Friction Numbers and Viscous Dissipation Heating for Laminar Flows of Water in Microtubes

2008 ◽  
Vol 130 (8) ◽  
Author(s):  
Mohamed S. El-Genk ◽  
In-Hwan Yang
Keyword(s):  
Experimental Data ◽  
Aspect Ratio ◽  
Viscous Dissipation ◽  
Temperature Rise ◽  
Strong Dependence ◽  
Velocity Slip ◽  
Laminar Flows ◽  
Water Flows ◽  
Aspect Ratios ◽  
Good Agreement

The friction numbers for laminar flows of water in microtubes, determined from the temperature rise due to the viscous dissipation heating assuming a velocity slip, show a strong dependence on the diameter and aspect ratio. The calculated values compare well with those determined from experimental data for water flows in glass and diffused silica microtubes (16–101μm in diameter D and aspect ratios L∕D=499–1479). With a slip, the friction number almost exponentially decreases as D decreases and, to a lesser extent, as L∕D increases. For D>400μm, the friction number approaches the theoretical Hagen–Poiseuille for macrotubes (64) when L∕D>∼1500, but higher values at smaller L∕D. The developed semiempirical analytical expression for calculating the friction number is in good agreement with the numerical and experimental results. The results suggest the presence of a velocity slip in the experiments and the plausible presence of a thin nanolayer at the walls of the microtubes. For D>200μm, this layer, if exists, is estimated to be ∼18.9nm, but increases to ∼21.5nm for D<200μm, when R¯e=800 and L∕D=1479.

Viscous Dissipation in Laminar Water Flows in Micro-Tubes

2007 ◽  
Author(s):  
In-Hwan Yang ◽  
Mohamed S. El-Genk
Keyword(s):  
Experimental Data ◽  
Viscous Dissipation ◽  
Temperature Rise ◽  
Experimental Results ◽  
Numerical Calculations ◽  
Analytical Expression ◽  
Water Flows ◽  
Good Agreement

Numerical calculations are performed to investigate the effect of viscous dissipation on the temperature rise and friction numbers for laminar water flows in micro-tubes. The calculated values are compared with those determined from reported experimental data for glass and diffused silica micro-tubes (D = 16 – 101 μm and L/D = 625 – 1479). The results confirm a definite slip at the wall with slip lengths of ∼ 0.7 μm and 1.0 μm, which decrease the friction number and the temperature rise in the micro-tubes, but their effect gradually diminishes as either D or L/D increases. The friction number decreases exponentially as D decreases and, to a lesser extent, as L/D increases. The effect of L/D on the friction number is insignificant for micro-tube diameters ≤ 20 μm. For D &gt; 400 μm, the friction number approaches that of Hagen-Posieuille of 64 for macro-tubes when L/D &gt; 1500, but approaches higher values at smaller L/D. The dimensionless analytical expression developed for calculating the friction number and the temperature rise for water flows in micro-tubes is in good agreement with both the numerical and experimental results.

A dispersion based model for anaerobic digestion of solid cattle wastes in a stratified thermophilic accumulation system

2005 ◽  
Vol 52 (1-2) ◽  
pp. 195-202 ◽  
Author(s):  
H.M. El-Mashad ◽  
W.K.P. van Loon ◽  
G. Zeeman ◽  
G.P.A. Bot ◽  
G. Lettinga
Keyword(s):  
Experimental Data ◽  
Anaerobic Digestion ◽  
Aspect Ratio ◽  
Dynamic Model ◽  
Model Validation ◽  
Methane Production ◽  
System Performance ◽  
Aspect Ratios ◽  
Accumulation System ◽  
Good Agreement

A dynamic model has been developed to describe the anaerobic digestion of solid cattle waste in an accumulation system (AC). To calibrate the model an experiment was carried out at a lab-scale AC at 50 °C. The predicted methane production shows a very good agreement (i.e. R2=0.998) with the experimental data. However less agreement is evident for the intermediates. After model validation the model was applied to study the effect of different aspect ratios on the system performance. An optimum aspect ratio of 2–3 could be determined.

