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Research Papers: Flows in Complex Systems

J. Fluids Eng. 2017;139(7):071101-071101-9. doi:10.1115/1.4036157.

Intense pressure pulsation, resulted from the flow structure shedding from the blade trailing edge and its interaction with the volute tongue and the casing, is detrimental to the stable operation of centrifugal pumps. In the present study, unsteady pressure pulsation signals at different positions of the volute casing are extracted using high response pressure transducers at flow rate of 0–1.55ΦN. Emphasis is laid upon the influence of measuring position and operating condition on pressure pulsation characteristics, and components at the blade passing frequency fBPF and root-mean-square (RMS) values in 0–20.66fn frequency band are mainly analyzed. Results clearly show that the predominant components in pressure spectra always locate at fBPF. The varying trends versus flow rate of components at fBPF differ significantly for different points, and it is considered to be associated with the corresponding flow structures at particular positions of the volute casing. At the near-tongue region, high pressure amplitudes occur at the position of θ = 36 deg, namely the point at the after tongue region. For different measuring points, angular distributions of amplitudes at fBPF and RMS values in 0–20.66fn frequency band are not consistent and affected significantly by the pump operating conditions.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(7):071102-071102-10. doi:10.1115/1.4036151.

We report the first systematic investigation of the phenomenon of “switching” between the two bistable axial jet (AJ) and precessing jet (PJ) flow modes in the fluidic precessing jet (FPJ) nozzle. While geometric configurations have been identified where the fractional time spent in the AJ mode is much less than that in the PJ mode, nevertheless, the phenomenon is undesirable and also remains of fundamental interest. This work was undertaken numerically using the unsteady shear stress transport (SST) model, the validation of which showed a good agreement with the experimental results. Three methods were employed in the current work to trigger the flow to switch from the AJ to the PJ modes. It is found that some asymmetry in either the inlet flow or the initial flow field is necessary to trigger the mode switching, with the time required to switch being dependent on the extent of the asymmetry. The direction and frequency of the precession were found to depend on the direction and intensity of the imposed inlet swirling, which will be conducive to the control of the FPJ flow for related industrial applications and academic research. The process with which the vortex skeleton changes within the chamber is also reported. Furthermore, both the rate of spreading and the maximum axial velocity decay of the jet within the nozzle are found to increase gradually during the switching process from the AJ to the PJ modes, consistent with the increased curvature within the local jet.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(7):071103-071103-7. doi:10.1115/1.4035948.

In this study, a commercial electromagnetic synthetic jet actuator (SJA) was characterized to determine its flow patterns and sphere of influence within a room. Its axisymmetry was also evaluated. The objective of this research is to explore the potential application of this commercial SJA for indoor air quality (IAQ) control and its limitations for such application. A small fan of similar size was also partially characterized. Results showed that the SJA is axisymmetric, and it uses a small power input to impact the airflow over 1 m from the jet exit. Further, while the SJA showed less flow output than the fan in this study, it had significantly greater dynamic pressure per unit power. This feature could be potentially useful for indoor air quality control applications.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(7):071104-071104-8. doi:10.1115/1.4036150.

Flow coefficients of intake valves and port combinations were determined experimentally for a compressed nitrogen engine under steady-state and dynamic flow conditions for inlet pressures up to 3.2 MPa. Variable valve timing was combined with an indexed parked piston cylinder unit for testing valve flows at different cylinder volumes while maintaining realistic in-cylinder transient pressure profiles by simply using a fixed area outlet orifice. A one-dimensional modeling approach describing three-dimensional valve flow characteristics has been developed by the use of variable flow coefficients that take into account the propagation of flow jets and their boundaries as a function of downstream/upstream pressure ratios. The results obtained for the dynamic flow cases were compared with steady-state results for the cylinder to inlet port pressure ratios ranges from 0.18 to 0.83. The deviation of flow coefficients for both cases is discussed using pulsatile flow theory. The key findings include the followings: (1) for a given valve lift, the steady-state flow coefficients fall by up to 21% with increasing cylinder/manifold pressure ratios within the measured range given above and (2) transient flow coefficients deviated from those measured for the steady-state flow as the valve lift increases beyond a critical value of approximately 0.5 mm. The deviation can be due to the insufficient time of the development of steady-state boundary layers, which can be quantified by the instantaneous Womersley number defined by using the transient hydraulic diameter. We show that it is possible to predict deviations of the transient valve flow from the steady-state measurements alone.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(7):071105-071105-10. doi:10.1115/1.4036159.

The interaction between multiple laminar thin vortex rings and solid surfaces was studied numerically so as to investigate flow patterns associated with near-wall flow structures. In this study, the vortex–wall interaction was used to investigate the tendency of the flow toward recirculatory behavior and to assess the near-wall flow conditions. The numerical model shows very good agreement with previous studies of single vortex rings for the case of orthogonal impact (angle of incidence, θ = 0 deg) and oblique impact (θ = 20 deg). The study was conducted at Reynolds numbers 585 and 1170, based on the vortex ring radius and convection velocity. The case of two vortex rings was also investigated, with particular focus on the interaction of vortex structures postimpact. Compared to the impact of a single ring with the wall, the interaction between two vortex rings and a solid surface resulted in a more highly energized boundary layer at the wall and merging of vortex structures. The azimuthal variation in the vortical structures yielded flow conditions at the wall likely to promote agitation of ground based particles.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(7):071106-071106-7. doi:10.1115/1.4036270.

