(142p) The Evolution of Kinetic Energy in Actively Forced Confined Mixing Layers

The
evolution of kinetic energy in actively forced    confined mixing layers

Wei
Zhao; Guiren Wang

Abstract The
receptivity of flow in the nozzle section of confined mixing layer is investigated
at different forcing frequencies and intensities. The results are compared to
the velocity fluctuations in the mixing chamber adjacent to the trailing edge.

It's
found the largest receptivity (at 5.3 Hz) in nozzle section doesn't necessarily
lead to highest velocity fluctuations. At 5.3 Hz, the velocity fluctuations
decrease fast in the mixing chamber along flow direction. This is contrary to
the unforced case. Besides, the role of mean vertical velocity in momentum
transfer and energy transport is still undetermined. Hence, the evolution of
kinetic energy in confined mixing layer is investigated. As the Reynolds number
we applied is only around 3000, close to the trailing edge, the homogeneous and
isotropic turbulence state cannot be achieved. Therefore, axisymmetric
approximation is adopted instead to simplify the dissipation terms.

 
 Experimental results indicate, compared
to the unforced case, the dissipation terms in forced flow (5.3 Hz) exhibit
small difference. Hence, the fast decreasing of the kinetic energy cannot be
the result of energy dissipation. Further investigations show that the total production
term in energy equation becomes highly negative, when the flow is forced. Its
magnitude is 25 times larger than the total dissipation term..
In the process, the term -v'²(dV/dy)
has the most significant contribution to the total production term, where v'², V and y are vertical
velocity fluctuation, average vertical velocity and vertical position
respectively. It implies the existence of V may inhibit the development of
turbulence and the mixing. This is questionable, because intuitively the varied
V in vertical direction should cause the stretching of spanwise
vortices and results in earlier break-down which should be propitious to the
mixing process. Further study is needed to understand the energy transport
phenomena in this complicated flow.