Direct numerical simulation of hot jets

The ultimate motivation of this work is to investigate the stability of two dimensional heated jets and its implications for aerodynamic sound generation from data obtained with direct numerical simulations (DNS). As pointed out in our last report, these flows undergo two types of instabilities, convective or absolute, depending on their temperature. We also described the limits of earlier experimental and theoretical studies and explained why a numerical investigation could give us new insight into the physics of these instabilities. The aeroacoustical interest of these flows was also underlined. In order to reach this goal, we first need to succeed in the DNS of heated jets. Our past efforts have been focused on this issue which encountered several difficulties. Our numerical difficulties are directly related to the physical problem we want to investigate since these absolutely or almost absolutely unstable flows are by definition very sensitive to the smallest disturbances and are very likely to reach nonlinear saturation through a numerical feedback mechanism. As a result, it is very difficult to compute a steady laminar solution using a spatial DNS. A steady state was reached only for strongly co-flowed jets, but these flows are almost equivalent to two independent mixing layers. Thus they are far from absolute instability and have much lower growth rates

Introduction to experimental aeroacoustics, Lecture 1

This introductory lecture gives an overview of the challenges and pitfalls of wind tunnel experiments in aeroacoustics. Most concepts introduced here will be developed in following lecture

Experimental investigation of the transient bleed valve noise

This study presents the first step of an experimental study of the transient bleed valve noise. It was carried out on a simplified TBV geometry composed of a cylindrical inlet pipe leading to a diaphragm or a perforated disk for the purpose of generating pressure drops. Numerous diaphragms and grids have been tested in order to identify parameters that influence the acoustic radiation of the TBV and for NPR (Nozzle pressure ratio) from 1.2 to 3.6 to cover both subsonic and supersonic regimes. A large number of acoustic behaviors have been identified. For diaphragms far field acoustic spectra are dominated by mixing noise for all NPR and by shock-associated noise (screech and broadband shock associated noise (BBSAN)) when the critical value of the NPR delimiting the subsonic and supersonic behavior (NPRc = 1.89)is exceeded. For grids the mixing noise is still present but is composed of two humps. The parametric study allowed to associate the first hump to the noise of an equivalent jet having the smallest diameter encircling the grid perforations while the second is associated to the noise of the outer isolated jets. A first prediction model has thus been proposed based on this double source. Furthermore, the grids other a significant noise reduction in the audible range with respect to a diaphragm of the same cross-section by shifting the radiation towards the high frequencies. The noise associated with supersonic phenomena (screech and BBSAN) are also strongly reduced and even suppressed in most of the tested cases

Zonal large-eddy simulation of a tip leakage flow

The flow induced by the clearance between the tip of an isolated airfoil and an end-plate is investigated numerically, using a zonal approach with large-eddy simulation in the region of interest. The results are analyzed in comparison with available experimental data, presented in a companion paper. The incoming boundary layer and the pressure distribution around the blade are evaluated. The description of the inflow-jet deviation, with an averaged approach, enables to represent the proper loading on the airfoil. Also, particular attention is paid to the powerful tipleakage vortex. The vortex characteristics are investigated using specific functions to locate its center and quantify its width. Overall, good results are obtained for the flow statistics and spectra. Furthermore, a very good description of the far-field pressure is achieved using the acoustic analogy, and the results confirm that the tip-flow essentially radiates in the central frequency range (0.7 kHz, 7 kHz)

Wavelet Analysis of a Blade Tip-Leakage Flow

The secondary flow generated by the clearance between an isolated airfoil tip and an end-plate is analyzed by means of a zonal large-eddy simulation, in comparison with available experimental data. The flow around the tip clearance is described with full large-eddy simulation, while Reynolds-averaged Navier-Stokes is employed in the rest of the computational domain in order to limit the computational cost. The various analyses of the flow characteristics (mean velocities, Reynolds stresses, spectra) show a very good agreement between the experiment and the simulation. Furthermore, a detailed analysis is carried out from the numerical results. The flow separations on the blade tip are related with the leakage distribution along the chord, which generates an intense tip-leakage vortex on the suction side. Finally, a hump in the pressure spectra at tip is investigated by means of a wavelet conditional average, and related to the unsteadiness of the aft tip separatio

Effect of DDES and IDDES modelling on the simulation of turbulent flows for aeroacoustics

The main objective of the present investigation is the analysis of the modelling capabilities of shear layer and boundary layer flows by Improved Delayed Detached Eddy Simulation (IDDES [4]) and standardDelayed Detached Eddy Simulation (DDES [3])

Airfoil Tip Leakage Aeroacoustics Predictions using a Lattice Boltzmann Based Method

