Acoustic Control of Turbulent Jets

Consider subsonic turbulent jets of incompressible fluid/gas. At the present time, the main laws of propagation of such jets are well studied in theory and experiment. This is true for mixing layers, plane, axisymmetric, and 3-D submerged jets, as well as for jets in accompanying flow. The jet partitioning into three sections (Fig. 1.1), initial, transition and main sections [1.1, 1.13], is generally recognized.

Aerodynamic characteristics of turbulent jets and mixing layers can be changed by periodical action on the jet initial section flow. Such action could be realized by generation of periodical change in fluid consumption through the nozzle, by the nozzle vibrations or by excitation of the mixing layer at the nozzle edge using a vibrating band. The listed control approaches are connected with mechanical action on the flow because they require direct action on geometry of a unit generating the jet flow [2.10]. The mechanism of their action on the flow is determined by the jet periodical excitation causing generation of annular periodical vortices at the circular nozzle outlet section. Their interaction with each other essentially changes the flow in the mixing layer of the jet initial region.

The changes in averaged and pulsation aerodynamic characteristics of jets under acoustical excitation should be accompanied by the corresponding changes in the jet natural acoustical characteristics determined by the flow aerodynamic parameters (cf. Chap. 1). A study of this phenomenon is not only of scientific but also of practical interest as it offers possibilities of the purposeful control of jet acoustical characteristics. Consider the effect of a harmonic acoustical signal on the field of pressure pulsations in a jet itself and in its near and far acoustical fields.

The approach to the control of aerodynamic and acoustical characteristics of subsonic turbulent jets by means of weak acoustical disturbances was demonstrated in Chap. 2 and 3. In the present chapter we consider some results of experimental study for effects of intensive periodical and, particularly, acoustical disturbances on aerodynamic characteristics of turbulent jets. We do not concern energy benefits of such in the approach to control of turbulent jets. It should only be noted that a number of researchers have performed experimental studies of turbulent jet characteristics under periodical excitation of high intensity. However, the comparison of their results is rather difficult because the periodical in time law of jet expenditure modulation was determined by constructional features of devices (flow interrupters) generating pulsations in jets. This situation hampers generalization of results in the published papers or their correlation, as the flow structure in an excited jet appears to be dependent on the spectral composition of the periodical velocity pulsations and the turbulence scale at the nozzle outlet section. This fact is confirmed by essential distinctions in the propagation laws for highly excited turbulent jets deduced in different authors’ papers [4.2, 4.3, 4.5, 4.6, 4.8].

The present chapter considers well-known approaches to mathematical simulation of subsonic turbulent jets under periodical harmonic excitation. Particular emphasis is placed on the description (in the framework of these approaches) of generation and interaction of large-scale coherent structures, as well as of their susceptibility to periodical excitation. The calculation results illustrating the turbulent mixing intensification (turbulence generation) in jets under low-frequency harmonic excitation and the mixing attenuation (turbulence suppression) under high-frequency excitation are presented. It is stressed that the mathematical simulation of turbulent jets under periodical excitation allows describing the laws of periodical (acoustical) excitation known from the experimental studies.

A supersonic anisobaric turbulent jet is a complex gas-dynamic object and is characterized by strong spatial heterogeneity of the velocity and pressure fields that are dictated by the presence of the shock system and shear layers with large velocity gradients. The strong spatial heterogeneity is favourable to the instability development leading to intense velocity and pressure pulsations. The feedback generation causes development of the self-excited oscillations resulting in intense discrete components of the pulsation spectra.

The urgency of the problem of jet noise reduction has been rising again at the present stage of aircraft technology development. The main reasons are the following. The studies of attenuation of turbo-machine noise have demonstrated that the jet is one of the main noise sources even in engines with high by-pass ratios. Moreover, up-to-date passenger airplanes often use engines with low by-pass ratios where the jet noise contributes predominantly to the total noise of the power engine.

The present chapter is devoted to the study of acoustical methods of suppression and generation of self-sustained oscillations in wind tunnels with the open test section based on sensitivity of the jet coherent structures to periodical excitation [9.1, 9.6 – 9.8]. The latter are attenuated under high-frequency excitation where the Strouhal number St_s = f _s d /u ₀ = 2 − 5 and are amplified under low-frequency excitation with St_s = 0.3 − 0.8. The most sensitive jet area to periodical excitation is the thin mixing layer in the immediate vicinity of the nozzle cut-off. Vortex disturbances are generated under acoustical excitation just there determining amplification or attenuation of coherent quasi-periodical structures.

The flow over a cavity on a plane surface generates under certain conditions self-sustained oscillations. Their cause is generation of an acoustical wave as a result of impact of the mixing layer vortices on the cavity trailing edge. This phenomenon is illustrated by comparison of the velocity pulsations in the mixing layer for two cases: flows over a ledge facing backwards and over a cavity of rectangular section (Fig. 10.1). The pulsation spectrum in the second case contains pronounced discrete components due to acoustical feedback with excitation of self-oscillations [10.3]. A cavity could serve for some frequencies as an acoustical resonator effecting on the nature of self-oscillation excitation [10.8]. As a result, the self-oscillation characteristics are determined by the cavity geometry, the Reynolds and Mach numbers, the flow regime (laminar or turbulent) in the boundary layer in front of the cavity, and the characteristic thickness of this layer.