Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session A33: Flow Control I: Coherent Structures and Turbulence |
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Chair: Anne-Marie Schreyer, RWTH Aachen University Room: 255 E |
Sunday, November 24, 2024 8:00AM - 8:13AM |
A33.00001: Spike formation theory in three-dimensional flow separation Sreejith Santhosh, Haodong Qin, Bjoern F Klose, Gustaaf B Jacobs, Jérôme Vétel, Mattia Serra We develop a frame-invariant theory of material spike formation during flow separation over a no-slip boundary in three-dimensional flows with arbitrary time dependence. Based on the exact evolution of the largest principal curvature on near-wall material surfaces, our theory identifies fixed and moving separation. Our approach is effective over short time intervals and admits an instantaneous limit. As a byproduct, we derive explicit formulas for the evolution of the Weingarten map and the principal curvatures of any surface advected by general three-dimensional flows. The material backbone we identify acts first as a precursor and later as the centrepiece of Lagrangian flow separation. We discover previously undetected spiking points and curves where the separation backbones connect to the boundary and provide wall-based analytical formulas for their locations. We illustrate our results on several steady and unsteady flows. |
Sunday, November 24, 2024 8:13AM - 8:26AM |
A33.00002: Application potential of periodically-forced air-jet vortex generators for separation control in supersonic flow Anne-Marie Schreyer, Robin Sebastian Shock-induced separation occurs in numerous circumstances and has strong detrimental effects on the aerodynamic behavior and functionality of aerospace transportation and propulsion systems. The associated low-frequency unsteadiness of the shock/separation-bubble system can induce structural vibrations that may severely damage the structure, particularly for lightweight designs, which are not entirely structurally stiff. Among the many separation-control methods, steady air-jet vortex generators (AJVGs) show excellent effectiveness in reducing the separation length, both over rigid and compliant surfaces. However, experimental analysis with digital image correlation showed that the surface deflections induced by low-frequency shock motions over compliant surfaces can be amplified due to the influence of the AJVGs, which is an undesired side effect. First large-eddy simulations of periodically-forced AJVGs applied to control the large-scale separation induced in a 24° compression-ramp interaction over a rigid surface at Mach 2.5 demonstrate the capability of these devices to shift the oscillation frequency of the shock/bubble system. This behavior is promising regarding the application of such forced AJVGs to control shock-induced separation over compliant surfaces. |
Sunday, November 24, 2024 8:26AM - 8:39AM |
A33.00003: Effect of local surface roughness on flows over airfoils at aerodynamically low Reynolds numbers Nianhua Liu, Serhiy Yarusevych The influence of a finite boundary layer trip on the flow over a NACA 0018 airfoil is investigated experimentally. Oil flow visualization along with particle image velocimetry (PIV) and direct force measurements, are conducted at aerodynamically low chord Reynolds numbers (~100,000) that produce a laminar separation bubble on a smooth model. Oil flow visualization is utilized to identify the spanwise extent of the effect of the roughness element and the changes in the baseline laminar separation bubble location and size. PIV measurements are carried in multiple planes to provide quantitative insight into the changes in the laminar separation bubble topology and dynamics due to the presence of the finite trip. A comparative analysis of the flow and direct force measurements is conducted to identify the associated effect on airfoil performance. |
Sunday, November 24, 2024 8:39AM - 8:52AM |
A33.00004: Resolvent-based estimation and control of airfoil wakes Junoh Jung, Aaron Towne We employ resolvent-based estimation and control tools to predict and mitigate velocity fluctuations in the wake of laminar and turbulent NACA 0012 airfoils. For the laminar airfoil ($Re=5000$), we apply random upstream perturbations to disrupt the periodic limit cycle caused by vortex shedding, inducing chaotic fluctuations. For the turbulent airfoil ($Re=23,000$), we consider natural turbulence on the mid-span plane and its spanwise-averaged flows. Estimation and control kernels are derived using both operator-based and data-driven approaches. The operator-based approach offers low computational costs without requiring \textit{a priori} model reduction and incorporates colored-in-time statistics of the nonlinear terms from the Navier-Stokes