USING DIFFERENT TURBULENT VISCOUS MODELS TO INVESTIGATE HYDRODYNAMIC PERFORMANCE OF A DUCTED PROPELLER

Chân vịt ống đạo lưu được biết đến như một loại ống phun, là một loại thiết bị đẩy sử dụng cho tàu thủy được đặt bên trong một ống bao cố định. Thiết bị này được sử dụng nhằm cải thiện hiệu suất đẩy cho chân vịt đối với một số loại tàu như tàu cá, tàu kéo, tàu ngầm, những loại tàu có trọng tải lớn hay những tàu bị hạn chế về đường kính chân vịt. Trong bài báo này, nhóm tác giả sử dụng ba mô hình rối khác nhau gồm RNG k-e, k-w SST và k-w transition, để khảo sát dòng bao quanh chân vịt thông qua sử dụng công cụ mô phỏng số thương mại CFD. Các đặc tính thủy động lực của một hệ thống chân vịt ống đạo lưu có kể đến ảnh hưởng của mô hình rối khác nhau trong tính toán mô phỏng sẽ được phân tích cụ thể. Một chân vịt cụ thể có đường kính 3,65m, vận tốc quay 200 vòng/phút, tỷ số truyền 0,1730 được sử dụng trong tính toán, ống đạo lưu với mặt cắt ngang có biên dạng Naca 4415 được sử dụng trong nghiên cứu. Thông qua sử dung CFD, mô hình chân vịt đạo lưu được xây dựng, chia lưới, hiệu chỉnh lưới và tính toán. Các kết quả về đặc tính thủy động lực của chân vịt đạo lưu được phân tích cụ thể với ba mô hình rối sử dụng khác nhau được trình bày. Tiếp theo, các kết quả thu được gồm các hệ số thủy động lực, phân bố áp suất và các hệ số đặc tính chân vịt và ống đạo lưu sẽ được phân tích và thảo luận.

Testing viscous disc theory using the balance between stellar accretion and external photoevaporation of protoplanetary discs

ABSTRACT The nature and rate of (viscous) angular momentum transport in protoplanetary discs (PPDs) have important consequences for the formation process of planetary systems. While accretion rates on to the central star yield constraints on such transport in the inner regions of a PPD, empirical constraints on viscous spreading in the outer regions remain challenging to obtain. Here, we demonstrate a novel method to probe the angular momentum transport at the outer edge of the disc. This method applies to PPDs that have lost a significant fraction of their mass due to thermal winds driven by UV irradiation from a neighbouring OB star. We demonstrate that this external photoevaporation can explain the observed depletion of discs in the 3–5 Myr old σ Orionis region, and use our model to make predictions motivating future empirical investigations of disc winds. For populations of intermediate-age PPDs, in viscous models we show that the mass flux outwards due to angular momentum redistribution is balanced by the mass-loss in the photoevaporative wind. A comparison between wind mass-loss and stellar accretion rates therefore offers an independent constraint on viscous models in the outer regions of PPDs.

Viscous effects in the dark sector of the Universe

Viscous properties are attributed to the dark sector of the Universe. They contribute to the accelerated expansion phase of the Universe and can alleviate existing tensions in the [Formula: see text]CDM model at small scales. We provide a short review of recent efforts on this topic. Different viscous models for the dark sector are analysed both from theoretical and observational point of view.

CFD Analysis of a Solid Propellant Retro Rocket Motor using Ansys Fluent

A Solid propellant Retro Rocket Motor with a C-D Nozzle is analyzed using Ansys Fluent 17.1 .Steady State analysis of a retro rocket motor has been done for Viscous models like Inviscid and k-ɛ (Realizable) with air and gas as working fluids. The dimensions and the boundary conditions of the nozzle are kept consistent for both the viscous models to compare the exit parameters to theoretical values obtained by using one dimensional equations. The results obtained shows that the exit temperature of the nozzle in Inviscid viscous model is higher than theoretical value by 2.2% and for k-ɛ viscous model it is higher by 5.4% with gas as working fluid.

Coronal energy release by MHD avalanches: Heating mechanisms

The plasma heating associated with an avalanche involving three twisted magnetic threads within a coronal loop is investigated using three-dimensional magnetohydrodynamic simulations. The avalanche is triggered by the kink instability of one thread, with the others being engulfed as a consequence. The heating as a function of both time and location along the strands is evaluated. It is shown to be bursty at all times but to have no preferred spatial location. While there appears to be a level of “background” heating, this is shown to be comprised of individual, small heating events. A comparison between viscous and resistive (Ohmic) heating demonstrates that the strongest heating events are largely associated with the Ohmic heating that arises when the current exceeds a critical value. Viscous heating is largely (but not entirely) associated with smaller events. Ohmic heating dominates viscous heating only at the time of the initial kink instability. It is also demonstrated that a variety of viscous models lead to similar heating rates, suggesting that the system adjusts to dissipate the same amount of energy.

Bianchi Type V Universe and Bulk Viscous Models with Time Dependent Gravitational Constant and Cosmological Constant in General Relativity

This paper deals with Bulk Viscous Models and Bianchi type- universe in the standard general relativity theory by assuming where constants, is the density. Einstein field equations ( ) have been solved in the presence of a variable gravitational constant as well as a variable cosmological constant together with a bulk viscous fluid. In order to find a deterministic solution of EFEs, a simple form of Hubble parameter being constant when where has been considered here that shows the signature flip from early deceleration to present acceleration. Moreover, the bulk viscous coefficient is allowed to vary with the density as . The physical and geometrical behavior are described in detail for the obtained model. The model obtained here is in good agreement with the present cosmological observations.

FRW viscous fluid cosmological model with time-dependent inhomogeneous equation of state

In this paper, we present two viscous models of non-perfect fluid by avoiding the introduction of exotic dark energy. We consider the first model in terms of deceleration parameter [Formula: see text] has a viscosity of the form [Formula: see text] and the other model in quadratic form of [Formula: see text] of the type [Formula: see text]. In this framework we find the solutions of field equations by using inhomogeneous equation of state of form [Formula: see text] with equation of state parameter [Formula: see text] is constant and [Formula: see text].