Near-field Development of a Turbulent Mixing Layer Periodically Forced by a Bimorph PVDF Film Actuator

Abstract

We have performed experiments in a turbulent mixing layer with periodic forcing introduced by a Piezo Film Actuator (PFA). Three different lengths of PFAs have been used, and the effects of various combinations of forcing amplitudes and frequencies are investigated. The forcing at the first and second sub-harmonic frequencies against the natural frequency enhances the development of the thickness of the mixing layer: the mixing layer spreads due to the forcing. On the other hand, the forcing near the natural frequency suppresses the development: the mean velocity gradient becomes steeper than the no control case. The vector pattern of the periodic velocity components indicated the formation of the vortical structure. By forcing at the natural and its first sub-harmonic frequencies, two counter-rotating vortices are clearly observed in one period of forcing. By forcing at second sub-harmonic frequency, the vortical structure is found only in the downstream region. The distribution of the periodic Reynolds shear stress significantly varies with the forcing frequency and it takes a positive value when forcing occurs near the natural frequency. However, the total value of the Reynolds shear stress remains negative due to the contribution of the turbulent components.

Direct numerical simulation of a turbulent mixing layer with a transversely oscillated inflow

“…The inflow conditions are summarized in Table 1 and compared with those in the experiment by Naka et al (2010). Due to the restrictions both in the experiment and the simulation, it was unable to match all the conditions.…”

“…In the present study, the periodic forcing using the piezo-film actuator (PFA) of Naka et al (2010) is numerically mimicked by transversely oscillating the inlet velocity field. To be concrete, the inflow field obtained by combining two cross-sections of turbulent boundary layers, u D i (y, z, t), is periodically oscillated and fed at the inlet of the mixing layer as…”

“…For more efficient utilization of these applications, control techniques for mixing enhancement or suppression have been investigated. One of the control strategies is to manipulate the flow in the upstream region of [-] turbulence: Tu f [%] thickness θ in /θ L in [-] number: Re θ Experiment (Naka et al, 2010) the mixing layer, e.g., using perturbations. Ho (1982) performed an experimental study of the mixing layer in which the flow rates of the inflows were perturbed.…”

“…They report that various modes of vortical structures can be introduced in the flow by changing the Strouhal number. Naka et al (2010) reported a mixing layer periodically forced by using a flap-type actuator made of a piezo-plastic (Polyvinylidene fluoride: PVDF) film aiming at both enhancement and suppression of mixing. They showed that, in addition to mixing enhancement, mixing suppression can also be achieved at a higher frequency.…”

“…The degree of mixing enhancement or suppression is evaluated by the momentum thickness and the vorticity thickness. The amplitude and the frequency of the periodic forcing are determined by following the experiment of Naka et al (2010), where mixing enhancement or suppression was confirmed. The mechanism of the control effects is discussed in detail by analyzing the turbulence statistics.…”