scholarly journals The effects of altered flow and bed sediment on macroinvertebrates in stream mesocosms

2017 ◽  
Vol 68 (3) ◽  
pp. 496 ◽  
Author(s):  
Ivor Growns ◽  
John F. Murphy ◽  
J. Iwan Jones
Keyword(s):  
Turbulent Flow ◽  
Fine Particles ◽  
Anthropogenic Disturbances ◽  
River Regulation ◽  
Laminar Flows ◽  
Ecological Impacts ◽  
Transitional Flow ◽  
Flow Rates ◽  
Altered Hydrology ◽  
Stream Mesocosms

River regulation and altered land use are common anthropogenic disturbances resulting in ecological impacts through siltation or altered hydrology. We tested the separate and combined effects of increased flow and fine particles (colmation) on macroinvertebrates in flume mesocosms. We hypothesised that increased flow would reduce any effects of colmation. We tested two bed types, namely clean and colmated sediment where fines were 10% by weight. Two flow rates were initially established, namely a turbulent flow in six mesocosms and a lower rate to create a transitional flow between turbulent and laminar flows in the remaining six mesocosms. After 30 days, macroinvertebrates were sampled and the flow in three turbulent-flow mesocosms and three transitional-flow mesocosms switched to the lower and higher flow rates respectively, thus creating four flow scenarios. The experiment was concluded after sampling macroinvertebrates again at Day 70. We demonstrated that colmation and decreased flows individually result in decreased density and richness of macroinvertebrates and altered assemblage and trait structure. However, our hypothesis that higher flows would ameliorate any effects of fines was not supported. Further research is required to evaluate whether lower thresholds of colmation have ecological impacts and determine the velocities required to ameliorate those impacts.

scholarly journals Accumulation of motile elongated micro-organisms in turbulence

2013 ◽  
Vol 739 ◽  
pp. 22-36 ◽  
Author(s):  
Caijuan Zhan ◽  
Gaetano Sardina ◽  
Enkeleida Lushi ◽  
Luca Brandt
Keyword(s):  
Turbulent Flow ◽  
Three Dimensional ◽  
Laminar Flows ◽  
Marine Life ◽  
Prolate Spheroids ◽  
Linear Shear ◽  
Local Shear ◽  
Micro Organisms ◽  
Hydrodynamic Effects ◽  
Preferential Alignment

AbstractWe study the effect of turbulence on marine life by performing numerical simulations of motile micro-organisms, modelled as prolate spheroids, in isotropic homogeneous turbulence. We show that the clustering and patchiness observed in laminar flows, linear shear and vortex flows, are significantly reduced in a three-dimensional turbulent flow mainly because of the complex topology; elongated micro-organisms show some level of clustering in the case of swimmers without any preferential alignment whereas spherical swimmers remain uniformly distributed. Micro-organisms with one preferential swimming direction (e.g. gyrotaxis) still show significant clustering if spherical in shape, whereas prolate swimmers remain more uniformly distributed. Due to their large sensitivity to the local shear, these elongated swimmers react more slowly to the action of vorticity and gravity and therefore do not have time to accumulate in a turbulent flow. These results show how purely hydrodynamic effects can alter the ecology of micro-organisms that can vary their shape and their preferential orientation.

Physical Interpretation of Flow and Heat Transfer in Preswirl Systems

2006 ◽  
Vol 129 (3) ◽  
pp. 769-777 ◽  
Author(s):  
Paul Lewis ◽  
Mike Wilson ◽  
Gary Lock ◽  
J. Michael Owen
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Turbulent Flow ◽  
Gas Turbines ◽  
Flow Parameter ◽  
Rotating Disk ◽  
Turbine Rotor ◽  
Scaled Model ◽  
Flow Rates

