An experimental study of the transverse injection of a particle laden liquid jet into a supersonic air stream was conducted. Five micron diameter silicon dioxide particles with a specific gravity of 2.35 were suspended in water and injected across a Mach 3.0 air stream. A stagnation pressure of 4.2 atm. and a stagnation temperature of 15° C were maintained throughout the tests. Particle loadings of up to 68% by mass were injected. The penetration and behavior of the jets were examined through a light extinction survey and through a series of streak arid nanoflash photographs. Some of the particles were found to agglomerate into clumps ranging up to 40 microns in diameter. The clumps separated from the liquid plume and penetrated up to 45% further into the air stream than the liquid. For mass loadings greater than 55%, the jet column appeared more rigid than those of lower or zero particle loadings. / Master of Science

Free jets are naturally unstable. As a result the jet which is initially laminar becomes turbulent. During this transition process, large-scale structures are formed and fluid is "induced" to join the jet from the surroundings. This induction of fluid is defined as entrainment and depends greatly upon the jet geometry. In particular, an elliptical jet has been found to entrain at a higher rate than its circular counterpart. Numerical simulations of elliptical jets have been conducted on NRL’s parallel CM-5 and CM-200 supercomputers using the Flux Corrected Transport algorithm and initialized with the results of a linear stability analysis with the objective of investigating the entrainment process and explaining the dependence of the entrainment on jet geometry. Through the medium of scientific visualization, mechanisms responsible for entrainment have been identified and associated with the results of a linear stability analysis to suggest passive means by which entrainment can be enhanced. In particular, it was found that increasing the aspect ratio, thinning the shear layer, and non-uniformly distributing the shear layer serve to increase entrainment. / Master of Science

Methods for the removal of heat from a sphere, via jet impingement by single and multiple jets was documented experimentally. Average heat transfer rates from a sphere maintained at constant temperature, by means of an internal electronic heater, and subjected to single or multiple jet impingements were obtained and related to the exit conditions of the impinging air jet(s) and to geometric parameters. The heat transfer rate was found to be insensitive to small changes in geometry. The heat transfer rate was found to increase with an increase in mass flow rate. The impingement of two jets was found not to be as efficient as a single jet using the same mass flow rate. Compressibility was found to decrease the heat transfer rate at high values of the Mach number. Attempts to increase the heat transfer rate by increasing the entrainment of the jet by acoustic or mechanical excitation or by the use of an elliptic orifice meet with no success. The decrease in velocity due to the increase in entrainment cancelled any benefit that was gained by increasing the entrainment of the jet. / Master of Science

Modeling of internal flow fields with hot, compressible fluids and sometimes combustion using cold flow techniques is discussed. The flow in a gas generator has been modeled using cold air. Experimental set up was designed and fabricated to simulate the unsteady flow with different configurations of inlet tubes. Tests were run for flow visualization and measurement of axial velocity at different frequencies ranging from 4 Hz to 12 Hz. Flow visualization showed that the incoming flow was a complex jet flow conformed to a cylindrical enclosure, while the outgoing flow resembled the venting of a pressurized vessel. The pictures show a complex flow pattern due to the angling of the jet towards the wall for the bent tube configurations and straightened flows with straight tube and other configurations with straighteners. Velocity measurements were made at an inlet Re of 8.1 x 10⁴ based on maximum velocity and inlet diameter using a single sensor hot wire anemometer at several locations in the plane of the inlet tube at 4 Hz, 8 Hz and 12 Hz for the straight tube and bent tube inlet configurations. The axial velocity near the entrance showed a strong component of the forcing frequency. Phase averaged mean velocities were observed to be well defined during charging and diminished during venting inside the cylinder. The jet flow penetrated most for the 4 Hz and least for the 12 Hz case. For the straight tube inlet comparison with a steady flow measurement of sudden expansion flow showed a qualitative similarity of the mean axial velocity distribution and centerline velocity decay during the charging phases. For the bent tube inlet case the contour plots showed the flow tendency towards the wall. Two cells were seen in the contours for the 8 Hz and 12 Hz cases. The deviation of the point of occurrence of maximum velocity in a radial profile was found to be about 6.5°. Entrance velocity profiles showed symmetry for the straight tube inlet while were skewed for the bent tube inlet. Contour plots of the phase averaged axial turbulence intensity for bent tube cases showed higher values in the core and near the wall in the region of impingement. Axial turbulence intensity measured for the straight tube case showed features as observed in an axisymmetric sudden expansion flow. / Ph. D.

