EXPERIMENTAL INVESTIGATION OF CHARACTERISTICS OF STEADY AND UNSTEADY CROSSFLOW-INSTABILITY MODES DEVELOPING IN A 35-DEGREE SWEPT-AIRFOIL BOUNDARY LAYER

The present study is a part of the 'RECEPT' Project of the 7 th European Frame- work Program devoted to investigation of the receptivity and amplitude-based transition prediction methods in swept-wing boundary layers. The experimental part of the Project assumes obtaining, in particular, the crossflow instability characteristics of a boundary layer developing on a swept airfoil, which are necessary (along with some other quantities) for examining characteristics of receptivity of this flow to free-stream vortices and surface nonuniformities. The results of a detailed investiga- tion of the various crossflow instability characteristics are presented in this paper broad for a range of disturbance parameters (frequencies, spanwise wavenumbers, and wave propagation angles). Introduction. The problem of turbulence onset in 3D boundary layers has been investigated rather intensively during many years because of its great fundamental and practical importance. At present, it is clear that the location of the boundary-layer transition on aircraft bodies and wings de- pends significantly on four main factors: (i) the spectra of most dangerous environmental disturb- ances, (ii) the receptivity coefficients, (iii) the boundary-layer stability characteristics, and (iv) the leading mechanisms of nonlinear disturbance interactions. The experimental part of the 'RECEPT' project concentrates basically on experimental and theoretical investigation of aspect (ii) of the problem which assumes also a detailed study of aspect (iii). In particular, the problems of investiga- tion of the swept-wing boundary-layer receptivity mechanisms responsible for excitation of steady and unsteady crossflow (CF) instability modes attracts great attention of experimentalists (see e.g. refs. (1, 2) for review). However, only small part of various possible ways of excitation of CF- modes has been studied at present. Moreover, the majority of quantitative physical experiments in this field have been carried out on swept plates with induced pressure gradient, rather than on real airfoils. The cross-flow instability represents one of the most important instabilities of the swept-wing boundary-layer. This is the main object of the present study. The results of a detailed experimental investigation of the linear-instability mechanism of development of CF-modes in a boundary layer of a swept airfoil are presented below. Experimental setup and base -flow. The measurements were performed in a low- turbulence MTL wind tunnel of the Royal Institute of Technology (Stockholm) at freestream speed of about 11 m/s. A section of a 35-degree swept-wing airfoil with chord length of 800 mm was mounted in the wind-tunnel test section at a −5-degree angle of attack (Fig. 1). In order to provide the spanwise base-flow uniformity (to meet the sweep condition) the model was equipped with con- toured sidewalls (see (3, 4)). Their shapes were calculated numerically. The experiments are performed at fully controlled disturbance conditions. The velocity pertur- bations were excited in the airfoil boundary layer by means of a special disturbance source (Fig. 2) located at chord position of 120 mm. The source represented a spanwise row of circular latex mem- branes oscillating under the influence of pressure fluctuations excited by a set of speakers, playing role of pumps. The speakers were located outside the wind-tunnel test section and were connected to the source by an array of plastic pipes. The spanwise step of the membranes was 'z = 8 mm. In different regimes of measurements either all membranes of part of them (including important case of one membrane) were activated.