Nicholas G. De Lucca

In-flight aero-optics of turrets

Abstract. Recent in-flight aero-optical measurements from the Airborne Aero-Optics Laboratory are provided, along with instrumentation and experimental set-up. Results of an extensive survey of the aero-optical environment at different viewing angles, for both flat-window and conformal-window turrets at different subsonic and low transonic speeds below M=0.65, are presented, compared and extensively discussed. A comparison between two turret geometries, hemisphere-on-cylinder and hemisphere only, plus the statistical analysis of wavefronts at different viewing angles, are also presented and discussed. Additionally, dynamics of a local shock appearing on the conformal-window turret at transonic Mach number are discussed.

The Study of Aero-Optical and Mechanical Jitter for Flat Window Turrets

Simultaneous local jitter, 2-D wavefronts and accelerometer measurements were performed in the flow over a flat-window turret at Mach numbers between 0.3 and 0.4 along a centerplane with a viewing angle varying between 90 and 118 degrees. Both local and global jitter imposed on a smalland large-aperture laser beams were measured using positionsensing devices and a high-speed 2-D wavefront sensor. A linear stochastic estimation technique was applied to separate the mechanically-related component of the jitter from the aero-optical component. Spectra of aero-optical jitter for different elevation angles and speeds were calculated and useful scaling laws were proposed. It was shown that aero-optical jitter was due to the presence of both the stationary and traveling aero-optical components. Detailed analysis of the global and the local jitter spectra and the spectral cross-correlation between them at different speeds and viewing angles is presented and discussed in details. Additionally, PIV system was used to measure the flow field over the flat window, simultaneously with the aero-optical jitter measurements.

Comparison of Aero-Optical Measurements from the Flight Test of Full and Hemispherical Turrets on the Airborne Aero-Optics Laboratory

The optical environment around both a hemisphere-on-cylinder turret and a hemisphereonly turret with flat and conformal windows was characterized at both subsonic and transonic speeds. Data was taken from Mach 0.4 to 0.65 at altitudes from 15,000 ft to 30,000 ft to analyze scaling laws, with a focus on Mach numbers of 0.5 and 0.65. A 25 kHz ShackHartmann wavefront sensor was used to measure the optical aberrations around the turret. Data were primarily collected while the two planes slewed by, giving statistics at several azimuthal/elevation angles with additional data taken at fixed angles. Additionally, dynamics of a local shock appearing on the conformal-window turret at the transonic Mach number are presented and discussed.

The Airborne Aero-Optics Laboratory, recent data

In this paper recent in-flight aero-optical measurements on the Airborne Aero-Optics Laboratory (AAOL) will be given. Instrumentation and experimental set-up will be presented. Results of an extensive survey of the aero-optical environment at different viewing angles for both flat-window and conformal-window turrets at different subsonic and low transonic speeds, below M = 0.65, will be presented, compared and extensively discussed. The statistical analysis of wavefronts at different viewing angles will is also presented and discussed.

Aero-Optical Investigation of Transonic Flow Features And Shock Dynamics on Hemisphere-On-Cylinder Turrets

Aero-optical environment around a hemisphere-on-cylinder turret with both flatand conformal windows was studied experimentally in-flight using AAOL-T for a range of subsonic and transonic Mach numbers between 0.5 and 0.8. Above M = 0.6, the local shock appeared near the top of the turret, causing additional aero-optical distortions at sidelooking angles. The shock dynamics was extracted from instantaneous wavefronts and analyzed in details. The mean shock location was found be near  = 80 degrees for both window types at M = 0.7 and 0.8. For M = 0.7, the shock motion exhibits multiple-harmonic behavior between StD = 0.15 and 0.8, while for M = 0.8, the shock has a lower single frequency peak at StD = 0.15, the same as for the unsteady separation line, indicating a possible lock-in mechanism between the shock and the separation region at high transonic Mach numbers.

