Presence Of Secondary Flow Research Articles

Secondary flows over artificial bed strips

Characteristics of open channel flows can be significantly modified in the presence of secondary flows. In this study, cellular secondary flows were artificially generated with alternate rough and smooth bed strips, which were aligned longitudinally in an open channel. Flow measurements were conducted with a two-dimensional Laser Doppler Anemometer system. Experimental results show that the distributions of the primary velocity and streamwise Reynolds shear stress that are altered in the presence of the secondary flows clearly deviate from those observed for two-dimensional open channel flows. The modified streamwise velocity can be represented by a function proposed in the log-wake form but varying periodically in the lateral direction. This study also demonstrates that the flow field can be linearized so that a flow quantity can be generally decomposed into two components, one being related to the average base flow and the other symbolizing the perturbation caused by the bed configuration. This perturbation-based approach finally provides an analytical description of the altered Reynolds shear stress distribution that differs from the linear profile used for two-dimensional open channel flows.

Residence Time Distributions and Flow Behaviour within Primary Crude Oil–Water Separators Treating well-head Fluids

Within primary crude oil separators used by the oil industries, the residence time distribution of both organic and aqueous phases has been obtained for the purpose of flow diagnostics. This paper describes the application of the Alternative Path Model developed by Simmons et al. (2002) to give a quantitative description of the hydrodynamics and mixing within several field separators. Parameters developed from the model are used to describe the degree of mixing within the vessels. The model shows that vessel performance is affected by the internal configuration (flow smoothing baffles and separation plates) and the primary separation duty (gas–liquid or oil–water). The presence of baffling is shown to reduce the turbulence within the flow for oil–water separation, but less so for gas–oil separation, which had the overall effect of increasing mixing levels, perhaps due to the buoyancy of the fast rising gas bubbles. The presence of secondary peaks on some of the measured residence time distributions indicates the presence of secondary flows within the main body of the separators. This was most noticeable when the differential velocity between the oil and water phases was high.

Microwave Remote Sensing of Tropical Cyclones from Space

This article reviews several microwave instruments employed in research and analysis of tropical cyclones (TCs), typhoons, and hurricanes. The instruments discussed include scatterometers, microwave radiometers, synthetic aperture radars (SARs), and rain radar from space. Examples of the particular contribution by one or more of these instruments in analysis of several storms illustrate the comprehensive new views provided by the SeaWinds scatterometers, the detailed high-resolution wind field provided by RADARSAT-1 SAR, particularly inside and in the vicinity of hurricane “eyes,” and the presence of secondary flows in the region between rainbands in TCs. The high spatial resolution of precipitation data from the Tropical Rainfall Measuring Mission's rain radar, combined with scatterometer or SAR data, give a significant improvement in the details that can be seen from space, at the surface, and in the precipitating areas of TCs. The microwave instruments provide a penetrating view below the upper level cirrus clouds.

A RHEOLOGICAL IN VITRO DEVICE FOR INVESTIGATING THE HEMOCOMPATIBILITY OF BIOMATERIALS

The hemocompatibility of artificial surfaces is in tight connection to the fluid dynamic characteristics of flowing blood. In addition to surface effects the growth rate of thrombi, cellular activation and adsorption of proteins are affected by blood flow conditions. We introduce an in vitro flow model (“Rheocoagulometer”) which allows coagulatory and cellular investigations with defined blood flow properties exposed to biomaterials based on Couette-flow principle. Two exchange-able co-axial cylinders rotate independently producing sheared laminar blood flow in the gap of both with and without secondary flows (Taylor-vortices). On example of polystyrene cylinders could be shown that the plasma protein adsorption was pronounced using laminar shear flow (G=600 s−1). Platelet drop as well as β-Thromboglobulin/PF4 release was surprisingly less affected in the presence of vortices in comparison to pure shear flow at the same shear rate. On the other hand flow cytometrical results demonstrate a higher activation of the fibrinogen receptors in the presence of secondary flows. The results demonstrate that the device opens a wide range of applications to investigate the complex blood/material interaction under specific rheological conditions for any given biomaterial and possible surface modifications.

Three‐dimensional pulsatile flow through a bifurcation

Pulsatile flow of an incompressible, Newtonian fluid through a symmetric bifurcated rigid channel was numerically analysed by solving the three‐dimensional Navier‐Stokes equations. The upstream flow conditions were taken from an experimentally measured human arterial pulse cycle. The bifurcation was symmetrical with a branch angle of 60° and a daughter to mother area ratio of 2.0. The predicted velocity patterns were in qualitative agreement with experimental measurements available in the literature. The effect of unsteadiness on the various flow characteristics was studied. The most drastic effect observed was on the flow reversal regions. There was no flow reversal at the highest inlet Reynolds number in the pulse cycle, whereas in the case of steady flow at the same Reynolds number, the flow reversal region was the largest. The presence of secondary flow was observed at all times during the pulse cycle. Shear stress was calculated along the outer and inner walls and the low and high time averaged shear stress regions correspond to the clinically observed sites of formation of atherosclerotic plaque and lesions.

