Search

Disturbance-Free-Payload is a novel spacecraft architecture proposed to achieve perfect vibration isolation performance of space applications. In this paper, the object under study is a DFP system in which a flexible umbilical connection between the support module (SM) and the payload module (PM) is introduced for data, power and fluid transfer. Bead model accounting for properties of mass and flexibility, is utilized to model the flexible umbilical. With incorporation of bead model, six degree-of-freedom multibody rigid-flexible dynamics for the DFP system is established via Newtonian mechanics. Along with rotational coupling between solar panels and SM, translational coupling, in particular, is taken into consideration since there is a stringent requirement on relative position between PM and SM. Based on the accurate motion equations of the DFP system, simulation studies are conducted to gain insight of the impact of umbilical connection and translational coupling on pointing performance of the DFP system. Simulation results validate effectiveness of DFP configuration in isolating vibrations transmitted from SM to PM. Besides, umbilical connection plays an important part in degrading pointing performance of PM whereas it exerts little impact on SM. Moreover, translational coupling leads to significant pointing performance degradation for both PM and SM.

Based on 5 requirements which are essential for stable OAM mode transmission, we propose an OAM fiber family based on a structure of circular photonic crystal fiber (C-PCF). The proposed C-PCF in the family is made of pure silica, with a big round air hole at the center, several rings of air-hole array as the cladding, and a ring shaped silica area in between as the core where the OAM modes propagate. We also provide a design strategy with which the optimized C-PCF can be obtained with optimum number of high quality OAM modes (up to 42 OAM modes), large effective index separation for corresponding vector modes over a wide bandwidth, relative small and flat dispersion, and low nonlinear coefficient compared with a conventional single mode fiber. The designed fiber can be used in MDM communications and other OAM applications in fibers.

In this work, the interactive process between separation bubble and impinging leading-edge vortices, which is superimposed in unsteady separated and reattaching flows over a finite blunt plate, was experimentally determined. We used an effective recognition strategy based on an offline data analysis by dynamic mode decomposition (DMD) and online DMD computation in a field-programmable gate array (FPGA). The approach was established for phase-locking particle image velocimetry (PIV) measurements of the underlying vortex dynamics. The DMD mode coefficients of wall-pressure fluctuations following the temporal evolution of the corresponding unsteady events clearly reflected the interactive process, in which three consecutive impinging vortices appeared in one period of the separation bubble. The global interactive process between the separation bubble and the impinging vortices was classified into three consecutive processes: an amplification process, a transition process and a preparation process. The phase-averaged PIV measurements revealed the following: (a) During the amplification process, the separation bubble and impinging vortices are in a synchronous enlargement process beginning from the shortest position of the separation bubble at x / D = 3.00 . (b) In both the transition and preparation processes, the interaction was characterized by a flapping separation bubble (which enlarged in the first half of the period and then shrank in the second half of the period) and by the impinging vortices constantly shedding to the downstream region. Finally, the underlying shear layer instabilities and vortex movements of these three interactive processes were analyzed.

Pointing accuracy and pointing stability play an important role in gravitational wave detection spacecraft. This paper presents dynamics model, pointing accuracy and stability analysis of a payload-spacecraft system connected using umbilical and non-contact actuators. Accurate model of payload-umbilical-spacecraft is proposed which takes into account the translational stiffness and damping effects, as well as the torsional stiffness effect. The Lagrangian approach is applied to derive the rigid-flexible coupling dynamics model. Both of the open-loop and closed-loop simulation studies are conducted to investigate the pointing accuracy and stability of the payload. The results indicate that the umbilical significantly degrades the vibration isolation performance of the payload at low frequencies between 0.01 Hz and 1 Hz. Simulation of two other cases, the payload without umbilical and the payload with umbilical considering only the translational stiffness effect, are also performed. The results of these three cases show that the umbilical greatly degrades the pointing accuracy of the payload and lowers the pointing stability. The torsional stiffness also has a slight effect on the pointing accuracy of the payload.

Design tools for circular PCF have been developed to analyze the vector modes by giving the modal map of the cutoff condition of eigenmodes and colormap of minimum effective refractive index separation between different eigenmodes. We also analyze the effect of fiber parameters for mode quality.

We propose a new orbital angular momentum (OAM) erbium-doped fiber amplifier based on a circular photonic crystal fiber, which can support a total of 18 modes (14 OAM modes) over C-band. A correction factor is proposed to modify the overlap factor, with the aim of evaluating the performance of the amplifier more accurately. We found that the confined doping profile can help optimize the differential model gain (DMG). Numerical simulations suggest that the proposed OAM fiber amplifier can provide a gain larger than 20 dB for all 14 OAM modes, with the small DMG less than 0.2 dB and the noise figure lower than 3.5 dB across the C-band.

Vortex dynamics and aeroacoustic energy transfer, which play essential roles in vortex-excited acoustic resonance inside straight channels with coaxial side-branches, were investigated by phase-locked particle image velocimetry (PIV) and Howe’s acoustic analogy. In the experiments, the periodic acoustic pressure fluctuations at the endplates of the side branches were used to trigger PIV via a field-programmable gate array control system. The results revealed that the spatiotemporal evolution of vortex shedding can be classified into three regions in response to the acoustic standing-wave propagations: the formation region, the convection region, and the collapse region, along with the flapping recirculation zone and the intermittent vertical flow streaks that occur inside the side branches. Further investigation was performed in terms of phase-dependent quantities such as the shear and normal stresses; the normal stress production, which was attributed to the evolution of vortex shedding, was found to be the major contributor to the kinematics and energetics of the self-sustained flow. Finally, Howe’s acoustic analogy was used to determine the instantaneous acoustic power and the accumulated aeroacoustic energy during one acoustic resonance cycle. The aeroacoustic energy extracted from the acoustic standing-wave propagations contributed to the formation and subsequent growth of the shedding vortex, whereas the decreased turbulent kinetic energy of the shedding vortex was transferred to the acoustic standing waves to maintain the longitudinal wave propagations.

The influence of vortex-excited acoustic resonance on flow dynamics in a straight channel with coaxial side branches was experimentally determined. The energetic vortex structures superimposed in the shear layer, formed at the intersection between the straight channel and the side-branches, are closely coupled with the natural acoustic modes of the side-branches, resulting in significantly intensified acoustic standing waves in the side-branches. Two configurations with symmetric (length-to-diameter ratio: L1/D = L2/D = 10) and asymmetric (L1/D = 10, L2/D = 5) arrangements of the side-branches were used to investigate the clearly different flow dynamics with and without acoustic resonance. In the experiments, wall pressure measurements and acoustic modal analyses were first conducted to determine the frequency lock-on range of the vortex-excited acoustic resonance. Subsequently, the flow fields in two configurations were measured with planar particle image velocimetry. The results indicated that the vortex-excited acoustic resonance resulted in significant flow fluctuations inside the branches together with expanded shear layers and the enlarged vortex structures. Vorticity fluctuations and Reynolds shear stresses were also increased significantly when subjected to acoustic modulation. In terms of the proper orthogonal decomposition analysis, two different motions of the energetic flow structures were identified as interacting with the acoustic resonance: the vertical synchronous oscillation mode and the streamwise vortex-shedding mode. Correspondingly, the phase-averaged low-order flow fields, determined by the mode coefficients of these two correlative modes, clearly indicated the flapping motion of the mainstream and the periodic oscillation of the shear layer.