Keywords: Contoured side-walls, design, experimental, laminar boundary layer, numerical, receptivity, stability, swept wing, wind tunnel.

Transcription

1 Applied Mechanics and Materials Vol. 390 (2013) pp Online available since 2013/Aug/30 at (2013) Trans Tech Publications, Switzerland doi: / Design and tests of wind-tunnel sidewalls for receptivity experiments on a swept wing Romano D. G. 1, a, Alfredsson, P.H. 2,b, Hanifi, A. 2-3,c,Örlü, R. 2,d, Tillmark, N. 2,e, Borodulin V.I. 4,f, Ivanov A.V. 4,g, Kachanov Y.S. 4,h, Minervino M. 5-1,i 1 Piaggio Aero Industries S.p.A., 34, Via Campi Flegrei, Pozzuoli (NA), Italy 2 Linné FLOW Centre, KTH Mechanics, Royal Institute of Technology, S Stockholm, Sweden 3 Swedish Defense Research Agency, FOI, S Stockholm, Sweden 4 Institute of Theoretical and Applied Mechanics, SB RAS, Novosibirsk, , Russia 5 C.I.R.A. (the Italian Aerospace Research Centre), SNC via Maiorise, Capua (CE), Italy a dromano@piaggioaero.it, b hal@mech.kth.se, c hanifi@kth.se, dramis@mech.kth.se, e nt@mech.kth.se, f bo@itam.nsc.ru, g andi@itam.nsc.ru, hkachanov@itam.nsc.ru, i m.minervino@cira.it Keywords: Contoured side-walls, design, experimental, laminar boundary layer, numerical, receptivity, stability, swept wing, wind tunnel. Abstract. This document explains in its first part the design procedure adopted to design the contoured sidewalls of a swept-wing airfoil section mounted in a wind tunnel in order to satisfy the infinite swept-wing approximation. In the second part, the experimental set-up is described as well as the first results of the experimental campaign. The sidewalls are shown to play their role properly and satisfactorily provide the infinite swept-wing conditions required for subsequent investigations of the most important vortex receptivity mechanisms responsible for excitation of crossflow and Tollmien-Schlichting instability modes in the airfoil boundary layer. Introduction The activities reported in the present report have been carried out inside the EU-funded research project RECEPT [1]. The aim of the RECEPT study, the first step of which is reported in this paper, is to clarify, through experimental wind tunnel (WT) test campaigns, the relative role of several receptivity mechanisms associated with the presence of freestream turbulence and surface nonuniformities together or separately. Moreover, the critical amplitudes of surface roughness leading to nonlinear receptivity phenomena providing abrupt transition beginning in the proximity of roughness elements at different freestream turbulence levels will be examined. To these aims, an experimental model of a swept wing that satisfies the infinite swept-wing approximation has been designed, manufactured and used for a WTT campaign. Design of the contoured sidewalls Design strategy. In order to satisfy the infinite swept-wing approximation, it is necessary to correctly design the sidewalls of the wind tunnel to have the streamlines turned in the correct direction (i.e. to have the isobars parallel to model s leading edge). To this aim, a design procedure based on seven steps has been defined and reported below. 1) Generation of a parametric CAD model for the swept-mounted airfoil installed in the wind tunnel with flat walls. 2) CFD mesh generation associated to the CAD model. 3) CFD flow simulations for the infinite span WT model, replacing solid sidewalls with periodic boundaries. 4) Extraction of typical streamlines shape from the CFD solution back to the CAD environment. 5) CAD design of the contoured sidewalls based on the reference streamline. 6) CFD mesh generation for the contoured WT configuration. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Pennsylvania State University, University Park, USA-10/03/15,07:45:17)

