Measurement Of Static Pressure Over Bodies In Hypersonic Shock Tunnel Using MEMS-Based Pressure Sensor Array

Abstract

Hypersonic flow is both fascinating and intriguing mainly because of presence of strong entropy and viscous interactions in the flow field. Notwithstanding the tremendous advancements in numerical modeling in the last decade separated hypersonic flow still remains an area where considerable differences are observed between experiments and numerical results. Lack of reliable data base of surface static pressures with good spatial resolution in hypersonic separated flow field is one of the main motivations for the present study. The experiments in hypersonic shock tunnels has an advantage compared to wind tunnels for simulating the total energy content of the flow in addition to the Mach and Reynolds numbers. However the useful test time in shock tunnels is of the order of few milliseconds. Hence in shock tunnel experiments it is essential to have pressure measurement devices which has special features such as small in size, faster response time and the sensors in array form with improved spatial resolutions. Micro Electro Mechanical Systems (MEMS) is an emerging technology, which holds lot of promise in these types of applications. In view of the above requirement, MEMS based pressure sensor array was developed to measure the static pressure distribution. The study is comprised of two parts: one is on the development of MEMS based pressure sensor array, which can be used for hypersonic application and other is on experimental static pressure measurement using MEMS based sensors in separated hypersonic flow over a backward facing step model. Initially a static pressure sensor array with 25 sensors was developed. The static calibration of sensor array was carried out to characterize the sensor array for various characteristic parameters. The preliminary experimental study with cluster of 25 MEMS sensor array mounted on the flat plate did not provide reliable and repeatable results, but gave valuable inputs on the typical problems of using MEMS sensors in short duration hypersonic ground test facilities like shock tunnels. Incidentally, to the best of our knowledge this is first report on use of MEMS based pressure sensors in hypersonic shock tunnel. Later cluster of 5 sensor array was developed with improved electronic packaging and surface finish. The experiments were conducted with flat plate by mounting 5 sensor array shows good agreement in static pressure measurement compared with standard sensors. In the second part of the study a backward facing step model, which simulates the typical gasdynamic flow features associated with hypersonic flow separation is designed. Backward facing step model with step height of 3 mm was mounted with sensor array along the length of model. Just after the step, static pressure measurements were carried out with MEMS sensors. It is important to note that, in the space available in backward facing step model we could mount only one conventional Kulite pressure transducer. The experiments were conducted at Mach number of 6.3 and at stagnation enthalpy of 1.5 MJ/kg in hypersonic shock tunnel (HST-5) at IISc. Based on the static pressure measurement on backward facing step, the location of separation and reattachment points were clearly identified. The static pressure values show that reattachment of flow takes place at about 7 step heights. Numerical simulations were carried out using commercial CFD code, FLUENT for flat plate and backward facing step models to compliment the experiments. The experimental tests results match well with the illustrative numerical simulations results.