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 E22: Flow Instability: Kelvin-Helmholtz and WakesInstabilities Shear layer
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Chair: Oleg Schilling, Lawrence Livermore National Laboratory Room: 708 |
Sunday, November 19, 2017 4:55PM - 5:08PM |
E22.00001: Reynolds-Averaged Navier-Stokes Modeling of Turbulent Free Shear Layers Oleg Schilling Turbulent mixing of gases in free shear layers is simulated using a weighted essentially nonoscillatory implementation of $\epsilon$- and $L$-based Reynolds-averaged Navier--Stokes models. Specifically, the air/air shear layer with velocity ratio $0.6$ studied experimentally by Bell and Mehta (1990) is modeled. The detailed predictions of turbulent kinetic energy dissipation rate and lengthscale models are compared to one another, and to the experimental data. The role of analytical, self-similar solutions for model calibration and physical insights is also discussed. It is shown that turbulent lengthscale-based models are unable to predict both the growth parameter (spreading rate) and turbulent kinetic energy normalized by the square of the velocity difference of the streams. The terms in the $K$, $\epsilon$, and $L$ equation budgets are compared between the models, and it is shown that the production and destruction mechanisms are substantially different in the $\epsilon$ and $L$ equations. Application of the turbulence models to the Brown and Roshko (1974) experiments with streams having various velocity and density ratios is also briefly discussed. [Preview Abstract] |
Sunday, November 19, 2017 5:08PM - 5:21PM |
E22.00002: Testing marginal stability in stratified shear layers Christopher Howland, Colm-cille Caulfield, John Taylor We perform two dimensional direct numerical simulations of a stratified shear layer to investigate the effect of variations in the minimum Richardson number ($Ri_m$) on the early evolution of Kelvin--Helmholtz (KH) instability. Using these simulations, we examine the development of KH billows up to the time when the perturbation energy saturates at its maximum value. We show that in the limit as $Ri_m\rightarrow1/4$ the perturbation growth rate tends to zero and the saturated perturbation energy becomes very small. Our results imply that `marginally unstable' flows with $Ri_m$ only slightly less than 1/4 are highly unlikely to become turbulent without additional forcing. [Preview Abstract] |
Sunday, November 19, 2017 5:21PM - 5:34PM |
E22.00003: Characterization of the transition of separated shear layers on 2D rectangular prisms Daniel Moore, Chris Letchford, Michael Amitay The flow field associated with a rectangular 2D prism, having aspect ratios from 1 to 5, was investigated experimentally using two dimensional Particle Image Velocimetry (2D-PIV), surface-mounted microphones and hot-wire anemometry. Specifically, the shear layer separating from the sharp-edged section, which originates from a laminar boundary layer along the windward face, was explored. Downstream of the sharp edge, the shear layer quickly experiences instabilities of the Kelvin-Helmholtz type that contribute directly to the transition process. The characterization of the flow field was evaluated using frequency analysis and the dominating terms (obtained from the PIV data) in the turbulent kinetic energy transport equation. Evaluating the spatial gradients reveals the locations and magnitudes of maximum turbulence production, advection, diffusion and dissipation within the shear layer. The effect of the prism's aspect ratio on the evolution of the shear layer will also be discussed in the presentation. [Preview Abstract] |
Sunday, November 19, 2017 5:34PM - 5:47PM |
E22.00004: Phase locking and unlocking of the symmetry-breaking mode of 3D bluff bodies turbulent wakes Guillaume Bonnavion, Olivier Cadot, Denis Sipp, Vincent Herbert, Sylvain Parpais, R\'emi Vigneron, Jean D\'elery The influence of small steady geometrical constraints such as a yaw or a pitch angle applied to the squareback Ahmed body ($Re=400000$) or wall proximity is investigated experimentally using unsteady base pressure, PIV and force measurements. The global turbulent wake properties are characterized with a polar description of the instantaneous base pressure gradient. Whatever the constraint, a coherent structure, called static symmetry-breaking mode (SB mode), is found with an almost constant modulus but with very different phase dynamics. The imposed geometrical constraints are associated with a deterministic component of the base pressure gradient and the phase dynamics adapts, when possible, to satisfy both the constant modulus and the deterministic component of the base pressure gradient. The phase dynamics result either in wake bi-stability, in phase locking or in a global unlocking with random explorations. A stochastic model adapted from that of the axisymmetric bluff body is applied to the base pressure gradient. The aerodynamic loading on the body is decoupled from the wake modes by means of a base cavity known to stabilize the SB mode. Both approaches are expected to provide the necessary ingredients to model the fluid force exerted on 3D bluff bodies with a blunt base. [Preview Abstract] |
Sunday, November 19, 2017 5:47PM - 6:00PM |
E22.00005: Three-dimensional short-wavelength instabilities in the near-wake of a circular cylinder Yogesh Jethani, Kamal Kumar, A. Sameen, Manikandan Mathur We perform local stability analysis of the near-wake region of two-dimensional flow past a circular cylinder for Reynolds number in the range $Re\in[10,300]$. The local stability equations that govern the leading-order amplitude of short-wavelength perturbations are solved along closed fluid particle trajectories in the numerically simulated flow-fields for both the steady ($Re\leq45$) and unsteady vortex-shedding ($Re>45$) regimes; the study is further complemented with analysis on time-averaged flows for $50\leq Re\leq300$. For steady and time-averaged flow, the inviscidly most unstable regions occur either at the core or at the edge of the separation bubble, with elliptic instability as the dominant mode for all $Re$. The effectiveness of viscous damping in eliminating the inviscid instabilities and the validity of the WKBJ approximation in the present context are studied. In the unsteady vortex-shedding regime, two types (I and II) of closed trajectories are identified for all $Re$ and the inviscid growth rates as a function of $Re$ are plotted for both. For type I trajectory, a bifurcation occurs at $Re\approx250$. Potential relevance of our results in understanding the transition from steady flow to vortex-shedding and the subsequent secondary instabilities are discussed. [Preview Abstract] |
Sunday, November 19, 2017 6:00PM - 6:13PM |
E22.00006: Frequency prediction by linear stability analysis around mean flow Yacine Bengana, Laurette Tuckerman The frequency of certain limit cycles resulting from a Hopf bifurcation, such as the von Karman vortex street, can be predicted by linear stability analysis around their mean flows. Barkley (2006) has shown this to yield an eigenvalue whose real part is zero and whose imaginary part matches the nonlinear frequency. This property was named RZIF by Turton et al. (2015); moreover they found that the traveling waves (TW) of thermosolutal convection have the RZIF property. They explained this as a consequence of the fact that the temporal Fourier spectrum is dominated by the mean flow and first harmonic. We could therefore consider that only the first mode is important in the saturation of the mean flow as presented in the Self-Consistent Model (SCM) of Mantic-Lugo et al. (2014). We have implemented a full Newton's method to solve the SCM for thermosolutal convection. We show that while the RZIF property is satisfied far from the threshold, the SCM model reproduces the exact frequency only very close to the threshold. Thus, the nonlinear interaction of only the first mode with itself is insufficiently accurate to estimate the mean flow. Our next step will be to take into account higher harmonics and to apply this analysis to the standing waves, for which RZIF does not hold. [Preview Abstract] |
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