D. J. Forliti

Vectoring Thrust in Multiaxes Using Confined Shear Layers

A fluidic scheme is described which exploits a confined countercurrent shear layer to achieve multiaxis thrust vector response of supersonic jets in the absence of moving parts. Proportional and continuous control of jet deflection is demonstrated at Mach numbers up to 2. for pitch vectoring in rectangular nozzles and multiaxis vectoring in axisymmetric nozzles. Secondary mass flow rates less than approximately 2% of the primary flow are used to achieve thrust vector angles exceeding 15 degrees. Jet slew rates up to 180 degrees per second are shown, and the fluidic scheme is examined in both static and wind-on configurations. Thrust performance is studied for external coflow velocities between Mach 0.3 and 0.7

Controlling turbulence in a rearward-facing step combustor using countercurrent shear

The present work describes the application of countercurrent shear flow control to the nonreacting flow in a novel step combustor. The countercurrent shear control employs a suction based approach, which induces counterflow through a gap at the sudden expansion plane. Peak turbulent fluctuation levels, cross-stream averaged turbulent kinetic energy, and cross-stream momentum diffusion increased with applied suction. The control downstream of the step operates via two mechanisms: enhanced global recirculation and near field control of the separated shear layer. The use of counterflow also enhances three dimensionality, a feature that is expected to be beneficial under burning conditions.

A Comparison of Fluidic and Physical Obstacles for Deflagration-to-Detonation Transition

Abstract : A fluidic obstacle has been proposed as an alternative to conventional deflagration-to-detonation transition (DDT) enhancement devices for use in a Pulsed Detonation Engine (PDE). Experimental results have been obtained utilizing unsteady reacting and steady non-reacting flow to gain insight on the relative performance of a fluidic obstacle. Using stoichiometric premixed hydrogen-air, transition to detonation has been achieved using solely a fluidic obstacle with comparable DDT distances to that of a physical orifice plate. Flame acceleration is achieved due to the intense turbulent mixing characteristics inherent of a high-velocity jet and the blockage created by the virtual obstacle. Turbulence intensity (T.I.) measurements, taken downstream of both obstacles with hot-film anemometry during non-reacting steady flow, show a conservative trend that a fluidic obstacle produces approximately a 240% increase in turbulence intensity compared to that of a physical obstacle. Ignition times were reduced approximately 45%, attributable to the increase in upstream T.I. levels relative to the fluidic obstacle during the fill portion of the PDEs cycle. Transition to detonation was obtained for injection compositions of both premixed stoichiometric hydrogen-air and pure air.

Flowfield Characteristics of a Confined Transverse Slot Jet

The current study explored the mean and turbulent flowfield features of a confined transverse slot jet. The slot jet spans 95 % of the full channel spanwise dimension, a geometrical feature found to result in a highly three-dimensional mean flowfield. The transverse slot jet produces a recirculation bubble that has similarities to that found for the flow downstream of a rearward-facing step. The flowfield results were compared with a 2:1 expansion ratio step flow and the observations were discussed in the context of how the transverse slot jet may provide advantages compared with a sudden expansion for subsonic combustors. High turbulence levels are achieved and large turbulent length scales are produced for strong transverse slot jets. The momentum ratio of the jet to that of the channel is found to be a governing parameter, and the dimensions of the recirculation zone scale with this parameter. A series of models constructed in ANSYS/CFX-10 were done to complement the experimental work and showed that the three-dimensionality of the mean flow disappears when the slot jet extends fully across the channel.

An experimental investigation of planar countercurrent turbulent shear layers

The spatial development of planar incompressible countercurrent shear layers was investigated experimentally. A facility was constructed to establish countercurrent shear layers without the formation of global stagnation in the flow. Particle image velocimetry was employed to obtain detailed measurements within the region of self-preservation for velocity ratios

Flame Holding and Combustion Characteristics of a Geometrical Flame Holder

U_{2}/U_{1}

On the mechanisms affecting fluidic vectoring using suction

between 0 and −0.3. The spatial growth rate of countercurrent shear layers was found to agree generally with simple analytical theory. At 30% counterflow, the growth rate was approximately twice as large as the case with no counterflow. Peak turbulence quantities, when normalized by the applied shear magnitude,

Chapter 8 – PREVAPORIZED JP-10 COMBUSTION AND THE ENHANCED PRODUCTION OF TURBULENCE USING COUNTERCURRENT SHEAR

\Delta U

On the influence of internal density variations on the linear stability characteristics of planar shear layers

, were found to be nominally constant for low levels of counterflow, but at counterflow velocities above 13% of the primary stream velocity, peak turbulence levels increased. The observed transition is accompanied by the development of mean flow three-dimensionality. The deviation occurs at a counterflow level that is in agree- ment with theoretical predictions for transition from convective to absolute instability.

Bias and precision errors of digital particle image velocimetry

Flame Stabilization in a high-speed premixed environment requires the presence of a mechanism to stabilize the flame. Bluff bodies or geometrical flame holders introduce a recirculation zone that anchor the flame. The current study considers the influence of equivalence ratio and the boundary layer state at the trailing edge of the flame holder on the flowfield and combustion characteristics. It was found that the recirculation zone is shortened as the equivalence ratio increases towards unity. A secondary shear region emerges downstream of the recirculation zone and is caused by the accelerated low-density combustion products. The emergence of the secondary shear region moves upstream with increasing equivalence ratio. Tripping the boundary layer causes a dramatic reduction in the length of the recirculation zone, and the secondary shear region is greatly augmented. Visualizations show that tripping the boundary layer resulted in a greatly disturbed flame near the trailing edge and large flame scales. Flowfield measurements suggest that the heat release is increased by approximately 50% when the boundary layer tripped.Copyright