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 T29: Separated Flows: Simulation, Modeling, and Wakes |
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Chair: Alexander Gehrke, Brown University Room: 255 A |
Monday, November 25, 2024 4:45PM - 4:58PM |
T29.00001: Pressure-gradient-based RANS model for predicting separation in transitional and turbulent flows Kevin P Griffin, Ganesh Vijayakumar, Bumseok Lee, Michael A Sprague Predicting flow separation poses a significant challenge for RANS models, particularly in transitional flows over airfoils. We propose a novel improvement to RANS models to predict incipient separation in both transitional and fully turbulent flows. Our approach modifies the eddy viscosity model in regions indicated by a pressure-gradient criterion that accounts for intermittency-determining whether the boundary layer is laminar or turbulent. This model demonstrates robust generalization across various airfoil shapes and Reynolds numbers. Applied to the NREL Phase VI wind turbine rotor, our model shows improved aerodynamic performance predictions compared to the baseline RANS model. |
Monday, November 25, 2024 4:58PM - 5:11PM |
T29.00002: 1-equation, 2-equation and Reynolds Stress Transport Modeling of Forward Facing Step Flow Robert F Kunz, Abdullah Geduk, Vishal A Wadhai, Shyam S Nair, Xiang I. A. Yang, Robert J Martinuzzi, Emil Larose Turbulent forward facing step (FFS) flows arise in many aero/hydrodynamic engineering venues. These flows are characterized by complex non-equilibrium turbulence dynamics including unsteady separation upstream and downstream of the step leading edge. Accordingly, these systems have been studied experimentally by many workers. CFD methods have also been deployed for decades using turbulence modeling methods ranging from Reynolds Averaged Navier-Stokes (RANS) though resolved turbulence simulations. |
Monday, November 25, 2024 5:11PM - 5:24PM |
T29.00003: Abstract Withdrawn
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Monday, November 25, 2024 5:24PM - 5:37PM |
T29.00004: Interaction of Taylor-Görtler and hairpin vortices during flow separation in a rotating channel flow Benjamin Sam Savino, Wen Wu DNS of a spanwise-rotating turbulent channel with a bump on the bottom wall was performed to explore the rotation effects on flow separation. This configuration represents surface imperfections in turbomachinery that may degrade the performance. Simulations were performed at $Re_b$=$U_b H/\nu$= 2500. A non-rotating case was compared with counter-clockwise (+) and clockwise (-) rotating cases at rotation numbers $Ro_b=2\Omega H/U_b$= $\pm$ 0.42 and $\pm$1.0. It was observed that G\"{o}rtler vortices on the anticyclonic side of the channel dominated the three-dimensionalization of the separating shear layer. Correlations were observed between absolute vorticity ratio ($S=\Omega/\Omega_s$ where $\Omega_s$ is the mean shear rotation) thresholds of $S$= -1 and $S$= -0.5 and regions of elevated Reynolds stresses. The one-dimensional destabilization layer ($-1<S<0$) in a smooth rotating channel was enlarged to two-dimensional regions downstream of the bump, due to near-wall deceleration caused by adverse pressure gradients (APGs). On the anticyclonic side of the channel, regardless of flow separation occurrence, turbulent hairpin vortices were ejected into these regions by the G\"{o}rtler vortices. The expanded destabilization region augmented the lifted hairpin vortices, allowing them to persist longer and extract significant energy from the mean flow. This resulted in elevated turbulence far away from the wall, persisting for more than 50 bump heights downstream of the bump. |
Monday, November 25, 2024 5:37PM - 5:50PM |
T29.00005: Characterization of Unsteady Separation on a Surrogate Cargo Aircraft Model with Edge Rounding Jacob Biesinger, Chitrarth Prasad, Datta V Gaitonde Aft sections of military cargo aircraft fuselages manifest high upsweep angles to accommodate ramp doors, yielding a wake dominated by an unsteady, counter-rotating longitudinal vortex pair that increases the risk of paratrooper injury, cargo drop errors, tail strikes, and drag. Streamwise oriented cylinders truncated at an angle have provided useful insights, but the sharp edge between the cylinder and the base localizes flow separation, potentially eliminating low-frequency unsteady features on aircraft fuselages which are smooth. This work uses Large Eddy Simulations to explore the effect of edge rounding which allows the separation line to adapt continually to local conditions, with accompanying effects on the wake, thus facilitating extraction of key distinctions from the corresponding sharp-edged configuration dynamics. The interpretation of instantaneous streamlines is achieved with meaningful, synchronized instantaneous wall shear stress components and connected to the visualization and characterization of separation locus dynamics. The range of separation line movement is relatively small near the upstream apex (start of the basal upsweep), but increases in the circumferential direction towards the downstream apex on the upper side. The separation locus is well described by Spectral Proper Orthogonal Decomposition modes at low frequencies, specifically St = 0.13, approximately an order of magnitude lower than key unsteady features in the sharp-edge case. |
