Moving tuft method for stagnation point location

Introduction T location of the forward stagnation point on a blunt nosed body in a flow stream is quite difficult when attempted by standard methods such as oil and lampblack on the surface, tufts attached to the surface, and static pressure measurements in the stagnation region. The source of this difficulty is the presence of very low dynamic pressures and pressure gradients in the region. As a consequence, the oil solution is affected more by gravity than by the flowfield, the surface tufts show considerable hysteresis due to the low flow velocities and differences in measured pressures are very small in the stagnation region. Of these three methods, the pressure measurement method is the most accurate and can be accomplished only with a very large number of pressure ports in a small region. The moving tuft method as described in this note is a simple, accurate procedure for the determination of a stagnation point. Although the method requires the development of some skill by the operator, based on the tests reported herein the necessary judgment can be developed in only a few hours. The moving tuft method as employed in this study involved the attachment of a single tuft to a piece of flat ribbon which was guided longitudinally around the model as shown in Fig. 1. Both ends of the ribbon were then routed outside the tunnel to an operator. By placing a surveyor's transit upstream of the open circuit tunnel inlet, the operator could view the tuft location relative to a grid attached to the model and could move the tuft in either direction by pulling on the ribbon. The transit was used for this study only because the stagnation region could not be directly observed. When the tuft is at the stagnation point it does not respond, but as it moves away, a slight movement can be detected. Then by moving the tuft away from the "dead" region in both directions until the movement was detected, the stagnation point could be bracketed and the midpoint of these two locations was taken as the actual stagnation point. Since the tuft was made of very fine thread, the region over which it did not respond was rather small. Also, after one or two hours of observation, it was found that an operator could obtain very consistent data and that different operators would produce quite consistent results.