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 T09: Separated Flows |
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Chair: Karen Mulleners, EPFL Room: North 124 A |
Tuesday, November 23, 2021 12:40PM - 12:53PM |
T09.00001: Flow separation in a variable-area subsonic and transonic turbulent channel flow Venkatesh Pulletikurthi, Joel J Redmond, Carlo Scalo, Luciano Castillo We have numerically investigated the flow separation dynamics in a subsonic and transonic channel flow undergoing a sinusoidal contraction of the cross-sectional-area. A block-spectral high-order unstructured code, H3AMR, is used to perform direct numerical simulations (DNS) for a frictional Reynolds number of Reτ, 375. The size of the separation bubble is studied for four different blockage ratios between 10%-40%. The extent of the flow separation is investigated as a function of the local Mach number and pressure-gradient parameter. The results show a longer separation bubble for transonic flow conditions. In addition, proliferation of Q-isosurfaces are observed for transonic Mach numbers which is consistent with the increase of turbulence intensity after the reattachment. |
Tuesday, November 23, 2021 12:53PM - 1:06PM |
T09.00002: Law of incipient separation for turbulent flows over airfoils as inferred by RANS Shao-Chi Huang, Abhiram B Aithal, Antonino Ferrante The law of incipient separation over ramps by Lu, Aithal & Ferrante (AIAA J., 2021) requires the knowledge of only few geometrical parameters of the ramp and the Reynolds number of the incoming flow. In that spirit, we have performed two-dimensional RANS wall-resolved simulations of incompressible turbulent flows over airfoils. We have also developed a new grid generation method, and have carried out verification and validation of RANS using the Spalart-Allmaras model against the experimental measurements by Wadcock (NASA-CR-177450, 1987) for a NACA4412 airfoil at Rec = 1.64×106. From such comparison, we have found that although the RANS prediction fails at α≧13○, the incipient separation occurs at only α≅1○.Thus, we have investigated the effects of angle of attack, airfoil thickness, and camber on the incipiency of flow separation for eleven different airfoils and Rec ∈[1.64×106, 6×106]. From the RANS results, we have determined a law for predicting incipient turbulent flow separation that relies only on a few key airfoil's geometrical parameters and the Reynolds number of the flow. |
Tuesday, November 23, 2021 1:06PM - 1:19PM |
T09.00003: Fractional Reynolds-averaged Navier Stokes equations (f-RANS) for modeling of separated boundary layers. Pavan Pranjivan Mehta, George E Karniadakis, Luis Bravo The duality of local and non-local regions observed in turbulent flows makes defining mathematically rigorously operators a cumbersome task with literature focused on either local or non-local modeling. Recently, we showed that a variable-order fractional operator can not only model both the regimes but also seamlessly transitions from local to non-local regimes. The effect of pressure gradients further complicates the problem, especially in separated flows, which has been a long standing problem in the turbulence modelling community. Thus, in this work we investigate the role of pressure coupled with Reynolds number effects for adverse pressure gradient boundary layers, including modelling of the recirculation bubble. Upon the formulation of one- and two-sided models using Caputo fractional derivatives, we found that only the two-sided model leads to physical solutions. This is not a surprise as non-locality at a given point is an aggregate effect of all directions and the two-sided model addresses this very fact. The predictive nature of this formulation presents a model free of ad-hoc tuning coefficients thereby providing the basis for a robust engineering tool. |
Tuesday, November 23, 2021 1:19PM - 1:32PM |
T09.00004: Dynamics of a hydrofoil in the wake of a cylinder Adrian Carleton, Todd M Currier, Yahya Modarres-Sadeghi We present the dynamics of a hydrofoil free to oscillate in a plane as it interacts with vortices that are shed from a cylinder placed upstream. When the cylinder is fixed, the shedding of vortices follows the Strouhal law, and the hydrofoil responds by following several different trajectories in all of which oscillations in the transverse direction follow the frequency predicted by the Strouhal law. When the cylinder is forced to rotate in one direction, at higher rotation rates, the shedding of vortices is suppressed, and the cylinder exhibits a mainly static response. When the cylinder is forced to rotate periodically, the frequency of the hydrofoil’s response in the transverse direction follows the frequency of the forced rotation rather than following the shedding frequency predicted by the Strouhal law. Based on these observations, we discuss methods of controlling the rotation of the cylinder such that we can obtain a predetermined trajectory for the hydrofoil’s response. |
Tuesday, November 23, 2021 1:32PM - 1:45PM |
T09.00005: Effect of Reynolds number and model chord on the wake structure of a blunt trailing edge profiled body Philippe Lavoie, Ross J Cruikshank The near wake of a bluff body, and in particular the vortex street, is closely associated with the mean and fluctuating forces experienced by the body. A blunt trailing edge (BTE) profiled body geometry is the focus of the present study. Despite the spanwise uniformity of the BTE body itself, its wake is highly three-dimensional once it transitions to turbulence. At large scales, this three-dimensionality manifests as oblique shedding and vortex dislocations, which tend to modulate the aerodynamic forces. The effect of Re and model chord length, c, on the three-dimensional wake structure is experimentally investigated using a combination of hot-wire anemometry and particle image velocimetry (PIV) measurements. All the cases in the 2,800 < Re < 36,000 range investigated in this study have a turbulent wake. An important distinction is made in this study between Re > 11,000 cases, where the boundary layer transitions upstream of the trailing edge and lower Re cases where the boundary layer is laminar over the body and transitions in the wake. The wake velocity field is shown to be much less correlated for the former case. Furthermore, in the laminar boundary layer regime the spanwise correlation drops as Re increases due to the growth in the magnitude of the velocity fluctuations in the separated shear layers. As the thickness of a turbulent boundary layer increases, the strength of the shed vortices decreases and the ratio between the velocity fluctuations in the shear layers to the shedding strength increases. Consequently, we find that the spanwise correlation drops as the thickness of a turbulent boundary layer increases. The changes in the spanwise correlation is fopund to be related to the spanwise phase drift of the vortex shedding and the incidence of dislocations. |
