Session J05: Aeroacoustics I

Analysis of the aeroacoustics of an ascending rocket at lift-off through high-fidelity simulations

Space vehicles' launches create a complex system of acoustic waves harmfully interacting with the ground facility and the vehicle itself. Previous engineering tools are unable to reconstruct accurately the intense aeroacoustic loads during the initial moments of a launch. Thus, it is crucial to devise innovative computational methods capable of capturing the impulsive flow resulting from the ignition of the rocket engines. In this study, we use high-fidelity, 3D large eddy simulations to simulate the flow field generated by the launch of a realistic space launcher. Our high-order, GPU-accelerated, compressible flow solver makes it possible to represent the highly turbulent interaction of the exhaust plume from the rocket's nozzle in a realistic environment. An immersed boundary method models the geometry of the launch pad and of the ascending space launcher. Results show the solver's ability to provide improved predictions compared to actual flight data measurements, despite the challenging conditions and complex – moving – geometry involved. Additionally, wavelet analysis characterizes effectively the hazardous ignition/duct overpressure mechanisms occurring during the lift-off.   

Spatial-temporal stability analysis of confined compressible shear layers of a vortex-sheet type

In many open-jet aeroacoustic wind tunnels, large-amplitude and low-frequency vibration and pressure fluctuations can occur at certain jet velocities. To investigate the role played by the surrounding chamber, a spatial-temporal stability analysis is performed on confined compressible shear flows based on vortex-sheet assumptions. Results show that at low Mach number (less than 0.3), the effects of confinement on compressible shear flows are close to those of incompressible flows, i.e. confinement destabilizes the flow and causes the transition from convective to absolute instability to occurs at lower shear values, but the increase of Mach number stabilizes the jet while destabilizes the wakes. At high Mach numbers (such as M = 1), the effect of confinement on jets are significantly different from those at low Mach numbers. The stability of jets at high Mach numbers hardly changes with confinement when the flow is strongly confined; however, for weakly confined jets, its stability first increases and then decreases when confinement decreases. Confinement on wakes at high Mach numbers tends to destabilizes the flow.   

A DNS Investigation of Aeroacoustic Noise Generation in Cold Spray Additive Manufacturing

Cold spray additive manufacturing (CSAM) technology is a solid-state deposition process applied to repair damaged components and fabricate individual components for aerospace, medical, and other important applications. CSAM uses a converging-diverging (CD) nozzle (exit diameter on the order of millimeters) to propel feedstock particles to a high velocity (higher than 300 m/s) and deposit the particles onto a substrate upon impact. In the CSAM process, the Mach number of the high-speed jet carrying powder feedstock can go beyond 2<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>M=2, the Reynolds number (Re_D) beyond 5×10⁵, and the sound pressure level generated from the high-speed jet beyond 100 dB. In this study, we numerically investigate the aeroacoustic sound pressure level generated from the fully-developed circular turbulent supersonic jet. The flow dynamic variables of the jet are obtained by solving the compressible Navier-Stokes equations using direct numerical simulation (DNS). The simulation is performed in a CD nozzle with a nozzle pressure ratio of 1.1. The dominant frequency modes identified in the flow field can be diminished with a suitable sound mitigation strategy.   

Quantification of Energy Transfer Processes in Supersonic Rectangular Screeching Jets

High-fidelity large-eddy simulation (LES) data of under-expanded rectangular jets are used to analyze energy transfer processes relevant to the generation of high-intensity screech tones. It is believed that interaction between downstream-traveling Kelvin-Helmholtz waves and the shock-cell structure produces an upstream-traveling guided jet mode that is important for sustaining screech. Waves involved in this process can be uniquely identified by frequency and streamwise wavenumber. An energy budget equation for the guided jet mode is derived through spatial filtering of the Navier-Stokes equations and evaluated using LES data. This method allows for identification of spectral energy fluxes across the guided jet mode wavenumber in the physical domain. Source terms involving Kelvin-Helmholtz waves and the shock-cell structure reveal a spatially-distributed energy transfer process to the guided jet mode in the shear layer and jet core. Regions in the jet where the guided jet mode loses significant energy to the slowly-varying mean flow are also identified. The energy equation is simplified by retaining dominant source terms and used to explain the spatial modulation of the guided jet mode amplitude. Stability properties of the guided jet mode are also assessed in this framework.   

A new instance of crackle noise

Intense crackle noise has been identified and studied in high temperature, supersonic jets from engines at high specific thrust. A new instance of crackle has been found in LES of cold, plane, perfectly expanded supersonic jets (Mach 1.5, Reynolds number 100,000) impinging of plane wedges. LES is by explicit filtering, with a new adaptive filtering method for shock capturing. Crackle fronts appear when a detached normal shock stands about a half jet width from the wedge tip and the jet column is about 4 jet-widths from the wedge tip. The edges of this oscillating shock interact with the jet's bounding shear layer structures to emit acoustic wave fronts that steepen as they travel into the far field. Pressure signals exhibit sudden rises followed by gentle relaxations, intermittently. Consistently, pressure distributions along lines in the ambient show the existence and arrival of a sequence of sharp compression fronts. Skewness exceeds 0.4 in the far field. Crackle fronts do not appear in other configurations of a short column (steady shock), short column and thin wedge (steady attached oblique shock), long column, thin wedge (large amplitude column oscillations).   

