Rayleigh–Taylor and Richtmyer-Meshkov instability induced flow, turbulence, and mixing. II

Noncircular jets have been the topic of extensive research in the last fifteen years. These jets were identified as an efficient technique of passive flow control that allows significant improvements of performance in various practical systems at a relatively low cost because noncircular jets rely solely on changes in the geometry of the nozzle. The applications of noncircular jets discussed in this review include improved large- and small-scale mixing in low- and high-speed flows, and enhanced combustor performance, by improving combustion efficiency, reducing combustion instabilities and undesired emissions. Additional applications include noise suppression, heat transfer, and thrust vector control (TVC). The flow patterns associated with noncircular jets involve mechanisms of vortex evolution and interaction, flow instabilities, and fine-scale turbulence augmentation. Stability theory identified the effects of initial momentum thickness distribution, aspect ratio, and radius of curvature on the initial flow evolution. Experiments revealed complex vortex evolution and interaction related to selfinduction and interaction between azimuthal and axial vortices, which lead to axis switching in the mean flow field. Numerical simulations described the details and clarified mechanisms of vorticity dynamics and effects of heat release and reaction on noncircular jet behavior.

Flow control with noncircular jets

We have experimentally studied the effect of microjets on the flow field of a Mach 0.9 round jet. Planar and three-dimensional velocity field measurements using particle image velocimetry show a significant reduction in the near-field turbulent intensities with the activation of microjets. The axial and normal turbulence intensities are reduced by about 15% and 20%, respectively, and an even larger effect is found on the peak values of the turbulent shear stress with a reduction of up to 40%. The required mass flow rate of the microjets was about 1% of the primary jet mass flux. It appears that the microjets influence the mean velocity profiles such that the peak normalized vorticity in the shear layer is significantly reduced, thus inducing an overall stabilizing effect. Therefore, we seem to have exploited the fact that an alteration in the instability characteristics of the initial shear-layer can influence the whole jet exhaust including its noise field. We have found a reduction of about 2 dB in the near-field overall sound pressure level in the lateral direction with the use of microjets. This observation is qualitatively consistent with the measured reduced turbulence intensities.

On the use of microjets to suppress turbulence in a Mach 0.9 axisymmetric jet

▪ Abstract It is classically assumed that the far field of a round turbulent jet discharging into quiescent fluid has a unique behavior characterized only by its momentum flux. However, there is now considerable evidence that different discharge conditions at the jet nozzle exit can give rise to very different far-field flows. Perhaps the most striking examples of these are the bifurcating and blooming jets produced by appropriate combinations of controlled axial and circumferential excitations at the nozzle exit. With the right excitations, a jet can be made to divide into two separate jets (bifurcating jet), each of which carries half the axial momentum and spreads in a manner similar to a single jet. Trifurcating jets can also be produced. Other excitations can produce blooming jets, in which the jet explodes into a shower of vortex rings, producing a far-field flow that is quite unlike a normal unexcited jet. Bifurcating and blooming jets exhibit much greater mixing than normal jets, suggesting possib...

Bifurcating and blooming jets

A detailed experimental study of supersonic, Mach 2, flow over a three-dimensional cavity was conducted using shadowgraph visualization, unsteady surface pressure measurements, and particle image velocimetry. Large-scale structures in the cavity shear layer and visible disturbances inside the cavity were clearly observed. A large recirculation zone and high-speed reverse flow was revealed in the cavity. In addition, supersonic microjets were used at the leading edge to suppress flow unsteadiness within the cavity. With a minimal mass flux (blowing coefficient B c = 0.0015), the activation of microjets led to reductions of up to 20 dB in the amplitudes of cavity tones and of more than 9 dB in the overall sound pressure levels. The microjet injection also modified the cavity mixing layer and resulted in a significant reduction in the flow unsteadiness inside the cavity as revealed by the shadowgraphs and the velocity-field measurements.

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Supersonic Cavity Flows and Their Control

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.

Effects of counterflow on the aeroacoustic properties of a supersonic jet

Effects of Counterflow on the Aeroacoustic Properties of a Supersonic Jet

A relatively new fluidic control technique, which employs secondary flow traveling in a direction opposite to the primary jet - namely, counterflow control - has been explored by Strykowski et al. at jet Mach numbers up to 2. These studies conclusively show that the primary jet can be continuously vectored up to angles approaching 20 deg. Futhermore, if implemented correctly, counterflow thrust vectoring (CFTV) does not suffer from the bistability problems encountered with earlier fluidic control schemes. We describe the results of a study in which a Mach 2 diamond-shaped jet was used to extend the single-axis vectoring concept of rectangular jets to multiaxis operation.

Multiaxis fluidic thrust vector control of a supersonic jet using counterflow

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.

Aeroacoustic properties of a supersonic diamond-shaped jet

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.

D. Washington

Papers

Effects of counterflow on the aeroacoustic properties of a supersonic jet

Effects of Counterflow on the Aeroacoustic Properties of a Supersonic Jet

Multiaxis fluidic thrust vector control of a supersonic jet using counterflow

Aeroacoustic properties of a supersonic diamond-shaped jet

Experimental study of a compressible countercurrent turbulent shear layer