Showing papers on "Supersonic speed published in 2016"

Input-output analysis of high-speed axisymmetric isothermal jet noise

Abstract: We use input-output analysis to predict and understand the aeroacoustics of high-speed isothermal turbulent jets. We consider axisymmetric linear perturbations about Reynolds-averaged Navier-Stokes solutions of ideally expanded turbulent jets with jet Mach numbers 0.6 < Mj < 1.8. For each base flow, we compute the optimal harmonic forcing function and the corresponding linear response using singular value decomposition of the resolvent operator. In addition to the optimal mode, input-output analysis also yields sub-optimal modes associated with smaller singular values. For supersonic jets, the optimal response closely resembles a wavepacket in both the near-field and the far-field such as those obtained by the parabolized stability equations (PSE), and this mode dominates the response. For subsonic jets, however, the singular values indicate that the contributions of sub-optimal modes to noise generation are nearly equal to that of the optimal mode, explaining why the PSE do not fully capture the far-fiel...

Investigation of tone generation in ideally expanded supersonic planar impinging jets using large-eddy simulation

Abstract: The generation of tones in a supersonic planar jet impinging on a flat plate normally has been investigated by performing compressible large-eddy simulations using low-dissipation and low-dispersion finite differences. At the exit of a straight nozzle of height $h$ , the jet is ideally expanded, and has a Mach number of 1.28 and a Reynolds number of 50000. Four distances between the nozzle and the plate between 3.94h and 9.1h have been considered. Flow snapshots and mean velocity fields are first presented. The variations of turbulence intensities and of the convection velocity in the jet shear layers are then examined. The properties of the jet near fields are subsequently described, in particular by applying Fourier decomposition to the pressure fields. Several coexisting tones appear to be generated by aeroacoustic feedback loops establishing between the nozzle lip and the flat plate, which also lead to the presence of hydrodynamic–acoustic standing waves. The tone frequencies are consistent with those given by the aeroacoustic feedback model and with measurements for high-aspect-ratio rectangular jets. The jet oscillation modes at these frequencies are characterized, and found to agree with experimental data. Their symmetric or antisymmetric natures are shown to be well predicted by a wave analysis carried out using a vortex sheet model of the jet, providing the allowable frequency ranges for the upstream-propagating acoustic waves. Thus, it is possible, for an ideally expanded impinging planar jet to predict both the frequencies of the tones and the symmetric or antisymmetric nature of the corresponding oscillation modes by combining the aeroacoustic feedback model and the wave analysis.

Aero-optical effects of an optical seeker with a supersonic jet for hypersonic vehicles in near space.

Abstract: The aero-optical effects of an optical seeker with a supersonic jet for hypersonic vehicles in near space were investigated by three suites of cases, in which the altitude, angle of attack, and Mach number were varied in a large range. The direct simulation Monte Carlo based on the Boltzmann equation was used for flow computations and the ray-tracing method was used to simulate beam transmission through the nonuniform flow field over the optical window. Both imaging displacement and phase deviation were proposed as evaluation parameters, and along with Strehl ratio they were used to quantitatively evaluate aero-optical effects. The results show that aero-optical effects are quite weak when the altitude is greater than 30 km, the imaging displacement is related to the incident angle of a beam, and it is minimal when the incident angle is approximately 15°. For reducing the aero-optical effects, the optimal location of an aperture should be in the middle of the optical window.

Composite materials for supersonic aircraft radomes with ameliorated radio frequency transmission-a review

Abstract: Encompassment of radar communication in modern avionics provides the backbone for safe flight and tactical warfare. In the past few decades radar technology has achieved commendable precision and sophistication, but efficient communication relies upon effective transmission of electromagnetic waves through materials shielding the antenna and exhibiting high transparency for radio/microwaves. Since the structure of radomes in supersonic aircraft is dominated by aerodynamic standards, numerous challenges are encountered while selecting the materials apposite for construction having superior mechanical strength and resilience while displaying the minimum influence on the transmission profile of radio waves. The radome structure of aircrafts is subjected to sudden acceleration, drag forces and erosion due to rain and dust, which change the transmission characteristics due to change in shape/thickness of the structure. The problems are further complicated by the advent of supersonic/hypersonic aircrafts and missiles where high strength preservation and structural integrity is necessary at temperatures in excess of 350 °C. This review systematically throws light on the composite materials and existing technologies employed for the fabrication of high strength, low dielectric loss sandwich radomes for supersonic aircrafts.

Neurofuzzy-Model-Based Unsteady Aerodynamic Computations Across Varying Freestream Conditions

Abstract: This paper presents a reduced-order modeling approach based on recurrent local linear neurofuzzy models for predicting generalized aerodynamic forces in the time domain. Regarding aeroelastic applications, the unsteady aerodynamic loads are modeled as a nonlinear function of structural eigenmode-based disturbances. In contrast to established aerodynamic input/output model approaches trained by high-fidelity flow simulations, the Mach number is considered as an additional model input to account for varying freestream conditions. To train the relationship between the input parameters and the corresponding flow-induced forces, the local linear model tree algorithm is adopted in this work. The proposed method is tested exemplarily with respect to the AGARD 445.6 configuration in the subsonic, transonic, and supersonic flight regimes. It is shown that good conformity is obtained between the reduced-order model results and the respective full-order computational-fluid-dynamics solution. A further comparative an...

