadji, Zhihua Xie, Omar K. Matar, Christopher C. Pain The generation of instabilities behind a bluff body bounded by a pipe wall and its effects on flow pattern transitions from separated to dispersed oil-water flows are studied. A cylindrical bluff body is located in the water phase and the transverse direction of the flow. Investigations are conducted for flow rates that result in stratified flow in the absence of the bluff body. A high-speed camera is used to track the interfacial waves while the velocity profile in the water phase is determined by PIV. Numerical studies on single-phase flow assist in designing new bluff bodies. The results showed that the choice of the bluff body and its location generated vortices with frequencies similar to unbounded flows that corresponded to Strouhal number of 0.2. In two-phase flows, the bluff body generates waves with frequencies similar to the von K\'{a}rm\'{a}n vortices in the water phase behind the cylinder. The formation of the waves depended on the distance of the bluff body from the oil-water interface. [Preview Abstract] |
Sunday, November 20, 2016 4:41PM - 4:54PM |
D18.00009: Flow behaviour and transitions in surfactant-laden gas-liquid vertical flows Ivan Zadrazil, Sourojeet Chakraborty, Omar Matar, Christos Markides The aim of this work is to elucidate the effect of surfactant additives on vertical gas-liquid counter-current pipe flows. Two experimental campaigns were undertaken, one with water and one with a light oil (Exxsol D80) as the liquid phase; in both cases air was used as the gaseous phase. Suitable surfactants were added to the liquid phase up to the critical micelle concentration (CMC); measurements in the absence of additives were also taken, for benchmarking. The experiments were performed in a 32-mm bore and 5-m long vertical pipe, over a range of superficial velocities (liquid: 1 to 7 m/s, gas: 1 to 44 m/s). High-speed axial- and side-view imaging was performed at different lengths along the pipe, together with pressure drop measurements. Flow regime maps were then obtained describing the observed flow behaviour and related phenomena, i.e., downwards/upwards annular flow, flooding, bridging, gas/liquid entrainment, oscillatory film flow, standing waves, climbing films, churn flow and dryout. Comparisons of the air-water and oil-water results will be presented and discussed, along with the role of the surfactants in affecting overall and detailed flow behaviour and transitions; in particular, a possible mechanism underlying the phenomenon of flooding will be presented. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700