equations, which are interpreted as a forcing on the linear dynamics, to account for complex nonlinear dynamics. The data-driven approach, which avoids constructing linearized Navier-Stokes operators, naturally includes these colored statistics of the nonlinear terms. The Wiener-Hopf formalism is used to ensure optimal causality in the estimator and controller, enhancing real-time estimation and control performance. Specifically, we achieve real-time estimation of velocity fluctuations across the entire wake region of the laminar and turbulent airfoils and closed-loop control reducing chaotic fluctuations induced by unsteady vortex shedding and external disturbances for the laminar airfoil. Furthermore, we explore effective sensor placement for the resolvent-based estimator using \textit{a priori} estimation errors. |
Sunday, November 24, 2024 8:52AM - 9:05AM |
A33.00005: Aerodynamic Control of Forebody Flow Asymmetries on a Slender Cylinder at High Incidence using Azimuthal Bleed Bojan Vukasinovic, Edward Lee, Ari N Glezer Inherent instability of the forebody vortices that form on a cylinder at high incidence in a uniform cross flow and their interactions with the cylinder's near wake leads to unpredictable asymmetric aerodynamic loads especially in yaw. This instability is controlled by using bleed of the surface flow over the forebody through segmented azimuthal surface porosity that leads to prescribed formation of the forebody vortices and thereby induced side forces of prescribed magnitude and direction. Small-scale features of the interaction of the autonomous bleed with cross flow over the forebody and the evolution of the forebody vortices are captured using the high-resolution PIV measurements in multiple planes normal to the cylinder's axis. It is shown that bleed flow driven merely by pressure differences along the forebody surface and regulated by an internal louver shell can prescribe stable asymmetries of the forebody vortices and thereby impose side force and yaw moment of desired magnitude and sense. Moreover, the time-dependent actuation of the bleed louver enables the temporal variations of the induced aerodynamic loads on 2-3 convective time scales of the model. |
Sunday, November 24, 2024 9:05AM - 9:18AM |
A33.00006: Feedforward control of very-large-scale motions using wall deformation Carson Plamondon, Sina Ghaemi This work experimentally investigated the active control capabilities of wall-normal surface deformations for targeting very-large-scale motions (VLSMs) of a turbulent boundary layer (TBL) at a friction Reynolds number of 2600. The control system used a feedforward control scheme based on velocity measurements using a real-time particle image velocimetry (RT-PIV). A circular active surface with a diameter D roughly equal to the boundary layer height δ was designed to achieve smooth deformations with a peak amplitude of 0.07δ. Preliminary investigations characterized the frequency response of the actuator, which was followed by characterizing the actuator’s impact on the TBL when operated at a constant amplitude and frequency. These tests included deformation amplitudes of 0.02δ, 0.04δ, 0.06δ and actuation frequencies of St = 0.05, 0.10, 0.15, 0.20 where the Strouhal number is based on D and the free stream velocity U∞. Finally, the feedforward control applied an opposition strategy with different gain values using the RT-PIV measurement of streamwise velocity fluctuations. The impact of the actuation on the TBL was analyzed using a spatial filter to separate the large-scale from the small-scale motions. The active surface showed the largest attenuation of the large-scale motions in the logarithmic region. |
Sunday, November 24, 2024 9:18AM - 9:31AM |
A33.00007: Evolution of Streamwise Vortices Formed by an Inclined, Yawed Round Surface Jet within a Turbulent Boundary Layer Bojan Vukasinovic, Derek A Nichols, Barnabas Toth, Matthew C DeFore, Christopher A Harris The interaction of a round surface jet with a flat plate turbulent boundary layer [Rex = O(106)] in a uniform stream is investigated in wind tunnel experiments. The jet whose diameter d is an order of magnitude smaller than the local characteristic boundary layer thickness issues at a range of prescribed pitch and yaw angles relative to the free stream. Of specific interest are spanwise distributions of cross stream momentum flux relative to the baseline boundary layer and the evolution of uneven counter-rotating axial vortex pairs that are induced by the shear between the cross flow and the jet. It is shown that momentum flux increment through streamwise-normal planes 75d downstream of the jet’s orifice relative to the baseline boundary layer decreases monotonically with yaw angle, and can even evolve into a slight momentum deficit. While the axial vortex on the straboard side of the yawed jet becomes weaker with increasing yaw angle as the leeward side vortex intensifies it nevertheless persists downstream and can be detected even at 50d. The dominant axial vortex intensifies (as measured by its circulation) with increasing yaw and lower pitch angles and often spawns a secondary axial vortex of opposite sense near the surface. |