Study of Unsteady Surface Pressure on a Turret via Pressure-Sensitive Paint

Fast-response pressure-sensitive paint (PSP) was used in this work for a study of the unsteady surface pressures resulting from the complex separated flow over a hemispherical turret model. The turret includes several distinct features such as a flat window and crevices that are based on functional requirements, but also introduce interesting additional compact unsteady flow features. Three high-speed cameras imaged the paint luminescence at 2 kHz, producing pressure time histories over the entire model. These pressure data were integrated over the model surface in order to determine unsteady loads. Fast-response PSP successfully resolved very small pressure features that had aperiodic fluctuations of at least several kilohertz.

The Estimation of the Unsteady Aerodynamic Force Applied to a Turret in Flight

The spatially-temporally-resolved pressure field on a surface of a hemisphere-on-cylinder optical turret was characterized in a wind tunnel at M = 0.33 using fast-response Pressure Sensitive Paint (PSP), simultaneously with 8 pressure sensors. A sparse unsteady pressure field was also obtained in-flight for subsonic speeds of M = 0.5 on the Airborne Aero-Optical Laboratory (AAOL) using 26 unsteady pressure sensors. Using the unsteady pressure data from PSP tests, the unsteady forces acting on the turret in the tunnel were calculated and discussed for different turret geometries and window viewing angles. Unsteady forces and moments acting on the turret in-flight were estimated using Linear Stochastic Estimation for different turret geometries and speeds and compared to the tunnel results. Using Proper Orthogonal Decomposition dominant surface pressure modes responsible for unsteady forces were identified and discussed.

Global Unsteady Pressure Fields Over Turrets In-Flight

Point pressure measurements were performed on a hemisphere-on-cylinder turret in-flight and in the wind tunnel at the University of Notre Dame. Measurements were performed at a total of 36 different locations on the turret. The pressure coefficients (cp) was analyzed versus viewing angle for each Mach number and were compared with previous experiments and theoretical values based on the potential flow. At transonic Mach numbers, the dominant frequencies of the motion of the local shock over the turret were investigated. The turret wake was also investigated and it was shown that the “smiles” cut-outs and the local shock can have a “regularizing” effect of decreasing higher frequency fluctuations while increasing low frequency fluctuations. Unsteady forces acting on the turret were computed from the pressure fields and their dependence on the Mach number and turret geometry was discussed.

Effect of Surface Unsteady Pressure Field on Global Beam Jitter for a Hemisphere-on-Cylinder Turret

Simultaneous point surface pressure measurements, accelerometer, optical wavefront and jitter measurements were performed using a hemisphere-on-cylinder turret with realistic geometric features like “smiles” at M = 0.33 and M = 0.4. A Linear Stochastic Estimation (LSE) technique was used to separate the aero-optical and the aero-mechanical jitter components. The unsteady pressure field on the turret was approximated using a combination of LSE technique and Proper Orthogonal Decomposition. To determine the driving factors of the aero-optical jitter, correlations were performed between the global POD modes, local pressure sensors and the measured aero-optical jitter. To further investigate sources of aero-optical jitter, correlations were performed between dominant wavefront modes and the measured jitter. It was found that different flow features, primarily the unsteady separation line, wake vertical global motion and the vortical structures formed in the separated wake influence the aero-optical jitter, both indirectly via Biot-Savart induction mechanism and directly, when crossing the laser beam. It was also found that different flow features affect the aero-optical jitter at different frequency ranges. Total jitter reduction based on the LSE technique is discussed and possible expansion of the technique using surface pressure measurements is investigated.

The Comparison of Unsteady Pressure Field over Flat- and Conformal-Window Turrets using Pressure Sensitive Paint

Spatially-temporally-resolved unsteady pressure fields on a surface of a hemisphere-oncylinder turret with either a flator a conformal-window were characterized using fastresponse pressure sensitive paint at M = 0.33 for several window viewing angles. Various statistical properties of pressure fields were computed and geometry effects on unsteady pressure, including an effect of either flator conformal-window, were analyzed and discussed. Proper orthogonal decomposition were also used to extract dominant pressure modes and corresponding temporal coefficients and to analyze and compare instantaneous pressure structures for different turret geometries and viewing angles. In addition, the effects of gaps and ‘smile’ cut-outs present on the turret were investigated.