An explanation for the phase lag in supersonic jet impingement

For the first time, a physical mechanism is identified to explain the phase lag term in Powell’s impinging feedback loop equation (Powell, J. Acoust. Soc. Am., vol. 83 (2), 1988, pp. 515–533). Ultra-high-speed schlieren reveals a previously unseen periodic transient shock in the wall jet region of underexpanded impinging flows. The motion of this shock appears to be responsible for the production of the acoustic waves corresponding to the impingement tone. It is suggested that the delay between the inception of the shock and the formation of the acoustic wave explains the phase lag in the aeroacoustic feedback process. This suggestion is quantitatively supported through an assessment of Powell’s feedback equation, using high-resolution particle image velocimetry and acoustic measurements.

A Realizable Version of the k-w Turbulence Model

This paper describes improvements in predictive quality to the original Wilcox k-ω turbulence model. A major disadvantage in the near-wall formulation of this model is usage of the large eddy inverse time-scale, ω ~ ε/k, even though small, dissipative eddies popular the immediate vicinity of walls. The present work suggests a correction to this problemthrough a realizable constraint introduced by the Kolmogorov time-scale. A second realizability constraint, the Schwarz condition, limits eddy viscosity magnitude in flow zones involving large normal strain gradients. Several examples demonstrate the improvements enabled due to these corrections, particularly in transonic and impinging flows where significant normal strain gradients occur.

Biofuel Emulsifier Using High Velocity Impinging Flows and Singularities in Micro-Channels

The objective of this work is to design an original microfluidic mixer for continuous emulsification of small fractions of water in a lipid phase. This system is aimed to be integrated on-line in the process so as to avoid the use of a surfactant. The targeted application is a better combustion of alternative biofuels in boilers, turbines or internal combustion engines in general. The developed micro-system which includes impinging flows and elbows, is performed on the basis of a specific design of micro-channels, adapted to the respective flow rates and the characteristics of the fluids to be emulsified (viscosity, surface tension). The variation of different parameters is tested in this study such as the nature of the lipid phase (viscosity, density, surface tension and components), the length of micro-channels in the elbow and the flow rate of the dispersed phase. The dispersion of water is much more efficient with this microsystem using gasoil rather than vegetable oil as the continuous phase.

Vortical structures and heat transfer in a round impinging jet

In order to gain a better insight into flow, vortical and turbulence structure and their correlation with the local heat transfer in impinging flows, we performed large-eddy simulations (LES) of a round normally impinging jet issuing from a long pipe at Reynolds number Re = 20000 at the orifice-to-plate distance H = 2D, where D is the jet-nozzle diameter. This configuration was chosen to match previous experiments in which several phenomena have been detected, but the underlying physics remained obscure because of limitations in the measuring techniques applied. The instantaneous velocity and temperature fields, generated by the LES, revealed interesting time and spatial dynamics of the vorticity and eddy structures and their imprints on the target wall, characterized by tilting and breaking of the edge ring vortices before impingement, flapping, precessing, splitting and pairing of the stagnation point/line, local unsteady separation and flow reversal at the onset of radial jet spreading, streaks pairing and branching in the near-wall region of the radial jets, and others. The LES data provided also a basis for plausible explanations of some of the experimentally detected statistically-averaged flow features such as double peaks in the Nusselt number and the negative production of turbulence energy in the stagnation region. The simulations, performed with an in-house unstructured finite-volume code T-FlowS, using second-order-accuracy discretization schemes for space and time and the dynamic subgrid-scale stress/flux model for unresolved motion, showed large sensitivity of the results to the grid resolution especially in the wall vicinity, suggesting care must be taken in interpreting LES results in impinging flows.