Research Papers: Flows in Complex Systems

J. Fluids Eng. 2018;141(5):051101-051101-26. doi:10.1115/1.4041229.

Collaboration is described on assessment of computational fluid dynamics (CFD) predictions for surface combatant model 5415 at static drift β = 0 deg and 20 deg using recent tomographic particle image velocimetry (TPIV) experiments. Assessment includes N-version verification and validation to determine the confidence intervals for CFD solutions/codes, and vortex onset, progression, instability, and turbulent kinetic energy (TKE) budget analysis. The increase in β shows the following trends. Forces and moment increase quadratically/cubically, and become unsteady due to shear layer, Karman and flapping instabilities on the bow. Wave elevation becomes asymmetric; its amplitude increases, but the total wave elevation angle remains same. The vortex strength and TKE increase by about two orders of magnitude, and for large β, the primary vortices exhibit helical mode instability similar to those for delta wings. Forces and moment for both β and wave elevation for β = 0 deg are compared within 4% of the data, and are validated at 7% interval. Wave elevation for β = 20 deg, and vortex core location and velocities for both β are compared within 9% of the data, and are validated at 12% interval. The vortex strength and TKE predictions show large 70% errors and equally large scatter and are not validated. Thus, both errors and scatter need reduction. TKE budgets show transport of turbulence into the separation bubble similar to canonical cases, but pressure transport is dominant for ship flows. Improved CFD predictions require better grids and/or turbulence models. Investigations of solution-adaptive mesh refinement for better grid design and hybrid Reynolds-averaged Navier-Stokes/large eddy simulation models for improved turbulent flow predictions are highest priority.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(5):051102-051102-14. doi:10.1115/1.4041507.

An experimental program was conducted to investigate turbulent flow of water over the stationary sand bed deposited in horizontal annuli. A large-scale horizontal flow loop equipped with the state-of-the-art particle image velocimetry (PIV) system has been used for the experiments. Experiments were conducted to measure the instantaneous local velocity profiles during turbulent flow and examine the impact of the presence of a stationary sand bed deposits on the local velocity profiles, Reynolds shear stresses and turbulence intensities. Results have shown that the existence of a stationary sand bed causes the volumetric flow to be diverted away from the lower annular gap. Increasing the sand bed height causes further reduction of the volumetric flow rate in the lower annulus. Velocity profiles near the surface of the bed deposits showed a downward shift from the universal law in wall units indicating that the flow is hydraulically rough near the sand bed. The equivalent roughness height varied with flow rates. At flow rates less than the critical flow rate, the Reynolds stress profile near the bed interface had slightly higher peak values than that of the case with no sand bed. At the critical flow rate, however, the peak Reynolds stress values for the flow over the sand bed was lower than that of the case with no bed. This behavior is attributed to the bed load transport of sand particles at the critical flow rate.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(5):051103-051103-11. doi:10.1115/1.4041455.

The study is a continuation of the authors' previous works on hydrodynamics and sediment transport modeling for Calcasieu Lake area (located in southwest Louisiana) (Zhang, N., Zheng, Z. C., and Yadagiri, S., 2011, “A Hydrodynamic Simulation for the Circulation and Transport in Coastal Watersheds,” Comput. Fluids, 47(1), pp. 178–88; Zhang, N., Kee, D., and Li, P., 2013, “Investigation of the Impacts of Gulf Sediments on Calcasieu Ship Channel and Surrounding Water Systems,” Comput. Fluids, 77, pp. 125–133; Yadav, P. K., Thapa, S., Han, X., Richmond, C., and Zhang, N., 2015, “Investigation of the Effects of Wetland Vegetation on Coastal Flood Reduction Using Hydrodynamic Simulation,” ASME Paper No. AJKFluids2015-3044). The major purposes of the study are: (1) to demonstrate the new model features and validate the model results, (2) to disclose the effects of Gulf Intracoastal Waterway (GIWW) and determine the boundary conditions for GIWW in the model, and (3) to use the model to analyze the effects of excessive freshwater withdrawals on the changes of hydrodynamics and salinity in the Calcasieu Lake system. Several new model features were added to the existing model framework, including the extension of modeling domain, vegetation model, salinity transport model, and pH calculation. Measurement data from NOAA and USGS are used as boundary conditions for the model. Simulation results were compared with measurement data from NOAA, USGS and other sources for validation. Due to lack of measured data for GIWW in the target area, the effect of GIWW flow conditions on the modeling results was investigated and appropriate GIWW boundary condition was determined based on numerical tests. Numerous petrochemical plants in the area use tremendous amount of fresh surface water. Recent industry expansions may further increase the demands of freshwater withdrawals. One of the purposes of the study is to use developed model to test and analyze the effects of increased freshwater withdrawal from Calcasieu River at the north boundary of the study area on the hydrodynamics and salinity in the downstream Calcasieu Lake system.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(5):051104-051104-15. doi:10.1115/1.4041523.