Three-Dimensional Thermocapillary Convection in Open Cylindrical Annuli

2000 ◽  
Author(s):  
Bok-Cheol Sim ◽  
Abdelfattah Zebib
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Critical Frequency ◽  
Thermocapillary Convection ◽  
Three Dimensional ◽  
Time Dependent ◽  
Infinite Layer ◽  
Aspect Ratios ◽  
Temperature Oscillations ◽  
Good Agreement

Abstract Three-dimensional, time-dependent thermocapillary convection in open cylindrical containers is investigated numerically. Results for aspect ratios (Ar) of 1, 2.5, 8, and 16 and a Prandtl number of 6.84 are obtained to compare the results of numerical simulations with ongoing experiments. Convection is steady and axisymmetric at sufficiently low values of the Reynolds number (Re). Transition to oscillatory states occurs at critical values of Re which depend on Ar. With Ar = 1.0 and 2.5, we observe, respectively, 5 and 9 azimuthal wavetrains travelling clockwise at the free surface near the critical Re. With Ar = 8.0 and 16.0, there are substantially more, but pulsating waves near the critical Re. In the case of Ar = 16.0, which approaches the conditions in an infinite layer, our results are in good agreement with linear theory. While the critical Reynolds number decreases with increasing aspect ratio in the case of azimuthal rotating waves, it increases with increasing aspect ratio in the case of azimuthal pulsating waves. The critical frequency of temperature oscillations is found to decrease linearly with increasing Ar. We have also computed supercritical time-dependent states and find that while the frequency increases with increasing Re near the critical region, the frequency of supercritical convection decreases with Re.

scholarly journals Numerical Simulation Using a Modified Solver within OpenFOAM for Compressible Viscous Flows

2020 ◽  
Author(s):  
Valiyollah Ghazanfari ◽  
Ali Akbar Salehi ◽  
Ali Reza Keshtkar ◽  
Mohammad Mahdi Shadman ◽  
Mohammad Hossein Askari
Keyword(s):  
Experimental Data ◽  
Viscous Dissipation ◽  
Energy Equation ◽  
Contact Discontinuity ◽  
Viscous Flows ◽  
Computational Time ◽  
Benchmark Tests ◽  
Compressible Viscous Flows ◽  
Good Agreement

In this work, we attempted to develop an Implicit Coupled Density-Based (ICDB) solver using LU-SGS algorithm based on the AUSM+ up scheme in OpenFOAM. Then sonicFoam solver was modified to include viscous dissipation in order to improve its capability to capture shock wave and aerothermal variables. The details of the ICDB solver as well as key implementation details of the viscous dissipation to energy equation were introduced. Finally, two benchmark tests of hypersonic airflow over a flat plate and a 2-D cylinder were simulated to show the accuracy of ICDB solver. To verify and validate the ICDB solver, the obtained results were compared with other published experimental data. It was revealed that ICDB solver has good agreement with the experimental data. So it can be used as reference in other studies. It was also observed that ICDB solver enjoy advantages such as high resolution for contact discontinuity and low computational time. Moreover, to investigate the performance of modified sonicFoam, a case study of airflow over the prism was considered. Then the results of the modified sonicFoam were compared with the ICDB, rhoCentralFoam and sonicFoam solvers. The results showed that the modified sonicFoam solver possesses higher accuracy and lower computational time in comparison with the sonicFoam and rhoCentralFoam solvers, respectively.

Mechanical Characterization of Graphite Platelet Nanocomposites Through Molecular Mechanics and Continuum Modeling

2006 ◽  
Author(s):  
J. Cho ◽  
J. J. Luo ◽  
I. M. Daniel
Keyword(s):  
Experimental Data ◽  
Molecular Mechanics ◽  
Elastic Constants ◽  
Strong Dependence ◽  
Micromechanical Model ◽  
Graphite Nanoplatelets ◽  
Inclusion Phase ◽  
Aspect Ratios ◽  
Molecular Force ◽  
Theoretical Predictions

