Research Article Flow Mode Magnetorheological Dampers with an Eccentric Gap Young-Tai Choi and Norman M. Wereley Smart Structures Laboratory, Department of Aerospace Engineering, University of Maryland, College Park, MD 20742, USA Correspondence should be addressed to Young-Tai Choi; nicechoi@umd.edu Received 6 April 2014; Accepted 23 June 2014; Published 17 July 2014 Academic Editor: Miao Yu Copyright © 2014 Y.-T. Choi and N. M. Wereley. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. his paper analyzes low mode magnetorheological (MR) dampers with an eccentric annular gap (i.e., a nonuniform annular gap). To this end, an MR damper analysis for an eccentric annular gap is constructed based on approximating the eccentric annular gap using a rectangular duct with a variable gap, as well as a Bingham-plastic constitutive model of the MR luid. Performance of low mode MR dampers with an eccentric gap was assessed analytically using both ield-dependent damping force and damping coeicient, which is the ratio of equivalent viscous ield-on damping to ield-of damping. In addition, damper capabilities of low mode MR dampers with an eccentric gap were compared to a concentric gap (i.e., uniform annular gap). 1. Introduction Magnetorheological (MR) dampers are attractive semiactive energy dissipators because their stroking loads are continu- ously controllable by simply adjusting a current input to an electromagnet. Also, the response time is fast (a few milli- seconds), and the required power consumption is low (tens of watts). As a result, MR dampers have been employed as vibra- tion mitigation devices for a number of applications, such as seismic dampers [1–7], seat dampers [8–11], and vibra- tion isolators [12–16]. More recently, MR dampers have been also applied to aerial and ground vehicles as crashworthiness devices, such as landing gear oleos [17–20], impact dampers [21–26], and energy absorbers [27–29]. Many studies of MR dampers have been conducted based on the low mode of operation because, for a given MR damper yield force, the design is more compact (less active volume) than for a typical shear mode device. In low mode MR dampers, MR luid lows through an MR valve, which is typically conigured as an annulus with two stationary walls and an electromagnetic coil to adjust the magnetic ield input. During motion of the large area piston, the MR luid lows through small area annular valve because of the induced pressure so that resistive damping force results. he damping force magnitude is continuously controllable by adjusting the intensity of the magnetic ield input. hus far, MR valves inside low mode MR dampers have been designed so as to have a uniform gap conigured as two concentric tubes. But the study of low mode MR dampers with an eccentric gap (i.e., nonuniform gap conigured as two eccentric tubes) has not much been examined. Moreover, to our knowledge, the analysis of low mode MR dampers with an eccentric gap has not yet been explored. herefore, in this study, the analysis of low mode MR dampers with an eccentric gap is presented. First, an eccentric gap inside low mode MR dampers was approximated as a rectangular duct or slit with variable height. A constitutive Bingham-plastic model was used in this study to represent MR luid low. Based on these assumptions, the analysis of low mode MR dampers with an eccentric gap was constructed. he ield-dependent damper force and the damping coeicient, which is the ratio of the equivalent viscous damping to the ield-of damping, were theoretically obtained in terms of a nondimensional preyield or plug thick- ness. Finally, the performance of low mode MR dampers with an eccentric gap was theoretically compared with that of MR dampers with a concentric gap (i.e., uniform gap). 2. Magnetorheological Dampers with an Eccentric Gap Figure 1 presents the piston head of a low mode MR damper with either a concentric gap or an eccentric gap. Note that, to simplify the analysis, the gas chamber typically used to compensate for the piston rod volume of single-ended Hindawi Publishing Corporation Advances in Mechanical Engineering Volume 2014, Article ID 931683, 7 pages http://dx.doi.org/10.1155/2014/931683