After transition in a soft-walled microchannel

2015 ◽  
Vol 780 ◽  
pp. 649-686 ◽  
Author(s):  
S. S. Srinivas ◽  
V. Kumaran
Keyword(s):  
Velocity Profile ◽  
Root Mean Square ◽  
Reynolds Stress ◽  
Fluid Velocity ◽  
Maximum Velocity ◽  
Reynolds Numbers ◽  
Dynamical Instability ◽  
Mean Square ◽  
Velocity Fluctuations ◽  
Soft Wall

In comparison to the flow in a rigid channel, there is a multifold reduction in the transition Reynolds number for the flow in a microchannel when one of the walls is made sufficiently soft, due to a dynamical instability induced by the fluid–wall coupling, as shown by Verma & Kumaran (J. Fluid Mech., vol. 727, 2013, pp. 407–455). The flow after transition is characterised using particle image velocimetry in the $x{-}y$ plane, where $x$ is the streamwise direction and $y$ is the cross-stream coordinate along the small dimension of the channel of height 0.2–0.3 mm. The flow after transition is characterised by a mean velocity profile that is flatter at the centre and steeper at the walls in comparison to that for a laminar flow. The root mean square of the streamwise fluctuating velocity shows a characteristic sharp increase away from the wall and a maximum close to the wall, as observed in turbulent flows in rigid-walled channels. However, the profile is asymmetric, with a significantly higher maximum close to the soft wall in comparison to that close to the hard wall, and the Reynolds stress is found to be non-zero at the soft wall, indicating that there is a stress exerted by fluid velocity fluctuations on the wall. The maximum of the root mean square of the velocity fluctuations and the Reynolds stress (divided by the fluid density) in the soft-walled microchannel for Reynolds numbers in the range 250–400, when scaled by suitable powers of the maximum velocity, are comparable to those in a rigid channel at Reynolds numbers in the range 5000–20 000. The near-wall velocity profile shows no evidence of a viscous sublayer for $(yv_{\ast }/{\it\nu})$ as low as two, but there is a logarithmic layer for $(yv_{\ast }/{\it\nu})$ up to approximately 30, where the von Karman constants are very different from those for a rigid-walled channel. Here, $v_{\ast }$ is the friction velocity, ${\it\nu}$ is the kinematic viscosity and $y$ is the distance from the soft surface. The surface of the soft wall in contact with the fluid is marked with dye spots to monitor the deformation and motion along the fluid–wall interface. Low-frequency oscillations in the displacement of the surface are observed after transition in both the streamwise and spanwise directions, indicating that the velocity fluctuations are dynamically coupled to motion in the solid.

Time-Resolved Velocity Measurements in a Matched Refractive Index Facility of Randomly Packed Spheres

2018 ◽  
Author(s):  
Ethan Kappes ◽  
Mateusz Marciniak ◽  
Andrew Mills ◽  
Robert Muyshondt ◽  
Stephen King ◽  
...  
Keyword(s):  
Root Mean Square ◽  
Experimental Studies ◽  
Reynolds Numbers ◽  
Index Of Refraction ◽  
Mean Square ◽  
Reynolds Stresses ◽  
Velocity Measurements ◽  
Mean Velocity ◽  
Time Resolved ◽  
Wall Boundary

Complex geometries and randomly connected void spaces within packed beds have hindered efforts to characterize the underlying transport phenomena occurring within. In this communication, we present our experimental studies on a facility of randomly packed spheres that can be a representative of sections within a reactor core in a nuclear power plant. The results of high-fidelity velocity measurements can be seen using Time-Resolved Particle Image Velocimetry (TR-PIV) at the pore scales and near the wall boundary in the Matched Index of Refraction (MIR) facility. The MIR approach allows for a non-invasive analysis of the flow within packed spheres at the microscopic scales with high temporal and spatial resolution. Flow characteristics obtained from the TR-PIV measurements at various Reynolds numbers are presented. The results include the first- and second-order flow statistics, such as mean velocity, root-mean-square fluctuating velocity and Reynolds stresses. Effects of the wall boundary and Reynolds numbers on flow patterns are currently being investigated. Comparisons of the mean velocities, root-mean-square fluctuating velocities, and Reynolds stress components show the increase of flow mixing and turbulent intensities within the gaps between spheres in the packed bed. Sizes of recirculation regions, however, seem to be independent of the increase of Reynolds numbers.

