Turbulent jet noise is a controversial fluid mechanical puzzle that has amused and bewildered researchers for more than half a century. Whereas numerical simulations are now capable of simultaneously predicting turbulence and its radiated sound, the theoretical framework that would guide noise-control efforts is incomplete. Wave packets are intermittent, advecting disturbances that are correlated over distances far exceeding the integral scales of turbulence. Their signatures are readily distinguished in the vortical, turbulent region; the irrotational, evanescent near field; and the propagating far field. We review evidence of the existence, energetics, dynamics, and acoustic efficiency of wave packets. We highlight how extensive data available from simulations and modern measurement techniques can be used to distill acoustically relevant turbulent motions. The evidence supports theories that seek to represent wave packets as instability waves, or more general modal solutions of the governing equations, and confirms the acoustic importance of these structures in the aft-angle radiation of high subsonic and supersonic jets. The resulting unified view of wave packets provides insights that can help guide control strategies.

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Wave Packets and Turbulent Jet Noise

https://www.eis.hu.edu.jo/deanshipfiles/pub10311352.pdf

Natural convection heat transfer enhancement in horizontal concentric annuli using nanofluids

https://eis.hu.edu.jo/deanshipfiles/pub10311347.pdf

Effects of variable viscosity and thermal conductivity of Al2O3-water nanofluid on heat transfer enhancement in natural convection

Numerical investigation of MHD effects on Al2O3–water nanofluid flow and heat transfer in a semi-annulus enclosure using LBM

In the present study the mechanisms of evolution of propeller tip and hub vortices in the transitional region and the far field are investigated experimentally. The experiments involved detailed time-resolved visualizations and velocimetry measurements and were aimed at examining the effect of the spiral-to-spiral distance on the mechanisms of wake evolution and instability transition. In this regard, three propellers having the same blade geometry but different number of blades were considered. The study outlined dependence of the wake instability on the spiral-to-spiral distance and, in particular, a streamwise displacement of the transition region at the increasing inter-spiral distance. Furthermore, a multi-step grouping mechanism among tip vortices was highlighted and discussed. It is shown that such a phenomenon is driven by the mutual inductance between adjacent spirals whose characteristics change by changing the number of blades.

Mechanisms of evolution of the propeller wake in the transition and far fields

Experimental data obtained in various turbulent flows are analysed by means of orthogonal wavelet transforms. Several configurations are analysed: homogeneous grid turbulence at low and very low Reλ, and fully developed jet turbulence at moderate and high Reλ. It is shown by means of the wavelet decomposition in combination with the form of scaling named extended self-similarity that some statistical properties of fully developed turbulence may be extended to low-Reλ flows. Indeed, universal properties related to intermittency are observed down to Reλ≃10. Furthermore, the use of a new conditional averaging technique of velocity signals, based on the wavelet transform, permits the identification of the time signatures of coherent structures which may or may not be responsible for intermittency depending on the scale of the structure itself. It is shown that in grid turbulence, intermittency at the smallest scales is related to structures with small characteristic size and with a shape that may be related to the passage of vortex tubes. In jet turbulence, the longitudinal velocity component reveals that intermittency may be induced by structures with a size of the order of the integral length. This effect is interpreted as the signature of the characteristic jet mixing layer structures. The structures identified on the transverse velocity component of the jet case turn out on the other hand not to be affected by the mixing layer and the corresponding shape is again correlated with the signature of vortex tubes.

Orthonormal wavelet decomposition of turbulent flows: intermittency and coherent structures

A vorticity-velocity method for the numerical solution of 3D incompressible flows

In the present study an experimental analysis of the velocity and pressure fields behind a marine propeller, in non-cavitating regime is reported. Particle image velocimetry measurements were performed in phase with the propeller angle, to investigate the evolution of the axial and the radial velocity components, from the blade trailing edge up to two diameters downstream. In phase pressure measurements were performed at four radial and eight longitudinal positions downstream the propeller model at different advance ratios. Pressure data, processed by using slotting techniques, allowed reconstructing the evolution of the pressure field in phase with the reference blade position. In addition, the correlation of the velocity and pressure signals was performed. The analysis demonstrated that, within the near wake, the tip vortices passage is the most important contribution in generating the pressure field in the propeller flow. The incoming vortex breakdown process causes a strong deformation of the hub vortex far downstream of the slipstream contraction. This process contributes to the pressure generation at the shaft rate frequency.

Analysis of the propeller wake evolution by pressure and velocity phase measurements

The fluid dynamic and thermal fields in a two-dimensional annulus between horizontally eccentric cylinders ( Ro/ Rin = 2.36) are numerically studied in laminar steady conditions. The Navier-Stokes equations with Boussinesq approximation are solved for a typical fluid with Pr = 0.71, 0.53 X 104 ≤ RaL ≤ 8.27 X 104, and a wide eccentricity range ( 0 ≤ e < 1 ). The governing equations are written in primitive variable form to avoid the enforcement of the additional integral condition for the pressure single valuedness. The horizontal eccentricity of the inner cylinder gives, in contrast with known numerical results, a nonzero azimuthal flow rate in the channel between the two cylinders and substantially alters the thermal field and the geometry of the plume. Comparisons with available numerical and experimental results for concentric and horizontally eccentric configurations are presented and discussed.

G. Guj

Papers

Orthonormal wavelet decomposition of turbulent flows: intermittency and coherent structures

A vorticity-velocity method for the numerical solution of 3D incompressible flows

Analysis of the propeller wake evolution by pressure and velocity phase measurements

Natural convection in horizontal eccentric annuli: numerical study

Acoustic identification of coherent structures in a turbulent jet