Observation and measurements of flow structures in the stagnation region of a wing-body junction
The behavior of a junction vortex formed around an obstacle in a boundary layer flow was studied experimentally in a water tunnel for two low speed cases. A wing consisting of a 3 : 2 elliptical nose and an NACA 0020 tail was used to simulate the junction vortex.
A visual study using a hydrogen bubble technique was extensively conducted to investigate the flow structures in the stagnation region of the wing. It was observed that a multiple vortex system exists in this region and shows an acyclic flow pattern.
LDV measurements were performed in the plane of symmetry upstream of the wing. The general behavior of the flow agrees with an earlier wind tunnel test of Devenport and Simpson which was conducted at higher speed. A low frequency, bistable flow structure was observed as in the wind tunnel measurements. The switching between two flow modes (a backfiow mode and a zero flow mode) was analyzed using LDV signals in the zone of a bimodal structure. A dimensionless frequency group (StT) was found to represent the average frequency of successive switches from a given mode to the other.
The visual evidence of acyclic flow pattern was consistent with the LDV measurements, and revealed that aperiodic stretching of the junction vortex appears responsible for the bimodal (double-peaked) structure in the velocity histograms.
An attempt to measure the three-dimensional instantaneous velocity field in this region was made. A unique PIDV (particle image displacement velocimetry) technique was developed using a multiple wire hydrogen bubble method and a high speed video system. A stereo vision approach was implemented to capture two orthogonal views simultaneously for the three-dimensional motion analysis.