Experimental and Numerical Calibration of Hotwire Anemometers for the Study of Stratospheric Turbulence

Understanding small-scale turbulence in the middle stratosphere is necessary for the characterization and prediction of boundary layer laminar to turbulent transition on hypersonic vehicles. A current research goal is to characterize stratospheric turbulence using in-situ measurements of velocity fluctuations using hotwire anemometers mounted on weather balloons that traverse through the stratosphere. An integral step in making these in-situ turbulence measurements is calibration of the hotwire instruments. This article discusses the experimental and numerical approach that is being used to calibrate and understand hotwires in stratospheric conditions. A low-speed, low-pressure wind tunnel developed to create the stratospheric conditions for hotwire calibration is described. Direct simulation Monte Carlo (DSMC) simulations of the rarefied flow around the hotwires supplement the experimental work. Recent modifications and improvements to the wind tunnel are discussed, and preliminary data from the tunnel are presented. Significant end-conduction effects are observed. Details about the numerical simulations are presented, and results show good agreement with published experimental data.