Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session A12: Separated Flows: General |
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Chair: Xiaofeng Liu, San Diego State University Room: 303 |
Saturday, November 23, 2019 3:00PM - 3:13PM |
A12.00001: Bursting of Laminar Separation Bubbles O. N. Ramesh, Abhijit Mitra The transition of a small laminar separation bubble (LSB) to a long LSB is called bursting, and this might be abrupt in some airfoils. Bursting is manifested as a significant departure of the surface pressure distribution from its inviscid pressure distribution, with a considerable drop in the peak suction pressure. In our present study of flow over Eppler387 at an angle of attack of 8 degrees, we have chosen three representative LSBs: long, transitional and short bubbles. Bursting has been often linked to the onset of absolute instability of the separated shear layer. On impulsively forcing these three bubbles, it is found that all the bubbles are convectively unstable in the high-frequency range. However, all of them, though an order of magnitude lesser in short bubbles, show tendencies of absolute instability in the low-frequency range. A self-limiting behavior for the unforced transitional and long LSBs is evident. An explosive growth of high-frequency activity is found to be strongly correlated to the low-frequency flapping motion of the longer bubbles. Negative production of turbulent kinetic energy is responsible for the runaway effect observed during bursting. [Preview Abstract] |
Saturday, November 23, 2019 3:13PM - 3:26PM |
A12.00002: Sudden kinks in proper orthogonal decomposition modes of vorticity field for a turbulent flow past a ship topside subject to oscillatory motion Xiaofeng Liu, Anish Sydney, Naipei Bi Complex flow fields above and around ship deck often exhibit a wide range of temporal and spatial features in fluid motion, which affects not only the ship performance, but also Launch and Recovery (L{\&}R) of air vehicles. To characterize the complex turbulent flow over a ship topside, Time-Resolved Particle Image Velocimetry (TR-PIV) is used in a subsonic wind tunnel test with both stationary and oscillatory modes of the ship model. The tests are conducted at a wind speed of 10.3 m/s and ship oscillations at a frequency of 2.0 Hz with an amplitude of 2.5 degrees. Proper Orthogonal Decomposition (POD) analysis, capable of extracting energetically and dynamically significant features of complex fluid flows, is applied to the time series of the planar velocity and vorticity distributions obtained by the PIV measurements. The POD analyses show that a novel phenomenon featuring significant sudden discontinuities in the vorticity POD mode pattern distributions dubbed as ``kinks'' is observed for the test model subject to oscillatory motion. In contrast, such kinks are not found in the vorticity POD modes for the test case without the model oscillatory motion, nor the velocity POD modes for both the stationary and the oscillatory tests. [Preview Abstract] |
Saturday, November 23, 2019 3:26PM - 3:39PM |
A12.00003: Dynamics of the shear layers detaching from the upstream corners of an elongated rectangular cylinder Maria Vittoria Salvetti, Alessandro Mariotti, Elena Pasqualetto, Benedetto Rocchio The behavior and the dynamics of the shear layers detaching from a rectangular cylinder, having chord-to-depth ratio equal to 5, are investigated through highly-resolved large-eddy simulations and experimental velocity and pressure measurements. The considered configuration, which is the object of the benchmark BARC, is characterized by flow reattachment on the cylinder sides in the rear part of the body. It is shown that the mean length of the separated region, and hence the reattachment length, are correlated with the position of the maximum of the fluctuating kinetic energy along the shear-layer edge. This is in turn linked with the location at which the shear layer rolls up in Kelvin-Helmotz vortexes and hence with the shear-layer transition length. The characteristic frequencies of the shear-layer dynamics are also investigated and compared with those characterizing the vortex shedding in the body wake and the unsteady aerodynamics loads. Finally, the effects of small rounding of the upstream corners on the previously mentioned quantities and phenomena are briefly analyzed. [Preview Abstract] |
Saturday, November 23, 2019 3:39PM - 3:52PM |
A12.00004: How dynamic is static stall? Julien Deparday, Karen Mulleners The static stall angle is the critical angle of attack above which the flow detaches from the airfoil's surface. When the static stall angle is exceeded, the transition from attached to separated flow is not instantaneous. The transient development is characterized by different stages and time-scales similar to the dynamic stall flow development stages and stall delays. Static stall can be seen as an extreme case of dynamic stall where there is no motion during the stall development. On a NACA0018 profile, we initiated static stall by an increase in the angle of attack from 0.3$^{\circ}$ below to 0.3$^{\circ}$ above the static stall angle in 10\% of a convective time unit. We measured the time-resolved flow field and aerodynamic forces and analysed their temporal evolution. The transition time from attached to separated flow is longer in the static stall case than in the dynamic stall cases even though the topological flow development is comparable. Special emphasis is directed towards linking the inception of stall to the motion of the leading edge stagnation point. These results will serve as a basis to improve low-order models based on indicial response. [Preview Abstract] |
