Rarefied planar jets into vacuum

This paper presents a fundamental gaskinetic study on high speed rarefied jets expanding into vacuum from a cluster of planar exits. Based on the corresponding exact expressions for one planar jet, this paper straightforwardly derives the combined multiple jet flowfield solutions of density and velocity components, however, for the combined temperature and pressure solutions, extra attention shall be practiced. Several direct simulation Monte Carlo simulation results are provided and they validate these analytical solutions of rarefied planar jet flows.     

Characteristics of impact-driven high-speed liquid jets in water

This paper describes a preliminary investigation of the characteristics of high-speed water jets injected into water from an orifice. The high-speed jets were generated by the impact of a projectile launched by a horizontal single-stage powder gun and submerged in a water test chamber. The ensuing impact-driven high-speed water jets in the water were visualized by the shadowgraph technique, and the images were recorded by a high-speed digital video camera. The processes following such jet injection into water, the jet-induced shock waves, shock wave propagation, the bubble behavior, bubble collapse-induced rebound shock waves and bubble cloud re-generation were observed. Peak over-pressures of about 24 and 35 GPa measured by a Polyvinylidence difluoride (PVDF) piezoelectric film pressure sensor were generated by the jet impingement and the bubble impingement, respectively. The peak over-pressure was found to decrease exponentially as the stand-off distance between the PVDF pressure sensor and the nozzle exit increases.     

Global mode-based control of laminar and turbulent high-speed jets

The emitted noise from round jets is reduced using linear feedback controllers designed using structural sensitivity analysis. Linear global modes inform the selection and placement of the controller, and Navier–Stokes simulations are used to demonstrate effectiveness in a Mach-1.5 cold axisymmetric jet and in a Mach-0.9 cold turbulent jet. In both jets, each fitted with a cylindrical nozzle, the control reduces the radiated noise and modifies the baseflow in a way that enhances the relative amplitudes of low-frequency $\mathit{St}\approx 0.05$ global modes that do not have significant support in the acoustic field.     

Experimental study on hydrodynamic behaviors of high-speed gas jets in still water

The present paper describes experimental investigation on the flow pattern and hydrodynamic effect of underwater gas jets from supersonic and sonic nozzles operated in correct- and imperfect expansion conditions. The flow visualizations show that jetting is the flow regime for the submerged gas injection at a high speed in the parameter range under consideration. The obtained results indicate that high-speed gas jets in still water induce large pressure pulsations upstream of the nozzle exit and the presence of shock-cell structure in the over- and under-expanded jets leads to an increase in the intensity of the jet-induced hydrodynamic pressure.     

Reduced-order modeling of high-speed jets controlled by arc filament plasma actuators

Arc filament plasma actuators applied to high-speed and high Reynolds number jets have demonstrated significant mixing enhancement when operated near the jet column mode (JCM) frequency. A feedback-oriented reduced-order model is developed for this flow from experimental data. The existent toolkit of stochastic estimation, proper orthogonal decomposition, and Galerkin projection is adapted to yield a 35-dimensional model for the unforced jet. Explicit inclusion of a "shift mode" stabilizes the model. The short-term predictive capability of instantaneous flow fields is found to degrade beyond a single flow time step, but this horizon may be adequate for feedback control. Statistical results from long-term simulations agree well with experimental observations. The model of the unforced jet is augmented to incorporate the effects of plasma actuation. Periodic forcing is modeled as a deterministic pressure wave specified on the inflow boundary of the modeling domain. Simulations of the forced model capture the nonlinear response that leads to optimal mixing enhancement in a small range of frequencies near the JCM.     

Microstructure anomalies in the deformation of barriers under high-speed penetration by plane jets

Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 155–158, September–October, 1989.     

