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 A21: Separated Flows I |
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Chair: Andrzej Domaradzki, University of Southern California Room: 30B |
Sunday, November 18, 2012 8:00AM - 8:13AM |
A21.00001: Numerical simulation of flow over three-dimensional dunes Ugo Piomelli, Mohammad Omidyeganeh We performed large-eddy simulations of the flow over a series of three-dimensional dunes at laboratory scale (the Reynolds number based on the average channel height and mean-streamwise velocity is 18,900). The three-dimensionality is imposed by shifting in the streamwise direction a standard two-dimensional dune shape according to a sine wave with an amplitude $A$ and a wavelength $\lambda$. The three-dimensional separation of flow at the crest-line alters the distribution of pressure gradient in the spanwise direction and may result in secondary flows across the stream. The secondary flow directs low-momentum fluid, near the bed, toward the ``lobe'' (the most downstream point on the crest-line) and high-momentum fluid, near the free surface, toward the ``saddle'' (the most upstream point on the crest-line). The behaviour of the reattachment length varies depending on the induced secondary flow. Three-dimensionality increases the drag in the channel and the turbulent-kinetic energy at constant flow discharge, but the normalized TKE by the wall stress is lower than the corresponding 2D dunes. The upward flow on the stoss side and higher deceleration of flow on the lee side, over the lobe plane, elevate and broaden the separated-shear layer, respectively, affecting the TKE. [Preview Abstract] |
Sunday, November 18, 2012 8:13AM - 8:26AM |
A21.00002: Numerical simulation of the flow over Barchan dunes Mohammad Omidyeganeh, Ugo Piomelli, Kenneth T. Christensen, Jim Best We performed large-eddy simulation of the turbulent flow over a typical barchan dune model. The configuration is similar to that of experiments carried out at the University of Illinois, but the Reynolds number based on the free-surface velocity and the dune height is one fifth of the experiment. The simulation adopts the volume-of-fluid technique to model the dune. The use of periodic boundary conditions in the streamwise and spanwise directions implies that we are considering a fully developed flow over one dune in an infinite array. The height of the domain is close to the thickness of the approaching boundary layer, upstream of the dunes in the experiment. The resolution used is close to a typical DNS; $\Delta x^+<20.7$, $\Delta y^+<0.8$, and $\Delta z^+<10.3$. The approaching flow to the dune accelerates over the stoss (upstream) side and rises up to the crest, while at the same time diverging slowly in the spanwise direction toward the closest horn. The separated flow either reattaches on the plane or moves helically inside the recirculation zone toward the closest horn. The separated shear-layer extends downstream and toward the free-surface and contribute to downstream dunes. The agreement of the turbulence statistics with the experiment is good. [Preview Abstract] |
Sunday, November 18, 2012 8:26AM - 8:39AM |
A21.00003: LES of a ducted propeller with rotor and stator in crashback Hyunchul Jang, Krishnan Mahesh A sliding interface method is developed for large eddy simulation (LES) of flow past ducted propellers with both rotor and stator. The method is developed for arbitrarily shaped unstructured elements on massively parallel computing platforms. Novel algorithms for searching sliding elements, interpolation at the sliding interface, and data structures for message passing are developed. We perform LES of flow past a ducted propeller with stator blades in the crashback mode of operation, where a marine vessel is quickly decelerated by rotating the propeller in reverse. The unsteady loads predicted by LES are in good agreement with experiments. A highly unsteady vortex ring is observed outside the duct. High pressure fluctuations are observed near the blade tips, which significantly contribute to the side-force. This work is supported by the United States Office of Naval Research. [Preview Abstract] |
Sunday, November 18, 2012 8:39AM - 8:52AM |
A21.00004: An investigation of the dynamics of marine propeller tip vortices using large-eddy simulations Seth Schroeder, Elias Balaras The ability to capture the dynamics of tip vortices, which are generated by marine propellers, is of major interest to naval hydrodynamics designers. The tip vortex of a propeller has a direct impact on performance and acoustics. Additionally, the tip vortex is a major source of erosion damage on downstream components such as rudders and stators. In the present study we utilize large-eddy simulations to compute the flow around a generic, 7-bladed, right-handed submarine propeller in open water testing configuration. We considered three different advance coefficients at Reynolds number (based on the radius and advance speed) of the order of 300,000. The governing equations are discretized on a structured grid in cylindrical coordinates and the boundary conditions on the surface of the propeller, which is not aligned with the grid lines, are introduced using an immersed boundary method. Approximately 1 billion points is used in the computation box. Tip vortices are identified by low pressure areas and the second invariant of the velocity gradient tensor (Q-criterium). In general, the vortex core radius contracts with the acceleration in the wake, and then maintains a constant radius for a certain distance before becoming unstable. Stability is affected by the advance ratio. [Preview Abstract] |
