The Michigan Prandtl System: An Instrument for Accurate Pressure Measurements in Convective Vortices

Abstract

his article describes a Prandtl tube system developed at the University of Michigan to measure the static pressure, the total (or stagnation) pressure, and the velocity in flows whose direction and intensity change rapidly. The ever-changing wind vectors in convective vortices are a challenge for making accurate measurements on them. Accurate measurements of the static pressure are particularly problematic because they require the sensor air intake to be aligned perpendicular to the wind direction. This article describes calibrations and tests of the Michigan Prandtl System (MPS) and, in particular, the characterization of the errors in the static pressure measurements as a function of misalignments between the Prandtl tube and the wind vector. This article shows that the MPS measures the pressure with a relative error of 3.5% for wind flows whose direction is within about 10° of the MPS tube direction. It also shows that the MPS adjusts to changes in wind direction of 90° in about 1.5 s, the average rate of change expected in a typical dust devil of about 15 m of radius traveling at 10 m s?1 (Rennó et al.). Field tests indicate that misalignments between the MPS and the wind vector are usually smaller than ~30° during measurements in dust devils and that these misalignments always cause increases in the static pressure measured and decreases in the total pressure measured.