276778 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.