Mechanical properties of nanocomposites consisting of epoxy substrate reinforced by randomly oriented graphite platelets are studied with Mori-Tanaka method in collaboration with molecular mechanics. Elastic constants of graphite nanoplatelets which are the inclusion phase of the micromechanical model are calculated based on their molecular force field. The calculated elastic constants are well compared with the both experimental data and other theoretical predictions in literatures. The results from Mori-Tanaka's method based on the graphite modulus calculation from molecular mechanics are found that nanocomposite moduli have strong dependence on the aspect ratios of reinforcing particles, but no direct size dependence. The predicted nanocomposite moduli compare favorably with modulus measurement of several graphite particles of various aspect ratios and sizes. The experimental data also shows that particle sizes have very weak effect on nanocomposite moduli.

The Deformational Behaviour of Low Aspect Ratio Multi-Web Wings

1962 ◽  
Vol 13 (1) ◽  
pp. 71-87 ◽  
Author(s):  
R. H. Gallagher ◽  
I. Rattinger
Keyword(s):  
Experimental Data ◽  
Structural Analysis ◽  
Aspect Ratio ◽  
Subsequent Paper ◽  
Plate Bending ◽  
Restricted Range ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Influence Coefficients ◽  
Geometric Factors

SummaryResults of a study of the accuracy attainable from various approaches to low aspect ratio wing deformational analyses are described. Seven model multi-web wings, representing a restricted range of sweep angles, aspect ratios and other geometric factors, were tested for deflection influence coefficients; Part I gave experimental data. This paper describes, applies, and compares certain elementary and plate bending theories. A subsequent paper will deal with discrete element idealisations commonly employed in matrix structural analysis.

An Experimental Study of Confined Masonry Walls with Varying Aspect Ratios

2015 ◽  
Vol 31 (2) ◽  
pp. 945-968 ◽  
Author(s):  
J. J. Perez Gavilan ◽  
L. E. Flores ◽  
S. M. Alcocer
Keyword(s):  
Aspect Ratio ◽  
Good Accuracy ◽  
Axial Stress ◽  
Masonry Walls ◽  
Inelastic Behavior ◽  
Ductility Demand ◽  
Confined Masonry ◽  
Aspect Ratios ◽  
Wide Range ◽  
Good Agreement

Results from an experimental series of seven full-scale confined masonry walls with height-to-length aspect ratios ( H/L) from 0.3 up to 2.2 are summarized. Results show that neither the level of axial stress nor the aspect ratio had a significant effect on lateral stiffness. Inelastic behavior of the walls, characterized by normalized stiffness degradation with ductility demand, can be estimated with good accuracy with a bilinear function for a ductility demand up to 4.5. A substantial increase in normalized shear strength was observed for walls with decreasing aspect ratio. A correction factor to the nominal cracking strength was deduced based on differences of the flexural deformations for squat and square walls. The factor was then compared to the experimental normalized strength with good agreement. A new expression for inclined cracking shear that can be used for a wide range of wall aspect ratios is proposed.

The Deformational Behaviour of Low Aspect Ratio Multi-Web Wings

1962 ◽  
Vol 13 (2) ◽  
pp. 143-166
Author(s):  
R. H. Gallagher ◽  
I. Rattingerj
Keyword(s):  
Experimental Data ◽  
Structural Analysis ◽  
Aspect Ratio ◽  
Plate Bending ◽  
Restricted Range ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Influence Coefficients ◽  
Geometric Factors ◽  
Analytical Approaches

SummaryResults of a study of the accuracy attainable from various approaches to low aspect ratio wing deformational analysis are described. Seven model multi-web wings, representing a restricted range of sweep angles, aspect ratios and other geometric factors, were tested for deflection influence coefficients and various analytical approaches were applied in the prediction of these results. Part I gave the experimental data; Part II dealt with analyses based on elementary and plate bending theories. This part describes and applies certain discrete element idealisations common to matrix structural analysis. The merits and shortcomings of the theories studied herein are reviewed and other promising approaches are discussed.

