Browsing by Author "Gregory, James W."

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INTERNAL FLOW PHYSICS OF A FLUIDIC OSCILLATOR SPRAY IN THE TRANSITION REGIME
(BEGELL HOUSE INC50 NORTH ST, DANBURY, CT 06810, 2016) Tomac, Mehmet N.; Gregory, James W.; 0000-0002-8589-8758; 0000-0003-1373-4639; AGÜ, Mühendislik Fakültesi, Makine Mühendisliği Bölümü; Tomac, Mehmet N.
An experimental investigation of the underlying flow physics of a dual-jet interaction fluidic oscillator spray has been conducted in the transition regime for a Reynolds number of 1680. The transition regime is defined as a narrow range of flow rates between two other operating modes of the fluidic oscillator. Particle image velocimetry (PIV) was used with refractive index-matching sodium iodide solution to minimize reflections from the spray geometry and obtain detailed internal velocity fields. PIV results show that the interaction of the two internal jets and the resultant vortices are responsible for the oscillation mechanism in the transition regime. Two side vortices sustain their existence throughout the oscillation period by altering their size, shape, and strength, and a dome vortex is created twice each oscillation period (once from each jet). The dome vortex plays a key role in the kinetic energy transfer mechanism inside the oscillator by means of jet bifurcations. The primary oscillation mechanism in the transition regime is that each internal jet's connection with the exiting jet is cut completely by the dome vortex in every period. This is in contrast to the low-flow rate oscillation mechanism, in which the oscillations are created by continuous collisions of the jets. Furthermore, the internal jets are observed to energize the side vortex on the opposite side of the chamber-a phenomenon that was not observed in the low-flow rate regime.
Internal jet interactions in a fluidic oscillator at low flow rate
(SPRINGER, 2014) Tomac, Mehmet N; Gregory, James W.; 0000-0003-1373-4639; AGÜ, Mühendislik Fakültesi, Makine Mühendisliği Bölümü; Tomac, Mehmet N
This study focuses on the internal jet interactions and the oscillation mechanism of the feedback-free fluidic oscillator at low flow rate, corresponding to a Reynolds number of 1,350 (based on exit nozzle width and average exit velocity). Particle image velocimetry (PIV) was used in this study with a refractive index-matched fluid to minimize reflections that would otherwise occur at the fluid-acrylic interface in the test setup. A simple microphone-tube sensor configuration generated a reference signal, with a phase-averaging method based on each quarter period for velocity time history reconstruction. PIV results revealed the existence of a vortex of fluctuating size, shape, and strength on each side of the oscillator; and two transient vortices that are formed in the dome region of the oscillator by each of the jets once per period. The dome vortices periodically bifurcate each of the jets and transfer some of the kinetic energy of that jet to the opposing jet. This kinetic energy transfer mechanism dictates the dominance of either jet at the exit, and this mechanism repeats itself to sustain the oscillations created by the fluidic oscillator. At this flow rate, the two jets form a continuous mutual collision, and the jets are never completely cut off from the exit. The oscillatory behavior at this flow rate is due to a complex combination of jet interactions and bifurcations, vortex-shear layer interactions, vortex-wall interactions, and saddle point formations.
Jet interactions in a feedback- free fluidic oscillator at low flow rate
(American Institute of Aeronautics and Astronautics Inc., 2013) Tomac, Mehmet Nazım; Gregory, James W.; 0000-0003-1373-4639; AGÜ, Mühendislik Fakültesi, Makine Mühendisliği Bölümü; Tomac, Mehmet Nazım
In this work, the internal fluid dynamics and frequency characteristics of feedback-free fluidic oscillators are investigated experimentally for low flow rates below 3.4 mL/s. The internal flow field of feedback-free fluidic oscillator was extracted using a refractive index-matched Particle Image Velocimetry (PIV) technique with the help of a problem-specific sensor setup for simultaneous frequency measurements in refractive index matching fluid. The oscillation mechanism for the low flow rate region was revealed with PIV measurements. Flow topologies extracted with the measurements were found to exhibit various flow features and the details of the jets interactions and vortical balance that lead to oscillatory behavior were discussed.
