Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session J02: Jets: Impinging, Swirling, and General (8:00am  8:45am CST)Interactive On Demand

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J02.00001: On the velocity field of the jets emerged from collapsing cavities Francisco J. BlancoRodriguez, Jose M. Gordillo A recent research work (Gordillo \& Rodr\'iguezRodr\'iguez, J. Fluid Mech., (2019)) had been demonstrated undoubtedly that the capillary waves was the responsible of the ejection of bubble bursting jets and that the velocity field at the ejection time can be approached using a line of sinks whose length $\ell_s$ is proportional to the wavelength $\lambda^*(Oh)$ of the capillary wave which is not attenuated by viscosity being $Oh=\mu/(\sqrt{\rho\,R\,\sigma})$ the Ohnesorge number. Here, comparing that theoretical framework with the numerical results computed by \texttt{GERRIS}, we provide an unified description of different physical scenarios in which liquid jets are expelled out of the bulk of a liquid as a consequence of the capillary collapse of an initially rounded cavity which is transformed into a truncated cone with an opening semiangle $\beta$. It is demonstrated here that the potentialflow velocity field created by a line of sinks with intensities fixed by the capillary velocities induced at the base of the conical cavity from which the jet is issued, is in remarkable agreement with those obtained from the numerical solution of the jets observed when either a bubble bursts at a free surface or when the cavity formed when a drop falls on a liquid pool, collapses. [Preview Abstract] 

J02.00002: Numerical bifurcation analysis of limit cycles and hysteresis in laminar swirling jets Christopher Douglas, Benjamin Emerson, Timothy Lieuwen This work details a nonlinear bifurcation analysis involving timeperiodic solutions in laminar swirling jets. Swirling jets are subject to a wide range of different coherent flow instabilities which manifest themselves in a variety of forms ranging from nearly stationary axisymmetric recirculation regions to unsteady asymmetric vortical structures. Even in the laminar regime, these flows have been shown to exhibit a significant degree of hysteresis among their various states which has largely obscured any comprehensive view of the swirling jet parameter space. To date, bifurcation analyses of strongly swirling jets are quite limited, with most directed toward the phenomenon of axisymmetric vortex breakdown and its stability toward threedimensional perturbations. We aim to fill this void by tracing branches of threedimensional nonlinear timeperiodic solutions in an unconfined swirling jet for $Re\leq200$. Our results reveal prebreakdown $m=2$ and postbreakdown $m=1$ limit cycle states which are consistent with previous experimental observations from the LadHyX group. In addition, the associated bifurcation diagrams offer a clear perspective of the $Re$$S$ parameter space, emphasizing the role of saddlenode bifurcations on the observed hysteresis behavior. [Preview Abstract] 

J02.00003: Topology of a Swirling Jet and Vortex Breakdown Reiley Weekes, Keiko Nomura Swirling jet flow is studied by analyzing properties of the velocity gradient field. Numerical simulations are performed for a 3D jet comparable to those in the experiments of Billant et al. (1998) at moderate $Re$. Invariants of the velocity gradient tensor, $Q$ and $R$, as well as the invariants of the strainrate and rotation rate tensors are evaluated. For relatively low swirl number, $S$, the flow begins with an unstrained vortex core $(Q>0, R \sim 0)$ and high strain at the periphery. A helical disturbance develops with alternating sign changes in $R$. For flows with high swirl, $S > S_{cr}$, a bubble type vortex breakdown is observed. The initial vortex core is compressed $(Q>0, R>0)$ with relatively high strain along the axis. The stagnation point and bubble occur where $Q$ changes sign. The JPDF of $Q$$R$ evaluated in the unsteady flow downstream of the bubble exhibits the characteristic teardrop shape previously reported in various turbulent shear flows. Details of the structure and topology of moderate to high swirl jets will be presented. [Preview Abstract] 

