Fan wake modelling for computational aeroacoustic simulations of turbulence-cascade interaction noise

The present work addresses the numerical modelling of fan wakes using synthetic turbulence and its influence on turbulence-cascade interaction noise predictions. Initial results show that cascade noise only depends on the circumferentially-averaged turbulence spectra that interact with the cascade. Consequently, isotropic turbulence produces noise predictions with approximately the same level of accuracy than fan wakes with cyclostationary variations in both turbulent kinetic energy and integral length scale. The paper also includes a parameter study on the effects of vane count and camber on cascade noise from thick aerofoils. Numerical results show that vane count may have a significant effect on noise predictions at low frequencies, whereas the effects of camber are negligible

Aeroacoustic source mechanisms of a wavy leading edge undergoing vortical disturbances

High-accuracy numerical simulations are performed to study aeroacoustic source mechanisms of wavy leading edges (WLEs) on a thin aerofoil undergoing vortical disturbances. This canonical study is based on a prescribed spanwise vortex travelling downstream and creating secondary vortices as it passes through the aerofoil’s leading edge. The primary aim of the study is to precisely understand the relationships between the vortex-induced velocity perturbation and the wall pressure fluctuation on the WLE geometry. It is observed that by increasing the size (amplitude) of the WLE the source strength at the peak region is reduced rapidly to a certain point, followed by a saturation stage, while at the root (trough) it remains fairly consistent regardless of the WLE size. This observation is demonstrated to be the consequence of three-dimensional vortex dynamics taking place along the WLE. One of the most profound features is that a system of horseshoe-like secondary vortices are created from the WLE peak region upon the impingement of the prescribed vortex. It is found that the horseshoe vortices produce a significantly non-uniform velocity perturbation in front of the WLE leading to the disparity in the source characteristics between the peak and root. The alterations to the impinging velocity perturbation are carefully analysed and related to the wall pressure fluctuation in this study. In addition, a semi-analytic model based on Biot–Savart’s law is developed to better understand and explain the role of the horseshoe vortex systems and the source mechanisms

Leading edge noise predictions using anisotropic synthetic turbulence

An advanced digital filter method is presented to generate divergence-free synthetic turbulence with homogeneous anisotropic velocity spectra. The resulting fluctuating velocity field is obtained through a superposition of anisotropic Gaussian eddies. This method is used to generate a two-dimensional turbulent flow with the key statistics of homogeneous axisymmetric turbulence. This type of turbulence has been reported in aero-engine intakes, fan wakes and open-jet wind tunnel experiments. The advanced digital filter method is implemented in a linearized Euler solver in order to investigate potential effects of anisotropic turbulence on leading edge noise. Computational aeroacoustic simulations are performed for anisotropic turbulence with streamwise-to-transverse length scale ratios ranging from 0.33 to 3 on a number of isolated airfoil configurations, including variations in mean flow Mach number, airfoil thickness and angle of attack. Noise reduction due to airfoil thickness is assessed on a NACA 0012 airfoil at zero angle of attack, showing similar trends for bothisotropic and moderately anisotropic turbulent flows. Effects of anisotropic turbulence on noise become evident for airfoil configurations at non-zero angle of attack

Aerofoil broadband and tonal noise modelling using Fast-Random-Particle-Mesh method and Large Eddy Simulation

PhD thesisThe aim of this work is to critically examine state-of-the art numerical methods used in computational aero-acoustics with the goal to further develop methods of choice that satisfy the industry requirements for aero-acoustic design, that is being fast, physical and potentially applicable to a variety of airframe noise problems. At the core of this thesis, two different modelling techniques are applied to benchmark aerofoil noise problems. One is based on a modern Fast Random Particle Mesh (FRPM) method with the mean flow and turbulence statistics supplied from the Reynolds-Averaged Navier-Stokes (RANS) simulation. The second technique is a Large Eddy Simulation (LES) method utilising the new in-house fast-turn-around GPU CABARET code. The novelty of the work presented herein consists in the development of new modifications to the stochastic FRPM method featuring both tonal and broadband noise sources. The technique relies on the combination of incorporated vortex-shedding resolved flow available from Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulation with the fine-scale turbulence FRPM solution generated via stochastic velocity fluctuations in the context of vortex sound theory. In contrast to the existing literature, proposed methodology encompasses a unified treatment for broadband and tonal acoustic noise sources at the source level, thus, accounting for linear source interference as well as possible non-linear source interaction effects. Results of the method’s application for two aerofoil benchmark cases, with sharp and blunt trailing edges are presented. In each case, the importance of individual linear and non-linear noise sources was investigated. Several new key features related to the unsteady implementation of the method were tested and brought into the equation. The source terms responsible for noise generation in accordance with the vortex sound theory are computed to assess the validity range of a digital filter calibration parameter used in the FRPM method for synthetic turbulence generation as compared to the same source reconstructed from the first principle LES solution. Such comparison at the source level has been achieved for the first time in the modelling literature, which allows for the physical interpretation of results obtained by the FRPM method. Finally, solutions of the FRPM method with the calibration parameter tailored in accordance with the LES are used for far-field noise predictions which are compared with experimental measurementsBAE Systems Ltd.Engineering and Physical Sciences Research Council (EPSRC)

