Laser-induced thermal acoustics 1LITA2 is a four-wave mixing technique that may be employed to measure sound speeds, transport properties, velocities, and susceptibilities of fluids. It is particularly effective in high-pressure gases 1.1 bar2. An analytical expression for LITA signals is derived by the use of linearized equations of hydrodynamics and light scattering. This analysis, which includes full finite-beam-size effects and the optoacoustic effects of thermalization and electrostriction, predicts the amplitude and the time history of narrow-band time-resolved LITA and broadband spectrally resolved 1mulitplex2 LITA signals. The time behavior of the detected LITA signal depends significantly on the detection solid angle, with implications for the measurement of diffusivities by the use of LITA and the proper physical picture of LITA scattering. This and other elements of the physics of LITA that emerge from the analysis are discussed. Theoretical signals are compared with experimental LITA data.

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. REPORT DATE (DD-MM-YYYY) SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)Air Force Research Laboratory (AFMC) AFRL/PRS SPONSOR/MONITOR'S Pollux Drive NUMBER(S)Edwards AFB CA 93524-7048 AFRL-PR-ED-TP-2007-164 14. ABSTRACT An experimental study on the effects of an externally-imposed transverse acoustic field in sub-, near-, and supercritical N2 coaxial jets is presented. Such fields and their interaction with the jets (i.e., breakup, mixing, etc.) is believed to play a critical role during combustion instabilities in liquid rocket engines. The shear coaxial injector used here is similar to those used in cryogenic liquid rockets. By using N2 as the working fluid, the chemistry effects on combsution instability are separated from the effects of a transverse acoustic field on coaxial jets. Furthermore, through this choice, ambiguities associated with compostion dependence on mixtures critical properties are eliminated. The acoustic oscillations are generated by a piezo-siren and have a frequency of ~3kHz. The pressures in the chamber range from 215-716 psia to span sub-, near-, and supercritical conditions. The outer to inner jet velocity ratio varies from ~1.2 to 23 and the momentum flux ratio varies from ~0.2 to 23. These ratios are mainly varied by changing the temperature and flow rates of the outer jet. At least 2000 backlit images were taken at 41kHz for each run. The main metric investigated is the length of the dark, or inner jet, core length. Both the axial length of the jet and the total, or curved, length are studied. A functional relation of the form A/MR n describes the behavior of the axial length with the exponent being 0.2 (A:20-25) for subcritical conditions and 0.5 (A:5-12) for near and supercritical conditions. These results agree with historical data. The standard deviation of the axial length, which due to the large number of data points is within 0.03% of the RMS of the jet lengths fluctuations, also decreases with velocity ratio, for sub-, near-, and supercritical conditions. For momentum flux ratios ~O(1) the axial and total length differences between ac...

Supercritical pressure conditions designate a situation where the working fluid pressure is above the critical point. Among these conditions, it is interesting to identify a transcritical range which corresponds to cases where the pressure is above the critical point, but the injection temperature is below the critical value. This situation is of special interest because it raises fundamental issues which have technological relevance in the analysis of flows in liquid rocket engines. This situation is here envisaged by analyzing the behavior of a nitrogen shear coaxial jet comprising an inner stream injected at temperatures close to the critical temperature and a coaxial flow at a higher temperature. Experiments are carried out both in the absence of external modulation and by imposing a large amplitude transverse acoustic field. Real gas large eddy simulations are performed for selected experiments. The combination of experiments and calculations is used to evaluate effects of injector geometry and operating parameters. Calculations retrieve what is observed experimentally when the momentum flux ratio of the outer to the inner stream \documentclass[12pt]{minimal}\begin{document}$J= (\rho _eu_e^2)/(\rho _iu_i^2)$\end{document}J=(ρeue2)/(ρiui2) is varied. Results exhibit the change in flow structure and the development of a recirculation region when this parameter exceeds a critical value. The instantaneous flow patterns for different momentum flux ratios are used in a second stage to characterize the dynamical behavior of the flow in terms of power spectral density of velocity and density fluctuations. Results obtained under acoustic modulation provide insight into mixing enhancement of coaxial streams with a view of its possible consequences in high frequency combustion instabilities. It is shown in particular that the presence of strong acoustic modulations notably reduces the high density jet core length, indicating an increased mixing efficiency. This behavior is more pronounced when the jet is placed at the location of maximum transverse velocity fluctuations.

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT 18. NUMBER OF PAGES 19a. NAME OF RESPONSIBLE PERSON

An experimental method for introducing carbon dioxide as a means of stabilizing a hypervelocity boundary layer over a slender bodied vehicle is investigated through the use of numerical simulations. In the current study, two different test bodies are examined. The first is a five-degree-half-angle cone currently under research at the T5 Shock Tunnel with a four cm porous wall insert used to transpire gas into the boundary layer. The second test body is a similar cone with a porous wall over a majority of the cone surface. Computationally, the transpiration is performed using an axi-symmetric flow simulation with wall-normal blowing. The effect of the injection and the transition location are gauged by solving the parabolized stability equations and using the semi-empirical e N method. The results show transition due to the injection for the first test body and a delay in the transition location for the second test body as compared to a cone without injection under the same free-stream conditions. The mechanism for the stabilizing effect of carbon dioxide is also explored through selectively applying non-equilibrium processes to the stability analysis. The results show that vibrational non-equilibrium plays a role in reducing disturbance amplification, however, other factors also contribute.