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### Research Papers: Flows in Complex Systems

J. Fluids Eng. 2014;137(2):021101-021101-8. doi:10.1115/1.4028186.

This article presents a numerical investigation of 2D turbulent flow within a double external gear pump. The configuration of the inlet and outlet ports is determined such that the double gear pump acts like the combination of two parallel pumps. The complex geometry of the double gear pump, existence of narrow gaps between rotating and stationary walls, and rapidly deforming flow domain make the numerical solution more complicated. In order to solve the mass, momentum, and energy conservation laws along with the k-ε turbulence model, a second-order finite volume method has been used over a dynamically varying unstructured mesh. The numerical results including pressure contours, velocity vectors, flow patterns inside the suction chamber, leakage paths, and time variation of volumetric flow rate are presented in detail. The flow rate characteristic curves with linear behavior are demonstrated at rotational speeds and outlet pressures in the range of 1500–4000 rpm and 2–80 bar, respectively. The effect of reducing the gear-casing gap-size on the augmentation of the net flow rate has been investigated. It is concluded that the minimum oil pressure within the gear pump occurs at the two places between contacting gears near the inlet ports. The contours of vapor volume fraction are also illustrated.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2014;137(2):021102-021102-12. doi:10.1115/1.4028123.

This study examines the effect of channel leak geometry on blast overpressure attenuation in the rear of a muzzle-loaded large caliber cannon. Effects of three primary geometric parameters including leak volume as well as number and length of channels are studied. Reduction in blast overpressure, and thus peak overpressure, is most influenced by the leak volume; however, leak volume needs to be selected carefully to limit the loss in the projectile exit velocity. Modification of the channel height in the current range has a minimal effect on peak overpressure, but the number of channels can have a significant effect due to the constriction experienced by the leaking flow, thereby limiting the attenuation. Two channel lengths are considered where the longer channel length, is found to be more effective. The best configuration showed over 50% reduction in peak overpressure at all monitored locations with about 4.8% loss in the projectile exit velocity.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2014;137(2):021103-021103-10. doi:10.1115/1.4028498.

Trailing edge slot film cooling is a widely used active cooling scheme for turbine blade trailing edges. Current Reynolds-Averaged Navier–Stokes (RANS) models are known to significantly overpredict the adiabatic effectiveness of these configurations. It is shown that this overprediction is due in part to the breakdown of the Reynolds analogy between turbulent shear stress and scalar transport in the near wall region. By examining previously reported direct numerical simulation (DNS) results for a wall-mounted cube in cross flow, it is seen that in a flow with a significantly perturbed outer boundary layer, the turbulent diffusivity is not as strongly damped as the turbulent viscosity in the viscous sublayer and buffer layer of the boundary layer. By removing the Van Driest damping function from the length scale model for the turbulent diffusivity, more accurate turbulent diffusivity predictions are possible. This near wall correction is applied to trailing edge slot film cooling flows and it is demonstrated that the predictive accuracy of the RANS models is significantly enhanced. Detailed comparisons between RANS results and experimental datasets for 15 different cases demonstrate that this correction gives significant improvement to the accuracy of the RANS predictions across a broad range of trailing edge slot film cooling configurations.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2014;137(2):021104-021104-9. doi:10.1115/1.4028534.

The piezoelectric valve-less pump is an attractive device to be used as a micropump for low flow rates. In these pumps, the nozzle/diffuser elements that have a preferential flow direction replace conventional valves, to direct the flow from the inlet to the outlet. This work is a study on the performance of such pumps when several of them (up to four) are combined for use in series and/or parallel arrangement. Two basic pumping configurations are considered: (a) pumping of fluid from low pressure to a higher pressure in an open circuit and (b) pumping of fluid in a closed circuit through a flow resistance. The performance analysis procedure developed is simple and quick and allows studying a wide range of operational conditions. Such an analysis is difficult to conduct using elaborate computational fluid dynamics (CFD) approach. The performance characteristics of the different combinations is reported and critically evaluated.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2014;137(2):021105-021105-8. doi:10.1115/1.4028501.

