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Stability of Hypervelocity Boundary Layers


Bitter, Neal Phillip (2015) Stability of Hypervelocity Boundary Layers. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9Q23X5Z.


The early stage of laminar-turbulent transition in a hypervelocity boundary layer is studied using a combination of modal linear stability analysis, transient growth analysis, and direct numerical simulation. Modal stability analysis is used to clarify the behavior of first and second mode instabilities on flat plates and sharp cones for a wide range of high enthalpy flow conditions relevant to experiments in impulse facilities. Vibrational nonequilibrium is included in this analysis, its influence on the stability properties is investigated, and simple models for predicting when it is important are described.

Transient growth analysis is used to determine the optimal initial conditions that lead to the largest possible energy amplification within the flow. Such analysis is performed for both spatially and temporally evolving disturbances. The analysis again targets flows that have large stagnation enthalpy, such as those found in shock tunnels, expansion tubes, and atmospheric flight at high Mach numbers, and clarifies the effects of Mach number and wall temperature on the amplification achieved. Direct comparisons between modal and non-modal growth are made to determine the relative importance of these mechanisms under different flow regimes.

Conventional stability analysis employs the assumption that disturbances evolve with either a fixed frequency (spatial analysis) or a fixed wavenumber (temporal analysis). Direct numerical simulations are employed to relax these assumptions and investigate the downstream propagation of wave packets that are localized in space and time, and hence contain a distribution of frequencies and wavenumbers. Such wave packets are commonly observed in experiments and hence their amplification is highly relevant to boundary layer transition prediction. It is demonstrated that such localized wave packets experience much less growth than is predicted by spatial stability analysis, and therefore it is essential that the bandwidth of localized noise sources that excite the instability be taken into account in making transition estimates. A simple model based on linear stability theory is also developed which yields comparable results with an enormous reduction in computational expense. This enables the amplification of finite-width wave packets to be taken into account in transition prediction.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Linear stability, hypersonic boundary layer, nonequilibrium flow, boundary layer transition, compressible flow, second mode
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Aeronautics
Awards:Rolf D. Buhler Memorial Award In Aeronautics, 2011; Ernest E. Sechler Memorial Award In Aeronautics, 2014; William F. Ballhaus Prize, 2015.
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Shepherd, Joseph E.
Group:Graduate Aeronautical Laboratories (Fluid Mechanics), GALCIT, Explosion Dynamics Laboratory
Thesis Committee:
  • Meiron, Daniel I. (chair)
  • Leonard, Anthony
  • McKeon, Beverley J.
  • Shepherd, Joseph E.
Defense Date:15 May 2015
Record Number:CaltechTHESIS:06052015-111128842
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:8995
Deposited By: Neal Bitter
Deposited On:26 Feb 2016 22:20
Last Modified:16 Jan 2021 00:34

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