Assessment of Digital Image Correlation as a method of obtaining deformations of a structure under fluid load

Abstract

Digital Image Correlation (DIC) is employed for the measurement of full-field deformation during fluid-structure interaction experiments in a wind tunnel. The methodology developed for the wind tunnel environment is quantitatively assessed. The static deformation error of the system is shown to be less than 0.8% when applied to a curved aerofoil specimen moved through known displacements using a micrometer. Enclosed camera fairings were shown to be required to minimise error due to wind induced camera vibration under aerodynamic loading. The methodology was demonstrated using a high performance curved foil, from a NACRA F20 sailing catamaran, tested within the University of Southampton RJ Mitchell, 3.5m x 2.4m, wind tunnel. The aerodynamic forces induced in the wind tunnel are relatively small, compared with typical hydrodynamic loading, resulting in small deformations. The coupled deflection and blade twist is evaluated over the tip region (80-100% Span, measured from the root) for a range of wind speeds and angles of attack. Steady deformations at low angles of attack were shown to be well captured however unsteady deformations at higher angles of attack were observed as an increase in variability due to hardware limitations in the current DIC system. It is concluded that higher DIC sample rates are required to assess unsteady deformations in the future. The full field deformation data reveals limited blade twist for low angles of attack, below the stall angle. For larger angles, however, there is a tendency to reduce the effective angle of attack at the tip of the structure, combined with an unsteady structural response. This capability highlights the benefits of the presented methodology over fixed-point measurements as the three dimensional foil deflections can be assessed over a large tip region. In addition, the methodology demonstrates that very small deformations and twist angles can be resolved.

Uncertainty assessment of coupled Digital Image Correlation and Particle Image Velocimetry for fluid-structure interaction wind tunnel experiments

“…In order to fully capture the structural response of the aerofoil, as concluded by Banks et al (2015), it was necessary to acquire the DIC images with high-speed cameras. Given the fixed dimensions of the wind tunnel and the level of accuracy needed for relatively large out-of-plane deflections, the maximum achievable stereo angle (ψ) of 41…”

“…Within the DIC software, the speckle pattern is mapped to calculate it's deformation, allowing the displacements and strains of the underlying structure to be measured (Rastogi and Erwin, 2012). DIC analysis of a foil structure within the working session of a wind tunnel was developed at the University of Southampton (Banks et al, 2015). This initial study showed that small deflections could be accurately measured but that the structure was too stiff to result in significant twist deformations.…”

“…Those findings were brought together in the current research to be able to accurately measure the deflections of a specimen with a fixed speckle size and density under varying lighting conditions. The DIC measurement error can be assessed through the use of motorised stages that provide known displacements of a specimen (Banks et al, 2015).…”

Developing tools for assessing the fluid structure interaction of passive adaptive composite foils

“…The speckle pattern, applied to a region of 450× 450 mm, was set to have a speckle size of approximately 7 pixels. This size allows each speckle to be described with reasonable confidence in the whole histogram intensity range, as described by Banks et al (2015). The tip vortex core position is investigated with high resolution PIV.…”

“…Experimental and numerical investigations conducted on a flexible NACA0015 foil are presented. The experimental methodology within the working section of a wind tunnel was first developed at the University of Southampton (Banks et al 2015). The numerical tools presented are validated against a well-known measurement technique (namely Digital Image Correlation) and the forces response measured by the wind tunnel balance.…”