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 A17: Aerodynamics: Fixed WingsAerodynamics
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Chair: Karen Mulleners, Ecole Polytechnique Federale de Lausanne Room: 605 |
Sunday, November 19, 2017 8:00AM - 8:13AM |
A17.00001: Effect of Reynolds Number on the Lift on a Steady Airfoil in Uniform Shear Flow Patrick Hammer, Caleb Barnes, Miguel Visbal, Ahmed Naguib, Manoochehr Koochesfahani Current understanding of airfoil aerodynamics is primarily based on a uniform freestream velocity approaching the airfoil, without consideration for possible presence of shear in the approach flow. Early inviscid theory by Tsien (1943) shows that a symmetric airfoil at zero angle of attack (AoA) experiences positive lift, i.e., a shift in the zero-lift angle of attack, in the presence of uniform positive mean shear in the approach flow. Our previous 2-D computations at a chord-Reynolds number \textit{Re} $=$ 12,000 showed that the sign of the lift at zero AoA was unexpectedly opposite to that obtained from the inviscid theory. To examine if this discrepancy is connected to the low Reynolds number of the earlier work, the current investigation utilizes 2-D and 3-D computations over \textit{Re} range of 2,000 -- 1,000,000. The results show that, indeed as \textit{Re }increases, the lift coefficient at zero AoA switches sign from positive to negative at \textit{Re} $\approx $ 100k. Furthermore, while the overall trend with Reynolds number is non-monotonic, the magnitude of the lift coefficient appears to asymptotically approach the value obtained from inviscid theory. To better understand this Reynolds number effect, the flow characteristics are examined in detail. [Preview Abstract] |
Sunday, November 19, 2017 8:13AM - 8:26AM |
A17.00002: Detailed comparison between DNS and wind tunnel experiment for an airfoil at Re $=$ 20,000 with a view towards control Joseph Tank, Gustaaf Jacobs, Geoffrey Spedding The reduction in size and weight of electronic devices in recent years has enabled the use of small flying devices that operate at Re \textless 1.5 x 10$^{\mathrm{5}}$ for a variety of applications. At these low Re, the boundary layer often separates before the trailing edge, even at low angles of attack, leading to aerodynamic behaviors that are not predicted by classical inviscid theories. There is currently no comprehensive database of airfoil data in this Re regime, where the sensitivity of the boundary layer behavior to small disturbances in the free stream often leads to discrepancies between results generated in different facilities. Here we provide experimental results generated in a wind tunnel with a low turbulence intensity for a NACA 65(1)-412 airfoil at Re $=$ 2 x 10$^{\mathrm{4}}$. Several unexpected phenomena are observed in force balance results and explanations are proposed based on PIV flow visualization. Qualitative and quantitative comparisons are made with results from a DNS code using higher-order discontinuous Galerkin methods. Internal acoustic forcing at locations dictated by Lagrangian Coherent Structure behavior is explored as a potential closed loop flow control strategy. [Preview Abstract] |
Sunday, November 19, 2017 8:26AM - 8:39AM |
A17.00003: On the Lateral Static Stability of Low-Aspect-Ratio Rectangular Wings Thomas Linehan, Kamran Mohseni Low-aspect-ratio rectangular wings experience a reduction in lateral static stability at angles of attack distinct from that of lift stall. Stereoscopic digital particle image velocimetry is used to elucidate the flow physics behind this trend. Rectangular wings of AR = 0.75, 1, 1.5, 3 were tested at side-slip angles $\beta = -10^\circ$ and $0^\circ$ with angle of attack varied in the range $\alpha = 10^\circ-40^\circ$. In side-slip, the leading-edge separation region emerges on the leeward wing where leading-edge flow reattachment is highly intermittent due to vortex shedding. The tip vortex downwash of the AR $< 1.5$ wings is sufficient to restrict the shedding of leading-edge vorticity, enabling sustained lift from the leading-edge separation region to high angles of attack. The windward tip vortex grows in size with increasing angle of attack, occupying an increasingly larger percentage of the windward wing. At high angles of attack pre-lift stall, the windward tip vortex lifts off the wing, resulting in separated flow underneath it. The downwash of the AR = 3 wing is insufficient to reattach the leading-edge flow at high incidence. The flow stalls on the leeward wing with stalled flow expanding upstream toward the windward wing with increasing angle of attack. [Preview Abstract] |
Sunday, November 19, 2017 8:39AM - 8:52AM |
A17.00004: A potential method for lift evaluation from velocity field data Guillaume de Guyon-Crozier, Karen Mulleners Computing forces from velocity field measurements is one of the challenges in experimental aerodynamics. This work focuses on low Reynolds flows, where the dynamics of the leading and trailing edge vortices play a major role in lift production. Recent developments in 2D potential flow theory, using discrete vortex models, have shown good results for unsteady wing motions. A method is presented to calculate lift from experimental velocity field data using a discrete vortex potential flow model. The model continuously adds new point vortices at leading and trailing edges whose circulations are set directly from vorticity measurements. Forces are computed using the unsteady Blasius equation and compared with measured loads. [Preview Abstract] |