Sunday, November 18, 2012 8:52AM - 9:05AM |
A21.00005: Large Eddy Simulation of a Gas-Turbine Model Combustor Yee Chee See, Matthias Ihme Gas-turbine combustors typically utilize swirling fuel-preparation strategies for flow-stabilization and flame-anchoring. Under such conditions, the flow inside the combustion chamber is highly unsteady and usually accompanied by dynamic flow structures such as precessing vortex cores. Due to this unsteadiness, steady-state flow solvers are not capable of accurately predicting the flow-field. In this study, simulations of a gas-turbine model combustor are performed using unsteady Reynolds-averaged Navier-Stokes (URANS) and large eddy simulations (LES). Simulation results are compared with experimental data to assess the capability of these modelling-techniques in predicting swirling flows under gas-turbine relevant flow-field environments. [Preview Abstract] |
Sunday, November 18, 2012 9:05AM - 9:18AM |
A21.00006: LES of Separated Flows Over an Airfoil at Moderate Reynolds Numbers G. Castiglioni, J.A. Domaradzki, M. Grilli, S. Hickel Separation effects strongly affect flows for unmanned aerial vehicles, micro air vehicles, and rotating machinery, e.g., wind turbines, propellers, and low pressure turbines. The Reynolds number for such flows are usually moderate and they can be accurately simulated using DNS. However, the large computational cost of DNS makes this technique unsuitable for industrial applications while less expensive RANS and LES techniques encounter difficulties in simulating such flows because they consist of a mixture of laminar, separated, transitional, and turbulent regions. In this work we investigate an ability of LES to accurately predict the behavior of such flows on a benchmark case of a laminar separation bubble on a NACA-0012 airfoil at $Re_c = 5 \times 10^4$ at 5 deg of incidence for which accurate DNS results are available (Jones et al., JFM 602, 175 (2008)). Using a conservative immersed boundary method with the Adaptive Local Deconvolution Method (ALDM) we have performed 2D and 3D simulations of this flow with resolution reduced drastically from that in the benchmark DNS. The results to date show good predictions for the pressure coefficient $C_p$ and the location of the separation point, but the friction coefficient $C_f$ is not predicted accurately. [Preview Abstract] |
Sunday, November 18, 2012 9:18AM - 9:31AM |
A21.00007: DNS and LES of Separated Flows at Moderate Reynolds Numbers F. Cadieux, J.A. Domaradzki, T. Sayadi, S. Bose, F. Duchaine Flows in rotating machinery, for unmanned and micro aerial vehicles, wind turbines, and propellers consist of different flow regimes. First, a laminar boundary layer is followed by a laminar separation bubble with a shear layer on top of it that experiences transition to turbulence. Subsequently, the separated turbulent flow reattaches and evolves downstream from a nonequilibrium turbulent boundary layer to an equilibrium one. Typical RANS and LES turbulence modeling methods experience difficulties when simulating such flows because they were developed for fully developed turbulent flows. This currently leaves DNS as the only reliable but computationally expensive alternative. Our work assesses the capability of LES to reduce the resolution requirements for such flows. Flow over a flat plate with suitable velocity boundary conditions away from the plate to produce a separation bubble is considered. Benchmark DNS data for this configuration was generated with the resolution of $50 \times 10^6$ mesh points; also used was a different DNS database with $15 \times 10^6$ points reported by Spalart and Strelets in JFM 403 (2000). Employing two codes, one using structured and another unstructured mesh, we concluded that accurate LES are possible using O(1\%) of the DNS resolution. [Preview Abstract] |
Sunday, November 18, 2012 9:31AM - 9:44AM |
A21.00008: Aero-optical analysis of a separated shear layer using large-eddy simulation Kan Wang, Meng Wang The aero-optical effects of a separated shear layer over a cylindrical turret are investigated based on the fluctuating density field obtained from large-eddy simulations. Good agreement of the velocity statistics and OPD$_{rms}$ with experimental data validates the simulation results. The optical aberrations are compared with those caused by the turbulent boundary layer prior to separation in terms of their magnitude and structure, underlying physical mechanisms and Reynolds number dependence. It is found that the distortions by the separated shear layer are five times larger and spatially less homogeneous than those by the attached boundary layer. Pressure fluctuations are confirmed to be the main cause of aero-optical distortions, in contrast to the dominant role of entropy fluctuations in the turbulent boundary layer. With turbulent separation, the optical distortions induced by the shear layer show only weak Reynolds number dependence, which again differs from the case of a turbulent boundary layer. [Preview Abstract] |
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