An axisymmetric, turbulent water jet with several very slow, coflowing external streams was measured with a frequency shifted laser Doppler anemometer. The objective was to approximate a jet submerged in an ambient fluid of infinite domain by using a confined jet in a uniform coflow. The coflow prevents flow reversal outside the jet, but if the coflow velocity is not small compared to the jet velocity, the jet will no longer be self-preserving. Thus, the objective is reached in the limit as the coflow approaches zero, but in the absence of reverse flow. In the present study, a jet with several slow coflows was examined to investigate this behavior, the data was extrapolated to the limit (U_s/U_j) to obtain the free jet results and reduce uncertainty in earlier data. The Reynolds number based on the jet diameter and exit velocity was 32100. Conservation of momentum of the jet was demonstrated up to the measurement limit of x/d = 100. Its distribution suggests that the near fleld axial pressure variation has significant effects on the momentum flux. The results also indicate that momentum flux measurements require accurate data to the edge of the jet. The similarity of mean and rms velocity profiles suggest the existence of a region of self-preservation. The entrainment rate, centerline velocity decay rate and spreading rate of the jet were determined and compared to previous measurements with and without a coflowing stream. The variation of these jet parameters with respect to the veIocity ratios was obtained. The limiting values of the jet parameters were determined by extrapolation to zero velocity ratio. This study indicates that a slow coflowing stream is an ideal way to eliminate the recirculating zone present outside jets without coflows. By reducing the coflow to a negligible velocity with constant Craya-Curtet number, researchers can greatly reduce the wide experimental variation in jet entrainment and spreading rates found in different facilities. The results also indicated that a confined jet with a very slow coflow without recirculation can asymptotically approach the conditions of a free jet. An estimate of the variation of the duct size versus the velocity ratio is obtained. It suggests that it is not possible to reduce the velocity ratio to an arbitrarily small value without backflow because the duct would become impractically large. / Ph. D.

A detailed study of the effects of injectant physical properties on the break-up and atomization of a transverse liquid jet in a supersonic airstream was conducted. The tests were run at Mach 3 with ambient stagnation temperature and stagnation pressure of 2.4 atm. Viscosity and surface tension of the injectant along with the injector diameter and the ratio of the jet to freestream dynamic pressures were individually varied (µ= 1.0 - 59.8 centipoise, σ = 15, 33.5, 73.0 dyne/cm., d = 0.45, 0.96, 1.5 nm., q̅ = 1.20) and their effect on the structure and the atomization processes of the jet were established. The investigation employed a short exposure (9 x 10⁻⁹ sec.) photographic technique to establish the instantaneous structure of the jet in the crossflow. Relatively long exposure (10⁻³ sec.) photographs were obtained to study the time averaged behavior of the jet in the crossflow. Two multi-exposure photographic techniques were used to study the velocities of the surface waves that lead to jet break-up along the windward edge of the jet. By employing the Diffractively Scattered Light Method, the mean droplet diameter resulting from atomization at various transverse and axial locations in the spray plume was investigated. The important results are: 1) jet penetration in the crossflow initially increases with increasing viscosity and then decreases, 2) jet penetration is essentially independent of surface tension, 3) for the cases of moderate viscosity and surface tension (values approximately those of water) wave growth and cross fracture of the jet column of the jet is the main mechanism of breakup and atomization, 4) for high viscosity (µ > 40 centipose) ligament formation is the principal mechanism of atomization, 5) increasing viscosity reduces wave growth on the jet surface, 6) wave speed initially increases with increasing viscosity then decreases, 7) wave speed and liquid clump velocities increase with decreasing surface tension, 8) liquid clump velocity decreases with increasing viscosity and surface tension, 9) wave propagation speed is independent of q̅, 10) mean droplet diameter as the injector diameter decreases (D₃₂ = 14 at x/d = 207.7, y/d = 12, dⱼ= 0.45 mm.), 11) increasing viscosity increases droplet diameter (D₃₂ = 16 at x/d = 93.2, y/d = 12.4 µ = 1 .0 to D₃₂ = 21 at x/d = 93.2, y/d = 10.4, µ = 10.0), 12) decreasing surface tension decreases the droplet diameter (D₃₂ = 14, σ = 73.0 dyne/cm., D₃₂ = 5, σ = 15 dyne/cm.). / Ph. D.

