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Abstract
An experimental study of turbulent planar offset attaching jets with and without a co-flowing jet was performed for jets with offset distances less than approximately 1.0 jet height and Reynolds numbers of 21 800 to 54 500. The development of the jets were characterized using the measurements of the flow velocities, static and fluctuating wall pressure, skin friction and heat transfer. The development of the large scale flow structures was also characterized using the two-point and two-time measurements of the wall pressure and velocity fluctuations.
The results show that the initial development of the offset attaching jets can be divided into 5 different regions, three associated with the reattachment process and two associated with the wall jet. The measurements showed three characteristic motions in the offset attaching jets. Upstream of the reattachment point, the fluctuations are predominantly determined by the downstream propagating large scale flow structures in the inner shear layer. There is also evidence of upstream propagating low frequency motions that seem to be in the recirculation region and a very low frequency flapping motion. After the reattachment, the inner shear layer structures gradually merge with wall jet like structures that formed away from the wall and these merged structures convect downstream similar to the flow structures in a developed planar wall jet.
The results showed that the effect of the co-flowing jet depends on the velocity, mass, momentum flux and the size of the jet. The fluctuations in the inner shear layer decrease when a co-flowing jet with a velocity less than 20% of the main jet velocity is added. When the velocity of the inner jet is more than 20% of the main jet velocity there is evidence that wake like structures form downstream of the splitter plate between the jets. In the case when the size of the inner jet is half of the main jet, the fluctuating wall pressure and the heat transfer continue to decrease when the inner jet velocity increases despite a large increase in the vertical fluctuating velocity in the attaching shear layer. This is different from the case where the inner jet height is 18% of the main jet height, where the fluctuating pressure and heat transfer increase with the inner jet velocity. In this case, the frequencies of the wake like structures and inner shear layer structures seem to coincide resulting in strong structures that interact with the wall, dramatically increasing the fluctuating wall pressure and heat transfer.
