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Controlling The Development of Coherent Structures in High Speed Jets and The Resultant Near Field

Speth, Rachelle Lea
http://rave.ohiolink.edu/etdc/view?acc_num=osu1447419083

Abstract Details

2015, Doctor of Philosophy, Ohio State University, Aero/Astro Engineering.
This work uses Large-Eddy Simulations to examine the effect of actuator parameters and jet exit properties on the evolution of coherent structures and their impact on the near-acoustic field without and with control. For the controlled cases, Localized Arc Filament Plasma Actuators (LAFPAs) are considered, and modeled with a simple heating approach that successfully reproduces the main observations and trends of experiments. A parametric study is first conducted, using the flapping mode (m=±1), to investigate the sensitivity of the results to various actuator parameters including: actuator model temperature, actuator duty cycle, and excitation frequency. It is shown by considering a Mach 1.3 jet at Reynolds number of 1 ×106 that the response of the jet is relatively insensitive to actuator model temperature within the limits of the experimentally measured temperature values. Furthermore, duty cycles in the range of 20%−90% were observed to be effective in reproducing the characteristic coherent structures of the flapping mode. The 90% duty cycle exhibits strengthened coherent structures and slightly higher jet growth along the flapping plane, but the overall dynamics remain almost identical to the lower duty cycle cases. However, a 100% duty cycle had no perceptible effect on the jet. Therefore, increasing the energy inserted into the flow via actuator temperature or duty cycle does not significantly alter the flow dynamics. The largest sensitivity was associated with excitation frequency, with the most significant effect associated with the column mode instability frequency (St ≅0.3), which results in alternating vortex rings for this mode. Higher and lower frequencies reduced the rate of decay of the centerline velocity. Although the higher frequencies increased the number of features observed in the phase-averaged data, their prominence is reduced due to their breakdown into smaller structures. These actuator parameter results confirm that the flow response and control authority is associated with manipulation of flow instability, rather than heat deposition. Next, jet flow parameters were explored to determine the control authority under different operating conditions. To begin, the effect of the laminar nozzle exit boundary layer thickness was examined by varying its value from essentially uniform flow to 25% of the diameter. In the absence of control, the distance between the nozzle lip and the initial appearance of breakdown is proportional to the boundary-layer thickness, which is consistent with theory and previous results obtained by other researchers at Mach 0.9. However, the subsequent growth towards the centerline is faster for the thicker boundary layers. For flapping mode control, increasing the thickness of the boundary layer has different effects on the flapping and non-flapping planes. The rapid spreading of the jet observed on the flapping plane with thin boundary layers is greatly diminished as the nozzle exit layer is thickened. Conversely, the rate of spreading on the non-flapping plane is increased. The characteristic vortical rings observed with thin layers in experiment and simulations become less prominent with increasing nozzle exit boundary layer thickness, indicating reduced control authority. Thus, increasing the boundary-layer thickness reduces the differences between controlled and uncontrolled cases. The second flow parameter studied was the effect of Reynolds number on a Mach 1.3 jet controlled by the flapping mode at an excitation Strouhal number of 0.3. The higher Reynolds number (Re=1,100,000) jet exhibited reduced control authority compared to the Re=100,000 jet. Like the effect of increasing the nozzle exit boundary layer thickness, increasing the Reynolds number cause a reduction in spreading on the flapping plane and an increase on the non-flapping plane. Therefore, these thicker layers and higher Reynolds number jets may require actuators with a higher energy input (i.e. higher duty cycle, higher actuator temperature, more actuators) to ensure the excitation of the flow instability. The final parameter studied is the effect of Mach number on the development and decay of large scale structures for no-control and control cases for Mach 0.9 and Mach 1.3 jets. For this exercise, the axisymmetric mode (m=0) was considered at excitation frequencies of St=0.05, 0.15, and 0.25, with emphasis on the evolution of coherent structures and their effects on the resultant near field pressure map. Without control, the two jets have similar shear layer growth until the end of the potential core length of the subsonic case, at which point the subsonic jet spreads at a higher rate. For the controlled cases, relatively larger streamwise hairpin vortices have been noted for the subsonic cases than the supersonic cases resulting in stronger entrainment of the ambient fluid. This increased entrainment in the subsonic cases causes a reduction in the normalized convective velocity resulting in similar normalized values to that of the supersonic cases. As the excitation frequency is increased, more hairpin vortices are present and the normalized convective velocity is reduced for both subsonic and supersonic cases. A detailed study of the connection between the coherent structures and the pressure in the near-acoustic field and lipline supports the theory that the successive interacting pulses produce a quasi-linear superposition of the impulse response of the jet to excitation. The velocity of waves just outside of the shear layer is very near the acoustic value due to the exponential decay of the hydrodynamic waves. In the nearfield, the acoustic influence on the convective velocity is greater on the sideline angles for the subsonic cases due to the directivity of the large scale structures and the lower acoustic signature of the subsonic jet compared to the supersonic jet. The supersonic jet region maintains higher correlations to the near acoustic field than the subsonic jet. The higher excitation frequencies cause a more directed propagation path to the downstream angles due to the consistent location of the large scale structure decay from the excitation.
Datta Gaitonde (Advisor)
Mo Samimy (Committee Member)
Mei Zhuang (Committee Member)
Jen-Ping Chen (Committee Member)
171 p.
Acoustics; Aerospace Engineering
acoustics; large eddy simulation; jets; plasma control

Recommended Citations

Citations

  • Speth, R. L. (2015). Controlling The Development of Coherent Structures in High Speed Jets and The Resultant Near Field [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1447419083

    APA Style (7th edition)

  • Speth, Rachelle. Controlling The Development of Coherent Structures in High Speed Jets and The Resultant Near Field. 2015. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1447419083.

    MLA Style (8th edition)

  • Speth, Rachelle. "Controlling The Development of Coherent Structures in High Speed Jets and The Resultant Near Field." Doctoral dissertation, Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1447419083

    Chicago Manual of Style (17th edition)

osu1447419083
272
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This open access ETD is published by The Ohio State University and OhioLINK.