Analysis of Length Scale Effects in Distortions of Laser Beams Propagating Through Turbulent Flows

Abstract

This paper presents results from analysis of the trends in the relevant length scales as applicable to Aero-Optic phenomena. Beam passage through the wake of a backward facing step with and without flow control are analyzed in terms of the relationships between the dominant length scales using Large Eddy Simulation results. The effect of optical scales such as the beam wavelength and aperture diameter and the turbulent length scales are examined to relate them to the effect they have on beam coherence, wander and spread in these turbulent flows. The overall goal is to obtain an understanding of the relationship between the length scales in the flow field and the beam distortions in order to be able to effectively control energy deposited on target. I. Introduction and Background The characterization and control of the optical or laser/turbulent-fluid interaction problem is critical to the successful implementation of DE weapons on aerial platforms. Deployment of these systems will require overcoming the large distortions of the beam as it passes through the near field turbulence. Due to the large range of scales of the turbulence in the near field, the distortions are difficult to characterize and may be beyond the capabilities of current Adaptive Optics systems. Hence, flow control may be required to reduce turbulent fluctuations in the flow and to minimize turbulent distortions of the beam. This can only be achieved through an understanding of the relationships between the relevant length scales of the beam and the flow field. The largest flow field length scales are governed by the geometry and the boundary conditions of the problem. The smallest ones are determined by the Reynolds number of the flow and the intermediate ones by the turbulent cascade process. For the beam, the dependence of the intensity on target for different wavelengths is reasonably understood 1 . An understanding of the relationships to the turbulent flow field length scales has been more elusive. However, in a practical system, the laser weapon may be housed within a turret or aerodynamic window and depending upon the integration selected, varied forms of turbulence may be encountered along the laser beam propagation path, e.g. turbulent boundary layer, free shear layer, or wake. Recently, Gordeyev et al. 2 observed that the optical aberrations grow faster in high speed flows by relating that the optical distortions grow approximately as a square of the incoming Mach number. In addition, the authors quantified the beam distortions due to flow past several configurations in an earlier work 3,4 . This paper represents another step along the same lines of these works. We address the issue of flow modification and its effect on flow control explicitly. In the present work, we analyze the effects of using flow control to significantly modify the length scales in a flow field using microjet actuators on beam distortion. The primary aim of this is to understand the trade-offs that exist in such problems, typically, reducing one kind of distortion can cause an increase in a different metric. Hence, the relationships between the distortions and the relevant flow and beam length scales are sought from this analysis.