High Reynolds Number Flow around Airfoil in Deep Stall

Flows Around Bodies

Underlying Flow Regime 2-11

Description

Introduction

The high Reynolds number flow around airfoils at large (beyond stall) angles of attack is a challenging CFD problem of significant importance for the aerospace industry. Nonetheless, up to the late 90s of the last century, the studies providing quantitative data on this type of flow were, first, exclusively experimental and, second, rather limited. The lack of CFD studies was caused by the inability of RANS turbulence models of any level of complexity to represent such massively separated flows, on one hand, and by an unaffordable computational cost of LES of such flows, on the other hand. The limited character of the early experimental studies is explained by the difficulties of measuring unsteady flow characteristics. For instance, the textbook of Hoerner [‌6] provides only the mean lift and drag coefficients. The same is to a considerable extent true for the later experiments of Sheldahl and Klimas (1981) [‌20] and of Raghunathan et al. (1988) [‌16] (see Table 1).

More systematic studies of the considered UFR, both computational and experimental, were started towards the end of the last century, when the growth of the computer power and emergence of appropriate modelling approaches (e.g., Detached-Eddy Simulation (DES) [‌24, 26] and Scale- Adaptive Simulation (SAS) [‌10, 11]) and measurement techniques capable of capturing unsteady flow features made this possible.

The key physics of this UFR is predominantly characterised by the unsteady, three-dimensional, massively-separated wake region. This takes the form of a nominally periodic shedding of large scale, coherent vortices in a vortex street pattern, which is overlaid with finer random turbulent fluctuations at higher frequencies and random modulation and intermittency at frequencies lower than the vortex shedding frequency. A visual impression of these features is given in Figure 1. It has been found that it is necessary to capture these key physical features in a simulation, not just for the prediction of unsteady quantities but even in order to reliably predict steady-state parameters such as the mean force coefficients.

Figure 1: Figures to accompany the description of the key flow physics of the UFR. Snapshot of vorticity magnitude from a finely-resolved DES of the UFR [‌13] (above) and experimental time traces of lift and drag coefficient [‌27, 28] (below).

The document starts from an overview of the past studies of the considered UFR and a justification of the choice of the primary test case (NACA0021 airfoil at 60° angle of attack, experiments of Swalwell et al. [‌27, 28]). A more detailed outline of the experiment is then given, after which a summary of CFD methods used, major results of the simulations and their comparison with the experimental data are presented. Following this, in the BPA section, the effects of the parameters of the simulations to which the results show the greatest sensitivity are discussed and BPA are formulated. A twofold purpose of the UFR and BPA are envisaged:

Provision of information regarding the best class of turbulence modelling strategy, accompanying numerical framework and simulation setup for the practitioner simulating analogous flows.
Thorough documentation of a test case suitable for the verification of either new developments or new implementations of turbulence-resolving approaches, e.g. in the hybrid RANS-LES family.

Due to the importance of the highly unsteady wake in this UFR, it is considered of relevance to this aspect of the following Application Challenges:

AC 1-05: Ahmed body
AC 1-08: L1T2 3 element airfoil
AC 2-01: Bluff body burner for CH4-HE turbulent combustion
AC 4-01: Wind environment around an airport terminal building

Review of UFR studies and choice of test case

Provide a brief review of past studies of this UFR which have included test case comparisons of experimental measurements with CFD results. Identify your chosen study (or studies) on which the document will focus. State the test-case underlying the study and briefly explain how well this represents the UFR? Give reasons for this choice (e.g a well constructed test case, a recognised international comparison exercise, accurate measurements, good quality control, a rich variety of turbulence or physical models assessed etc.) . If possible, the study should be taken from established data bases. Indicate whether of not the experiments have been designed for the purpose of CFD validation (desirable but not mandatory)?

Contributed by: Charles Mockett; Misha Strelets — CFD Software GmbH and Technische Universitaet Berlin; New Technologies and Services LLC (NTS) and Saint-Petersburg State University