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Laser Induced Thermal Acoustics for Flow Diagnostics

Laser-Induced Thermal Acoustics (LITA) is a nonlinear optical technique for remotely measuring the sound speed and transport properties of a fluid with accuracy comparable to the best conventional intrusive methods. LITA excels in high pressure environments, where many other diagnostics fail. The development of the technique has addressed issues including the need to make single-pulse measurements in challenging environments.

LITA has been demonstrated to be an accurate tool for measuring gas properties. An analytical expression for the signal accurately models experimental signals. Over a wide range of pressure and in gases ranging from He to near-critical carbon dioxide, LITA measurement of the sound speed featured ~0.1% accuracy. Thermal diffusivity measurements at elevated pressures can feature ~1% accuracy under ideal experimental conditions. The ability to measure transport properties is important both for fundamental and applied studies, since transport is of primary importance in phenomena ranging from combustion to boundary layer behavior in hypersonic flows. LITA may also be applied for making other measurements in systems of scientific or engineering interest.

LITA schematic

Schematic diagram of the narrow-band LITA experimental arrangement.

LITA scattering involves driving of perturbations of a medium via opto-acoustic effects and coherent scattering of light off these perturbations into a coherent signal beam via acousto-optic effects. In the driving step, light from a pulsed laser is split into two phase-coherent beams that intersect at a shallow angle in the medium. The region of intersection of the beams defines a sample volume of the LITA measurement. Interference of the driver beams creates an electric field intensity grating in the sample volume. The medium responds to the intensity of these driver beams. In an effect called thermalization, light energy absorbed by the medium subsequently heats the medium locally over a short time. The medium expands thermally upon heating. If the thermal expansion speed is comparable to the sound speed, sound waves are generated. A second effect is called electrostriction, in which molecules of the medium are accelerated toward or away from regions of intense light, depending on molecular properties. If the driving is rapid, the acceleration is impulsive and sound waves are created. If the driving is slow, molecules diffuse into or away from the regions of intense light, creating species concentration gradients. Through these and other generalized opto-acoustic effects, the driver laser can perturb the bulk properties of a medium.

The perturbations induced by the driver laser create perturbations in the susceptibility of the medium via acousto-optic effects. These perturbations therefore have the appearance of volume diffraction gratings. In the coherent scattering step, a CW or long-pulse source laser is trained on the sample volume at the Bragg angle of the gratings. Light from the source laser is scattered off the gratings into a coherent signal beam that is detected through large f-number optics. The signal is modulated in time by the evolution of the laser-induced perturbations. From this modulation, accurate physical properties of the medium may be inferred, including the sound speed, thermal diffusivity, and other properties that affect the evolution of the laser-induced gratings.

Gas property measurements are extracted from the recorded signals by a process of optimal nonlinear filtering. The nonlinear filter is a global least-squares fit between theory and experiment with the gas properties as adjustable parameters. This filtering vastly reduces the effect of signal noise and improves measurement accuracy.

LITA in operation

Photo of LITA beams entering test cell in APRI's device.

Inquiries concerning APRI's LITA program should be directed to Dr. Thomas Sobota at thomas.sobota@apri.com


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