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 G17: Separated FlowsBoundary Layers

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Chair: Mattia Serra, ETH  Zurich Room: 605 
Monday, November 20, 2017 10:35AM  10:48AM 
G17.00001: Benchmark Smooth Body Flow Separation Experiments Daniel Simmons, Flint Thomas, Thomas Corke The accurate and repeatable computational fluid dynamics (CFD)based prediction of turbulent flow separation relevant to offdesign aerodynamic configurations remains an important challenge. There is a clear need for improved models that can accurately capture the essential flow physics associated with smooth body, adverse pressure gradient (APG) generated flow separation and reattachment. In this paper archival benchmark smooth body flow separation experiments are described that will be used for model development. These include: (1) attached smooth body APG turbulent boundary layer (TBL) flow, (2) APG TBL flow with incipient separation, (3) APG TBL separation with a smallscale separation bubble and (4) TBL separation with a large region of separated flow extending over a significant fraction of the smooth body surface. The model geometry is fully twodimensional and consists of a flat TBL development plate followed by a large convex ramp surface with APG controlled by a fully adjustable flexible top tunnel ceiling. Experimental results characterizing the largescale flow separation case are highlighted. [Preview Abstract] 
Monday, November 20, 2017 10:48AM  11:01AM 
G17.00002: Effects of surface roughness on an adversepressuregradient separating turbulent boundary layer Wen Wu, Ugo Piomelli Separating turbulent boundary layers over smooth and rough flat plates are investigated by largeeddy simulations. A suctionblowing velocity distribution is imposed at the top boundary to produce an adversetofavourable pressure gradient and a closed separation bubble. Sandgrain roughness in the fullyrough regime is modelled by an immersed boundary method. In the roughwall case, streamline detachment occurs earlier and the separation region is substantially larger due to the momentum deficit caused by the roughness. The adverse pressure gradient decreases the form drag and causes a thin reversedflow region below the roughness crest, so that $C_f = 0$ does not coincide with the detachment of the flow from the surface. The wake regions behind roughness elements affect the intermittency of the nearwall flow, so that upstream of the detachment point the flow can be reversed half of the time, but its average is positive. The separated shear layer exhibits higher turbulent kinetic energy (TKE); the growth of the TKE there begins earlier relative to the separation point, and the peak TKE occurs close to the separation point. The momentum deficit caused by the roughness, again, plays a critical role in these changes. [Preview Abstract] 
Monday, November 20, 2017 11:01AM  11:14AM 
G17.00003: Exact Theory of Material Spike Formation in Flow Separation Mattia Serra, George Haller We develop a frameinvariant theory of material spike formation during flow separation over a noslip boundary in twodimensional flows with arbitrary time dependence. This theory identifies both fixed and moving separation, is effective also over shorttime intervals, and admits a rigorous instantaneous limit. Our theory is based on topological properties of material lines, combining objectively stretching and rotationbased kinematic quantities. The separation profile identified here serves as the theoretical backbone for the material spike from its birth to its fully developed shape, and remains hidden to existing approaches. Finally, our theory can be used to rigorously explain the perception of offwall separation in unsteady flows, and more importantly, provide the conditions under which such a perception is justified. We illustrate our results in several examples including steady, timeperiodic and unsteady analytic velocity fields with flat and curved boundaries, and an experimental dataset. [Preview Abstract] 
Monday, November 20, 2017 11:14AM  11:27AM 
G17.00004: Lowfrequency dynamics of pressureinduced turbulent separation bubbles Julien Weiss, Abdelouahab MohammedTaifour, Arnaud LeFloch We experimentally investigate a pressureinduced turbulent separation bubble (TSB), which is generated on a flat test surface through a combination of adverse and favorable pressure gradients imposed on a nominally twodimensional, incompressible, turbulent boundary layer. We probe the flow using piezoresistive pressure transducers, MEMS shearstress sensors, and highspeed, 2D2C, PIV measurements. Through the use of Fourier analysis of the wallpressure fluctuations and Proper Orthogonal Decomposition of the velocity fields, we show that this type of flow is characterized by a selfinduced, lowfrequency contraction and expansion  called breathing  of the TSB. The dominant Strouhal number of this motion, based on the TSB length and the incoming velocity in the potential flow, is of the order of 0.01. We compare this motion to the lowfrequency dynamics observed in laminar separation bubbles (LSBs), geometryinduced TSBs, and shockinduced separated flows. [Preview Abstract] 
Monday, November 20, 2017 11:27AM  11:40AM 
G17.00005: ABSTRACT WITHDRAWN 
Monday, November 20, 2017 11:40AM  11:53AM 