2 Applied Mechanics and Materials Vol ) Flow simulation for the contoured WT configuration. Parametric CAD model. A fully parametric CAD model developed in CATIA v5 R19 [2] is used to design the sidewalls. The starting swept-mounted airfoil is obtained through a best fitting of about 600 points defining the airfoil shape. In particular, a 5 th order NURBS with 19 control points has been used. Fig. 1 shows a view of the parametric CAD model, whose dimensions are listed below. 1) Airfoil chord normal to leading edge: 0.8 m. 2) WT test section length: 7.0 m. 3) WT test section width: 1.2 m. 4) WT test section height: 0.8 m. The model is mounted vertically, and positioned at mid-length of the test section. The rotation axis for angle of attack modification is parallel to the leading edge. Two different angle of attacks (AoA) have been considered: -5 and Fig. 1: Overview of the CAD model (left side), airfoil curvature (top right) and CFD mesh (bottom right). CFD Simulations. CFD evaluations have been performed using CFD++, the commercial CFD code used by Piaggio Aero Industries, which is based on the finite volume method. A 2 nd order spatial discretization has been used with a multigrid technique (4 W-cycles on 20 levels) to accelerate the convergence. The boundary layer has been integrated directly to the wall around all solid boundaries. The 3-equations Goldberg s κ-ε-rt turbulence model has been applied [3]. Both an unstructured and a structured meshe have been used and in both cases the y + was of order of unity around all solid adiabatic viscous walls. Sea level ISA conditions have been considered to define the values of the ambient temperature and pressure. A WT inlet mass flow rate equal to kg/s has been imposed. Transition has been imposed at 70% of chord length on the upper side and at 10% on the lower side. In Fig. 2 the wall-shear-stress distribution (τ x ) for the design at -5 is reported for the fully turbulent, fully laminar and with imposed transition cases. LAMINAR TURBULENT Fig. 2: Wall shear stress for the design at -5 (left side) and boundary layer profiles in the transition region (right side).

3 98 Mechanical and Aerospace Engineering IV The ideal design would be obtained by designing a 3D surface enveloping all the streamlines normal to the model. However, to simplify the manufacturing of sidewalls, only one reference streamline for the upper and another one for the lower side of the model have been considered and should provide approximate satisfaction of the sweep condition in the vicinity of the airfoil surface. Besides, only a limited length of the WT side walls has been modified (about 2m). The above reported simplification causes a reduction of the parallelism of isobars to the leading edge, which is not preserved close to the intersection of the model to the wind tunnel walls. However, this is not a big issue, since to perform the present WT tests it is enough that the isobars are parallel to the leading edge in the central area of the model itself. This condition is respected by both the designs carried out at AoA of -5 and In particular, in the first design the isobars look perfectly parallel on the upper side, but not on the lower, while in the second design isobars are parallel on the lower side but not on the whole upper side (see Fig.3). Further simulations for slightly modified configuration were performed by FOI using the EDGE code [4]. (a) (b) (c) Fig. 3: Isobars on the (a) lower model side for a rotation of -5, (b) upper model side for a rotation of -5, (c) lower model side for a rotation of +1.5, (d) upper model side for a rotation of (d) Design of the measuring and installation equipment Experimental equipment. The measurement and installation equipment for RECEPT has been designed and manufactured by ITAM and KTH. The equipment consists of an airfoil support including a structure to allow angle of attack adjustment in the wind tunnel, a cross hot-wire calibration device, a new traversing mechanism (the Komarik ) for boundary-layer hot-wire measurements and a dedicated software for its control and data acquisition as well as electrical and electronic devices and software (see Fig. 4) to operate the mechanical part of source of freestream vortices and surface nonuniformities. The Komarik traverse has been designed to have as small flow blockage as possible and also the motors mounted on traverse were specially chosen to have large torque but small dimensions. The components of the surface disturbance source are: a multi-channel digitally controlled generator and a power amplifier, a set of powerful loudspeakers, and a disturbance source body including membranes.

4 Applied Mechanics and Materials Vol Fig. 4: Traversing system Komarik with its controller (upper insert), a detail of the photo cell (lower insert), installation in WT (top right) and detail (bottom right). Wind tunnel tests Experimental set-up. Wind tunnel tests have been carried out using the MTL wind tunnel facility [5] at KTH Mechanics (see Fig.5) considering the design performed at AoA = -5. The abovementioned wind tunnel is a closed loop circuit WT with a 7 m long test section with a cross section area of 1.2 x 0.8 m 2. The maximum speed is around 70 m/s and the contraction ratio is equal to 9. A small gap between the test section and the first diffuser is used to have a value of the static pressure in the test section close to the atmospheric one. The free-stream turbulence level is below to 0.025% at the flow speeds of the present experiments. Fig.6 shows the model installed in the wind tunnel test section. The experiments were carried out by imposing transition through a transition strip on the model at 70% on the upper side and at 10% on lower side. Fig. 5: KTH s MTL Wind Tunnel layout. Fig. 6: Model installed in the MTL wind tunnel and surface disturbance source.