Dynamics of revolving wings for various aspect ratios

2014 ◽  
Vol 748 ◽  
pp. 932-956 ◽  
Author(s):  
D. J. Garmann ◽  
M. R. Visbal
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Pressure Gradient ◽  
Centrifugal Force ◽  
Vortex Breakdown ◽  
Delta Wing ◽  
Strong Dependence ◽  
Tip Vortex ◽  
Aspect Ratios ◽  
Gradient Forces

AbstractHigh-fidelity, direct numerical simulations (DNSs) are conducted to examine the vortex structure and aerodynamic loading of unidirectionally revolving wings in quiescent fluid. Wings with aspect ratios $({\mathit{AR}}) = 1$, 2 and 4 are considered at a fixed root-based Reynolds number of 1000. Each wing is shown to generate a coherent leading-edge vortex (LEV) that remains in close proximity to the surface and provides persistent suction throughout the motion. Towards the tip, the LEV lifts off as an arch-like structure and reorients itself along the chord through its connection with the tip vortex. The substantial and sustained aerodynamic loads achieved during the motion saturate with aspect ratio resulting from the chordwise growth of the LEV along the span eventually becoming geometrically constrained by the trailing edge. Further, for ${\mathit{AR}}=4$, substructures develop in the feeding sheet of the LEV, which appear to directly correlate with the local, span-based Reynolds number achieved during rotation. The lower-aspect-ratio wings do not have sufficient spans for these transitional elements to manifest. In contrast, vortex breakdown, which occurs around midspan for each aspect ratio, shows a strong dependence on the spanwise pressure gradient established between the root and tip of the wing and not local Reynolds number. This independent development of shear-layer substructures and vortex breakdown parallels very closely with what has been observed in delta wing flow. Next, the centrifugal, Coriolis and pressure gradient forces are also analysed at several spanwise locations across each wing, and the centrifugal and pressure gradient forces are shown to be responsible for the spanwise flow above the wing. The Coriolis force is directed away from the surface at the base of the LEV, indicating that it is not a contributor to LEV attachment, which is contrary to previous hypotheses. Finally, as a means of emphasizing the importance of the centrifugal force on LEV attachment, the ${\mathit{AR}}=2$ wing is simulated with the addition of a source term in the governing equations to oppose and eliminate the centrifugal force near the surface. The initial formation and development of the LEV is unhindered by the absence of this force; however, later in the motion, the outboard lift-off of the LEV moves inboard. Without the opposing outboard-directed centrifugal force to keep the separation past midspan, the entire vortex eventually separates and moves away from the surface.

scholarly journals Analysis of Aspect Ratio Effects on Transonic Rotor Performance Using a 3D Viscous Solver

1998 ◽  
Author(s):  
Cecil R. Buchanan ◽  
Paul R. Emmerson ◽  
Michael Spruce
Keyword(s):  
Aspect Ratio ◽  
Numerical Study ◽  
Navier Stokes ◽  
Test Cases ◽  
Compressor Rotor ◽  
Aspect Ratios ◽  
Radial Profiles ◽  
Ratio Change ◽  
The One ◽  
Good Agreement

This paper presents the results of a numerical study into the effects of aspect ratio on compressor rotor performance. The test cases studied are NASA rotors 37 and 38, which have aspect ratios of 1.19 and 1.63 respectively. A 3D, single-passage steady flow Navier-Stokes solver was used to predict complete performance characteristics, including the numerical instability point, for both rotors. The predictions are generally in good agreement with the test data (characteristics, radial profiles and rotor over tip measurements) at all conditions modelled for rotor 37. The performance for rotor 38 is overpredicted, with slightly less than half of the measured performance difference between the two rotors being captured. The effect of a pure aspect ratio change (divorced from the rotor inlet to exit area changes present in the rotor 37/38 comparisons) was also investigated, and a case with an aspect ratio double that of rotor 37 was also modelled. The results indicated that the code predicted little effect on rotor performance due to an aspect ratio change alone (from 1.19 to 2.38). This is surprising and it raises doubts about the ability of current codes (or at least the one used in the study) to predict this important aspect of a compressor design adequately.

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