Saturday, November 23, 2019 3:52PM - 4:05PM |
A12.00005: Role of an active trailing-edge flap of a pitching airfoil undergoing dynamic stall Guosheng He, Lars Siegel, Arne Henning, Karen Mulleners The flow around a pitching NACA0015 airfoil with an active trailing-edge flap is investigated using two-dimensional time-resolved particle image velocimetry and surface pressure measurements. The Reynolds number based on the chord length is about $5.5\times 10^5$. The airfoil is pitching around the static stall angle of attack of $20^{\circ}$ and the flap is either oscillating around the symmetrical plane at $\beta=0^{\circ}$ or fixed at a constant deflection angle. The pitching of the airfoil and the deflection of the flap can be individually controlled in terms of mean angle, oscillating amplitude, frequency and initial phase angle. Below the stall angle, the lift of the static airfoil increases proportionally with the increase of flap deflection angle in the investigated range of $-20^{\circ}\leq\beta\leq20^{\circ}$. Variations of the phase delay between the oscillations of the main airfoil and the flap lead to a rotation, expansion, or contraction of the dynamic stall lift curves. Higher-order harmonic flap oscillations for the pitching airfoil result in bending or twisting of the lift curves. Quantitative evidence has been extracted from the PIV data to help elucidate the modified aerodynamic characteristics of the pitching airfoil manipulated by the active flap. [Preview Abstract] |
Saturday, November 23, 2019 4:05PM - 4:18PM |
A12.00006: Enhancement of Slat Airfoil Configuration using Invasive Weed Optimization Framework coupled with Artificial Neural Networks. Sushrut Kumar, Priyam Gupta, Raj Kumar Singh This research attempts to develop an optimization scheme by integrating Genetic Algorithm and Artificial Neural Network (ANN) - surrogate model which was successfully implemented to optimize Leading Edge Slat shape and configuration. The optimization model used Invasive Weed Optimization. Bezier curves were used as an aerodynamic shape parameterization method to ensure the generation of smooth-contoured slat profiles. Geometry, Overhang, Depth and Deflection were taken as the defining parameters for each individual. These parameters were varied within a range which is 3.4{\%} to 15{\%} of slat chord for Overhang, -5{\%} to 4{\%} of slat chord for Depth, -5{\%} to 5{\%} of initial deflection for Deflection and the ordinate of shape control points had a variation of y$+$3 to y-3. The standard deviation decreases non linearly from 6 to 0.005 with a modulation index of 3. Multiple Computational Fluid Dynamics simulations were run for each individual under various operating conditions to evaluate their fitness (lift to drag ratio). The data generated from this process was used as training and test sets for the ANN. Shape control points, angle of attack, Reynolds number and operating conditions were taken as input parameters for the neural network to predict lift to drag ratio. The developed technique showed approximately 85{\%} improvement in the time taken and allowed algorithm to better explore design space. [Preview Abstract] |
Saturday, November 23, 2019 4:18PM - 4:31PM |
A12.00007: Resolvent analysis of laminar separation bubble over an airfoil at high Reynolds number using discounting and randomized linear algebra Chi-An Yeh, Stuart Benton, Kunihiko Taira, Daniel Garmann We perform resolvent analysis to examine the perturbation dynamics over the laminar separation bubble (LSB) that forms near the leading edge of an airfoil at Re$=$5E5. While we focus on the LSB residing over 6{\%} of the chord, the resolvent operator is constructed about the global mean flow over the airfoil, avoiding issues due to domain truncation. Moreover, randomized SVD is adopted in the analysis to relieve the computational cost for the high-Re global baseflow. To examine the local physics over the LSB, we consider the use of exponential discounting to limit the time horizon that allows for the instability to advect along the baseflow. In addition to discounting, we also examine the use of a spatial mask that highlights the LSB in the analysis. We find that discounting is required to reveal the stability characteristics of the LSB due to the unstable baseflow, regardless of the use of the mask. With discounting, the gain distribution over frequency accurately captures the spectral content over the LSB obtained from LES. The peak frequency also agrees with previous flow control results on suppressing dynamic stall. We show that discounted analysis is the appropriate approach for unstable baseflow and that randomized approach can expand global resolvent analysis to high-Re flows. [Preview Abstract] |
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