Showing papers on "Supersonic speed published in 2017"

Unstructured Large-Eddy Simulations of Supersonic Jets

Abstract: Experience gained from previous jet noise studies with the unstructured large-eddy simulation flow solver “Charles” is summarized and put to practice for the predictions of supersonic jets issued f...

Feedback loop and upwind-propagating waves in ideally expanded supersonic impinging round jets

Abstract: The aeroacoustic feedback loop establishing in a supersonic round jet impinging on a flat plate normally has been investigated by combining compressible large-eddy simulations and modelling of that loop. At the exit of a straight pipe nozzle of radius r0, the jet is ideally expanded, and has a Mach number of 1.5 and a Reynolds number of 60000. Four distances between the nozzle exit and the flat plate, equal to 6r0, 8r0, 10r0 and 12r0, have been considered. In this way, the variations of the convection velocity of the shear-layer turbulent structures according to the nozzle-to-plate distance are shown. In the spectra obtained inside and outside of the flow near the nozzle, several tones emerge at Strouhal numbers in agreement with measurements in the literature. At these frequencies, by applying Fourier decomposition to the pressure fields, hydrodynamic-acoustic standing waves containing a whole number of cells between the nozzle and the plate and axisymmetric or helical jet oscillations are found. The tone frequencies and the mode numbers inferred from the standing-wave patterns are in line with the classical feedback-loop model, in which the loop is closed by acoustic waves outside the jet. The axisymmetric or helical nature of the jet oscillations at the tone frequencies is also consistent with a wave analysis using a jet vortex-sheet model, providing the allowable frequency ranges for the upstream-propagating acoustic wave modes of the jet. In particular, the tones are located on the part of the dispersion relations of the modes where these waves have phase and group velocities close to the ambient speed of sound. Based on the observation of the pressure fields and on frequency–wavenumber spectra on the jet axis and in the shear layers, such waves are identified inside the present jets, for the first time to the best of our knowledge, for a supersonic jet flow. This study thus suggests that the feedback loop in ideally expanded impinging jets is completed by these waves.

Dynamic stability analysis of a pressurized FG-CNTRC cylindrical shell interacting with supersonic airflow

Abstract: The objective of this research is to investigate the aeroelastic buckling and flutter instability of a pressurized functionally graded carbon nanotube reinforced composite (FG-CNTRC) cylindrical shell subjected to supersonic airflow. The dynamic model of the FG-CNTRC cylindrical shell is established in accordance with the first-order shear deformation theory, Donnell kinematic theory along with the von Karman geometrical nonlinearity. The quasi-steady Krumhaar's modified piston theory by considering the effect of the panel curvature is used to estimate the aerodynamic pressure induced by the supersonic airflow. The dynamic equations are discretized using trigonometric expansion through the circumferential direction and harmonic differential quadrature (HDQ) method through the meridional direction. Effects of boundary conditions, geometrical parameters, volume fraction and distribution of CNTs and the Mach number on the flutter instability, onset of the buckling and deformation shapes of the cylindrical shell are put into evidence through a set of parametric studies. The simulation indicates that the critical flutter dynamic pressure may be significantly enhanced through functionally graded distribution of CNTs in a polymer matrix. Furthermore, it is found that presence of the aerodynamic pressure may completely change deformation shapes of the FG-CNTRC cylindrical shell.

An investigation on the aeroelastic flutter characteristics of FG-CNTRC beams in the supersonic flow

Abstract: The present research is dedicated to the aeroelastic analysis of functionally graded carbon nanotube reinforced composite (FG-CNTRC) beams subjected to aerodynamic load and axial compression, simultaneously. The nonlinear dynamic equations of FG-CNTRC beams are obtained according to the von Karman type of geometrical nonlinearity along with the first-order shear deformation theory. The aerodynamic pressure is estimated in accordance with the quasi-steady supersonic piston theory. Harmonic differential quadrature method (HDQM) is applied to discretize the dynamic equations in the spatial domain. The aeroelastic flutter and buckling bounds are calculated via the derivations of natural frequencies and bifurcation points. Influences of boundary conditions, geometrical parameters, volume fraction and distribution of carbon nanotubes (CNTs) and Mach number on the stability boundaries and deformation shapes are put into evidence through a set of parametric studies. It is found that the presence of the aerodynamic pressure not only increases the critical buckling load of the FG-CNTRC beam, but also alters deformation configurations of the beam. Furthermore, the results indicate that aeroelastic characteristics of FG-CNTRC beams may be noticeably enhanced through FG-X distribution of the CNTs.

