1 3 Exp Fluids (2016) 57:183 DOI 10.1007/s00348-016-2267-9 RESEARCH ARTICLE A molecular Rayleigh scattering setup to measure density fluctuations in thermal boundary layers J. Panda 1 Received: 24 June 2016 / Revised: 13 September 2016 / Accepted: 20 October 2016 © Springer-Verlag Berlin Heidelberg (outside the USA) 2016 fluctuations in scalars such as density and temperature are as important as that of velocity; however, existing tools, such as particle image velocimetry, and intrusive probes, cannot measure density fluctuations. As for the measure- ment of temperature fluctuations, commercially available fine thermocouple probes are capable of a frequency range of a couple of 100 Hz, and a cold-wire constant current probe extends that to a couple of kHz, albeit for a small temperature range. The current endeavor is a part of an effort to develop a particle-free, non-intrusive, molecu- lar Rayleigh scattering-based technique to simultaneously measure density, velocity, and temperature fluctuations in wind tunnel applications. Multiple obstacles exist for suc- cessful measurement. The present work, using a demon- stration unit in a low-speed research tunnel, attempts to find the means to overcome these obstacles and provide a step- ping stone for larger applications. The molecular Rayleigh scattering technique provides a fundamental way of measuring flow properties of gases. For the present work, the measurement of air density is of interest. In the author’s experience, in a new setup, once all steps for density measurement are implemented, opti- cal spectral analysis to measure velocity and temperature becomes easier. The intensity of the Rayleigh scattered light is directly proportional to the molecular number density, which in turn is related to bulk density. The pro- portionality constant between the bulk density and the Rayleigh intensity is obtained via a calibration procedure. Once the time variation of the scattered light intensity was measured, a Fourier transform and an application of the calibration constants provided spectra of air density fluc- tuations. The technique has been used extensively in high- speed free jets (Panda and Seasholtz 1999, 2002; Mielke et al. 2009 among others), and premixed combusting flows (Pitts and Kashiwagi 1984; Ng et al. 1982; Robben Abstract A Rayleigh scattering-based density fluctuation measurement system was set up inside a low-speed wind tunnel of NASA Ames Research Center. The immediate goal was to study the thermal boundary layer on a heated flat plate. A large number of obstacles had to be overcome to set up the system, such as the removal of dust particles using air filters, the use of photoelectron counting elec- tronics to measure low intensity light, an optical layout to minimize stray light contamination, the reduction in tunnel vibration, and an expanded calibration process to relate photoelectron arrival rate to air density close to the plate surface. To measure spectra of turbulent density fluc- tuations, a two-PMT cross-correlation system was used to minimize the shot noise floor. To validate the Rayleigh measurements, temperature fluctuations spectra were cal- culated from density spectra and then compared with tem- perature spectra measured with a cold-wire probe operated in constant current mode. The spectra from the downstream half of the plate were found to be in good agreement with cold-wire probe, whereas spectra from the leading edge dif- fered. Various lessons learnt are discussed. It is believed that the present effort is the first measurement of density fluctuations spectra in a boundary layer flow. 1 Introduction Measurement of various turbulence statistics in high-speed, compressible flows and heated flows is extremely diffi- cult using current experimental techniques. In such flows * J. Panda Jayanta.Panda-1@nasa.gov 1 NASA Ames Research Center, Moffett Field, CA, USA