Research Papers: Flows in Complex Systems

J. Fluids Eng. 2017;139(10):101101-101101-10. doi:10.1115/1.4036715.

In petroleum industry, the stability of multiphase pumping is highly disturbed by the gas' presence with high content and variable working conditions. This paper is focused on studying the whole working cycle of the novel three-cylinder double-acting reciprocating multiphase pump. Based on the theoretical analysis, the method of computational fluid dynamics (CFD) is adopted to simulate the oil–gas flow in reciprocating multiphase pump. The numerical methodology, involving multiphase model, dynamic grid technique and user defined functions (UDF), is used to deal with in the calculation. The transient flow characteristics in pump cavity are obtained, and the flow ripples of reciprocating multiphase pump are analyzed. Furthermore, the effects of different operating parameters, such as suction and discharge pressures, inlet gas volume fraction (GVFi) on the capacity, and stability of pump, are studied. The results could help to develop and optimize the high-efficiency multiphase pump system.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(10):101102-101102-9. doi:10.1115/1.4037037.

A low-speed wind tunnel investigation is presented characterizing the impact of Gurney flaps on an elliptical airfoil. The chordwise attachment location and height of the flaps were varied, as was the Reynolds number. The results showed strong nonlinearities in the lift curve which were present for all tested geometries. Flap effectiveness was seen to diminish as the flap was moved closer to the trailing edge stemming from flap submersion in separated flow. For the tested cases, the measured lift coefficients showed a weak Re dependency. The upper airfoil surface was shown to carry approximately 80% of the total lift load. The top surface caused a pitching moment reversal associated with nonlinearity in the lift curve.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(10):101103-101103-17. doi:10.1115/1.4036823.

Nozzle aspect ratio effect on the mixing of Mach 2 elliptic free jet, issuing from convergent–divergent elliptic nozzles of aspect ratios 2, 3, and 4, in the presence of adverse and marginally favorable pressure gradients at the nozzle exit has been studied experimentally. The results show that AR4 jet enjoys better mixing than AR2 and AR3 jets at all nozzle pressure ratios. The AR2 and AR3 jets displayed axis switching, whereas there is no axis switching for AR4 jet. The shadowgraph shows that the waves in AR4 jet are weaker than those in AR2 and AR3 jets.

Topics: Pressure , Jets , Nozzles
Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(10):101104-101104-12. doi:10.1115/1.4036827.

The intakes of modern aircraft are subjected to ever-increasing demands in their performance. Particularly, they are expected to carry out diffusion with the highest isentropic efficiency while subjected to aggressive geometry requirements arising from stealth considerations. To avoid a penalty in engine performance, the flow through intake needs to be controlled using various methods of flow control. In this study, a serpentine intake is studied experimentally and its performance compared with and without boundary layer suction. The performance parameters used are nondimensional total pressure loss coefficient and standard total pressure distortion descriptors. The effect is observed on surface pressure distributions, and inferences are made regarding separation location and extent. A detailed measurement at the exit plane shows flow structures that draw attention to secondary flows within the duct. Suction is applied at three different locations, spanning different number of ports along each location, comprising of ten unique configurations. The mass flow rate of suction employed ranges from 1.1% to 6.7% of mass flow rate at the inlet of the intake. The effect is seen on exit total pressure recovery as well as circumferential and radial distortion parameters. This is examined in the context of the location of the suction ports and amount of suction mass flow, by the deviation in surface pressure distributions, as well as the separation characteristics from the baseline case. The results show that applying suction far upstream of the separation point together with a modest amount of suction downstream results in the best performance.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2017;139(10):101201-101201-8. doi:10.1115/1.4036673.

Three-dimensional (3D) simulations with ansys cfx 16.1 as well as measurements of the cavitating flow in a low specific speed centrifugal pump (nq = 12 min−1) are performed for different operation conditions and varying surface roughness. Surface roughness is considered by wall functions in the flow simulations. Good agreement between measured and calculated head is achieved for noncavitating flow. Net positive suction head (NPSH3%) rises toward overload due to incidence, flow separation, and vapor zones at the volute tongue. The NPSH3% rise is slightly higher for rough walls according to measurements and significantly overestimated by the wall function approach, irrespective of the roughness level in the simulation. A low-Reynolds number approach at the volute tongue leads to a more accurate prediction of NPSH3% than wall functions, at the cost of high computational effort.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(10):101202-101202-6. doi:10.1115/1.4036674.

