PolyU IR Collection:
http://hdl.handle.net/10397/379
2015-11-27T10:06:29ZVortex dynamics of in-line twin synthetic jets in a laminar boundary layer
http://hdl.handle.net/10397/7613
Title: Vortex dynamics of in-line twin synthetic jets in a laminar boundary layer
Authors: Wen, Xin; Tang, Hui; Duan, Fei
Abstract: An experimental investigation is conducted on the vortices induced by twin synthetic jets (SJs) in line with a laminar boundary layer flow over a flat plate. The twin SJs operating at four different phase differences, i.e., Δϕ = 0°, 90°, 180°, and 270°, are visualized using a stereoscopic color dye visualization system and measured using a two-dimensional particle image velocimetry (PIV) system. It is found that depending on the phase difference of twin SJs, three types of vortex structures are produced. At Δϕ = 90°, the two hairpin vortices interact in a very constructive way in terms of the vortex size, strength, and celerity, forming one combined vortex . At Δϕ = 270°, the two individual hairpin vortices do not have much interaction, forming two completely separated hairpin vortices that behave like doubling the frequency of the single SJ case. At Δϕ = 0° and 180°, the two hairpin vortices produced by the twin SJ actuators are close enough, with the head of one hairpin vortex coupled with the legs of the other, forming partially interacting vortex structures. Quantitative analysis of the twin SJs is conducted, including the time histories of vortex circulation in the mid-span plane as well as a selected spanwise-wall-normal plane, and the influence of the twin SJs on the boundary layer flow filed. In addition, dynamic mode decomposition analysis of the PIV data is conducted to extract representative coherent structures. Through this study, a better understanding in the vortex dynamics associated with the interaction of in-line twin SJs in laminar boundary layers is achieved, which provides useful information for future SJ-array applications.2015-01-01T00:00:00ZFlow structure behind two staggered circular cylinders. Part 2. Heat and momentum transport
http://hdl.handle.net/10397/7556
Title: Flow structure behind two staggered circular cylinders. Part 2. Heat and momentum transport
Authors: Hu, J. C.; Zhou, Yu
Abstract: This work aims to study flow structures, heat and momentum transport in the wake of two staggered circular cylinders. In order to characterize heat transport in the flow, both cylinders were slightly heated so that heat generated could be treated as a passive scalar. The velocity and temperature fluctuations were simultaneously measured by traversing a three-wire (one cross-wire plus one cold wire) probe across the wake, along with a fixed cross-wire, which acted to provide a reference signal. Four distinct flow structures, i.e. two single-street modes (S-I and S-II) and two twin-street modes (T-I and T-II), are identified based on the phase-averaged vorticity contours, sectional streamlines, and their entrainment characteristics. Mode S-I is characterized by a vortex street approximately antisymmetric about the centreline. This mode is further divided into S-Ia and S-Ib, which differ greatly in the strength of vortices. The vortex street of Mode S-II is significantly asymmetric about the centreline, the strenth of vortices near the downstream cylinder exceeding by 50% that on the other side. Mode T-I consists of two alternately arranged vortex streets; the downstream-cylinder-generated street is significantly stronger than that generated by the upstream cylinder. In contrast, Mode T-II displays two streets approximately antisymmetrical about the wake centreline. Free-stream fluid is almost equally entrained from either side into the wake in Modes S-Ia and T-II, but largely entrained from the downstream cylinder side in Modes S-II and T-I. The entrainment motion in Mode S-Ib is very weak owing to the very weak vortex strength. Vortices decay considerably more rapidly in the twin-street modes, under vigorous interactions between the streets, than in the single-street modes. This rapid decay is particularly evident for the inner vortices near the wake centreline in Modes T-II and T-I. Other than flow structures, heat and momentum transport characteristics are examined in detail. Their possible connection to the initial conditions is also discussed.2008-07-25T00:00:00ZLocal SVD inverse of robot Jacobians
http://hdl.handle.net/10397/7555
Title: Local SVD inverse of robot Jacobians
Authors: Yuan, Jing
Abstract: This study presents a fast inverse kinematics algorithm for a class of robots, including PUMA and SCARA. It decomposes a robot Jacobian into a product of sub-matrices to locate singularities. Singular value decomposition (SVD) is applied to each singular sub-matrix to find a local leastsquares inverse. Perfect inverses are derived for all non-singular sub-matrices. The proposed algorithm is extremely fast. A total inverse requires 54 flops for PUMA and 43 for SCARA. Simulation and experiment are conducted to test the accuracy and real-time speed of the algorithm.2001-01-01T00:00:00ZEquilibrium states of turbulent homogeneous buoyant flows
http://hdl.handle.net/10397/7554
Title: Equilibrium states of turbulent homogeneous buoyant flows
Authors: Jin, L. H.; So, Ronald M. C.; Gatski, T. B.
Abstract: The equilibrium states of homogeneous turbulent buoyant flows are investigated through a fixed-point analysis of the evolution equations for the Reynolds stress anisotropy tensor and the scaled heat flux vector. The mean velocity and thermal fields are assumed to be two-dimensional. Scalar invariants formed from the Reynolds stress anisotropy tensor, the scaled heat flux vector, and the strain rate and rotation rate tensors are governed by a closed set of algebraic equations derived for the stress anisotropy and scaled heat flux under a (weak) equilibrium assumption. Six equilibrium state variables are identified for the buoyant case and contrasted with the corresponding two state variables obtained for the non-buoyant homogeneous turbulence case. These results, while dependent on the functional forms of the models for the pressure–strain rate correlation tensor and the pressure–scalar-gradient correlation and viscous dissipation vector, can be used as in the non-buoyant case to either calibrate new closure models or validate the performance of existing models. In addition, since the analysis only involves the turbulent time scales (both velocity and thermal) and their ratio, the results of the analysis are independent of the specific models for the dissipation rates of the turbulent kinetic energy and the temperature variance. The analytical results are compared with model predictions as well as recent direct numerical simulation (DNS) data for buoyant shear flows. Good agreement with DNS data is obtained.2003-05-01T00:00:00Z