# An efficient optical pressure measurement in compressible flows: Laser-induced iodine fluorescence

- Received:
- Accepted:

DOI: 10.1007/BF00189703

- Cite this article as:
- Lemoine, F. & Leporcq, B. Experiments in Fluids (1995) 19: 150. doi:10.1007/BF00189703

## Abstract

The understanding of complex phenomena, the experimental validation of calculation codes, and the construction of data bases in fluid mechanics require the development of non-intrusive experimental techniques. The laser-induced fluorescence of a gaseous molecule seeded into a gas flow can be related to the flowfield thermodynamic parameters, such as pressure and temperature. An experimental method is described, that allows the removal of the temperature dependence of the fluorescence signal. A narrow-bandwidth single-line laser is tuned to the center of an absorption line, whose temperature dependence of the Boltzmann fraction can be neglected. The experimental set-up requires a single-line dye laser and a high resolution spectral analysis device.

The accuracy of the method, checked in a static vessel, appears to be better than 5%. The method has been successfully tested with a supersonic jet issuing from an underexpanded nozzle.

The experimental results have been compared to those of an Euler calculation. A mean difference of 14% has been observed, but a major part of this can be attributed to the difference between inviscid and real gas calculation.

### List of Symbols

*A*_{21}spontaneous emission Einstein coefficient

*B*_{v}rotational energy of the

*J*level for the*v*vibrational level*B*_{e}first order approximation of

*B*_{v}*c*velocity of light

*C*_{opt}optical constant

*E(v)*vibrational energy of the

*v*level*f*_{1}Boltzmann fraction

*FCF*_{i}Franck-Condon factor of the

*i*th line*f*_{v}vibrational fraction

*tf*_{r}rotational fraction

*g*efficient spectral power density

*h*Planck constant

*I*_{2}*X*iodine molecule in the

*X*state*I*_{2}*B*iodine molecule in the

*B*state- [
*I*_{2}*X*] molecular concentration of

*I*_{2}*X*- [
*I*_{2}*B*] molecular concentration of

*I*_{2}*B**I*_{i}relative intensity of the ith absorption line

*J*″rotational level of the fundamental state

*J*′rotational level of the excited state

*k*Boltzmann constant

*k*_{c}collisional broadening coefficient

*m*iodine molecular mass

*n*_{0}initial concentration of iodine in the absorbing state

*P*pressure

*P*_{laser}laser power

*P*_{l(v)}laser power spectral density

*P*_{sl2}iodine vapor pressure

*Q*quenching rate

*Q*_{v}vibrational partition function

*T*temperature

*v*″vibrational level of the fundamental state

*v*′vibrational level of the excited state

*V*_{c}collection volume

*X*_{l}_{2}iodine molar fraction

*λ*wave length

*Δv*_{D}Doppler linewidth, at half maximum

*Δv*_{c}collisional linewidth at half maximum

*Vl*laser frequency

*δ(v)*Dirac function

*Δv*_{i}spectral location of a line, referenced to the laser frequency