PhD Student, Indian Institute of Technology Bombay
Title:Effect of Geometric Parameters of Chevron Nozzle on Production of Streamwise Vortex in JetFlow
Jet noise is one of the major concern for communities near airports with growing Aviation industry since the beginning of the jet era. While take-off aircraft creates most of its noise through an exhaust jet of the engine. Research is going on from the last several decades to reduce jet noise.
Among the several techniques examined, chevron is considered to be the better trade off between noise reduction and thrust loss. It is a saw tooth like serrations on the lip of the nozzle, which can be seen in Boeing 787 Dreamliner. Numerous researchers are working on jet acoustics, but it is still not completely understood how it exactly works. Our research is focused on the same problem of chevron nozzles.
It is not clear from open literature how different geometric parameters of chevrons affects flow characteristics of a jet. in the present work effect of various geometric parameters of chevron on jet flow is investigated computationally. This provides curcial information regarding chevron geometry selection while designing a nozzle for jet engines.
Abstract: To mitigate jet-mixing noise emitted from a Mach 0.85 hot-air lance in iron manufacturing, we examined the effect of chevron tip geometry on large turbulence structure noise and fine-scale turbulence noise. The parametric variables are tip count (0, 4, 8 and 16) and tip shape (original chevron, tetragonal tip and round tip). To determine the tip effect quantitatively, we proposed the corrected jet noise spectrum equation and coefficient factor. Increasing the chevron tip count induced efficient jet mixing due to an increase in downstream jetmixing interface area. When the tip count was eight, large turbulence noise was reduced to the most. When the tip count was fixed at eight for round and tetragonal tips, jet mixing and noise reduction efficiency are both decreased owing to the non-uniform tip area decrease in the nozzle’s axial direction.
Pub.: 22 Jan '15, Pinned: 31 Jul '17
Abstract: The flow and acoustic fields of subsonic turbulent hot jets exhausting from three divergent nozzles at a Mach number M=0.12 based on the nozzle exit velocity are conducted using a hybrid CFD-CAA method. The flow field is computed by highly resolved large-eddy simulations (LES) and the acoustic field is computed by solving the acoustic perturbation equations (APE) whose acoustic source terms are determined by the LES. The LES of the computational domain includes the interior of the nozzle geometry. Synthetic turbulence is prescribed at the inlet of the nozzle to mimic the exit conditions downstream of the last turbine stage. The LES is based on hierarchically refined Cartesian meshes, where the nozzle wall boundaries are resolved by a conservative cut-cell method. The APE solution is determined on a block structured mesh. Three nozzle geometries of increasing complexity are considered, i.e., the flow and acoustic fields of a clean geometry without any built-in components, a nozzle with a centerbody, and a nozzle with a centerbody plus struts are computed. Spectral distributions of the LES based turbulent fluctuated quantities inside the nozzle and further downstream are analyzed in detail. The noise sources in the near field are noticeably influenced by the nozzle built-in components. The centerbody nozzle increases the overall sound pressure level (OASPL) in the near field with respect to the clean nozzle and the centerbody-plus-strut nozzle reduces it compared to the centerbody nozzle due to the increased turbulent mixing. The centerbody perturbed nozzle configurations generate a remarkable spectral peak at St=0.56 which also occurs in the APE findings in the near field region. This tone is generated by large scale vortical structures shed from the centerbody. The analysis of the individual noise sources shows that the entropy term possesses the highest acoustic contribution in the sideline direction whereas the vortex sound source dominates the downstream acoustics.
Pub.: 18 Sep '16, Pinned: 31 Jul '17
Abstract: A 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 application of the 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.
Pub.: 04 Mar '17, Pinned: 31 Jul '17
Abstract: In the present study, the flow fields generated by two synthetic jets with a chevron and a conventional circular nozzle exits are studied and compared. For both configurations, the devices are operated at the same input electrical power, thus leading to Reynolds and Strouhal numbers equal to 5600 and 0.115 (for the circular exit) and 6000 and 0.106 (for the chevron exit). Phase-locked stereoscopic particle image velocimetry measurements are used to reconstruct the three-dimensional coherent vortex structures. Time-averaged and phase-averaged mean and turbulent statistics are analysed and discussed. The flow field strongly depends on the exit geometry. In presence of the chevron exit, the conventional vortex ring issued through the circular nozzle exit, is replaced by a non-circular vortex ring with additional streamwise vortices. The mutual interaction between these structures prevents the axis-switching of the non-circular vortex ring during its convection. These streamwise vortices disappear convecting downstream and the vortex ring assumes a circular shape. Comparing the two configurations, the chevron exit generates a larger time-averaged streamwise velocity along the centreline but with lower turbulent kinetic energy intensity. Differences are also present between the notch and the apex planes of the chevron exit. In the notch plane, both the time-averaged axial velocity component profile in the spanwise direction and the shearlayer width are wider than in the apex plane. Furthermore, the presence of the streamwise vortices causes a flow motion towards the jet axis in the apex plane and an opposite motion in the notch plane.
Pub.: 11 Nov '16, Pinned: 31 Jul '17