Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session R16: Aerodynamics V |
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Chair: Karen Mulleners, Leibniz University Room: 304 |
Tuesday, November 26, 2013 1:05PM - 1:18PM |
R16.00001: Unsteady Aerodynamics of Static Airfoils in Reverse Flow Andrew Lind, Anya Jones Wind tunnel experiments have been conducted on two-dimensional blunt and sharp trailing edge airfoils held at static angles of attack in reverse flow for three Reynolds numbers. The current work is aimed at advancing the understanding of fully developed reverse flow for high-speed helicopter applications, and evaluates the potential for blunt trailing edge airfoils to mitigate unsteady rotor blade airloads in this flow regime. Time-resolved particle image velocimetry measurements at post-stall angles of attack have revealed the evolution of a trailing edge vortex formed by the roll-up of vorticity generated in a separated shear layer. Proper orthogonal decomposition (POD) was applied to the flow field measurements to improve the identification and tracking of dominant flow structures. Unsteady force balance measurements have captured non-structural vibrations with frequency content which correlates well with that of the temporal coefficients for the first two POD spatial modes. These vibrations vary in frequency with angle of attack and are shown to be linked with trailing edge vortex shedding. The findings presented here give fundamental insight towards the development of efficient rotor blades for high-speed helicopters. [Preview Abstract] |
Tuesday, November 26, 2013 1:18PM - 1:31PM |
R16.00002: Experimental Investigation of Dynamic Stall on an Airfoil with Leading Edge Tubercles John Hrynuk, Douglas Bohl Humpback whales are unique in that their flippers have leading edge ``bumps'' or tubercles. Past work on airfoils modeled after whale flippers has centered on the static aerodynamic characteristics of these airfoils. In the current work, NACA 0012 airfoils modified with leading edge tubercles are investigated to determine the effect of the tubercles on the dynamic characteristics, specifically on dynamic stall vortex formation, of the airfoils. Molecular Tagging Velocimetry (MTV) is used to measure the flow field around the modified airfoils at nondimensional pitch rates of $\Omega = $ 0.1, 0.2, and 0.4. The results show that the characteristics of the dynamics stall vortex are dependent on the location relative to the peak or valley of the leading edge bumps. These characteristics are also found to be different than those observed in dynamic stall on a smooth leading edge airfoil. In specific, the location of the dynamic stall vortex appears to form further aft on the airfoil for the tubercle case versus the smooth case. [Preview Abstract] |
Tuesday, November 26, 2013 1:31PM - 1:44PM |
R16.00003: Flow Structure and Forces on an Airfoil Pitching Asymmetrically at High Reduced Frequency Patrick Hammer, Ahmed Naguib, Manoochehr Koochesfahani Previous experimental work has shown that non-sinusoidal oscillation of a pitching airfoil can greatly alter the vortical flow structure in the wake. The current study focuses on characterizing the corresponding changes in the resulting force on the airfoil. High-order computations are carried out using the FDL3DI solver developed by Visbal's group at the Air Force Research Laboratory. We will describe the influence of various computational parameters on the ability to capture with high fidelity the vortical flow structure observed experimentally. Results will be presented for the history of lift and drag forces on the airfoil, along the with their mean values, and their connection to the motion history. [Preview Abstract] |
Tuesday, November 26, 2013 1:44PM - 1:57PM |
R16.00004: Influence of the vortex shedding on the time evolution of instantaneous pressure fields and forces in rotating airfoils Arturo Villegas, Francisco J. Diez Time-resolved measurements of instantaneous pressure fields and aerodynamic loads are obtained for rotating airfoils. These allowed evaluating temporal variations in the flow field and were able to capture the evolution of vortex shedding in the wake of the rotating blade. The results show the influence of vortex shedding in the instantaneous loads. These measurements involve obtaining first the velocity field from TR-PIV. This is used to calculate the pressure field from the Poisson pressure equation, and later the forces from the integral momentum equation. The robustness of the measurements is analyzed by calculating the PIV uncertainty, and the independence of the calculated forces. Experimental mean aerodynamic forces are compared to theoretical predictions from the Blade Element Momentum theory (BEM) showing good agreement. The instantaneous pressure varied with time only in the wake due to vortex shedding. This is the first time the evolution of the instantaneous pressure field has been resolved for a rotating airfoil. The contribution to the instantaneous forces from each term in the integral momentum equation is evaluated. The analysis shows that the larger contributions to the normal force coefficient are from the unsteady and the pressure terms while the larger contribution to the tangential force coefficient is from the convective term. The method can be used to measure unsteady forces in rotating airfoils, providing useful information not just for computational studies, but also for aerodynamics, material and structural optimization and safety purposes. [Preview Abstract] |
