Impact of shear flow turbulence on EM signals

Summary form only given. Flow associated with high speed vehicle can get ionized because of the heat emanating from the shock that it creates in the atmosphere. Depending on the degree of ionization and the signal frequency the flow can be overdense or underdense. It is well known that the overdense condition leads to blackout. In the underdense condition the EM signal can propagate through the flow. However, the turbulence in the flow can significantly distort the EM signals. Past studies on this problem were primarily concerned with attenuation and integrated phase shifts of the signals. Our study shows that the impact of shear flow turbulence can be far more severe than those reported thus far. Since our flow is partially ionized the character of turbulence is quite different from that of purely hydrodynamic turbulence. We consider the situation when the magnetic field is negligible. The primary source of turbulence is caused by ion acoustic wave (IAW) instabilities. On numerically solving the nonlinear system of equations governing the IAW fluctuations we determined the spectrum of electron density fluctuations in the flow. We assume that these fluctuations are stationary and statistically homogeneous. Our primary interest is in the reception of EM signals through this turbulent flow. The EM signal will interact with turbulent flow and create current fluctuations which in turn will distort the received EM signals. It is convenient to decompose the received signals into two parts: coherent and diffuse, viz, the first and second moment of the received signals. We find that the coherent part has the same spectrum as that of the incident signal. However, it undergoes attenuation because of scattering. This attenuation is dispersive and hence leads to signal distortion. The impact of turbulent flow on the diffuse part is quite different and can be far greater significance. It is expressed as a convolution (in wavenumber and frequency) of the source signal with the s- ectrum of electron density fluctuations. This is a constrained convolution since the spectrum has to satisfy the IAW dispersion relation. Thus the diffuse part of the received signal can get significantly altered because of the turbulent flow. A quantity that characterizes the flow is the mean free path (MFP). When the MFP is large compared to the thickness of the flow, the layer is considered as “thin”. In this case the coherent part is significant. If the MFP is larger than the thickness of the flow, the layer is called “thick”. Here the diffuse part is the dominant part of the received signal. We thus find this classification scheme to be useful for understanding the nature of the impact of turbulent flow on EM signals.