Underexpanded Axisymmetric Jet Research Articles

Azimuthal mode analysis of broadband shock-associated noise in an under-expanded axisymmetric jet

Broadband shock-associated noise (BBSAN) is a high frequency noise generated in imperfectly expanded jets from the interaction of turbulent structures and shock-cells. This paper presents an azimuthal mode analysis of BBSAN in the jet, near-field and far-field from an unstructured large-eddy simulation of a non-screeching supersonic under-expanded jet. The studied supersonic jet is issued from a round convergent nozzle with a perfectly expanded Mach number of 1.15 and Reynolds number based on the nozzle diameter of 1.25 × 106. Aerodynamic and far-field acoustic numerical results are validated against experiments. Pressure fluctuations represented in the frequency-wavenumber domain follow the neutral waves from a vortex-sheet model of an ideally expanded jet. The far-field pressure field is decomposed in azimuthal modes based on Fourier coefficients, showing three different regions: broadband behaviour of the mixing noise for downstream angles below 50° with the first modes containing most of the energy, BBSAN with similar amplitudes for the first 4 azimuthal modes at central angles between 50° and 130°, and distinct modes at upstream angles above 130°. A decomposition in negative/positive phase-velocity waves of the jet and near-field pressure field allows the identification of these characteristic modes. The complexity of the flow is illustrated with the analysis of the root-mean-square and spectral contents of the first 4 azimuthal modes. The modes exhibit peaks at different shock-cells and radial positions as well as depicting particular radiation patterns that depend on both the mode and frequency.

Influence of Nozzle Exit Conditions on the Near-Field Development of High Subsonic and Underexpanded Axisymmetric Jets

Detailed knowledge of jet plume development in the near-field (the first 10–15 nozzle exit diameters for a round jet) is important in aero-engine propulsion system design, e.g., for jet noise and plume infrared (IR) signature assessment. Nozzle exit Mach numbers are often high subsonic but improperly expanded (e.g., shock-containing) plumes also occur; high Reynolds numbers (O (106)) are typical. The near-field is obviously influenced by nozzle exit conditions (velocity/turbulence profiles) so knowledge of exit boundary layer characteristics is desirable. Therefore, an experimental study was carried out to provide detailed data on nozzle inlet and exit conditions and near-field development for convergent round nozzles operated at Nozzle Pressure Ratios (NPRs) corresponding to high subsonic and supersonic (underexpanded) jet plumes. Both pneumatic probe and Laser Doppler Anemometry (LDA) measurements were made. The data revealed that internal nozzle acceleration led to a dramatic reduction in wall boundary layer thickness and a more laminar-like profile shape. The addition of a parallel wall extension to the end of the nozzle allowed the boundary layer to return to a turbulent state, increasing its thickness, and removing vena contracta effects. Differences in nozzle exit boundary layers exerted a noticeable influence but only in the first few diameters of plume development. The addition of the exit extension removed the vena contracta effects of the convergence only design. At underexpanded NPRs, this change to nozzle geometry modified the shock cell pattern and shortened the potential core length of the jet.

Near Field of an Axisymmetric Underexpanded Jet and an Adjacent Parallel Surface

An underexpanded axisymmetric jet exhausting across an adjacent rigid planar surface generates a unique flowfield that has received little attention in the literature. To date, most published research within this area has been motivated by acoustics, and consequently little flowfield data exist. This study aims to provide such a data set for an axisymmetric jet operating at a nozzle pressure ratio of 5 and shows how variation in the plate-to-surface separation distance results in large-scale changes in the flowfield. High-resolution planar particle image velocimetry data were obtained on the geometrical symmetry plane. These data are complemented by schlieren imagery, surface oil-flow visualization, and steady pressure-sensitive paint measurements. Data were obtained for , 0.55, 0.60, and for the isolated jet (for comparison purposes). The data indicate that the flowfield is dominated by a shock/boundary-layer interaction generated due to the presence of the plate and that the shock/boundary-layer interaction subsequently alters the structure of the jet. The size, strength, and location of the shock/boundary-layer interaction vary as increases. The presence of the surface was also found to inhibit the screech cycle observed in the isolated jet case. The data presented herein form a baseline data set for an ongoing study in which the rigid surface is replaced with a compliant surface.

