181 research outputs found
Simplified permeable surface correction for frequency-domain Ffowcs Williams and Hawkings integrals
No full textA simplified surface correction formulation is proposed to diminish the far-field spurious sound generated by the quadrupole source term in Ffowcs Williams and Hawkings (FW-H) integrals. The proposed formulation utilizes the far-field asymptotics of the Green's function to simplify the computation of its high-order derivatives, which circumvents the difficulties reported in the original frequency-domain surface correction formulation. The proposed formulation has been validated by investigating the benchmark case of sound generated by a convecting vortex. The results show that the proposed formulation successfully eliminates the spurious sound. The applications of the proposed formulation to flows with some special parameters are also discussed. (C) 2021 The Author(s). Published by Elsevier Ltd on behalf of The Chinese Society of Theoretical and Applied Mechanics
Nonlinear disturbance evolution in a two-dimensional boundary layer along an elastic plate and induced radiated sound
Get PDFThe interaction between a boundary-layer flow and an elastic plate is addressed by direct numerical simulation, taking into account the full coupling between the fluid flow and the flexible wall. The convectively unstable flow state is harmonically forced and two-dimensional nonlinearly saturated wavelike disturbances are computed along archetype-plates with respect to stiffness and natural frequencies. In the aim of determining the low-Mach-number radiated sound for the system, the simulation data are used to solve the Lighthill’s equation in terms of a Green’s function in the wavenumber-frequency space. Different degrees of fluid-structure coupling are implemented in the radiated sound model and the resulting acoustic pressure levels are compared. The sound radiation levels are shown to be increased in the presence of flexible walls with however significant differences in the radiated pressure levels for different coupling assumptionsThe authors gratefully acknowledge Thales Underwater Systems and DCNS for their financial support to this work
Modelling of turbulent jets and wall layers: extensions of Lighthill's acoustic analogy with application to computational aeroacoustics
No full textTwo extensions to Lighthill’s aeroacoustic analogy are presented. First, equivalent sources due to initial conditions are derived that supplement those due to boundary conditions, as given by Ffowcs Williams & Hawkings. The resulting exact inhomogeneous wave equation is then reformulated with pressure rather than density as the wave variable, and the right-hand side is rearranged using the energy equation with no additional assumptions. Applications to computational aeroacoustics are discussed, and illustrated with examples based on 2D and 3D simulations
Former Sheriffs of Denton County
No full textGroups portrait of all of the ex-sheriffs of Denton County at the time, except W. S. (Bill) Fry, who was not in town at the time. Standing in the top row, from left to right: Sam Hawkings and W. E. (Lish) Durbin. Sitting in the bottom row, from left to right: Charles Alexander "C. A." (Alex) Williams, Christopher Columbus "C. C." (Lum) Daugherty, Walter F. "W. F." Egan, and Captain Robert Harris "R. H." (Bob) Hopkins
Toward inclusion of atmospheric effects in the aircraft community noise predictions
Get PDFThis paper presents an atmospheric propagation model, based on ray acoustics, that accounts for realistic weather conditions in the evaluation of the noise footprint of an aircraft. Noise sources, obtained using the Ffowcs Williams and Hawkings acoustic analogy applied to scale-resolved flow simulation data, are stored on a hemisphere surrounding the vehicle. These noise sources are propagated using a propagation model that takes into account the vertical variability of air temperature and wind velocity. The electric vertical takeoff and landing aircraft, presented by Casalino, van der Velden, and Romani [(2019). in Proceedings of the AIAA Scitech 2019 Forum, January 7-11, San Diego, CA, pp. 1834-1851], is used as a case study; noise footprints, obtained considering various vertically varying temperature and wind velocity distributions, are compared. It is shown that weather conditions in the acoustic wave propagation can contribute to mismatch up to 4 dBA in the illuminated zone and a significant drop in the refractive shadow zone caused by the vertical air temperature and wind velocity gradients. This work constitutes the first accomplishment in including realistic atmospheric effects in aircraft community noise prediction based on scale-resolved flow simulations.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Wind Energ
Real-time reconstruction of unsteady rotating forces acting by rotor blades in moving medium
No full textA time-domain inverse aeroacoustic method based on the convective Ffowcs Williams–Hawkings equation is presented. The method allows to determine, in real-time, the unsteady forces exerted on rotating blades in the presence of a moving medium. The inversion procedure is based on a space-time regularization with a mixed l1,2-norm, which guarantees accuracy and smoothness of the solution. The method is initially verified through synthetic acoustic signals emitted by rotating sources in a constant flow, up to a convective Mach number of about 0.88. Then the method is validated through signals generated by a propeller immersed in a wind-tunnel jet flow, up to a Mach number of 0.06. Due to the reduced convective Mach number, the leading aeroacoustic effect is derived from a variation of the blade loading. It is argued that the onset of flow separation at high values of the rotor advance ratio is responsible for the onset of force fluctuations that the inverse method is able to retrieve both qualitatively and quantitatively.Wind Energ
