Browsing by Subject "Jets -- Fluid dynamics"
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Item Computational modeling of synthetic jets(Texas Tech University, 2004-12) Jeyisanker, KalyaniThis thesis presents a CFD model that employs a novel moving grid methodology to investigate the interaction of an isolated synthetic jet in a crossflow. The current moving grid methodology is said to be 'novel' in the sense of formulation of equations in strong conservative form on a general, nonorthogonal grid system with moving boundaries. In addition, it also conserves space. This moving grid methodology can be easily extended and applied to other problems involving moving boundaries in the system. In the current study, numerical simulation was performed to investigate a timedependent behavior of a simplified configuration, which includes a two-dimensional, single synthetic jet interaction with a turbulent boundary layer. A simple, uniform, nonstaggered Cartesian grid of 270x74 was used. To employ an unsteady boundary condition, a moving diaphragm was introduced to a side wall of the cavity. Unsteady, Reynolds-Averaged, Navier-Stokes equations were solved numerically by a finitevolume method and applied to study the flow. The two-dimensional computational fluid code, HEAD2D was equipped with the standard k-e model. Programs were developed in Fortran. Ensight® was used to visualize the flow patterns. The current study focuses on phase averaged velocity fields, which allows one to study the evolution of the synthetic jet essentially as a function of time. Results on several phase-averaged velocity profiles obtained with this novel moving grid methodology considerably agree with the trend of data obtained from experimental setup by NASA Langley Research Center [13].Item Laser-induced breakup of a small liquid jet(Texas Tech University, 1990) Gritzo, Louis AlanNot availableItem Local instantaneous convective heat transfer characteristics from mechanical and supply pulsed radial reattaching nozzles(Texas Tech University, 1999-08) Furlow, John ScottThe heat transfer characteristics of combined mechanically and supply pulsated radial reattaching, CPRJR, nozzles were documented as a function of nozzle exit angle (0° and 20°), non-dimensional gap height (0.05 and 0.13), non-dimensional flow guide height (0.8 and 1.16), mechanical pulsation rate (5 and 10 Hz), supply pulsation rate (10, 20, 30, and 40 Hz), phase angle (0° and 180°), and Reynolds number (1683 and 2366). Air was forced through a supply pulsation mechanism and then through a pulsated nozzle diverter apparatus. The air impinged on a heated steel plate whereupon instantaneous heat flux and surface temperature measurements were collected and analyzed on instantaneous, time-averaged, and integrated area bases. Frequency analysis was made, via an FFT algorithm, of local instantaneous surface temperature and heat flux data. Nozzle exit velocity data was taken and a frequency analysis was performed. CPRJR nozzle heat transfer can be characterized as a frequency interference of mechanical and supply pulsation effects. CPRJR heat transfer values were similar to their mechanically pulsed equivalent excepting areas of low heat transfer that corresponded to the "flow off' portion of the supply pulsation cycle. The flow was blocked in the "flow off" portion of the pulsation cycle in an attempt to scour the boundary layer; however, air was entrained thus preventing any scouring of the boundary layers. For measured CPRJR cases, flow rate measurements were based upon the average flow rate over one mechanical pulsation cycle where the "on slot" supply pulsed flow rate was twice the mechanical flow rate. Variation in nozzle exit angle decreased the reattachment radius and caused a wide range of Nu number variation. Increasing of non-dimensional gap height and non-dimensional flow guide height induced a 50% increase in reattachment radius and a 50% drop in Nu number values. Increase in mechanical pulsation from 5 Hz to 10 Hz has little influence on reattachment radius and induces slightly higher heat transfer. Variation of phase angle and supply pulsation frequency produced a wide range of increased and degraded heat transfer values. Heat transfer from CPRJR nozzles was found to be consistently higher, up to 68% than the corresponding mechanically pulsated nozzle, for a supply-to-mechanical pulsation ratio of 1:1 with a phase angle, ct;=180°. A brief investigation was made to evaluate the basing of the "on slot" flow rate equal to the mechanical flow rate. Heat transfer results from this flow rate measurement scheme produced heat transfer variations generally less, 3% to 53%, than the corresponding PRJR cases. From the these results, it is concluded that supply pulsation does not enhance convective heat transfer.Item The convective velocity of large-scale structures in a turbulent two-dimensional jet(Texas Tech University, 1982-08) Wu, Sheng-chengSome of the deterministic characteristics of the coherent structures in the similarity region of a two-dimensional jet were investigated experimentally. The presence of the large-scale vortex-like structures within the flow field was confirmed with longitudinal velocity fluctuation cross-correlation