Aerodynamic noise measurement in anechoic wind tunnel of rod-airfoil with leading edge serrations

Turbulent flow interaction with the leading edge of an airfoil generates a well-known leading edge noise. The present work is an experimental investigation of reducing the leading edge noise by adapting serrations on the leading edge of NACA0012 airfoil. A rod is placed upstream of the airfoil to magnify the turbulence flow-leading edge interaction. Locations and the size of the serrations are two parameters being investigated for the noise reduction. The study found that a noise reduction up to 3.5 dB is obtained when the size of the serration is wider. Additionally, more sound reduction can be obtained when the angle of attack is increased

Aerodynamic noise from rigid trailing edges with finite porous extensions

This paper investigates the effects of finite flat porous extensions to semi-infinite impermeable flat plates in an attempt to control trailing-edge noise through bio-inspired adaptations. Specifically the problem of sound generated by a gust convecting in uniform mean steady flow scattering off the trailing edge and permeable-impermeable junction is considered. This setup supposes that any realistic trailing-edge adaptation to a blade would be sufficiently small so that the turbulent boundary layer encapsulates both the porous edge and the permeable-impermeable junction, and therefore the interaction of acoustics generated at these two discontinuous boundaries is important. The acoustic problem is tackled analytically through use of the Wiener-Hopf method. A two-dimensional matrix Wiener-Hopf problem arises due to the two interaction points (the trailing edge and the permeable-impermeable junction). This paper discusses a new iterative method for solving this matrix Wiener-Hopf equation which extends to further two-dimensional problems in particular those involving analytic terms that exponentially grow in the upper or lower half planes. This method is an extension of the commonly used "pole removal" technique and avoids the needs for full matrix factorisation. Convergence of this iterative method to an exact solution is shown to be particularly fast when terms neglected in the second step are formally smaller than all other terms retained. The final acoustic solution highlights the effects of the permeable-impermeable junction on the generated noise, in particular how this junction affects the far-field noise generated by high-frequency gusts by creating an interference to typical trailing-edge scattering. This effect results in partially porous plates predicting a lower noise reduction than fully porous plates when compared to fully impermeable plates.Comment: LaTeX, 20 pp., 19 graphics in 6 figure

