Article

Measurement of the gradient field of a turbulent free surface

Experiments in Fluids (impact factor: 1.74). 04/2012; 41(4):629-640. DOI:10.1007/s00348-006-0186-x pp.629-640

ABSTRACT We study the free surface above a turbulent channel flow. We describe a laser scanning technique that can be used to measure the space–time turbulent surface gradient field along a line. A harmonically swiveling laser beam is focused on the surface and its angle of refraction is measured using a position sensing device. The registered signals can be converted easily to the desired gradient field, and spectra and correlations can be measured. Examples of measured spectra and correlation functions of the surface above a turbulent channel flow (Reynolds number R
λ ≈ 250) demonstrate the viability of the technique. We further assess the validity of Taylor’s frozen turbulence hypothesis that implies that time-dependent signals measured along a line that is oriented perpendicularly to the mean channel velocity can be interpreted as 2D measurements of the surface slope. While Taylor’s hypothesis works for a turbulent velocity field, it does not work for its free surface.

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Keywords

2D measurements
 
correlation functions
 
correlations
 
desired gradient field
 
Examples
 
free surface
 
laser beam
 
laser scanning technique
 
mean channel velocity
 
perpendicularly
 
refraction
 
registered signals
 
Reynolds number R
 
space–time turbulent surface gradient field
 
surface slope
 
Taylor’s
 
Taylor’s hypothesis works
 
turbulence hypothesis
 
turbulent channel flow
 
turbulent velocity field
 

Ralph Savelsberg