Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session P24: Vortex Dynamics and Vortex Flows: Propulsion and Wakes |
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Chair: Mark Stremler, Virginia Tech Room: North 224 B |
Monday, November 22, 2021 4:05PM - 4:18PM |
P24.00001: Effect of Mounting System Friction on Simultaneous Thrust and Power Production for an Airfoil in the Wake of a Cylinder Morgan L Hooper, Beverley J McKeon An airfoil interacting with the wake shed by an upstream circular cylinder is able to extract both net power and net thrust from the flow when allowed to react passively to oncoming vorticity. This has important implications for the design of small-scale passive energy harvesting devices; however such devices are engineering systems, and their real dynamics may include nonidealities such as friction. To characterize the effect of mounting system friction on the flow past the airfoil, and therefore on the thrust and power production of the system, two sets of experiments were performed. Firstly, the effects of friction were directly observed from a mechanically-mounted airfoil. Secondly, an airfoil was driven through an idealized sinusoidal trajectory, exhibiting no frictional effects. The power and thrust production for both systems are compared, and the flow structures giving rise to the observed characteristics are interrogated. Implications for small-scale energy harvesting devices are explored. |
Monday, November 22, 2021 4:18PM - 4:31PM |
P24.00002: Flow-induced vibration of a circular cylinder placed in the wake of an upstream cylinder, forced to oscillate in the crossflow direction Sarah Dulac, Hamed Samandari, Banafsheh Seyed-Aghazadeh Flow-induced vibration (FIV) of a flexibly-mounted cylinder, placed in a tandem arrangement with an upstream cylinder, is studied experimentally. The upstream cylinder was forced to oscillate with a peak-to-peak amplitude of 0.5 times the cylinder’s diameter, with frequencies in the range of 0.5 to 3 times the downstream cylinder’s natural frequency. Amplitudes and frequencies of oscillation, as well as flow forces were studied in the reduced velocity range of U* = 2.9 – 18.1, corresponding to a Reynolds number range of Re = 565 – 3,500. Additionally, qualitative and quantitative flow field measurements were conducted using hydrogen bubble imaging and a volumetric Particle Tracking Velocimetry (PTV) technique, respectively. |
Monday, November 22, 2021 4:31PM - 4:44PM |
P24.00003: Vortex patterns in the wake of a transversely oscillating circular cylinder at low Reynolds number Emad Masroor, Mark A Stremler The flow patterns that form in the wake of an oscillating cylinder give us important information about the nature and extent of vortex-induced vibrations (VIV) of cylindrical structures such as oil risers, power lines and undersea cables. To investigate these patterns at low Reynolds number, we oscillated a circular cylinder transversely to a uniform free stream in a gravity-driven soap film over a large portion of the frequency-amplitude parameter space and visualized the resulting wakes using a high-speed camera. A rich variety of vortex patterns were found in the primary synchronization region of the parameter space, including the 2S, 2P, P+S, 2T and coalescing wake patterns. The presence of 2P wakes in a (nominally) two-dimensional flow suggests that spanwise instabilities in the cylinder wake are not necessary for the formation a 2P wake, since these instabilities are suppressed in our experiments. We will discuss these results, as well as the results of a follow-up study involving the oscillation of an extremely thin circular disk, instead of a cylinder, which was conducted to test alternative hypotheses for the formation of the 2P wake. |
Monday, November 22, 2021 4:44PM - 4:57PM |
P24.00004: Influence of operating conditions on wake recovery of a yawed tidal stream turbine Pranav Kashyap Modali, Arindam Banerjee Yawed inflow results in performance degradation and downstream wake deflection of a tidal stream turbine (TST). Previous studies have explored performance degradation and wake recovery for yawed TSTs operating at rated rotational speed (λrated). However, array operators often need to operate the turbines at speeds slower than λrated to reduce fatigue load on blades and increase the operational life of the turbine. Momentum and energy recovery mechanisms under those off-design conditions are not well known. We will discuss detailed near-wake laboratory measurements for a 30° yawed turbine at three different operating conditions (λrated, 0.5λrated, and 0.1λrated). Increasing λ was observed to directly influence the wake deflection magnitude for yawed inflow, increasing the cross-stream advection, which is the largest contributor to momentum recovery. We will discuss various insights from our study using an acoustic Doppler velocimeter on the dependence of turbine rotational speed on wake recovery for yawed inflows. |
