Highly localized pressure perturbations induced by laserabsorptive heating in the shear layer of a gas jet
D. C. Fourguette and M. B. Long
Yale University, Center for Laser Diagnostics and Department of Mechanical Engineering, New Haven, Connecticut 06520
Received August 22,1983
A technique to create a localized pressure perturbation by laser heating the absorbing gas inside the flow field is
described. The effect of the laser-induced pressure pulse is to trigger the development of turbulent structures,which then evolve in time. With such external perturbation, the nozzle gas concentration is measured simulta-neously at 104 points within a plane intersecting the flow by using two-dimensional imaging of the Lorenz-Mie-scattered light from aerosols seeded in the flow. The instantaneous and time-averaged spatial distributions of theperturbed flow field are presented as is the rms fluctuation of the concentration.
It is well known that gas jets are sensitive to periodicdisturbances.1"2 Rayleigh3 first introduced the stro-boscopic illumination technique to freeze the motionof a forced cold jet, and, since that time, differentmethods of excitation have been used to perturb jets.Jets can be forced with sinusoidal acoustic wavesemerging from a loudspeaker placed in the vicinity ofthe nozzle4 or connected to the chamber upstream of thenozzle.5 The common approach to all these tech-niques6 -9 is that the acoustic perturbations are appliedat the boundary layer of the jet, close to the nozzle.Organized turbulent structures therefore result fromthe periodic density perturbations at the boundarylayer.
Density perturbations can also be achieved by rapidheat addition in a small volume,10 which results in eithera pressure wave or, if enough heat is added, a shockwave. By using the optoacoustic phenomenon,1 it ispossible to create a temperature increase in a gas byfocusing infrared-laser radiation into'a volume con-taining optically absorbing gas molecules. This opto-acoustic phenomenon has been used previously tomeasure simultaneously the velocity and temperatureof a cold jet12 as well as to induce a sound wave in flamesby seeding the reacting flow with optically absorbing Naatoms.'3
We report a new technique that enables us to createa localized, short-duration pressure perturbation on theshear layer at any distance downstream from the nozzleexit. To create a localized perturbation over the entirejet cross section, the laser beam must be focused into asheet whose thickness is small compared with the nozzlediameter. The pressure perturbation must also beshort relative to the characteristic time scale of the flow.Under these conditions, the effect of the pulsed per-turbation on the shear layer can be studied before theportion of flow disturbed by the reflection of the pres-sure pulse at the nozzle exit appears in the region ofobservation. This allows the acoustic perturbation onthe shear layer to be distinguished from that on theboundary layer. To create the pressure pulse, a
broadband C02 laser beam was focused into a jet ofethylene, which, because of its vibrational levels,strongly absorbs the laser energy. Unlike in the twoprevious studies,' 2' 1 3 no atomic seeding or gas break-down was necessary.
A pulsed C02 laser with output wavelength at 10.6 Am(Tachisto Tac II 150 XR) was equipped with broadbandcavity mirrors. The laser provided 125 mJ of energy ina 40-nsec pulse. The laser spot size (1.6 cm X 0.4 cm)was focused onto the jet (nozzle diameter, d = 4 mm)with a spherical lens (focal length, 700 mm) into a sheet1 mm thick and 4 mm wide, which was orthogonal to thedirection of the flow. Optoacoustic studies of ethylenegas at high laser fluences in the 10.6-gm region14 "15 haverevealed an absorption profile different from the linearabsorption case. The laser power (78 MW/cm2) usedin this experiment lies in the nonlinear range of ab-sorption of ethylene, indicating that multiphoton ab-sorption processes occurred. A radiative transition at5 pm, which becomes populated through multiplephoton absorption, dissipates a portion of the energy,and the other portion is transformed into translationalenergy. The relative efficiency of the two processes wasnot measured in the current experiment.