Structure improvements and numerical simulation of supersonic separators

Abstract: In the separation process of the conventional supersonic separator, the shockwave easily occurs in the nozzle expanding section before the cyclone, the low temperature section is short and the cooling effect is not satisfied, and the swirling flow occurs in subsonic conditions with poor efficiency. Based on this, structure improvement of the supersonic separator was carried out by reducing the nozzle expand angle and extending the length of expanding section, which helps to improve the refrigeration performance and separation efficiency of the separator. The flow characteristics of natural gas in the improved supersonic separator were obtained by three-dimensional numerical simulation. The results show that the shockwave occurs in the back of the cyclone and the improved supersonic separator can achieve a wider range of deep cooling. The shockwave moves back with the increase of the pressure loss ratio, when the pressure loss ratio increases to 70%, the position of shockwave moves back to the diffuser. The numerical results show that when the inlet pressure is 600 kPa and the pressure loss ratio is 47.5%, the improved supersonic separator has a good cooling and separation performance.

Entropic lattice Boltzmann model for gas dynamics: Theory, boundary conditions, and implementation

Abstract: We present in detail the recently introduced entropic lattice Boltzmann model for compressible flows [N. Frapolli et al., Phys. Rev. E 92, 061301(R) (2015)]. The model is capable of simulating a wide range of laminar and turbulent flows, from thermal and weakly compressible flows to transonic and supersonic flows. The theory behind the construction of the model is laid out and its thermohydrodynamic limit is discussed. Based on this theory and the hydrodynamic limit thereof, we also construct the boundary conditions necessary for the simulation of solid walls. We present the inlet and outlet boundary conditions as well as no-slip and free-slip boundary conditions. Details necessary for the implementation of the compressible lattice Boltzmann model are also reported. Finally, simulations of compressible flows are presented, including two-dimensional supersonic and transonic flows around a diamond and a NACA airfoil, the simulation of the Schardin problem, and the three-dimensional simulation of the supersonic flow around a conical geometry.

On the convection velocity of source events related to supersonic jet crackle

Abstract: An image analysis method is developed and applied to shadowgraph images of supersonic jet flow to measure shock front propagation angles at numerous interrogation points distributed throughout the quiescent region outside of the jet shear layer. These shock fronts manifest in acoustic measurements of jet noise as steepened temporal waveforms that have been linked to the perception of crackle. The analysis method uses the Radon transform to quantitatively determine a local shock front propagation angle at each point. The dataset of angles is subsequently used to determine the locations and convection velocities of the sources inside the jet shear layer. The results indicate that the shock-like waves emerge immediately from the jet shear layer and are created by the supersonic convection of coherent structures. The statistical distribution of convection velocities follows an extreme value distribution, indicating that the shock front emitting sources are maxima of the underlying turbulence. A noise reduction method known to reduce the convection velocities in the jet shear layer is applied to the jet to investigate the effect on the shock front emission. The shock front angles change in concert with the reduction in convection velocity giving further evidence that the source of crackle is a flow field event.

Vorticity dynamics after the shock–turbulence interaction

Abstract: The interaction of a shock wave with quasi-vortical isotropic turbulence (IT) represents a basic problem for studying some of the phenomena associated with high speed flows, such as hypersonic flight, supersonic combustion and Inertial Confinement Fusion (ICF). In general, in practical applications, the shock width is much smaller than the turbulence scales and the upstream turbulent Mach number is modest. In this case, recent high resolution shock-resolved Direct Numerical Simulations (DNS) (Ryu and Livescu, J Fluid Mech 756:R1, 2014) show that the interaction can be described by the Linear Interaction Approximation (LIA). Using LIA to alleviate the need to resolve the shock, DNS post-shock data can be generated at much higher Reynolds numbers than previously possible. Here, such results with Taylor Reynolds number approximately 180 are used to investigate the changes in the vortical structure as a function of the shock Mach number, $$M_{s}$$ , up to $$M_{s}=10$$ . It is shown that, as $$M_{s}$$ increases, the shock interaction induces a tendency towards a local axisymmetric state perpendicular to the shock front, which has a profound influence on the vortex-stretching mechanism and divergence of the Lamb vector and, ultimately, on the flow evolution away from the shock.

Coalescence in the Sound Field of a Laboratory-Scale Supersonic Jet

Abstract: The spatial evolution of acoustic waveforms produced by a laboratory-scale Mach 3 jet are investigated using both 1/4 and 1/8 in. pressure field microphones located along rays emanating from the postpotential core where the peak sound emission is found to occur. The measurements are acquired in a fully anechoic chamber, where ground or other large surface reflections are minimal. Various statistical metrics are examined along the peak emission path, where they are shown to undergo rapid changes within 2 m from the source region. An experimentally validated wave-packet model is then used to confirm the location where the pressure amplitude along the peak emission path transitions from cylindrical to spherical decay. Various source amplitudes, provided by the wave-packet model, are then used to estimate shock formation distance and Gol’dberg numbers for diverging waves. The findings suggest that cumulative nonlinear distortion is likely to occur at laboratory scale near the jet flow, where the waveform amp...

Effects of Bleed Position on the Stability of a Supersonic Inlet

Abstract: Effects of various boundary-layer bleed locations on the stability of an axisymmetric supersonic inlet has been investigated experimentally. The bleeds were located on the inlet compression cone and were designed to improve the stability margin of the inlet. The main objective of the study was to identify the boundary-layer bleed location and its effects on the improvement of the stability margin of the inlet. In addition, a buzz precursor detection methodology based on the rms level has been introduced. Experiments have been carried out on a mixed-compression inlet with a design Mach number of 2.0 and three different bleed locations at three freestream Mach numbers of 1.8, 2.0, and 2.2. All tests were conducted at an angle of attack of 0 deg. The results show that, based on the individual design of an inlet, a location can be found that the spilled normal shock stands at this location and consequently separation starts at the buzz onset. If the bleed slot is located in this place, the subcritical perform...