Abstract: Structure and unsteady behaviour of the separation flow
arising when an inner jet is injected from a nozzle of diameter d
into outer coflow confined by a pipe of diameter D
are studied using experimental and numerical methods. Investigations have been
conducted for large inner to outer velocity ratio at the flow rate ratio and the Reynolds number Measurements of passive scalar
distributions in radial planes at various distances from the nozzle have been
carried out by Laser Induced Fluorescence (LIF). Velocity fields have been
measured using Laser Doppler Velocimetry (LDV). Calculations have been done with
Large Eddy Simulation using a novel inflow generator and a clipping procedure
proposed by the authors. Numerical and experimental results reveal the formation
of the recurculation zone in a short distance behind the nozzle. The flow in the
recirculation zone is highly unsteady and nearly periodic. The scalar field
experiences long period temporal oscillations which are in nearly opposite
temporal phase at symmetrical points with respect to the pipe centerline.
Detailed analysis of vorticity and scalar fields obtained from LES calculations
shows that the reason for such flow behaviour are coherent vortex structures
appearing on the boundary between the jet and the coflow. They induce
oscillating motions accross the pipe and transport the scalar against the main
flow direction within the recirculation zone.
