NASA Scientific and Technical Aerospace Reports

The effect of far-field boundary conditions on the evolution of a finite-amplitude two-dimensional wave in the Blasius
boundary layer is assessed. With the use of the parabolized stability equations (PSE) theory for the numerical computations,
either asymptotic, Dirichlet, Neumann or mixed boundary conditions are imposed at various distances from the wall. The
results indicate that asymptotic and mixed boundary conditions yield the most accurate mean-flow distortion and unsteady
instability modes in comparison with the results obtained with either Dirichlet or Neumann conditions.
Author
Boundary Conditions; Boundary Layer Transition; Far Fields; Blasius Flow
20040111995 Manchester Univ., UK
The Nonlinear Evolution of the Inviscid Secondary Instability of Streamwise Vortex Structures
Blackaby, Nicholas D.; Hall, Philip; [1994]; 21 pp.; In English
Contract(s)/Grant(s): NAS1-19480; No Copyright; Avail: CASI; A03, Hardcopy
The weakly nonlinear evolution of an inviscid marginally unstable wave growing on a boundary layer supporting a
streamwise vortex structure is investigated. The nonlinear growth of the wave is found to be controlled by the diffusion layer
located at the edge of the critical layer associated with the wave. The evolution equation is found to depend on the upstream
history of the wave and the solution of the equation suggests that the wave either restructures the mean state so as to make
it stable or develops a singularity at a finite distance downstream of the point of neutral stability.
Author
Inviscid Flow; Nonlinearity; Stability; Vortices
20040111996 NASA Langley Research Center, Hampton, VA, USA
Active Control of Instabilities in Laminar Boundary-Layer Flow, Part 2, Use of Sensors and Spectral Controller
Joslin, Ronald D.; Nicolaides, R. A.; Erlebacher, Gordon; Hussaini, M. Yousuff; Gunzburger, Max D.; [1994]; 10 pp.; In
English
Contract(s)/Grant(s): NAS1-19480; No Copyright; Avail: CASI; A02, Hardcopy
This study focuses on the suppression of instability growth using an automated active-control technique. The evolution
of 2D disturbances that are spatially growing in a flat-plate boundary layer are computed with a spatial DNS code. A controller
receives wall sensor information (pressure or shear) as input and provides a signal that controls an actuator response as output.
The control law assumes that wave cancellation is valid. The results indicate that a measure of wave cancellation can be
obtained for small- and large-amplitude instabilities without feedback; however, feedback is required to optimize the control
amplitude and phase for exact wave cancellation.
Author
Active Control; Automatic Control; Boundary Layer Flow; Laminar Boundary Layer; Stability
20040111998 NASA Langley Research Center, Hampton, VA, USA
Crossflow-Vortex Breakdown on Swept Wings: Correlation of Nonlinear Physics
Joslin, R. D.; Streett, C. L.; [1994]; 29 pp.; In English; No Copyright; Avail: CASI; A03, Hardcopy
The spatial evolution of cross flow-vortex packets in a laminar boundary layer on a swept wing are computed by the direct
numerical simulation of the incompressible Navier- Stokes equations. A wall-normal velocity distribution of steady suction
and blowing at the wing surface is used to generate a strip of equally spaced and periodic disturbances along the span. Three
simulations are conducted to study the effect of initial amplitude on the disturbance evolution, to determine the role of
traveling cross ow modes in transition, and to devise a correlation function to guide theories of transition prediction. In each
simulation, the vortex packets first enter a chordwise region of linear independent growth, then, the individual packets coalesce
downstream and interact with adjacent packets, and, finally, the vortex packets nonlinearly interact to generate inflectional
velocity profiles. As the initial amplitude of the disturbance is increased, the length of the evolution to breakdown decreases.
For this pressure gradient, stationary modes dominate the disturbance evolution. A two-coeffcient function was devised to
correlate the simulation results. The coefficients, combined with a single simulation result, provide sufficient information to
generate the evolution pattern for disturbances of any initial amplitude.
Author
Cross Flow; Nonlinearity; Swept Wings; Vortex Breakdown
20040111999 NASA Langley Research Center, Hampton, VA, USA
Metamorphosis of a Hairpin Vortex into a Young Turbulent Spot
Singer, Bart A.; Joslin, Ronald D.; March 17, 1995; 61 pp.; In English
Contract(s)/Grant(s): NAS1-20059; No Copyright; Avail: CASI; A04, Hardcopy
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