Seasonal glacier surface velocity fluctuation and contribution of the Eastern and Western Tributary Glaciers in Amery Ice Shelf, East Antarctica

2019 ◽  
Vol 9 (1) ◽  
pp. 49-60
Author(s):  
Shridhar Digambar Jawak ◽  
Shubhang Kumar ◽  
Alvarinho Joaozinho Luis ◽  
Prashant Hemendra Pandit ◽  
Sagar Filipe Wankhede
Keyword(s):  
Remote Sensing ◽  
Root Mean Square ◽  
Winter Season ◽  
Maximum Velocity ◽  
Surface Velocity ◽  
Mean Square ◽  
Ice Shelf ◽  
Amery Ice Shelf ◽  
Rms Error ◽  
Western Side

Glaciers play a crucial role in the study of the climate change pattern of the Earth. Remote sensing with access to large archives of data has the ability to monitor glaciers frequently throughout the year. Therefore, remote sensing is the most beneficial tool for the study of glacier dynamics. Fed by many tributaries from different sides, the Amery Ice Shelf (AIS) is one of the largest ice shelves that drains ice from the Antarctic ice sheet into the Southern Ocean. This study focuses on the eastern and the western tributaries of the AIS. The primary objective of the study was to derive the velocity of the tributary glaciers and the secondary objective was to compare variations in their velocities between the summer and winter season. This study was carried on using the European Space Agency’s (ESA) Sentinel-1 satellite’s Synthetic Aperture Radar (SAR) data acquired from the Sentinel data portal. Offset tracking method was applied to the Ground Range Detected (GRD) product of the Sentinel-1 interferometric wide (IW) swath acquisition mode. The maximum velocity in summer was observed to be around 610 m/yr in the eastern tributary glacier meeting the ice shelf near the Pickering Nunatak, and around 345 m/yr in the Charybdis Glacier Basin from the western side. The maximum velocity in the winter was observed to be 553 m/yr in the eastern side near the Pickering Nunatak whereas 323 m/yr from the western side in the Charybdis Glacier Basin. The accuracy of the derived glacier velocities was computed using bias and root mean square (RMS) error. For the analysis, the publicly available velocity datasets were used. The accuracy based on RMS error was observed to be 85-90% for both seasons with bias values up to 25 m/yr and root mean square error values up to 30 m/yr.

An Analytical Study of Turbulent Dispersion of Bubbles

2001 ◽  
Author(s):  
Anthony L. Lawson ◽  
Ramkumar N. Parthasarathy
Keyword(s):  
Root Mean Square ◽  
Time Scale ◽  
Liquid Velocity ◽  
Added Mass ◽  
Mean Square ◽  
Rise Velocity ◽  
Velocity Fluctuations ◽  
Bubble Rise ◽  
Integral Time Scale ◽  
Bubble Rise Velocity

Abstract An analysis of the dispersion of bubbles in homogenous and isotropic turbulent liquid flows was performed to study the effects of bubble and flow characteristics on their dispersion. Bubbles were assumed to be spherical and to follow the fluid motion in the mean. No mass transfer occurred between the bubble and liquid; also, there was no interaction between individual bubbles. It was found that for accurate prediction of bubble dispersion requires a simultaneous consideration of the inertia of the added mass of liquid (because the inertia of the bubble itself is small) and the bubble rise velocity. Normalized bubble diffusivity, root-mean-square fluctuating velocity, and Lagrangian integral time scale were related to two non-dimensional parameters: ratio of the added mass response time to the liquid flow integral time scale, and the ratio of the bubble rise velocity to the root-mean-square liquid velocity fluctuation. In general, the bubble Lagrangian velocity auto-correlations decreased as the rise velocity ratio increased. The dependence of the autocorrelations on the time-scale ratio was complex. A surprising result was that the bubble velocity fluctuations could exceed the liquid velocity fluctuations for certain conditions because of their low inertia.

CFD Model Validation for a Benchmark Data of Texas A&M 1/16th Scaled VHTR Upper Plenum

2020 ◽  
Author(s):  
Prasad Vegendla ◽  
Rui Hu ◽  
Ling Zou
Keyword(s):  
High Temperature ◽  
Root Mean Square ◽  
Reynolds Stress ◽  
Gas Flow ◽  
Independent Study ◽  
Mean Square ◽  
Reactor Performance ◽  
Mean Velocity ◽  
Benchmark Data ◽  
Cfd Models

Abstract In High Temperature Gas-cooled Reactors (HTGR), gas flow patterns are very complex and reduced order models (1D or 2D) may be too simplified to predict accurate reactor performance. 3D Computational Fluid Dynamics (CFD) models can help provide the detailed information needed to optimize the reactor thermal performance. The main objective of this work is to validate the CFD models with data of a 1/16th scaled Very High Temperature Reactor (VHTR) upper plenum measured at Texas A&M University. In this paper, the flow characteristics of a single isothermal jet discharging into the upper plenum was investigated using Nek5000 Large-Eddy Simulation (LES) CFD tool. Several numerical simulations were performed for various jet Reynolds numbers ranging from 3,413 to 12,819. Grid independent study was performed. The numerical results of mean velocity, root-mean-square fluctuating velocity, and Reynolds stress were validated with the benchmark data. Good agreement was obtained between simulated and measured data for axial mean velocities, except near the upper plenum hemisphere. The maximum predicted errors for axial mean velocities at various normalized coolant channel diameter heights of 1, 5 and 10 are 1.56%, 1.88% and 3.82%, respectively. Also, the predicted root-mean-square fluctuating velocity and Reynolds stress are qualitatively agreed with the experimental data.