International audienceThe noise produced by rotating systems such as fans and turbo machines is of growing importance in the academic and engineering communities. The prediction and understanding of the physical mechanisms associated with noise generation are required in order to develop innovative solutions able to efficiently reduce radiated acoustics levels. The flow-induced noise generation mechanisms related to rotating devices are various and complex, and one of them is related to the blade tip flow. The tip flow noise, or tip leakage noise, is particularly important for free-tip configurations, for which the tip flow induced by the pressure gradient between the suction and pressure sides can be particularly intense. The experimental investigation of this mechanism is practically challenging. Consequently, a simplified non-rotating representative configuration has been proposed, and has been previously investigated experimentally. In this paper, transient, compressible, and time-explicit Computational Fluid Dynamics/Computational Aero-Acoustics (CFD/CAA) simulations of an airfoil tip leakage flow for this simplified geometry are performed using a Lattice Boltzmann Method (LBM) based approach. The studied configuration is a NACA 5510 airfoil profile at high Reynolds number flow conditions, for which a variable size gap is introduced between the airfoil and one of the end plates, modeling the tip gap encountered in free-tip fans. First, the simulation results are compared with experimental results to validate the numerical approach. Further investigation of the numerical results underlines the connection between the tip vortex structures and noise radiation, including a parametric study on the Angle of Attack (AoA) and the tip gap width

Prediction of Noise from Low Reynolds Number Rotors with Different Number of Blades using a Non-Linear Vortex Lattice Method

The demand of micro air vehicles (MAV) with multiple rotors is increasing in both military and civil applications because of their versatility on various missions. However, the use of MAVs for some missions still has limited success because of their noise pollution. One of the main noise sources is aeroacoustic sound produced by the low Reynolds number flows around the rotors. There have been many previous studies about small-scale rotor systems of MAVs during the past decades, but they mainly focused on investigations of the aerodynamics rather than the acoustics. Several studies considering the acoustics have started recently. However, only steady loading forces computed by using Blade Element Momentum Theory (BEMT) were considered in the previous studies, and the noise from unsteady flow phenomena were not taken into account. The main objective of the current study is to investigate the noise mechanisms and further to find ways to reduce the noise levels in unsteady low Reynolds number flows. A Non-linear Vortex Lattice Method (NVLM) is used to simulate the unsteady low Reynolds number flow and model the corresponding noise sources. The tonal components of far-field noise is predicted by using an acoustic analogy based on Ffowcs Williams-Hawkings (FW-H) equations. These numerical methods are applied to low Reynolds number propeller and rotor cases, and validated upon experimental data. Then, they are used to investigate the influence of the number of blades on both aerodynamic and aeroacoustic performance and provide further insight into prominent sources of noise

Towards silent Micro-Air Vehicles: optimization of a low Reynolds number rotor in hover

The demand in Micro-Air Vehicles (MAV) is increasing as well as their potential missions. Either for discretion in military operations or noise pollution in civilian use, noise reduction of MAV is a goal to achieve. Aeroacoustic research has long been focusing on full scale rotorcrafts. At MAV scales however, the hierarchization of the numerous sources of noise is not straightforward, as a consequence of the relatively low Reynolds number that ranges typically from 5,000 to 100,000 and low Mach number of approximately 0.1. This knowledge however, is crucial for aeroacoustic optimization and blade noise reduction in drones. This contribution briefly describes a lowcost,numerical methodology to achieve noise reduction by optimization of MAV rotor blade geometry. Acoustic power measurements show a reduction of 8 dB(A). The innovative rotor blade geometry allowing this noise reduction is then analyzed in details, both experimentally and numerically with Large Eddy Simulation using Lattice Boltzmann Method (LES-LBM). Turbulence interaction noise is shown to be a major source of noise in this configuration of low Reynolds number rotor in hover, as a result of small scale turbulence and high frequency unsteady aeroadynamics impinging the blades at the leading-edge

Zonal large-eddy simulation of a fan tip-clearance flow, with evidence of vortex wandering

The flow in a fan test-rig is studied with combined experimental and numerical methods, with a focus on the tip-leakage flow. A zonal RANS/LES approach is introduced for the simulation: the region of interest at tip is computed with full large-eddy simulation (LES), while Reynolds-averaged Navier–Stokes (RANS) is used at inner radii. Detailed comparisons with the experiment show that the simulation gives a good description of the flow. In the region of interest at tip, a remarkable prediction of the velocity spectrum is achieved, over about six decades of energy. The simulation precisely captures both the tonal and broadband contents. Furthermore, a detailed analysis of the simulation allows identifying a tip-leakage vortex (TLV) wandering, whose influence onto the spectrum is also observed in the experiment. This phenomenon might be due to excitation by upstream turbulence from the casing boundary layer and/or the adjacent TLV. It may be a precursor of rotating instability. Finally, considering the outlet duct acoustic spectrum, the vortex wandering appears to be a major contribution to noise radiation