Separation-Bubble Velocity Measurements Using an Oscillating-Hot-Wire System.

Abstract

A system is developed to allow measurement of both the mean and disturbance flow velocities in separated regions. An oscillating-arm assembly provides a directional bias to the hot-wire probe, along with a linear-step assembly, which steps the probe through the boundary layer. This system is used to investigate the flow field associated with laminar separation bubbles. A series of velocity profiles are measured on an FX63-137 airfoil in the chord-Reynolds-number range: 150,000 < R, < 300,000, at angles of attack of a = 120 and a = 14'. Data are presented on the size of the separation bubble and on the location of transition to turbulence. Interests in the use of low-Reynolds-number airfoils has established demands for better qualitative and quantitative understanding of the flow associated with these airfoils. In particular, detailed boundary-layer information is needed to aid in both physical understanding and theoretical development. At the lower range of chord Reynolds numbers, Rc, down to Rc = 10,000, the airfoil performance (measured by lift-to-drag ratio) rapidly deteriorates, as shown by Miley 1 . The drop in performance is due to increased skin friction (thus an increase in the drag) and the fact that the airfoil can behave like a flat plate. To account for the increase in drag, airfoils have been designed with strong adverse pressure gradients to increase lift. These strong adverse pressure gradients, however, stimulate laminar boundary layer separation and violent stall. A thorough review of all aspects of work in this area is given by Mueller 2 .Associated with laminar separation is the formation of a free shear layer. Under most conditions, the inherently unstable character of the free shear layer results in transition to turbulence. In the case where the turbulent shear layer reattaches as a turbulent boundary layer, a separation bubble is formed. With the presence of a separation bubble, lift is maintained and the characteristics of stall are avoided. On the other hand, if velocity disturbance amplitudes are sufficiently high, transition will occur first and separation will be avoided.Because of the low Reynolds numbers associated with these airfoils, separation is in a laminar boundary layer or a transitional boundary layer. Individually, separation and transition are phenomena that are very sensitive to environmental conditions such as free-stream turbulence, roughness, and noise. Therefore it is important to study the behavior of these airfoils in low-disturbance environments characteristic of flight. This is clearly a stability problem involving adverse pressure gradients. Although the problem is not new, most earlier work has dealt with a two-dimensional back step geometry (Lochtenberg 3 , Browand 4 , Roshko 5 , Goldstein, Ericksen, Olsen and Eckert 6 ) or the exit of axisymmetric free jets (Freymuth 7 , Moore', Davies and Baxter'). More recently, LeBlanc, Blackwelder, and Liebeck 10 have studied the free shear layer resulting from separation over a low Reynolds number airfoil.In orde...