Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session H17: Flow Instability: Boundary Layers II |
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Chair: Anatoli Tumin, The University of Arizona Room: 205 |
Monday, November 23, 2015 10:35AM - 10:48AM |
H17.00001: On role of kinetic fluctuations in laminar-turbulent transition in chemically nonequilibrium boundary layer flows Anatoli Tumin Zavol’skii and Reutov (1983), Luchini (2008, 2010), Fedorov (2010, 2012, 2014) explored potential role of kinetic fluctuations (KF) in incompressible and calorically perfect gas boundary layer flows. The results indicate that role of KF is comparable with other disturbance sources in flight experiments and in quiet wind tunnels. The analysis is based on the Landau and Lifshitz (1957) concept of fluctuating hydrodynamics representing the dissipative fluxes as an average and fluctuating parts. We are extending analysis of the receptivity problem to the fluctuating dissipative fluxes in chemically reacting nonequilibrium boundary layer flows of binary mixtures. There are new terms in the energy, and the species equations. The species conservation equation includes the dissipative diffusion flux and the species generation due to dissociation. The momentum equation includes fluctuating stress tensor. The energy equation includes fluctuating heat flux, energy flux due to diffusion of the species, and fluctuating dissipative flux due to viscosity. The effects are compared for the cases stemming from constraints of the HTV project (Klentzman and Tumin, AIAA Paper 2013-2882). [Preview Abstract] |
Monday, November 23, 2015 10:48AM - 11:01AM |
H17.00002: Optimal frequency-response sensitivity of compressible flow over roughness elements Miguel Fosas de Pando, Peter J. Schmid In this work we investigate the global frequency-reponse characteristics of compressible flow over a flat plate with roughness elements, as well as sensitivity characteristics to changes in flow and geometry parameters. Wu (2011) previously considered this configuration as a model for sound generation in wall bounded flows, where acoustic feedback effects were analyzed using asymptotic theory. In the approach followed here, the flow response is assessed by the calculation of the frequency response to optimal forcings for varying frequency at a given flow parameter choice. Even though the computational cost associated with the resolvent analysis renders parametric studies intractable, first-order sensitivity information can still be obtained from a weighted inner product between the optimal forcing and optimal response. This information will allow us to investigate the destabilizing effects of acoustic feedback and baseflow changes. [Preview Abstract] |
Monday, November 23, 2015 11:01AM - 11:14AM |
H17.00003: Transitional Flows in Imperfect Millimeter-Scale Channels Charles Lissandrello, Le Li, Kamil L. Ekinci, Victor Yakhot The majority of workers studying transition to turbulence in pipes have been interested in the flow response to perturbations in otherwise perfect pipes. Conversely, the “fuzzy” problem involving inlet disturbances, pipe imperfections, and pipe roughness has not attracted as much attention. Here, we investigate both experimentally and theoretically the transition to turbulence in imperfect millimeter-scale channels. For probing the flows, we use microcantilever sensors embedded in the channel walls. We perform experiments in two nominally identical channels. We quantify growing perturbations near the channel wall by their spectra and statistical properties, including probability densities and low- and high-order moments. The different sets of imperfections in the two channels result in two random flows in which the high-order moments of the near-wall fluctuations differ by orders of magnitude. Surprisingly, however, the lowest-order statistics in both cases appear to be qualitatively similar and can be described by a proposed noisy Landau equation for a slow mode. The noise, regardless of its origin, regularizes the Landau singularity of the relaxation time and makes transitions driven by different noise sources appear similar. [Preview Abstract] |
Monday, November 23, 2015 11:14AM - 11:27AM |
H17.00004: On the influence of free-stream turbulence length scales on boundary-layer transition Jens Fransson, Shahab Shahinfar A measurement campaign on the free-stream turbulence (FST) induced boundary layer transition has been carried out in the Minimum-Turbulence-Level wind tunnel at KTH. Previous numerical investigations where the turbulence intensity ($Tu$) has been kept constant, while the integral length scale ($\Lambda_x$) has been varied, have shown that the transition location is advanced for increasing $\Lambda_x$. The present measurement campaign has been carried out using hot-wire anemometry and consists of 42 unique FST conditions with thorough measurements throughout the transitional region. Unlike other extensive FST induced transition measurements the free-stream velocity was here kept constant for all cases, implying that the boundary layer scale is locked up to transition onset. Our measurements confirm previous results on the advancement of the transition location with increasing $\Lambda_x$ for low to moderate $Tu$ levels, but show the {\emph{opposite effect}} for higher levels, i.e. a delay in the transition location for larger $\Lambda_x$, which to the knowledge of the present authors so far is unreported. In addition, the common belief that the FST length scales have a negligible effect on the transition location with regards to the $Tu$ level does not seem to be fully true. [Preview Abstract] |
Monday, November 23, 2015 11:27AM - 11:40AM |
H17.00005: Primary instabilities in the rotating-disk boundary-layer flow Ellinor Appelquist, Philipp Schlatter, P. Henrik Alfredsson, R.J. Lingwood For the flow over a rotating disk, also called the von K\'arm\'an flow, there is an exact similarity solution to the Navier--Stokes equations (NSE). This solution is open to a theoretical analysis and there are two types of instabilities present in the flow, convective and an absolute instability.\footnote{Lingwood, R. {\emph{J. Fluid. Mech. }}{\bf{299}}, 1995} The primary convective instability is of the same type as the instabilities one finds on a swept wing, called the crossflow instability. Here the development of this flow is investigated by direct numerical simulations (DNS) using both the linearised and fully nonlinear NSE. The main goal is to map out the instabilities and structures in the flow to investigate how the flow becomes turbulent. Linear simulations are already finalized,\footnote{Appelquist, E., et al. {\emph{J. Fluid. Mech. }}{\bf{765}}, 2015} and further nonlinear simulations allow investigation of the transition to turbulence of the realistic spatially-developing boundary layer, and these simulations can be directly compared with physical experiments of the same case. However, in contrast to experiments, the DNS provides an opportunity to eliminate certain instabilities in the flow field such that other instabilities can be investigated separately. [Preview Abstract] |
