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We consider a single server queueing system with a finite capacity buffer in which the server is subject to breakdowns and repairs. At the time of a breakdown, the customers, if any, present in the system will either decide to stay or abandon according to a probabilistic mechanism. This decision is made by every customer present in the system independent of the others. We assume that the arrivals of customers occur according to a Markovian arrival process (MAP). The shocks that create breakdowns of the servers are also assumed to follow a MAP independent of the other MAP. The service and the repair times of the server are assumed to follow (possibly) different phase type (PH) distributions. An arriving customer finding the buffer full will be lost. Further, an arriving customer finding the server under repair may decide to balk or join the queue at that instant. This queueing system, which generalizes some of the well-known queueing models in the literature, is studied as a GI/ M/ 1- type with a finite state space. The model is analyzed in steady-state and illustrative numerical examples including an optimization problem of interest are presented. [ABSTRACT FROM AUTHOR]

The mixing field in a supersonic cold flow past a set of compression-expansion ramps mounted at the downstream end of a strut, with staged injection at two tandem locations on the strut upstream of the ramps, has been experimentally investigated. Air and helium have been used to inject into a primary seam of air at an inlet Mach number of 2.0. Two test section geometries, one having a flow cross section twice that of the other, have been considered. The larger test section has wall struts as well as a middle strut, whereas the smaller one has the strut only in the middle. Terminal bulk pressure and wall static pressure measurements, and Schlieren visualization have been performed. The Mie scattering technique has been used to obtain the concentration contours of the injectant across the flow cross section at several axial locations downstream of the injection points. The results indicate better mixing with helium injection and less influence of the boundary layer along the test section walls with the larger test section, as expected. DOI: 10.1061/(ASCE)0893-1321(2005)18:4(197) CE Database subject headings: Ramps; Mixing; Combustion; Experimentation.

The focus of the present work is the acoustic oscillations exhibited by confined supersonic flow past a rectangular cavity of varying length-to-depth (L/D) ratio, with a view to identify optimal dimensions for application in scramjet combustors. Experiments were conducted to study the acoustic oscillations induced by supersonic flow at a Mach number of 1.5 past a rectangular cavity of variable dimensions mounted on one wall of a rectangular duct. The effect of L/D ratio of the cavity on the dominant acoustic modes registered on the wall of the duct opposite to the cavity is investigated. The range of LID ratio varied is 0.25-6.25. The dominant acoustic modes and the amplitudes are observed to be quite sensitive to L/D ratio in the above range. Shifts in the dominant acoustic modes are observed predominantly for L/D [approximately equal to] 0.94 and L/D [approximately equal to] 1.5. The variation of the Strouhal number with L/D ratio indicates a transition in the modal content in the 0.94 < L/D < 1.5 range. Further shifts in the dominant frequencies are observed in the 1.5 < L/D < 5.0 range. Peak amplitudes occur at L/D ratios of around 0.75 and 2.25, with over twice the magnitude at the former than at the latter condition. Time-averaged schlieren visualization indicates the presence of quasi-steady shocks at about 0.75 the length of the cavity for L/D [less than or equal to] 1 as opposed to being nearly at the trailing edge for higher L/D ratio. Instantaneous phase-locked schlieren images show the quasi-steady shocks are due to the movement of vortices and compression waves along the length of the cavity. It is also observed that the number of vortices in the shear layer roll up along the length of the cavity increases corresponding to the mode shifts for cavities with L/D > 1. Such distinct streamwise oscillations are also observed for cavities with L/D < 1, when the length is appreciable. The presence of higher modes in the acoustic oscillations is correlated with shocks produced at the lip of the cavity at a different frequency than the compression waves inside the cavity. [DOI: 10.1115/1.1949642] Keywords: Cavity-Induced Oscillations, Supersonic Flow, Mode Shifts, Shear Layer Instability, L/D Ratio

A 2-D computational study of flow over a fully open sunroof of a car with a passenger compartment has been done on a scalar workstation. The critical velocity for resonance in the passenger compartment has been determined. A wind deflector angle study has been done to determine an adequate angle to suppress the resonance. A validation case of computing the resonance condition of the flow over a 3:1 depth to length rectangular cavity has also been done. These resonance studies show the potential of using scalar workstations for a CFD design study.

The use of a time-accurate compressible Navier-Stokes solver and a 1/16 scale model of the J-25 rocket engine helps numerical analysis of the startup and throttle-down nozzle flows for overexpanded flow conditions. The resulting flowfield is different from that obtained by inviscid analysis. The study reveals three patterns of flow structures. Hysteresis occurs for different pressure ratios. The experiments help determine the location of separation point and the lower end turning point of hysteresis.

