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 A19: Boundary Layers: General 
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Chair: Rene VanHout, Technion Israel Institute of Technology Room: 207 
Sunday, November 22, 2015 8:00AM  8:13AM 
A19.00001: Timeresolved TomoPIV measurements of the interaction between a stationary held sphere and a turbulent boundary layer. Rene van Hout, Jerke Eisma, Edwin Overmars, Gerrit Elsinga, Jerry Westerweel Time resolved tomographic PIV measurements (acquisition rate 250Hz) were performed in a turbulent boundary layer (TBL) on the side wall of an open channel, water flow facility (cross section 60x60cm, $W$x$H)$, 3.5m downstream of the inlet at a bulk flow velocity of $U_{b}=$0.17m/s (Re$_{b}=U_{b}H$/$\nu =$102x10$^{\mathrm{3}}$, $\delta _{\mathrm{0.99}} \quad =$ 5.0cm, Re$_{\mathrm{\theta }}=$891). The measurement volume was a horizontal slab (6x1.5x6cm$^{\mathrm{3}}$, $l$x$w$x$h)$ extending from the side wall, 30cm above the bottom. The TomoPIV setup comprised four highspeed ImagerPro HS cameras (2016x2016pixels), a highspeed laser (Nd:YLF, Darwin Duo 80M, Quantronix), optics/prisms and data acquisition/processing software (LaVision, DaVis8.2). A sphere with diameter, $D=$6mm ($D^{+}=$51, ``$^{\mathrm{+}}$'' denotes inner wall scaling), was positioned at $y=$37.5 and 5.4mm ($y^{+}=$319 and 46) from the wall (measured from the sphere's center). The latter position covers most of the buffer layer while the former is well in the outer layer. Sphere Reynolds numbers based on $D$ and the average streamwise velocity at the sphere's center were 984 ($y^{+}=$319) and 684 ($y^{+}=$46). Results show the interaction between the coherent turbulence structures in the TBL and those generated in the sphere's wake. Total and partial destruction of the loglaw layer is observed when the sphere is positioned in the buffer and outer layer, respectively. [Preview Abstract] 
Sunday, November 22, 2015 8:13AM  8:26AM 
A19.00002: Effect of viscosity stratification on stability of axisymmetric boundary layer Nirmal Jayaprasad, Vinod Narayanan Stability analysis of heated axisymmetric boundary layers explores the stability characteristics of different types of fluids flowing over a heated cylindrical body. In this work, an incompressible, laminar flow over a heated and cooled cylinder, where the flow direction is parallel to the axis of cylinder at steady state is numerically simulated by solving the coupled Navier Stoke's equation and energy equation in cylindrical coordinates. Parallel flow assumption is used to obtain the stability equations. The viscosity variation with temperature is incorporated by using an empirical relation. Since air and water show opposite trend of viscosity variation with temperature, these fluids are considered. The analysis is performed for a range of Reynolds numbers and different wave numbers. The results show that heating stabilizes the flow of water but it has a destabilizing effect on air flow. The effect of Peclet number on the stability characteristics of the flow is also studied. Neutral stability curves of axisymmetric flow of air and water for various temperatures of the cylindrical body are also computed. More detailed results will be presented at the time of conference. [Preview Abstract] 
Sunday, November 22, 2015 8:26AM  8:39AM 
A19.00003: Boundary layer similarity flow driven by powerlaw shear over a nonlinearly stretching surface Daniel Kubitschek, Patrick Weidman Similarity solutions of the boundary layer equations describing wallbounded flow driven by rotational velocities, $U(y) = y^\alpha$, as $y \to \infty$, over a nonlinearly stretching surface, $U(x) = \lambda x^\sigma$ for permissible exponents, $\sigma = \alpha/(\alpha + 2)$, are presented. An exact solution is presented for $\alpha = 1/2$ in terms of Airy functions. Numerical results for the wall shear stress and sample velocity profiles in the range $2/3 < \alpha \le 5/4$ are computed. The limiting values for $\lambda < 0$ are determined, for each value of $\alpha$, beyond which no solutions are found. The existence of solutions in the range $2/3 < \alpha < 1/2$ is confirmed and the necessary condition, given by M. Guedda (2007), is shown to be satisfied. [Preview Abstract] 
Sunday, November 22, 2015 8:39AM  8:52AM 
A19.00004: The effect of thin liquid films on boundarylayer separation Radu Cimpeanu, Demetrios Papageorgiou, Marina Kravtsova, Anatoly Ruban In this study we develop the theory for understanding the process of boundarylayer separation in the presence of a thin liquid film. The investigation is physically motivated by the accumulation of water on aircraft surfaces as a result of flying during adverse weather conditions, with implications in aircraft safety, certification and performance. We present an extension of the asymptotic framework of viscousinviscid interaction and formulate a modified tripledeck model accounting for the strong density and viscosity contrast between the fluids in the system. The primary goal of the study is to address the question of whether the thin liquid layer acts to suppress or promote boundarylayer separation. We find that an increase in liquid film height (within its asymptotic scaling) contributes to a delay in the onset of separation. Furthermore, the main flow features, represented by local extrema in the perturbed flow quantities, are shifted further downstream within the interaction region. The consequences of the presence of the liquid film are illustrated through two typical examples encountered in flows past aircraft wings, namely surface roughness elements and corners/flap junctions. [Preview Abstract] 
