Generation of sound due to laminar flow past a circular cylinder performing rotary oscillations has been studied using a direct numerical simulation approach. Two-dimensional, unsteady, compressible Navier-Stokes equations are directly solved using high resolution, physical dispersion relation preserving schemes. In this work, modifications in the flow induced acoustic noise due to imposed rotary oscillations have been discussed in detail. Simulations have been performed for a Reynolds number Re = 150 and a Mach number M = 0.2 over a wide range of forcing frequencies and amplitudes of rotary oscillation, specifically in the synchronization region. Rotary oscillating motion of a cylinder modifies the vortex shedding patterns in the wake region as compared to the case of flow past a stationary cylinder. The frequency and strength of shed vortices determine the nature of aerodynamic forces acting on the cylinder as well as sound generation. Reduction in sound generation has been observed for some of the forced oscillation cases as compared to the flow past a stationary cylinder case. The Doak’s decomposition methodology has been used to segregate the acoustic and hydrodynamic modes from the momentum density field to understand changes in the radiated sound field for different forcing conditions. Furthermore, disturbance pressure fields have been decomposed into a number of modes based on their significance, using a proper orthogonal decomposition (POD) technique in order to identify and quantify the contribution of the lift and drag dipoles to the sound field. In addition, POD modes of disturbance vorticity fields as well as noise source structures based on approximate Lighthill’s stress tensor are also obtained and related to the generated sound fields. This analysis concludes that the frequency of rotary oscillation dictates the frequency content of the flow induced sound field. Low frequency rotary oscillations trigger sound waves with low frequencies and large wavelengths. As the forcing frequency increases, the corresponding sound field displays shorter wavelengths. Directivity of the sound field is affected by the amplitude of rotary oscillation. A case with higher forcing amplitude distributes sound energy more evenly in all directions as compared to a lower forcing amplitude case. Prescription of rotary oscillations to the circular cylinder significantly alters the frequency, amplitude, and directivity of the generated sound field.

Analysis of sound generation by flow past a circular cylinder performing rotary oscillations using direct simulation approach

The calculation of acoustic field solutions due to aeroacoustic sources is performed for a large number of observer locations. Sound is predicted by a hybrid method that uses direct calculation for near-field source computations and the Ffowcs Williams―Hawkings equation as the acoustic analogy formulation. The integrations of surface dipole and volume quadrupole source terms appearing in the Ffowcs Williams―Hawkings formulation are accelerated by a three-dimensional wideband multilevel adaptive fast multipole method. The three-dimensional wideband fast multipole method presented here applies a plane-wave expansion formulation with fast spherical filtering and interpolation and diagonal translation for calculations in the high-frequency regime and a partial-wave expansion formulation with rotational―coaxial translation in the low-frequency regime. The method is described for the solution of a three-dimensional Green's function that incorporates convective effects. The sound generated by the unsteady flow past a cylinder in the proximity of a NACA0012 airfoil is studied and the interaction of both geometries is analyzed for different cylinder positions. The developed numerical capability allows the analysis of each noise source individually. Therefore, the effects of dipoles and quadrupoles for each configuration are investigated. Results for acoustic field solutions obtained by the accelerated Ffowcs Williams―Hawkings formulation are up to 2 orders of magnitude faster than with the conventional computation of Ffowcs Williams―Hawkings equation.

Aeroacoustic Integrals Accelerated by Fast Multipole Method

Hybrid prediction of the aerodynamic noise radiated by a rectangular cylinder at incidence

Direct simulations of sound generation due to two-dimensional, unsteady, laminar flow past a wedge at several angles of incidence have been performed using a highly accurate, physical dispersion relation preserving scheme We have considered a uniform flow past a wedge at a Mach number of M = 02 and a Reynolds number of Re = 100, at thirteen different angles of incidence 0° ≤ α ≤ 60° Results show that the vortex shedding phenomena which in turn strongly depend on the angle of incidence are responsible for triggering negative and positive pressure pulses We have in particular focused our attention on a special case α = 30° where the mean drag attains a lowest value among all angle of incidence cases and also reports a highest root mean square value for the lift coefficient We have closely related the effects of the fluctuations in flow field parameters on the frequency and amplitudes of generated sound waves The generated sound field displays dipolar nature The lift dipole contributes more to the sou

Direct simulation of sound generation by a two-dimensional flow past a wedge

Aerodynamic sound from a square cylinder with a downstream wedge

Mach number and thermal effects on the mechanisms of sound generation and propagation are investigated in spatially evolving two-dimensional isothermal and non-isothermal mixing layers at Mach number ranging from 0.2 to 0.4 and Reynolds number of 400. A characteristic-based formulation is used to solve by direct numerical simulation the compressible Navier-Stokes equations using high-order schemes. The radiated sound is directly computed in a domain that includes both the near-field aerodynamic source region and the far-field sound propagation. In the isothermal mixing layer, Mach number effects may be identified in the acoustic field through an increase of the directivity associated with the non-compactness of the acoustic sources. Baroclinic instability effects may be recognized in the non-isothermal mixing layer, as the presence of counter-rotating vorticity layers, the resulting acoustic sources being found less efficient. An analysis based on the acoustic analogy shows that the directivity increase w...

Numerical study of Mach number and thermal effects on sound radiation by a mixing layer

An iterative algorithm is developed for the computation of aeroacoustic integrals in the time domain. It is specially designed for the generation of acoustic images, thus giving access to the wavefront pattern radiated by an unsteady flow when large size source fields are considered. It is based on an iterative selection of source-observer pairs involved in the radiation process at a given time-step. It is written as an advanced-time approach, allowing easy connection with flow simulation tools. Its efficiency is related to the fraction of an observer grid step that a sound-wave covers during one time step. Test computations were performed, showing the CPU-time to be 30 to 50 times smaller than with a classical non-iterative procedure. The algorithm is applied to compute the sound radiated by a spatially evolving mixing-layer flow: it is used to compute and visualize contributions to the acoustic field from the different terms obtained by a decomposition of the Lighthill source term.

An iterative algorithm for computing aeroacoustic integrals with application to the analysis of free shear flow noise

https://hal.archives-ouvertes.fr/hal-00707329/document

High Accuracy Volume Flow Rate Measurement Using Vortex Counting

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