Effect of rotation on massively separated turbulent flows

16 apr - 11 may 2007

Turbulent flows in rotating systems are both of scientific and practical concern, in geophysical fluid dynamics as well as in turbomachinery applications. The Coriolis body force arising from the rotation of the frame-of-reference has several important implications on the flow field. Not only is the mean flow pattern altered by the presence of the Coriolis force, but the turbulence is also affected qualitatively as well as quantitatively. These effects have been studied before in rotating ducts with rectangular cross-sections. In the project we aimed to investigate the flow pattern and the turbulence activity in a rotating duct with a partially blocked rectangular cross-section. The obstruction created a massive separation zone downstream of the blockage and the focus was on the rotational effects on the recirculating flow within the separation ‘bubble’ and the vortex dynamics in the mixing-layer emanating from the downstream corner of the obstruction.

Participants

Name	Position	Nationality
H. Andersson	Professor	Norwegian
B. A. Petterson	Principal Scientist	Norwegian
J. H. Visscher	PHD student	German
J. Hussong	PHD student	German
B. Mustafa	PHD student	Jordanian

Summary of Achievements

A novel set-up was designed for flow visualisations and turbulence measurements in rotating high-aspect-ratios ducts. An 8m long straight duct was positioned transversely across the 13m diameter Coriolis turntable. Stereoscopic PIV-measurements were made over a wide range of rotation or Rossby numbers (Ro) and over a modest range of Reynolds numbers (Re). At the lowest Re considered, the measured data in the clear duct were in accordance with earlier DNS data (Kristoffersen & Andersson, J. Fluid Mech. 1993) at roughly the same Re. The highest rotation number (lowest Ro) which was reached was nearly twice that attained in earlier laboratory studies. For the first time all three velocity components have been measured in a rotating duct and the instantaneous vorticity vector perpendicular to the measurement plane could be deduced. Distinct asymmetries between the cyclonic and anti-cyclonic sides of the duct were observed and some sample results are shown in an ETMM7-paper.

: Hyrodynamic loop with the test channel on the Coriiolis platform

PIV measurements were thereafter made downstream of a severe obstruction. On the lee side of the 1:2 one-sided expansion of the duct, a massive separation zone occurred. The streamwise length of the separation bubble varied substantially with the rate of rotation and in particular with the sense of rotation. Special attention in the forthcoming data analysis will be paid to the turbulence in the mixing layer which emanates from the trailing edge of the obstruction.

Conference Publications

“Effects of rotation on clear and obstructed turbulent channel flows”, 7th International ERCOFTAC Symposium on Engineering Turbulence Modeling and Measurements, Cyprus, June 4th – June 6th, 2008.

Helge Andersson

Professor

Fluids Engineering Strømningstekn*211, K Hejes Vei 2

+47 73593556

helge.i.andersson@ntnu.no