Visualization and force measurement of high-temperature,supersonic impulse jet impinging on baffle plate

The flow visualization and force measurement of a supersonic impinging jet on a center-holed vertical baffle plate were investigated. Center-holed baffle plates of 2d to 5d in diameter, with a 1d center hole were tested, where d is the bore of the launch tube. The standoff distance of the baffle plates from the open end of the launch tube were varied to be from 2d to 5d. The supersonic impulse jet, with an incident shock wave of Mach 2.89 was produced by a high-enthalpy blast-wave simulator. The direction-indicating color schlieren method produced a two-dimensional density gradient of the flow field around the baffle plate. In addition, the flow fields were numerically analyzed, using two-dimensional asymmetric Euler equations. The results of the numerically-analyzed and the experimentally-visualized flow field agreed well. The visualized flow field indicates that the baffle plate should be at least 3.5d in diameter to deflect the supersonic impinging jet at an angle greater than a right angle. We have concluded that the representative method of designing muzzle brakes for military purpose accurately predicts the force yielded by the supersonic impinging impulse jet on the vertical baffle plate only when there is a large ratio of the baffle-plate diameter to the bore of the launch tube.     

A PIV study of a supersonic impinging jet

The characteristics of supersonic impinging jets are investigated using Particle Image Velocimetry (PIV). The purpose of the experiments is to understand the jet induced forces on STOVL aircraft while hovering close to the ground. For this purpose, a large diameter circular plate was attached at the nozzle exit. The oscillations of the impinging jet generated due to a feedback loop are captured in the PIV images. The instantaneous velocity field measurements are used to describe flow characteristics of the impinging jet. The important flow features such as oscillating shock waves, slipstream shear layers and large scale structures are captured clearly by the PIV. The presence of large scale structures in the impinging jet induced high entrainment velocity in the near hydrodynamic field, which resulted in lift plate suction pressures. A passive control device is used to interfere with the acoustic waves travelling in the ambient medium to suppress the feedback loop. As a consequence, the large scale vortical structures disappeared completely leading to a corresponding reduction in the entrainment.     

Plasma and heat flux characteristics of a supersonic ammonia or nitrogen/hydrogen-mixture direct-current plasma jet impinging on a flat plate

Spectroscopic and electrostatic probe measurements were made to examine plasma characteristics with or without a titanium plate under nitriding for a 10-kW-class direct-current arc plasma jet generator with a supersonic expansion nozzle in a low-pressure environment. Heat fluxes into the plate from the plasma were also evaluated with a Nickel slug and thermocouple arrangement. Ammonia and mixtures of nitrogen and hydrogen were used as a working gas. The NH/sub 3/ and N/sub 2/+3H/sub 2/ plasmas in the nozzle and in the downstream plume without a substrate plate were in thermodynamical nonequilibrium states. As a result, the H-atom electronic excitation temperature and the N/sub 2/ molecule-rotational excitation temperature intensively decreased downstream in the nozzle although the NH molecule-rotational excitation temperature did not show an axial decrease. Each temperature was kept in a small range in the plume without a substrate plate except for the NH rotational temperature for NH/sub 3/ working gas. On the other hand, as approaching the titanium plate, the thermodynamical nonequilibrium plasma came to be a temperature-equilibrium one because the plasma flow tended to stagnate in front of the plate. The electron temperature had a small radial variation near the plate. Both the electron number density and the heat flux decreased radially outward, and an increase in H/sub 2/ mole fraction raised them at a constant radial position. In cases with NH/sub 3/ and N/sub 2/+3H/sub 2/, a radical of NH with a radially wide distribution was considered to contribute to the better nitriding as a chemically active and non heating process.     

The connection between sound production and jet structure of the supersonic impinging jet

An experimental investigation into the sound-producing characteristics of moderately and highly underexpanded supersonic impinging jets exhausting from a round convergent nozzle is presented. The production of large plate tones by impingement on a square plate with a side dimension equal to 12 nozzle exit diameters is studied using random and phase-locked shadowgraph photography. Discrete frequency sound is produced in the near-wall region of the jet when a Mach disk occurs upstream of the standoff shock wave. Tones cease when the plate distance is approximately 2.2 free-jet cell lengths and the first and second shock waves are located in the free-jet positions. The production of impulsive sound appears to be associated with the collapse of the standoff shock wave during a portion of the oscillation cycle. Results from unsteady plate-pressure measurements indicate that plane-wave motion occurs in the impingement region and a secondary pressure maximum is observed on the plate adjacent to the flow region where sound appears to originate.     

