Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session A34: Flow Instability: Transition to Turbulence |
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Chair: Jae Sung Park, UNL Room: 616 |
Saturday, November 23, 2019 3:00PM - 3:13PM |
A34.00001: Transition in Pulsatile Flows with Flow Reversals Joan Gomez, Yiannis Andreopoulos Pulsatile flows are of interest because for certain range of Reynolds and Womersley numbers they exhibit flow reversals while at the same time they display laminar and turbulent behavior at different times of the pulsating cycle. Addressing how turbulence appears, decays and is suppressed in such environments is challenging due to the flow unsteadiness and flow-wall interactions. An experiment was setup to replicate pulsatile motion of water flowing in a clear, rigid pipe. The flow is driven by a piston-motor assembly controlled by a computer to induce cyclic motion of the mean flow. Time-Resolved Particle Image Velocimetry (TR-PIV) techniques are used to acquire velocity data on the plane of a CW laser illumination sheet. Simultaneous acquisition of time-dependent PIV data and wall pressure measurements, obtained from pressure sensors installed along the length of the pipe, allow the estimation of the instantaneous wall shear-stress which was used as a metric for the appearance of reverse and forward flow regions. It was found that the reverse flow region is formed close to the wall and it is bounded from the forward flow region by a counter-flowing shear-layer. Transition to turbulence occurs within this shear layer which prevents propagation of disturbances from the near wall region towards the center of the pipe. [Preview Abstract] |
Saturday, November 23, 2019 3:13PM - 3:26PM |
A34.00002: The impact of slip surfaces on exact coherent states: insight into the transition-to-turbulence Ethan Davis, Jae Sung Park The effect of slip surfaces on the laminar-turbulent separatrix, or transition, in a channel geometry is studied by direct numerical simulation. Firstly, turbulence lifetimes, or the likelihood that turbulence persists, is investigated. Slip surfaces decrease the likelihood of sustained turbulence when compared with the no-slip case, and the likelihood is further decreased with increasing slip length. Secondly, a more deterministic approach is used to investigate the effect of slip surfaces on the transition to turbulence. To this end, exact coherent states, specifically nonlinear traveling wave solutions to the Navier-Stokes equations, are used as initial conditions. Lower-branch solutions are insightful for the study of transition as they lie along the laminar-turbulent boundary in state-space. Two solution families, P3 (core mode) and P4 (critical layer mode), are considered. For P3, slip surfaces induce earlier transition with negligible effect on the instability of the solution. For P4, on the other hand, slip surfaces delay the transition while weakening the instability. Beyond a critical slip length, the instability is totally eliminated, and flow is laminarized. Flow dynamics and structures are further discussed for these transition events. [Preview Abstract] |
Saturday, November 23, 2019 3:26PM - 3:39PM |
A34.00003: ABSTRACT WITHDRAWN |
Saturday, November 23, 2019 3:39PM - 3:52PM |
A34.00004: Growth and Saturation of Instabilities of Stratified Schlichting Jets with Varying Aspect Ratio Alexander Martell, John Taylor, Qi Zhou We investigate the stability of jets in a linearly stratified Boussinesq fluid. The base flow considered has a two-dimensional (2D) velocity profile which resembles Schlichting’s profile but is not necessarily axisymmetric. We examine a range of Richardson number, as well as the horizontal-to-vertical aspect ratio of the base flow profile, and perform linear stability analysis to identify the most unstable three-dimensional (3D) normal-mode perturbations to the 2D base flow. We compare our results to previously published linear stability analyses of the Schlichting jet and its planar counterpart, the Bickley jet, most of which, to our knowledge, either require perturbations to be axisymmetric (Schlichting jet) or use 2D perturbations, which do not allow for spanwise variation (Bickley jet). The 3D normal modes are then investigated using DNS to understand the nonlinear saturation (in the sense of growth and maximization of perturbation energy) and the transition of the flow to a fully turbulent state. [Preview Abstract] |
Saturday, November 23, 2019 3:52PM - 4:05PM |
A34.00005: Spatial amplification of coupled disturbances in bluff body shear layers Daniel Moore, Michael Amitay Detailed experimental campaigns into separated shear layers have been carried out on a series of rectangular sections over a Reynolds number range from 13,400 to 118,00, based on the body height, at a range of angles of attack. Recent work by this same group has demonstrated the level of coupling that occurs between the convective shear layer instability at the leading and the global, large-scale instability associated with wake shedding of these sharp-edged rectangular sections. Building upon these findings, the spatial amplification of the separated shear layers is experimentally derived and compared to other shear flows including the classical planar mixing layer. Results show that the effect of a coupled shear layer manifests itself via a relatively broad spectrum of unstable frequencies and significantly elevated growth rates. On the other hand, reducing the coupling the same two instabilities has the opposite effect. It is further shown that the coupling is directly tied to a turbulent transition length, reinforcing the notion of a globally unstable flow field. [Preview Abstract] |
Saturday, November 23, 2019 4:05PM - 4:18PM |
A34.00006: Temporal development of hairpin vortex sequences in turbulent puffs Kyle WINTERS, Ellen Longmire Puffs are characterized by intermittent swirling structures that develop from sufficient perturbations to pipe flow around $1800 < Re < 2700$. By conducting stereo-PIV on circular cross sections, the authors examined hairpin-shaped vortices near the trailing edge of many puffs. To study the temporal evolution of the hairpins, planar-PIV was conducted on an axial-wall normal plane inside a $44.8mm$ diameter, $D$, $8.8m$ ($180D$) downstream of a disturbance ring. In certain records, the measurement plane coincided with the plane of symmetry of a hairpin. This allowed for comparison with the stereo-PIV records and examination of the hairpin development. The hairpin heads and associated velocity fluctuations grew in strength as they moved both downstream and away from the wall. Eventually the original hairpin spawned a new hairpin upstream close to the wall at the same azimuthal position. The axial spacing between the two hairpins agreed with that observed in the stereo-PIV records. Further, the spacing grew as they continued to propagate downstream at velocities that generally agree with the propagation of axial fluctuations noted by Shimizu and Kida (2008). The overall development process was similar to that outlined by Jodai and Elsinga (2016) in a turbulent boundary layer. [Preview Abstract] |
Saturday, November 23, 2019 4:18PM - 4:31PM |
A34.00007: The onset of turbulence in channel flow Bjorn Hof, Vasudevan Mukund, Chaitanya Paranjape, Philip Sitte In channel flow turbulence arises at Reynolds numbers where the laminar state is linearly stable. In order to define the critical point where turbulence first becomes sustained we identify the growth and decay processes of individual turbulent stripes in experiments. As shown, the critical point is reached when the (continuous) entrainment of laminar fluid at the stripes downstream tip outweighs the(stochastic) shedding of turbulence at their upstream tip. For growing stripes, the probability to collapse decreases while the probability to split increases in time. Consequently, neither collapse nor splitting are memoryless and unlike in pipes and unlike in directed percolation, the contamination rate changes in time. The coupling between the growth of individual stripes and the creation of new stripes leads to a significantly lower critical point than most earlier studies suggest. [Preview Abstract] |
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