Sunday, November 24, 2024 9:31AM - 9:44AM |
A33.00008: Enhancing Pressure Recovery in a Serpentine Duct using Active Flow Control Ozgur Tumuklu, Michael Amitay The turbulent flow fields in a three-dimensional (3D) diffuser with massive flow separation are being computationally investigated using high-fidelity Large Eddy Simulations across a spectrum of Mach numbers from Ma = 0.4 to 0.8. For verification and validation, the unsteady characteristics and 3D flow structures are compared against temporally- and spatially-resolved stereoscopic PIV experiments conducted at the Center for Flow Physics and Control at Rensselaer Polytechnic Institute. This study examines the growth and dynamics of large-scale vortical structures as produced by multiscale physics. Similar to the experimental setup, the effects of pulsed jet arrays, segmented jets, and 2D steady and unsteady jets, along with side-wall suction, on the mean and turbulent properties will be meticulously investigated. Specifically, the turbulent intensities and turbulent kinetic energies are analyzed both with and without flow control, and their outcomes are benchmarked against experimental data. The primary objective of this research is to enhance pressure recovery and minimize flow distortion using active flow control techniques. This marks unique active control strategies for internal flows which is validated and verified experimentally, with additional determination of actuation amplitude and frequency in numerical simulations under various conditions. Following validation, further simulations will be conducted in higher Mach number regimes where experimental data is lacking, an approach anticipated to be highly beneficial for manipulating flows limited by design constraints. |
Sunday, November 24, 2024 9:44AM - 9:57AM |
A33.00009: Design and Characterization of a 3D Gust Generator Facility Carson Brashear, Manish Timsina, Agastya Balantrapu, Toby Bookman Urban air mobility (UAM) aircraft suffer from highly three-dimensional, anisotropic unsteady loads from atmospheric turbulence, coherent building vortices and upstream vehicle wakes. Existing 1D and 2D gust models that feed into autonomous flight do not accurately represent this urban turbulence and more studies are needed to develop and validate the urban gusts. We propose to generate three-dimensional highly controllable gusts using a distributed flow facility comprised of 80 mm fans that can be controlled simultaneously. We present results from preliminary experiments, using laser based diagnostics, that characterize the facility and reveal the repertoire of gusts that can be generated. The results will lay a foundation for being able to replicate the turbulent airflow profile such as helical vortices that exist in urban environments that can be used with UAM aircraft in a larger test facility. |
Sunday, November 24, 2024 9:57AM - 10:10AM |
A33.00010: Vortex generators alter particle transport in solar photovoltaics Sarah E Smith, Marc Calaf, Henda Djeridi, Martin Obligado, Raúl Bayoán B Cal Extreme heat, high winds, and airborne debris threaten production and lifespan of solar photovoltaic (PV) systems. Impacts of such hazards are dependent on local and plant-scale arrangement, where turbulent coherent structure enhancement modifies heat transfer and particle distributions for large-scale PV arrays. In lieu of costly design changes, panel-mounted vortex generators (VGs) and flow control (FC) devices promote panel cooling, but little is known relating turbulent structure formation to cooling and particle transport toward harmful impact and soiling. This work presents new perspectives on VG and FC-imposed turbulence, highlighting the role of coherent structures in transport of heat and debris in PV systems. Scaled wind tunnel experiments were developed ranging VG and FC designs. Comparing neutrally-buoyant to inertial particle-laden flow (φv ∈ [0 → 2.1 × 10-5]), time-resolved particle image velocimetry uncovers spatio-temporal variations as modified by VG and FC presence, and allows for temporal analysis of particle spatial heterogeneity through lacunarity. Embedded thermocouples connect panel cooling to structure enhancement. Results inform the complex role of turbulence in PV systems, introducing panel-scale flow manipulation as a unique solution to hazard mitigation. |
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