Experiments in ground-test facilities are required to develop and mature aero-optic platforms. However, structural vibrations of the facility can introduce significant error in optical experiments, making it difficult to separate wind-tunnel dynamics from the aero-optic measurements of interest. As a means of performing this separation, a Kalman filtering scheme, tuned using the autocovariance least-squares method, is implemented to measure the dynamic motion of a generalized aero-optic model geometry. An example is presented for the case of a laser diode mounted on a flexible post undergoing vibration. Using a single three-axis accelerometer, combined with a priori knowledge of the system's structural dynamics, pitch angle fluctuations on the order of 0.01 deg were successfully estimated within 0.002 deg uncertainty.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(7):071107-071107-11. doi:10.1115/1.4036186.

The control of flow in the wake of a circular cylinder by an attached permeable plate having various porosity ratios was analyzed experimentally using both particle image velocimetry (PIV) and dye visualization techniques. The force measurements were also done in order to interpret the effect of control method on drag coefficient. The diameter of the cylinder and length to diameter ratio of the plate were kept constant as D = 50 mm and L/D = 1.0, respectively. The porosity ratio, β, which can be defined as the ratio of open surface area to the whole body surface area, was taken as β = 0.4, 0.5, 0.6, 0.7, and 0.8 (permeable plates). The study was performed considering deep water flow conditions with a constant Reynolds number of ReD = 5000 based on the cylinder diameter. Each permeable plate was attached on the separation point and the results were compared with the results of cylinder without permeable plate (plain cylinder) in order to understand the control effect. Both qualitative and quantitative results revealed that the permeable plates of 0.4 ≤ β ≤ 0.6 are effective on controlling the unsteady flow structure downstream of the cylinder, i.e., the vortex formation length was increased, turbulent statistics was reduced and vortex shedding frequency was diminished when the permeable plate attached normal to the cylinder surface from the lower separation point. However, the drag force acting on the cylinder was found to be increased due to the increased cross-sectional area.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2017;139(7):071201-071201-10. doi:10.1115/1.4035950.

A comprehensive study on the flow structure of an ensemble-averaged fluidic precessing jet (FPJ) flow is reported. This study is based on the concepts of critical point theory, previous experimental data, and validated simulation results. The unsteady k–ω shear stress transport (SST) turbulence model was adopted for the simulation, which provided high resolution flow details. The numerical model successfully reproduced the four main flow features of the FPJ flow. The predicted equivalent diameter and the centerline velocity of the phase-averaged FPJ flow were compared against the measured results and achieved reasonable agreement. The streamlines, velocity, and vorticity contours in a series of cross-sectional planes are presented. The calculated streamlines at the surfaces of the nozzle and the center-body (CB) are compared with previously deduced surface flow patterns. With these methods, a vortex skeleton with six main vortex cores of the FPJ flow within the nozzle is identified for the first time. This skeleton, which is illustrated diagramatically, is deduced to be responsible for the jet precession.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(7):071202-071202-8. doi:10.1115/1.4036163.

Direct numerical simulations (DNS) are performed to investigate the transient growth of a steady disturbance induced by a numerically generated Gaussian rough wall in a laminar boundary layer. In the calculation of the interaction between the rough wall and the fluid, the multiple direct force and immersed boundary method (MDF/IBM) are adopted. The evolution of the streak structures and the energy of the disturbances generated by the rough wall are presented. A similar evolution into an almost sinusoidal modulation for the cylindrical roughness element is found for the current irregular rough wall, and the disturbance energy also undergoes the classical transient growth mode. Moreover, the influences of the skewness, kurtosis, and correlation length on the evolution of spanwise harmonics are also analyzed. The results show that the effects of skewness and kurtosis are on the distribution of energy among the wavelengths and the subsequent growth processes, while the wavelengths of the harmonics are linked to both the streamwise and spanwise correlation lengths of the rough wall.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(7):071203-071203-9. doi:10.1115/1.4036164.

The spray characteristics of a liquid sheet contribute much to the investigation of atomization efficiency. Considering the jet contracting effect of elliptical jets, an improved model of elliptical power-law fluid jets is proposed herein to derive the spray characteristics. Some experiments have been conducted to verify its feasibility, and the results show a good agreement with theoretical predictions. The effect of the aspect ratio on sheet shape and thickness has been studied to interpret the phenomenon that liquid sheets formed by the impinging elliptical jets are more likely to disintegrate. The relationships between rheological parameters (K and n) and the spray features are also discussed.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(7):071204-071204-15. doi:10.1115/1.4036162.