The maintenance of Reynolds stress in turbulent shear flow

1967 ◽  
Vol 27 (1) ◽  
pp. 131-144 ◽  
Author(s):  
O. M. Phillips
Keyword(s):  
Shear Flow ◽  
Velocity Profile ◽  
Reynolds Stress ◽  
Mixing Layer ◽  
Turbulent Shear ◽  
Turbulent Shear Flow ◽  
Mean Flow ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
The Mean

A mechanism is proposed for the manner in which the turbulent components support Reynolds stress in turbulent shear flow. This involves a generalization of Miles's mechanism in which each of the turbulent components interacts with the mean flow to produce an increment of Reynolds stress at the ‘matched layer’ of that particular component. The summation over all the turbulent components leads to an expression for the gradient of the Reynolds stress τ(z) in the turbulence\[ \frac{d\tau}{dz} = {\cal A}\Theta\overline{w^2}\frac{d^2U}{dz^2}, \]where${\cal A}$is a number, Θ the convected integral time scale of thew-velocity fluctuations andU(z) the mean velocity profile. This is consistent with a number of experimental results, and measurements on the mixing layer of a jet indicate thatA= 0·24 in this case. In other flows, it would be expected to be of the same order, though its precise value may vary somewhat from one to another.

Transitions to different kinds of turbulence in a channel with soft walls

2017 ◽  
Vol 822 ◽  
pp. 267-306 ◽  
Author(s):  
S. S. Srinivas ◽  
V. Kumaran
Keyword(s):  
Reynolds Number ◽  
Travelling Waves ◽  
Fluid Velocity ◽  
Rectangular Channel ◽  
Wall Turbulence ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
Wall Displacement ◽  
Experimental Resolution ◽  
Soft Wall

The flow in a rectangular channel with walls made of polyacrylamide gel is experimentally studied to examine the effect of soft walls on transition and turbulence. The bottom wall is fixed to a substrate and the top wall is unrestrained. As the Reynolds number increases, two different flow regimes are observed. The first is the ‘soft-wall turbulence’ (Srinivas & Kumaran,J. Fluid Mech., vol. 780, 2015, pp. 649–686). There is a large increase in the magnitudes of the velocity fluctuations after transition and the fluid velocity fluctuations appear to be non-zero at the soft walls, although higher resolution measurements are required to establish the nature of the boundary dynamics. The fluid velocity fluctuations are symmetric about the centreline of the channel, and they show relatively little downstream variation. The wall displacement measurements indicate that there is no observable motion perpendicular to the surface to within the experimental resolution, but displacement fluctuations parallel to the surface are observed after transition. As the Reynolds number is further increased, there is a second ‘wall-flutter’ transition, which involves visible downstream travelling waves in the top (unrestrained) wall alone. Wall displacement fluctuations of frequency less than approximately$500~\text{rad}~\text{s}^{-1}$are observed both parallel and perpendicular to the wall. The mean velocity profiles and turbulence intensities are asymmetric, with much larger turbulence intensities near the top wall. The transitions are observed in sequence from a laminar flow at Reynolds number less than 1000 for a channel of height 0.6 mm and from a turbulent flow at a Reynolds number greater than 1000 for a channel of height 1.8 mm.

The influence of a drag-reducing surfactant on a turbulent velocity field

1999 ◽  
Vol 388 ◽  
pp. 1-20 ◽  
Author(s):  
MICHAEL D. WARHOLIC ◽  
GAVIN M. SCHMIDT ◽  
THOMAS J. HANRATTY
Keyword(s):  
Shear Stress ◽  
Velocity Field ◽  
Root Mean Square ◽  
Reynolds Shear Stress ◽  
Rectangular Channel ◽  
Streamwise Velocity ◽  
Laser Doppler Velocimeter ◽  
Shear Stresses ◽  
Mean Square ◽  
Velocity Fluctuations

A two-component laser-Doppler velocimeter, with high spatial and temporal resolution, was used to study how the introduction of a drag-reducing surfactant to water changes the fully-developed velocity field in an enclosed rectangular channel. Measurements were made for four different Reynolds numbers, Re = 13300; 19100; 32000, and 49100 (based on the bulk viscosity, the half-height of the channel, and the viscosity of water). For a fixed volumetric flow the pressure drop was reduced by 62 to 76% when compared to a Newtonian flow with an equal wall viscosity. Measurements were made of the mean streamwise velocity, the root mean square of two components of the fluctuating velocity, the Reynolds shear stress and the spectral density function of the fluctuating velocity in the streamwise direction. The Reynolds shear stress is found to be zero over the whole channel and the spectra of the streamwise velocity fluctuations show a sharp cutoff at a critical frequency, fc. The ratio of the cutoff frequency to the root mean square of the streamwise velocity fluctuations is found to be approximately equal to 1 mm−1. The observation of a zero Reynolds shear stress indicates the existence of additional mean shear stresses (or mean transfers of momentum) that are not seen with a Newtonian fluid. Furthermore, the presence of a random fluctuating velocity field suggests a production of turbulence by a mechanism other than that usually found for a fully developed flow. Possible explanations for this behaviour are presented.

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