Downstream Velocity Distribution Research Articles

Study on reduction in pressure losses in pipe bends using guide vanes

The pipe bends are common elements in pipeline network of mechanical systems. The losses in pipes and bend are generally considered as insignificant and unavoidable. The recent trends in achieving a higher order of performance in machines led to optimization by reduction in minor energy losses. The usage of components with the higher surface finish is the main methodology adopted for minimizing frictional loss. In bends, especially in the case of turbulent flows, the losses are significant when comparing the frictional pipe losses. The viscous interaction between fluid layers is more in bends due to the presence of flow separation secondary swirling motion. The reduction in these interactions can be achieved by installing turning vane (guide vanes) inside the pipe bend. The current literature available lacks detailing of many included flow physics in bends and vaned bends. The present work focuses on flow characteristics on a 90° curved bend. Incompressible isothermal RANS solutions were performed using turbulence treatment with the k − ω SST model. The initial validation study is carried out with available experimental results. The bend downstream velocity distribution and bend wall pressure distribution are carried out. Further, the computational methodology is extended to bend with different vane configurations and for different Reynolds numbers. The velocity, wall pressure, swirl strength, turbulent kinetic energy, etc. are reported for downstream locations. The comparison of pressure loss through different cases and improvements in flow through bend due to the inclusion of vanes are discussed in this work.

The influence of scale, slope and channel geometry on the flow dynamics of submarine channels

Submarine channels are major morphological features of the sea floor and are important in the transport of sediment to the deep ocean. Although much is known concerning the large-scale distribution of sediment within and surrounding submarine channels, there is little understanding of the fluid dynamic processes that control this sedimentation. Direct measurement of flow velocities and concentrations has proved to be extremely difficult within submarine channels, with the resultant paucity of direct observations making physical laboratory modelling a critical technique for examining the processes that operate in, and control, submarine channel development. Recent experimental and numerical studies have proposed a new model of secondary circulation within submarine channel bends, characterised by a reversal in the orientation of the secondary circulation cell relative to that found in meandering rivers. This new paradigm for submarine channels thus predicts basal flow from the inside to the outside of the bend at a bend apex, with an upper return flow directed towards the inner bend. The reversal in orientation of the secondary flow cell has been linked to the vertical distribution of downstream velocity and associated changes in centrifugal and pressure gradient forces. However, previous work has additionally proposed that shearing of the within-channel flow by overbank flow may also generate secondary flow reversal. This study assesses the applicability of the proposed submarine bend flow model against a range of key channel parameters. We demonstrate that the sense of secondary circulation is the same for all experimental conditions, strongly supporting the new model of secondary flow in submarine channels. Furthermore, investigation of overbank shear induced secondary circulation confirms for the first time that this mechanism can occur, and identifies the channel styles most likely to exhibit this effect. Such shear-induced circulation is, however, shown to be a secondary mechanism, with the vertical distribution of downstream velocity the principal mechanism. In certain channel configurations, the two mechanisms may act to augment one another.

Analytic distribution functions for an ion plasma crossing an MHD shock

Context. Very recently, we derived the Boltzmann equation for a collisionless plasma crossing an MHD shock, such as that found in astrophysical situations. Aims. We aim to solve the Boltzmann equation analytically, to obtain exact downstream velocity distribution functions for a given upstream distribution function and an MHD compression ratio. Methods. We apply the general Boltzmann equation to the case of a purely perpendicular shock and solve it using a separation Ansatz. Results. We obtain a simple,'analytical relation between the upstream and downstream velocity distribution functions, which is valid for all physical plasmas. Having obtained the desired relation, we also derive an analytical relation for MHD velocity moments of arbitrary order.

The orientation of helical flow in curved channels

AbstractThe planform patterns of meandering submarine channels and subaerial fluvial bends show many similarities that have given rise to strong analogies concerning the fluid dynamics of these channel types. Existing models of helical motion in open‐channel bends depict flow that is characterized by surface flow towards the outer bank, and basal flow towards the inner bank. This paper investigates and compares, through an analytical model and physical experiment, flows within fluvial meanders, and submarine channel bends that contain density‐driven gravity currents. The results indicate that the sense of helical motion can be reversed in submarine bends that contain density currents when compared with fluvial bends, and that the orientation of the helical flow is dependent on the vertical distribution of downstream velocity. Specifically, the sense of helical motion is reversed in bends when the maximum downstream velocity is near the bed, resulting in near‐bed flow towards the outer bank. These findings suggest that the dynamics of sediment transport and deposition in curved channels with such velocity profiles will be fundamentally different to those currently assumed from sinuous open‐channels.

Flow Characteristics in Channel with Vegetation

It is very important in river management works to take into account the influences of the vegetation to the flow during the flood. Logarithmic or parabolic law can be a good assumption for the vertical profile of downstream velocity distribution in open channel flow without vegetation. However, these assumptions cannot be applied to the flow through the vegetation because of the strong effect of the resistance caused by trees.In this paper, the vertical distribution of flow through the vegetation is predicted theoretically. The bed drag coefficient has to be changed in order to calculate the depth averaged downstream velocity accurately.

Outlet Structure Design for Conduits and Tunnels

This paper describes the analysis and experimental work leading to the development of design criteria for an outlet structure. The recommended design consists of a transition and stilling basin with straight diverging sidewalls. The rate of sidewall divergence is expressed as a function of the width ratio and Froude number. Design values were determined from model tests carried out for both round and square conduits. Design criteria were based upon observations on the effective position of the hydraulic grade line at the outlet, head losses in the transition, depth and velocity distribution in the transition, stability of the hydraulic jump, downstream velocity distribution and scour tendencies in the discharge channel. A major portion of the paper is devoted to details of the recommended design procedure, including the rationale behind the various recommendations. An example is worked to illustrate the procedure, and the performance data for one of the test models is included.

A model study of tidal barrage sluices for minimum energy loss

As part of the pre-feasibility study for the Severn Barrage, hydraulic tests were carried out on half models of three sluice passageway designs based on a vertical gated Venturi passage. Froude law scaling relationships were used throughout and the models were tested over a range of water levels corresponding to the expected sea level variation at the preferred barrage site. The sluices were tested under simulated in-service conditions, the flow being in one direction only, i.e. from the open sea. The quantitative results are presented as diffuser efficiency values. Flow visualisation identified regions of flow separation and measurements were also made of the downstream velocity distribution in a vertical plane. The three models tested here represent progressive modification and improvement to the original design and their performance, as represented by diffuser efficiency, was improved from 0.55 to 0.75 and finally to 0.85.

Simulation of velocity profiles by shaped gauze screens.

In this paper results of a theoretical and experimental investigation of axisymmetric flow through shaped wire gauze screens are presented. The theoretical analysis for steady, inviscid, rotational flow in which the highly viscous nature of the flow processes at the screen is introduced into the theory by empirical values of gauze resistance and deflection coefficient, produces linearized equations which relate the velocity distributions far upstream and downstream of the screen, the screen shape and the gauze coefficients. Given the screen shape, the gauze coefficients and the upstream velocity distribution, the downstream velocity distribution can be determined or alternatively given the upstream and downstream velocity distribution a screen shape can be specified for certain values of the resistance and deflection coefficients. The previously accepted term in the linearized formulation which accounts for the resistance variation due to screen angle is shown to produce erroneous results. A comparison between experimental and theoretical profiles is presented omitting this term for four different screen shapes and reasonable agreement is obtained.