Planar Laser-induced Iodine Fluorescence Research Articles

Investigations of Peripheral Propulsive Deceleration Jets on a Mars Science Laboratory Aeroshell

The objective to send more massive landed missions to the surface of Mars necessitates further research and development for ways to adequately decelerate the lander, such as propulsive deceleration. Experimental measurements using planar laser-induced iodine fluorescence provide qualitative visualizations and quantitative propulsive decelerator jet mole fraction measurements over a 0.22% scaled Mars Science Laboratory aeroshell. Peripheral (off-centerline) sonic and supersonic propulsive decelerator jet models, with jet exit velocities of Mach 1.0 and 2.66, were studied in Mach 12 flow and compared with numerical results obtained using computational fluid dynamics. Experimental visualizations were obtained for various thrust coefficients ranging from 0.5 to 3.0 in increments of 0.5. Experimental results indicate that, for both sonic and supersonic jets, the bow shock is preserved between the peripheral jets and, as thrust coefficient increases, the bow shock is pushed farther from the aeroshell forebody, with the supersonic jets having a greater shock standoff distance than sonic models at thrust coefficient greater than 2.0. Comparisons of the experimental visualizations to computed streamlines and shock locations show good agreement. Computed propulsive decelerator jet mole fractions for both sonic and supersonic thrust coefficient of 1.5 closely match experimental results through the jet cores and in cross-sectional cuts.

Propulsion Deceleration Studies Using Planar Laser-Induced Iodine Fluorescence and Computational Fluid Dynamics

Future high-mass spacecraft entering the thin Martian atmosphere will exceed the capabilities of supersonic and subsonic parachutes and will incorporate other means of deceleration. Propulsive deceleration is one technology that is being considered. The interaction of the spacecraft aerodynamics with the propulsion deceleration jets has been shown to cause a decrease in drag coefficient with increasing thrust coefficient, which is not desirable for deceleration. Planar laser-induced iodine fluorescence images for a single-propulsion deceleration jet showed a lifting of the vehicle bow shock away from the aeroshell. Flowfield calculations showed that this lifting provided a shielding effect, preventing the freestream mass and momentum flux from reaching the aeroshell surface, creating a low-pressure region between the jet boundary and the aeroshell. With four peripheral propulsive deceleration jets, planar laser-induced fluorescence images and computational fluid dynamics calculations showed that the vehicle bow shock is maintained between the jets as the thrust coefficient is increased. This bow shock is responsible for greater drag preservation with the peripheral jets. The calculations also showed that high pressure is maintained between the peripheral jets. These results suggest that using a few peripheral propulsion-deceleration jets located near the aeroshell shoulder would provide the greatest drag preservation when using propulsive deceleration.

Quantitative Imaging of Injectant Mole Fraction and Density in a Supersonic Mixing

The fluorescence ratio technique for processing planar laser-induced fluorescence data was generalized for quantitative imaging of the injectant mole fraction and extended to quantify the density distributions in a nonreacting supersonic mixing flowfield. The original fluorescence ratio approach was first developed by Hartfield et al. (Hartfield, R. J., Jr., Abbitt, J. D., III, and McDaniel, J. C., Injectant Mole Fraction Imaging in Compressible Mixing Flow Using Planar Laser-Induced Iodine Fluorescence, Optics Letters, Vol. 1, No. 16, Aug. 1989, pp. 850-852.) for tests in a special closed-loop wind tunnel to eliminate the effects of thermodynamic property variations on planar laser-induced fluorescence signals in compressible flowfields. This approach provided us a quantitative means of planar mole-fraction measurement; however, it implicitly assumed that the tracer molecules were seeded at the same fraction in both the main and the secondary flows. In the present study, we generalized the Hartfield et al. method by considering differences in the tracer-seeding rates for obtaining planar images of mole fraction and density. Experimental validation of the new method was carried out in a mixing flowfield formed by sonic transverse injection into a Mach 1.9 supersonic airstream. The injectant mole-fraction distribution obtained from planar laser-induced fluorescence data processed by our new approach showed better agreement with the gas-sampling data than one based on the Hartfield et al. method. The density distribution was verified by comparison with the theoretical density ratio across the oblique shock wave.

Computational fluid imaging for iodine fluorescence in compressible flows

Computational Fluid Imaging (CFI) is a new field developing out of computational fluid dynamics (CFD). The effort described herein is the development and validation of a scheme for numerically imaging planar laser-induced iodine fluorescence in a compressible flow. The pressure and temperature required for the calculation of the fluorescence intensity are computed using a SPARK three-dimensional Reynolds averaged Navier-Stokes code developed at NASA Langley. The details of the fluorescence model are included herein and a comparison between a calculated image and an experimentally acquired image is shown.