5 100 Mechanical and Aerospace Engineering IV Experimental results. The measurements of the base-flow around the model have been performed using cross hot-wire probes for two components of the potential-flow velocity vector field. Measurements were made in three (x,z)-planes oriented vertical and parallel to the incident flow velocity. Fig.7 shows that at large distances from the airfoil (bottom plot) the contour-lines of the streamwise velocity components are not parallel to the swept-wing leading edge (this direction is marked with white dashed lines). Meanwhile, close to the airfoil surface (top plot in Fig.7) the base-flow becomes uniform in the spanwise direction, a result that was expected since in the 2D sidewall design the reference streamline was chosen close to the airfoil surface. Fig.8 and Fig.9 display that the streamwise velocity component measured in the potential flow close to the boundary-layer edge increases in the streamwise direction but is independent of the spanwise direction as it should be for the infinite-span swept wing. In particular, the spanwise variation of the streamwise velocity component does not exceed 1% at each fixed chordwise coordinate (Fig.9). Moreover, the spanwise potential-flow velocity component W' (parallel to the airfoil leading edge) is found to be practically independent of both the chordwise and the spanwise coordinate (Fig.8). The constancy of this velocity component is the inherent property of infinite-span swept wings and is often directly attributed to the satisfaction of the sweep condition. Fig.10 shows the comparison between the computed and the measured pressure distributions for the test case described. In particular, the lines reported in the above mentioned figure are the pressure coefficients at 11 cross sections in a distance of 20 cm around the midspan. It is possible to notice that CFD evaluations are in good agreement with the experimental measures, especially in the upper side of the airfoil. Y = 16 mm Y = 166 mm Fig. 7: Contours of streamwise velocity component U'/U o measured in potential flow at two different distances from the airfoil section. Fig. 8: Downstream distributions of normalized streamwise (U') and spanwise (W') velocity components measured in the potential flow at y 10 mm at three different spanwise locations.

6 Applied Mechanics and Materials Vol Fig. 9: Set of spanwise distributions of normalized streamwise (U) velocity component measured in the potential flow at y 10 mm at several chordwise locations. The negative peaks correspond to laminar wakes produced by thin wires placed on the wing and is an excellent method to trace the streamlines. Fig. 10: Computed (lines) and measured (circles) pressure distributions for the test case analyzed. Conclusions and future works. Design and manufacture of contoured sidewalls to perform experimental tests aimed to study the most important receptivity mechanisms responsible for excitation of disturbances leading to turbulent transition in 3D boundary layers of a typical swept-wing geometry have been made. It is found that the developed sidewalls do provide the base-flow characteristics, which correspond to those typical for an infinite-span swept wing in the vicinity of the airfoil-section surface. Thus, the developed experimental model has been found to be suitable for measurements of linear receptivity characteristics of a swept-wing boundary layer associated with excitation of cross-flow instability modes by fully controlled unsteady free-stream vortices due their scattering on localized controlled surface nonuniformities. Similar results are expected for another AoA (+1.5 ) at which the linear-receptivity problem of excitation of 3D Tollmien-Schlichting waves by freestream vortices and surface nonuniformities will be investigated.

7 102 Mechanical and Aerospace Engineering IV Acknowledgement. This work has been supported by the European Commission through the FP7 project RECEPT (Grant Agreement no. ACPO-GA ). References [1] [2] Dassault Systemes CATIA v5 R19 SP5, Product Documentation. [3] Metacomp Technologies Inc., CFD++User Manual Version 8.1, pages [4] P. Eliasson, in: EDGE, a Navier-Stokes Solver for Unstructured Grids, Proceedings to Finite Volumes for Complex Applications III, 2002, pages [5] B. Lindgren, A. V. Johansson, in: Evaluation of the Flow Quality in the MTL Wind Tunnel, Technical Reports from Royal Institutes of Technology Department of Mechanics, SE Stockholm, Sweden, pages 1-7.

8 Mechanical and Aerospace Engineering IV / Design and Tests of Wind-Tunnel Sidewalls for Receptivity Experiments on a Swept Wing /