Eric J. Jumper

AERO-OPTICAL CHARACTERISTICS OF COMPRESSIBLE, SUBSONIC TURBULENT BOUNDARY LAYERS

An extensive experimental study of optical aberrations due to propagation through fully-developed turbulent boundary layers at high subsonic Mach numbers was performed. Time-resolved, high- bandwidth, direct optical measurements of the dynamic aberrations were made using a Malley probe. The probe was used to obtain the convective speeds of the optically-significant turbulence structures and to measure the optical path differences. Measurements were made over a range of boundary layer thicknesses and Mach numbers. Optical distortions were found to scale linearly with boundary layer thickness and freestream density, and to go as the square of the freestream Mach number.

A Preliminary Study in Regularizing the Coherent Structures in a Planar, Weakly-Compressible, Free Shear Layer

This paper presents our first attempt at regularizing the coherent, vortical structures in a planar, relatively-high-Mach-number free shear layer. The purpose of regularizing the structures is to reduce the bandwidth required by an adaptive optic system to correct for the aero-optic aberrations imposed on a laser beam passing through the shear layer. In order to regularize the structures, plasma actuators were used. Analysis is given that suggests that the plasma actuators are best suited for this regularization. Results are presented for a single actuator design operated in two modes using four ac waveforms. This configuration showed mixed but somewhat encouraging results.

Forcing of a Two-Dimensional, Weakly-Compressible Subsonic Free Shear Layer

Forcing of a two-dimensional, weakly-compressible subsonic shear layer has been experimentally demonstrated. The measurements were made in a new compressible shearlayer facility that mixes highand low-speed flows with speeds up to M = 1. The forcing was performed using voice-coil actuators mounted at the trailing edge of the splitter plate separating the highand low-speed flows at the inlet to the mixing section. The ability of the forcing actuator to organize the shear layer into large-amplitude vortical structures was verified from Malley-probe measurements of shear-layer turbulent spectra, flow visualization, and optical wavefront measurements. The ramifications of the results on adaptive-optic control of the optical aberrations imposed by the shear layer are discussed.

REARWARD FORCING OF AN UNSTEADY COMPRESSIBLE CASCADE

Phase-locked, steady and unsteady, surface pressure measurements were made on the vanes of a compressible cascade, for inlet Mach numbers up to —0.7. The cascade vanes were production hardware stator vanes from an AlliedSignal F109 turbofan engine. Data are presented from which it may be concluded that the cascade produces reasonably good two-dimensional flow through the vane row. Unsteady forcing within the cascade can be produced either upstream or downstream of the vane row. In the present paper, flow unsteadiness in the cascade was produced using shedding from circular cylinders positioned at a distance equal to 80 % vane chords downstream of the vane row. This rearward forcing of the vane row resulted in unsteady surface pressures of the same order as those produced in similar experiments with forward forcing. Decomposition of the ensembled, phase-locked, pressure-response signals for rearward forcing provided insight into both the nature of the unsteady disturbance and the physics of its propagation within the vane row.

Upstream-Propagating Potential Disturbances Interacting with A Compressible Cascade

Phase-locked, unsteady surface-pressure measurements were made at eight suction-side and six pressure-side chord locations on the turning vanes of a compressible cascade. Cascade inlet Mach numbers ranged from 0.427 to 0.500, resulting in a maximum local Mach number through the cascade vane row of 0.730. Unsteady forcing of the cascade was obtained by von Karm´ an vortex shedding from a row of e ve circular cylinders positioned 0.80 vane chords downstream of the cascade trailing edge. The resulting measured surface pressures were found to be on the same order of magnitude as those measured previously in forward-forcing studies, in which the circular cylinders were placed 0.80 vane chords upstream of the vane-row leading edge. Frequency decomposition of the rearward-forced surface-pressure signals provided for the construction of disturbance position-vs-phase maps, allowing the measured disturbances to be unambiguously interpreted as potential in nature, originating at the forcing cylinders, and propagating upstream through the vane row at acoustic speeds. The effect of these potential disturbancesonthevane-rowpressureresponsewastoelicittwo,previouslyunexpectedamplie cationmechanisms: a large-magnitude pressure rise near the vane-row trailing edge and a Mach-number-related, midchord pressure increase known as acoustic blockage.

