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.

Thesis. 1976. M.S.--Massachusetts Institute of Technology. Dept. of Mechanical Engineering. / Microfiche copy available in Archives and Engineering. / Bibliography: p.163-164. / M.S.

In this analysis, the efficacy of cavitation jets for heat transfer enhancement was demonstrated. The cavitation jet was formed from a cluster of cavitation bubbles that are the result of a submerged piezoelectric diaphragms oscillating about a given velocity threshold Two different heaters operating in two different flow environments were examined. For each heater in each environment, the cavitation jet significantly increased the heat transfer

A numerical study of axisymmetric synthetic jet flow was conducted. The synthetic jet cavity was modeled as a rigid chamber with a piston-like moving diaphragm at its bottom. The Shear-Stress-Transportation (SST) k-omega and #61559; turbulence model was employed to simulate turbulence. Based on time-mean analysis, three flow regimes were identified for typical synthetic jet flows. Typical vortex dynamics and flow patterns were analyzed. The effects of changes of working frequency, cavity geometry (aspect ratio), and nozzle geometry were investigated. A control-volume model of synthetic jet cavity was proposed based on the numerical study, which consists of two first-order ODEs. With appropriately selected parameters, the model was able to predict the cavity pressure and average velocity through the nozzle within 10% errors compared with full simulations. The cavity model can be used to generate the boundary conditions for synthetic jet simulations and the agreement to the full simulation results was good. The saving of computational cost is significant. It was found that synthetic jet impingement heat transfer outperforms conventional jet impingement heat transfer with equivalent average jet velocity. Normal jet impingement heat transfer using synthetic jet was investigated numerically too. The effects of changes of design and working parameters on local heat transfer on the impingement plate were investigated. Key flow structures and heat transfer characteristics were identified. At last, a parametric study of an active heat sink employing synthetic jet technology was conducted using Large Eddy Simulation (LES). Optimal design parameters were recommended base on the parametric study.

Enhanced heat transfer in electronic hardware by direct, small-scale actuation is investigated experimentally in two test bed configurations. The first configuration exploits the unsteady motions induced by a vibrating reed embedded within a heated duct (in contact with hardware that needs cooling) to enhance forced convection transport heat from the duct surfaces. The flow within the duct is either exclusively driven by the reed or, for higher heat flux, is augmented by an induced core flow. The time harmonic motion of the reed results in the regular shedding of vortical structures that interact with the inner surfaces in the absence and presence of a core flow. The second configuration focuses on the effects of small scale motions induced by a synthetic jet on heat transfer within an advanced heat sink. The synthetic jets emanate directly through the base of the heat sink and induce a recirculating flow between the fins, resulting in a lower thermal resistance than what is typically achieved with traditional fans. The unsteady flow characteristics in both configurations are investigated using particle image velocimetry (PIV). Of particular interest are the effects of small-scale motions and enhanced mixing on heat transfer compared to conventional time-invariant flows at similar or higher Reynolds numbers.

