Phase averaged velocity field in an axisymmetric jet subject to a sudden velocity decrease

Abstract

An unsteady transient axisymmetric turbulent jet was studied experimentally. The initial flow perturbation consisted of a sudden and large decrease in the ejection velocity. The temporal evolution of the mean and fluctuating unsteady velocity field was measured by using X hot-wire probes. In the jet far field, adaptation of the externally imposed unsteadiness to the local jet time scale is confirmed quantitatively. The main features of the phase averaged velocity field are presented and comments are made about the instantaneous state of the turbulence energetics. Transient mean radial velocities are deduced and an important increase of the instantaneous rate of entraining external fluid into the jet is found. Finally, we show that the pressure effect due to radial impusle terms plays an important role in the propagation of the mean perturbation. The longitudinal adaptation of the perturbation time scale driven by the local jet time scale provides a turbulent flow that is intermediate to quasi-static flows and rapidly distorted flows.

Self-similar properties of decelerating turbulent jets

“…Figure 3(a) shows that the mean axial velocity profile in the stopping jet transitions from the near-Gaussian steady-state profile to a slightly fuller profile in the decelerating region, similar to the change of shape recorded by Borée et al (1996) in the statistically unsteady region after halving the jet flow rate. The overall width of the profile remains unchanged, supporting the assumption of a constant jet spreading angle used in the model by Musculus (2009).…”

“…On the contrary, the measurements by Borée et al (1996) indicate that the radial profiles of phase-averaged axial velocity and other velocity moments deviate from the self-similar profiles of a statistically steady jet as the jet decelerates, and that the profiles vary axially through the confined deceleration region (Borée et al 1996(Borée et al , 1997. Here, we hypothesise that the flow field in a decelerating jet asymptotes towards a new temporally stationary self-similar state in which the radial profiles of normalised velocity moments are axially uniform.…”

“…If the integrated mass flux Q η remains constant, as assumed by Musculus (2009), the scaled mass entrainment rate in the decelerating self-similar jet (n = 1) is predicted to be three times the rate in a statistically stationary jet (n = −1). However, the mean axial velocity profile and therefore the integrated mass flux Q η are also subject to change within the decelerating region of the jet (Borée et al 1996), further affecting the scaled entrainment rate given by (2.12). To be shown in (2.14), the unsteady self-similar h-profile depends on the corresponding f -profile, indicating that Q η and h are not independent each other.…”

“…Here, we hypothesise that the flow field in a decelerating jet asymptotes towards a new temporally stationary self-similar state in which the radial profiles of normalised velocity moments are axially uniform. The experiment described by Borée et al (1996) does not achieve this state fully because the deceleration region is confined between the initial and final statistically steady jet flows. Instead, we investigate self-similarity in decelerating jets through theoretical analysis and statistical analysis of a stopping jet using new DNS data where the inflow stops abruptly.…”

Effect of post injections on mixture preparation and unburned hydrocarbon emissions in a heavy-duty diesel engine

“…This rapid reduction in equivalence ratio near the injector is driven by advection and enhanced mixing that arises from the reduction of flow through the nozzle approaching the end of the injection. Changes in mixing, or entrainment, during rapid velocity variations at the nozzle have been measured in turbulent gas jets by several authors [16][17][18][19] . For diesel jets, Musculus and coworkers [20][21][22] showed how charge temperature and density as well as the profile of end-of-injection transient affect the evolution and propagation of large-scale structures downstream in the jet to increase entrainment.…”