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Daniel CRUNTEANU Petre Claudiu COJAN 54, Table 1 Characteristics and dimension of the four chevrons all dimensions are in centimeters. Code Type of chevron, For the two dimensional simulation of the flow the governing equations for Ansys. Fluent 12 1 are based on Navier Stokes equations The equations which describe the best our. case are the following ones,t U x Fx y Fy t U x Fx y Fy. G x x yG y,x x G yG y,INCAS BULLETIN Volume 5 Issue 3 2013. 55 Acoustic characteristics of the flow over different shapes of nozzle chevrons. The viscous tensors are,xx 2 u x u x v y v y,yy 2 v y v y u x u x.

y u y v x v x,Fig 1 Studying domain h chevron s length. 3 NUMERICAL SIMULATION AND RESULTS, On the first hand we placed each chevron inside the studying domain and after that we. realized a mesh for each case using Gambit 2 4 6, The resulted meshes were structured ones They were made selecting Map option for the. type of mesh and Quad option for the elements They also contain around 2 5 105 faces. and 12000 nodes, For obtaining proper results we chose to make a denser grid next to the chevron s walls. This option is justified because our interest is confined to find best results close to the. chevron area,INCAS BULLETIN Volume 5 Issue 3 2013,Daniel CRUNTEANU Petre Claudiu COJAN 56.

Fig 2 Structured meshes a Gambit 2 4 6 b Ansys Fluent 12 1. Because we were looking for a higher accuracy of the solution we have selected a. double precision method and density based solver because this was originally designed for. high speed flows, The density based explicit and implicit formulations solve the equations for additional. scalars e g turbulence or radiation quantities sequentially. The implicit and explicit density based formulations differ in the way that they linearize. the coupled equations, Due to broader stability characteristics of the implicit formulation a converged steady. state solution can be obtained much faster using the implicit formulation rather than the. explicit formulation, However the implicit formulation requires more memory than the explicit formulation. The governing equation for density based solver which involves a scalar quantity is the. following one,dV v dA dA V,where density,v velocity vector v ui vj for 2D. A area vector,diffusion coefficient for,gradient of i j for 2D.

S source of, Regarding boundary conditions we selected the following types for all of our four. simulations,WALL solid walls for the chevron profile. VELOCITY INLET 250 m s for the inlet area,PRESSURE OUTLET for lateral and outlet area. INCAS BULLETIN Volume 5 Issue 3 2013, 57 Acoustic characteristics of the flow over different shapes of nozzle chevrons. Because we searched for accuracy results regarding the acoustic field we enabled the. Broadband Acoustic Source, This model uses for calculation Proudman s equation Lilley s equation and Goldstein s.

theory All these approaches are based on Lighthill s equation for turbulent flows which. tells us that the acoustic power emitted by a spherical source is. K 0u 4l 3 1 M c2,4 a05 4 1 M 2 4 4 21, Beside this we needed to select a proper turbulence model which should be a. characteristic of each flow, Our option was standard k epsilon turbulence model because this is one of the most. popular models which have been tested for a large area of applications One of his major. advantages is the fact that this model doesn t need a large amount of time for doing the. simulation, This is the reason why our simulations had a convergent solution somewhere between. 28000 and 30000 iterations the maximum time spent for a complete simulation being around. The obtained results shown in the next figures took into consideration the fact that. noise production is closely connected with flow velocity and turbulent kinetic energy. Fig 3 Contours of Velocity Magnitude m s, As we can see in Fig 3 the chevron shape does not have quite an influence regarding. air flow velocity, So taking into consideration that the noise magnitude is proportional with the air flow.

velocity for now we cannot tell which of the shapes is more efficient in noise reduction. INCAS BULLETIN Volume 5 Issue 3 2013,Daniel CRUNTEANU Petre Claudiu COJAN 58. Fig 4 Contours of Turbulent Intensity, Studying the turbulent intensity we can observe that the chevron shapes with the lower. intensity are CH 2 and CH 3 So these two shapes should be taken into consideration for the. following studies which involve noise reduction,Fig 5 Contours of Acoustic Power Level dB. Going further and analyzing the acoustic power level we can find a difference of two. decibels between chevrons CH 2 CH 3 125 dB and chevron CH 4 127 dB and a. difference of one decibel between chevrons CH 2 CH 3 125 dB and chevron CH 1 126. INCAS BULLETIN Volume 5 Issue 3 2013, 59 Acoustic characteristics of the flow over different shapes of nozzle chevrons. 4 CONCLUSIONS, In this numerical simulation the importance of the air flow velocity and turbulence effects on.

the acoustic noise generation and propagation was obviously presented Beside this we also. presented the influence of chevron shape design on the acoustic noise reduction finding that. this aspect may contribute to a decrease of acoustic power level by 2 dB. The technology which has been used for simulation was not a very high one this being. the reason why the differences between the four shapes of chevrons are not so evident. Despite this we can tell that the most efficient shape of chevron from all points of view. noise reduction turbulent intensity and air flow velocity is chevron CH 3. REFERENCES, 1 E Marvin Goldstein Aeroacustics McGraw Hill International Book Company New York USA 1976. 2 S Sarkar M Y Hussaini Computation of the sound generated by isotropic turbulence NASA Contractor. Report 191543 ICASE Report No 93 74 October 1993, 3 Umesh Paliath Numerical simulation of jet noise ISBN 0542810077 9780542810077 Pennsylvania State. University USA 2006, 4 F Alton Everest The master handbook of acoustics fourth edition McGraw Hill International Company. 5 C K W TAM Numerical Methods in Computational Aeroacoustics AIAA Short Course Seattle. Washington 8 9 May 1999, 6 T J CHUNG Computational fluid dynamics ISBN 0521594162 Cambridge University Press New York. 7 ANSYS FLUENT 12 1 documentation, 8 Vance Dippold III CFD Analyses and Jet Noise Predictions of Chevron Nozzles with Vortex Stabilization.

Glenn Research Center Cleveland Ohio USA 2008,INCAS BULLETIN Volume 5 Issue 3 2013.

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