This paper compares heat transfer measurements from a preswirl rotor–stator experiment with three-dimensional (3D) steady-state results from a commercial computational fluid dynamics (CFD) code. The measured distribution of Nusselt number on the rotor surface was obtained from a scaled model of a gas turbine rotor–stator system, where the flow structure is representative of that found in an engine. Computations were carried out using a coupled multigrid Reynolds-averaged Navier-Stokes (RANS) solver with a high Reynolds number k-ε∕k-ω turbulence model. Previous work has identified three parameters governing heat transfer: rotational Reynolds number (Reϕ), preswirl ratio (βp), and the turbulent flow parameter (λT). For this study rotational Reynolds numbers are in the range 0.8×106<Reϕ<1.2×106. The turbulent flow parameter and preswirl ratios varied between 0.12<λT<0.38 and 0.5<βp<1.5, which are comparable to values that occur in industrial gas turbines. Two performance parameters have been calculated: the adiabatic effectiveness for the system, Θb,ad, and the discharge coefficient for the receiver holes, CD. The computations show that, although Θb,ad increases monotonically as βp increases, there is a critical value of βp at which CD is a maximum. At high coolant flow rates, computations have predicted peaks in heat transfer at the radius of the preswirl nozzles. These were discovered during earlier experiments and are associated with the impingement of the preswirl flow on the rotor disk. At lower flow rates, the heat transfer is controlled by boundary-layer effects. The Nusselt number on the rotating disk increases as either Reϕ or λT increases, and is axisymmetric except in the region of the receiver holes, where significant two-dimensional variations are observed. The computed velocity field is used to explain the heat transfer distributions observed in the experiments. The regions of peak heat transfer around the receiver holes are a consequence of the route taken by the flow. Two routes have been identified: “direct,” whereby flow forms a stream tube between the inlet and outlet; and “indirect,” whereby flow mixes with the rotating core of fluid.

Investigation of Flow Dynamics Over Transitional-Type Microcavity

2018 ◽  
Vol 140 (7) ◽  
Author(s):  
Paulius Vilkinis ◽  
Nerijus Pedišius ◽  
Mantas Valantinavičius
Keyword(s):  
Turbulent Flow ◽  
Flow Regime ◽  
Recirculation Zone ◽  
Transitional Flow ◽  
Navier Stokes ◽  
Flow Topology ◽  
Rans Equations ◽  
Recirculation Flow ◽  
Turbulent Flow Regime ◽  
Transitional Type

Flow over a transitional-type cavity in microchannels is studied using a microparticle image velocimetry system (μPIV) and commercially available computational fluid dynamics (CFD) software in laminar, transitional, and turbulent flow regimes. According to experimental results, in the transitional-type cavity (L/h1 = 10) and under laminar flow in the channel, the recirculation zone behind the backward-facing step stretches linearly with ReDh until the reattachment point reaches the middle of the cavity at xr/L = (0.5 to 0.6). With further increase in ReDh, the forward-facing step lifts the reattaching flow from the bottom of the cavity and stagnant recirculation flow fills the entire space of the cavity. Flow reattachment to the bottom of the cavity is again observed only after transition to the turbulent flow regime in the channel. Reynolds-averaged Navier–Stokes (RANS) equations and large eddy simulation (LES) results revealed changes in vortex topology, with the flow regime changing from laminar to turbulent. During the turbulent flow regime in the recirculation zone, periodically recurring vortex systems are formed. Experimental and computational results have a good qualitative agreement regarding the changes in the flow topology. However, the results of numerical simulations based on RANS equations and the Reynolds-stress-baseline turbulence model (RSM-BSL), show that computed reattachment length values overestimate the experimentally obtained values. The RSM-BSL model underestimates the turbulent kinetic energy intensity, generated by flow separation phenomena, on the stage of transitional flow regime.