Tuesday, November 26, 2013 1:57PM - 2:10PM |
R16.00005: Aerodynamics of S809 Airfoil at Low and Transitional Reynolds Numbers Jaime J. Carreras, Nader Laal-Dehghani, Serdar Gorumlu, Faraz Mehdi, Luciano Castillo, Burak Aksak, Jian Sheng The S809 is a thick airfoil extensively used in wind turbine design applications and model studies in wind tunnel. With increased interests in reducing energy production cost and understanding turbulence and turbine interactions, scaled down models (Re$\sim10^3$) are often used as an alternative to full scale field experimentation (Re $>10^6$). This Reynolds number discrepancy raises the issue of scaling for the airfoil performance from laboratory studies to field scale applications. To the best of our knowledge, there are no studies existing in literature to characterize the lift- and drag-coefficients of S809 airfoil at Re less than 3$\times10^5$. This study is to fill the deficit in the current state of knowledge by performing high resolution force measurements. The lift and drag measurements are carried out in Texas Tech Wind Tunnel Facility using an in-house developed dual-cell force balance. The configuration eliminates the large torque and torsion often accompanied by conventional mounts. This unique design allows us to reach a measurement accuracy of 0.02N (0.1\%). Comparative studies are performed on a two-dimensional airfoil with a smooth- as well as a well-engineered surface covered by micro-pillar array to simulate the surface conditions of a real life airfoil. [Preview Abstract] |
Tuesday, November 26, 2013 2:10PM - 2:23PM |
R16.00006: Large eddy simulation with periodic forcing of low-frequency flow oscillation near airfoil stall. Jaber Almutairi, Ibraheem AlQadi, Elteyeb Eljack The effect of periodic forcing on the airfoil performance is investigated in the present study. A large eddy simulation with a flow control technique of periodic forcing is used to remove the low-frequency flow oscillation of the NACA-0012 airfoil at a Reynolds number of 130,000 and incidence of 11.5$^{\circ}$ and thus enhancing the performance of the airfoil. The periodic forcing is introduced into the laminar boundary layer just upstream of the natural flow separation to produce perturbations in the near wall region. The amplitude of the forcing is set to be 0.3{\%} of the freestream velocity while several different frequencies based on the detected frequency of the vortex trailing edge revealed from the simulation of the natural low-frequency flow oscillation are used. It was found that periodic forcing removes the low-frequency flow oscillation, and as a consequence improve the performance of the airfoil. It has been seen that the large fluctuation of the lift and drag coefficients are entirely eliminated and the enlargement of the turbulent boundary layer that usually occurs after separation and the associated increase of the normal velocity component are reduced sharply when the periodic forcing is added. [Preview Abstract] |
Tuesday, November 26, 2013 2:23PM - 2:36PM |
R16.00007: Analysis of Dynamic Stall Through Chirp Signal Pitch Excursions Kyle Heintz, Dustin Coleman, Michael Wicks, Thomas Corke, Flint Thomas An augmentation of the typical pitching airfoil experiment has been performed where the pitching frequency and amplitude are dynamically varied in a short-time event to produce a ``chirp" trajectory, $\alpha(t)=\alpha_0 + \alpha_1(t)\sin(t\omega(t))$. The frequency evolution followed a Schroeder-phase relation, $\omega(t)=\omega_{min}+K(\omega_{max}-\omega_{min})$. The frequencies ranged from 0.5Hz to 30Hz, resulting in reduced frequencies from 0.02 to 0.1. The free-stream Mach number ranged from Mach 0.4 to 0.6, giving chord Reynolds numbers from $5\times 10^5$ to $3\times 10^6$. The airfoil was a NACA 23012 section shape that was fully instrumented with 31 flush-mounted high-bandwidth pressure transducers. The pressure transducer outputs were simultaneously sampled with the instantaneous angle of attack, $\alpha(t)$. The motivation for this study was to compare dynamic stall under non-equilibrium conditions. A particular interest is on the flow features that occur when dynamically passing between light and deep stall regimes. The results include phase analysis of aerodynamic loads, wavelet-based spectral analysis, and the determination of the intra-cycle aerodynamic damping factors. [Preview Abstract] |
Tuesday, November 26, 2013 2:36PM - 2:49PM |