Flow of a fluid of grade two between two eccentric rotating spheres

The isothermal, stationary and isochoric flow of a fluid of grade two between a pair of rotating eccentric spheres is investigated. The equations of motion of first and second order are formulated and solved for the first order only. However, the equation of second order indicates the presence of secondary flow. The stress distributions are computed and used to determine the resultant forces and torques acting on the stationary outer sphere. An important result for rheometry is that the resultant torques can be used to determine the coefficient of viscosity, while the resultant force in the direction of the axis of symmetry may be employed to determine the second normal stress difference.

An experimental investigation of the flow of dilute polymer solutions through corrugated channels

Measurements of the velocity using Laser Doppler Velocimetry and normal stress are made for the flow of dilute polymer solutions through a channel with corrugated top and bottom plates. (Since we are dealing with non-Newtonian fluids, there can be significant contributions to the normal stress from non-linear terms in the constitutive expression, even when the flow is slow. The measurements being made are the normal stresses and not the “pressure”.) The surfaces of the plates are sinusoidal. A Reynolds number based on half the average plate spacing as the length scale and the characteristic velocity as the velocity scale was used and the range of Reynolds numbers studied was 50 < Re < 1000. The centerline velocities indicate that the experiments were performed in the inertial regime, as confirmed by the asymmetry of the centerline velocities along the channel length. The velocity profiles at the trough near the wall, for a channel with wavelength of 2.54 cm, indicate the presence of secondary flow. Sinusoidal plates with nearly identical aspect ratios ( a λ ) allowed for dramatic changes in the way in which the friction factor varied with Reynolds number, in that, in one case the friction factor associated with the fluid without polymer was higher than the friction factor associated with the fluid with polymer, while in others it was just the opposite. This would call into question the use of aspect ratio as an appropriate parameter for studying such problems. Changes in plate wavelength either increased or decreased the friction factor depending on the Reynolds number. Increasing plate amplitude increased the friction factor of the fluid for the range of values for the Reynolds number that was considered. The amplitude associated with the dimensionless normal stress increased with decreasing wavelength, for particular Reynolds numbers, irrespective of the fluid studied. Increasing the polymer concentration in the fluid decreased the difference in the amplitude of the dimensionless normal stress, the Reynolds number being fixed. Increasing the plate amplitude increased the amplitude of the normal stress, while an increase in plate wavelength decreased the amplitude of the normal stress.

Velocity and boundary shear in a wide compound duct

Mean flow distributions of primary velocity and boundary shear stress were measured in a wide closed duct with a range of asymmetric compound cross-sections. The cross-sectional geometries included three different wall angles, vertical, 1:1 and 1:2 (vertical:horizontal) at the interface between the shallow and deep subsections. Results are presented for the effect of relative depth and wall angle upon primary velocity, boundary shear stress and friction factors. The particularly detailed boundary shear stress measurements indicate the presence of secondary flow on the flood plain even at the lowest relative depths, where it would not normally be identified by coarser spacing of the data.

Heat transfer to a viscoelastic fluid in laminar flow through a rectangular channel

The measured local and mean Nusselt numbers for a viscoelastic fluid in laminar flow through a rectangular channel are found to be much higher than those of a purely viscous fluid or a Newtonian fluid. The differences cannot be explained on the basis of a superimposed free convection effect. Rather, the increase is due primarily to secondary flows which are induced in the viscoelastic fluid as a result of the normal force differences acting at the boundaries which are unique to elastic fluids. The pressure drop behavior is unaffected by the presence of secondary flows and predictions based on a purely viscous power law model give good agreement with the measured values.

Homogeneous Instability under Free Convection in a Tilted Nematic Sample

Abstract Exact calculations on homogeneous instability (HI) threshold under free convection in a tilted nematic sample are presented. Inclusion of the effect of secondary flow leads to HI threshold value in close agreement with the experimental result for MBBA. Effects of stabilizing and destabilizing magnetic fields on the HI threshold are studied. Calculations are extended to HBAB where it is shown that under the joint action of the stabilizing effect of free convection and the destabilizing effect of magnetic field, HI may occur in the sample. At high enough values of destabilizing field it may be possible to observe a mode (involving net secondary flow) of HI which is generally unfavorable. The presence of secondary flow may lead to a loss of scaling for materials which show a divergence of twist viscosity near the nematicsmectic A transition.

Experimental Investigation of Flow Resistance and Wall Shear Stress in the Interior Subchannel of a Triangular Array of Parallel Rods

A simulated model of a triangular array of rods with pitch to diameter ratio of 1.1 with air flow was used to study the hydraulic parameters of the liquid metal fast breeder reactor (LMFBR) fuel geometry. The wall shear stress distribution, static pressure distribution, turbulence intensity, and friction factor were measured in the central subchannel from Reynolds numbers of 4 × 103 to 36 × 103. Our results show that the maximum wall shear stress occurs at the largest flow area, the static pressure is not uniform around the rod periphery, there is no detectable presence of secondary flow from the wall shear stress measurements, and the friction factor derived from the measured wall shear stress is less than the common friction factor derived from pressure drop measurement.

