Turbulent transport modelling of separating and reattaching shear flows

The improvement of capabilities for computer simulation of turbulent recirculating flows was investigated. Attention has been limited to two dimensional flows and principally to statistically stationary motion. Improvement of turbulence modeling explored the treatment of the near wall sublayer and of the exterior fully turbulent region, working within the framework of turbulence closures requiring the solution of transport equations for the turbulence energy and its dissipation rate. The work on the numerical procedure, based on the Gosman-Pun program TEACH, addressed the problems of incorporating the turbulence model as well as the extension to time dependent flows, the incorporation of a third order approximation of convective transport, and the treatment of non-orthogonal boundaries

Combustion of hydrogen-air jets in local chemical equilibrium: A guide to the CHARNAL computer program

A guide to a computer program, written in FORTRAN 4, for predicting the flow properties of turbulent mixing with combustion of a circular jet of hydrogen into a co-flowing stream of air is presented. The program, which is based upon the Imperial College group's PASSA series, solves differential equations for diffusion and dissipation of turbulent kinetic energy and also of the R.M.S. fluctuation of hydrogen concentration. The effective turbulent viscosity for use in the shear stress equation is computed. Chemical equilibrium is assumed throughout the flow

Modelling windage power loss from an enclosed spur gear

Within a gearbox the majority of transmission losses can be attributed to bearing losses, meshing losses, or losses due to windage/churning. In this paper the commercial computational fluid dynamics (CFD) code Fluent 6.2.16 is applied in a two-dimensional study of windage power loss (WPL) from a single spur gear rotating in air. By comparing CFD data to published experimental data appropriate grid density and modelling parameters are identified. The model is used to investigate how peripheral shrouding affects WPL and whether WPL can be reduced through minor modifications to tooth tip geometry. Non-dimensional shroud spacings (ratio of gap to gear PCD) of between 0.005 and 0.05 were investigated at shaft speeds between 5000 and 20 000 r/min. Although CFD data compared reasonably well to experimental data, trends were not reproduced and an optimum shroud could not be identified. A full three-dimensional study is recommended. Modifying the tooth tip by adding a small chamfer on the leading edge reduced WPL by ~6 per cent. A small fillet increased total WPL by a similar amount suggesting that WPL may increase as a gear wears. This preliminary study suggests further work in this area would be beneficial

Journal off Fluids Sneering PSL-An Economical Approach to the Numerical Analysis of Near-Wall, Elliptic Flow

The paper points out that, in the numerical computation of elliptic or three-dimensional turbulent flows, the neglect of pressure-variations across the very thin viscosity affected region near the wall allows a fine-grid analysis of this sublayer without prohibitive penalties in core or computational time. The scheme has been successfully applied to the threedimensional flow around a U-bend

Phenomenology of Wall Bounded Newtonian Turbulence

We construct a simple analytic model for wall-bounded turbulence, containing only four adjustable parameters. Two of these parameters characterize the viscous dissipation of the components of the Reynolds stress-tensor and other two parameters characterize their nonlinear relaxation. The model offers an analytic description of the profiles of the mean velocity and the correlation functions of velocity fluctuations in the entire boundary region, from the viscous sub-layer, through the buffer layer and further into the log-layer. As a first approximation, we employ the traditional return-to-isotropy hypothesis, which yields a very simple distribution of the turbulent kinetic energy between the velocity components in the log-layer: the streamwise component contains a half of the total energy whereas the wall-normal and the cross-stream components contain a quarter each. In addition, the model predicts a very simple relation between the von-K\'arm\'an slope $\kappa$ and the turbulent velocity in the log-law region $v^+$ (in wall units): $v^+=6 \kappa$. These predictions are in excellent agreement with DNS data and with recent laboratory experiments.Comment: 15 pages, 11 figs, included, PRE, submitte

Influence of reaction atmosphere (H2O, N2, H2, CO2, CO) on fluidized-bed fast pyrolysis of biomass using detailed tar vapor chemistry in computational fluid dynamics