Monday, November 22, 2021 4:57PM - 5:10PM |
P24.00005: wake-induced vibration of two cylinders with time-varying gap distances Yaqing Jin, Pengyao Gong, Dhanush Bhamitipadi Suresh, Matthew J Zaksek The flow-induced vibration of an elliptical cylinder in the wake of a moving circular cylinder with time-varying gap distances was experimentally investigated. A time-resolved particle image velocimetry was used to characterize the wake dynamics and vibrations of elliptical cylinder across various initial gap distances and upstream cylinder moving speeds. Results show that after the movement of upstream cylinder, the time-series of downstream cylinder vibration can be divided into two regions. In the first region, the vibration patterns remain almost the same as those with constant gaps due to the time required for wake transport. In the second region, the vibration intensity gradually decreases with time, where the decaying rate increases with the upstream cylinder moving speed and decreases with the initial gap distances. Considering the time-varying vorticity magnitudes impinging the elliptical cylinder, a theoretical framework is applied to predict its vibrations across time. The uncovered wake-induced-vibration patterns with time-varying gap distances provide insights to understand the fluid-structure interaction mechanisms within unsteady flow environments. |
Monday, November 22, 2021 5:10PM - 5:23PM |
P24.00006: The wake of two side-by-side elliptic cylinders Prabakaran Rajamanickam, Sarah E Morris, Abbishek Gururaj, Brian S Thurow, Vrishank Raghav When two or more cylinder-like structures are placed in close proximity, the resulting interactions between the wakes are complex and known to alter the wake pattern significantly from those behind a single cylinder. One commonly investigated arrangement of two cylinders is the side-by-side configuration, wherein two identical cylinders are placed parallel to the incoming flow. In this work, we consider two oval-shaped (elliptical) cylinders with large values of eccentricities. The flow past these cylinders is studied experimentally in a water-tunnel setup by parametrically investigating the wake pattern for various values of gap ratio between the cylinders (0 - 2), Reynolds numbers (100 - 2000) and two eccentricity values. The three well-known wake regimes (single-body wake, asymmetric wake, two parallel wakes) are first quantified by dye visualization and then by planar Particle Image Velocimetry (PIV) technique. A bifurcation diagram, having the measured values of Strouhal numbers at specified locations in the wake and the gap ratios as coordinates, is presented, that identifies various wake modes of the side-by-side cylinders. |
Monday, November 22, 2021 5:23PM - 5:36PM |
P24.00007: Effect of harmonic inflow perturbation on the wake vortex dynamics of a 2-DOF cylinder undergoing vortex-induced vibration near a plane boundary Maziyar Hassanpour, Christopher R Morton, Robert J Martinuzzi The flow development over a fixed circular cylinder is highly sensitive to the presence of harmonic waves superposed on the inflow. Vortex-shedding ‘lock-on’ occurs when the wave frequency (fp) is within about 1.8-2.2 times the natural Strouhal vortex shedding frequency (fo). In contrast, the effect of incipient harmonics waves on an elastically mounted circular cylinder remains largely unexplored. In this work, vortex-induced vibration (VIV) of a 2DOF cylinder with superposed harmonic waves is investigated numerically at a constant Reynolds number of 200 for 1.5fo < fp < 3.5fo. The results show that adding harmonic waves to the inflow can induce a type of ‘lock-on’ involving excitation of a new mode involving highly complex and irregular vortex interactions. The Strouhal frequency, the system’s natural frequency and their inter-harmonics persist in the structural response for all cases, demonstrating that the observed dynamics differ fundamentally from those observed for a fixed cylinder. The harmonic wave frequency is detectable only in the x-direction vibration spectrum, indicating that the inline degree of freedom plays a critical role in triggering nonlinear interactions between perturbations and the shedding dynamics. |
