IMPACT OF NEW CHEVRON CONFIGURATIONS ON MIXING ENHANCEMENT IN SUBSONIC JETS

A major contributor to the overall noise of an aircraft is jet noise – the noise generated by the gases exiting the exhaust nozzle of a jet engine. One approach to mitigate jet noise is through the implementation of chevron nozzles. In the present context, first, a baseline axisymmetric separate-flow nozzle, termed the 3BB model, with an external plug having a bypass ratio of 5 is analyzed. The specifications of this nozzle are taken from an acoustic study carried out at the NASA John H. Glenn Research Center. Then, various chevron configurations are added to the core and fan nozzles to produce three chevron nozzles. Of these, two are presented as modified versions of the conventional chevron nozzle and form the essence of this work. The third chevron nozzle represents the conventional chevron nozzle in use today. For all the nozzles considered in this study, the flow conditions used represent the takeoff environment of a contemporary subsonic aircraft. The fan nozzle total pressure is set to 1.8 atm while the core nozzle total pressure is 1.65 atm. The total temperature inside the fan nozzle is set to 333.3 K while the core nozzle has a total temperature of 833.3 K. The freestream conditions are given as: static pressure = 0.98 atm, total pressure = 1.04 atm, total temperature = 298.8 K and Mach number = 0.28. For the three chevron nozzles, the core and fan nozzles have 12 chevrons each. Each chevron extends over a sector of 30 degrees of the circumference. To carry out the study presented herein, first, computer-aided design (CAD) models of the four nozzles are created. These models are then used to carry out computational fluid dynamics (CFD) simulations with the conditions stated above. The CFD simulations are performed on STAR-CCM+. The results of the simulations carried out for the baseline nozzle are compared with existing experimental and numerical data to validate the use of STAR-CCM+ as a tool for studying jet flows. Once this step is complete, numerical simulations are carried out for the three chevron nozzles. The results from these are compared with those obtained for the baseline nozzle. The turbulent kinetic energy (TKE) and the mean axial velocity are the two main parameters that represent mixing enhancement and are focused on in this work. Since the TKE levels for a given nozzle are directly linked to the jet noise generated, the TKE is an important indication of the jet noise produced by a given nozzle. Other jet mixing parameters such as the centerline total temperature decay and the centerline velocity of the jet flow exiting each nozzle are also analyzed. A 2-D axisymmetric grid is produced for the 3BB nozzle while a 3-D mesh is generated for each of the chevron nozzles. To reduce the computation cost, only a 30° sector of the chevron nozzles is modeled. Since the Shear Stress Transport (SST) k-ω turbulence model has been widely used in several aerospace applications, it is chosen for all simulations here as well. The numerical analysis shows that STAR-CCM+ can successfully be used for the study of jet flows. Although some shortcomings do exist, the simulations provide a reasonable understanding of jet flows. Of the three chevron nozzles studied, the simulations demonstrate that in comparison to the baseline nozzle, all three chevron nozzles register peak values of the turbulent kinetic energy that are lower than that observed for the 3BB nozzle. The regions of highest turbulence also appear further upstream for the chevron nozzles. Compared to the conventional chevron nozzle, the two parametric designs presented in this work show a potential reduction in the peak values of the turbulent kinetic energy in their respective flows. A slight reduction in the mean axial velocities is also observed for these nozzles. Further, a close inspection of the turbulent flowfield of one of the parametric designs shows that the highest intensity turbulence in the flow is first observed at the most upstream location for this nozzle. The high levels of TKE are also confined to a smaller region in this case. Based on these results, the two parametric chevron nozzle designs demonstrate a potential to produce lower jet noise than what is observed in case of a conventional chevron nozzle. Finally, a study of the turbulent flowfields of all the nozzles shows that the mixing between the fan and freestream shear layers still dominates the mixing in the jet flow. However, the chevrons are able to add streamwise vortices to the flow that enhance mixing between the core and fan shear layers to some extent. This promotes better mixing and as a result, the turbulence in the jet plume is reduced