Experimental study of gas contamination impact on aerodynamic heating and force measurements in shock tunnel

Abstract

Impulse facilities such as shock tunnels are hypersonic ground test facilities capable of compressing test gases to high temperatures and pressures using strong shock waves. The compression method using shock waves can form test flows over a relatively wide range of stagnation temperatures and has minimal flow contamination during the compression process. However, even in shock tunnels, certain factors can degrade the quality of the test flow. The disturbances that can contaminate the flow and measurements in shock tunnel tests can be categorized into physical contamination caused by the diaphragm and chemical contamination due to the driver gas. In this study, an experimental study investigated the measurement and improvement of the two main types of contamination occurring in shock tunnel tests.

First, a new double-layer diaphragm with minimal debris generation was developed to mitigate physical flow contamination. The designed diaphragm features a double-layer structure to minimize debris, consisting of polyethylene film pre-cut in eight directions and sealed with copper tape. The effect of diaphragm debris on pressure measurement was analyzed by comparing two types of diaphragms: a thin single-layer diaphragm and the double-layer diaphragm. To analyze the impact of diaphragm debris, shadowgraph flow visualization and pitot pressure measurements were conducted. The experimental results for both diaphragms aligned well with theoretical predictions, and the degree of flow contamination was evaluated by analyzing normalized disturbances, representing fluctuations in pitot pressure measurements. The experimental results confirmed that the double-layer diaphragm effectively reduced debris, improving flow quality.

A technique was developed to detect chemical contamination caused by the mixing of driver gas with the test gas. This technique measures the amount of driver gas mixed into the test gas over time using pressure transducer and pressure sensitive paint (PSP). The pressure transducer records the total pressure applied to the transducer regardless of the gas composition, while the PSP detects the partial pressure of oxygen based on the oxygen quenching mechanism. By utilizing the difference between these two measurement techniques and applying Dalton’s law of partial pressures, the mixing ratio of driver gas within the test flow can be determined. In this study, the degree of test gas contamination was measured under two conditions with different incident shock wave Mach numbers, and the effective test time was determined. The effective test time was deduced based on the measured effective test time and the drainage time of the shock tunnel.

An experimental study was conducted to investigate the impact of flow contamination on aerodynamic forces in the shock tunnel. An accelerometer-based force balance was designed to measure drag and rolling moment. The balance was calibrated independently for both axial and rotational directions. The drag and rolling moment measured during the effective test time were found to agree with the computational fluid dynamics (CFD) analysis results within the acceptable margin of error. Physical contamination from diaphragm debris induced vibrations and measurement errors, significantly affecting aerodynamic measurements using the dynamic measurement system. However, during the steady pitot pressure, chemical contamination had minimal impact on the aerodynamic forces, which are primarily determined by pressure force.

Finally, an experimental study was conducted to investigate the impact of flow contamination on aerodynamic heating measurements in the shock tunnel. Stagnation point heat flux measurements were performed using a hemispherical test model. A fast-response coaxial surface-junction thermocouple was used to measure the heat flux within the short test time of the shock tunnel. Shadowgraph flow visualization techniques were employed to analyze the effects of both physical and chemical contamination on heat flux measurements. The flow visualization results confirmed that physical contamination from diaphragm debris influenced the structure of the shock wave and flow around the test model, thereby affecting the aerodynamic heating measurements. Chemical contamination from the driver gas altered the flow characteristics, increasing the shock stand-off distance and reducing heat flux due to the mixing of cold driver gas.

This study categorized test gas contamination in the shock tunnel into physical and chemical contamination and developed methods to mitigate and detect contaminations. Additionally, the impact of flow contamination on both aerodynamic forces and heating was analyzed, providing foundational data for the future application of hypersonic ground test facilities.