A novel experimental facility to impose unsteady pressure gradients on turbulent boundary layers, A simplified photogrammetry procedure in oil-film interferometry for accurate skin-friction measurement over arbitrary geometries
Authors: Saxton-Fox Theresa , Gluzman Igal 
Published: October 2022

A novel experimental facility to impose unsteady pressure gradients on turbulent boundary layersThe design and characterization of a new removable wind tunnel installation to impose unsteady pressure gradients (PGs) on flat plate turbulent boundary layers (TBLs) are presented. An electropneumatic actuation mechanism was used to rapidly deform a flat ceiling section into an inverted convex bump, producing a temporally strengthening favorable and adverse PG in spatial sequence. The design allowed the vertical extent of deformation and the speed of deformation of the ceiling to be independently varied in a controlled manner to access a series of spatial and temporal strengths of PGs. High-frequency pressure measurements were carried out to characterize the spatio-temporal pressure distributions in the test area for 18 test cases. The resulting range of PGs is presented in terms of non-dimensional parameters relevant to PG TBLs: the acceleration parameter (K), which varied in the range [3,−2.5]×10−6, the Clauser PG parameter (β), in the range ±7, and the non-dimensional gradient of pressure coefficient (dCpd(x/L)), in the range ±2.6. The temporal rates of change of PGs are presented in terms of the reduced frequency (k) and are in the range [0.19, 2.75]. The current and future potential for using this facility to impose a wide range of steady and unsteady PGs in a wind tunnel to enable fundamental studies of various engineering flows of interest are discussed.A simplified photogrammetry procedure in oil-film interferometry for accurate skin-friction measurement over arbitrary geometriesAn accurate and straightforward skin-friction measurement procedure over a three-dimensional (3D) bump geometry is proposed. The procedure employs oil-film interferometry (OFI) measurements coupled with photogrammetry techniques. Photogrammetry is vital for accurate quantitative skin friction measurements over curved surfaces and at different oil flow imaging settings. However, it requires information on surface geometry obtained from multiple reference points on the surface and placing the camera in the exact location and orientation before and after the wind-tunnel run, which complicates the application of OFI. A new simplified method reported here is based on camera calibration algorithms and a flexible small checkerboard. The proposed procedure removes the elaborate steps typically performed in photogrammetry while retaining the same measurement accuracy. In particular, it allows obtaining OFI data without a priori knowledge of model geometry and only taking images after experiments at arbitrary undocumented camera locations. The simplified method was first validated with the Clauser method over a flat surface in a canonical turbulent boundary layer. Then the method was applied to obtain the skin-friction distribution for turbulent flow over 3D Gaussian bump geometry, which exhibits smooth body flow separation. The obtained high-quality benchmark data over the bump geometry are aimed to serve as validation data for numerical simulations for the challenging task of predicting flow separation.