VELOCITY STATISTICS ALONG THE STAGNATION LINE OF AN AXISYMMETRICAL STAGNATING TURBULENT-FLOW

Velocity statistics along the stagnation line of an axi-symmetric wall stagnating turbulent flow are studied experimentally. A low turbulenc e, uniform air flow from a nozzle type air supply with an exit diamete r of 50 mm stagnates at a wall located 50 mm downstream. A flow veloci ty is set to 3 m/s, 10 mm downstream from the exit of the air supply. Instantaneous values of streamwise and radial velocities are measured by laser-Doppler velocimetry. The turbulence level in the air flow is changed by use of turbulence generator. When the turbulence generator is not installed in the air supply, the mean velocity profile in the s treamwise direction fits well with that of a laminar viscous flow with the rms value of velocity fluctuations low near the wall. With the tu rbulence generator installed, a significant turbulence structure appea rs near the wall. When the wall is approached, the rms value of veloci ty fluctuations in the streamwise direction decreases monotonically wh ile the profile of the rms value in the radial direction reaches a max imum near the wall. The increase in the rms value of velocity fluctuat ions in the radial direction near the wall is attributed to the bi-mod al histogram of the fluctuating velocity in the radial direction. Near the wall, the instantaneous stagnation streamline fluctuates and the probability of the mean location of the stagnation point reaches a max imum not at the stagnation line but on a circle around the stagnation line, resulting in the bi-modal histogram. Turbulence statistics, the rms value of velocity fluctuation and the turbulent kinetic energy, ca n be normalized successfully by similarity parameters based on the str ain rate and the reference turbulent kinetic energy introduced by Cham pion and Libby.