Lagrangian compressible dynamics in a self-similar incompressible jet

摘要: A large-scale experimental Lagrangian study based on Particle Tracking Velocimetry (PTV) has been completed in an incompressible turbulent round water jet freely spreading into water with a Taylor-based Reynolds number Reλ≃ 230. The vertical jet is ejected from a round nozzle with a diameter D= 4 mm and the measurement volume spans 80 mm= 20D in the self-similar region. The jet is seeded with neutrally buoyant spherical polystyrene tracers with a diameter of 250 microns. The particularity of this study is that the jet is seeded only through the nozzle (inhomogeneous seeding called nozzle seeding). The Lagrangian flow of tracers therefore does not contain any contribution from particles entrained into the jet from the surrounding fluid. Tracers are tracked with three high speed cameras at 6000 fps, then PTV methods are used to obtain tracer trajectories. New calibration [1] and matching [2] algorithms are especially used.We will report here results on the mean velocity field Uϕ of tracked particles which we compare to the well-known self-similar velocity field U of the jet [3]. We show that Uϕ is essentially undistinguishable from U for the axial velocity while important discrepancies are found for the radial velocity. These discrepancies are interpreted and analysed by considering the flow of particles as effectively compressible. Indeed, as particles entrained into the jet are not tracked, even if the jet flow is incompressible (v· U= 0), the nozzle seeded flow is not divergence-free (v· Uϕ= 0). To account for this apparent compressibility, a new mass conservation equation is proposed by considering the mean tracer density field ϕ: v·(ϕ Uϕ)= 0. This …