Study of Underexpanded Sonic Jets by Numerical Simulation

This paper deals with the numerical simulation of sonic underexpanded axi-symmetric jets, issuing from a convergent nozzle, at different nozzle pressure ratios. The simulations have been carried out by solving the unsteady Reynolds Average Navier-Stokes (RANS) equations utilizing the realizable k-ε (epsilon) two equations turbulence model in the commercial software FLUENT (version 6.3.26). The flow is assumed to be axi-symmetric and turbulent. The computational domain included a convergent nozzle of length 50 mm and a rectangular domain of size 25 times the nozzle exit diameter (D) verses 4D, which have been modeled and meshed in GAMBIT (version 2.46). The grid consisted of 81020 cells, while gradient grid adaptions were performed later to pronounce the accuracy of the results and visualize the shock-cell structures. From the simulations, it is inferred that, for correctly expanded jets, the potential core extends upto 6D, where D is the exit diameter of the nozzle. For underexpanded jets, the shock-cell structures in the jet flow field were captured properly. A pattern of ‘shock-diamonds’ or shock-cells were observed in the flow at low NPR. The formation of shock-cell is found to change considerably and also its strength increases with increase in nozzle pressure ratio (NPR). From this study, it is clear that the effect of favorable pressure gradient in the underexpanded jet influences the jet characteristics significantly. At high NPRs, ‘Mach disc’ formation is observed in the flow field. The flow after the Mach disc becomes subsonic and regains energy from the surrounding fluid.

1314 不足膨張軸対称噴流中の衝撃波のヒステリシス現象による噴流構造の遷移(流体力学III)

1314 不足膨張軸対称噴流中の衝撃波のヒステリシス現象による噴流構造の遷移(流体力学III)

Numerical Simulation of Highly Underexpanded Axisymmetric Jet with Runge-Kutta Discontinuous Galerkin Finite Element Method

Highly underexpanded axisymmetric jet was simulated using the Runge-Kutta Discontinuous Galerkin (RKDG) finite element method, which, based on two-dimensional conservation laws, was used to solve the axisymmetric Euler equations. The computed results show that the complicated flow field structures of interest, including shock waves, slipstreams and the triple point observed in experiments could be well captured using the RKDG finite element method. Moreover, comparisons of the Mach disk location exhibit excellent agreements between the computed results and experimental measurements, indicating that this method has high capability of capturing shocks without numerical oscillation and artificial viscosity occurring near the discontinuous point.

1605 不足膨張軸対称噴流中における衝撃波形状のヒステリシス現象(S35-2 圧縮性流れの基礎と応用(2),S35 圧縮性流れの基礎と応用)

In two-dimensional under-expanded supersonic jet, it is reported that the hysteresis phenomenon for the reflection type of shock wave in the jet is occurred under the quasi-steady flow in the recent studies. The transitional pressure ratio between the regular reflection and Mach reflection in the jet is affected by this phenomenon. Some papers have been described the hysteresis phenomenon for under-expanded supersonic jet. However the phenomenon under the under-expanded axi-symmetric jet has not been investigated satisfactorily. The purpose of this study is to clarify the hysterisis phenomena for the reflection type of shock wave at the under-expanded axisymmetric jet experimentally and to discuss the characteristics of hysteresis phenomenon.

Streamwise vortices in an underexpanded axisymmetric jet

This Letter describes an experimental investigation demonstrating the existence of streamwise vortices in an underexpanded jet issuing from a convergent axisymmetric nozzle at a moderate pressure ratio of 5.1, and at a Reynolds number of 1.8×106. These vortices are quite strong and are located in the supersonic region of the jet between the intercepting shock and the jet boundary. They seem to play a significant role by enhancing mixing between the ambient and jet fluids in the supersonic region of the jet. The scale of these vortices increases with downstream distance by a process of vortex merging.