The cochlea is the most important part of the hearing system, due to the fact that it transforms sound guided through air, bone, and lymphatic fluid to vibrations of the cochlear partition which includes the organ of Corti with its sensory cells. These send nerve impulses to the brain leading to hearing perception. The work presents the wave propagation in rigid ducts filled with air or water including viscous-thermal boundary layer damping. In extension, a mechanical box model of the human cochlea represented by a rectangular duct limited by the tapered basilar membrane at one side is developed and evaluated numerically by the finite element method. The results match with rare experiments on human temporal bones without using the physically unfounded assumption of Rayleigh damping. A forecast on the concept of the traveling wave parametric amplification is given to potentially explain the high hearing sensitivity and otoacoustic emissions.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(10):101203-101203-16. doi:10.1115/1.4036670.

This paper concerns the study of the influence of an external magnetic field on the reverse flow occurring in the steady mixed convection of two Newtonian immiscible fluids filling a vertical channel under the Oberbeck–Boussinesq approximation. The two isothermal boundaries are kept either at different or at equal temperatures. The velocity, the temperature, and the induced magnetic field are obtained analytically. The results are presented graphically and discussed for various values of the parameters involved in the problem (in particular, the Hartmann number and the buoyancy coefficient) and are compared with those for a single Newtonian fluid. The occurrence of the reverse flow is explained and carefully studied.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(10):101204-101204-13. doi:10.1115/1.4036717.

The maximum skin friction and flow field are experimentally measured on a planar impinging gas jet using oil film interferometry (OFI) and particle image velocimetry (PIV), respectively. A jet nozzle width of W = 15 mm, impingement ratios H/W = 4, 6, 8, 10, and a range of jet Reynolds numbers Rejet = 11,000–40,000 are tested to provide a parametric map of the maximum skin friction. The maximum skin friction predictions of Phares et al. (2000, “The Wall Shear Stress Produced by the Normal Impingement of a Jet on a Flat Surface,” J. Fluid Mech., 418, pp. 351–375) for plane jets agree within 5% of the current OFI results for H/W = 6, but deviates upward of 28% for other impingement ratios. The maximum skin friction is found to be less sensitive to changes in the impingement ratio when the jet standoff distance is roughly within the potential core length of the jet. PIV measurements show turbulence transition locations moving toward the nozzle exit with increasing Reynolds number, saturation in the downstream evolution of the maximum axial turbulence intensity before reaching a maximum peak upon impingement, followed by sudden damping at the plate surface. As the flow is redirected, there is an orthogonal redistribution of the fluctuating velocity components, and local peaks in both the axial and transverse turbulence intensity distributions at the plate locations of the maximum skin friction.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(10):101205-101205-12. doi:10.1115/1.4037036.

The transition from a laminar to turbulent boundary layer on a wing operating at low Reynolds numbers can have a large effect on its aerodynamic performance. For a wing operating in ground effect, where very low pressures and large pressure gradients are common, the effect is even greater. A study was conducted into the effect of forcing boundary-layer transition on the suction surface of an inverted GA(W)-1 section single-element wing in ground effect, which is representative of a racing-car front wing. Transition to a turbulent boundary layer was forced at varying chordwise locations and compared to the free-transition case using experimental and computational methods. Forcing transition caused the laminar-separation bubble, which was the unforced transition mechanism, to be eliminated in all cases and trailing-edge separation to occur instead. The aerodynamic forces produced by the wing with trailing-edge separation were shown to be dependent on trip location. As the trip was moved upstream the separation point also moved upstream, this led to an increase in drag and reduction in downforce. In addition to significant changes to the pressure field around the wing, turbulent energy in the wake was considerably reduced by forcing transition. The differences between free- and forced-transition wings were shown to be significant, highlighting the importance of modeling transition for ground-effect wings. Additionally, it has been shown that while it is possible to reproduce the force coefficient of a higher Reynolds-number case by forcing the boundary layer to a turbulent state, the flow features, both on-surface and off-surface, are not recreated.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(10):101206-101206-11. doi:10.1115/1.4036824.