Monday, November 25, 2024 5:50PM - 6:03PM |
T29.00006: An experimental investigation of a high Reynolds number turbulent wake generated by a vehicle-like bluff body SAMARESH MIDYA, Sean P Symon Flow past bluff bodies is common in nature and has significant scientific and practical importance. The majority of goods transporters have bluff bodies, and the volume they must transport puts a limit on aerodynamic shape optimization. Owing to the adverse pressure gradient, the boundary layer developed on bluff bodies separates easily and creates a great deal of pressure drag. Oftentimes, these flows develop large-scale quasi-periodic structures, thus creating unsteady forces that affect the stability of the body itself. Despite being the subject of several basic studies, there aren't many experimental studies that completely charaterize each velocity component of a bluff body's turbulent wake at high Reynolds numbers. A study was conducted utilising stereo-particle image velocimetry (PIV) to fully characterise the wake of a vehicle-like bluff body at Reynolds number 3.43x105 (defined as zU∞/ν, where z is the spanwise width of the model (=514.3 mm), U∞ is the free stream velocity, and ν is kinematic viscosity). Every instantaneous velocity component was measured at cross-stream planes (with a resolution of 1 mm2) at different streamwise locations (100 mm apart), starting from the trailing edge of the model. The flow is visualized, and the basic features are extracted (e.g., the separation bubble, corner vortices) and characterized using different techniques (e.g., Singular Value Decomposition). The effect of these structures on the unsteady forcing on the body is discussed, and comments are made on possible control scenarios. This work was supported by the EPSRC (Grant No. EP/W009935/1). |
Monday, November 25, 2024 6:03PM - 6:16PM |
T29.00007: Kirigami sheets in disturbed flow Yahya Modarres-Sadeghi, Adrian G Carleton It has been shown in the past that kirigami sheets placed in uniform fluid flow can produce predictable wakes and potentially be used as flow control devices. In practical applications, however, it is expected that there will often be non-uniform incoming flow interacting with the kirigami sheets. The question then arises of how the response of the kirigami sheet and therefore its wake changes when it is placed in non-uniform incoming flow. In this work, we investigate the response of kirigami sheets with different cut patterns in disturbed flow by placing the kirigami sheets in the wake of a cylinder whose rotation can be imposed externally such that the vortices that are shed in its wake can be controlled in their shedding frequency and strength. We investigate the influence of non-uniform incoming flow on the response of kirigami sheets by quantifying the sheet's displacement synchronized with the PIV wake visualizations for different types of kirigami patterns (ribbon cuts, fractal cuts, and closed cuts) exposed to a variety of unsteady incoming flow, such as vortices at different frequencies and strength, and uniform flow approaching the sheet at an angle. |
Monday, November 25, 2024 6:16PM - 6:29PM |
T29.00008: Experimental investigation of the characteristics of the wake structure behind a structured porous square cylinder Chansoo Seol, Taehoon Kim When the flow passes two-dimensional porous cylinders, the downstream wake is influenced by the mutual interaction between longitudinal and lateral bleedings. In this context, the characteristics of the wake were experimentally explored. The present porous cylinders, whose cross-sections are square (D = 24-42 mm, where D is the cylinder diameter), consist of a periodic and scalable structure based on a simple cubic lattice. By scaling the unit cell with a consistent ratio of the unit-cell length to the strut size, cylinder permeability is successfully isolated from the effect of porosity. Consequently, the structural features of the bleeding flows and their corresponding wake characteristics are systematically investigated under different levels of permeability and compared to the flow past a solid body with an equivalent diameter. |
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