JOINT PROBABILITY DENSITY ANALYSIS OF THE STRUCTURE AND DYNAMICS OF THE VORTICITY FIELD OF A TURBULENT BOUNDARY-LAYER

An experimental study of a turbulent boundary layer at R-theta approxi mate to: 1070 and R-tau approximate to 543 was conducted. Detailed mea surements of the velocity vector and the velocity gradient tensor with in the near-wall region were performed at various distances from the w all, ranging from approximately y(+) = 14 to y(+) = 89. The measured m ean statistical properties of the fluctuating velocity and vorticity c omponents agree well with previous experimental and numerically simula ted data. These boundary layer measurements were used in a joint proba bility density analysis of the various component vorticity and vortici ty-velocity gradient products that appear in the instantaneous vortici ty and enstrophy transport equations. The vorticity filaments that con tribute most to the vorticity covariance <(Omega(x)Omega(y))over bar> in this region were found to be oriented downstream with angles of inc lination to the wall, when projected on the streamwise (x, y)-plane, t hat decrease with distance moving from the buffer to the logarithmic l ayer. When projected on the planview (x, z)- and cross-stream (y, z)-p lanes, the vorticity filaments that most contribute to the vorticity c ovariances <(Omega(x)Omega(z))over bar> and <(Omega(y)Omega(z))over ba r> have angles of inclination to the z-ordinate axis that increase wit h distance from it. All the elements of the Omega(i)Omega(j)partial de rivative U-i/partial derivative x(j) term in the enstrophy transport e quation, i.e. the term that describes the rate of increase or decrease of the enstrophy by vorticity filament stretching or compression by t he strain-rate field, have been examined. On balance, the average stre tching of the vorticity filaments is greater than compression at all y (+) locations examined here. However, some individual velocity gradien t components compress the vorticity filaments, on average, more than t hey stretch them.