Visualization of high-speed gas jets and their airblast sprays of cross-injected liquid

A planar and instantaneous visualization study of high-speed gas jets and their airblast sprays was performed to qualitatively examine the different atomization performances of different gas nozzles. For the visualization of high-speed gas jets (with no liquid injected), Nd:YAG pulsed laser sheets imaged the clustered vapor molecules in the Rayleigh range (d?λ), condensed from the natural humidity during the isentropic gas expansion through a nozzle. This method visualized both underexpanded sonic gas jets from a converging nozzle (SN-Type) and overexpanded supersonic gas jets from a converging-diverging nozzle (CD-Type). When liquid is cross-injected, the same laser sheet images the spray droplets of relatively large sizes (d?λ). The present visualization results show that the SN-Type nozzle develops a wider spray than the CD-Type nozzle, quite probably because the SN-Type nozzle has a wider gas jet (in the absence of liquid) than the CD-Type. Also, the wider spray of the SN-Type nozzle lowers the probability of droplet coalescence and generates finer sprays compared to the CD-Type nozzle. These visualization results qualitatively agree with the previous quantitative finding of the different atomization characteristics of the two types of nozzles (Park et al. 1996).     

Space–time correlation measurements of high-speed axisymmetric jets using optical deflectometry

An optical deflectometry system is used to provide unique space–time correlation measurements at two positions separated by varying axial distances within a high-speed jet shear layer. The measurements were made for both pure air and for helium/air mixture jets at Mach numbers M=0.9 and M=1.5. The jets issue from round nozzles and the sensing volumes at the two measurement positions consist of small light filaments along spanwise lines that are tangential to the annular jet shear layer. Applying this technique to obtain measurements detailing the level of correlation, spectral content, and convection velocity for jet flows in these flow regimes near the end of the potential core is particularly important in the understanding and prediction of jet noise. Measurements near the end of the potential core along the jet lip line exhibit distinct cross-correlation curves for the pure air jet cases. However, helium/air mixture jets display much lower levels of correlation and little evidence of large-scale structure in the measured spectra. It is believed that the thick visual density gradients dominated by smaller scales throughout the shear layer of the helium/air mixture jets effectively mask the large-scale structure, thus, reflecting a limitation of this optical deflectometer. Finally, a decrease in normalized convection velocity with helium addition is observed.     

Spatial distribution of CO2 dimers in axisymmetric gas jets expanding in a vacuum

Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 56–62, January–February, 1989.     

Application of novel pressure-sensitive paint formulations for the surface flow mapping of high-speed jets

The principle of pressure-sensitive paints (PSPs) is based upon excitation of the luminophore molecules at a certain wavelength and the emission of this absorbed energy at a higher wavelength. By isolating these two wavelengths we insure that the results obtained are not affected by any background radiation. Various international research groups, such as: the Central Aero-Hydrodynamic Institute (Russia), the University of Washington, NASA Ames, Boeing and McDonnell Douglas (USA), have developed their PSP formulations and some are commercially available.Two paints, which have been developed in-house at the Aero-Physics Laboratory (APL) at the University of Manchester, are studied here. One formulation uses hydrochloric acid (PSP1–HCl) and the other acetone as the solvent (PSP2–Ace). The current study employs the well known schlieren photography technique together with the relatively new PSP method, with comparison to discrete measurements, to examine the flow through a two-dimensional air-ejector system and examines the efficacy of the PSP formulations in providing an accurate global pressure field of the aforementioned setup. Detailed analysis of the errors and drawbacks involved in PSP measurements along with possible solutions to overcome them are also presented. Fully expanded jet Mach numbers in the range of 0.52 ? M_j ? 1.36 were examined.     