Monday, November 22, 2021 5:36PM - 5:49PM |
P24.00008: Influence of the Boundary Layer State on the Wake of a Cantilevered Square Cylinder Robert J Martinuzzi, Ali Mohammadi, Christopher R Morton Differences in the mean wake topology of a cantilevered square cylinder of aspect ratio 4 are related to the influence of the thin, on-coming boundary layer state on interaction involving periodically shed vortices. Surface pressure and particle-image velocimetry measurements are conducted for a laminar boundary layer (LBL), of thickness-to-obstacle height ratio of δ/h≈0.05 , and a turbulent boundary layer (TBL) of δ/h≈0.18 at Reynolds number ∼104, based on the obstacle width. A dipole, consisting of a pair of counter-rotating streamwise vortices extending from the obstacle free-end region, characterizes the TBL wake. An additional vortex pair descending from the dipole exists for the LBL wake. The descending vortices arise due to the interaction between successive shed vortices from opposing sides. Along the descending vortices, the Reynolds stress and turbulence production rates are significantly higher than observed for the TBL case. In contrast, the interaction between the horseshoe and shed vortices in the obstacle-ground junction region are only observed for the TBL case. Complementary oil-film visualisations of the surface flow patterns show evidence of the mean signature of these interactions and differences in the attachment point topology between the two wakes. |
Monday, November 22, 2021 5:49PM - 6:02PM |
P24.00009: Spanwise Wake Structure of a Yaw-Oscillating Cylinder at Subcritical Flow Vahid Nasr Esfahani, Ronald E Hanson, Alis Ekmekci An experimental study is carried out on the spanwise variation of the near wake of a yaw-oscillating circular cylinder. The yaw angle during oscillations varies from θ = 0o (exposing cylinder directly to crossflow) to θ = 30o at two Reynolds numbers of 5×103 and 1.5×104. Two different length-to-diameter ratios of 20 and 13 are tested. Planar Particle Image Velocimetry measurements were performed in vertical symmetry planes and horizontal planes orthogonal to the axis of the non-yawed cylinder to investigate the flow structure in the near wake region of the yaw-oscillating cylinder. The range of oscillation frequencies, which when expressed non-dimensionally in terms of reduced frequencies (K) vary between K = 0.5 to 4. For a cylinder undergoing yaw oscillation, the near wake is highly three-dimensional and the spanwise variation with the phase of oscillation is significantly dependent on the yaw angle and the direction of motion of the cylinder. Increasing yaw angle in the first half of the oscillation cycle leads to a shorter wake closure length and occasional suppression of the mean recirculation region (depending on the reduced frequency) near the upper-middle section of the cylinder whereas the wake closure length is generally elongated on the lower-middle section. This trend is reversed when the cylinder returns to the cross-flow position. Moreover, in the return cycle, at yaw angles in the range of θ = 15o to 30o, the strong axial flow generated from the upstream end of the cylinder develops on the lower half of the span of the cylinder intensifies the suppression of the mean recirculation region. The axial flow extends more rapidly on the span of the cylinder with the higher length-to-diameter ratio at moderate reduced frequencies. However, at high reduced frequencies, the role of axial flow is shown to be diminished and the suppression of the mean recirculation region along a large section of the span occurs under the impact of the cylinder rapid motion. |
Monday, November 22, 2021 6:02PM - 6:15PM |
