Boundary layer interacting with wakes under adverse pressure gradient - NLR 7301 high lift configuration

Underlying Flow Regime 3-01 © copyright ERCOFTAC 2004

Description

Introduction

The design of high-lift systems is an important issue in the aircraft industry, since it has a large impact on the overall cost and safety of the airplane. The flow around a high lift system is very complex, in which viscous flow phenomena dominate. Flow features include boundary-layer transition, separation bubbles, boundary-layer wake interaction, flow re-attachment, adverse pressure gradients, and possible shock/boundary layer interactions. RANS turbulence models are in general unable to describe simultaneously all the flow physics of a high&endash;lift system [1, 2], and the numerical simulation of the flow over a high-lift system is considered to be a very challenging CFD problem, see also the QNET-CFD TA1 Application Challenge L1T2 multi element airfoil (AC TA1-08).

Here we will focus on one flow phenomena found in the flow over high-lift systems: the interaction of a boundary layer with a wake under adverse pressure gradients. Note that unsteady boundary layer wake interactions, as for example occurring in turbo-machines, will not be considered here.

The key physics of boundary layer-wake interaction in context of this UFR can be summarized as follows. The wake of the flow over one element interacts with the boundary layer developing on a downstream element. Both the wake and the boundary layer are subject to curvature and adverse pressure gradient effects, which generate higher turbulence intensities and enhance the merging of the wake with the boundary layer developing on the downstream element.

Review of UFR studies and choice of test case

A review on experimental studies on wake boundary layer interaction prior to 1989 can be found in [3]. Test cases considered ranged from the interaction of flat plate boundary layers to the interaction of the wake of an airfoil with the boundary layer developing on a flat plate. One particular interesting experiment is the one by Zhou and Squire [4]. In a specifically designed boundary layer wind tunnel, they studied the interaction of a wake coming from an airfoil (with sharp and blunt trailing edges) with the boundary layer developing on the bottom wall of the tunnel. An adverse pressure gradient could be created by adding a gauze resistance at the tunnel exit. The incidence angle of the airfoil, and the height between airfoil and flat plate was varied. In all experiments, it was ensured that the boundary layers were fully turbulent by the use of trip wires. Measured data included pressure profiles, mean and fluctuating velocities, and skin friction. Some CFD calculations are presented in [3, 4], but, to our knowledge, the test case by Zhou and Squire has not been used for CFD validation exercises, nor is the experimental data readily available.

Liu et al. [5] studied experimentally the influence of pressure gradients on a developing wake. The experimental set-up was specifically designed to provide data for CFD code validation, and consisted of a splitter plate mounted in a tunnel, the contours of which could be modified to impose a pressure gradient. Measured data consisted of mean velocities and turbulence quantities, and a comparison with CFD can be found in [1]. However, this test case was specifically set-up to study only the development of the wake under various pressure gradients, not the interaction of a wake with a boundary layer, and for this reason it is not further considered here.

Kyriakides et al. [6] performed experiments on the interaction between the wake of a large-aspect ratio cylinder and a flat plate boundary layer. Measurements were made for a range of Reynolds numbers, but it is not clear how transition occurs at the lowest Reynolds numbers. Measured data consisted of mean and fluctuating velocities. DNS and LES simulations were made by Piomelli et. al. [7], but were limited to the low Reynolds number cases.

Van den Berg [8] performed measurements on the NLR 7301 wing-flap configuration which was designed such that no flow separation occurred, apart from a small laminar separation bubble on the wing nose. The main flow features are the growth of boundary layers and the mixing of such layers with wake flow under the action of adverse pressure gradients. The NLR 7301 wing-flap configuration has been used in a GARTEUR Action Group studying inviscid/viscous interaction methods. This was performed in 1983 but little information has emerged into the public domain. The configuration was subsequently studied in three separate projects funded by the EC: the EUROVAL project (1990-1992) [9], the ECARP project (1993-1995) [10] and the FLOWNET Thematic Network (1998-2002) [11]. In all these, the same two geometries were considered, namely a) the gap between the wing and flap set at 1.3% of the chord length, and an incidence angle of 6 degrees and b) the gap set at 2.6% of the chord length, and an incidence angle of 13.1 degrees. The Mach and Reynolds numbers did not change between the two cases.

SAAB Military Aircraft kindly made multi block structured grids used in FLOWNET for these 2 cases available. They can be downloaded here.

The NLR 7301 test case was specifically designed for CFD code validation for high-lift configurations and in all three afore mentioned EC projects comparisons of CFD results with experimental data are provided. In addition, the experimental data is publicly available. For these reasons this case was selected as the most appropriate UFR. It should be mentioned that the NLR 7301 multi element airfoil is one of the most computed multi element configurations currently available [2].

Contributors: Jan Vos - CFS Engineering SA