Sunday, November 22, 2015 8:52AM  9:05AM 
A19.00005: The motion induced between radial extensional plates with one or both plates shrinking Patrick Weidman, Enrico Perocco Flow between the radial extensional motion of parallel plates is studied when one plate stretches at rate $a$ while the other shrinks at rate $b$, and also when both plates shrink. The flow is governed by the stretching ratio $\sigma = b/a$ and the Reynolds number $R = ah^2/\nu$, where $h$ is the plate separation distance and $\nu$ is the fluid kinematic viscosity. When both plates shrink one can find solutions in the region $\sigma < 1$ from those found in the region $1 \le \sigma \le 0$. This feature is not available when one plate stretches and the other shrinks, and thus $\sigma$ must be varied over the region $\sigma \le 0$. The $R = 0$ solutions and their large$R$ asymptotic behaviors are determined. Using two numerical techniques, no bifurcated solutions were encountered. Results are presented for upper and lower wall shear stresses, radial pressure gradients, and radial velocity profiles for these axisymmetric flows. A region of zero wall shear stress exists for stretching/shrinking plates whilst the wall shear stresses for shrinking/shrinking plates are never zero. An interesting singular limit in solution behavior as $R \to \infty$ is found for the shrinking/shrinking plate flow. [Preview Abstract] 
Sunday, November 22, 2015 9:05AM  9:18AM 
A19.00006: Investigation of turbulence structure over impermeable and permeable rough walls with identical topography Taehoon Kim, Gianluca Blois, Jim Best, Kenneth Christensen Turbulent flow over complex topographies, both impermeable and permeable, is encountered in a broad range of natural and engineering systems. Wall permeability gives rise to significant modifications of the underlying flow structure owing to modified wall boundary conditions: slip and penetration. Across this interface, remarkable flow interactions occur and govern significant mass and momentum exchange resulting in apparent modification of the turbulence. In addition, the topography (roughness) of the surface modifies the nearwall flow. The current investigation explores the role of permeability and topography in turbulent flow through the use of spherebased impermeable (single layer of hemispheres) and permeable (two layers of spheres) walls. Flow across the permeable interface was accessed using a refractiveindex matching technique, that permitted high resolution particleimage velocimetry (PIV) measurements to be made in the streamwisewallnormal ($x$ x $y)$ plane. This paper will detail analysis of the first and second order velocity statistics associated with these two different cases. [Preview Abstract] 

A19.00007: ABSTRACT WITHDRAWN 
Sunday, November 22, 2015 9:31AM  9:44AM 
A19.00008: Using refractive index matching to image flow above and within a highlypermeable laboratory stream bed Derek Lichtner, Jim Best, Gianluca Blois, Taehoon Kim, Kenneth Christensen Turbulent flow over a rough, porous gravel bed is investigated with particle image velocimetry (PIV) and refractive index matching (RIM). A model stream bed was constructed with 4224 precast acrylic spheres (D $=$ 1.27 cm) in a fixed cubic pattern. The flow above and within the bed was measured in the streamwisewallnormal plane at Re$_{\mathrm{b}}=$ 3.20 $\times$ 10, with an image resolution of 11 Mpixel, and the flow was seeded with silvercoated hollow glass spheres ($\rho =$ 1700 kg m$^{3}$). The highpermeability of the interface in these experiments permits large, instantaneous, nearbed streamwise momentum due to vertical exchange viaturbulence. The mean velocity flow structure exhibitsa significant slip velocity at the bed interface. In the pore spaces, mean velocities are nearhorizontal and 510{\%} of the maximum free stream velocity. High Reynolds stresses near the bed, particularly around the crests of spherical roughness elements, suggest turbulence is produced by flow separation and the shedding of vortices from streambed grains. The dimensions of turbulent flow structures, determined via twopoint correlations and Galilean decompositions, appear similar to those of hairpin vortices, although the resemblance remains unconfirmed without timeresolved data. [Preview Abstract] 
Sunday, November 22, 2015 9:44AM  9:57AM 
A19.00009: The lawofthewall in mixed convection flow in a vertical channel Duncan Sutherland, Daniel Chung, Andrew Ooi, Elie BouZeid Direct numerical simulations (DNS) of mixed convection in a plane vertical channel are conducted over a range of Richardson numbers. The direction of the buoyant forces are parallel and anti parallel to the direction of the imposed mean flow resulting in buoyancyaided flow on the hot wall and buoyancy opposed flow on the cold wall. In the absence of buoyant forces the mean normal velocity profile is logarithmic and independent of the orientation of the wall. In the case of a horizontal channel, where buoyancy is orthogonal to the direction of mean flow, a correction to the loglaw for the mean normal velocity is given by MoininOhbukov similarity theory in terms of empirically determined universal functions of momentum and temperature. We attempt an an analysis of the lawofthewall for the aiding and opposing flows near the walls in a differentially heated vertical channel and develop analogous universal functions for temperature and momentum from the DNS data. [Preview Abstract] 
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