Large-scale structures in a plane turbulent mixing layer are studied through the use of
the proper orthogonal decomposition (POD). Extensive experimental measurements
are obtained in a turbulent plane mixing layer by means of two cross-wire rakes
aligned normal to the direction of the mean shear and perpendicular to the mean
flow direction. The measurements are acquired well into the asymptotic region. From
the measured velocities the two-point spectral tensor is calculated as a function of
separation in the cross-stream direction and spanwise and streamwise wavenumbers.
The continuity equation is then used for the calculation of the non-measured components
of the tensor. The POD is applied using the cross-spectral tensor as its kernel.
This decomposition yields an optimal basis set in the mean square sense. The energy
contained in the POD modes converges rapidly with the first mode being dominant
(49% of the turbulent kinetic energy). Examination of these modes shows that the
first mode contains evidence of both known flow organizations in the mixing layer, i.e.
quasi-two-dimensional spanwise structures and streamwise aligned vortices. Using the
shot-noise theory the dominant mode of the POD is transformed back into physical
space. This structure is also indicative of the known flow organizations.
The true global source of the farfield sound radiated from a subsonic jet is the entire dynamic found within the confines of the hydrodynamic field, a dynamic which comprises an unsteady compressive excitation of the medium resulting from turbulent mixing and unsteady temperature fluctuations, these being driven by a wide range of turbulence scales. Improved understanding, and subsequent modelling of the space-time character of the global source term has been acheived in the past through study of the different physical mechanisms implicated in its dynamic. The phenomena which have to date been accepted as important in terms of the radiated sound are (i) turbulent mixing and shear, (ii) fluctuating entropy, (iii) convective amplification and (iv) refraction and scattering of sound by the mean and turbulent components of the velocity field. Once identified as important, specific modelling strategies can and have been developed in order to deal with these phenomena. The existence of coherent structures in turbulent jets was identified as important in the 1970's and their role in the production of sound has received considerable attention in more recent years. However, direct identification of the causal link between this component of the turbulence and its sound field is a delicate matter due to the virtual impossibility of directly measuring the source dynamic of the flow, this being due to an overwhelming dominance of hydrodynamic energy in the source region, and a total absence of any hydrodynamic signature in the linear, acoustic region. As a result, modelling strategies where this component of the source term is concerned have remained, at best, highly empirical.An interesting region of the flow where our understanding of the relationship between the hydrodynamic cause and its acoustic effect is concerned is the near pressure field, found just outside the rotational region of the flow. In this region the signature of the sound production mechanism and its resultant sound field are both present, coincident in space and in time.
The temporal dynamics of large-scale structures in a plane turbulent mixing layer
are studied through the development of a low-order dynamical system of ordinary
differential equations (ODEs). This model is derived by projecting Navier–Stokes
equations onto an empirical basis set from the proper orthogonal decomposition
(POD) using a Galerkin method. To obtain this low-dimensional set of equations, a
truncation is performed that only includes the first POD mode for selected streamwise/spanwise
(k1/k3) modes. The initial truncations
are for k3 = 0; however, once
these truncations are evaluated, non-zero spanwise wavenumbers are added. These
truncated systems of equations are then examined in the pseudo-Fourier space in
which they are solved and by reconstructing the velocity field. Two different methods
for closing the mean streamwise velocity are evaluated that show the importance
of introducing, into the low-order dynamical system, a term allowing feedback between
the turbulent and mean flows. The results of the numerical simulations show a
strongly periodic flow indicative of the spanwise vorticity. The simulated flow had the
correct energy distributions in the cross-stream direction. These models also indicated
that the events associated with the centre of the mixing layer lead the temporal
dynamics. For truncations involving both spanwise and streamwise wavenumbers,
the reconstructed velocity field exhibits the main spanwise and streamwise vortical
structures known to exist in this flow. The streamwise aligned vorticity is shown to
connect spanwise vortex tubes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.