Investigation of a supersonic free jet by means of a laser anemometer

The present paper reports on experimental investigations conducted by means of a laser anemometer in order to analyze a supersonic free jet escaping from a non-divergent nozzle into the free atmosphere. As the free jet is generated in a shock tube, thereby presenting a blow time of the order of 1 ms only, the laser anemometer also proved to be very suitable as a measuring probe to study transient phenomena involving high flow velocities. The experimental results achieved are compared to theoretical predictions.     

Visualization of developing high temperature supersonic impulse jet induced by blast wave simulator

In order to investigate the developing characteristics of a high temperature supersonic impulse jet under an atmospheric ambient condition, we generate jets induced by an incident shock wave at Mach number 2.89, emerging from the open end of a high enthalpy blast wave simulator (HEBS). Developing high temperature supersonic impulse jets were visualized by the direction-indicating color schlieren method and analyzed by numerical calculations. A comparison of the experimental results with literature shows that the unsteady boundary area generated by the HEBS, starts to grow in half the time that the unsteady boundary area needs after the incident shock wave emerges in the literature.     

Flow structures around an inclined substrate subjected to a supersonic impinging jet in laser cutting

In laser cutting, the flow structure around a substrate significantly affects the material removal rate, the cutting depth and the surface finish of the cutting front. In this paper, the phenomena of shock wave that is induced by a supersonic impinging jet emanating from a straight nozzle onto a substrate with varying inclined angles has been simulated numerically and visualized experimentally. The numerical model offers fairly good prediction in comparison with the experiments. It transpires that the angle of inclination has a significant and dramatic effect on the flow structure and that a large wall pressure with a steep gradient can be built up when the angle is large.     

Determination of the vibrational temperature of OCS in a supersonic free jet

The rotational temperature of OCS in the v₂ = 1 excited state and the vibrational temperature in supersonic free jets seeded in Ar were measured by a diode-laser spectrometer. From the hot band associated to the ν₁ band, the rotational temperature in the v₂ = 1 state was obtained to be 5.7 K and the vibrational temperature obtained for this state was 213 K.     

Isotope-selective infrared multiphoton dissociation of CF3Br in a supersonic free jet

The carbon-isotope selectivity in the multiphoton dissociation of CF₃Br is studied in the collisional region of supersonic free jet. The isotopic abundance of¹²C and¹³C in C₂F₆ formed by recombination of the dissociation products is measured with a quadrupole mass spectrometer. An enrichmet factor of 9.4 is obtained for¹²C with the 9R(30)CO₂ laser line while the factor of 6.9 is obtained for¹³C with the 9P(16) line.     

The fluorescence excitation spectrum of s-tetrazine cooled in a supersonic free jet

The fluorescence excitation spectrum of the ${}^1\text{B}{}_{\mathrm{3u}}\text{(v′ = 0) ←}{}^1\text{A}{}_{\mathrm{g}}\text{(v″ = 0)}$ transition in s-tetrazine has been observed and measured. The sample was cooled to a rotational temperature of <1 K by expansion in a supersonic free jet. In this way the rotational structure arising from asymmetry split low J lines could be observed. The rotational A and B axes of the ²H₁¹²C₂¹⁴N₄ isotope were observed to interchange upon electronic excitation and a theory describing the effect of this interchange upon the optical selection rules has been developed. Analysis of the resolved rotational structure suggests that the geometry change upon electronic excitation is smaller than that deduced from previous analysis of the room temperature optical spectrum.     

Spectrum of the sound produced by a jet impinging on the gas-water interface of a supercavity

An analysis is made of the sound generated by the impingement of an air jet on the gas-water interface of a supercavity. The water is in uniform low Mach number motion over the interface. The interface is rippled by the jet, which produces an unsteady surface force on the water that behaves as a dipole or monopole acoustic source, respectively, at high and low frequencies. In a first approximation the very large difference in the gas density and that of water implies that the surface force is similar to that occurring when a jet impinges on a rigid wall. Data from recent measurements by Foley (2009, Ph.D. Dissertation, Department of Mechanical Engineering, Boston University) of the frequency spectrum of the surface force produced by the impact of a turbulent jet on a wall are used to formulate an analytical representation of the spectrum and thence to predict the sound produced in water when the same jet impinges on the cavity interface. The prediction is used to estimate the characteristics of gas jet impingement noise for an experimental supercavitating vehicle in use at the Applied Research Laboratory of Penn State University.