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# Internal Report

## Design and evaluation of a low-speed wind-tunnel with expanding corners

A new low-speed closed circuit wind-tunnel has been designed and built at the Royal Institute of Technology, Stockholm. It has a test section with a cross section area of $0.5\times 0.75\;\mathrm{m}^2$ and a length of 4.2 m. The maximum speed is about 48 m/s with empty test section. The contraction ratio is 9. A heat exchanger, a honeycomb and 5 screens are included to minimize flow disturbances in the test section. The streamwise turbulence intensity in the test section is less than $0.04\%$ in the core with similar values for the cross stream turbulence intensities. The variation in total pressure is less than $\pm 0.1\%$ and the variation in temperature is less than $\pm 0.07{}^\circ$C over the cross section area and $\pm 0.03{}^\circ$C in time in the centre of the test section. The concept of expanding corners, with a substantially larger outlet than inlet cross section area, ($e=1.316$), has been implemented successfully with a total pressure-loss coefficient in the first corner downstream the test section of 0.047 at a guide-vane chord Reynolds number of 205000. This value is a two-dimensional estimate obtained from measurements in a centre plane downstream the vanes. It compares favorably with the total pressure-loss coefficient in most wind-tunnels using non-expanding corners. Together with the good flow quality found in the test section this proves the usefulness of expanding corners in modern wind-tunnels to minimize the overall size for a given test section length.