The aero-optical effects of an optical seeker with a supersonic jet for hypersonic vehicles in near space were investigated by three suites of cases, in which the altitude, angle of attack, and Mach number were varied in a large range. The direct simulation Monte Carlo based on the Boltzmann equation was used for flow computations and the ray-tracing method was used to simulate beam transmission through the nonuniform flow field over the optical window. Both imaging displacement and phase deviation were proposed as evaluation parameters, and along with Strehl ratio they were used to quantitatively evaluate aero-optical effects. The results show that aero-optical effects are quite weak when the altitude is greater than 30 km, the imaging displacement is related to the incident angle of a beam, and it is minimal when the incident angle is approximately 15°. For reducing the aero-optical effects, the optimal location of an aperture should be in the middle of the optical window.

Aero-optical effects of an optical seeker with a supersonic jet for hypersonic vehicles in near space.

We investigated the influence of altitude on aero-optic imaging quality degradation of the hemispherical optical dome. Boundary conditions for the aerodynamic heating effect of the optical dome were calculated by solving the Reynolds-averaged Navier–Stokes equations provided by FLUENT. The finite element model and the thermal-structure simulation results of the optical dome were obtained using ANSYS. The 3D nonuniform refractive index field of the optical dome was obtained according to the thermal-optical effect. The optical tracking method based on the fourth-order Runge–Kutta algorithm was adopted to simulate the optical transmission through the optical dome. The Strehl ratio (SR), encircled energy, distorted target images, and peak signal-to-noise ratio were presented for imaging quality evaluation. The variation rules of these imaging quality evaluation parameters were obtained in the altitude range of 0–45 km. The results showed that, in the same flight conditions, with the increase of altitude, peak signal-to-noise ratio (PSNR) of the distorted image, and SR result were increased, and radiuses of dispersion spots, including 80% energy, were decreased; therefore, the influence of aero-optics effect on imaging quality degradation was gradually weakened.

Influence of altitude on aero-optic imaging quality degradation of the hemispherical optical dome.

A correction strategy for correcting the aero-optical wavefront aberration of the supersonic mixing layer is presented based on flow control that is implemented by periodic pulse forcing. The flow field of the controlled supersonic mixing layer is simulated by large eddy simulation, and the optical transmission trajectory of the incident beam is computed by the ray-tracing method. The key component of the correction strategy is a compensation signal employed to correct the aberrated wavefront, which is produced according to flow and control parameters. Three numerical examples, including cases of an incident beam at different streamwise locations, flow control with different pulse periods, and vortex evolution in a pulse period, are performed to test performance of the correction strategy. The numerical results indicate that the correction strategy has a great effect in perfecting the aberrated wavefront again, and hence the far-field beam quality is also improved remarkably. In the end, the possibility of experimental implementation of the presented correction strategy is discussed.

Correcting aero-optical wavefront aberration of the supersonic mixing layer based on periodic pulse forcing.

Airborne platform effects on lasers and warning sensors (ALWS) has been a European collaborative research project to investigate the effects of platform-related turbulence on optical countermeasure systems, especially missile approach warning systems (MAWS) and directed infrared countermeasures (DIRCM). Field trials have been carried out to study the turbulence effects around a hovering helicopter and behind a turboprop aircraft with engines running on the ground. In addition different methods for modelling the effects have been investigated. In the helicopter trials significant beam wander, scintillations and beam broadening were experienced by narrow divergence laser beams when passing through the down-wash of the hot engine exhaust gases. The measured effects considerably exceed the effects of atmospheric turbulence. Extraction of turbulence parameters for modelling of DIRCM-relevant scenarios show that in most cases the reduction of jamming power and distortion of jamming waveform can be expected to be small. The reduction of effects of turbulence is mainly related to the larger beam divergence and shorter Rayleigh length of DIRCM lasers compared to the experimental probe beams. Measurements using the turboprop platform confirm that tolerable effects on laser beam properties are found when the laser beam passes through the exhaust 15 m behind the outlet where the exhaust gases are starting to cool down. Modelling efforts have shown that time-resolved computational fluid dynamics (CFD) calculations can be used to study properties of beam propagation in engine exhaust-related turbulence. Because of computational cost and the problem of validating the CFD results the use for system performance simulations is however difficult. The hot exhaust gases emitted from aircraft engines create extreme optical turbulence in a local region. The effects on countermeasure system performance depend both on the system parameters and on the threat characteristics. With present-day DIRCM systems, the effects of even severe turbulence are often tolerable.

Airborne platform effects on lasers and warning sensors

We present results of calculations of the mean intensity of a beam which passes in the beginning of the path through a shock wave which is formed during a supersonic air flow around the turret and propagates further in a homogeneous medium. It is shown that the spatial inhomogeneity of the air refractive index in the region of a shock wave can lead to strong anisotropic distortions of a beam intersecting the wave; the distortions result in focusing and defragmentation of the beam at comparatively short distances from the turret and rapid degradation of the beam in the process of its further propagation.

Laser Beam Distortions Caused by a Shock Wave near the Turret of a Supersonic Aircraft

The mean intensity and the displacement from the initially given propagation direction of the optical beam passed through the shock wave have been calculated. It has been shown that the spatial inhomogeneity of the refractive index of air caused by the shock wave arising in supersonic flow flowing a conical body can cause the focusing of the beam and strong anisotropic distortions of the intensity distribution in its cross section. The angular displacement of the optical beam from the initially given propagation direction owing to the shock wave depends only on the height above the Earth’s surface at which the shock wave is formed. As the height increases, the influence of the shock wave on the optical beam propagating through it decreases.

Optical beam distortions induced by a shock wave.

This paper presents results of the analysis of the mean intensity, intensity fluctuations, and regular and random shifts of optical beams propagating through a shock wave resulting from the supersonic motion of a conical body in a turbulent atmosphere It is shown that aero-optical effects caused by a shock wave are suppressed with an increase in the optical turbulence Quantitative data illustrating the degree of the manifestation of aero-optical effects for paths with different geometry and length depending on turbulent conditions of light propagation are presented

Manifestation of aero-optical effects in a turbulent atmosphere in supersonic motion of a conical body

The results of experimental researches of intensity fluctuations of a laser beam propagating through a supersonic flooded jet are presented. The laser beam propagates across the jet at various distances from a Vitoshinskii nozzle. The experiment was carried out in an Eiffel chamber at pressure ratios from 1.7 to 7 at two modes of the outflow of the jet: with the use of chevrons on the outlet nozzle and without chevrons. Dependencies of the dispersion and fluctuation spectra on pressure and distances from the nozzle are determined. The inner scale of turbulence is estimated.

Intensity fluctuations of a laser beam propagating through a supersonic flooded jet

The results of retrieval of wind speed and direction from measurements of radial velocity by a Stream Line pulsed coherent Doppler lidar using the duo-beam method and conical scanning are presented. These results are compared with data of a sonic anemometer (point sensor).

A. A. Sukharev

Papers

Optical beam distortions induced by a shock wave.

Manifestation of aero-optical effects in a turbulent atmosphere in supersonic motion of a conical body

Laser Beam Distortions Caused by a Shock Wave near the Turret of a Supersonic Aircraft

Intensity fluctuations of a laser beam propagating through a supersonic flooded jet

Stream Line Doppler lidar measurements of wind speed and direction with the duo-beam method in the surface air layer