The nozzle gas concentration distribution with andwithout C0 2-laser-induced acoustic perturbation wasdetected by a concentration mapping technique capableof making instantaneous two-dimensional measure-ments.16 The ethylene gas was seeded with submi-crometer-sized particles, which did not affect the gen-eration of the pressure pulse in the ethylene gas. Anargon-ion-laser beam (514.5 nm) was formed into asheet 20 mm X 20 mm X 0.2 mm by two cylindrical re-flectors, which refocused the argon laser beam uponeach reflection. The illumination sheet was parallel tothe flow direction (Fig. 1) and intersected the center ofthe jet. The elastically scattered light intensity wasproportional to the number of particles in a given vol-ume and therefore proportional to the amount of eth-ylene gas present in that same volume. The scatteredlight from the microparticles in the flow field was col-
Fig. 1. Experimental apparatus used for obtaining thetwo-dimensional gas concentration of a flow field forced bya highly localized, laser-induced perturbation.
obtained under similar conditions.16 The near top-hat-shaped profile of the average concentration ischaracteristic of a flow in which the formation oflarge-scale structures does not occur in the region beingobserved. The ensemble average of the 60 frames re-corded with the perturbation caused by the CO2 laserbeam is shown in Fig. 4(a). The concentration distri-bution for this averaged case is clearly similar to theinstantaneous realization shown in Fig. 2. The Tms
C- -I Am 5d
limated by one lens and then imaged with a second lensonto the face of a low-light-level computer-controlledTV camera. The camera was operated in pulsed mode(35 gsec), and the cw argon-ion-laser beam was choppedby an acousto-optic modulator (also 35 psec). Withinthis time, the flow field can be considered to be frozen.Each detected flow-field image was digitized in a 100 X100 point format, yielding a resolution of 0.12 mm X 0.12mm X 0.2 mm for each point. The flow-field image wascorrected for the electronic background noise and forthe nonuniformity of the illuminating sheet. Onedigitized image recorded the gas concentration over theportion of the flow field between 5 and 8 nozzle diame-ters downstream. The undisturbed flow had an exitvelocity of 4 m/sec and showed no large-scale structuresin the region under observation.
The C02-laser beam intersected the jet at 5 nozzlediameters downstream from the nozzle exit. Figure 2shows the flow field 4 isec after the pressure pulse hasoccurred. Organized turbulent structures can be clearlyobserved 7 diameters downstream. These structuresare the result of the perturbation of the shear layer atthe laser focal volume by the original pressure pulse.The effect of the interaction of the upstream -travelingacoustic pulse with the boundary layer at the nozzle lipcould be seen only 8-20 msec after the main pulse.
In order to check the repeatability of the concentra-tion distribution displayed in Fig. 2, two sets of 60 in-dividual frames were recorded under the same experi-mental conditions as described above, one set withlaser-induced perturbation and the other set without.The ensemble average and the rms fluctuation of the gasnozzle concentration were calculated for both of thesesets of data. The ensemble average is shown in Fig.3(a), and Fig. 3(b) shows the rms fluctuation of theconcentration without laser-hnduced perturbation.These results are in agreement with those previously
Fig. 2. Instantaneous two-dimensional gas concentrationof the flow with laser-induced perturbation.
(a)
r
3
--5§
Fig. 3. (a) Average nozzle gas concentration without laserperturbation and (b) normalized rnms deviation of the gasconcentration for the 60 instantneous shots Aithout laser-induced perturbation.
Fig. 4. (a) Ensemble average over 60 instantaneous shotsforced with the laser-induced perturbation and (b) normalizedrms deviation of the gas concentration for the 60 instanta-neous shots with laser-induced perturbation.
fluctuation of the gas concentration with laser-inducedperturbation is shown in Fig. 4(b). Here, the maximumrms fluctuation occurs along the edge of the large-scalestructure, indicating that some small variation in thelocation of the structures does exist on a shot-to-shotbasis. The magnitude of these fluctuations, however,is similar to that observed in the unforced case in whichthe jet is nearly laminar in the region observed.
Results described above indicate that laser heatingmakes possible the creation of a highly localized and
short-duration pressure pulse inside a flow field.Further, the large-scale structures induced by laserheating are repeatable, which should make the tech-nique valuable for studying the temporal evolution ofstructures induced in this way. The localized natureof this perturbation technique makes it possible to in-terpret more readily the mechanisms of interactionbetween the pressure wave and the various regions ofthe shear layer. Additional experiments are under wayto obtain a better understanding of the nature of theacoustic-wave, flow-field interaction.
We thank Richard K. Chang and Boa-Teh Chu formany useful discussions and gratefully acknowledge thepartial support of this work by the National ScienceFoundation (grant no. CPE81-05976) and the U.S. Of-fice of Naval Research, Project SQUID (contract no.N00014-79-C-0254).
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