Numerical Investigation on Flame Stabilization in DLR Hydrogen Supersonic Combustor with Strut Injection

Abstract: Flame stabilization in the DLR hydrogen supersonic combustor with strut injection was numerically investigated by using an in-house large eddy simulation code developed on the OpenFoam platform. To facilitate the comparison and analysis of various hydrogen oxidation mechanisms with different levels of mechanism reduction, the proposed 2D calculation model was validated against both the 3D simulation and the experimental data. The results show that the 2D model can capture the DLR flow and combustion characteristics with satisfactorily quantitative accuracy and significantly less computational load. By virtue of the flow visualization and the analyses of species evolution and heat release, the supersonic combustion in the DLR combustor can be divided into three stages along the streamwise direction: the induction stage where ignition occurs and active radicals are produced, the transition stage through which radicals are advected to the downstream, and the intense combustion stage where most heat r...

Application of POD on time-resolved schlieren in supersonic multi-stream rectangular jets

Abstract: In this paper, we present an experimental investigation of a supersonic rectangular nozzle with aft deck used for three-stream engines. The jet utilizes a single expansion ramp nozzle (SERN) configuration along with multiple streams, operating at a bulk flow Mj,1 = 1.6 and bypass stream Mj,3 = 1.0. This idealized representation consists of two canonical flows: a supersonic convergent-divergent (CD) jet and a sonic wall jet. Time-resolved schlieren experiments were performed up to 100 kHz. Proper orthogonal decomposition (POD), as suggested by Lumley for structure identification in turbulent flows, is applied to the schlieren images and the spatial eigenfunctions and time-dependent coefficients are related to the flow structures. This research seeks to lay a foundation for fundamental testing of multi-stream SERNs and the identification of the flow physics that dominate these modern military nozzles.

Quasi-DC electrical discharge characterization in a supersonic flow

Abstract: A Quasi-DC (Q-DC) electrical discharge generates a highly transient filamentary plasma in high-speed airflow. Major specific properties of this type of discharge are realized due to a strong coupling of the plasma to the moving gas. The plasma, supplied by a DC voltage waveform, demonstrates a pulsed-periodic pattern of dynamics significantly affecting the flow structure. In this study, the dynamics and plasma parameters of the Q-DC discharge are analyzed in the Supersonic Test Rig (SBR-50) at the University of Notre Dame at Mach number M = 2, stagnation pressure P 0 = (0.9–2.6) × 105 Pa, stagnation temperature T 0 = 300 K, unit Reynolds number ReL = 7–25 × 106 m−1, and plasma power W pl = 3–21 kW. The plasma parameters are measured with current–voltage probes and optical emission spectroscopy. An unsteady pattern of interaction is depicted by high-speed image capturing. The result of the plasma-flow interaction is characterized by means of pressure measurements and schlieren visualization. It is considered that the Q-DC discharge may be employed for active control of duct-driven flows, cavity-based flow, and for effective control of shock wave–boundary layer interaction.

Direct numerical simulations of supersonic turbulent channel flows of dense gases

Abstract: The influence of dense-gas effects on compressible wall-bounded turbulence is investigated by means of direct numerical simulations of supersonic turbulent channel flows Results are obtained for PP11, a heavy fluorocarbon representative of dense gases, the thermophysics properties of which are described by using a fifth-order virial equation of state and advanced models for the transport properties In the dense-gas regime, the speed of sound varies non-monotonically in small perturbations and the dependency of the transport properties on the fluid density (in addition to the temperature) is no longer negligible A parametric study is carried out by varying the bulk Mach and Reynolds numbers, and results are compared to those obtained for a perfect gas, namely air Dense-gas flow exhibits almost negligible friction heating effects, since the high specific heat of the fluids leads to a loose coupling between thermal and kinetic fields, even at high Mach numbers Despite negligible temperature variations across the channel, the mean viscosity tends to decrease from the channel walls to the centreline (liquid-like behaviour), due to its complex dependency on fluid density On the other hand, strong density fluctuations are present, but due to the non-standard sound speed variation (opposite to the mean density evolution across the channel), the amplitude is maximal close to the channel wall, ie in the viscous sublayer instead of the buffer layer like in perfect gases As a consequence, these fluctuations do not alter the turbulence structure significantly, and Morkovin’s hypothesis is well respected at any Mach number considered in the study The preceding features make high Mach wall-bounded flows of dense gases similar to incompressible flows with variable properties, despite the significant fluctuations of density and speed of sound Indeed, the semi-local scaling of Patel et al (Phys Fluids, vol 27 (9), 2015, 095101) or Trettel & Larsson (Phys Fluids, vol 28 (2), 2016, 026102) is shown to be well adapted to compare results from existing surveys and with the well-documented incompressible limit Additionally, for a dense gas the isothermal channel flow is also almost adiabatic, and the Van Driest transformation also performs reasonably well The present observations open the way to the development of suitable models for dense-gas turbulent flows

