Donnell Washington

Effects of Counterflow on the Aeroacoustic Properties of a Supersonic Jet

The influence of counterflow on the mixing and acoustic characteristics of a Mach 1.4 rectangular jet operated at on- and off-design conditions were studied experimentally for different levels of counterflow. The results show that counterflow significantly enhances shear-layer mixing and reduces the jet potential core length under all operating conditions. Significant changes in both shock-cell spacing and strength were ohserved when counterflow was applied to nonideally expanded jets. Consequently, screech tones were either reduced or totally eliminated, and broadband shock-associated noise was shifted to higher frequencies. In the underexpanded mode, a Mach disk was formed at certain levels of counterflow, which substantially weakened the subsequent periodic shock-cell structure and reduced the broadband shock-associated noise and the overall sound pressure level (OASPL) by as much as 3 dB. Interestingly, it was also discovered that a jet operating at overexpanded conditions could be decelerated nearly isentropically by applying the proper amount of counterflow. This modification led to a 4 dB reduction in OASPL. Based on the present study, it is suggested that counterflow warrants further investigation as a potential noise reduction technique.

Experimental study of a compressible countercurrent turbulent shear layer

A countercurrent shear layer with a nominal convective Mach number (M c ) of 2 is generated using a unique arrangement. The shear layer growth rates were found to be twice that of conventional, coflowing shear layers at comparable M c . It is hypothesized that the countercurrent shear layer becomes self-excited, leading to a modification of the turbulent properties. Instantaneous flow visualization images clearly reveal the presence of large, highly convoluted, turbulent structures believed to be responsible for the enhanced growth rates. The flowfield was also examined for the presence of eddy shocklets that have been observed in numerical simulations under similar compressible flow conditions. Even though the countercurrent flowfield appears to satisfy the criteria for the creation of shocklets, a careful and thorough search revealed no evidence of their presence.

Aeroacoustic properties of a supersonic diamond-shaped jet

The potential for using a novel diamond-shaped nozzle, which may allow superior mixing and noise characteristics of supersonic jets without significant thrust losses, is explored. Flow visualization and pressure measurements indicate the presence of distinct structures in the shear layers, not normally observed in shear layers of ideally expanded axisymmetric and rectangular jets. The characteristics of these features suggest that they are a manifestation of significant streamwise vorticity in the shear layers. Despite the distinct nature of the flowfield structure of the diamond jet, the far-field noise characteristics are quite similar to those of the corresponding axisymmetric jet, except moderate mixing-noise reduction in the aft quadrant. Furthermore, the global shear-layer growth rates are also very similar to those of the axisymmetric and two-dimensional counterparts. These observations lead one to believe that the presence of streamwise vorticity may not play a significant role on the overall mixing and noise generation of a supersonic jet.

Multi-Axis Fluidic Thrust Vectoring of a Supersonic Jet Using Counterflow

The most common techniques currently used to efficiently vector supersonic jets require external flaps and or pivoting devices. Fluidic thrust vectoring using counterflow eliminates the need for such complex hardware. Thus, the promise of decreases in both weight and drag as well as increased maneuverability makes this technique an attractive alternative. This technique has been successfully employed to achieve single axis fluidic thrust vectoring of a Mach 2 rectangular jet. To better compete with contemporary systems the current study extends this technique to multi-axis thrust vectoring of a Mach 2 diamond-shaped jet by applying counterflow to one of its four sides. To evaluate the performance of this technique the Planar Laser Scattering (PLS) technique is used to show the continuous vectoring of the diamond jet up to 20 degrees. Also, cross-stream PLS images are acquired to show the vectoring can be achieved off all four surfaces of the diamond jet.

The use of counterflow for noise suppression of a supersonic rectangular jet

The effect of counterflow on the mixing and acoustic characteristics of a Mach 1.44 rectangular jet operated at on and off design conditions is studied experimentally for different levels of counterflow. The results show that counterflow significantly enhances shear layer mixing and reduces the jet potential core length under all operating conditions. Changes in both shock cell spacing and strength occur when counterflow is applied to nonideally expanded jets. Consequently, screech tones are either reduced or totally eliminated, and breadband shock associated noise is observed to shift to higher frequencies. In the under-expanded mode, a Mach disk is formed at certain levels of counterflow, which significantly weakens the subsequent periodic shock cell structure and reduces the breadband shock associated noise and the overall sound pressure level (OASPL) by as much as 3 dB. Significantly, it was also discovered that a jet operating at over-expanded conditions can be decelerated nearly isentropically by applying the proper amount of counterflow. This modification leads to a 4-dB reduction in OASPL. It is suggested that counterflow should be further investigated as a potential noise reduction technique in the future. (Author)