On the Use of Reynolds Number as the Scaling Parameter for the Performance of Plasma Actuator in a Weakly Compressible Flow

Dimensional analysis of momentum integral equation of boundary-layer flow in the presence of body force was performed to determine the scaling law for plasma body force for an application in boundary layer flow control. The motivation was to relate experiments on a model in a low-speed flow to the prototype in a high-speed, weakly compressible, flow. The analysis showed that the presumption that the matching of Reynolds number will match the effect of the body force on the boundary layer velocity profile was not substantiated. The scaling suggested a requirement for a change in the body force per unit span for a change in the size of the prototype in order to produce similar effect on boundary layer velocity profile.

An experimental verification of the weakly-compressible model

This paper has a narrow purpose; it addresses the principle question lying at the heart of the weaklycompressible model: whether or not large static-pressure fluctuations are present in a high-Mach, free shear layer. Had these pressure fluctuations not been present, the weakly-compressible model could not be correct; however, as the paper describes, such pressure fluctuations are, indeed, present and in good agreement with the predictions of the weakly-compressible model. This vindication of the model is important because it explains the principle cause for such devastating effects on the focusability of a laser beam that is made to propagate through a near-transonic free shear layer. Further, the weakly-compressible model affords a foundation for generalizing specific experimental results for optical propagation through a shear layer at one set of flow conditions to other conditions at different Mach numbers and altitudes.

Atmospheric Propagation Vs. Aero -Optic s

For decades the parameters of Cn , the Fried parameter (r0), and the Greenwood frequency (fG) have been used to characterize optically distorting flows and to guide the design of systems to correct the resulting distortions. However, their use has become so widespread that few stop to consider the origins and underlying assumptions of these terms. In recent years interest has grown in use of airborne optical systems, but it is questionable whether the aforementioned parameters can be applied to the conditions in the immediate vicinity of such systems in flight in a meaningful way. Despite this, there are ways of finding and expressing the relevant factors of such flows.

Upstream-propagating acoustic waves interacting with a compressible cascade

Phase-locked, unsteady, surface-pressure measurements were made on the vanes of a compressible cascade, for inlet Mach numbers up to 0.5, which resulted in Mach numbers through the vanes of up to 0.73. Flow unsteadiness was produced by shedding from circular cylinders positioned at a distance of 80 percent chord downstream of the vane row (rearward forcing). When the unsteady pressure response from this rearward forcing was compared with the response from forward forcing, rearward forcing resulted in the same order of unsteady surface pressure response as forward forcing. Decomposition of the ensembled, phase-locked signals for rearward forcing demonstrated unambiguous position-vs-phase curves that showed that the unsteady disturbances originate from downstream and propagate forward through the vane row at acoustic speeds into the oncoming flow. These acoustically-radiated disturbances appear to have elicited two previously-unexpected response amplification mechanisms: a possible trailing-edge singularity; and a Mach number-related amplification that may be evidence of acoustic blockage. (Author)

Upstream-Propagating Potential Disturbances in the F109 Turbofan Engine Inlet Flow

An experimental investigation of the unsteady-velocity e eld upstream of the single-stage axial fan in a F109 turbofan enginewas performed. Phase-locked, three-component velocity measurements were collected at multiple axial, radial, and azimuthal locations directly forward of the fan, out to 1.0 fan-blade chords upstream. The measured velocity data were ensemble averaged and resolved into mean and unsteady components, where the unsteady components were further decomposed into elements of amplitude, frequency, and phase for the primary and e rst-harmonicfrequencies. Analysis of the primary-frequency phaseinformation demonstrated the measured unsteady velocities to be potential in nature, generated by the fan, and to propagate upstream at acoustic speeds into the engine inlet e ow. Furthermore, the axial component of the unsteady velocity data was found to exhibit peak-to-peak e uctuations of nearly 20% of the mean-axial velocity very near the fan, whereas the azimuthal (swirl)component exhibited e uctuations reaching nearly 50% of the mean-axial velocity at thesamemeasurement locations.Acomputationalsimulationofthevelocitye eldupstreamoftheF109fandemonstratedgoodquantitative agreement with the measured unsteady velocity amplitudes.