Criticality of Edge Flows and Vortex Shedding in Two and Three Dimensions
Abstract
Vortex shedding from leading edges of lifting surfaces, which is prevalent in flight vehicles, engineering devices, and nature, is governed by flow criticality at the edges. In particular, our recent work has resulted in the so-called leading-edge suction parameter (LESP), which reaches a critical value at the onset of leading-edge vortex (LEV) shedding on an airfoil or a wing section. Further, criticality of LESP also governs vortex shedding from round leading edges in reverse flow. In this effort, we use a combination of low-order methods and Reynolds-averaged Navier Stokes (RANS) computations at NCSU, experiments at UIUC, and higher-fidelity computations from collaborators in order to advance the predictive capability of theoretical methods for unsteady separated flows. The theoretical work will focus on exploring the criticality of edge boundary layers, modeling of the leading-edge decambering, and finite-wing integration. In the experimental effort, pressures and hot-film measurements on the surface and off-body velocimetry data will be collected during various pitch motions of airfoils and wings to study the connections between flow features, LESP, and three-dimensional effects. The simulations will use combinations of both RANS approaches, to generate large volumes of test cases for further theoretical development, and detached eddy simulation (DES) and large eddy simulations (LES) data, which will serve to validate the RANS approach and provide additional insights into the flow phenomena of interest. We seek to answer several outstanding questions regarding the connections between the various edge-flow parameters and the flow physics of vortex shedding. In particular, we are intrigued by whether it is possible to supersede the LESP with an Òedge boundary-layer parameterÓ (EBP), whose criticality will be independent of not just motion kinematics, but also airfoil shape and Reynolds number in forward and reverse flow. The development of this EBP is our first research aim. The second research aim is to gain a deeper understanding of how the LESP varies during the vortex shedding. Our results show that there is a sharp drop in LESP after initiation, which corresponds well with the behavior of the leading-edge outer-flow streamline curvature. These flow conditions will be studied in wind-tunnel experiments using the airfoil model. Flow measurements near the leading edge using PIV will be correlated with surface measurements of critical points using the hot-film array to deduce the shapes of the outer-flow streamlines. These shapes will be incorporated as Òvirtual decamberingÓ in an unsteady panel method to develop a low-order model for this flow behavior. Our third research aim focuses on extension of the LESP concept to LEV formation and dynamics on finite wings. Experiments using the finite-wing configurations will be used to study the LEV dynamics for various sweep angles, while the low-order formulation will focus on a novel ÒLEV horse-shoe vortex elementÓ to augment an unsteady vortex lattice method to model finite-wing LEV shedding. Our final research aim seeks to apply the LESP concept to complex, full-rotor flows involving stall. Our partnership with Rohit Jain of US Army will enable us to access relevant datasets from published full-scale rotor experiments and companion DES computations. By calculating LESP variations from these datasets, we hope to determine the applicability and limitations of the LESP formulations for these flows, and determine the new flow physics questions that will guide future research toward understanding these complex flows. We also plan to collaborate with researchers at the US Air Force Research Labs to use some of their LES datasets to support the RANS and experimental studies. Using this integrated theoretical, experimental, and computational effort, we hope to answer key fundamental questions and develop improved prediction capability for these vortex-dominated flows.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Mar 08, 2023
- Source ID
- W911NF2310109
Entities
People
- Ashok Gopalarathnam
Organizations
- Army Contracting Command
- North Carolina State University
- United States Army