Wed November 2nd 2016
16:00 – 16:30
ZH286
Seminar Wavepacket models for turbulent jet flows by means of numerical simulations
Mengqi Zhang

Details:

In this thesis, we study wavepacket models for turbulent jet flows by means of numerical simulations. We first developed a numerical code to solve the Linearized Euler equation (LEE) subject to a harmonic inlet forcing to evaluate the capability of the linear framework to describe the statistics of wavepackets in the turbulent jet. While the linear models provide a good description of the second-order statistics of wavepacket up to the end of the potential core, farther downstream they fail to correctly capture the dynamics. They are also found to hugely underestimate the level of acoustic radiation. The latter is attributed to the fact that the two- point coherence, which decays when the real flow is considered, is, by construction, unity in the linear models, even when the LEE is driven by a broadband forcing, as we show by coupling full unsteady data from a Large Eddy Simulation (LES) with the LEE solver. The coherence decay is due to a scrambling of the phase of wavepackets as they evolve downstream, a phenomenon which has been given the denomination ’jitter’.
We perform a series of analyses aimed at clarifying the mechanisms responsible for the amplification of wavepackets downstream of the end of the potential core, and for the decay of the two-point coherence, both of which are believed to be important where the acoustic e ciency of wavepackets is concerned.
A locally parallel transient growth analysis performed downstream of the end of the potential core suggests that the mismatch between the linear models and what is observed in experiments and numerical simulations can be understood in terms of non-normal growth mechanisms that become important when the Kelvin-Helmholtz (K-H) mode becomes stable. The analysis suggests that turbulence is required in order to sustain this mechanism. This is confirmed in a non-parallel framework by driving the LEE using data taken from the LES. The analysis shows, interestingly, that while coherence decay cannot occur in the linear framework in the region upstream of the end of the potential core, where the K-H mode dominates wavepacket evolution (as mentioned in the first paragraph), in the region downstream of the potential core coherence decay similar to that observed in the LES can occur in the absence of sustained volume forcing by turbulence.
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The 10th Complex Motion in Fluids 2020
Max Planck Gesellschaft
MCEC
Twente
Centre for Scientific Computing
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