Computer-controlled multiparameter flowfield measurements using planar laser-induced iodine fluorescence

A measurement technique for the mapping of complex compressible flowfields using planar laser-induced iodine fluorescence (PLIIF) is presented. Time-averaged values of all relevant flowfield properties are found : pressure, temperature, velocity, and, for mixing flowfields, injectant mole fraction. Several modifications have been made to a previous version of the PLIIF technique that allow multiple planes of data, needed for studying highly three-dimensional flowfields, to be collected and processed in greatly reduced time and with improved measurement accuracies. The technique is demonstrated with measurements in the flowfield of a Mach 2, rearward-facing step. The technique presented represents a unique tool that allows complex, compressible, three-dimensional flowfields to be mapped out completely and nonintrusively.

Complete three-dimensional multiparameter mapping of a supersonic ramp fuel injector flowfield

Planar laser-induced iodine fluorescence is used to map out the nonreacting mixing flowfield of an unswept ramp fuel injector using air injected at Mach 2.0 into a Mach 2.9 freestream. A fully automated test setup is used to measure time-averaged pressure, temperature, velocity, and injectant mole fraction on 21 crossflow planes and 7 axial planes. The measurement uncertainties are 5-8% for temperature, 4-10% for pressure, 10-20 m/s for velocity, and 2-3% for injectant mole fraction depending on the thermodynamic conditions. The measurements allow any desired gasdynamic quantity to be determined on a three-dimensional grid that spans the entire wind-tunnel test section. The experimental data set is comparable to the completeness of results normally available only from a computational fluid dynamics simulation. Results showing detailed flow features on specific planes, as well as overall quantities, such as global conservation checks, mixing performance, and flowfield losses, are presented. Mass, momentum, and energy flux, determined at the crossflow plane locations of the data set, show about a 2% standard deviation. The results are compared to a simulation using a three-dimensional Navier-Stokes solver. Agreement is reasonable with the exception of measurements in regions very close to walls, where the intensity of scattered light is high or where optical access is limited. The ability to generate extensive data sets, such as the one presented here, demonstrates that the planar laser-induced iodine fluorescence technique can be used 1) to generate detailed test cases for the validation of computational fluid dynamics codes and 2) as an alternative to computational fluid dynamics for performing design studies and performance evaluation in complex compressible flows.

Experimental and numerical study of swept ramp injection into a supersonic flowfield

Time-averaged measurements of pressure, temperature, velocity, and injectant mole fraction are presented using the planar laser-induced iodine fluorescence technique in the complex three-dimensional compressible flowfield around a swept ramp fuel injector. Within the range of thermodynamic conditions present in the test case studied, the technique's accuracy is estimated to be 4% for pressure, temperature, and velocity and 3% for injectant mole fraction. Comparisons with numerical simulations using the SPARK three-dimensional Navier-Stokes computer code with an algebraic turbulence model are made at the centerplane of the flowfield as well as on three crossflow planes downstream of the injector. Calculations and measurements are in good agreement throughout the flowfield, with deviations on the order of 5%; however, in specific regions, such as in the base of the ramp, deviations are larger. A weak asymmetry in the incoming flowfield appears to be amplified by boundary-layer separation occurring when the ramp-generated shock reflects off the tunnel walls. Ramp-generated vortices are weaker in the calculated results due to the effects of numerical viscosity in the vortex cores. This leads to less turning and mixing of the jet plume than observed in the experiments. The rate of decay of the maximum injectant mole fraction with streamwise distance is greater for the present ramp injection scheme than for previously measured transverse injection schemes. In this recirculation region at the base of the injector, laminar calculations show better agreement with the measurements than turbulent calculations.

Injectant mole-fraction imaging in compressible mixing flows using planar laser-induced iodine fluorescence

A technique is described for imaging the injectant mole-fraction distribution in nonreacting compressible mixing flow fields. Planar fluorescence from iodine, seeded into air, is induced by a broadband argon-ion laser and collected using an intensified charge-injection-device array camera. The technique eliminates the thermodynamic dependence of the iodine fluorescence in the compressible flow field by taking the ratio of two images collected with identical thermodynamic flow conditions but different iodine seeding conditions. The resulting images are, to our knowledge, the first quantitative planar measurements of mole-fraction distributions in a nonreacting compressible flow field and allow mixing to be studied directly.