The near field of coaxial jets: A numerical study

Abstract

The near-field behavior of coaxial jets is studied through direct numerical simulation ͑DNS͒ with a particular focus on the influence of the inner shear layer steepness characterized by its momentum thickness 01 thus mimicking the variation in the lip thickness of a real jet nozzle. We investigate the two distinct jet regimes r u Ͼ r uc for which a recirculation bubble is present near the jet inlet and r u Ͻ r uc without any recirculation bubble, r u being the velocity ratio between the outer jet and inner jet velocities. It is shown that small values of 01 lead to a fast transition to turbulence. The various mechanisms leading to this transition are investigated in detail: the three-dimensionality growth, the appearance of secondary vortices superimposed on the main ring vortices, and the subsequent longitudinal stretching of streamwise vortices. This stretching mechanism is shown to play a dominant role in the transition processes towards a fully developed turbulent state. For high enough values of r u , a pinching of the jet is observed near the inlet and it is shown that this pinching phenomenon lasts on a shorter downstream distance for small values of 01 due to a more efficient turbulent mixing. In the r u Ͼ r uc case, variations of 01 strongly affect the shape and the downstream extent of the recirculation bubble. The DNS allow to show the strong dependency of the inner and outer potential core lengths and of the critical value r uc on the jet inlet velocity profile. We finally revisit the theoretical model originally proposed by Rehab, Villermaux, and Hopfinger ͓"Flow regimes of large-velocity-ratio coaxial jets," J. Fluid Mech. 345, 357 ͑1997͔͒ first aimed at the prediction of the variations of various jet characteristics as a function of r u . The model is extended to determine the dependency of the jet characteristics with 01 . A very good correspondence between the theoretical predictions and the numerical results is obtained.

Dynamics of Jets Issuing from Trailing-Edge Modified Nozzles

“…This was followed by a study by Villermaux and Rehab [91] where they looked into the mixing characteristics of such large velocity-ratio coaxial jets in terms of the jet shear layer behaviour, velocity differences and gap thickness, among others. The presence of the recirculation bubble was later confirmed and explored in greater detail by numerical studies performed by Reynier and Ha Minh [71] and Balarac and Metais [5]. In particular, the effects on the interactions between the two jet streams, as well as the recirculation bubble, due to variations in the nozzle lip thickness was investigated by the latter study as well.…”

Modelling of a multiphase reacting turbulent jet: Application to supersonic carbon injection in siderurgic furnaces

“…The flow generated by a coaxial arrangement of streams has been extensively studied in the literature, both in the case of incompressible fluid (see Rehab et al, 1997;Balarac and Metais, 2005, among others) and in the case of compressible fluid (Dahal and Morris, 1997). The flow is controlled by the growth rate of the shear mixing layer at the interface between the inner and the outer jet.…”

Numerical Study of Turbulence Models in the Computation of Blood Flow in Cannulas

“…Balarac and Métais [14] performed a Direct Numerical Simulation (DNS) for an array of coaxial jets, which resembles closely the characteristics of the flow at cannulas, as it can be seen in Figure 3. In both cases, there is a shear layer between the inner and outer streams.…”

“…The free-stream velocity was set to U 3 /U 2 = 0.04. Further details about the boundary conditions used by Balarac and Métais can be found on [14].…”

“…The boundary conditions for the RANS simulations were set to resemble exactly those from the DNS simulations of [14]: a laminar inlet profile according to Equation 4 and as shown on Although the inlet velocity profile is laminar, turbulence parameters should be specified, because a turbulence model is being used during the simulation. The most critical of these parameters is the turbulent-to-molecular viscosity ratio.…”