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CONTENTS
Volume 1, Number 4, October 2014
 

Abstract
This article discusses the microvibration analysis of a cantilever configured reaction wheel assembly. Disturbances induced by the reaction wheel assembly were measured using a previously designed platform. Modelling strategies for the effect of damping are presented. Sine-sweep tests are performed and a method is developed to model harmonic excitations based on the corresponding test results. The often ignored broadband noise is modelled by removing spikes identified in the raw signal including a method of identifying spikes from energy variation and band-stop filter design. The validation of the reaction wheel disturbance model with full excitations (harmonics and broadband noise) is presented and flaws due to missing broadband noise in conventional reaction wheel assembly microvibration analysis are discussed.

Key Words
microvibration; reaction wheel; disturbance model; broadband noise; cantilever configured

Address
Zhe Zhang : University of Southampton, Southampton, Hampshire, SO17 1BJ, United Kingdom
Guglielmo S. Aglietti, Daniele Addari : Surrey Space Centre, University of Surrey, Guildford, Surrey, GU2 7XH, United Kingdom
Weijia Ren : Chinese Academy of Sciences, Beijing, 100094, People

Abstract
The flowfields inside a contour and a conical nozzle exhausting into a straight cylindrical supersonic diffuser are computed by solving numerically axisymmetric turbulent compressible Navier-Stokes equations for stagnation to ambient pressure ratios in the range 20 to 34. The diffuser inlet-to-nozzle throat area ratio and exit-to-throat area ratio are 21.77, and length-to-diameter ratio of the diffuser is 5. The flow characteristics of the conical and contour nozzle are compared with the help of velocity vector and Mach contour plots. The variations of Mach number along the centre line and wall of the conical nozzle, contour nozzle and the straight supersonic diffuser indicate the location of the shock and flow characteristics. The main aim of the present analysis is to delineate the flowfields of conical and contour nozzles operating under identical conditions and exhausting into a straight cylindrical supersonic diffuser.

Key Words
CFD simulation; supersonic diffuser; compressible flow; nozzle

Address
R.C. Mehta : Department of Aeronautical Engineering, Noorul Islam University, Kumaracoil, 629180, Tamil Nadu, India
G. Natarajan : Department of Mechanical Civil Engineering, Kumaracoil, 629180, Tamil Nadu, India

Abstract
Converging flows of a gas and a liquid at a microchannel cross junction, under proper conditions, can result in the formation of periodic, dispersed microslugs. This microslug formation phenomenon has been proposed as the basis for a fuel injection system in a novel, \'discrete\' monopropellant microthruster designed for use in next-generation miniaturized satellites. Previous experimental studies demonstrated the ability to generate fuel slugs with characteristics commensurate with the intended application during steady-state operation. In this work, numerical and experimental techniques are used to study the effect of valve actuation on slug characteristics, and the results are used to compare with equivalent steady-state slugs. Computational simulations of a valve with a 1 ms valve-actuation cycle show that as the ratio of the response time of the valve to the fully open time is increased, transient effects can increase slug length by up to 17%. The simulations also demonstrate that the effect of the valve is largely independent of surface tension coefficient, which is the thermophysical parameter most responsible for slug formation characteristics. Flow visualization experiments performed using a miniature valve with a 20 ms response time showed less than a 1% change in the length of slugs formed during the actuation cycle. The results of this study indicate that impulse bit and thrust calculations can discount transient effects for slower valves, but as valve technology improves transient effects may become more significant.

Key Words
microfluidics; micropropulsion; CFD

Address
M. Ryan McDevitt and Darren L. Hitt : Mechanical Engineering Program, School of Engineering, University of Vermont, Burlington, VT 05405, USA

Abstract
Orbit-raising is an important step to place spacecraft from parking orbits into working orbits. Attitude control system design is crucial in the success of orbit-raising. Several text books have discussed this design and focused mainly on the traditional methods based on single-input single-output (SISO) transfer function models. These models are not good representations for many orbit-raising control systems which have multiple thrusters and each thruster has impact on the attitude defined by all outputs. Only one published article is known to use a more suitable multi-input multi-output (MIMO) Euler angle model in spacecraft orbit-raising attitude control system design. In this paper, a quaternion based MIMO model for the orbit-raising attitude control system design is proposed. The advantages of using quaternion based model for orbit-raising control system designs are (a) there is no need for mathematical transformations because the attitude measurements are normally given by quaternion, (b) quaternion based model does not depend on rotational sequences, which reduces the chance of human errors, and (c) the singular point of reduced quaternion model is the farthest from the operational point where linearization is performed. We will show that performance of quaternion model based design will be as good as the performance of Euler angle model based design for orbit-raising problem.

Key Words
reduced quaternion model; spacecraft; orbit-raising; attitude control; pulse width modulation

Address
Yaguang Yang : Division of Engineering, Office of Research, US Nuclear Regulatory Commission, 21 Church Street, Rockville, MD 20850, USA

Abstract
An experimental study was conducted to investigate the effect of Reynolds number on compressible convex-corner flows, which correspond to an upper surface of a deflected flap of an aircraft wing. The flow is naturally developed along a flat plate with two different lengths, resulting in different incoming boundary layer thicknesses or Reynolds numbers. It is found that boundary layer Reynolds number, ranging from 8.04x104 to 1.63x105, has a minor influence on flow expansion and compression near the corner apex in the transonic flow regime, but not for the subsonic expansion flow. For shock-induced separated flow, higher peak pressure fluctuations are observed at smaller Reynolds number, corresponding to the excursion phenomena and the shorter region of shock-induced boundary layer separation. An explicit correlation of separation length with deflection angle is also presented.

Key Words
convex corner; Reynolds number; shock; pressure fluctuations; boundary layer separation

Address
Kung-Ming Chung : Aerospace Science and Technology Research Center, National Cheng Kung University, 2500 Section 1, Chung-Cheng South Road, Guiren district, Tainan, 711, Taiwan
Po-Hsiung Chang and Keh-Chin Chang : Department of Aeronautics and Astronautics, National Cheng Kung University, 1 University Road, East district, Tainan, 701, Taiwan

Abstract
High-precision autonomous localization technique is essential for future Mars rovers. This paper addresses an innovative integrated localization algorithm using a multiple information fusion approach. Firstly, the output of IMU is employed to construct the two-dimensional (2-D) dynamics equation of Mars rover. Secondly, radio beacon measurement and terrain image matching are considered as external measurements and included into the navigation filter to correct the inertial basis and drift. Then, extended Kalman filtering (EKF) algorithm is designed to estimate the position state of Mars rovers and suppress the measurement noise. Finally, the localization algorithm proposed in this paper is validated by computer simulation with different parameter sets.

Key Words
Mars rover; localization; multi-information fusion; radio measurement; terrain image matching; extended Kalman filter

Address
Xiuqiang Jiang, Shuang Li, Ting Tao and Bingheng Wang : College of Astronautics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China


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