Magnetic Field Amplification by a Plasma Cavitation Instability in Relativistic Shock Precursors

Plasma streaming instabilities play an important role in magnetic field amplification and particle acceleration in relativistic shocks and their environments. However, in the far shock precursor region where accelerated particles constitute a highly relativistic and dilute beam, streaming instabilities typically become inefficient and operate at very small scales when compared to the gyroradii of the beam particles. We report on a plasma cavitation instability that is driven by dilute relativistic beams and can increase both the magnetic field strength and coherence scale by orders of magnitude to reach near-equipartition values with the beam energy density. This instability grows after the development of the Weibel instability and is associated with the asymmetric response of background leptons and ions to the beam current. The resulting net inductive electric field drives a strong energy asymmetry between positively and negatively charged beam species. Large-scale particle-in-cell simulations are used to verify analytical predictions for the growth and saturation level of the instability and indicate that it is robust over a wide range of conditions, including those associated with pair-loaded plasmas. These results can have important implications for the magnetization and structure of shocks in gamma-ray bursts, and more generally for magnetic field amplification and asymmetric scattering of relativistic charged particles in plasma astrophysical environments.

Hydrodynamic improvement by adding inlet baffles on centrifugal pump for reducing cavitation instabilities

Cavitation instability is a common phenomenon that causes vibration and noise of turbomachinery. In this study, an attempt is made to suppress the cavitation instability. A high-speed centrifugal pump with inducer is taken as the research objective. Four baffles are evenly arranged at the inlet of the inducer as a hydrodynamic improvement. The energy characteristics of the pump are measured on a closed hydraulic test rig. The pressure, vibration, and noise under different flow rates and different cavitation number are acquired for comparative analyses. Experimental results show that the energy characteristics changed after hydrodynamic improvement. The original pump is mainly affected by y-direction vibration and is clearly suppressed in the new pump. The low-frequency pressure pulsation under partial flow rate condition can be effectively suppressed. The baffles can also reduce the broadband center frequency at the pump outlet and change the relationship between center frequency and cavitation number. These results show that the hydrodynamic improvement at the inlet helps the suppression of cavitation instability of the high-speed centrifugal pump.

Numerical investigation on cavitation instability and flow-induced vibration of liquid rocket engine inducer

The objective of this paper is to numerically investigate the unsteady cavitating flow around a four-blade inducer, with focus on the cavitation instability and the flow-induced vibration characteristics. In the numerical simulation, the modified rotation/curvature correction turbulence model and the Zwart cavitation model are used for the simulation of the flow field. The tightly coupled algorithm is adopted for the precise prediction of the fluid-structure interaction, including the calculation of the hydrodynamic loads based on the multiphase fluid dynamics and the computation of the structural displacement via the Finite Element Method (FEM). The results showed that good agreement has been obtained between the experimental and numerical results. The fluctuation of cavity volume is the main cause of the change in the head of the inducer, and the backflow vortex cavitation has little effect on that at this flow condition. The backflow vortex cavity develops and rotates with the blades of the inducer, but with a much lower rotational velocity than that of the blades. The flow-induced vibration of the inducer caused by the unsteady cavitating flow mainly manifests as a first-order bending mode. The backflow vortex cavitation has a significant impact on the vibration of both the blades and the guide-water cone. Besides, a cavitation auto-oscillation at the inlet of the inducer has also been detected based on the phase correlation analysis.

Full Three-Dimensional Cavitation Instabilities Using a Non-Quadratic Anisotropic Yield Function

Full three-dimensional cell models containing a small cavity are used to study the effect of plastic anisotropy on cavitation instabilities. Predictions for the Barlat-91 model (Barlat et al., 1991, “A Six-Component Yield Function for Anisotropic Materials,” Int. J. Plast. 7, 693–712), with a non-quadratic anisotropic yield function, are compared with previous results for the classical anisotropic Hill-48 quadratic yield function (Hill, 1948, “A Theory of the Yielding and Plastic Flow of a Anisotropic Metals,” Proc. R. Soc. Lond. A193, 281–297). The critical stress, at which the stored elastic energy will drive the cavity growth, is strongly affected by the anisotropy as compared with isotropic plasticity, but does not show much difference between the two models of anisotropy. While a cavity tends to remain nearly spherical during a cavitation instability in isotropic plasticity, the cavity shapes in an anisotropic material develop toward near-spheroidal elongated shapes, which differ for different values of the coefficients defining the anisotropy. The shapes found for the Barlat-91 model, with a non-quadratic anisotropic yield function, differ noticeably from the shapes found for the quadratic Hill-48 yield function. Computations are included for a high value of the exponent in the Barlat-91 model, where this model represents a Tresca-like yield surface with rounded corners.

Visual experiment and numerical simulation of cavitation instability in a high-speed inducer

Cavitation instabilities in a high-speed inducer at a design flow rate were investigated for different cavitation numbers in numerical simulations and visual experiments. On the basis of a shear stress transport k–ω turbulence model and Zwart–Gerber–Belamri cavitation model, the transient cavitating flow in a high-speed centrifugal pump with an inducer is numerically simulated using ANSYS-CFX 15.0 software. Visual experiments were carried out to capture the evolution of cavitating flow in the inducer by using a high-speed camera. The performance and cavitation characteristic curves from numerical simulation agree with those from experiment. With a decreasing cavitation number, the cavitation development in the high-speed inducer goes through incipient cavitation, developing cavitation, critical cavitation, and deteriorated cavitation and presents vortex cavitation, sheet cavitation, cloud cavitation, backflow cavitation, and a cavitation surge. The region having a high vapor volume fraction basically coincides with the region of low local pressure at the same cavitation number. The position of largish blade loading on the inducer changes with the development of cavitation. A cavitation surge as one type of cavitation instability appears in the inducer at lower cavitation numbers. The drop or rise of the head coefficient is affected by an increasing or decreasing cavitation area in the cycle of a cavitation surge.