DOI: 10.7274/28792508.v1
title: Global Measurements of Inlet/Isolator Dynamics and Optimization of Unstart Detection

description: Dual-mode scramjets are a promising technology to expand the capabilities of hypersonic vehicles. Streamtraced inlets are an attractive inlet concept for high performance dual-mode scramjets. These inlets generate complex shock-wave/boundary-layer interactions, which have not been widely studied in the literature, especially with high-resolution measurements. Scramjet unstart is a phenomenon that results in a severe reduction in inlet mass capture and occurs during operation at off-design conditions. Unstart can occur in milliseconds, and the flow physics governing it are extremely complex, involving numerous shock-wave/boundary-layer interactions. Improved prediction and detection of unstart is required to inform proper engine design and allow for effective flow control to avoid unstart.

In this study, global surface measurements are applied to axisymmetric and streamtraced inlet/isolators to better measure shock-wave/boundary layer interactions and unstart. Tests were conducted in the AFOSR--Notre Dame Large Mach-6 Quiet Tunnel under Mach 5.7 flow for a variety of unit Reynolds numbers. Transverse jet injection of air at the aft portion of the isolator induced unstart of the inlet under cold-flow conditions. Fast pressure-sensitive paint (PSP) was used to measure the time-resolved pressure field on the inside of the isolator. The PSP was temperature compensated using temperature-sensitive paint measurements. Background-oriented schlieren was utilized for visualizing the inlet shock structure during unstart. 

The utility of PSP for investigating the dynamics of internal SWBLI was demonstrated using a variety of data-analysis techniques. The mean shock locations in the isolator were determined to within 0.1~mm. Significant spanwise variations were seen in the inlet-generated conical shock fronts. A downstream propagating disturbance influencing SWBLI unsteadiness was seen, and its velocity and frequency were quantified. The pressure fields of a half- and full-scale streamtraced inlet/isolator were found to be similar, with some differences attributed to viscous effects. Varying the unit Reynolds number had a limited effect on the pressure rise and SWBLI locations within the isolator. Global pressure fields, valuable for validation of computations, were measured over a region 8.5 to 12.5 isolator heights long.

High-resolution measurements of the isolator pressure profiles and shock-train leading edge (STLE) position were made during the unstart process. The shape of the STLE front was measured with high spanwise resolution, and showed minimal spanwise variation in an rectangular isolator with curved corners. Time-resolved measurements show that the isolator pressure profile varies quickly in time, which presents challenges for sophisticated unstart detection methods. The unstarted streamtraced inlet flow had oscillatory or non-oscillatory modes, which were dependent on the mass flow rate of transverse air injection. 

Optimization of pressure transducer configuration for unstart detection was conducted. The PSP's high spatial resolution and lack of a priori decisions on where to collect pressure data were instrumental in the optimization procedure. The optimized configurations had 50.2% lower mean error and 33.5% lower maximum error in STLE location prediction on average. A relationship between the number of transducers used for STLE tracking and the error of the unstart detection method was found.