Mass transfer in ideal and geometrically deformed open tubes. : I. Ideal and coiled tubes with circular cross-section

A summary of the hydrodynamics relevant to open tubes is presented. The band broadening of unretained samples in ideal and coiled tubes with circular cross-sections (0.25–4 mm I.D.) was measured with n-heptane and water as eluents. The lack of any appreciable interference from instrumental factors is substantiated by the experimentally determined diffusion coefficients and critical Reynold's numbers. It is shown that carefully prepared copper tubes behave as ideally as the corresponding glass tubes. With decreasing tube diameter, wall roughness leads to greater radial mass transfer. This effect is evident in 0.25-mm I.D. copper tubes and 0.5-mm (or even 0.75-mm) I.D. stainless-steel tubes, and can be used to advantage in constructing heat exchangers, connecting tubes or reaction detectors in liquid chromatography. It is shown for the first time that the onset of turbulence can be detected with considerably higher sensitivity by means of h versus u curves than by the K versus u curves used previously. It is shown that at the onset of turbulence the specific permeability decreases by a factor of about 3. Band broadening in the turbulent region was found to be ten times greater than that calculated on the basis of the friction theory of Taylor, apparently because laminar conditions prevail in a substantial portion of the tube cross-section. If an ideal tube is coiled, its specific permeability decreases with increasing velocity. Furthermore, coiling stabilizes “laminar” flow and the beginning of the turbulent region is shifted to higher Reynold's number ( i. e., higher velocities). It is shown that, as the inner diameter of the open tube decreases, higher linear velocities are required in order to decrease band broadening substantially ( e. g., by a factor of 5) for coiled tubes than for ideal tubes. For example, a linear velocity of 250 cm/sec is necessary to achieve this factor of 5 with 0.25-mm I.D. tubing, even though the presence of secondary flow is indicated by the K versus u curve at u > 10 cm/sec. For liquid chromatographic separations tubes narrower than 0.25 mm I.D. are essential. Therfore, at acceptable linear velocities (<50 cm/sec), the coiling of an

Calculation of heat transfer to turbine blading in the presence of secondary flow

A previous paper [1]has described measurements of local heat-transfer coefficients over the surface of a gas turbine blade in a cascade where strong secondary flow was generated. Details of calculations are now given, the work being considered in two parts: (a) the estimation of secondary flow velocities and (b) the nature of the blade boundary layer and related heat-transfer properties. In the first part, calculations for the induced stream wise vorticity are based on the method of Hawthorne [2]. The calculations are made with three assumptions, of various degrees of refinement, about the main flow. In the simplest case a free vortex flow is assumed, in the next a known two-dimensional velocity field about the blades is used, and finally, this is corrected to allow for displacement of the main stream. The results do not differ greatly from each other, and agreement with experiment is generally good. Boundary-layer calculations make use of the momentum integral relations in the form due to Mager [3] for three-dimensional flow. Adaptations are made to allow for mainstream vorticity where this is significant, and to incorporate a more general skin friction law. Heat transfer in the laminar layer is calculated by Squire's method [4]and for the turbulent boundary layer by using Von Kármán's extension of Reynolds analogy. The difficulty of predicting conditions following a laminar separation has made it necessary to base the initial data for a reattaching turbulent layer on an empirical rule, which is tentatively suggested for wider application, obtained from measured heat-transfer data. Comparison with experiment is good where the boundary-layer is laminar, but the heat transfer is overestimated in turbulent regions, particularly in the presence of a favourable pressure gradient.

Heat transfer to turbine blading in the presence of secondary flow

As part of a study of heat transfer to gas turbine blading, local heat-transfer coefficients were measured round chordwise profiles of one blade of a cascade when heated to a few degrees above the air stream temperature, each measuring profile being made isothermal in turn. The flow at inlet had boundary layers artificially thickened by spoilers to about one third of the blade span, so that strong secondary flows were set up in the deflecting stream. Over the uniform-flow region at the centre of the span results are shown to be virtually two-dimensional. Comparison with the earlier work of Wilson and Pope [1] shows some differences, particularly at zero incidence. One important mechanism by which variation of total pressure along the span may increase the heat transfer is by forward movement of the boundary-layer transition to the site of a local pressure reversal near the leading edge of the section, under conditions when, with two-dimensional flow, the layer would remain laminar after the pressure reversal. A further effect is the contribution of the spanwise velocities at the blade surface, produced by secondary flows, which increase heat transfer and skin friction. The resulting increase in mean heat-transfer rate due to the severe non-uniformity of upstream conditions was 6 per cent at design incidence. Measurements of secondary flow made in the blade passages were compared with calculations of various degrees of refinement based on methods due to Hawthorne [2] and to Squire and Winter [3]. The correlation is reasonably good in all cases.