TIME-RESOLVED PIV MEASUREMENTS OF TURBULENT FLOW IN ROTATING RIB-ROUGHENED CHANNEL WITH CORIOLIS AND BUOYANCY FORCES

The present contribution addresses the effect of Coriolis and buoyancy forces in a rotating rectangular channel having one wall provided with ribs perpendicular to the flow direction. Time-resolved PIV measurements are performed in a rotating facility where both the channel model and the measurement system rotate on a turntable at 134 rpm. Air is used as working fluid. The Reynolds number defined by the bulk velocity and the hydraulic diameter is 15000, and a rotation number of 0.38 is obtained both in clockwise and counter-clockwise sense. The ribbed wall, machined out of copper, is heated to a uniform temperature of about 100 degrees C by means of electrical resistances. This allows to obtain a centrifugal buoyancy number of 0.31. Velocity fields are measured along the channel symmetry plane using a continuous laser diode and a high-speed camera. Both ensemble-averaged and time-resolved measurements are performed. In the latter case the realizations are acquired at 3.3 KHz, allowing to resolve the fine temporal flow scales. The effects of the rotational buoyancy with respect to the action of the Coriolis forces alone are highlighted. Particular attention is drawn to the extension of the separated area behind the rib, as well as to the length scales and time scales of the structures generated on the separated shear layer behind the ribs. Vortical structures are identified as regions of strong swirl having both spatial and temporal coherence. The rotational buoyancy near the heated wall affects strongly the physical characteristics, distribution and trajectory of such structures, which are critical for the turbulent transport and heat transfer.