published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

Particle-laden jets are an important type of multiphase flow which can be found in various natural and technical processes. This study focuses on the flow mechanisms in a horizontally discharging sediment-laden jet that is of particular interest in environmental science and engineering. Experimental techniques and mathematical models are developed to investigate horizontal sediment-laden jets, both for the buoyant and non-buoyant jet discharge cases. In the laboratory, the separation of images of the fluid and the particulate phases is achieved by harnessing light signals of visualization at different wavelengths. Whole field measurements of velocities of the two phases are made by the adoption of particle image velocimetry (PIV) algorithms. Numerical models are developed in two approaches with regard to the treatment of the particulate phase. In the Lagrangian approach, individual sediment particles are tracked while the flow field of the fluid phase is computed with large-eddy simulation (LES). This simulation successfully captures the transient nature of the particle-laden flow. In the Eulerian approach, a two-phase model is used to obtain steady flow simulations in a much shorter computation time. The experimental and numerical results for the horizontal momentum jets show that, at low initial particle concentrations, the sediment particles generally follow the jet flow but with some levels of deficit velocities. In the upper layer of the jet the particles do not follow the fluid flow as well as in its lower layer. More particles are observed in the lower layer than in the upper one. For the momentum-dominated zone of a horizontal buoyant jet, the flow exhibits similar behaviors as the horizontal particle-laden momentum jet, except that there are some slight modifications from the effects of buoyancy. In the bending zone of the buoyant jet, the effects of buoyancy become significant. Notably, the locations of maximum velocity magnitude and those of maximum turbulence intensity are well separated in this zone. A strong correlation of particle abundance and high turbulence intensity is observed in the lower outer jet layer in this bending zone. Significant modifications to the global behaviors of horizontal sediment jets are observed as the particle concentration increases to relatively high levels. The jet trajectories are brought downwards by the particle loads and the jet widths are also increased. For the flow regime being investigated, turbulence intensity in the fluid flow is found to be increased by the presence of sediment particles. The results suggest that turbulence helps suspend sediment particles in horizontally discharging jets. A Stokes number is proposed to represent the ability of particles to follow the fluid flow. It is defined as St=W_s/U_j , where ws is the particle settling velocity in still fluid and Uj is the jet exit velocity, which indirectly governs the turbulence characteristics of the jet flow. The advecting large eddies in a turbulent jet are found to play the role of organizing particles in patches. Interaction and coalescence between particle-concentrated eddies may result in the sudden drop of a group of particles, which contributes to sediments falling from a horizontal jet in the form of particle-rich “fingers”. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

Sediment-laden turbulent buoyant jets are commonly encountered in the natural and man-made environments. Examples of sediment-laden buoyant jets include volcanic eruptions, deep ocean hydrothermal vents (“black smokers”), ocean dumping of dredged spoils and sludge, and submarine discharge of wastewater effluent. It is important to understand the fluid mechanics of sediment jets for environmental impact assessment, and yet there is currently no general model for predicting the mixing of sediment-laden jets. This study reports a theoretical and experimental investigation the sediment mixing, fall-out and deposition from sediment-laden buoyant jets. It is well known that turbulence generates fluctuations to the particle motion, modulating the particle settling velocity. A general three-dimensional (3D) stochastic particle tracking model is developed to predict the particle settling out and deposition from a sediment-laden jet. Particle velocity fluctuations are modelled by a Lagrangian velocity autocorrelation function that accounts for the loitering and trapping of sediment particles in turbulent eddies which results in the reduction of settling velocity. The model is validated against results of independent experimental studies. Consistent with basic experiments using grid-generated turbulence, the model predicts that the apparent settling velocity can be reduced by as much as 30% of the stillwater settling velocity. The mixing and deposition of sediment-laden horizontal momentum jets are studied using laboratory experiments and 3D computational fluid dynamics (CFD) modelling. It is shown that there is a significant settling velocity reduction up to about 25-35%, dependent on jet turbulent fluctuations and particle properties. The CFD approach necessitates an ad hoc adjustment/reduction on settling velocity and lacks generality. Using classical solutions of mean velocity, and turbulent fluctuation and dissipation rate profiles derived from CFD solutions, 3D particle tracking model predictions of sediment deposition and concentration profiles are in excellent agreement with measured data over a wide range of jet flow and particle properties. Unlike CFD calculations, the present method does not require any a priori adjustment of particle settling velocity. A general particle tracking model for predicting sediment fall-out and deposition from an arbitrarily inclined buoyant jets in stagnant ambient is successfully developed. The model incorporates the three flow regimes affecting the sediment dynamics in a buoyant jet, namely turbulent jet flow, jet entrainment-induced external flow and surface spreading current. The jet mean flow velocity is determined using a well-validated jet integral model. The external jet-induced irrotational flow field is computed by a distribution of point sinks along the jet trajectory. The surface spreading current is predicted using an integral model accounting for the interfacial shear. The model is validated against experimental data of sediment deposition from vertical and horizontal sediment-laden buoyant jets. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

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