The Wind computational fluid dynamics code was used to perform a series of simulations on two offset stream nozzle concepts for jet noise reduction. The first concept used an S-duct to direct the secondary stream to the lower side of the nozzle. = nozzle plenum total temperature u = local axial velocity ujet = mass-averaged axial velocity of primary jet x, y, z = coordinate system ( ) = used to denote difference in discharge or thrust coefficient of the current configuration from the baseline configuration
A series of three convergent, round-to-rectangular high aspect ratio (HAR) nozzles were designed for acoustic testing at the NASA Glenn Research Center Nozzle Acoustic Test Rig (NATR). The HAR nozzles had exit area aspect ratios of 8:1, 12:1, and 16:1. The nozzles were designed to mimic a distributed propulsion system array with a slot nozzle. The nozzle designs were screened using Reynolds-Averaged Navier-Stokes (RANS) simulations. In addition to meeting the geometric constraints required for testing in the NATR, the HAR nozzles were designed to be free of flow features that would produce unwanted noise (e.g., flow separations) and to have uniform flow at the nozzle exit. Multiple methods were used to generate HAR nozzle designs. The final HAR nozzle designs were generated in segments using a computer code that parameterized each segment. RANS screening simulations showed that intermediate nozzle designs suffered flow separation, a normal shockwave at the nozzle exit (caused by an aerodynamic throat produced by boundary layer growth), and nonuniform flow at the nozzle exit. The RANS simulations showed that the final HAR nozzle designs were free of flow separations, but were not entirely successful at producing a fully uniform flow at the nozzle exit. The final designs suffered a pair of counter-rotating vortices along the outboard walls of the nozzle. The 16:1 aspect ratio HAR nozzle had the least uniform flow at the exit plane; the 8:1 aspect ratio HAR nozzles had a fairly uniform flow at the nozzle exit plane.
The wind computational fluid dynamics code was used to perform a series of analyses on a single-flow plug nozzle with chevrons. Air was injected from tubes tangent to the nozzle outer surface at three different points along the chevron at the nozzle exit: near the chevron notch, at the chevron mid-point, and near the chevron tip. Three injection pressures were used for each injection tube location-10, 30, and 50 psiggiving injection mass flow rates of 0.1, 0.2, and 0.3 percent of the nozzle mass flow. The results showed subtle changes in the jet plume's turbulence and vorticity structure in the region immediately downstream of the nozzle exit. Distinctive patterns in the plume structure emerged from each injection location, and these became more pronounced as the injection pressure was increased. However, no significant changes in centerline velocity decay or turbulent kinetic energy were observed in the jet plume as a result of flow injection. Furthermore, computational acoustics calculations performed with the JeNo code showed no real reduction in jet noise relative to the baseline chevron nozzle. Nomenclature Ddiameter of nozzle exit, taken as the average of the nozzle diameters at the chevron valley and chevron peak D jet equivalent jet diameter based on flow area as calculated by the JeNo code EPNdB effective perceived noise in dB f frequency of noise in Hz k turbulent kinetic energy NPR nozzle pressure ratio SPL sound pressure level, at a given frequency St Strouhal number u local axial velocity u jet mass-average axial velocity of jet at nozzle exit u jet,JeNo maximum axial velocity at jet exit as calculated by the JeNo code W magnitude of vorticity x, y, z orthogonal coordinate system IntroductionIn recent years, nozzles with chevrons have been developed as a method to reduce aircraft jet noise. Chevrons work by strengthening streamwise vortices that increase mixing within the plume to hasten jet potential core decay. Enhanced mixing usually increases the smaller scales of motion, and thus adds to the high frequency noise. However, the breakdown of the larger scale turbulence into small scale turbulence reduces the low frequency noise at its peak directivity angle near the downstream jet axis and subsequently reduces the overall sound pressure level. Callender, Gutmark, and Martens (refs. 1 to 3) conducted extensive tests to study the near-field and far-field noise reduction benefits of chevrons nozzles in dual-stream jets. They found that the presence of chevrons moved the peak noise directivity angle away from the streamwise NASA/TM-2008-215150 2 axis and reduced its level. Additionally, increasing chevron penetration tends to further strengthen its role in reshaping the noise spectrum, i.e., increasing the high frequency noise and reducing the low frequency noise. Bridges and Brown (ref. 4), used a single flow nozzle with chevrons to study the impact of the chevron penetration on the noise. They found that chevrons reduce noise equally well in heated and unheated jets. Koch, Khavaran, and Bri...
This article investigates the role of a free jet on the sound radiated from a jet. In particular, the role of an infinite wind tunnel, which simulates the forward flight condition, is compared to that of a finite wind tunnel. The second configuration is usually used in experiments, where the microphones are located in a static ambient medium far outside the free jet. To study the effect of the free jet on noise, both propagation and source strength need to be addressed. In this work, the exact Green's function in a locally parallel flow is derived for a simulated flight case. Numerical examples are presented that show a reduction in the magnitude of the Green's function in the aft arc and an increase in the forward arc for the simulated flight condition. The effect of finite wind tunnel on refraction is sensitive to the source location and is most pronounced in the aft arc. A Reynolds-averaged Navier-Stokes solution (RANS) yields the required mean flow and turbulence scales that are used in the jet mixing noise spectrum calculations. In addition to the sound/flow interaction, the separate effect of source strength and elongation of the noise-generating region of the jet in a forward flight is studied. Comparisons are made with experiments for the static and finite tunnel cases. Finally, the standard free-jet shear corrections that convert the finite wind tunnel measurements to an ideal wind tunnel arrangement are evaluated. I. IntroductionForward flight is generally believed to reduce the jet noise emission due to the reduced shear. Experimentally, simulation of the flight and its effect on jet noise is carried out in either an open wind tunnel (OWT) or an ideal wind tunnel (IWT). In the first configuration, the primary jet is surrounded by a co-flow (free-jet) with Mach number M ∞ that simulates the flight Mach number, while the microphones are positioned outside the free stream in the stationary atmosphere. In the second arrangement, also referred to as an infinite wind tunnel, the microphones are positioned inside the co-flow to avoid the ambiguities associated with the refraction correction. In order for the microphones to be in the far field of the acoustic sources, the tunnel flow needs to extend far enough in the span-wise direction. Obviously, in either case, the measurements are carried out with fixed microphones.In practice, the OWT measurements are corrected for the secondary shear layer refraction, using semi-empirical corrections equations. The usual practice (ref. 1) is to replace the free jet shear layer with a vortex-sheet and to consider the source along the jet center line-and use a three-step calibration procedure for angle, distance, and sound amplitude. This procedure takes advantage of the high frequency geometric acoustics arguments, i.e., conservation of acoustic energy along a ray tube, and converts the noise measured outside the free jet to the levels that would be measured in an IWT. These simplifications, among other factors, ignore the difference between the on-axis and offaxis sourc...
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