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# Accepted Manuscripts

BASIC VIEW  |  EXPANDED VIEW
research-article
J. Fluids Eng   doi: 10.1115/1.4039158
The Parallel Plate Flow Chamber (PPFC) has gained popularity due to its applications in fields such as biological tissue engineering. However, most of the studies using PPFC refer to theoretical relations for estimating the wall shear stress (WSS) and, hence, the accuracy of such quantifications remains elusive for anything other than steady laminar flow. In the current study, a laser Doppler velocimetry (LDV) method was used to quantify the flow in a PPFC (H = 1.8 mm × W = 17.5 mm, Dh = 3.26 mm, aspect ratio=9.72) under steady Re=990, laminar pulsatile (carotid Re0-mean=282 as well as a non-zero-mean sinusoidal Re0-mean=45 pulse) and low-Re turbulent Re=2750 flow conditions. A mini-LDV probe was applied and the absolute location of the LDV measuring volume with the respect to the wall was determined using a signal monitoring technique with uncertainties being around ±27 µm. The uniformity of the flow across the span of the channel, as well as the WSS assessment for all the flow conditions, were measured with the uncertainties all being less than 16%. At least two points within the viscous sublayer of the low-Re turbulent flow were measured (with the y+ for the first point<3) and the WSS was determined using two methods with the differences between the two methods being within 5%. This paper for the first time presents the experimental determination of WSS using LDV in a small-scale PPFC under various flow conditions, the challenges associated with each condition and a comparison between the cases. The present data will be useful for those conducting biological or numerical modeling studies using such devices.
TOPICS: Flow (Dynamics), Turbulence, Laser Doppler anemometry, Shear stress, Uncertainty, Computer simulation, Laminar flow, Biological tissues, Probes, Signals
research-article
J. Fluids Eng   doi: 10.1115/1.4039159
Flow over a transitional-type cavity in microchannels is studied using a microparticle image velocimetry system and commercially available computational fluid dynamics 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 equations (RANS) and large eddy simulation 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 RSM-BSL turbulence model, 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.
TOPICS: Flow (Dynamics), Turbulence, Cavities, Vortices, Reynolds-averaged Navier–Stokes equations, Topology, Microchannels, Microparticles, Large eddy simulation, Computer simulation, Kinetic energy, Laminar flow, Navier-Stokes equations, Computational fluid dynamics, Computer software, Flow separation
research-article
David A. Hullender
J. Fluids Eng   doi: 10.1115/1.4039120
Transient pressure peak values and decay rates associated with water hammer surges in fluid lines are investigated using an analytical method that has been formulated, in a previous publication, to simulate pressure transients in turbulent flow. The method agrees quite well with MOC simulations of unsteady friction models and has been verified with experimental data available for Reynolds numbers out to 15,800. The method is based on the formulation of ordinary differential equations from the frequency response of a pressure transfer function using an inverse frequency algorithm. The model is formulated by dividing the line into n-sections to distribute the turbulence resistance along the line at higher Reynolds numbers. In this paper it will be demonstrated that convergence of the analytical solution is achieved with as few as 5 to 10 line sections for Reynolds numbers up to 200,000. The method not only provides for the use of conventional time domain solution algorithms for ordinary differential equations but also provides empirical equations for estimating peak surge pressures and transient decay rates as defined by eigenvalues. For typical sets of line and fluid properties, the trend of the damping ratio of the first or dominate mode of the pressure transients transfer function is found to be an approximate linear function of a dimensionless parameter that is a function of the Reynolds number. In addition, a reasonably accurate dimensionless trend formula for estimates of the normalized peak pressures is formulated and presented.
TOPICS: Turbulence, Transients (Dynamics), Water hammer, Reynolds number, Pressure, Algorithms, Differential equations, Fluids, Transfer functions, Surges, Engineering simulation, Eigenvalues, Frequency response, Damping, Friction, Simulation
research-article
J. Fluids Eng   doi: 10.1115/1.4039117
Fluid distributors are widely used in various industrial and ventilation applications. For the appropriate design of such distributors, the discharge coefficient has to be known to predict the energy and fluid distribution performance. Despite the vast amount of experimental data published, no generally applicable equations are available. Therefore, a new equation is presented for sharp edged circular side outlets, which can be widely used for calculating the discharge coefficient. The equation is developed by regression with non-linear least squares combined with genetic algorithm on experimental data available in the literature. The equation covers a wider range than the others presented in the literature.
TOPICS: Discharge coefficient, Fluids, Ventilation, Design, Genetic algorithms
research-article
J. Fluids Eng   doi: 10.1115/1.4039130
The hydrodynamic entrance length, pressure drop analysis, viscosity and fully developed velocity profile in horizontal pipe for crude oil with and without water and surfactant was studied in a 2" ID horizontal pipe of length 2.5 m experimentally. Hydrodynamic entry length and fluid characteristics have been examined by varying temperature, water fraction and flow rates. Temperature was varied 25°C - 40°C, flow rates 40 - 60 LPM and water 0% - 15% v/v and Madhuca longifolia from 500 - 2000 ppm. Triton X-100 was mixed with water to increase the emulsion capability during crude oil-water flows. The results showed significant influence of water, flow rate and temperature on the hydrodynamic entry region length, pressure drop, viscosity and velocity profiles along with natural surfactant. Pressure drop was reduced by 93.75 %, 94.18%, and 93.02% with 15%water and 2000 ppm surfactant at 40ºC for 40 LPM, 50 LPM and 60 LPM respectively. Viscosity of the crude oil during flowing is greatly influenced by water and addition of surfactant. After addition of 2000 ppm surfactant and 15% water at 40ºC, Viscosity reduced by about 94%. Hydrodynamic entry region length increased from 0.0354 to 0.2014 m, 0.0368 to 0.2336 m and 0.0384 to 0.2641m during transportation of crude oil after addition of 2000 ppm surfactant and 15% water at 40°C for 40 LPM, 50 LPM and 60 LPM flow respectively.
TOPICS: Flow (Dynamics), Hydrodynamics, Pipes, Crude oil, Surfactants, Water, Viscosity, Pressure drop, Temperature, Fluids, Emulsions, Transportation systems
research-article
J. Fluids Eng   doi: 10.1115/1.4039087
The Spalart-Allmaras (SA) is one of the most popular turbulence models in the aerospace CFD community. In its original (low-Reynolds number) formulation it requires a very tight grid spacing near the wall to resolve the high flow gradients. However, the use of wall functions with an automatic feature of switching from the wall function to the low-Reynolds number approach is an effective solution to this problem. In this work, we extend Menter's automatic wall treatment (AWT), devised for the k$\omega$-SST, to the SA model in our in-house developed 3-D Unstructured grid density-based CFD solver. It is shown, for both momentum and energy equations, that the formulation gives excellent predictions with low sensitivity to the grid spacing near the wall, and allows the first grid point to be placed at $y^{+}$ as high as 150 without loss of accuracy, even for the curved walls. In practical terms, this means a near-wall grid 10-30 times as coarse as that required in the original model would be sufficient for the computations.
TOPICS: Turbulence, Computational fluid dynamics, Computation, Curved walls, Aerospace industry, Density, Momentum, Flow (Dynamics)