Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session A22: Flow Instability: Boundary Layers - TransitionBoundary Layers Instabilities
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Chair: Olaf Marxen, University of Surrey Room: 708 |
Sunday, November 19, 2017 8:00AM - 8:13AM |
A22.00001: Transition to turbulence in the rotor boundary layer Eric Serre, Eunok Yim, Denis Martinand, Jean-Marc Chomaz This work brings new insights on the way that turbulence occurs in the rotating disk boundary layer. It takes part in the debate initiated by the pioneering work of Lingwood in the ninetees suggesting the possibility of a direct route. The rotating disk boundary layer of a closed rotor-stator cavity is investigated here using direct numerical simulation and linear stability analysis. The mean flow along the rotor matches the von Karman self-similarity solution, except at the edge, where the flow is more unstable due to shear and centrifugal effects, eventually leading to a strong source of perturbations. DNS shows that the transition is governed at moderate rotation rates by edge-driven global modes, below the c/a transition, and at large rotation rates by self-sustained rotor layer global modes. These latter, are the superposition of various unstable modes that account for a gentler front than expected theoretically. This result explains the discrepancy observed in the growth rates between LSA and DNS in azimuthal sectors on the one side, and experiments and present simulations on the other side. Furthermore, these results show that fully nonlinear simulations are mandatory to reproduce experimental observations. [Preview Abstract] |
Sunday, November 19, 2017 8:13AM - 8:26AM |
A22.00002: Turbulent wedge spreading dynamics and control strategies. Saikishan Suryanarayanan, David Goldstein, Garry Brown Turbulent wedges are encountered in some routes to transition in wall bounded flows, particularly those involving surface roughness. They are characterized by strongly turbulent regions that are formed downstream of large disturbances, and spread into the non-turbulent flow. Altering the wedge spreading mechanism is a possible drag reduction strategy. Following recent studies of Goldstein, Chu and Brown (Flow Turbul. Combust. 98(1), 2017) and Kuester and White (Exp. Fluids 57(4), 2016), we explore the relation between the base flow vorticity field and turbulent wedge spreading using immersed boundary direct numerical simulations. The lateral spreading rate of the wedges are similar for high Reynolds number boundary layers and Couette flow, but differences emerge in wall normal propagation of turbulence. We also attempt to utilize the surface texture based strategy suggested by Strand and Goldstein (J. Fluid Mech. 668, 2011) to reduce the spreading of isolated turbulent spots, for turbulent wedge control. The effects of height, spacing and orientation of fins on the dynamics of wedge evolution are studied. The results are interpreted from a vorticity dynamics point of view. [Preview Abstract] |
Sunday, November 19, 2017 8:26AM - 8:39AM |
A22.00003: The effect of shape and shedding on the dynamics of the pendulum. Martin Obligado, Martin Gonzalez Ramirez, Christos Vassilicos The simple pendulum is a paradigm for many problems in physics (oscillators, resonances, instabilities, dissipative systems, etc.). A recent study (Obligado et al, JFM, 2013) performed an experimental investigation of the influence of the turbulence level on the aerodynamic forces acting on a circular plate facing a mean stream by measuring the equilibrium position of the plate when it is mounted on a pendulum capable of moving in the streamwise direction. It was found that the equilibrium of a pendular disk facing a flow exhibits bi-stability and hysteresis, caused by the well known mechanism of detachment/reattachment of the boundary layer of the blob. In this work we propose a systematic study of the influence of the geometry of the pendulum blob on this behaviour and some characterisation of the turbulent wake it generates. Pendulum blobs with regular peripheries (such as squares, triangles and disks) and with fractal/irregular peripheries (as used in Nedic et al, Fluid Dyn Res, 2013) are studied. We find that all the plates with irregular/fractal peripheries present smooth mean angle profiles and low, constant fluctuations for all positions tested in these experiments. Irregular/fractal peripheries therefore inhibit the reattachment mechanism.~ [Preview Abstract] |
Sunday, November 19, 2017 8:39AM - 8:52AM |
A22.00004: Boundary-layer transition via spatially growing oblique waves Olaf Marxen External laminar flow over the surface of a slender body is often convectively unstable so that high-frequency traveling streamwise waves in the boundary layer grow exponentially downstream. In particular, spatially growing oblique waves may initiate laminar-turbulent boundary-layer transition if the boundary layer is separated or at supersonic speeds. Such a transition scenario is investigated by means of direct numerical simulations for these two conditions. Despite certain similarities characteristic for breakdown caused by oblique waves, such as the non-linear growth of steady streamwise streaks, the breakdown mechanism is found to be fundamentally different in the two cases. The incompressible separated boundary layer undergoes wave breakdown, i.e. non-linear waves assume the form of spanwise vortices and are susceptible to instabilities of the vortex core or the braid region, causing breakdown. In the supersonic case, streak breakdown occurs instead, leading to a sinuous motion of the low-speed streak. [Preview Abstract] |
Sunday, November 19, 2017 8:52AM - 9:05AM |