Secondary pyrolysis in fluidized bed fast pyrolysis of biomass is the focus of this work. A novel computational fluid dynamics (CFD) model coupled with a comprehensive chemistry scheme (134 species and 4169 reactions, in CHEMKIN format) has been developed to investigate this complex phenomenon. Previous results from a transient three-dimensional model of primary pyrolysis were used for the source terms of primary products in this model. A parametric study of reaction atmospheres (H2O, N2, H2, CO2, CO) has been performed. For the N2 and H2O atmosphere, results of the model compared favorably to experimentally obtained yields after the temperature was adjusted to a value higher than that used in experiments. One notable deviation versus experiments is pyrolytic water yield and yield of higher hydrocarbons. The model suggests a not overly strong impact of the reaction atmosphere. However, both chemical and physical effects were observed. Most notably, effects could be seen on the yield of various compounds, temperature profile throughout the reactor system, residence time, radical concentration, and turbulent intensity. At the investigated temperature (873 K), turbulent intensity appeared to have the strongest influence on liquid yield. With the aid of acceleration techniques, most importantly dimension reduction, chemistry agglomeration, and in-situ tabulation, a converged solution could be obtained within a reasonable time (∼30 h). As such, a new potentially useful method has been suggested for numerical analysis of fast pyrolysis

PDF model based on Langevin equation for polydispersed two-phase flows applied to a bluff-body gas-solid flow,

The aim of the paper is to discuss the main characteristics of a complete theoretical and numerical model for turbulent polydispersed two-phase flows, pointing out some specific issues. The theoretical details of the model have already been presented [Minier and Peirano, Physics Reports, Vol. 352/1-3, 2001 ]. Consequently, the present work is mainly focused on complementary aspects, that are often overlooked and that require particular attention. In particular, the following points are analysed : the necessity to add an extra term in the equation for the velocity of the fluid seen in the case of twoway coupling, the theoretical and numerical evaluations of particle averages and the fulfilment of the particle mass-continuity constraint. The theoretical model is developed within the PDF formalism. The important-physical choice of the state vector variables is first discussed and the model is then expressed as a stochastic differential equation (SDE) written in continuous time (Langevin equations) for the velocity of the fluid seen. The interests and limitations of Langevin equations, compared to the single-phase case, are reviewed. From the numerical point of view, the model corresponds to an hybrid Eulerian/Lagrangian approach where the fluid and particle phases are simulated by different methods. Important aspects of the Monte Carlo particle/mesh numerical method are emphasised. Finally, the complete model is validated and its performance is assessed by simulating a bluff-body case with an important recirculation zone and in which two-way coupling is noticeable.Comment: 23 pages, 10 figure

Structure of turbulent sonic underexpanded free jets

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76109/1/AIAA-10145-898.pd

Backflow air and pressure analysis in emptying a pipeline containing an entrapped air pocket

