Abstract
M.Ing. (Mechanical Engineering)
The requirements that a wind tunnel balance needs to satisfy have become increasingly stringent. These requirements, as set out by the wind tunnel testing community at large, include: improved static force accuracy, improved resolution, increased stiffness, temperature compensation and immunity to electromagnetic interference (EMI). In order to satisfy these requirements, wind tunnel balance design philosophy needs to be expanded to include employing alternative strain sensing technologies and alternative manufacturing techniques. This study sets to investigate the design and development of a six component side wall balance using optical fibre Bragg grating sensors (OFBGs). This is done in an attempt to address these stringent requirements by expand on traditional balance design philosophy by determine the viability of using OFBGs in wind tunnel balances.
Traditional strain gauge side wall balances (SWB) fundamentally rely on measuring strain, being induced by an applied load, on a balance. For this reasons, the regions of the balance which are intended to be gauged are intentionally made thin. This is done to achieve a high strain gauge output. This however compromises balance stiffness and induces a high level of stress in the balance material at the gauged regions.
In order to employ OFBGs on a SWB, the two groove strain sensing method must be employed. The basic working principles of this method is that an optical fibre is spanned between two points across a groove. As a load is applied, the groove deforms. This deformation (displacement) of the groove induces a strain into the optical fibre spanning the groove and a shift in the Bragg grating wavelength. Therefore the design of this SWB is fundamentally based on measuring the displacement between points. Based on these fundamentals, several design iterations were generated and analysed using FEM. The selected design underwent an iterative refinement process, aimed at optimising the design. This balance was manufactured and gauged with OFBGs.
The manufactured balance was calibrated at the Counsel for Scientific and Industrial Research (CSIR) wind tunnel testing division in Pretoria, South Africa. The balance was evaluated based on uncertainty, hysteresis and interactions and repeatability...