Computational analysis of underexpanded jets in the hypersonic regime

Underexpanded axisymmetric jets are studied numerically using a full Navier-Stokes solver. Emphasis has been given to supersonic and hypersonic jets in supersonic and hypersonic ambient flows, a phenomenon previously overlooked. It is demonstrated that the shear layers and shock patterns in a jet plume can be captured without complicated viscous/inviscid and subsonic/supersonic coupling schemes. In addition, a supersonic pressure relief effect has been identified for underexpanded jets in supersonic ambient flows. While it is well known that an underexpanded jet in a quiescent ambience (or subsonic ambience) contains multiple shock cells, the present study shows that because of the supersonic pressure relief effect, an underexpanded jet in a supersonic or hypersonic ambience contains only one major shock cell.

Numerical investigation of ?lateral? interaction with an obstacle of an axisymmetric jet exhausting into vacuum

The problem of the interaction of a strongly underexpanded axisymmetric jet with an obstacle for which the normal to the surface makes an angle near π/2 with the jet axis is rather laborious for numerical solution due to the high disequilibrium of the gas-dynamic parameters in the peripheral part of the jet and the three-dimensional nature of the flow in the interaction region. Therefore, the results at present available have mainly been obtained experimentally [1, 2]. Among the theoretical studies made in this direction, it is necessary to mention Ivanov and Nazarov's [3], which gives the results of numerical investigation of “lateral” interaction of a jet with obstacles of various shapes in the case of weakly underexpanded jets when the flow in the interaction region is everywhere supersonic. In the present paper, a study is made of the case when a jet exhausts into vacuum and in front of the obstacle there is a detached shock wave, behind which there is mixed subsonic and supersonic three-dimensional flow.

The impingement of underexpanded, axisymmetric jets on perpendicular and inclined flat plates

This paper reports on an extensive experimental study of the flows due to under-expanded axisymmetric jets impinging on flat plates. The range of plate locations extends to a point where the jet is just subsonic but the main emphasis is on the behaviour in the first shock cell. Plate inclinations from 90° to 30° were investigated by means of comprehensive surface pressure measurements and shadowgraph pictures. Wherever possible, the main features of the results have been reconstructed using inviscid analyses of the wave interactions.The flows are shown to be extremely complex due to the local structure of the free jet and, particularly, due to interactions between shock waves in the free jet and those created by the plate. In the near field, these interactions tend to be the controlling factors but at larger distances from the nozzle, mixing effects become increasingly important.The maximum pressure on the plate when it is inclined can be very much larger than when the plate is perpendicular, owing to the possibility of high pressure recoveries through multiple shock systems. Correlations are presented for some of the main features on perpendicular plates and it is shown that the integrated pressure loads for both normal and inclined plates can be predicted well by a simple momentum balance.

The impingement of underexpanded axisymmetric jets on wedges

This paper reports an experimental investigation into the impingement of underexpanded axisymmetric jets on each of three wedges arranged symmetrically in the jets. Three jets were used; two were produced by a convergent-divergent nozzle of exit Mach number 2.2 which was operated at underexpansion ratios of 1·2 and 2, while the third jet was produced by a convergent nozzle operated at an underexpansion ratio of 4. The base widths of the wedges equalled the exit diameter of the convergent-divergent nozzle, their apex angles were 90°, 60° and 45° and they were situated within or just downstream of the first cell of each jet.Detailed front-face pressure distributions were obtained for 15 different configurations. Shadowgraph photographs were taken of these and other cases. The results show a variety of possible flow patterns. The major factors determining which flow pattern occurs are the combination of the centre-line Mach number and wedge apex angle and the jet shock strength and position. It was found that observed shock intersections can be reconstructed by means of shock polars, but that a four-shock confluence will not always occur when it is possible according to the shock polars. In one case there is evidence of the existence of a β triple-shock confluence. An interesting and unusual flow involving a stagnation bubble was obtained when the wedges were situated downstream of the free-jet Mach disk.

Underexpanded jet noise reduction using radial flow impingement.

Reduction of noise from underexpanded axisymmetric jet flows using radial jet flow impingement

Boundary of underexpanded axisymmetric jets in still air.

Boundary of underexpanded axisymmetric jets in still air.

Boundary of underexpanded axisymmetric jets issuing into still air

Boundary of underexpanded axisymmetric jets issuing into still air