The transient effects of the breakup and atomization of liquid jets in a crossflow on the size of droplets within the spray plume was experimentally determined. Water and water/methanol mixtures were injected normal to a high velocity air stream at Mach numbers of 0.48 and 3.0 with ambient stagnation temperature and respective stagnation pressures of 1.4 and 4.3 atm. The liquids were injected at liquid-to-gas momentum flux ratios ranging from 4 to 12. Droplet size distributions were obtained using a Fraunhofer diffraction technique at sampling rates of up to 9 kHz. Liquid mass flow rates were inferred from measurements of the extinction of a laser beam traversing the plume. The droplet sizes were found to fluctuate with frequencies of the order of 1 to 10 kHz. The fluctuations were characterized by a sudden and relatively brief increase in the mean diameter of the droplets caused by the passage of fractured clumps through the spray plume. Also evident in the droplet size distributions was the very small size of the droplets that had been sheared off the windward surfaces of the jet. The jet fracture frequency was related to the frequency of waves propagating along the initial jet column. The column waves are postulated to have been caused by jet perturbations created by vortices in the air flow around the jet column. / Ph. D.

No description available.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

The impingement of supersonic jets on surfaces is of interest because of its important application to jet blast deflectors (JBD), and short takeoff and vertical landing aircraft (STOVL) during hover. Typically, on an aircraft carrier deck, the impingement of the jet blast on the deflector generates impingement tones, and structural vibrations, not only on the JBD but also on the ship deck. Therefore, apart from direct transmission of jet noise to the gallery level, there is a component of noise transmitted due to the impingement of the jet on the JBD. The objectives of this work are to study the pressure spectra (i) on a flat plate, and separately on a cone due to axisymmetric impingement of a supersonic underexpanded cold jet issuing from a convergent-divergent nozzle and (ii) on a plane jet impinging on a finite plate and an adjoining ground plane due to the impingement of a planar jet on the plate. The characteristics of the surface pressure fluctuations are numerically investiga ted using WIND-US 2.0. The time-dependent, compressible Euler equations for perfect gas are employed for the present computations. The impingement distance between the jet nozzle and the deflector plate, and the plate inclination with respect to the incident jet are varied. The impingement zone stagnation bubble and a high-speed radial jet with several embedded structures (shocklets) were identified on the perpendicular plate. Flows involving cones reveal the presence of detached cone shocks, enclosing a recirculation zone. The location and magnitude of the peak pressure on the cone surface are a strong function of the cone apex angle. For the two-dimensional jet impingement on angled plate the peak value of pressure occurs at normal jet impingement. The pressure at the intersection point of the plate and the ground plane is sometimes higher than the peak pressure on the plate. Beyond this point there is a sharp decrease in pressure. As the flow accelerates, an oblique shock is / Beyond this point there is a sharp decrease in pressure. As the flow accelerates, an oblique shock is sometimes formed in this grazing flow region. A recirculation region at the lower lip of the nozzle was observed for all the separation distances and plate inclinations. / by Binu Pundir. / Thesis (Ph.D.)--Florida Atlantic University, 2011. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2011. Mode of access: World Wide Web.