Passive pumping using gravity-driven flow is a fascinating approach for microfluidic systems. When designing a passive pumping system, generated flow rates should be known precisely. While reported models used to estimate the flow rates do not usually consider capillary forces, this paper shows that their exclusion is unrealistic in typical gravity-driven systems. Therefore, we propose a new analytical model to estimate the generated flow rates. An extensive set of measurements is used to verify that the proposed model provides a remarkably more precise approximation of the real flow rates compared to the previous models. It is suggested that the developed model should be used when designing a gravity-driven pumping system.

Commentary by Dr. Valentin Fuster

### Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2014;137(2):021201-021201-10. doi:10.1115/1.4027953.

Direct numerical simulations (DNSs) of rotating turbulent Poiseuille flows are performed to study the effects of both cyclonic and anticyclonic system rotation on the kinematics of the quasi-streamwise vortices. By using the second invariant of the deformation tensor, a number of streamwise vortices are detected and averaged in the wall vicinity where the intense sweep motion, i.e., the inrush motion of high-speed fluid toward the wall, is related to the quasi-streamwise vortices. The effects of the system rotation on the angle of vortex axis are clearly observed as studied in longitudinal vortices of the homogeneous shear flow. Moreover, by calculating the probability of the emergence of the counterclockwise vortices (CCVs) around a clockwise vortex (CV), we find that with increase in the anticyclonic system rotation, the probability increases and decreases in the ejection and sweep sides of a CV, respectively. In contrast, cyclonic system rotation attenuates CCVs in both sides of a CV, though it increases at the top of the CV. This distribution of CCVs is found to affect sweep motion related to the quasi-streamwise vortices.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2014;137(2):021202-021202-10. doi:10.1115/1.4028122.

Various corrections were previously proposed to account for wall roughness with the k–ω and shear stress transport (SST) models. A simplified analysis, based upon the wall region analysis, is proposed to characterize the behavior of these roughness corrections. As this analysis points out some deficiencies for each correction, two new corrections are proposed for the SST model, to reproduce different behaviors, mainly in the transition regime. The correction development is based upon a previously developed strategy. A large set of boundary layer experiments is used to compare the different roughness corrections, confirm the failures of previous proposals, and validate the present ones. Moreover, it assesses the proposed simplified analysis. It also evidences the difficulty to determine the equivalent sand grain roughness for a given surface. The Colebrook based correction is recommended while the Nikuradse based one can add information about the envelope of possible behaviors in the transition regime.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2014;137(2):021203-021203-12. doi:10.1115/1.4028402.

An experimental investigation was carried out to study the turbulent flow over a flat plate in a wind tunnel. The turbulence was generated by a plate with diamond-shaped perforations mounted perpendicular to and on the leading edge of the flat plate. Unlike conventional grid turbulence studies, this perforated plate had a finite height, and this height was explored as a key independent parameter. Instantaneous velocity measurements were performed with a 1D hot-wire anemometer to reveal the behavior of the flow a short distance downstream of the perforated plate (X/D = 10–30). Different perforated plate heights (H = 3, 7, 11 cm) and free stream velocities (U = 4.5, 5.5, 6.5 m/s) have been studied.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2014;137(2):021204-021204-10. doi:10.1115/1.4028500.

The fluid dynamic interaction between a uniform free stream flow and the rotation induced flow from a sharp edged body is numerically investigated. A two-dimensional (2D) finite volume based computation is performed in this regard to simulate the laminar fluid flow around a rotating square cylinder in an unconfined medium. Body fitted grid system along with moving boundaries is used to obtain the numerical solution of the incompressible Navier–Stokes equations. The Reynolds number based on the free stream flow is kept in the range $10≤Re≤200$ with a dimensionless rotational speed of the cylinder in the range $0≤Ω≤5$. At low $Re=10$, the flow field remains steady irrespective of the rotational speed. For $50≤Re≤200$, regular low frequency Kármán vortex shedding (VS) is observed up to a critical rate of rotation ($Ωcr$). Beyond $Ωcr$, the global flow shows steady nature, although high frequency oscillations in the aerodynamic coefficients are present. The rotating circular cylinder also shows likewise degeneration of Kármán VS at some critical rotational speed. However, significant differences can be seen at higher rotation. Such fluid dynamic transport around a spinning square in an unconfined free stream flow is reported for the first time.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2014;137(2):021205-021205-10. doi:10.1115/1.4028595.

