Dynamic airfoil stall investigations

Experimental and computational investigations of the dynamic stall phenomenon continue to attract the attention of various research groups in the major aeronautical research laboratories. There are two reasons for this continued research interest. First, the occurrence of dynamic stall on the retreating blade of helicopters imposes a severe performance limitation and thus suggests to search for ways to delay the onset of dynamic stall. Second, the lift enhancement prior to dynamic stall presents an opportunity to achieve enhanced maneuverability of fighter aircraft. A description of the major parameters affecting dynamic stall and lift and an evaluation of research efforts prior to 1988 has been given by Carr. In this paper the authors' recent progress in the development of experimental and computational methods to analyze the dynamic stall phenomena occurring on NACA 0112 airfoils is reviewed. First, the major experimental and computational approaches and results are summarized. This is followed by an assessment of our results and an outlook toward the future

Evaluation of Turbulence Models for Unsteady Flows of an Oscillating Airfoil

Unsteady flowfields of a two-dimensional oscillating airfoil are calculated using an implicit, finite-difference, Navier Stokes numerical scheme. Five widely used turbulence models are used with the numerical scheme to assess the accuracy and suitability of the models for simulating the retreating blade stall of helicopter rotor in forward flight. Three unsteady flow conditions corresponding to an essentially attached flow, light-stall, and deep-stall cases of an oscillating NACA 0015 wing experiment were chosen as test cases for computations. Results of unsteady airloads hysteresis curves, harmonics of unsteady pressures, and instantaneous flowfield patterns are presented. Some effects of grid density, time-step size, and numerical dissipation on the unsteady solutions relevant to the evaluation of turbulence models are examined. Comparison of unsteady airloads with experimental data show that all models tested are deficient in some sense and no single model predicts airloads consistently and in agreement with experiment for the three flow regimes. The chief findings are that the simple algebraic model based on the renormalization group theory (RNG) offers some improvement over the Baldwin Lomax model in all flow regimes with nearly same computational cost. The one-equation models provide significant improvement over the algebraic and the half-equation models but have their own limitations. The Baldwin-Barth model overpredicts separation and underpredicts reattachment. In contrast, the Spalart-Allmaras model underpredicts separation and overpredicts reattachment

Simulation of flow diversion in cerebral aneurysms

Intracranial aneurysms are abnormal focal enlargements of the vascular walls that necessitate surgical intervention once detected. Emerging stent technology involves an innovative type of finely-braided stents, called flow diverters, which abruptly impede the arterial flow into the aneurysm, upon deployment, and induce thrombosis, vascular remodelling and complete aneurysm occlusion in under a year [1]. The understanding of the dynamics of blood flow within this radically modified environment is thought to be pivotal in increasing the efficacy of both stent design and prolonged treatment. The aim of this study is to numerically simulate the blood flow within stented arterial segments and to evaluate critical hemodynamic factors around the aneurysm neck, validated with clinical and experimental data [1,2]. These objectives create many geometric challenges around the flow diverter due to the tessellation and resolution of features with very large ratio (artery-to-stent) in the input i.e., medical images and stent models. Following a novel Body-Centric Cubic (BCC) mesh generation method [2], high-fidelity tetrahedral meshes of aneurysmal dilatations that incorporate flow diverters across the aneurysm neck are now possible with an accurate image- to-mesh (I2M) conversion scheme from micro-CT images (Fig. 1a,b). Preliminary results involve arterial segments both with and without flow diverters (Fig.1c), utilising the CFD software OpenFOAM® to solve the incompressible Navier-Stokes equations, under steady and physiologically-correct pulsatile flow conditions

Compressive properties of min-mod-type limiters in modelling shockwave-containing flows

The long-ignored compressive properties of Min-mod-type limiter is investigated in this manuscript by demonstrating its potential in numerically modelling shockwave-containing flows, especially in shock wave/boundary layer interaction (SWBLI) problems. Theoretical studies were firstly performed based on Sweby’s total variation diminishing (TVD) limiter region and Spekreijse’s monotonicity-preserving limiter region to indicate Min-mod-type limiters’ compressive properties. The influence of limiters on the solution accuracy was evaluated using a hybrid-order analysis method based on the grid-independent study in three typical shockwave-containing flows. The conclusions are that, Min-mod-type limiter can be utilized as a dissipative and/or compressive limiter, but depending on the reasonable value of the compression parameter. The compressive Min-mod limiter tends to be more attractive in modelling shockwave-containing flows as compared to other commonly preferred limiters because of its stable computational process and its high-resolution predictions. However, the compressive Min-mod limiter may suffer from its slightly poor convergence, as that observed in other commonly accepted smooth limiters in modelling SWBLI problems. © 2020, The Author(s)