458 Experimental study on disturbance structures in a transitional boundary layer subjected to weak free stream turbulence

2009 ◽  
Vol 2009.58 (0) ◽  
pp. 273-274
Author(s):  
Kota TAKAICHI ◽  
Masaharu MATSUBARA
Keyword(s):  
Boundary Layer ◽  
Experimental Study ◽  
Free Stream ◽  
Free Stream Turbulence ◽  
Transitional Boundary Layer

An Investigation of the Scales in Transitional Boundary Layers Under High Free-Stream Turbulence Conditions

2002 ◽  
Author(s):  
Ralph J. Volino
Keyword(s):  
Boundary Layer ◽  
Free Stream ◽  
Dominant Frequency ◽  
Intermittent Flow ◽  
Self Similarity ◽  
Free Stream Turbulence ◽  
Transitional Boundary Layer ◽  
Streamwise Location ◽  
Transition Models ◽  
Dominant Frequencies

The scales in a transitional boundary layer subject to high (initially 8%) free-stream turbulence and strong acceleration (K as high as 9×10−6) have been investigated using wavelet spectral analysis and conditional sampling of experimental data. The boundary layer shows considerable evolution through transition, with a general shift from the lower frequencies induced by the free-stream unsteadiness to higher frequencies associated with near wall generated turbulence. Within the non-turbulent zone of the intermittent flow, there is considerable self-similarity in the spectra from the beginning of transition to the end, with the dominant frequencies in the boundary layer remaining constant at about the dominant frequency of the free-stream. The frequencies of the energy containing scales in the turbulent zone change with streamwise location and are significantly higher than in the non-turbulent zone. When normalized on the local viscous length scale and velocity, however, the turbulent zone spectra also show good self-similarity throughout transition. Turbulence dissipation occurs almost exclusively in the turbulent zone. The velocity fluctuations associated with dissipation are isotropic, and their normalized spectra at upstream and downstream stations are nearly identical. The distinct differences between the turbulent and non-turbulent zones suggest the potential utility of intermittency based transition models in which these zones are treated separately. The self-similarity noted in both energy containing and dissipation scales in both zones suggests possibilities for simplifying the modeling for each zone.

Conditional Sampling in a Transitional Boundary Layer Under High Free-Stream Turbulence Conditions

2001 ◽  
Author(s):  
Ralph J. Volino ◽  
Michael P. Schultz ◽  
Christopher M. Pratt
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Free Stream ◽  
Streamwise Velocity ◽  
Turbulent Shear ◽  
Conditional Sampling ◽  
Mean Velocity ◽  
Free Stream Turbulence ◽  
Transitional Boundary Layer ◽  
Turbulent Shear Stress

Conditional sampling has been performed on data from a transitional boundary layer subject to high (initially 9%) free-stream turbulence and strong K=ν/U∞2dU∞/dxas high as9×10-6 acceleration. Methods for separating the turbulent and non-turbulent zone data based on the instantaneous streamwise velocity and the turbulent shear stress were tested and found to agree. Mean velocity profiles were clearly different in the turbulent and non-turbulent zones, and skin friction coefficients were as much as 70% higher in the turbulent zone. The streamwise fluctuating velocity, in contrast, was only about 10% higher in the turbulent zone. Turbulent shear stress differed by an order of magnitude, and eddy viscosity was three to four times higher in the turbulent zone. Eddy transport in the non-turbulent zone was still significant, however, and the non-turbulent zone did not behave like a laminar boundary layer. Within each of the two zones there was considerable self-similarity from the beginning to the end of transition. This may prove useful for future modeling efforts.

Quadrant analysis of a transitional boundary layer subject to free-stream turbulence

2010 ◽  
Vol 658 ◽  
pp. 310-335 ◽  
Author(s):  
K. P. NOLAN ◽  
E. J. WALSH ◽  
D. M. McELIGOT
Keyword(s):  
Boundary Layer ◽  
Coherent Structures ◽  
Particle Image ◽  
Free Stream ◽  
Quadrant Analysis ◽  
Shear Stresses ◽  
Free Stream Turbulence ◽  
Transitional Boundary Layer ◽  
Image Velocimetry ◽  
Spanwise Vorticity

This paper presents analyses of particle image velocimetry measurements from a boundary layer on a flat plate subject to grid-generated free-stream turbulence. The pre-transition region and early stages of breakdown to turbulent spots are explored by means of quadrant analysis and quadrant hole analysis. By isolating the contributors to the Reynolds shear stresses, it is possible to identify coherent structures within the flow that are responsible for the production of TKE. It is found that so called ‘ejection’ events are the most significant form of disturbance, exhibiting the largest amplitude behaviour with increased negative spanwise vorticity. Sweep events become increasingly large close to the wall with increased Reynolds number and intermittency.

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