Oscillation characteristics of mutually impinging dual jets in a mixing chamber
(AMER INST PHYSICS, 1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA, 2018) Tomac, Mehmet N.; Gregory, James W.; AGÜ, Mühendislik Fakültesi, Makine Mühendisliği Bölümü;
In this study, we consider the oscillatory behavior of mutually impinging jets in an enclosed, domeshaped mixing chamber. The frequency of the impinging jet oscillations is dictated by the flow rate, with the oscillatory behavior being grouped into three regimes: a low flow rate regime (Re < 1500), a transition regime (1500 < Re < 2000), and a high flow rate regime (Re > 2000). The detailed characteristics of the oscillations in the high flow rate regime (Re = 6800 in the present study) are investigated through simultaneous frequency and refractive-index-matched particle image velocimetry measurements. The oscillation mechanism in the high flow rate regime was found to be similar to that of the other two regimes, where jets collide and interact in an oscillatory manner. However, in the high flow rate regime, there is a distinct and phase-evolving process of saddle point formation and jet bifurcation that is not present at the lower flow rates. The jet bifurcation process is also distinctly related to the balance of vortical structures inside the mixing chamber, and saddle point formation plays a key role in the internal and external flow field of this configuration. The external sweep angle of the exiting jet increases with the flow rate throughout the low and transition flow rate regimes, but a constant sweep angle was found to persist in the high flow rate regime. Thus, formation and location of the internal saddle point is directly linked to the external sweep angle of the jet. (C) 2018 Author(s).
Phase-Synchronized Fluidic Oscillator Pair
(AMER INST AERONAUTICS ASTRONAUTICS, 1801 ALEXANDER BELL DRIVE, STE 500, RESTON, VA 22091-4344 USA, 2019) Tomac, Mehmet N; Gregory, James W.; 0000-0003-1373-4639; AGÜ, Mühendislik Fakültesi, Makine Mühendisliği Bölümü
The relative phase of oscillating jets from a pair of fluidic oscillators was synchronized in this work. The means for this synchronization was mutual interaction through a shared feedback channel between the two oscillators. Flow visualization and hot-wire measurements indicated a strong correlation and phase synchronization between the two oscillators. A numerical analysis offered better understanding of the internal flow physics that led to the synchronization phenomenon. A portion of the output jet from one fluidic oscillator was redirected and crossed over into the adjacent oscillator, leading to momentum transfer between the two oscillators. A portion of this cross-oscillator flow was directed into the shared feedback channel and constituted the main feedback flow. In this process, one of the shared feedback channel outlets was blocked by a vortex, allowing only one oscillator to receive feedback flow. The primary mechanism for in-phase synchronization was the cross-oscillator flow, which was divided into phase-modulated momentum injection to the primary jet and modulated flow input to the shared channel feedback channel.
A review of fluidic oscillator development and application for flow control
(2013) Gregory, James W.; Tomac, Mehmet Nazım; 0000-0003-1373-4639; AGÜ, Mühendislik Fakültesi, Makine Mühendisliği Bölümü; Tomac, Mehmet Nazım
This review provides a detailed discussion of the historical development of fluidic oscillators and their application to flow control. Fluidic oscillators were initially developed in the 1960's for a variety of applications, and have seen resurgent interest for their suitability for modern flow control applications. The devices produce an oscillating jet of fluid over a wide fan angle and have no moving parts, making them an attractive actuator concept. This review aims to highlight the most important historical papers of relevance to modern fluidic oscillator development. The reviewed works will extend from the early 1960's to the most recent investigations, with a focus on the fundamental operating mechanisms of fluidic oscillators. The authors present this review as a short synopsis of fluidic oscillators for flow control, while a more comprehensive review will be submitted for archival publication in the near future.