Tuesday, November 23, 2021 1:45PM - 1:58PM |
T09.00006: Experimental Investigation of Low-frequency Unsteadiness in Trailing-edge Separation Sen Wang, Sina Ghaemi The low-frequency unsteadiness related to flow separation on wings is known for producing force-fluctuations. The present investigation explored the low-frequency unsteadiness of the turbulent separated flow near the trailing-edge of a NACA 4418 airfoil at a chord-based Reynolds number of 720,000 and angle-of-attack of 9.7°. Time-resolved planar particle image velocimetry was performed at the trailing edge section in a streamwise-wall-normal measurement plane at the midspan and a streamwise-spanwise plane parallel to the airfoil surface. The turbulent separation bubble was found to breathe at a low frequency with a Strouhal number on the order of 0.01. The spectral analysis of the flow unsteadiness and space-time correlations between the separation line and near-wall streaks suggested that the streaks were most likely responsible for the breathing motion. Proper orthogonal decomposition analysis revealed that the breathing motion consisted of two types of energetic motions: large-scale expansion and contraction, and small-scale undulation of the separation line. |
Tuesday, November 23, 2021 1:58PM - 2:11PM |
T09.00007: Effect of corner rounding and angle of attack on the dynamics of the shear layers detaching from the upstream corners of a 5:1 rectangular cylinder Gianmarco Lunghi, Elena Pasqualetto, Alessandro Mariotti, Maria V Salvetti The high Reynolds number flow around a rectangular cylinder, having a chord-to-depth ratio equal to 5, is the object of the international benchmark BARC. This configuration is characterized by shear layer separation at the upstream corners and mean-flow reattachment on the cylinder side. In Rocchio et al. (J Wind Eng. Ind. Aerod., 2020), highly-resolved LES showed that even very small values of the corner curvature radius have a dramatic impact on the numerical solution on the cylinder sides, leading to a better agreement with available experiments. In this work, we experimentally investigate the sensitivity of the flow pattern to the degree of rounding of the upstream corners and, also, to small angles of attack of the free-stream flow. A small value of the corner roundings does not produce significant differences in the shear-layer dynamics and the mean-flow reattachment point. On the contrary, for large values of the roundings the reattachment point moves upstream. Small angle of attacks produces a significative variance of the length of the two recirculation regions. As an example, for an angle of 2 degrees, the distance between the mean reattachment points on the windward and leeward sides of the cylinder is about 0.2 times the length of the side. |
Tuesday, November 23, 2021 2:11PM - 2:24PM |
T09.00008: The scales of the leading-edge separation bubble Ignazio Maria Viola, James A Smith, Gabriele Pisetta We discuss the length scales of the leading-edge separation bubble that occurs at the sharp edge of bluff and streamlined bodies at an angle of attack with the stream. We consider a flat plate with a blunt leading edge and with the chord aligned with the stream, and a thin plate at an angle of attack. We suggest predictive models of the reattachment length based on the free-streamline theory and the growth ratio of the mixing layer. The model predictions compare well with experimental data of various authors. We focus on turbulent flow conditions, where transition occurs in the separated shear layer at a negligible distance from the point of separation, viz. at thickness- and chord-based Reynolds numbers of 104 and 105, respectively. For a plate with thickness t aligned with the stream, we show that the reattachment length xR increases with the chord (c) up to xR ≈ 4.8t for c / t > 12. For a plate at an angle of attack (α), we find that xR / c = π σ α2, where σ ≈ 7.9 is the inverse of the growth rate of a turbulent mixing layer. These results provide new insights on the governing mechanism underlying flow reattachment, and allow the prediction in the first approximation of the reattachment length of separated shear layers on bluff and streamlined bodies. |
Tuesday, November 23, 2021 2:24PM - 2:37PM |
T09.00009: A Cluster-Based Network Analysis of Post Stall Load Fluctuations Fatma AYANCIK, Karen Mulleners The cycle-to-cycle variability of the vortex shedding and the associated fluctuations of the aerodynamic force coefficients during the dynamic stall are physical phenomena. This phenomenon always drops beneath the researcher’s radar due to phase-averaged presentation of velocity fields and lift histories. A correct representation of the maximal and minimal bounds of aerodynamic force fluctuations is important for the accurate prediction of the aerodynamic efficiency and the dynamic stall hysteresis of wind turbines or helicopter blades. We used cluster-based analysis to identify the chain of events leading to flow separation and associated post-stall lift fluctuations and their bounds. A feature space is spanned by the aerodynamic forces. The feature space of trajectories is then partitioned into discrete clusters with similar aerodynamic characteristics using unsupervised clustering. Each cluster corresponds to a characteristic phase of the flow in the feature space, and it is represented with a cluster centroid. Statistical properties of cycle-to-cycle variations are determined at each cluster centroid. We propose an inverse model to rebuild the experimental data from the information obtained from clustered statistical properties of cycle-to-cycle variations. This model would be used to introduce post-stall fluctuations to dynamic stall models for the accurate prediction of dynamic stall hysteresis without the requirement of additional second-degree terms. |
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