Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session M21: Turbulence Shear Layer |
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Chair: Joseph Katz, Johns Hopkins University Room: 30B |
Tuesday, November 20, 2012 8:00AM - 8:13AM |
M21.00001: ABSTRACT WITHDRAWN |
Tuesday, November 20, 2012 8:13AM - 8:26AM |
M21.00002: Momentum balance in wall jets T. Gunnar Johansson, Faraz Mehdi, Jonathan W. Naughton A plane wall jet experiment has been done to study its momentum balance. Two component laser Doppler anemometry was used to simultaneously measure the axial and wall-normal velocity components in 6 axial positions (x/H= 25, 50, 75, 100, 125 and 150) spanning from the wall all the way well into the ambient stagnant area. In this way not only the mean velocity components and Reynolds normal and shear stresses but also all their spatial derivatives were determined. In addition the wall shear stress was measured in all six axial positions using oil film interferometry. From these data all terms in the x-momentum equation, except the pressure term, could be evaluated. Later also the pressure was measured in the same profiles, and thereby also the pressure term was included in the balance. Contrary to common belief it was found that the pressure was not constant in the wall jet. The complete momentum balance is discussed and used to evaluate the roles played by the different contributing terms in different regions of the flow field in an effort to improve on our understanding of the mechanics of wall jets. [Preview Abstract] |
Tuesday, November 20, 2012 8:26AM - 8:39AM |
M21.00003: Statistics of the local turbulent/nonturbulent interface thickness in jets and boundary layers Rodrigo Taveira, Carlos da Silva, Guillem Borrell, Javier Jimenez Direct numerical simulations (DNS) of turbulent planar jets and boundary layers are used to assess and compare the differences/similarities in the turbulent/nonturbulent (T/NT) interface separating the turbulent from the irrotational flow regions in these flows. Specifically, we focus on the local thickness and local vorticity magnitude across the T/NT interface. Probability density functions of the local T/NT interface thickness in jets show the existence of a plateau between 3 to 10 Kolmogorov micro-scales wide, while the mean thickness is of the order of the Taylor micro-scale, as observed in recent studies. Between 60 to 70\% of the local thickness values are among this range. On the other hand only 5\% of the local vorticity magnitudes are within the range designated by intense vorticity structures (``worms'') and around 60\% of the local vorticity magnitudes after the T/NT interface is in the range of background to weak turbulence. The goal of the work is to characterise what type of structures (vortices/sheets) and in what fraction do they make up the T/NT interface. A similar analysis for a boundary layer is under way. [Preview Abstract] |
Tuesday, November 20, 2012 8:39AM - 8:52AM |
M21.00004: Eulerian and lagrangian statistics across the turbulent/non-turbulent interface in turbulent jets Jos\'e Alexandre Diogo, Rodrigo Taveira, Carlos da Silva Direct numerical simulations (DNS) of turbulent planar jets are used to obtain Eulerian and Lagrangian statistics of the flow across the turbulent/nonturbulent (T/NT) interface, separating the turbulent and the irrotational flow regions. The Eulerian statistics used here consist in conditional statistics made in relation to the distance from the T/NT interface that eliminate the large scale intermittence affecting classical eulerian statistics near the jet edge. For the Lagrangian statistics we use tracers and study their statistics during the entrainment. We focus on the enstrophy characteristics and on the mechanism of enstrophy generation and associated time and length scales as well as on the relative importance between engulfing and nibbling mechanisms. [Preview Abstract] |
Tuesday, November 20, 2012 8:52AM - 9:05AM |
M21.00005: Small scale entrainment characteristics in variable density turbulent jets Pedro Costa, Carlos da Silva Direct numerical simulations (DNS) of turbulent planar jets with different densities in the irrotational and in the turbulent region are used to assess the small scale characteristics associated with the entrainment in turbulent jets. The mean thickness of the enstrophy jump at the turbulent/nonturbulent (T/NT) interface separating the turbulent and the irrotational flow regions is slightly bigger in variable density than in incompressible jets, which may be attributed to the more elongated shape of the coherent vortices at that location. Moreover, the characteristic velocity jump is also bigger which can be explained by a momentum balance made in a control volume near the T/NT interface, as done in recent works for the incompressible case. Finally, variable density jets also display smaller (negative) mean entrainment conditional velocity profiles compared to the incompressible case. [Preview Abstract] |
Tuesday, November 20, 2012 9:05AM - 9:18AM |