P24.00010: Response of an acoustically forced flame to vortex shedding of a circular cylinder Eirik Æsøy, Girish K Jankee, Srikar Y Venkata, Nicholas A Worth, James R Dawson Understanding the characteristics of structures arising from vortex shedding can be of great importance in combustors, where controlling thermoacoustic instabilities is primarily desired. An experimental study is conducted on the interaction of a circular cylinder with an oscillating flow and the subsequent response of a bluff body stabilised CH4/H2 flame to such form of acoustic forcing. The flow has a non-zero mean velocity, ¯u of 10 m/s at the exit with forcing frequency ranging from 0.05 − 1 kHz and forcing amplitude maintained at 5 % of ¯u. Measurements of the flow and flame properties were carried out through hot-wire anemometry and planar particle image velocimetry (PIV), and by photomultiplier tubes and high-speed imaging capturing OH*-chemiluminescence, respectively. The findings confirm the classical case of vortex lock-on of the upstream cylinder and reveal that the flame reacts correspondingly as the heat release rate also locks onto the sub-harmonic, akin to the vortex shedding. These results highlight the potential in controlling the onset of thermoacoustic instabilities through the effect of acoustic/convective interference between vortex shedding behind cylinders and acoustic forcing of a bluff body stabilised flame. |
Monday, November 22, 2021 6:15PM - 6:28PM Not Participating |
P24.00011: Influence of a 3D near-wall plate on turbulent boundary layer structure Dimitry Ivanov, Valery Zhdanov, Igor Kukharchuk Three dimensional numerical investigation of a velocity field in a turbulent boundary layer disturbed by near-wall thin plate were made by the LES method. The plate was located at 10% of the boundary layer thickness (\delta) apart of the water channel surface. The length of the plate chord and its spanwise size were 0.01 m and 0.024 m, respectively. The Reynolds number based on the velocity at the channel centerline and its half-height was 7750. The wall-normal profile of the mean velocity behind the plate increased in the log region up to x/\delta ≤ 3.8 (where \delta ) and fluctuations decreased within an interval 0 ≤ x/\delta ≤ 0.8 behind the plate. The longitudinal fluctuations became minimal at x/\delta = 1.8 whereas the normal and spanwise ones at this locations were higher than in the undisturbed boundary layer. The numerical data correlated with the experimental ones obtained at the similar initial and boundary conditions. The analysis of the velocity field within the buffer region revealed an impact mechanism of the plate wake and edge vortices on the near-wall structure transformation. The entrainment of a near-wall fluid into a lower shear layer of the plate wake resulted in the decrease of a velocity gradient and the wall-shear stress. The shear stress reduces locally by ~ 30% and was observed up to ~ 4\delta downstream the plate. The entrainment into the upper shear layer of the wake resulted in a downward high velocity fluid from the log region to the buffer region. The edge vortices caused nonuniform distributions of the spanwise velocity in the buffer region that creates the formation conditions for a new vorticity in the region. This vorticity (small-scale longitudinal vortices) blocks the contact of a high velocity fluid, entered from the log region, with the wall, so the shear stress rise was delayed within the interval 1.2 ≤x/\delta ≤ 2.8. The shear stresses increased downstream when the longitudinal vortices system decayed. |
Monday, November 22, 2021 6:28PM - 6:41PM |
P24.00012: The law of the wake: revisited and modeled using an offset from the wall Gregoire Winckelmans, Matthieu Duponcheel We examine the wake function F(Y) with Y = y/δ: the excess velocity above the logarithmic profile of the law of the wall in wall-bounded turbulence at high Reynolds number. It is first measured using DNS data of channel flow at Reτ ≈ 5200 (which has a distinguishable overlap layer with 1/κ ≈ 2.61). The generating function Q(Y) = Y F'(Y) is seen to be significant only beyond Yc ≈ 0.16, and also linear (with a slope α ≈ 1.15) up to Y ≈ 0.5. Our Q(Y) model for that first part is then a linear ramp function that starts at Yc. Our model is furthermore extended for the second part by subtracting a quadratic term which satisfies the boundary condition at Y=1. The analytical integration of the Q(Y) model finally provides our complete F(Y) wake function model with offset Yc: it is seen to fit very well the DNS data, over the full range. The first part of our model is also usefully compared to the "extended law of the wall model" of Bernardini et al. (a model without offset). |
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