Forced-convective turbulent-flows through horizontal ducts with isosceles-triangular internal cross-sections

This experimental investigation into the nature of free-surface flows was to study the effects of surfactants on the wake of a surface-piercing cylinder. A better understanding of the process of vorticity generation and conversion at a free surface due to the absence or presence of surfactants has been gained. Surfactants, or surface contaminants, have the tendency to reduce the surface tension proportionally to the respective concentration at the free surface. Thus when surfactant concentration varies across a free surface, surface tension gradients occur and this results in shear stresses, thus altering the boundary condition at the free surface. A low Reynolds number wake behind a surface-piercing cylinder was chosen as the field of study, using digital particle image velocimetry (DPIV) to map the velocity and vorticity field for three orthogonal cross-sections of the flow. Reynolds numbers ranged from 350 to 460 and the Froude number was kept below 1.0. In addition, a new technique was used to simultaneously map the free surface deformation. Shadowgraph imaging of the free surface was also used to gain a better understanding of the flow. It was found that, depending on the surface condition, the connection of the shedding vortex filaments in the wake of the cylinder was greatly altered with the propensity for surface tension gradients to redirect the vorticity near the free surface to that of the surface-parallel component. This result has an impact on the understanding of turbulent flows in the vicinity of a free surface with varying surface conditions.

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Turbulent Flow in the Micro Structures of Porous Media

ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels ◽

10.1115/icnmm2015-48074 ◽

2015 ◽

Author(s):

Heinz Herwig ◽

Yan Jin ◽

Marc-Florian Uth ◽

Andrey V. Kuznetsov

Keyword(s):

Turbulent Flow ◽

Turbulent Flows ◽

Cross Sections ◽

Turbulence Modeling ◽

Porous Matrix ◽

Solid Matrix ◽

Turbulence Structure ◽

Turbulence Structures ◽

Turbulent Flow Regime ◽

Micro Structures

Some fundamental issues with respect to turbulent flows through a porous matrix are addressed by analyzing DNS results (DNS: direct numerical simulation, i.e. no turbulence modeling). In a porous matrix with pore sizes of micro or even nano dimensions turbulent flow may occur when the (local) Reynolds number is sufficiently large. An open question, however, is whether turbulence structures are restricted in size by the pore size dimensions or not. This is an important aspect that immediately affects the way turbulence has to be modelled. In order to find out which influence the solid matrix has on the turbulence a generic matrix built from a large number of bars with square cross sections is investigated. Two different DNS approaches are used, a finite volume one and a Lattice-Boltzmann approach. From both DNS calculations detailed flow field information about the influence of the solid matrix on the turbulence structure are obtained. Finally the extension of Darcy’s friction law by the Forchheimer term is investigated with respect to the question whether this extended law may be used in the fully turbulent flow regime.

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Experimental Acoustic Flow Analysis Inside a Section of an Acoustic Waveguide

Archives of Acoustics ◽

10.2478/aoa-2013-0025 ◽

2013 ◽

Vol 38 (2) ◽

pp. 211-216 ◽

Cited By ~ 6

Author(s):

Stefan Weyna ◽

Witold Mickiewicz ◽

Michał Pyła ◽

Michał Jabłoński

Keyword(s):

Turbulent Flows ◽

Cross Sections ◽

Flow Analysis ◽

Sound Intensity ◽

Separated Flow ◽

Noise Abatement ◽

Circular Duct ◽

Practical Concern ◽

Piping Systems ◽

Particle Velocities

Abstract Noise propagation within ducts is of practical concern in many areas of industrial processes where a fluid has to be transported in piping systems. The paper presents experimental data and visualization of flow in the vicinity of an abrupt change in cross-section of a circular duct and on obstacles inside where the acoustic wave generates nonlinear separated flow and vortex fields. For noise produced by flow wave of low Mach number, laminar and turbulent flows are studied us- ing experimental sound intensity (SI) and laser particle image velocimetry (PIV) technique adopted to acoustics (A-PIV). The emphasis is put on the development and application of these methods for better understanding of noise generation inside the acoustic ducts with different cross-sections. The intensity distribution inside duct is produced by the action of the sum of modal pressures on the sum of modal particle velocities. However, acoustic field is extremely complicated because pressures in non-propagating (cut-off) modes cooperate with particle velocities in propagating modes, and vice versa. The discrete frequency sound is strongly influenced by the transmission of higher order modes in the duct. By under- standing the mechanism of energy in the sound channels and pipes we can find the best solution to noise abatement technology. In the paper, numerous methods of visualization illustrate the vortex flow as an acoustic velocity or sound intensity stream which can be presented graphically. Diffraction and scattering phenomena occurring inside and around the open-end of the acoustic duct are shown.