Turbulent flow over superhydrophobic surfaces with streamwise grooves

2014 ◽  
Vol 747 ◽  
pp. 186-217 ◽  
Author(s):  
S. Türk ◽  
G. Daschiel ◽  
A. Stroh ◽  
Y. Hasegawa ◽  
B. Frohnapfel
Keyword(s):  
Boundary Conditions ◽  
Turbulent Flow ◽  
Drag Reduction ◽  
Secondary Flow ◽  
Slip Length ◽  
Laminar Flows ◽  
Superhydrophobic Surfaces ◽  
Mean Velocity ◽  
Effective Slip Length ◽  
Effective Slip

AbstractWe investigate the effects of superhydrophobic surfaces (SHS) carrying streamwise grooves on the flow dynamics and the resultant drag reduction in a fully developed turbulent channel flow. The SHS is modelled as a flat boundary with alternating no-slip and free-slip conditions, and a series of direct numerical simulations is performed with systematically changing the spanwise periodicity of the streamwise grooves. In all computations, a constant pressure gradient condition is employed, so that the drag reduction effect is manifested by an increase of the bulk mean velocity. To capture the flow properties that are induced by the non-homogeneous boundary conditions the instantaneous turbulent flow is decomposed into the spatial-mean, coherent and random components. It is observed that the alternating no-slip and free-slip boundary conditions lead to the generation of Prandtl’s second kind of secondary flow characterized by coherent streamwise vortices. A mathematical relationship between the bulk mean velocity and different dynamical contributions, i.e. the effective slip length and additional turbulent losses over slip surfaces, reveals that the increase of the bulk mean velocity is mainly governed by the effective slip length. For a small spanwise periodicity of the streamwise grooves, the effective slip length in a turbulent flow agrees well with the analytical solution for laminar flows. Once the spanwise width of the free-slip area becomes larger than approximately 20 wall units, however, the effective slip length is significantly reduced from the laminar value due to the mixing caused by the underlying turbulence and secondary flow. Based on these results, we develop a simple model that allows estimating the gain due to a SHS in turbulent flows at practically high Reynolds numbers.

Laminar and transitional flow of drilling muds and various suspensions in circular tubes

1967 ◽  
Vol 30 (3) ◽  
pp. 449-464 ◽  
Author(s):  
Bernard Le Fur ◽  
Madeleine Martin
Keyword(s):  
Reynolds Number ◽  
Laminar Flow ◽  
Rheological Behaviour ◽  
Simple Expression ◽  
Transitional Flow ◽  
Transition Flow ◽  
Flow Rates ◽  
Circular Tubes ◽  
Head Losses ◽  
Drilling Muds

Most suspensions exhibit a rheological behaviour which cannot be represented by either Bingham's or Ostwald–De Waele's law. In studying such cases a very simple expression with only three parameters may be used. Starting with an intermediate law of this sort, this paper gives velocity profiles and head losses in laminar flow, which have been computed and plotted on diagrams in non-dimensional co-ordinates.It has been found that transition flow rates in circular tubes for data taken from the literature and from experiments conducted on drilling muds at the Institut Français du Pétrole, are efficiently predicted by an empirical criterion (Ryan & Johnson 1959) which establishes a relation between a generalized Reynolds number and a generalized Hedström number.

A Functional Scaling Relationship for Laminar to Turbulent Flow Regimes

2011 ◽  
Author(s):  
George Papadopoulos
Keyword(s):  
Turbulent Flow ◽  
Stokes Equations ◽  
Initial Conditions ◽  
Flow Characteristics ◽  
Flow Region ◽  
Flow Regimes ◽  
Transitional Flow ◽  
Turbulent Regime ◽  
Flow Profile ◽  
Scaling Relationship

A dimensional analysis that is based on the scaling of the two-dimensional Navier-Stokes equations is presented for correlating bulk flow characteristics arising from a variety of initial conditions. The analysis yields a functional relationship between the characteristic variable of the flow region and the Reynolds number for each of the two independent flow regimes. A linear relationship is realized for the laminar regime, while a nonlinear relationship is realized for the turbulent regime. Both relationships incorporate mass-flow profile characteristics to fully capture the effects of initial conditions on the variation of the characteristic variables. The union of these two independent relationships is formed utilizing the concept of flow intermittency to further expand into a generic scaling relationship that incorporates transitional flow effects to fully encompass solutions spanning the laminar to turbulent flow regimes. The results of the analysis are discussed within the context of several flow phenomena (e.g. pipe flow, jet flow & separated flow) resulting from various initial and boundary conditions.