This article describes a direct comparison between two symmetrical airfoils undergoing dynamic stall at high, unsteady reduced frequencies under otherwise identical conditions. Particle image velocimetry (PIV) was performed to distinguish the differences in flow structure between a NACA 0021 and a NACA 0012 airfoil undergoing dynamic stall. In addition, surface pressure measurements were performed to evaluate aerodynamic load and investigate the effect of laminar separation bubbles and vortex structures on the pressure fields surrounding the airfoils. Airfoil geometry is shown to have a significant effect on flow structure development and boundary layer separation, with separation occurring earlier for thinner airfoil sections undergoing constant pitch-rate motion. Inertial forces were identified to have a considerable impact on the overall force generation with increasing rotation rate. Force oscillation was observed to correlate with multiple vortex structures shedding at the trailing-edge during high rotation rates. The presence of laminar separation bubbles on the upper and lower surfaces was shown to dramatically influence the steady-state lift of both airfoils. Poststall characteristics are shown to be independent of airfoil geometry such that periodic vortex shedding was observed for all cases. However, the onset of deep stall is delayed with increased nondimensional pitch rate due to the delay in initial dynamic-stall vortex.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(5):051105-051105-13. doi:10.1115/1.4041642.

The flow in the injector's sac volume has been reported to influence diesel-injector nozzle flow, but few studies have characterized sac volume. Our study modeled flow in the sac volume using a large Eddy simulation (LES) approach to gain better insight into the complexity of the flow dynamics. It focused on the effect of fixed needle lifts on sac-volume internal flow of a single-hole injector with emphasis on large-scale unsteadiness; three-dimensional proper orthogonal decomposition (POD) was used to analyze the flow. The near-wall turbulence resolution of the elaborated computational fluid dynamics (CFD) model has been validated with direct numerical simulation (DNS) results in the canonical case of fully developed channel flow. The main findings are: (1) an enlarging flow jet entering the sac volume with decreasing small scales of turbulence was observed as needle lift increased. (2) three-dimensional POD revealed that the mean flow energy was nearly constant at low needle lifts (6%, 8%, and 10%) and decreased twofold at the higher needle lift of 31%. (3) The analysis of fluctuating modes revealed that flow restructuring occurred with increasing needle lift as three different energy distributions were observed with the lowest (6%), intermediary (8%, 10%, and 16%), and highest needle lifts (31%). (4) Finally, the analysis of the POD-reduced-order model has shown that the lowest frequency of mode 1, which carries the highest fluctuating energy, is responsible for the oscillation of the main rotating structure within the sac volume that causes fuel-jet enlarging/narrowing with time. This oscillation of the main structure was found to decrease with increased needle lift.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(5):051106-051106-9. doi:10.1115/1.4041560.