G17.00006: Wallresolved Largeeddy simulation of flow past a grooved cylinder up to $Re_D=6\times 10^4$ Wan Cheng, Dale Pullin, Ravi Samtaney We present wallresolved, largeeddy simulations (LES) of flow past a groovewalled circular cylinder. Periodic spanwise boundary conditions are implemented with span up to $3\,D$. The stretchedvortex subgrid scale model is utilized in the whole domain, including regions of largescale separated flow. The circumferential cylinder surface comprises $32$ sinusoidal, spanwise groves of equal height $\epsilon$. For the first set of LES, with $Re_D=3.9 \times 10^3$ fixed and $0\le \epsilon \le 1/32$, some properties of the mean flow behave similarly to changes in the smoothcylinder flow when $Re_D$ is increased, such as shrinking meanflow recirculation length and nearconstant pressure coefficient. A second set of LES uses fixed $\epsilon/D=1/32$ and varies $Re_D$ from $3.9 \times 10^3$ to $6 \times 10^4$, the latter value reaching the beginning of the transcritical flow regime. Comparison with experiment as well as wallfriction and Q plots, will be discussed. [Preview Abstract] 
Monday, November 20, 2017 11:53AM  12:06PM 
G17.00007: The computation of the drag coefficient of the unbounded flow around a circular cylinder in the limit of zero Reynolds number Nikolaos Malamataris, Ioannis Sarris, Dimitrios Pazis, Anastasios Liakos The behaviour of the drag coefficient in the flow around a circular cylinder in the limit of zero Reynolds number is still unknown. A numerical experiment has been contacted to investigate this behaviour. This experiment is designed like a laboratory set up with tow tank boundary conditions along the top and the bottom of the computational domain. The two dimensional Navier Stokes equations are solved with Galerkin finite elements. It turns out that the domain should very long at the inlet and the transverse direction of the flow, in order for the results to be valid for the unbounded flow. Hence, supercomputing facilities are required, in order to solve the discretized system of equations which is of the order of 50 million unknowns. Results are given of Reynolds numbers ranging from $1\cdot 10^{10}$ up to 40., which is the upper limit of the steady state flow conditions for this benchmark. The numerical results are compared with the available experimental measurements in the range of $0.1 \leq Re \leq 40$. The good agreement of our computations with laboratory data add credibility to our investigation in the limit of zero Reynolds number. It is the first time that a code produces results for this flow in such a wide range of values. [Preview Abstract] 
Monday, November 20, 2017 12:06PM  12:19PM 
G17.00008: Visualization of Surface Flow on a Prolate Spheroid Model Suspended by Magnetic Suspension and Balance System Takumi Ambo, Yuki Nakamura, Taku Ochiai, Taku Nonomura, Keisuke Asai In this study, the surface flow on a 6:1 prolate spheroid model was visualized by oil flow method in the magnetic suspension and balance system (MSBS). The MSBS is a supportfree system for windtunnel test in that a model is levitated by magnetic force. In this experiment, the 0.3m MSBS was installed in the lowspeed wind tunnel. The Reynolds number was 0.5 million and the angle of attack was set 0 and 5 degrees. In addition to freelevitation tests, a thin rod simulating disturbance of a support system was placed on the model surface and the influence of support interference was evaluated. The obtained results indicate that complicated separation patterns are present even at zero angle of attack. At $\alpha \quad =$ 5\textdegree , separation pattern becomes more complicated than that at $\alpha \quad =$ 0\textdegree and the streamlines form a highly threedimensional structure. A characteristic pattern of open separation is observed and a focal point is formed at the end of the separation line. In evaluation of the support interference, the separation is delayed in the downstream of the rod, suggesting that the change of separation pattern is caused by the transition of laminar boundary layer behind the rod. These results indicate that one must take particular care to the support interference in studying threedimensional separation on a prolate spheroid. [Preview Abstract] 
Monday, November 20, 2017 12:19PM  12:32PM 
G17.00009: SupportFree Measurements of Aerodynamic Characteristics of Axial Circular Cylinders with Fineness Ratio from 0.50 to 0.75 Hayato Nagaike, Hiroyuki Okuizumi, Yasufumi Konishi, Hideo Sawada, Taku Nonomura, Keisuke Asai In this study, aerodynamic characteristics of axial circular cylinders having the fineness ratio (length to diameter, $L/D)$ of 0.50, 0.67 and 0.75 were measured using the 1m Magnetic Suspension and Balance System (MSBS) in the LowTurbulence Wind Tunnel at Tohoku Univ. The MSBS supports and controls the model using magnetic forces. All the tests were conducted at \textit{Re }$=$ 100,000 and the models were aligned with the free stream. The results of force measurements show that the drag decreases gradually in the $L/D$ range from 0.50 to 0.75 and connects continuously to the data for higher fineness ratios. This indicates that a local maximum of the drag does not exist in this range. The previous studies show that, for axial circular cylinders having $L/D$ from 1.0 to 2.0, the drag measured using a MSBS differs substantially from the value measured with sting support, however this study shows that a circular cylinder of $L/D =$ 0.50 has a drag close to that measured with sting support. This suggests that the influence of support interference is significant when a shear layer separated from the leading edge reattaches on the body or interacts near the base, but is insignificant when a separated shear layer is away from the base area. [Preview Abstract] 
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