Supersonic Core Length Research Articles

Computational Investigation of Swirling Supersonic Jets Generated Through a Nozzle-Twisted Lance

The dynamic characteristics of supersonic swirling jets generated through a nozzle-twisted lance are numerically studied. The essential features of the swirling jets are identified by defining a deviation angle. The effects of nozzle twist angle (NTA) on swirling flow intensity, coalescence characteristics, and dynamic parameter distributions of the jets are discussed. The rotational flow characteristics are revealed. The results show that the jets from the nozzle-twisted lance are imparted to a circumferential rotating movement around the lance axis, and such swirling flow is enhanced by increasing NTA. The enhanced swirling flow causes weaker coalescence of the jets, faster attenuations of the axial velocity, and higher heat transfer rate between the jets and surroundings. The supersonic core length, however, is found to be less sensitive to the swirling flow intensity. The radial spreading of the jets, changing non-monotonically with NTA, arrives at its maximum at 5 deg of NTA. Furthermore, the swirling flow induces a considerable tangential velocity component, and as a result, a holistic and effective horizontal swirling flow field develops. The y-vorticity distribution range and the corresponding magnitude turn larger with increasing NTA, which promote the vortex motion of the local fluid element and thus intensify the local mixing.

Supersonic axisymmetric microjets: structure and laminar–turbulent transition

The supersonic core length of microjets and the influence of the laminar–turbulent transition on the core length are considered. Axisymmetric mini- and micronozzles with diameters from 341 to 10.4 μm are used. The microjet is studied with the use of a Pitot microtube, shadow flow visualization and hot-wire anemometry. It is demonstrated that the laminar–turbulent transition in the jet mixing layer exerts a dominating effect on the supersonic core length. The increasing of the supersonic core length is associated with the laminar flow in microjet. Decreasing of the supersonic core length is connected with the laminar–turbulent transition in microjet. Based on experimental results, a chart of microjet regimes is constructed. The influence of the Pitot tube diameter on the accuracy of supersonic core length determining is considered. The effect of the nozzle edge roughness on the supersonic core length is examined.

An experimental study of the structure of supersonic flat underexpanded microjets

We have experimentally studied the structure of supersonic flat underexpanded room-temperature air jets escaping from micro nozzles with characteristic heights from 47 to 175 μm and widths within 2410–3900 μm in a range of Reynolds numbers of 1280–9460. The dimensions of the first shock cell are established. The supersonic core length of supersonic flat underexpanded air jets has been determined for the first time. A flow regime with a large supersonic core length has been observed for air jets escaping from a 47-μm-high nozzle.

Study on supersonic rectangular microjets using molecular tagging velocimetry

In the present study, the characteristics of supersonic rectangular microjets are investigated experimentally using molecular tagging velocimetry. The jets are discharged from a convergent–divergent rectangular nozzle whose exit height is 500 μm. The jet Mach number is set to 2.0 for all tested jets, and the Reynolds number Re is altered from 154 to 5,560 by changing the stagnation pressure. The experimental results reveal that jet velocity decays principally due to abrupt jet spreading caused by jet instability for relatively high Reynolds numbers (Re > ~450). The results also reveal that the jet rapidly decelerates to a subsonic speed near the nozzle exit for a low Reynolds number (Re = 154), although the jet does not spread abruptly; i.e., a transition in velocity decay processes occurs as the Reynolds number decreases. A supersonic core length is estimated from the streamwise distribution of the centerline velocity, and the length is then normalized by the nozzle exit height and plotted against the Reynolds number. As a result, it is found that the normalized supersonic core length attains a maximum value at a certain Reynolds number near which the transition in the velocity decay process occurs.

Investigation of the structure of supersonic nitrogen microjets

Results of research on microjets escaping into an ambient space from nozzles with diameters of 341–10.4 μm are described. A special Pitot microtube is used for studying the structure of supersonic microjets. The main feature of this microtube is a small diameter of the intake hole (12 μm). The main parameters of supersonic underexpanded microjets are identified, including the size of shock cells and the supersonic core length of the microjet. The results show a significant increase in the supersonic core length of microjets compared to macrojets. The Reynolds numbers of the laminar–turbulent transition in microjets are found to be in the range of 1,100–2,100. In addition, a classification of supersonic underexpanded axisymmetric microjets escaping into the ambient space is proposed.

Effect of Co-Flow on Near Field Shock Structure

This paper presents the experimental results of underexpanded jets delivered from a central nozzle surrounded by a co-flow. The co-flow is found to be effective in elongating the supersonic core length of the central jet, at all levels of underexpansion. However, the expansion level of the central jet dictates the elongation caused by the co-flow.

Effect of cross-wire and tabs on sonic jet structure

An experimental study has been conducted to investigate the effectiveness of passive controls in the form of small tabs and a cross-wire projecting normally into the flow at the nozzle exit, on the characteristics of an axisymmetric sonic jet operated at three underexpansion levels. In this investigation, NPR 3, 5 and 7 were studied. The cross-wire and tabs were found to reduce the amplitude and axial extent of Pitot pressure oscillations and also the supersonic core length. Both cross-wire and tabs were found to be effective in modifying the jet structure significantly, resulting in faster characteristic decay of the jet at all the NPRs. Shadowgraph pictures captured the widening of the supersonic zone in the direction normal to the tabs/cross-wire.