EXPERIMENTAL INVESTIGATION OF THE WAKE BEHIND A CASCADE OF AIRFOILS AND ISOLATED AIRFOIL

The study of the wake characteristics of an isolated airfoil and two dimensional cascade of airfoils has significant engineering applications. For isolated airfoils and cascade of airfoils, experimental data on the asymmetric character of the wake, due to airfoil loading, are scarce, in particular for symmetric airfoils at relatively low Reynolds number. The objective of the present paper is to experimentally investigate the mean velocity field in the near and far wake regions of an isolated symmetric airfoil and a cascade of symmetric airfoils at relatively low Reynolds number and at different loading conditions; operation at different incidences provides the asymmetric character of the wake. For the isolated airfoil case, the wake measurements were performed for five incidences (0,5,10,17 and 22 deg. ). For the cascade case, the inlet flow angle was kept zero and the stagger angle was taken as 0,3,5,6,10 deg. for 0.5 space-to-chord ratio, and as zero for space-to-chord ratio of 0.75. The velocity profile exploration was conducted at four downstream stations from the trailing edge, (x/c =0.1,0.3,0.5, and 1.0 ). Similarity of the mean velocity profile was verified by reducing the velocity profiles to a single curve. Self similarity assumption was also checked against the present measurements. Variation of the wake centreline velocity defect with downstream distance from the trailing edge are given for different incidence angles; this showed that the decay rate is faster at lower incidence angles in the near wake region and that the inverse is the case in the far wake region. Also, the wake centreline velocity defect was found to attain higher values with an increase in cascade solidity. Wake width variation with downstream distance is also given. Wake integral parameters, momentum thickness and shape factor, for the isolated airfoil and cascade at different incidences are calculated and represented as function of the downstream distance. The present experimental wake measurements, using symmetric blades at low Reynolds number, is compared with other experimental data carried out using different blade sections at higher Reynolds numbers. The comparison shows similar trends; however, the present measurements yield higher values of momentum thickness. Besides, the present measurements provide data for validating and testing the available numerical methods of computation.