R16.00008: LES of turbulent separated flow over NACA0015 at Reynolds number 1,600,000--toward the separation control by a DBD plasma actuator Makoto Sato, Kengo Asada, Taku Nonomura, Soshi Kawai, Hikaru Aono, Aiko Yakeno, Kozo Fujii Large eddy simulation of a separated flow over NACA0015 at Reynolds number 1,600,000 with angle of attack 20.1 deg. is conducted to clarify the feature of turbulent separation at high Reynolds number. The grid point is approximately 1 billion, and a high order scheme is used in this simulation. The LES result agrees with experiment data in terms of the laminar-separation bubble region, the locations of reattachment point and second separation point and $C_{p}$ distribution. In the turbulent separated flow of this simulation, the laminar-separation bubble is formed near the leading edge with turbulent transition, then turbulent boundary layer develops over the airfoil surface and the flow is separated as turbulent separation. Here, streamwise velocities in the attached region correspond to the profile of turbulent boundary layer. In addition, flow structures at \textit{Re}$=$1,600,000 are compared to those at \textit{Re}$=$63,000 about the turbulent transition, separation behavior, the space scale, time scale and so on. The most unstable frequency of the laminar separation flow at \textit{Re}$=$1,600,000 is 10-20 times of that of \textit{Re}$=$63,000 The flow scale at transition point of \textit{Re}$=$1,600,000 is about 1/15 times of that of \textit{Re}$=$63,000. [Preview Abstract] |
Tuesday, November 26, 2013 2:49PM - 3:02PM |
R16.00009: Direct Numerical Simulation of Turbulent Flows over Microstructured Surfaces at \textit{Re}$_{\tau} =$ 180 Jee Hann Ng, Rajeev Kumar Jaiman, Tee Tai Lim Direct numerical simulations (DNSs) are used to investigate drag-reducing performance of microstructured surfaces (i.e. riblets) in a turbulent channel flow. The present numerical study aims to analyze systematically the influence of riblet geometry on the near wall properties and flow structures. In previous studies, a reduction in the skin friction drag was attributed to two mechanisms: (i) riblets shield the wall from the action of near wall streamwise vortices, (ii) mitigate the cross-stream motions of these eddy structures. The present work aims to investigate the validity of these propositions for a various set of riblet configurations. To begin with, a standard V-groove riblet geometry with six different heights and spacings, spanning both drag-reducing and drag-increasing regimes, is considered. These simulations are conducted in a channel flow at a friction Reynolds number \textit{Re}$_{\tau} =$ 180, where riblets are mounted on one of the walls. The results from these simulations reveal the interactions of the near wall structures with riblets, and the modifications of these flow structures to enable the drag reduction. Finally, the effects of spacing and height of the riblets are summarized for \textit{Re}$_{\tau} =$ 180. [Preview Abstract] |
Tuesday, November 26, 2013 3:02PM - 3:15PM |
R16.00010: Impact of surface roughness on the turbulent wake flow of a turbine blade Karen Mulleners Roughened aero engine blade surfaces lead to increased friction and reduced efficiency of the individual blades. The surface roughness also affects the wake flow of the blade and thus the inflow conditions for the subsequent compressor or turbine stage. To investigate the impact of surface roughness on a turbulent blade wake, we conducted velocity field measurements by means of stereo Particle Image Velocimetry (PIV) in the wake of a roughened turbine blade in a linear cascade wind tunnel. The turbine blade was roughened at different chord-wise locations. We examined the influence of the chord-wise location of the added surface roughness by comparing their impact on the width and depth of the wake, the positions and distribution of vortical structures and the overall circulation in the wake. The associated variations in the wake's turbulence characteristics including Reynolds stresses were also explored. [Preview Abstract] |
Tuesday, November 26, 2013 3:15PM - 3:28PM |
R16.00011: Cessna-172R Airplane in Cruise and Landing Configurations: A Numerical Study of the Wing Loads and Wake Pankaj Jha The present work deals with the analysis of flight test data on a Cessna 172R airplane near University Park airport in Pennsylvania. Several tests pertaining to rate-of-climb, cruise, stall and landing were performed. Those of aerodynamic nature will be discussed. The wing loads for the cruise as well as landing configurations with various flap angles were computed using a vortex method considering horse-shoe and bound vortices. The stall speed and maximum lift coefficient of the airplane for these flap settings at a particular altitude were determined. The comparison against the processed flight data was generally very good. A detailed study will be presented. A CFD approach inspired by the author's work (Jha et al, 2013) to model wind turbine blades and wakes and classical aerodynamics problems was taken to model the airplane wings. The simulation results were also compared against the flight data. In addition, these simulations facilitated visualization and analysis of flow features of interest, like wing tip trailing vortices and their turbulence characterization. [Preview Abstract] |
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