J02.00004: Liquid Jet impingement on superhydrophobic metalmesh substrates Shashwata Moitra, Tamal Roy, Ranjan Ganguly, Constantine M Megaridis Liquid jetimpact on permeable substrates has a variety of applications especially in heat transfer, liquid fuel atomization, incontinence products and solid substrate erosion. We study liquidjet impact on superhydrophobic metal meshes and investigate the radial spreading and throughflow of the liquid. The effect of liquid properties (density, surface tension and viscosity) and the permeability of the mesh on the prebreakthrough hydraulic jump, breakthrough velocity and the postbreakthrough distribution of the liquid were studied. The hydraulic jump radius on the prebreakthrough side of the mesh increases with rising jet velocity and is independent of the liquid properties or mesh geometry. The breakthrough velocity increases with the surface tension of the liquid and decreases with the mesh opening diameter and the viscosity of the liquid. The theoretical prediction of the breakthrough velocities from a simple analytical model is in accordance with the experimental observations. In the postbreakthrough regime (mesh underside), the liquid flow rate through the pores showed an initially steep increase, followed by a subsequent plateauing with increasing jet velocity, probably as the impact area of the mesh underwent a CassieBaxtertoWenzel transition. [Preview Abstract] 

J02.00005: Impinging planar jets: hysteretic behaviour and origin of the selfsustained oscillations Alessandro Bongarzone, Arnaud Bertsch, Philippe Renaud, Fran\c cois Gallaire In a recent experimental and numerical investigation we studied the oscillatory regime induced by the interaction of two impinging jets in feedbackfree microfluidic devices. The physical mechanism behind these selfsustained oscillations remains still undetermined. The present paper focuses on an twodimensional (2D) fluidic oscillators. The linear global stability analysis performed confirms the existence of an oscillating mode, whose spatial structure qualitatively coincides with the one observed in our experiments, suggesting that the physical mechanism from which the oscillations originate is predominately 2D. The interaction of the oscillating mode with a steady symmetrybreaking mode is examined making use of the weakly nonlinear theory, which shows how the system exhibits hysteresis in a certain range of aspect ratios. The theory of sensitivity analysis is exploited to identify the wavemaker region associated with the instability modes, whose accurate examination allows us to spot the core of the symmetrybreaking instability at the stagnation point and to propose the KelvinHelmholtz instability as the main candidate for the origin of the flow oscillations observed in both 2D and 3D fluidic devices. [Preview Abstract] 

J02.00006: Mechanism of formation of coherent structures in underexpanded supersonic impinging jets Shahram Karami, Daniel EdgingtonMitchell, Vassilis Theofilis, Julio Soria Largeeddy simulations of underexpanded impinging jets are performed to study the mechanism by which the initial highfrequency instabilities change to lowfrequency coherent structures within a short distance. The spectral characteristics of the Mach energy norm is utilised to obtain the spatial growth of instabilities. Linear spatial instability analysis with streamwise varying mean flow profiles is also performed. Crosscorrelation of velocity and pressure show that hydrodynamic wavepackets form approximately one jet diameter downstream of the nozzle lip. No evidence has been found to support the 'collective interactive' mechanism of Ho {\&} Nosseir (JFM, Vol. 105, p. 119142, 1981). The 'vortex pairing' of Winant {\&} Browand (JFM, Vol. 63, p. 237255, 1974) is observed near the nozzle; however, it has an insignificant role in the sharp reduction of the most unstable frequency of disturbances. Nonetheless, both Mach energy norm and linear spatial instability analyses show that the most unstable frequency of disturbances decreases rapidly in a very short distance from the nozzle lip in the nearnozzle region through the spatial growth of instabilities where linear instability analysis overpredicts the frequency of the most unstable instabilities. [Preview Abstract] 

J02.00007: Receptivity at the nozzle lip of underexpanded supersonic impinging jets Shahram Karami, Vassilios Theofilis, Julio Soria The receptivity of an underexpanded supersonic impinging jet flow at the sharp nozzle lip to acoustic impulse disturbances is investigated as a function of geometric and flow parameters. Receptivity is defined as the internalisation of an external disturbance into the initial condition that either initiates or sustains a vortical instability. In the case of underexpanded impinging jet flow subjected to an acoustic disturbance receptivity is located at the nozzle lip and amenable to an impulse response analysis using the linearised compressible threedimensional NavierStokes equations. Underexpanded supersonic jets emanate from an infinitelipped nozzle with a nozzle pressure ratio of 3.4 for two nozzletowall distances (h) of 2 and 5 jet diameters have been studied. It is found that for both cases, acoustic disturbances located at angles greater than 80$^o$ from the jet centreline have the highest receptivity for all azimuthal modenumbers, except the azimuthal modenumber 2 for h=5d. For h=5d there is also high receptivity to acoustic disturbances located at angles 15$^o$  50$^o$ from the jet centreline for all azimuthal modenumbers. [Preview Abstract] 
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