The effect of the nozzle top lip thickness on a two-dimensional wall jet was examined experimentally in a wind tunnel using hot-wire anemometry. Lip thicknesses of 0.125b, 0.5b, 1b, and 2b, where b is the jet nozzle height, were considered at a Reynolds number of 30,700 based on the jet nozzle height and jet velocity. Noticeable differences in the flow profiles were observed at the jet outlet, but by 10b downstream these differences became insignificant. Different lip thicknesses resulted in different maximum velocity decay rates. The spread of the wall jet was found to be insensitive to the lip thickness.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(5):051107-051107-13. doi:10.1115/1.4041600.

Guide vanes (GVs) improve the performance of a turbine in terms of efficiency, torque, or operating range. In this work, a concept of different orientations of GVs in between a two-row biplane wells turbine (BWT) was introduced and analyzed for the performance improvement. The fluid flow was simulated numerically with a commercial software ANSYS CFX 16.1. The Reynolds-averaged Navier–Stokes equations with the k-ω turbulence closure model were solved for different designs and flow conditions. For the base model, the results from simulation and experiments are in close agreement. Among the designs considered, the configuration, where the blades are in one line (zero circumferential angle between blades of two plane) and the midplane guide vane has concave side to the leading edge of the blade, performed relatively better. However, the performance was still less compared to the base model. The reason behind the reduction in performance from the base model is attributed to the blockage of flow and the change of flow path occurring due to the presence of the midplane GVs. The flow analysis of different cases and the comparison with the base model are presented in the current study.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(5):051108-051108-16. doi:10.1115/1.4041614.

Porous disks are commonly encountered in experimental studies dealing with flow through objects such as wind turbines, parachutes, and fluidic devices to regulate pressure and/or downstream turbulence. Perforations are typically staggered and only porosity is altered to attain the required disk drag coefficient, despite a documented influence of topology. Few works have reported, however, to which extent the spatial distribution of the circular perforations affect the mean flow pertaining freestanding disks, and for this reason, this work presents a first, more systematic study focused on the effect of azimuthally varying hole topology and porosity on disk drag and near-wake characteristics. An experimental study performed in airflows of negligible freestream turbulence at Reynolds numbers in the order of 105 is reported and related to the existing literature to ensure reliability. Complementary to drag measurements, near-wake surveys have been performed on a variety of perforation layouts using two-component laser Doppler velocimetry and two-component particle image velocimetry. It is shown that minor changes in perforations can cause drastic changes in near-wake flow topology and no perforation layout can be consistently associated with highest drag. Explicit empirical expressions for drag coefficient linked with the simplified topologies considered have been derived.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2018;141(5):051201-051201-8. doi:10.1115/1.4041232.

Determination of friction factor (f) in pipe flow is necessary for various applications dealing with fluid flow. The Colebrook–White equation is the most accepted technique for the f-values estimation in turbulent flow. The biggest problem with this equation is that it can only be solved using numerical iteration methods. This paper contributes two new formulas based on the Colebrook–White equation to calculate f for the turbulent flow regime. To determine the new correlations, several equations were first suggested and then their coefficients were determined using the curve fitting method. Thereafter, based on various statistical error calculations, two equations with the highest accuracies were selected for the further modification. The advantages of the proposed correlations are that they are explicit in f so they do not need any iteration to compute friction factor and the results of calculating f-values reveal that the two new equations are of maximum absolute percent errors (APE) of 0.91% and 3.49% over the entire applicability range of Colebrook–White equation.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(5):051202-051202-9. doi:10.1115/1.4041392.

This paper deals with the dynamic stability of a flexible liquid-filled rotor. On the basis of three-dimensional flow, the fluid perturbation motion is analyzed and the fluid–structure interaction equation is established, combining with continuity equation, the expression of fluid force exerted on rotor is derived in terms of Fourier series expansion. Considering the complex nonlinear relationship between fluid dynamic pressure and the rotor deformation function, they are expanded in terms of the eigenfunction of a dry rotor. The whirling frequency equation of a flexible rotor partially filled with liquid is obtained based on the rotor static equilibrium equation. Finally, the numerical technique is used to analyze the dynamic stability of the rotor system, and the influences of system parameters on unstable region are discussed.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(5):051203-051203-8. doi:10.1115/1.4041561.