Investigation on the generation process of impact-driven high-speed liquid jets using a CFD technique

High-speed liquid jets have been applied to many fields of engineering, science and medicine. It is therefore of benefit to all these areas to investigate their characteristics by modern and inexpensive methods using a computational fluid dynamics (CFD) technique. Previously, high-speed liquid jets have been studied experimentally using a momentum exchange method, called the “impact driven method (IDM)”, by which the impact of a high-velocity projectile on the liquid package contained in the nozzle cavity produced the jet. The shock pulse reflections in the cavity caused by the impact then drove a multiple pulsed jet from the nozzle exit. In this study, a two-fluid simulation consisting of liquid and air can be successfully calculated by using a two-phase flow mixture model and a moving mesh for the projectile motion. The CFD results show good agreement to the results of previous experimental studies, both quantitatively and qualitatively. For the first time, the wave propagation within the liquid in the nozzle has been captured and analyzed, thereby demonstrating the dynamic characteristics of multiple pulsed high-speed liquid jets initiated by the IDM. This provides a breakthrough in the simulation of the supersonic injection of a liquid into air by using a well-known and user-friendly CFD software. It is useful fundamental knowledge for future studies of high-speed injection with applications in all its related fields.     

Impact of vacuum degree on the aerodynamics of a high-speed train capsule running in a tube

Motivated by the growing scientific and engineering interest in evacuated tube railway transportation systems, in this paper we numerically study the influence of the vacuum degree on the flow field around a train capsule running in an evacuated tube with circular section. The vacuum degree is increased by lowering the nominal pressure inside the tube. The numerical simulations are fully verified by wind-tunnel experimental data of supersonic flows around a blunt body and in a scramjet combustion chamber, as well as by several numerical results in other related studies. The flow around the train capsule is characterized by a compression region in front of the train, a chocked flow near the train, and a complex highly unsteady region behind the train, where expansions waves and reflecting oblique shock waves exist. The total aerodynamic drag and the vacuum degree are found to be linearly related, revealing that lowering the nominal pressure can have a significantly beneficial effect on the aerodynamic performance of the train capsule. The aerodynamic heating due to compressibility effects and the increased pressure are more prominent along the centreline of the tube than on the tube wall. As the vacuum degree increases, the temperature and pressure differences between the front and the tail of the train and the intensity of the reflected shock waves become less significant, so that the extension of the expansion region in the train wake shortens.     

Characterization of a liquid-metal microdroplet thermal interface material

This work presents the characterization of a thermal interface material consisting of an array of mercury microdroplets deposited on a silicon die. Three arrays were tested, a 40 × 40 array (1600 grid) and two 20 × 20 arrays (400 grid). All arrays were assembled on a 4 × 4 mm² silicon die. An experimental facility which measures the thermal resistance across the mercury array under steady state conditions is described. The thermal interface resistance of the arrays was characterized as a function of the applied load. A thermal interface resistance as low as 0.253 mm² K W⁻¹ was measured. A model to predict the thermal resistance of a liquid-metal microdroplet array was developed and compared to the experimental results. The contact resistance of the mercury arrays was estimated based on the experimental and model data. An average contact resistance was estimated to be 0.14 mm² K W⁻¹.     

A comparison between synthetic jets and continuous jets

Experimental measurements and flow visualization of synthetic jets and continuous jets with matched Reynolds numbers are described. Although they have the same profile shape, synthetic jets are wider and slower than matched continuous jets. The synthetic jets are examined at higher Reynolds numbers than previously studied and over a large range of dimensionless stroke lengths.     

Acoustic-velocity measurement across the diameter of a liquid-metal column

Present techniques for measuring sound velocity in liquid metals have been limited by the use of transducers which cannot survive in extreme-temperature conditions. These methods also require relatively long measurement times. An optical noncontacting method has been developed which may be used for extremely short experimental times and very high temperatures and pressures. This technique is being incorporated into an isobaric-expansion apparatus in which a 1-mm-diam wire sample in a high-pressure argon-gas environment is resistively heated to melt within a time period of only a few microseconds. Before instability of the liquid column occurs, thermal expansion, enthalpy and temperature are measured. The addition of the sound-velocity measurement permits a more complete determination of the thermophysical properties of the liquid metal.