Noise Amplification Effects due to Jet-Surface Interaction
No full textA numerical investigation on jet-installation noise is performed using a software based on the Lattice-Boltzmann Method. A flat plate is placed in the irrotational hydrodynamic field of the jet, i.e. outside of the plume, resulting in noise increase due to the scattering of instability waves at the plate trailing edge. The configuration adopted in this work replicates the one used for a benchmark study at NASA Glenn, against which the numerical results are validated. Far-field noise spectra from the isolated and installed jet cases, obtained through the Ffowcs-Williams Hawkings analogy, are compared at different polar angles. The results show a low-frequency noise amplification, occurring mainly in the directions normal to the plate and upstream of the jet axis. It is also shown that strong pressure fluctuations are generated at the trailing-edge of the plate and then propagate to the far-field as acoustic waves. The effects of the plate geometry (length and radial position) and jet flow characteristics (velocity and temperature) on the installation noise are also addressed. Pressure fluctuations on the plate are also computed and correlated to the overall far-field results, showing that, in the installed case, the surface dipoles are the dominant sources at low frequenciesGreen Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Wind Energ
Internal and near-wall flow fields around a structured porous coated cylinder and their role in passive flow and noise control
No full textView Video Presentation: https://doi-org.tudelft.idm.oclc.org/10.2514/6.2021-2226.vidThe mechanisms responsible for vortex shedding reduction, due to the application of a porous coating to a smooth cylinder placed in uniform flow, are still uncertain despite several numerical and experimental studies. To help understand these mechanisms, a transparent Structured Porous Coated Cylinder (SPCC) was manufactured to investigate the internal and near-wall flow fields. The SPCC mimics the more commonly used porous materials such as metal foam and polyurethane, that possess randomized structures preventing a clear line-of-sight along the span and circumference of the porous layers. Tomographic Particle Image Velocimetry was used in a water-tunnel facility to investigate three small regions of an SPCC, on the windward, mid-region and leeward sides. In this paper, previously unseen experimentally obtained internal flow fields of a porous coated cylinder are presented. The following summarize the key observations (1) stagnation from freestream flow to the inner cylinder diameter in the windward region, (2) boundary layer profiles within the porous layer in the circumferential mid-region and (3) bleeding from the porous layer into the wake in the leeward region. These results provide key experimental findings for comparison and validation of numerical simulations of bulk porous media, such as the Darcy-Forscheimer method used in conjunction with the Ffowcs Williams-Hawkings acoustic analogy.Wind Energ
Spectral analysis of unsteady flow simulation in a small VAWT
Get PDFThe vertical axis wind turbine studied in this paper combine two rotations: one rotating movement of each blade around its own axis and one rotating movement around turbine’s axis. The aim of this paper is to analyse the effect of this two combine movements on fields of pressure and on global forces on each blade with time. Preliminary calculations showed, for some initial blade stagger angles (angle between blade 1 and x axis), that flow is highly unsteady and sometimes hardly periodic. The main goal here is to present spectral analysis of unsteady results like temporal pressure on specific points in the domain and temporal forces on blades and to show the influence of the two combine movements for two different blade stagger angles for elliptic blades
Performance and noise prediction of low-Reynolds number propellers using the Lattice-Boltzmann method
No full textThis paper proposes a CFD/CAA-based approach to predict the aerodynamic performances and tonal/broadband noise radiation of low-Reynolds number propellers at engineering level. Broadband self-noise prediction of low-Reynolds number propellers is particularly challenging, due to the requirement for the employed computational method to emulate the complexity of the laminar/turbulent boundary-layer behavior on the blade. In this study, the numerical flow solution is obtained by using the Lattice-Boltzmann/Very Large Eddy Simulation method, whereas far-field noise is computed through the Ffowcs-Williams & Hawkings' acoustic analogy applied on the propeller surface. A zig-zag transition trip on the propeller blades is used in the numerical setup to reproduce resolved turbulent pressure fluctuations in boundary-layer for broadband noise computation at a relatively low computational cost. The effect of using a transition trip to simulate low-Reynolds number propellers, as well as the impact of its chordwise position on the calculation of performances and radiated noise, is outlined. The trip position marginally affects the thrust and to a slightly larger extent the torque prediction. Tonal noise at the blade-passing frequencies does not show a relevant sensitivity to it, whereas broadband noise is found to be slightly more influenced by the chordwise position of the trip, especially at high advance ratios. The low sensitivity of the numerical results to the trip location, as well as their good agreement with loads and noise measurements carried out in the A-Tunnel of TU-Delft, demonstrates the robustness of the proposed approach for industrial applications.Wind Energ
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