Jonathan W. Naughton | Search NASA

NASA NTRSDate not supplied

A Method for Obtaining Surface Flow Vectors and Its Implementation in Interferometric Skin Friction Measurement

A new method was developed to extract surface flow vectors from an oilflow visualization image that has oil streaklines. The method is analogous to the PIV processing where the image is divided into interrogation windows. A representative flow direction is obtained for each interrogation window using image processing with a line-detection algorithm. Repeating the process for the entire image, one can find the surface vector field. The Hough transformation, Radon transformation and Machine Learning were used as line-detection algorithms. The obtained vector field is then postprocessed to filter spurious vectors and to apply smoothing. The method was tested on 2D and 3D models with different flow complexities. The method was able to predict surface flow vectors for all cases tested. The predicted surface flow vectors were used in obtaining the surface skin friction. The successful implementation of the surface flow vectors enables interferometric skin friction measurements on surfaces beneath 3D complex flows. In addition, the surface flow vectors can be superimposed on the oilflow visualization images to better explain the surface flow topology.

Surface flow vectors↗

NASA NTRSDate not supplied

Skin Friction Measurement on a Swept NACA 0015 Wing Using Oil Film Interferometry

An application of oil film interferometry (OFI) to measure skin friction on the 3D flow over an NACA 0015 airfoil with 30° of sweep is presented. Local flow direction must be known to apply OFI to complex 3D flows such as this. A recently developed surface flow vector extraction method was integrated into the OFI analysis algorithm. Surface flow vectors were used to calculate the surface streamlines and hence provided local flow direction to the OFI analysis. After calibration using photogrammetry, the surface flow vectors also enabled decomposition of the skin friction coefficient into different components (such as chordwise and spanwise). The presented method successfully measured skin friction at locations with different flow characteristics such as high skin friction regions, strong spanwise flow regions, and 3D flow separated regions. In the current OFI analysis, multiple interferogram images acquired at different times were analyzed for the same location. The multiple interferogram analysis enabled averaging, which reduced the scatter the skin friction data. The results presented herein demonstrate the capability of the current approach, thereby extending OFI skin friction measurements to complex 3D flows.

Skin friction measurement↗

NASA NTRSDate not supplied

Skin Friction Measurement on a Swept NACA 0015 Wing Using Oil Film Interferometry

An application of oil film interferometry (OFI) to measure skin friction on the 3D flow over an NACA 0015 airfoil with 30° of sweep is presented. Local flow direction must be known to apply OFI to complex 3D flows such as this. A recently developed surface flow vector extraction method was integrated into the OFI analysis algorithm. Surface flow vectors were used to calculate the surface streamlines and hence provided local flow direction to the OFI analysis. After calibration using photogrammetry, the surface flow vectors also enabled decomposition of the skin friction coefficient into different components (such as chordwise and spanwise). The presented method successfully measured skin friction at locations with different flow characteristics such as high skin friction regions, strong spanwise flow regions, and 3D flow separated regions. In the current OFI analysis, multiple interferogram images acquired at different times were analyzed for the same location. The multiple interferogram analysis enabled averaging, which reduced the scatter the skin friction data. The results presented herein demonstrate the capability of the current approach, thereby extending OFI skin friction measurements to complex 3D flows.

Skin friction measurement↗