Monday, November 23, 2015 11:40AM - 11:53AM |
H17.00006: Characteristics of the laminar-turbulent edge in transitional boundary layers Jin Lee, Tamer Zaki Characteristics of the boundary separating the laminar and turbulent regions in a transitional boundary layer are examined using a time series of three-dimensional flow fields extracted from direct numerical simulations (DNS). In order to accurately mimic boundary-layer experiments perturbed by grid turbulence, the current simulation includes the leading edge of the flat plate and the incoming homogeneous isotropic turbulence. The Reynolds number based on the momentum thickness reaches up to 1400, and high-resolution three-dimensional flow fields of the DNS data will be publicly accessible via the Johns Hopkins Turbulence Database (JHTDB). The laminar-turbulence discrimination algorithm isolates the turbulence spots within the transition zone and the bounding surface of the fully-turbulent flow. Attention is placed on the cross-stream surface between the transition zone and fully-turbulent boundary layer. The shape of this interface is dictated by a balance between downstream advection, destabilization of upstream flow and merging of turbulence spots. Conditionally sampled statistics are examined across the interface, and are also compared to the downstream equilibrium turbulent boundary layer. [Preview Abstract] |
Monday, November 23, 2015 11:53AM - 12:06PM |
H17.00007: The Effect of Surface Waviness on the Growth and Development of TS Waves Christian Thomas, Shahid Mughal, Richard Ashworth The growth and development of TS wave disturbances on an infinitely swept wing are investigated, where surface waviness is imposed along the chordwise direction. Boundary layers are extracted directly from Reynolds Averaged Navier-Stokes solutions, which allows a stability analysis to be undertaken for many flow systems that may include regions of boundary layer separation. Stability analysis is then carried out using both PSE and LNS methods. The effects of varying the wavelength, amplitude and phase of the waviness are considered and the impact on the development of the boundary layer and TS wave disturbances are investigated. It is found that wavy surfaces can significantly affect the amplification rates of the TS wave disturbances, causing large variations in both the onset of the instability and transition. [Preview Abstract] |
Monday, November 23, 2015 12:06PM - 12:19PM |
H17.00008: Flat-plate boundary-layer receptivity to high amplitude, spanwise-oriented, vortical disturbances in the free-stream Richard Bosworth, Jonathan Morrison Extending previous experimental work on boundary-layer receptivity to harmonic free-stream forcing with roughness, the current study focuses on the high amplitude (Tu\textgreater 1{\%}), 2D case. It is shown that at these high amplitudes the boundary layer response is markedly different to the low amplitude case and that roughness is not required for this behaviour to appear. The behaviour within the boundary layer takes the form of low-frequency, streaky structures characteristic of `Klebanoff-modes' and the beginnings of bypass transition rather than 2D Tollmien-Schlichting waves. Traditionally, high turbulence levels are created with turbulence grids, however, the current set up creates qualitatively similar behaviour in a different manner allowing easy variation of the incoming disturbance amplitudes. This may provide a novel way to study two different routes to transition - simply via the addition or subtraction of a roughness strip - in a single controllable experiment. The free-stream disturbances are created via a vibrating ribbon placed above and upstream of the plate leading edge and the disturbance signature is measured using a single cross-wire. Correlation measurements taken within the boundary layer provide details of the streaky structures present. [Preview Abstract] |
Monday, November 23, 2015 12:19PM - 12:32PM |
H17.00009: Frequency response of the swept-wing three-dimensional boundary layer Gianluca Meneghello, Thomas Bewley Three dimensional boundary layers are well known for being subject to the growth of perturbations and transition to turbulence well below the linear stability threshold. Both their linear stability and the temporal transient growth have been analyzed in previous studies. In this work we focus on the response of the swept-wing attachment-line boundary layer to periodic external forcing — e.g. incoming turbulence or wall roughness — and we analyze the response amplitude in multiple system norms. In particular, the induced $||\cdot||_2$ norm (aka singular value norm) identifies the peak gain at a given forcing frequency. The input-output gain and the spacial structures associated with each frequency are identified by singular value decomposition and discussed. [Preview Abstract] |
Monday, November 23, 2015 12:32PM - 12:45PM |
H17.00010: Non-parallel Flow Effects of Stationary Crossflow Vortices at their Genesis Adam Butler, Xuesong Wu We investigate the linear stability of stationary Crossflow vortices whose spanwise wavenumber is sufficiently small that non-parallel flow effects play a leading order role in determining their growth rate. The chordwise and spanwise variations of the baseflow and the perturbation are of equal importance, and so must both be accounted for. Neutral modes can occur in this regime, which lies close to the leading edge. If the effective pressure minimum occurs within this regime, a new scaling for the lower deck must be determined along with a new governing equation for the perturbation. When the mode from the non-parallel regime is continued through the pressure minimum, it passes into a critical layer in the form of a Cowley, Hocking, \& Tutty instability. Downstream of the effective pressure minimum, this critical layer will eventually pass into the main body of the boundary layer. This CHT instability can occur in a more general setting, when the first three derivatives of the effective velocity profile are zero at the wall. [Preview Abstract] |
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