M21.00006: Extreme events in a vortex gas simulation of a turbulent half-jet Saikishan Suryanarayanan, Gokul Pathikonda, Roddam Narasimha Extensive simulations [arXiv:1008.2876v1 [physics.flu-dyn], BAPS.2010.DFD.LE.4] have shown that the temporally evolving vortex gas mixing layer has 3 regimes, including one which has a universal spreading rate. The present study explores the development of spatially evolving mixing layers, using a vortex gas model based on Basu et al (1995 Appl. Math. Modelling). The effects of the velocity ratio (r) are analyzed via the most extensive simulations of this kind till date, involving up to 10000 vortices and averaging over up to 1000 convective times. While the temporal limit is approached as r approaches unity, striking features such as extreme events involving coherent structures, bending, deviation of the convection velocity from mean velocity, spatial feedback and greater sensitivity to downstream and free stream boundary conditions are observed in the half-jet (r = 0) limit. A detailed statistical analysis reveals possible causes for the large scatter across experiments, as opposed to the commonly adopted explanation of asymptotic dependence on initial conditions. [Preview Abstract] |
Tuesday, November 20, 2012 9:18AM - 9:31AM |
M21.00007: A revisit of the low-frequency flapping mechanism in an open cavity shear layer flow Xiaofeng Liu, Joseph Katz Causes for low-frequency flapping in an open cavity shear layer flow are revisited using correlations among experimentally obtained time-resolved velocity and pressure distributions. Interactions of the shear layer with the downstream corner cause substantial local variations in pressure and velocity, but correlations show little evidence that these phenomena affect the flow around the upstream corner. Instead, it seems that the flapping occurs due to changes in the streamwise pressure gradients (\textit{$\partial $p'/$\partial $x}) associated with vertical motion of the shear layer in the vicinity of the upstream corner. When the highly correlated initial part of the shear layer and the boundary layer upstream of the corner are thin, \textit{$\partial $p'/$\partial $x} increases. The elevated adverse pressure gradients increase the thickness of the boundary layer, causing a downstream propagating increase in shear layer elevation. As the shear layer rises, \textit{$\partial $p'/$\partial $x} around the upstream corner decreases, causing thinning of the boundary layer. This process persists at \textit{St=fL/U}$_{\infty }$ of 0.052, an order of magnitude lower than that associated with transport of shear layer vortices. Oscillations in the vertical velocity along the upstream cavity wall are weakly correlated with the flapping except for a small separation bubble near the corner. [Preview Abstract] |
Tuesday, November 20, 2012 9:31AM - 9:44AM |
M21.00008: Linear analysis and temporal DNS of compressible mixing layers Mona Karimi, Sharath Girimaji We perform linear analysis and temporal direct numerical simulations (DNS) of high-speed compressible mixing layers. The DNS solver is based on Gas Kinetic Method (GKM) and has been validated in a variety of high-speed shear flows and against rapid distortion theory results. The objective is to examine the effect of compressibility on Kelvin-Helmholtz instability. We perform modal and statistical analysis. The difference in the behavior of two-dimensional and oblique modes is investigated. The action of pressure on 2D and oblique modes is differentiated. The stabilizing influence of compressibility on mixing layer growth (quantified by the so-called Langley Curve) in investigated from fundamental principles. [Preview Abstract] |
Tuesday, November 20, 2012 9:44AM - 9:57AM |
M21.00009: Time-resolved PIV measurements of flow around and through a permeable rectangular prism G.L. Blois, J.M. Barros, J.L. Best, K.T. Christensen The unsteady turbulent wake dynamics generated by flow around and through a permeable rectangular prism is experimentally investigated using a refractive-index-matching (RIM) approach. A 7-mm diameter cylindrical flow passage was drilled through the center of an acrylic rectangular prism (25.5-mm thick; 51.0-mm long) along its streamwise axis. This permeable prism was then immersed in an aqueous solution ($\sim 63$\% by weight) of sodium iodide (NaI) within a recirculating flow loop wherein the refractive index of the NaI was accurately matched to that of the acrylic prism via control of the NaI concentration and solution temperature. This RIM approach enabled simultaneous optical interrogation of the flow both around and within the permeable prism with time-resolved particle-image velocimetry at $\mathrm{Re} > 10^4$. The interaction between the flow exiting the passage and the vortices shed from the model dramatically modifies the dynamics of the wake compared to that of a solid prism of identical dimensions. While the flow through the passage was found to be relatively steady, it generated a pulsating jet that penetrated into the wake, yielding strong internal--external flow interactions. [Preview Abstract] |
Tuesday, November 20, 2012 9:57AM - 10:10AM |
M21.00010: Measurement of the flow past a cactus-inspired cylinder Ghanem F. Oweis, Adnan M. El-Makdah Desert cacti are tall cylindrical plants characterized by longitudinal u- or v-shaped grooves that run parallel to the plant axis, covering its surface area. We study the wake flow modifications resulting from the introduction of cactus-inspired surface grooves to a circular cylinder. Particle image velocimetry PIV is implemented in a wind tunnel to visualize and quantify the wake flow from a cactus cylinder in cross wind and an equivalent circular cylinder at Re O(1E5). The cactus wake exhibits superior behavior over its circular counterpart as seen from the mean and turbulent velocity profiles. The surface flow within the grooves is also probed to elucidate the origins of the wake alterations. Lastly, we use simple statistical analysis based only on the wake velocity fields, under the assumption of periodicity of the shedding, to recover the time varying flow from the randomly acquired PIV snapshots. [Preview Abstract] |
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