[EN] The prediction of the pressure inside the air pocket in water pipelines has been the topic for a lot of research works. Several aspects in this field have been discussed, such as the filling and the emptying procedures. The emptying process can affect the safety and the efficiency of water systems. Current research presents an analysis of the emptying process using experimental and computational results. The phenomenon is simulated using the two-dimensional computational fluid dynamics (2D CFD) and the one-dimensional mathematical (1D) models. A backflow air analysis is also provided based on CFD simulations. The developed models show good ability in the prediction of the sub-atmospheric pressure and the flow velocity in the system. In most of the cases, the 1D and 2D CFD models show similar performance in the prediction of the pressure and the velocity results. The backflow air development can be accurately explained using the CFD model.This work was supported by the Fundação para a Ciência e a Tecnologia (FCT), Portugal under grant number PD/BD/114459/2016.Besharat, M.; Coronado-Hernández, OE.; Fuertes-Miquel, VS.; Viseu, MT.; Ramos, HM. (2018). Backflow air and pressure analysis in emptying a pipeline containing an entrapped air pocket. Urban Water Journal. 15(8):769-779. https://doi.org/10.1080/1573062X.2018.1540711S769779158Benjamin, T. B. (1968). Gravity currents and related phenomena. Journal of Fluid Mechanics, 31(2), 209-248. doi:10.1017/s0022112068000133Besharat, M., Teresa Viseu, M., & Ramos, H. (2017). Experimental Study of Air Vessel Behavior for Energy Storage or System Protection in Water Hammer Events. Water, 9(1), 63. doi:10.3390/w9010063Besharat, M., Tarinejad, R., & Ramos, H. M. (2015). The effect of water hammer on a confined air pocket towards flow energy storage system. Journal of Water Supply: Research and Technology-Aqua, 65(2), 116-126. doi:10.2166/aqua.2015.081Besharat, M., Tarinejad, R., Aalami, M. T., & Ramos, H. M. (2016). Study of a Compressed Air Vessel for Controlling the Pressure Surge in Water Networks: CFD and Experimental Analysis. Water Resources Management, 30(8), 2687-2702. doi:10.1007/s11269-016-1310-1Coronado-Hernández, O., Fuertes-Miquel, V., Besharat, M., & Ramos, H. (2017). Experimental and Numerical Analysis of a Water Emptying Pipeline Using Different Air Valves. Water, 9(2), 98. doi:10.3390/w9020098Coronado-Hernández, O. E., Fuertes-Miquel, V. S., Besharat, M., & Ramos, H. M. (2018). Subatmospheric pressure in a water draining pipeline with an air pocket. Urban Water Journal, 15(4), 346-352. doi:10.1080/1573062x.2018.1475578Edmunds, R. C. (1979). Air Binding in Pipes. Journal - American Water Works Association, 71(5), 272-277. doi:10.1002/j.1551-8833.1979.tb04348.xEscarameia, M. (2007). Investigating hydraulic removal of air from water pipelines. Proceedings of the Institution of Civil Engineers - Water Management, 160(1), 25-34. doi:10.1680/wama.2007.160.1.25Izquierdo, J., Fuertes, V. S., Cabrera, E., Iglesias, P. L., & Garcia-Serra, J. (1999). Pipeline start-up with entrapped air. Journal of Hydraulic Research, 37(5), 579-590. doi:10.1080/00221689909498518Kader, B. A. (1981). Temperature and concentration profiles in fully turbulent boundary layers. International Journal of Heat and Mass Transfer, 24(9), 1541-1544. doi:10.1016/0017-9310(81)90220-9Laanearu, J., Annus, I., Koppel, T., Bergant, A., Vučković, S., Hou, Q., … van’t Westende, J. M. C. (2012). Emptying of Large-Scale Pipeline by Pressurized Air. Journal of Hydraulic Engineering, 138(12), 1090-1100. doi:10.1061/(asce)hy.1943-7900.0000631Leon, A. S., Ghidaoui, M. S., Schmidt, A. R., & Garcia, M. H. (2010). A robust two-equation model for transient-mixed flows. Journal of Hydraulic Research, 48(1), 44-56. doi:10.1080/00221680903565911Martins, N. M. C., Delgado, J. N., Ramos, H. M., & Covas, D. I. C. (2017). Maximum transient pressures in a rapidly filling pipeline with entrapped air using a CFD model. Journal of Hydraulic Research, 55(4), 506-519. doi:10.1080/00221686.2016.1275046Martins, S. C., Ramos, H. M., & Almeida, A. B. (2015). Conceptual analogy for modelling entrapped air action in hydraulic systems. Journal of Hydraulic Research, 53(5), 678-686. doi:10.1080/00221686.2015.1077353Pozos, O., Gonzalez, C. A., Giesecke, J., Marx, W., & Rodal, E. A. (2010). Air entrapped in gravity pipeline systems. Journal of Hydraulic Research, 48(3), 338-347. doi:10.1080/00221686.2010.481839Ramezani, L., Karney, B., & Malekpour, A. (2016). Encouraging Effective Air Management in Water Pipelines: A Critical Review. Journal of Water Resources Planning and Management, 142(12), 04016055. doi:10.1061/(asce)wr.1943-5452.0000695Richards, R. T. (1962). Air Binding in Water Pipelines. Journal - American Water Works Association, 54(6), 719-730. doi:10.1002/j.1551-8833.1962.tb00883.xTijsseling, A. S., Hou, Q., Bozkuş, Z., & Laanearu, J. (2015). Improved One-Dimensional Models for Rapid Emptying and Filling of Pipelines. Journal of Pressure Vessel Technology, 138(3). doi:10.1115/1.4031508Triki, A. (2015). Water-hammer control in pressurized-pipe flow using an in-line polymeric short-section. Acta Mechanica, 227(3), 777-793. doi:10.1007/s00707-015-1493-1Vasconcelos, J. G., & Wright, S. J. (2008). Rapid Flow Startup in Filled Horizontal Pipelines. Journal of Hydraulic Engineering, 134(7), 984-992. doi:10.1061/(asce)0733-9429(2008)134:7(984)Wang, H., Zhou, L., Liu, D., Karney, B., Wang, P., Xia, L., … Xu, C. (2016). CFD Approach for Column Separation in Water Pipelines. Journal of Hydraulic Engineering, 142(10), 04016036. doi:10.1061/(asce)hy.1943-7900.0001171Zhou, F., Hicks, F. E., & Steffler, P. M. (2002). Transient Flow in a Rapidly Filling Horizontal Pipe Containing Trapped Air. Journal of Hydraulic Engineering, 128(6), 625-634. doi:10.1061/(asce)0733-9429(2002)128:6(625)Zhou, L., Liu, D., & Karney, B. (2013). Investigation of Hydraulic Transients of Two Entrapped Air Pockets in a Water Pipeline. Journal of Hydraulic Engineering, 139(9), 949-959. doi:10.1061/(asce)hy.1943-7900.0000750Zhou, L., Liu, D., & Ou, C. (2011). Simulation of Flow Transients in a Water Filling Pipe Containing Entrapped Air Pocket with VOF Model. Engineering Applications of Computational Fluid Mechanics, 5(1), 127-140. doi:10.1080/19942060.2011.11015357Zhou, L., Wang, H., Karney, B., Liu, D., Wang, P., & Guo, S. (2018). Dynamic Behavior of Entrapped Air Pocket in a Water Filling Pipeline. Journal of Hydraulic Engineering, 144(8), 04018045. doi:10.1061/(asce)hy.1943-7900.0001491Zukoski, E. E. (1966). Influence of viscosity, surface tension, and inclination angle on motion of long bubbles in closed tubes. Journal of Fluid Mechanics, 25(4), 821-837. doi:10.1017/s002211206600044