Sunday, November 19, 2017 8:52AM - 9:05AM |
A17.00005: Force production and time-averaged flow structure around thin, non-slender delta wings Han Tu, Melissa Green Experimental force measurement and time-averaged three dimensional flow visualization of low Reynolds number baseline cases have been carried out on a steady flat plate delta wing. Current data will serve as steady reference for future unsteady flow and actuation cases. The comprehensive study will compare force production in highly unsteady environments, which is necessary to consider in unmanned combat aerial vehicle (UCAV) control strategies. Force measurements are carried out at angles of attack 10, 15, 20, 25 and 30 degrees. The coefficient of drag increases with angle of attack, while the coefficient of lift reaches a maximum value at 20 degrees. Time-averaged flow visualization conducted at angles of attack of 20, 25 and 30 degrees shows vortices with larger magnitude that persist farther into wake are generated at higher angles of attack. These results compare analogously with similar steady baseline experiment results of high Reynolds number conducted by collaborators. [Preview Abstract] |
Sunday, November 19, 2017 9:05AM - 9:18AM |
A17.00006: ABSTRACT WITHDRAWN |
Sunday, November 19, 2017 9:18AM - 9:31AM |
A17.00007: An Experimental Investigation of Flow past a Wing at high Angles of Attack Vipul Dalela, Rinku Mukherjee The aerodynamic characteristics for post-stall angles of attack past a single and/or multiple 3D wing(s) have been studied using a novel ‘decambering technique’ assuming the flow to be steady. It is expected that the location of separation as well as the strength of the separated flow is unsteady. The objective of this work therefore is to investigate flow at high angles of attack considering unsteady behavior.\\ The numerical technique used for this purpose that accounts for loss in camber due to flow separation is termed as ‘decambering’. Two linear functions are used to define the ‘decambering’ for the steady case, located at the leading edge and anywhere between $50\%$-$80\%$ chord.\\ Wind tunnel experiments are to be conducted to study the unsteady nature of separated flow using flow visualization techniques. An estimation of the unsteady wake will be of paramount importance. It is expected to get an experimental corroboration for the numerical decambering. A NACA 4415 wing section is being tested for a range of Reynolds numbers. It is observed from the preliminary results that the drag becomes more dominant after increasing the Reynolds number from $Re$ = $0.093\times 10^6$ to $Re$ = $0.128\times 10^6$ resulting a gentle decrease in the lift coefficient,$C_l$. [Preview Abstract] |
Sunday, November 19, 2017 9:31AM - 9:44AM |
A17.00008: PIV Study on Flow around Leading-Edge Slat of 30P30N Airfoil Ryosuke Ando, Yusaku Onishi, Jun Sakakibara We measured flow velocity distribution around leading-edge slat using PIV. Simultaneously, noise measurement using microphone was also performed. A leading-edge slat and main wing model having a chord length of 160 mm was placed in the tunnel with free stream velocity of about 26m/s and chord Reynolds number of 2.8 x 10$^{\mathrm{5}}$. Angle of attack was changed from 4 degrees to 10 degrees at two degree intervals. In this experiment, we investigated the relationship between the unsteady flow condition and the noise. At 4 degrees in the angle of attack, vortices shedding from the slat cusp were moved to the downstream. At 6 degrees or more, flow velocity distributions show that vortices were reattached on the slat lower surface and the flow in the slat cove recirculated. In FFT analysis of noise measurement, at 6 degrees in the angle of attack, there were some peaks on low frequency area and dominant peak on high frequency area was found. At 8 degrees or more, there were also some peaks on low frequency area. But dominant peak on high frequency area disappeared. [Preview Abstract] |
Sunday, November 19, 2017 9:44AM - 9:57AM |
A17.00009: An experimental study on suppressing Tip Vortex Cavitation by means of a Flexible Thread Jeonghwa Seo, Ali Amini, Seung Jae Lee, Jongyeol Park, Honggu Yeo, Mohamed Farhat, Shin Hyung Rhee Tip vortex cavitation (TVC) is an important issue in hydraulic machines and efforts to reduce it are required. The present study aims to mitigate the intensity of a TVC by attaching a flexible thread at the hydrofoil tip. As the test model, a hydrofoil with elliptical planform with NACA 16-020 cross section was used. Reynolds number, incidence angle, and cavitation number were varied to examine the cavitation suppression effects under different flow conditions. It is observed that the size of the cavitating core of the tip vortex is drastically reduced in comparison to the same flow conditions without the thread. The vortex roll-up and formation was disturbed by the fluctuations of the flexible string around the TVC, resulting in cavitation intensity reduction. In addition, the thread diameter and length were varied, to show that the technique was proved effective even for strings as short as half of the root chord length, which acted like a stiff thread. Our results demonstrate that there exists a critical thread length of , where the decrease in the cavitating core diameter converges. It is also found that this recipe ameliorates the desinent cavitation index of the TVC. [Preview Abstract] |
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