measurements. The structures were found to have an asymmetric instantaneous distribution of longitudinal velocity fluctuation with streamwise periodicity and coherence over the entire width of the flow. Structural passage frequencies at various streamwise stations were measured on the basis of two-point longitudinal velocity fluctuation cross-correlations. The streamwise variation of this frequency parameter was found to be compatible with the similarity scaling relationships for the two-dimensional jet. The structural convective velocity in the fully turbulent region was measured and found to be a unique percentage of the local mean centerline velocity. This is in contrast to the results of previous investigations where the structural convective velocity appeared to be functionally dependent on the cross-stream coordinate.Item The effect of a piezoelectric perturbation on the spray distribution of an atomizing jet(Texas Tech University, 1990-08) Fernandez, Melvyn LThe effect of a finite, piezoelectrically induced, perturbation on the characteristics of a fluid stream was investigated in this study. The fluid stream characteristics considered were the mean drop size and the standard deviation of the spray distribution. Three parameters were considered to affect the spray distribution in this investigation: static supply pressure; frequency of the piezoelectric disturbance; and the amplitude of the piezoelectric disturbance. Mean and standard deviation data were collected for amplitudes of 6 and 15 volts. For each amplitude, all combinations of frequencies of 0, 250, 500, 750, 800, 900, 950, and 1000 kHz and pressures of 100, 200, 300, 400, 500 and 600 psi were used. Similarly, data were collected for frequencies of 500 and 1000 kHz, for all combinations of amphtudes of 0, 3, 6, 9, 12, and 15 volts and the same range of pressures. Experimental results show that there is a definite decrease in the mean and standard deviation of a spray distribution with an increase in pressure. Also, at pressures of 400, 500 and 600 psi the standard deviation decreased with higher amplitudes and frequencies. At 100 psi there was no effect of either frequency or amplitude on the mean and standard deviation of the spray distribution.Item The instability of liquid jets subjected to small perturbations(Texas Tech University, 1992-08) Gritzo, Louis AlanNumerous theories have been developed since the late 1800's in an attempt to predict jet behavior. However, the results of such theories have been difficult to apply to the physical problem. Existing theories require an accurate characterization of the flow at the nozzle exit for use as a boundary or initial condition. Furthermore, present methods of analyzing jet response only describe the nature of the flow near the breakup point, a location where jet behavior is highly nonline2Lr, and therefore most theories become increasingly inaccurate. For the purposes of examining the response of liquid jets to small naturally occurring disturbances inherent in a flow system, a new optical tool is developed. This non-intrusive technique allows the behavior of the jet to be examined near the nozzle exit, and upstream of the breakup point. Application of this tool allows the spatial growth rates and temporal variation of small naturally produced disturbances to be measured with significantly improved resolution and increased sampling rates. The development of this tool includes a derivation of the intensity distribution resulting from the incidence of collimated coherent monochromatic light on a dielectric cylinder. Two separate approaches are presented, the region of applicability being determined by the jet radius to light wavelength ratio. Using this tool, the stability of a vertical liquid jet exiting from a simple nozzle with a fully developed laminar velocity profile into ambient air is investigated. The magnitude 2uid frequency of temporal jet diameter variations are obtained at points along the axis of the jet. Spatial growth rates are also analyzed and compared to the predictions of existing theories. From these results, it was possible to conclude that liquid jet instability is not well represented by the transformed predictions of temporal instability analyses. Furthermore, two separate spatial regions of jet behavior were identified. For the conditions of this investigation, approximately the first 35% of the jet is dominated by the influence of velocity profile relaxation and appears to be characterized by the slow spatial growth of disturbances associated with low frequency variations in jet diameter. The results indicate that these low frequency disturbances are related to the formation of the smooth sinusoidal disturbances which are visible in photographs slightly upstream of the breakup point. The last 25% of the jet appears to be governed by the exponential growth of high frequency temporal disturbances. These high frequency changes in jet diameter are associated with the small short bulges which appear near the breakup point. The above two regions are separated by a transitional region which exhibits behavior indicative of the interference between disturbances corresponding to low and high frequency temporal variations in jet diameter.