Simulation and control of stationary crossflow vortices

Turbulent flow and transition are some of the most important phenomena of fluid mechanics and aerodynamics and represent a challenging engineering problem for aircraft manufacturers looking to improve aerodynamic efficiency. Laminar flow technology has the potential to provide a significant reduction to aircraft drag by manipulating the instabilities within the laminar boundary layer to achieve a delay in transition to turbulence. Currently prediction and simulation of laminar-turbulent transition is con- ducted using either a low-fidelity approach involving the stability equations or via a full Direct Numerical Simulation (DNS). The work in this thesis uses an alternative high-fidelity simulation method that aims to bridge the gap between the two simulation streams. The methodology uses an LES approach with a low-computational cost sub-grid scale model (WALE) that has inherent ability to reduce its turbulent viscosity contribution to zero in laminar regions. With careful grid spacing the laminar regions can be explicitly modelled as an unsteady Navier-Stokes simulation while the turbulent and transitional regions are simulated using LES. The methodology has been labelled as an unsteady Navier-Stokes/Large Eddy Simulation (UNS/LES) approach. Two test cases were developed to test the applicability of the method to simulate and control the crossflow instability. The first test case replicated the setup from an experiment that ran at a chord-based Reynolds number of 390, 000. Two methods were used to generate the initial disturbance for the crossflow vortices, firstly using a continuous suction hole and secondly an isolated roughness element. The results for this test case showed that the approach was capable of modelling the full transition process, from explicitly modelling the growth of the initial amplitude of the disturbances to final breakdown to turbulence. Results matched well with the available experimental data. The second test case replicated an experimental setup using a custom- designed aerofoil run at a chord-based Reynolds number of 2.4 million. The test case used Distributed Roughness Elements (DRE) to induce crossflow vortices at both a critical and a control wavelength. By forcing the crossflow vortices at a stable (control) wavelength a delay in laminar-turbulent transition can be achieved. The results showed that the UNS/LES approach was capable of capturing the initial disturbance amplitudes due to the roughness elements and their growth rates matched well with experimental data. Finally, downstream a transitional region was assessed with low-freestream turbulence provided using a modified Synthetic Eddy Method (SEM). The full laminar-turbulent transition pro- cess was simulated and results showed significant promise. In conclusion, the method employed in this thesis showed promising results and demonstrated a possible route to high-fidelity transition simulation run at more realistic flow conditions and geometries than DNS. Further work and validation is required to test the secondary instability region and the final breakdown to turbulence

Synthetic turbulence methods for leading edge noise predictions

An advanced digital filter method to generate synthetic turbulence is presented for efficient two- and three-dimensional leading edge noise predictions. The technique, which is based on the Random Particle-Mesh method, produces a turbulent inflow that matches a target isotropic energy spectrum. The discretized equations for the synthetic eddies, and the input parameters needed to recover the desired turbulence statistics, are presented. Moreover, a simple and fast implementation strategy, which does not require an additional boundary condition, is presented under the frozen turbulence assumption. The method is used in a linearized Euler solver to predict turbulence-airfoil interaction noise from a number of configurations, including variations in airfoil thickness, angle of attack and Mach number. For the first time, noise predictions from a digital filter method are directly compared to those provided by synthetic turbulence based on a summation of Fourier modes. The comparison indicates that the advanced digital filter method gives enhanced performance in terms of computational cost and simulation accuracy. In addition, initial tests show that this method is capable of reproducing experimental noise measurements within 3 dB accuracy

Large eddy simulations of inflow turbulence noise: application to tidal turbines

Marine anthropogenic noise is increasing, along with concern about its impact on the environment. Hence minimising noise within engineering design is important, including in applications such as ships, submarines and turbines. The desire to mitigate noise may also be related to reducing the detectability of certain types of marine craft. Noise reduction typically focuses on rotating machinery such as propellers, due to the high velocity of the blades.A common source of broadband noise in engineering scenarios is often termed inflow turbulence noise. Resulting from upstream turbulence impinging onto rotor blades, this source typically dominates the low to mid range of the frequency spectrum. This is due to the high turbulence intensity and large length scales present in the inflow turbulence, which exceed those generating competing noise sources.This thesis uses a library of numerical tools to simulate broadband inflow turbulence noise. Synthetic turbulence is generated numerically within the simulations. Turbulence is resolved on the grid by solving the filtered Navier-Stokes equations. Based on the assumption of incompressible flow, noise sources may be predicted without resolving acoustic waves on the grid. This decoupling of hydrodynamic and acoustic processes means that radiated noise may be estimated using an acoustic analogy.Validation of two inflow turbulence generators revealed the importance of obtaining the prescribed turbulence statistics, as well as minimising artificial pressure fluctuations. This is used to simulate homogeneous isotropic turbulence impinging onto a foil, allowing acoustic sources to be located. The far-field sound prediction is in good agreement with experimental measurement data for low frequencies. It is then shown that the effect of foil thickness on noise can successfully be predicted using the proposed methodology.Noise radiation from a tidal turbine is then estimated by fully resolving all turbine blades, both spatially and temporally, in the simulation. A good agreement is seen in comparison to an analytical model, demonstrating that the simulation captures the dominant flow features which affect the acoustic spectrum. These spectral ‘humps’ are a result of turbulence-rotor interaction, which is implicitly included. Full scale noise estimates made from the simulations are then used to inform environmental impact assessment; the turbine hydrodynamic noise is not expected to be an issue in this regard