An experimental study was conducted to investigate the effect of nozzle geometries on the statistical properties of free orifice jets at low and moderate Reynolds numbers. The studied cross sections were round, square, and ellipses with aspect ratios of 2 and 3. For each jet, detailed velocity measurements were made using a particle image velocimetry (PIV) system at Reynolds numbers of 2500 and 17,000. The results showed that at both Reynolds numbers, the elliptic jets had relatively higher velocity decay and jet spreading; however, the nozzle geometry effects were more pronounced at Re = 17,000 than at Re = 2500. Analysis of the swirling strength revealed that the rotational motions induced by vortices within the minor planes of the elliptic jets were stronger than observed in the major planes, square and round jets which were consistent with the relatively higher spreading observed in the minor planes. It was observed that the streamwise locations of the switchover points were independent of Reynolds number but are a strong function of aspect ratio. Based on the present results and those documented in the literature, a linear correlation was proposed for the location of axis-switching in orifice jets. Due to the axis-switching phenomena, a sign change was observed in the distribution of the Reynolds shear stress in the major planes of the elliptic jets. This results in the existence of regions with negative eddy viscosity in the near field regions, an observation that has an important implication for the predictive capabilities of standard eddy viscosity models.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2017;139(10):101301-101301-8. doi:10.1115/1.4036711.

The bulk modulus of hydraulic fluid is dependent on the quantity of entrained gas in the fluid. In this paper, an effective fluid bulk modulus model that captures dynamic gas absorption during pressure transients is derived from the overall mass transfer theory. Optical measurement of a microgas bubble volume is used to determine the interfacial mass transport. Compared to traditional models, the proposed model is able to capture the 10% gap in the pressure profile between the first and second cycles, when simulating multiple compression cycles of an oil sample with 0.65% entrained gas by volume at 8 MPa.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(10):101302-101302-12. doi:10.1115/1.4036669.

Unstable cavitation presents an important speed barrier for underwater vehicles such as hydrofoil craft. In this paper, the authors concern about the physical problem about the cloud cavitating flow that surrounds an underwater-launched hydrofoil near the free surface at relatively high-Froude number, which has not been discussed in the previous research. A water tank experiment and computational fluid dynamics (CFD) simulation are conducted in this paper. The results agree well with each other. The cavity evolution process in the experiment involves three stages, namely, cavity growth, shedding, and collapse. Numerical methods adopt large eddy simulation (LES) with Cartesian cut-cell mesh. Given that the speed of the model changes during the experiment, this paper examines cases with varying constant speeds. The free surface effects on the cavity, re-entry jet location, and vortex structures are analyzed based on the numerical results.

Commentary by Dr. Valentin Fuster

Research Papers: Techniques and Procedures

J. Fluids Eng. 2017;139(10):101401-101401-10. doi:10.1115/1.4036668.

In this work, the suitability of pressure probes, commonly known as Irwin probes, to determine the local wall shear stress was evaluated for steady turbulent flow in rectangular ducts. Pressure measurements were conducted in the fully developed flow region of the duct and both the influence of duct aspect ratio (AR) (from 1:1.03 to 1:4.00) and Reynolds number (from 104 to 9 × 104) on the mean characteristics of the flow were analyzed. In addition, the sensitivity of the longitudinal and transversal placement of the Irwin probes was verified. To determine the most appropriate representation of the experimental data, three different characteristic lengths (l*) to describe Darcy's friction coefficient were investigated, namely: hydraulic diameter (Dh), square root of the cross section area (√A), and laminar equivalent diameter (DL). The comparison of the present experimental data for the range of tested Re numbers against the results for turbulent flow in smooth circular tubes indicates similar trends independently of the AR. The selection of the appropriate l* to represent the friction coefficient was found to be dependent on the AR of the duct, and the three tested scales present similar performance. However, the hydraulic diameter, being the commonly employed to compute turbulent flow in rectangular ducts, is the selected characteristic length scale to be used in the present study. A power function-based calibration equation is proposed for the Irwin probes, which is valid for the range of ARs and Reynolds numbers tested.

Commentary by Dr. Valentin Fuster

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