Revision as of 10:27, 5 September 2011

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.

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