Analysis of noise from a transcritical N2 jet using a hybrid CFD/CAA solver for simulating real fluid flows

Fuel combustion under transcritical conditions in high-pressure combustors offers high thermal efficiency but is also accompanied by loud noise, which has the potential to trigger thermo-acoustic instabilities that can damage the combustor. As a first step toward understanding noise from real fluid flows under such severe conditions, a non-reacting N₂ jet under transcritical conditions is simulated in this work. A hybrid Computational Fluid Dynamics (CFD)/Computational Aero-Acoustics (CAA) solver developed for simulating real fluid flows and the noise radiated by them is demonstrated and used to analyze the generation and radiation of noise from this transcritical N₂ jet. In this hybrid simulation, the turbulent flow field of the jet is predicted using Large-Eddy Simulation (LES) which employs an efficient and robust pressure-based semi-implicit algorithm tailored for compressible real fluid flows (i.e., the CFD simulation), and the LES results are validated against experimental data. To describe the generation and radiation of noise from this N₂ jet, the Linearized Euler Equations (LEE) reformulated to consider real fluid effects are solved in the CAA simulation. Source terms of the LEE are computed from the solution of the LES which offers a spatiotemporal description of the noise-generating sources. Then, to characterize the far-field noise radiation, the acoustic field predicted by the LEE solver is analyzed in terms of sound pressure amplitudes, spectral content, and directivity behavior.   

Aeroacoustics of sonic ESTS lobed nozzles

Elliptic Sharp Tipped Shallow (ESTS) lobed nozzles are established as a potential candidate for mixing in supersonic flows. These nozzles have the ability to reduce centre-line total pressure loss, increase jet spreading through efficient streamwise vortices generation, and promote far-field centre-line velocity decay. However, the aeroacoustic characteristics of these nozzles are not available in the open literature. Hence, a rigorous experimental campaign is undertaken to study the ESTS lobed nozzles' aeroacoustic characteristics in an anechoic environment. The study uses different lobe numbers (n = 3, 4, 5, 6, 7, 8) and nozzle pressure ratios (ζ = 1.5 to 6.5). Preliminary microphone measurements taken at an angle of φ = 135° from the jet axis are done for all the cases at varying ζ. A circular sonic jet is kept as the baseline case for comparison. High-speed Schlieren imaging resolving apparent streamwise density gradient is used to identify the spatiotemporal modes using modal analysis. The findings from both experiments reveal the presence of different jet oscillation modes at varying intensities across the ESTS lobed nozzles. Particularly, the difference between the overall sound pressure level (η) for one of the ESTS lobed nozzles (n=8) is ∼12 dB less than the baseline case. Moreover, increments in the lobe numbers cull the variations in η across different ζ.   

Influence of Large-Scale Coherent Structures on Lift Loss in Supersonic Impinging Jets

Supersonic impinging jets are essential components of Short Take-Off and Vertical Landing (STOVL) aircraft operation. These impinging jets are prone to resonance via a feedback mechanism, yielding highly unsteady flowfields characterized by large-scale coherent structures. These coherent structures are associated with the generation of intense acoustic waves, leading to loud noise. Concurrently, these high-speed jets entrain the ambient air, forming a region of negative pressure beneath the fuselage that prompts a loss of lift. This induced lift loss can be significantly augmented in the presence of large-scale coherent structures. The present study aims to explore the relationship between lift loss and noise using a supersonic round jet (Mach 1.5). The lift loss is quantified by pressure measurements beneath a lift plate simulating a fuselage. The lift loss measurements are performed simultaneously with acoustic measurements over a range of impingement distances, encompassing cases with both microjet actuation and uncontrolled jet conditions. Evaluation of the results is facilitated through schlieren flow visualization and Particle Image Velocimetry (PIV) at selected impingement distances.   

Nozzle flow acoustics with wall-friction and entropy two-dimensionality

The behavior of one-dimensional linear flow disturbances in an isentropic nozzle flow can be solved in the zero frequency limit using the Marble & Candel (1977) relations. Frequency dependence be incorporated by rearranging the governing linearized Euler equations in terms of the flow invariants i.e. the perturbation quantities that are matched across an area change in the zero frequency limit. Semi-analytical solution using the Magnus expansion then follows, with frequency as an expansion parameter (Duran & Moreau 2013). Last year, we presented the first inclusion of flow non-isentropicity as a further Magnus expansion parameter – accounting for mean heat transfer along the nozzle (Yeddula et al. 2022).  

The present work extends this to nozzle flows with wall friction and two-dimensionality in which entropy perturbations have both radial and axial dependence. The analytical framework is developed, with frequency, wall-friction coefficient and an entropy “two-dimensionality” coefficient all incorporated as Magnus expansion parameters. Resulting predictions of entropy noise are validated against numerical simulations. The agreement confirms that the Magnus expansion is a powerful semi-analytical tool suitable for replacing full numerical simulations across a broad range of nozzle acoustic problems.