A22.00005: Nonlinear growth of unsteady streaks caused by free-stream vorticity in a compressible boundary layer Pierre Ricco, Elena Marensi, Xuesong Wu The nonlinear response of a compressible boundary layer to unsteady free-stream vortical fluctuations is investigated theoretically and numerically. We focus on low-frequency streamwise-elongated perturbations, known as streaks or Klebanoff modes. The nonlinear streak evolution is described through the nonlinear unsteady compressible boundary-region equations. The free-stream flow is studied by including the boundary-layer displacement effect and is matched asymptotically with the boundary-layer flow. The nonlinear interactions inside the boundary layer drive an unsteady two-dimensional flow of acoustic nature in the outer region through the displacement effect. A close analogy with the flow over a thin oscillating airfoil is exploited to find analytical solutions. In the subsonic regime the disturbances propagate in all directions, while at supersonic speeds the fluid ahead of the body is undisturbed and the perturbations are confined within the Mach dihedron. Nonlinearity stabilizes the velocity and temperature streaks. Increasing the Mach number inhibits the kinematic fluctuations but enhances the thermal streaks. An abrupt deviation of the nonlinear solution from the linear one is observed in the case pertaining to a supersonic wind tunnel. [Preview Abstract] |
Sunday, November 19, 2017 9:05AM - 9:18AM |
A22.00006: Nonlinear growth and resonance of second-mode waves Joseph Kuehl, Carlo Scalo A fundamental mechanism for second-mode wave growth in a Mach 6 hypersonic boundary layers is proposed. It is shown that second-mode behavior is consistent with a standing-wave quarter-wavelength thermoacoustically driven instability. The unstable modes are shown to resonant in an acoustic impedance well between the wall (infinite impedance) and near the sonic line (secondary peak in impedance). A Lagrangian approach is adopted to show that such resonant standing-waves behavior is sustained from the base flow through thermoacoustic Reynolds stress, that result from the divergence of acoustic power inside the impedance well, and thermodynamic work. While this treatment does not represent a complete energy closure, it does provide insight towards the fundamental energy source and physical mechanisms driving Mack's acoustic second-mode. [Preview Abstract] |
Sunday, November 19, 2017 9:18AM - 9:31AM |
A22.00007: Direct Numerical Simulation of Acoustic Noise Generation from the Nozzle Wall of a Hypersonic Wind Tunnel Junji Huang, Lian Duan, Meelan Choudhari Direct numerical simulations (DNS) are used to examine the acoustic noise generation from the turbulent boundary layer on the nozzle wall of a Mach 6 Ludwieg Tube. The emphasis is on characterizing the freestream acoustic pressure disturbances radiated from the nozzle-wall turbulent boundary layer and comparing it with acoustic noise generated from a single, flat wall in an unconfined setting at a similar freestream Mach number to assess the effects of noise reverberation. In particular, the numerical database is used to provide insights into the pressure disturbance spectrum and amplitude scaling with respect to the boundary-layer parameters as well as to understand the acoustic source mechanisms. Such information is important for characterizing the freestream disturbance environment in conventional (i.e., noisy) hypersonic wind tunnels. [Preview Abstract] |
Sunday, November 19, 2017 9:31AM - 9:44AM |
A22.00008: Patterned Roughness for Cross-flow Transition Control at Mach 6 Alexander Arndt, Eric Matlis, Michael Semper, Thomas Corke Experiments are performed to investigate patterned discrete roughness for transition control on a sharp right-circular cone at an angle of attack at Mach 6.0. The approach to transition control is based on exciting less-amplified (subcritical) stationary cross-flow (CF) modes that suppress the growth of the more-amplified (critical) CF modes, and thereby delay transition. The experiments were performed in the Air Force Academy Ludwieg Tube which is a conventional (noisy) design. The cone model is equipped with a motorized 3-D traversing mechanism that mounts on the support sting. The traversing mechanism held a closely-spaced pair of fast-response total pressure Pitot probes. The model utilized a removable tip to exchange between different tip-roughness conditions. Mean flow distortion x-development indicated that the transition Reynolds number increased by 25\% with the addition of the subcritical roughness. The energy in traveling disturbances was centered in the band of most amplified traveling CF modes predicted by linear theory. The spatial pattern in the amplitude of the traveling CF modes indicated a nonlinear (sum and difference) interaction between the stationary and traveling CF modes that might explain differences in $Re_{trans}$ between noisy and quiet environments. [Preview Abstract] |
Sunday, November 19, 2017 9:44AM - 9:57AM |
A22.00009: Stability and sensitivity analysis of hypersonic flow past a blunt cone Joseph W. Nichols, David Cook, Joseph M. Brock, Graham V. Candler We investigate the effects of nosetip bluntness and low-level distributed roughness on instabilities leading to transition on a 7 degree half-angle blunt cone at Mach 10. To study the sensitivity of boundary layer instabilities to bluntness and roughness, we numerically extract Jacobian matrices directly from the unstructured hypersonic flow solver US3D. These matrices govern the dynamics of small perturbations about otherwise laminar base flows. We consider the frequency response of the resulting linearized dynamical system between different input and output locations along the cone, including close to the nosetip. Using adjoints, our method faithfully captures effects of complex geometry such as strong curvature and roughness that lead to flow acceleration and localized heating in this region. These effects violate the assumption of a slowly-varying base flow that underpins traditional linear stability analyses. We compare our results, which do not rely upon this assumption, to experimental measurements of a Mach 10 blunt cone taken at the AEDC Hypervelocity Ballistic Range G facility. In particular, we assess whether effects of complex geometry can explain discrepancies previously noted between traditional stability analysis and observations. [Preview Abstract] |
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