1,721,246 research outputs found
Some aspects of RANS based jet noise prediction
No full textEThOS - Electronic Theses Online ServiceGBUnited Kingdo
Non-axisymmetric acoustic radiation from a transversely oscillating rigid sphere above a rigid/compliant planar boundary
No full textHarmonic acoustic radiation from a transversely oscillating rigid sphere within a fluid halfspace, while its polar axis is oriented either normal or parallel to the rigid/compliant halfspace boundary, is studied using the classical method of separation of variables. The translational addition theorem for spherical wave functions is initially employed to analyze the non-axisymmetric acoustic coupling between a pair of arbitrarily positioned, axisymmetric, finite-sized spherical sources undergoing harmonic modal surface vibrations in an unbounded acoustic medium. Subsequently, the method of images is used to efficiently take the presence of the rigid/compliant boundary into account. The analytical results are illustrated with numerical examples in which the spherical source, vibrating in the horizontal-dipole and vertical-dipole modes, is positioned near the rigid (compliant) boundary of a water-filled acoustic halfspace. Subsequently, the basic acoustic field quantities such as the modal acoustic radiation impedance load and the radiation intensity distribution are evaluated for representative values of the parameters characterizing the system
Sound radiation from a fluid-filled spherical acoustic lens with an internal eccentric baffled spherical piston
No full textAn exact study on reradiation of an acoustic field due to radial/axial vibrations of a baffled spherical piston, while eccentrically positioned within a fluid-filled thin spherical elastic shell, into an external fluid medium is presented. This configuration, which is a realistic idealization of a liquid-filled spherical acoustic lens with focal point inside the lens when used as a sound projector, is of practical importance with multitude of possible applications in ocean engineering and underwater acoustics. The formulation utilizes the appropriate wave field expansions along with the translational addition theorems for spherical wave functions to develop a closed-form solution in form of infinite series. Numerical results reveal that in addition to frequency, cap angle, radiator position (eccentricity), cap surface velocity distribution, and dynamics of the elastic shell can be of significance in sound radiation
Airfoil trailing edge noise reduction by the introduction of sawtooth and slitted trailing edge geometries
No full textThis paper compares the measurements of the trailing edge self noise reduction obtained using sawtooth and slit serrations on a NACA651210 airfoil. This work is relevant to reducing the noise from aircraft engines, aircraft wings and wind turbines. A detailed experimental study conducted in the ISVR's open-jet wind tunnel reveals noise reduc- and the boundary layer has been tripped so as to become turbulent. Measurements of the static pressure coefficient distribution along the chord of the airfoil are also reported. This is to allow the effects on lift to be assessed. Noise measurements for the sawtooth serrations are compared to the theory derived by Howe. Howe's theory is extended to include a series of slits and compared to experiments. It is shown theoretically that for a sawtooth profile high levels of noise reduction can be achieved, either when the serration wavelength γ is smaller than the boundary layer thickness 8 or when the root-to-tip distance h is larger than δ. It is shown theoretically that the slit serrations are not an effective noise reduction treatment since the noise reduction asymptotes to zero at high frequencies. Experimental measurements of the noise reduction obtained using trailing edge sawtooth and slits are shown to be significantly less than that predicted. The noise is shown to increase at frequencies above some critical frequency, which is shown to depend only on f 0δ U c ≈ 1 and independent of serration geometry.</p
Simulation of atmospheric boundary layer in the wind tunnel facility at University of Bristol
No full textInteraction between a flat plate and a circular subsonic jet
No full textThis paper reports an extensive near- and far-field analysis of the noise generated by an isothermal, subsonic, circular jet in the presence of a solid, flat plate shield. Far-field polar and azimuthal acoustic images are presented initially to characterize the interaction noise source. Near-field streamwise microphone phase analysis along the plate trailing edge reveals a deeper understanding of the link between the jet hydrodynamic field (both linear and non-linear regions) and the mechanisms behind interaction noise generation. Near-field point spectrum data have also been used successfully to validate Amiet's far-field trailing edge dipole prediction code for low-speed jet acoustic Mach numbers.</p