In this paper, we propose an integral force approach for potential flow around two-dimensional bodies with external free vortices and with vortex production. The method can be considered as an extension of the generalized Lagally theorem to the case with continuous distributed vortices inside and outside of the body and is capable of giving the individual force of each body in the case of multiple bodies. The lift force formulas are validated against two examples. One is the Wagner problem with vortex production and with moving vortices in the form of a vortex sheet. The other is the lift of a flat plate when there is a standing vortex over its middle point. As a first application, the integral approach is applied to study the lift force of a flat plate induced by a bounded vortex above the plate. This bounded vortex may represent a second small airfoil at incidence. For this illustrative example, the lift force is found to display an interesting distance-dependent behavior: for a clockwise circulation, the lift force acting on the main airfoil is attractive for small distance and repulsive for large distance.

Commentary by Dr. Valentin Fuster

### Research Papers: Multiphase Flows

J. Fluids Eng. 2014;137(2):021301-021301-9. doi:10.1115/1.4028337.

A theoretical framework to model the dynamics of acoustically driven microbubble inside a rigid tube is presented. The proposed model is not a variant of the conventional Rayleigh–Plesset category of models. It is derived from the reduced Navier–Stokes equation and is coupled with the evolving flow field solution inside the tube by a similarity transformation approach. The results are computed, and compared with experiments available in literature, for the initial bubble radius of Ro = 1.5 μm and 2 μm for the tube diameter of D = 12 μm and 200 μm with the acoustic parameters as utilized in the experiments. Results compare quite well with the existing experimental data. When compared to our earlier basic model, better agreement on a larger tube diameter is obtained with the proposed coupled model. The model also predicts, accurately, bubble fragmentation in terms of acoustic and geometric parameters.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2014;137(2):021302-021302-11. doi:10.1115/1.4028283.

The paper deals with solid particle erosion downstream of a sharp-edged orifice commonly found in many chemical processing industries. The orifice is installed in a pipe that is long enough to ensure fully developed turbulent flow in both upstream and downstream directions. Both the k-ε model and the Lagrangian particle-tracking technique were used for predicting solid particle trajectories. Gambit 2.2 was used to construct the computational grid and the commercial Fluent 12.1 code was used to perform the calculations. The available erosion correlations were used for determination of erosion characteristics considering carbon steel and aluminum pipes. The investigation was carried out for a flow restricting orifice of fixed geometry and pipe flow velocities in the range 1–4 m/s using solid particle of diameters 50–500 μm. The results indicated two critical erosion regions downstream of the orifice: the first is in the immediate neighborhood of the orifice plate and the second is in the flow reattachment zone. The results showed also a strong dependence of erosion on both particle size and flow velocity.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Fluids Eng. 2014;137(2):024501-024501-4. doi:10.1115/1.4027432.

A currently unexplored mechanical application of nanowires is near-wall active flow manipulation, with potential uses mixing and filtering chemicals, enhancing convective heat transfer, and reducing drag. Here, we present experimental evidence that it is possible to introduce persistent perturbations into turbulent flow with active nanowires. A $TiO2$ nanowire array was fabricated and installed in the bounding wall of a turbulent channel flow, and the array was oscillated by external actuation. Measurements indicated that the array increased turbulent kinetic energy throughout the entire wall layer. These findings suggest that dynamically actuated nanowires can potentially be used to implement near-wall flow control.

Commentary by Dr. Valentin Fuster

### Discussion

J. Fluids Eng. 2014;137(2):025501-025501-1. doi:10.1115/1.4027830.

The comments present an assessment of formulations for frictional pressure gradient correlation of moderately viscous lubricating oil–water two-phase flow through a horizontal pipe of 0.025 m internal diameter, which was obtained by Dasari et al. [1]. Their modified Lockhart–Martinelli correlation wasDisplay Formula

(1)$φW2=0.019X3+0.06X2-0.006X+1.397$

Commentary by Dr. Valentin Fuster

### Errata

J. Fluids Eng. 2014;137(2):027001-027001-1. doi:10.1115/1.4027446.

The figures containing image sequences are missing dimensionless time stamp information. This information is listed below.

Commentary by Dr. Valentin Fuster

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