Heat Transfer and Pressure Drop Correlations for Twisted-Tape Inserts in Isothermal Tubes: Part I—Laminar Flows

1993 ◽  
Vol 115 (4) ◽  
pp. 881-889 ◽  
Author(s):  
R. M. Manglik ◽  
A. E. Bergles
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Swirl Flow ◽  
Laminar Flows ◽  
Low Flow ◽  
Published Data ◽  
Twisted Tape ◽  
Flow Rates ◽  
Uniform Wall Temperature ◽  
Uniform Wall

Laminar flow correlations for f and Num are developed based on experimental data for water and ethylene glycol, with tape inserts of three different twist ratios. The uniform wall temperature condition is considered, which typifies practical heat exchangers in the chemical and process industry. These and other available data are analyzed to devise flow regime maps that characterize twisted-tape effects in terms of the dominant enhancement mechanisms. Depending upon flow rates and tape geometry, the enhancement in heat transfer is due to the tube partitioning and flow blockage, longer flow path, and secondary fluid circulation; fin effects are found to be negligible in snug- to loose-fitting tapes. The onset of swirl flow and its intensity is determined by a swirl parameter, Sw=Resw/y, that defines the interaction between viscous, convective inertia, and centrifugal forces. Buoyancy-driven free convection that comes into play at low flow rates with large y and ΔTw is shown to scale as Gr/Sw2≫ 1. These parameters, along with numerical baseline solutions for laminar flows with y = ∞, are incorporated into correlations for f and Num by matching the appropriate asymptotic behavior. The correlations describe the experimental data within ±10 to 15 percent, and their generalized applicability is verified by the comparison of predictions with previously published data.

Physical Interpretation of Flow and Heat Transfer in Pre-Swirl Systems

2006 ◽  
Author(s):  
Paul Lewis ◽  
Mike Wilson ◽  
Gary Lock ◽  
J. Michael Owen
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Turbulent Flow ◽  
Gas Turbines ◽  
Flow Parameter ◽  
Swirl Flow ◽  
Rotating Disc ◽  
Scaled Model ◽  
Flow Rates

This paper compares heat transfer measurements from a pre-swirl rotor-stator experiment with 3D steady state results from a commercial CFD code. The measured distribution of Nusselt number on the rotor surface was obtained from a scaled model of a gas turbine rotor-stator system, where the flow structure is representative of that found in an engine. Computations were carried out using a coupled multigrid RANS solver with a high-Reynolds-number k-ε/k-ω turbulence model. Previous work has identified three parameters governing heat transfer: rotational Reynolds number (Reφ), pre-swirl ratio (βp) and the turbulent flow parameter (λT). For this study rotational Reynolds numbers are in the range 0.8×106 &lt; Reφ &lt; 1.2×106. The turbulent flow parameter and pre-swirl ratios varied between 0.12 &lt; λT &lt; 0.38 and 0.5 &lt; βp &lt; 1.5, which are comparable to values that occur in industrial gas turbines. At high coolant flow rates, computations have predicted peaks in heat transfer at the radius of the pre-swirl nozzles. These were discovered during earlier experiments and are associated with the impingement of the pre-swirl flow on the rotor disc. At lower flow rates, the heat transfer is controlled by boundary-layer effects. The Nusselt number on the rotating disc increases as either Reφ or λT increases, and is axisymmetric except in the region of the receiver holes, where significant two-dimensional variations are observed. The computed velocity field is used to explain the heat transfer distributions observed in the experiments. The regions of peak heat transfer around the receiver holes are a consequence of the route taken by the flow. Two routes have been identified: “direct”, whereby flow forms a stream-tube between the inlet and outlet; and “indirect”, whereby flow mixes with the rotating core of fluid. Two performance parameters have been calculated: the adiabatic effectiveness for the system, Θb,ab, and the discharge coefficient for the receiver holes, CD. The computations show that, although Θb,ab increases monotonically as βp increases, there is a critical value of βp at which CD is a maximum.