On the equations of mathematical hydraulics

The relation between classical hydraulics and modern turbulence modelling is discussed for the case of two-dimensional open channel flow down an inclined plane. A second order turbulence model describing the flow is treated asymptotically for the parameter range F ≥ O (1), δ ≪1, β ≪1, and δ = O ( β 2 ), where F is the Froude number, δ is the aspect ratio, and β is the square root of a characteristic drag coefficient. The Chezy law formulation of mathematical hydraulics is derived as the lowest order approximation to the solution for the flow outside bore regions, and the transverse variation of the longitudinal velocity component is determined at the next stage of the analysis. It is shown that flow discontinuities calculated using the equations of mathematical hydraulics are resolved in bore regions of transverse length scale O ( H o ), where H o is the characteristic fluid depth. The bore structure is found to consist of a highly turbulent outer region with transverse length scale O ( H o ) in which the turbulence intensity is O (1), and a bottom boundary layer of transverse length scale O ( β 2 H o ), in which the turbulent stresses decrease rapidly to satisfy the bottom boundary conditions. The jump conditions of mathematical hydraulics at flow discontinuities are verified, and it is inferred that classical hydraulics provides an acceptable approximation to the flow outside bore regions for the parameter range considered in the theory.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43446/1/33_2004_Article_BF00945957.pd