The role of hydrodynamics resulting in the mixed-mode oscillations (MMO) of unsteady pressure in a backward facing step combustor with partially premixed flame is investigated using time-resolved particle image velocimetry and CH* chemiluminescence imaging. This mode of pressure oscillations comprises switching between high-amplitude oscillations dictated by acoustic mode and/vortex mode and low-amplitude oscillations. The high-amplitude oscillations are marked by intense flow and flame modulation, while the low-amplitude oscillations are accompanied by small-scale structures on the shear layer, that are sedate in driving the heat release oscillations. The small-scales override on the elongated flame that resides in the recirculation zone of the step. The key aspect of the present work is in identifying the transitional events that result in the decay/ onset of high-amplitudes. This is achieved by tracking vortex cores at each instant during the transition. The small-scale shear layer structures, that are present during low-amplitude events merge as they convect with the flow. The identified vortex cores on the shear layer and in the recirculation zone reduce in their multiplicity as the merger progresses. This subsequently results in the genesis of a large-scale structure that modulates the flame coherently, thereby triggering high-amplitude oscillations. This is corroborated with identifying the presence of a single vortex core resulting in high amplitude events. [ABSTRACT FROM AUTHOR]

The present work involves the use of Laser Doppler Velocimetry (LDV) technique to measure the acoustic admittance of aluminized composite solid propellant (AP/HTPB/Al) by experimentally measuring the instantaneous velocities of the particles of solid propellant during burning while the acoustic pressure oscillations are being imposed. Both velocity fluctuations and pressure fluctuations are measured synchronously. The probe volume of the laser is maintained at a constant distance from the burning surface by using a linear actuator. Tests were conducted for two aluminized composite solid propellants at low frequency ranges (130-312 Hz) of the longitudinal acoustic modes and mean chamber pressures ranging from 1 to 4 MPa. A rotary valve is designed to acoustically excite the chamber to the required resonant frequency. It was found that admittance rises with frequency and peaks around 260 Hz and then falls. The use of linear actuator mechanism significantly improves the velocity data rate (number of data points). Maintaining the probe volume at 800 to 900 µm above the propellant surface is very crucial to determine the admittance accurately. Test results revealed that a good agreement between the steady state mean burning rates with the window bomb experiments is achieved. The measured mean burn rates under imposed acoustic pressure oscillations shows the augmentation up to 55%. The admittance measured for the metallized composite propellant using this technique and the comparison with T-burner shows good agreement. The comparison of responses with LDV and T-burner and the augmentation factor due to acoustic oscillations using LDV are reported for the first time. [ABSTRACT FROM AUTHOR]

The atomization process and fuel placement inside a gas turbine fuel injector are investigated at realistic operating conditions to understand the role of individual flow processes and hardware components. A high pressure, high temperature spray test facility is developed that can simulate a wide range of aero-engine operating conditions. The fuel injector is a swirl cup - dual orifice nozzle combination which is used in the rich dome combustors of various aero-engines. The internal flow field is captured using time-resolved laser induced fluorescence imaging and phase Doppler interferometry measurements. A stage-wise characterization is adopted to track the spray formation as the flow evolves through the swirl cup. The fuel placement at the cup exit is mainly decided by pilot nozzle spray characteristics and the relative momentum exchange between the gas and liquid phases. At the swirl cup exit, two class of droplets are formed: (1) droplets originated directly from the pilot nozzle, which are convected downstream by the primary air swirl, (2) droplets originated from the rim, which are atomized by the shear layer. The relative percentage of these two droplet classes depends on the pilot nozzle operation at low and high power conditions. The effect of primary spray and wall-filming are minimal towards improving the atomization. In both low and high power operating cases, the counter-rotating shear layer interactions are identified as the major mechanism that is improving the atomization process in swirl cup. [ABSTRACT FROM AUTHOR]

Abstract This paper reports the effect of inlet flow turbulence intensity on the combustion instability characteristics in a backward facing step combustor. The inlet turbulence intensity is varied by a turbulence generator. Unsteady pressure measurements and OH* chemiluminescence images are recorded over a wide range of operating conditions at different inlet turbulence intensities. The study shows an early onset of instability at low turbulence level, i.e., higher turbulence postpones the onset of instability to higher Reynolds number Re and/or higher equivalence ratio Φ. The early onset of instability in the Re and Φ parameter spaces is due to the change in system parameters such as flame speed and size of the recirculation zone downstream of the step at different turbulence levels. Further, the onset is characterized as subcritical bifurcation. At low Re , the hysteresis zone width is small for low turbulence levels and it is large at higher turbulence levels; and at higher Re , the hysteresis width remains constant at all turbulence levels. Investigation of instability characteristics reveals that there are momentary slippages from limit cycle orbit into brief silent regimes in an intermittent manner. The frequency of occurrence of the momentary silent regimes increases with reduction in turbulence, indicating that higher turbulence helps in maintaining the system in a stable limit cycle orbit. High-speed chemiluminescence imaging reveals the necessity of the vortex rollup in the recirculation zone to grow up to the top wall by dilatation from the heat release for the onset of instability. Considerations of the effect of turbulence on both the flame speed and the recirculation zone size together explain all the observed bifurcation trends. These results suggest that inlet flow turbulence should not just be considered as background noise. The turbulence effects on both the flame and flow should be considered in predicting the instability characteristics. [ABSTRACT FROM AUTHOR]