scholarly journals The benefits of turbine endwall profiling in a cascade

2005 ◽  
Vol 219 (1) ◽  
pp. 49-59 ◽  
Author(s):  
G Ingram ◽  
D Gregory-Smith ◽  
N Harvey
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Turbulent Flow ◽  
Secondary Flow ◽  
Surface Flow ◽  
Transitional Flow ◽  
Loss Reduction ◽  
Small Reduction ◽  
Laminar And Turbulent Flow ◽  
Computational Fluid Dynamics Cfd

Non-axisymmetric profiled endwalls have been shown to reduce losses and secondary flow both in cascades and in rig tests. This paper presents experimental results which quantify the benefits of loss reduction in the cascade with particular attention to accuracy. The paper compares the benefits achieved in experiment to the results predicted by computational fluid dynamics (CFD). The results show that both the experiment and CFD give significant reductions in secondary flow. A reduction of 31 per cent in secondary loss has been measured for the best case, but the CFD gives only a small reduction in loss. Previous studies on the planar endwall have shown significant areas of transitional flow, so the surface flow has been studied with the aid of surface-mounted hot films. It was concluded that the loss reductions were not due to changes in regions of laminar and turbulent flow.

Multilaboratory Particle Image Velocimetry Analysis of the FDA Benchmark Nozzle Model to Support Validation of Computational Fluid Dynamics Simulations

2011 ◽  
Vol 133 (4) ◽  
Author(s):  
Prasanna Hariharan ◽  
Matthew Giarra ◽  
Varun Reddy ◽  
Steven W. Day ◽  
Keefe B. Manning ◽  
...  
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Particle Image Velocimetry ◽  
Turbulent Flow ◽  
Medical Device ◽  
Particle Image ◽  
Transitional Flow ◽  
Flow Conditions ◽  
Image Velocimetry ◽  
Flow Case

This study is part of a FDA-sponsored project to evaluate the use and limitations of computational fluid dynamics (CFD) in assessing blood flow parameters related to medical device safety. In an interlaboratory study, fluid velocities and pressures were measured in a nozzle model to provide experimental validation for a companion round-robin CFD study. The simple benchmark nozzle model, which mimicked the flow fields in several medical devices, consisted of a gradual flow constriction, a narrow throat region, and a sudden expansion region where a fluid jet exited the center of the nozzle with recirculation zones near the model walls. Measurements of mean velocity and turbulent flow quantities were made in the benchmark device at three independent laboratories using particle image velocimetry (PIV). Flow measurements were performed over a range of nozzle throat Reynolds numbers (Rethroat) from 500 to 6500, covering the laminar, transitional, and turbulent flow regimes. A standard operating procedure was developed for performing experiments under controlled temperature and flow conditions and for minimizing systematic errors during PIV image acquisition and processing. For laminar (Rethroat=500) and turbulent flow conditions (Rethroat≥3500), the velocities measured by the three laboratories were similar with an interlaboratory uncertainty of ∼10% at most of the locations. However, for the transitional flow case (Rethroat=2000), the uncertainty in the size and the velocity of the jet at the nozzle exit increased to ∼60% and was very sensitive to the flow conditions. An error analysis showed that by minimizing the variability in the experimental parameters such as flow rate and fluid viscosity to less than 5% and by matching the inlet turbulence level between the laboratories, the uncertainties in the velocities of the transitional flow case could be reduced to ∼15%. The experimental procedure and flow results from this interlaboratory study (available at http://fdacfd.nci.nih.gov) will be useful in validating CFD simulations of the benchmark nozzle model and in performing PIV studies on other medical device models.

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