Mach 10 Bow-Shock Unsteadiness Modeled by Linear Combination of Two Mechanisms.

This paper presents mechanisms to explain, as well as mathematics to model, time-averaged spatially resolved amplitude observations of number density and number density unsteadiness in a Mach 10 flow as it transitions from the freestream, through a bow-shock wave, and into the gas cap created by a blunt-body model. The primary driver for bow-shock unsteadiness is freestream unsteadiness or "tunnel noise." Primary unsteadiness is bow-shock oscillation. It scales spatially with the number density first derivative and is modeled using a sech2(z) term. Secondary weaker unsteadiness begins as freestream unsteadiness and increases linearly in direct proportion to the gas number density across the bow shock and into the gas cap. This is the well-known amplification of the freestream turbulent kinetic energy mechanism and is modeled using a tanh(z) term. Total unsteadiness [fit using tanh(z) term + sech2(z) term] is expressed as the number density standard deviation and modeled as a linear combination of these two independent, simultaneous, and nonlinear unsteadiness mechanisms. Relationships between mechanism coefficients and various flowfield and wind-tunnel parameters are discussed. For example, bow-shock and gas cap oscillation amplitudes are linearly correlated with stagnation pressure and, by deduction, freestream unsteadiness.

Pressure measurement in supersonic air flow by differential absorptive laser-induced thermal acoustics.

Nonintrusive, off-body flow barometry in Mach 2 airflow has been demonstrated in a large-scale supersonic wind tunnel using seedless laser-induced thermal acoustics (LITA). The static pressure of the gas flow is determined with a novel differential absorption measurement of the ultrasonic sound produced by the LITA pump process. Simultaneously, the streamwise velocity and static gas temperature of the same spatially resolved sample volume were measured with this nonresonant time-averaged LITA technique. Mach number, temperature, and pressure have 0.2%, 0.4%, and 4% rms agreement, respectively, in comparison with known free-stream conditions.

Common-path heterodyne laser-induced thermal acoustics for seedless laser velocimetry.

We demonstrate the use of a novel technique for the detection of heterodyne laser-induced thermal acoustic signals that allows the construction of a highly stable seedless laser velocimeter. A common-path configuration is combined with quadrature detection to provide flow direction, to greatly improve robustness to misalignment and vibration, and to give reliable velocity measurement at low-flow velocities. Comparison with Pitot tube measurements in the free stream of a wind tunnel shows root-mean-square errors of 0.67 m/s over the 0-55-m/s velocity range.