A ray tracing method applied to the propagation of jet noise
No full textAircraft noise remains a major obstacle for air traffic growth as new aircraft must meet ever more stringent certification requirements for noise emissions around airports. A large part of aircraft noise arises from the engine and manufacturers have taken steps to make them quieter. In the case of turbofan aircraft this has been achieved largely by increasing the bypass ratio. Today, there is little room left for further reduction of noise in this way and more novel solutions must be found. One such method is to redesign exhaust nozzles in order manipulate the flow in a way that gives an acoustic benefit, for example in non-circular nozzles a larger flow field may shield the noise sources more efficiently. In turn, this brings a requirement for more advanced noise prediction tools. In this paper, a jet noise prediction method based on Lighthill’s Acoustic Analogy coupled with a Ray-Tracing theory is presented. A full 3D Ray-Tracing method is developed which provides information about the refraction effects due to wave propagation in a non-axisymmetric jet flow. Using the turbulence information obtained from a RANS CFD simulation and the refractions effects obtained from the Ray-Tracing, the method calculates the far-field noise using a modified Lighthill’s equation (LRT). The classic jet noise prediction method, known as MGBK, is also used here for comparison. Results are presented for subsonic single-stream jets operating at Mach 0.75, 0.9 and different temperatures. The refraction results obtained using the ray tracing method are compared with those found using Lilley’s wave propagation equation. Comparisons have shown that the ray tracing method works well for all polar angles outside the zone of silence. The far-field noise comparisons have shown that the LRT method is capable of capturing the peak frequency much better than the MGBK method. The general trend of the spectrum at high and low frequencies as obtained using the new method is also better than those found using MGBK. <br/
Prediction of jet mixing noise with Lighthill’s Acoustic Analogy and geometrical acoustics
No full textA computational aeroacoustics prediction tool based on the application of Lighthill’s theory is presented to compute noise from subsonic turbulent jets. The sources of sound are modeled by expressing Lighthill’s source term as two-point correlations of the velocity fluctuations and the sound refraction effects are taken into account by a ray tracing methodology. Both the source and refraction models use the flow information collected from a solution of the Reynolds-Averaged Navier-Stokes equations with a standard k-epsilon turbulence model. By adopting the ray tracing method to compute the refraction effects a high-frequency approximation is implied, while no assumption about the mean flow is needed, enabling the authors to apply the new method to jet noise problems with inherently three-dimensional propagation effects. Predictions show good agreement with narrowband measurements for the overall sound pressure levels and spectrum shape in polar angles between 60 and 110 for isothermal and hot jets with acoustic Mach number ranging from 0.5 to 1.0. The method presented herein can be applied as a relatively low cost and robust engineering tool for industrial optimization purposes
High-Fidelity Propeller Broadband Noise Prediction using SU2
No full textA comprehensive numerical study is conducted to assess aerodynamic loads and far-field noise radiation of a 9x9 propeller under two distinct flow conditions: hover and forward flight. The simulations are performed on the UK National High Performance Computing cluster ARCHER2 using the open-source solver SU2. Two mesh configurations are employed: a coarse mesh (G1) consisting of 127 million elements and a finer mesh (G2) consisting of 251 million elements. The study commences with a Reynolds-Averaged Navier-Stokes (RANS) simulation to achieve a steady state, which is then used to initiate an Implicit Large Eddy Simulation (ILES) simulation with a dual time-stepping scheme. Far-field noise is predicted using the solid-surface Ffowcs Williams-Hawkings acoustic analogy at 21 observer locations, corresponding to the microphone positions from an experimental study conducted at the University of Bristol aeroacoustic facilities. The numerical results display good agreement with experimental data for torque and thrust predictions at both G1 and G2 meshes. Although the numerical noise prediction method does not consider quadrupole noise, it shows satisfactory agreement with experimental data for broadband noise between 1 kHz and 10 kHz and good agreement with experimental overall sound pressure level, especially in the forward flight case. The results validate the SU2 solver's ability to perform high-fidelity noise predictions for rotating propellers
- …