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On the influence of turbulent kinetic energy level on accuracy of k − ε and LRR turbulence models

Theoretical and Applied Mechanics ◽

10.2298/tam171201009n ◽

2018 ◽

Vol 45 (2) ◽

pp. 139-149

Author(s):

Djordje Novkovic ◽

Jela Burazer ◽

Aleksandar Cocic ◽

Milan Lecic

Keyword(s):

Kinetic Energy ◽

Theoretical Analysis ◽

Turbulent Kinetic Energy ◽

Turbulent Flows ◽

Cross Sections ◽

Turbulence Models ◽

Velocity Profiles ◽

Navier Stokes ◽

Test Case ◽

Reynolds Stresses

This paper presents research regarding the influence of turbulent kinetic energy (TKE) level on accuracy of Reynolds averaged Navier?Stokes (RANS) based turbulence models. A theoretical analysis of influence TKE level on accuracy of the RANS turbulence models has been performed according to the Boussinesq hypothesis definition. After that, this theoretical analysis has been investigated by comparison of numerically and experimentally obtained results on the test case of a steady-state incompressible swirl-free flow in a straight conical diffuser named Azad diffuser. Numerical calculations have been performed using the OpenFOAM CFD software and first and secondorder closure turbulence models. TKE level, velocity profiles and Reynolds stresses have been calculated downstream in four different cross sections of the diffuser. Certain conclusions about modeling turbulent flows by ?? ??? and LRR turbulence models have been made by comparing the velocity profiles, TKE distribution and Reynolds stresses on the selected cross sections.

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Visualizing cross sections of 3D turbulent flows using a modified white light Lau interferometer

10.1117/12.194327 ◽

1994 ◽

Cited By ~ 1

Author(s):

Leslie S. Mair ◽

Peter Waddell ◽

Mathew Stickland ◽

Steven Mason ◽

Stuart McKay

Keyword(s):

Turbulent Flows ◽

Cross Sections ◽

White Light

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Modeling Inspiratory Flow in a Porcine Lung Airway

Journal of Biomechanical Engineering ◽

10.1115/1.4038431 ◽

2018 ◽

Vol 140 (6) ◽

Cited By ~ 3

Author(s):

Peshala P. T. Gamage ◽

Fardin Khalili ◽

M. D. Khurshidul Azad ◽

Hansen A Mansy

Keyword(s):

Secondary Flow ◽

Turbulent Flows ◽

Cross Sections ◽

Animal Studies ◽

Inspiratory Flow ◽

Secondary Flows ◽

Inlet Flow Rate ◽

Simulation Results ◽

Human Airways ◽

Viscous Pressure

Inspiratory flow in a multigeneration pig lung airways was numerically studied at a steady inlet flow rate of 3.2 × 10−4 m3/s corresponding to a Reynolds number of 1150 in the trachea. The model was validated by comparing velocity distributions with previous measurements and simulations in simplified airway geometries. Simulation results provided detailed maps of the axial and secondary flow patterns at different cross sections of the airway tree. The vortex core regions in the airways were visualized using absolute helicity values and suggested the presence of secondary flow vortices where two counter-rotating vortices were observed at the main bifurcation and in many other bifurcations. Both laminar and turbulent flows were considered. Results showed that axial and secondary flows were comparable in the laminar and turbulent cases. Turbulent kinetic energy (TKE) vanished in the more distal airways, which indicates that the flow in these airways approaches laminar flow conditions. The simulation results suggested viscous pressure drop values comparable to earlier studies. The monopodial asymmetric nature of airway branching in pigs resulted in airflow patterns that are different from the less asymmetric human airways. The major daughters of the pig airways tended to have high airflow ratios, which may lead to different particle distribution and sound generation patterns. These differences need to be taken into consideration when interpreting the results of animal studies involving pigs before generalizing these results to humans.

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LES for Turbulent Flows through Ducts of Regular-Polygon Cross-Sections

Journal of Mechanics Engineering and Automation ◽

10.17265/2159-5275/2020.05.002 ◽

2020 ◽

Vol 10 (5) ◽

Author(s):

Masayoshi Okamoto

Keyword(s):

Turbulent Flows ◽

Cross Sections ◽

Regular Polygon

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Direct numerical simulation of turbulent flows with cloud-like off-source heating

Journal of Fluid Mechanics ◽

10.1017/s0022112099004280 ◽

1999 ◽

Vol 385 ◽

pp. 199-228 ◽

Cited By ~ 42

Author(s):

A. J. BASU ◽

R. NARASIMHA

Keyword(s):

Turbulent Flows ◽

Cross Sections ◽

Numerical Solutions ◽

Stokes Equations ◽

Streamwise Vortex ◽

Turbulent Jets ◽

Boussinesq Approximation ◽

Mean Flow ◽

Volumetric Heating ◽

Flow Acceleration

Direct numerical solutions of the incompressible Navier–Stokes equations have been obtained under the Boussinesq approximation for the temporal evolution of a turbulent jet-like flow subjected to off-source volumetric heating, of the kind that occurs in a cloud due to latent heat release on condensation of water vapour. The results show good qualitative agreement with available experimental data on spatially growing jets. Thus, heating accelerates the flow and arrests jet growth; and turbulence velocities increase with heating but not as rapidly as mean velocities, so normalized intensities drop. It is shown that the baroclinic torque resulting from the heating enhances the vorticity dramatically in all three directions, with a preferential amplification at the higher wavenumbers that results in a rich fine structure at later times in the evolution of the jet. Streamwise vortex pairs, rendered stronger by mean flow acceleration, appear to be responsible for large expulsive motions at certain transverse cross-sections in the ambient fluid near the heated flow; together with the disruption of the toroidal component of the coherent vorticity achieved by heating, this results in an entraining velocity field that is qualitatively different from that around unheated turbulent jets. This mechanism may provide a plausible explanation for the experimentally observed drop in entrainment with off-source heating.

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