Head and flow rate are the important parameters for proper selection of centrifugal pump. However, the reversed operation of centrifugal pump leads to the off-design characteristic of head and flow rate. This paper presents an analytical model developed by using the system curves and velocity relations derived from pump application. Also, the differential technique is applied to the analytical model to develop the off-design characteristics of head ratio and flow rate ratio relations. The off-design characteristic relations were compared with literature and available conversion methods. Then, the analytical model coefficient (AMC) with the range between −4 and +4 was developed from the off-design characteristics of head ratio and flow rate ratio relations. The AMC value was equal to 1 when the pump operates in turbine mode and pump mode at the pump best efficiency point (BEP) and extended to either side up to ±4 when tested with literature data. Therefore, the analytical model consists of the off-design head and flow rate characteristics, when simplified leading to the AMC that could be applied to select the possible boundary limits of head and flow rate for different pumps.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2018;141(5):051301-051301-12. doi:10.1115/1.4041565.

In this paper, the effect of water, air, and their combined injection from two different injection points is studied in order to reduce vorticity effects in a draft tube of prototype turbine working at three operating points. The flow from spiral case to the end of draft tube is simulated using the shear stress transport k–ω turbulence and two-phase models. Using an appropriate validation method, acceptable results were obtained under the noninjection condition. To determine suitable number of points and inlet flow rate for air injection as well as the appropriate nozzle diameter for air and water injection, a new method which considers the ratio of total loss to the pressure recovery factor is used, in addition to using the traditional method which calculates the total loss in the draft tube. Comparing results of the three types of injections shows air injection in the operating range greater than 70% of turbine design flow rate, is much more effective than water injection or the combination of air and water injection. However, in the operating range below 70%, either water or air injections are not suitable, but combination of these two fluids can improve system performance.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(5):051302-051302-12. doi:10.1115/1.4041667.

Stratified gas–liquid flow is a flow regime typically encountered in multiphase pipelines. The understanding and modeling of this regime is of engineering importance especially for the oil and gas industry. In this work, simulations have been conducted for stratified air–water flow in pipes. We solved the Reynolds-averaged Navier–Stokes (RANS) equations with the volume of fluid (VOF) method. The aim of this work was to evaluate the performance of the k–ω shear stress transport (SST) turbulence model with and without damping of the turbulence at the gas–liquid interface. Simulation results were compared with some of the latest experimental results found in the literature. A comparison between the simulated velocity and kinetic energy profiles and the experimental results obtained with the particle image velocimetry (PIV) technique was conducted. The characteristics of the interfacial waves were also extracted and compared with the experiments. It is shown that a proper damping of the turbulence close to the interface is needed to obtain agreement with the experimental pressure drop and liquid hold-up. In its current form, however, RANS with the k–ω turbulence model is still not able to give an accurate prediction of the velocity profiles and of the interface waves.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Fluids Eng. 2018;141(5):054501-054501-6. doi:10.1115/1.4041562.

We investigate the role of eddy viscosity variation and the effect of zonal enforcement of the mass flow rate on the log-layer mismatch problem observed in turbulent channel flows. An analysis of the mean momentum balance shows that it lacks a degree-of-freedom (DOF) when eddy viscosity is large, and the mean velocity conforms to an incorrect profile. Zonal enforcement of the target flow rate introduces an additional degree-of-freedom to the mean momentum balance, similar to an external stochastic forcing term, leading to a significant reduction in the log-layer mismatch. We simulate turbulent channel flows at friction Reynolds numbers of 2000 and 5200 on coarse meshes that do not resolve the viscous sublayer. The second-order turbulence statistics agree well with the direct numerical simulation benchmark data when results are normalized by the velocity scale extracted from the filtered velocity field. Zonal enforcement of the flow rate also led to significant improvements in skin friction coefficients.

Commentary by Dr. Valentin Fuster

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