Aerodynamic Heating in Wall-Modeled Large-Eddy Simulation of High-Speed Flows

Abstract: Aerospace vehicles flying at supersonic and hypersonic speeds are subject to increased wall heating rates caused by viscous friction with the gas environment. This extra heat is commonly referred t...

Exploring Physics and Control of Twin Supersonic Circular Jets

Abstract: The closely spaced twin-jet configuration often seen in military aircraft is distinct from the single jet in both the flowfield and acoustic field. The twin-jet plumes interact with each other weakly or strongly, depending upon the distance between the two jets, the jet operating flow regime, and the Mach number. In the current study, the interaction mechanisms and coupling of a supersonic twin jet exhausting from biconical converging–diverging nozzles with a design Mach number of 1.23 at a separation distance of two nozzle exit diameters are investigated using near-field pressure measurements and phase-locked flow visualization. Across a series of jet operating Mach numbers, the twin-jet plumes feature three major jet azimuthal modes: axisymmetric, helical, and flapping. The jet flapping mode is strongly augmented in the twin-jet configuration compared to the single-jet case. Along the twin-jet plane, which is also the plane of the jet flapping direction, jet plumes with coherent flow structures are obse...

Direct Numerical Simulation of Foreign-Gas Film Cooling in Supersonic Boundary-Layer Flow

Abstract: The cooling-film behavior in an adiabatic supersonic air main flow over a flat plate is investigated using direct numerical simulations. Air and helium are employed as cooling gases and are injected with a low rate in the wall-normal direction through a single infinite spanwise slit into the laminar or turbulent boundary-layer flow. The blowing is realized by prescribing a fixed distribution of the cooling-gas mass flux, mass fraction, and temperature at either the orifice location without the cooling-gas channel (modeled blowing) or at the lower end of the included blowing channel (simulated/interacting blowing), thus allowing for an interaction of the main and cooling-gas flows. The influence of the modeling, the main-flow boundary-layer state, and the cooling-gas type on the film-cooling effectiveness and the skin-friction alteration is scrutinized; and valuable data for the validation of less costly computational fluid dynamics methods employing turbulence models are gained from this fundamental study.

Direct numerical simulation of supersonic turbulent boundary layer subjected to a curved compression ramp

Abstract: Numerical investigations on a supersonic turbulent boundary layer over a longitudinal curved compression ramp are conducted using direct numerical simulation for a free stream Mach number M∞ = 2.9 and Reynolds number Reθ = 2300. The total turning angle is 24°, and the concave curvature radius is 15 times the thickness of the incoming turbulent boundary layer. Under the selected conditions, the shock foot is transferred to a fan of the compression wave because of the weaker adverse pressure gradient. The time-averaged flow-field in the curved ramp is statistically attached where the instantaneous flow-field is close to the intermittent transitory detachment state. Studies on coherent vortex structures have shown that large-scale vortex packets are enhanced significantly when the concave curvature is aligned in the spanwise direction. Consistent with findings of previous experiments, the effect of the concave curvature on the logarithmic region of the mean velocity profiles is found to be small. The intensi...

Laminar-turbulent transition in supersonic boundary layers with surface heat transfer: A numerical study

Abstract: Through high-resolution direct numerical simulations, the present study aims to investigate several laminar-to-turbulent transition scenarios in the presence of wall heat transfer for supersonic boundary layers over strongly heated/cooled and adiabatic flat plates. The laminar boundary layer is tripped using a suction and blowing technique with a single-frequency, multiple-spanwise wavenumber excitation. The results are evaluated and compared with linear stability theory to isolate the effect of wall heat transfer, as well as forcing parameters, on the transition. It was found that increasing the disturbance amplitude as well as perturbation frequency moves the transition upstream. Also, the effect of wall heating was seen to stabilize the flow and to postpone the transition, contrary to the wall cooling.