This paper considers the effects of multiphase parameters on a shock-driven particle-laden hydrodynamic instability using simulations performed with the hydrocode FLAG, developed at Los Alamos National Laboratory. The classic sinusoidal interface common in instability literature is created using water particles seeded in a nitrogen–water vapor mixture. The simulations model a shock tube environment as the computational domain, to guide future experimentation. Multiphase physics in FLAG include momentum and energy coupling, with this paper discussing the addition of mass coupling through evaporation. The multiphase effects are compared to a dusty gas approximation, which ignores multiphase components, as well as to a multiphase case which ignores evaporation. Evaporation is then further explored by artificially changing parameters which effect the rate of evaporation as well as the amount of total evaporation. Among all these experiments, the driving force of the hydrodynamic instability is a shock wave with a Mach number of 1.5 and a system Atwood number of 0.11 across the interface. The analysis is continued into late time for select cases to highlight the effects of evaporation during complex accelerations, presented here as a reshock phenomenon. It was found that evaporation increases the circulation over nonevaporating particles postshock. Evaporation was also shown to change the postshock Atwood number. Reshock showed that the multiphase instabilities exhibited additional circulation deposition over the dusty gas approximation. Mixing measures were found to be affected by evaporation, with the most significant effects occurring after reshock.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2017;139(7):071301-071301-12. doi:10.1115/1.4036160.

Foam structures have been a subject of intensive research since the last decade. The pore space in open-cell foam is interconnected, forming perforated channels of varying cross-sectional areas where fluid can flow. Knowledge of pressure drop induced by these foam matrices is essential for successful design and operation of high-performance industrial systems. In this context, analytical correlations were derived for the determination of Darcian permeability (KD) and Forchheimer inertia coefficient (CFor) in open-cell foams of different strut shapes. It has been shown that the flow law characteristics are strongly dependent on strut shape, strut characteristic dimension, and length. The applicability of new correlations was examined by comparing and validating the numerical and experimental flow law characteristics data against the predicted ones. An excellent agreement has been observed for the foam structures of different materials and variable texture in a wide range of porosity and Reynolds number.

J. Fluids Eng. 2017;139(7):071302-071302-9. doi:10.1115/1.4036158.

This investigation presents both theoretical and experimental studies on the size of a growing bubble in power-law non-Newtonian liquids. At first, some previous works on the prediction of bubble size in Newtonian liquids have been extended by considering the balance of forces acting on the bubble at the moment of separation. Predicted bubble sizes were validated against the experimental results for a wide range of operating conditions, including different gas flow rates and needle diameters as well as a wide range of physical properties of the Newtonian liquids. Furthermore, in order to determine the size of the bubbles formed in power-law non-Newtonian liquids with a similar analysis, the effective shear rate of bubble growth was calculated in which the rheological properties of fluid were taken into account and subsequently the viscosity of the fluid was modified. Theoretically obtained bubble sizes for non-Newtonian liquids are in a good agreement with our experimental high-speed video observations of three carboxyl methyl cellulose (CMC) solutions.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(7):071303-071303-14. doi:10.1115/1.4036165.

Eucalyptus and Pine suspensions flow in a pipe was studied experimentally and numerically. Pressure drop was measured for different mean inlet flow velocities. Electrical impedance tomography (EIT), was used to evaluate the prevailing flow regime. Fibers concentration distribution in the pipe cross section and plug evolution were inferred from EIT tomographic images. A modified low-Reynolds-number k–ε turbulence model was applied to simulate the flow of pulp suspensions. The accuracy of the computational fluid dynamics (CFD) predictions was significantly reduced when data in plug regime was simulated. The CFD model applied was initially developed to simulate the flow of Eucalyptus and Pine suspensions in fully turbulent flow regime. Using this model to simulate data in the plug regime leads to an excessive attenuation of turbulence which leads to lower values of pressure drop than the experimental ones. For transition flow regime, the CFD model could be applied successfully to simulate the flow data, similar to what happens for the turbulent regime.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Fluids Eng. 2017;139(7):074501-074501-4. doi:10.1115/1.4036161.

The characteristics of pressure fluctuations in a prototype reversible pump turbine (RPT) is investigated within a wide range of load conditions with a focus on the low-load condition (e.g., 25% of rated power) with the aid of pressure signals obtained at several typical recording points (i.e., spiral casing, vaneless space, draft tube cone, and draft tube elbow). Our findings reveal that at the low-load condition, the pressure fluctuation is quite significant (e.g., above 12% in terms of nondimensional values), especially in the vaneless space and spiral casing with the dominant frequency being the blade passing frequency. Furthermore, based on the characteristics of pressure fluctuation, the investigated load range is divided into three zones. For zone I (with low load), the amplitude of pressure fluctuation is highest and the dominant mechanism is the rotor–stator interaction in the vaneless space with the blade passing frequency. For zone II (with medium load), the amplitude of pressure fluctuation is less prominent (below 5%) and the dominant mechanism is the low-frequency fluctuations induced by the swirling vortex rope. For zone III (with high load), the amplitude of pressure fluctuation is quite limited (less than 3%) and the dominant mechanism is still the rotor–stator interaction but with the dominant frequency being the harmonics of blade passing frequency. Detailed examples for all three zones are given and discussed with quantitative descriptions of propagation mechanism of fluctuation.

Commentary by Dr. Valentin Fuster

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