Application of a Hybrid RANS-LES CFD Methodology to Primary Atomization in a Coaxial Injector

Non-zonal hybrid RANS-LES models, i.e. those which do not rely on user-prescribed zones for activating RANS or LES, have shown promise in accurately resolving the energy-containing and highly anisotropic large-scale motions in complex separated flows. In particular, the recently proposed dynamic hybrid RANS-LES (DHRL) approach, a method which relies on the continuity of turbulence production through the RANS-to-LES transition zone, has been validated for several different compressible and incompressible single phase flow problems and has been found to be accurate and relatively insensitive to mesh resolution. Time-averaged source terms are used to augment the momentum balance. An added benefit of the DHRL is the ability to directly couple any combination of RANS and LES models into a hybrid model without any change to numerical treatment of the transition region. In this study, an attempt is made to extend the application of this model to multiphase flows using two open literature coaxial two-stream injectors involving non-Newtonian liquids. For the first time, the new model has been successfully implemented in a multiphase framework, combining the SST RANS model with MILES LES approach. Favre averaging is used to ensure consistency between the momentum equations and the density fluctuations. It was found that the momentum source terms must be density weighted in order to ensure stability of the solution. Primary atomization findings with a stable model are encouraging. The spray character with the new model was somewhere between that of a RANS model and the LES result. Droplet sizes, which are indicative of the shear layer energy, for the RANS model were greater than the hybrid results, which were comparable to the LES result and matched the experimental expectation. Additionally, the new approach showed a liquid core breakup length close to that expected from the literature.

The Evolution of Streamwise Counter-Rotating Vortices in Flat Plate Boundary Layer With Leading Edge Pattern

The evolution of streamwise counter-rotating vortices induced by different leading edge patterns is investigated quantitatively using hot-wire anemometer. A notched and triangular leading edge with the same wavelength and amplitude were designed to induce streamwise vortices over a flat plate at Reynolds number (based on the wavelength of the leading edge patterns) of 3080 corresponding to free-stream velocity of 3 m/s. The streamwise velocity at different streamwise locations collected and analyzed using a single wire probe hot-wire anemometer showed reveal different characteristics of boundary layer flow due to the presence of these two leading edge patterns. The major difference is the appearance of an additional streamwise vortex between the troughs of the notched pattern. Such vortices increase the mixing effect in the boundary layer as well as the velocity profile.

On the Scaling of Bubble Cluster Collapse in Cloud Cavitating Flow Around a Slender Projectile

The scaling law of bubble cluster collapse in cloud cavitating flow around a slender projectile is investigated in the present paper. The influence of compressibility is mainly discussed. Firstly the governing parameters are obtained by dimensional analysis, and the numerical method is established in order to verify the similarity law and obtain the influence of parameters based on a mixture approach with Singhal cavitation model. Moreover, the similarity law is validated by numerical simulations. Two main factors of compressibility of mixture fluid, including compressibility of non-condensable gas and phase change, are studied, respectively. Results indicated that the phase change has little influence on both flowing and collapse pressure. In the condition that the variation range of the mixture compressibility is small, the compressibility of non-condensable gas has notable impact the local collapse pressure peaks, however the macroscopic flow pattern does not change.

Dynamic Stall Simulation of Flow Over NACA0012 Airfoil at 1 Million Reynolds Number

The paper is concerned with the prediction and analysis of dynamic stall of flow past a pitching NACA0012 airfoil at 1 million Reynolds number based on the chord length of the airfoil and at reduced frequency of 0.25 in a three dimensional flow field. The turbulence in the flow field is resolved using large eddy simulations with the dynamic Smagorinsky model at the sub grid scale. The development of dynamic stall vortex, shedding and reattachment as predicted by the present study are discussed in detail. This study has shown that the downstroke phase of the pitching motion is strongly three dimensional and is highly complex, whereas the flow is practically two dimensional during the upstroke. The lift coefficient agrees well with the measurements during the upstroke. However, there are differences during the downstroke. The computed lift coefficient undergoes a sharp drop during the start of the downstroke as the convected leading edge vortex moves away from the airfoil surface. This is followed by a recovery of the lift coefficient with the formation of a secondary trailing edge vortex. While these dynamics are clearly reflected in the predicted lift coefficient, the experimental evolution of lift during the downstroke maintains a fairly smooth and monotonic decrease in the lift coefficient with no lift recovery. The simulations also show that the reattachment process of the stalled airfoil is completed before the start of the upstroke in the subsequent cycle due to the high reduced frequency of the pitching cycle.

Numerical and Experimental Study of Turbulent Flows Around Clark Y-14 Aerofoil

Experimental Fluid Dynamics (EFD) and Computational Fluid Dynamics (CFD) have been instrumental in Fluid Mechanics to help solve scientific and engineering problems. This research attempts to use both techniques to perform a parametric study of turbulence flow around airfoil ClarkY-14 at various velocity and angle of attack (AoA). Clark Y-14 airfoil was designed in the 1920’s. It demonstrated good overall performance at low and moderate Reynolds numbers. With the progress in the aviation field, its performance was sub-optimal for newer aircraft designs. However, with the advent of RC airplanes and model aircrafts, there is a renewed interest in this airfoil. Various research projects have been conducted using this airfoil, but there hasn’t been a combined EFD and CFD study of the performance characteristics of the airfoil itself, which still finds real world applications today. One important aspect of this research included the investigation of the effects of a Force Measurement Device/Sensor, which is typically used in scaled/full-size wind tunnels to mount the test model as well as measure the forces/moments acting on it during the testing. The presence of such a device could affect the quality of the data obtained from the wind tunnel testing when compared to a real world application scenario where the aforementioned device may not be present. To the best of the author’s knowledge, no detailed study has been published on the effects of such devices. In this study, the results with and without the measuring device were generated by using CFD simulations. The results were then compared to see to what extent the inclusion of these devices will affect the results. The methodology used for this research was experimental as well as computational. In the present research, a commercially available CFD software STAR-CCM+ was employed to simulate the flows around airfoil Clark Y-14. The experimental data was obtained from wind tunnel tests using AEROLAB Educational Wind Tunnel (EWT) and compared with the simulation data from the CFD. The two data sets were in good agreement. Both experimental and simulation results were used to understand the effects of the measurement device/sensor used in the scaled wind tunnel on the lift and drag coefficients of the airfoil. Two separate CFD simulation setups were designed to model the presence and absence of the measurement device/sensor. These setups replicated the wind tunnel setup. The airfoil was tested and simulated at different speeds as well as different AoA. The comparative study gave a useful insight on the accuracy of the CFD simulations in relation to the actual testing. The analysis of results concluded that the force measurement device/sensor had insignificant effects on the accuracy and quality of data collected through wind tunnel testing.

Effect of Time Dependent Excitation Signals on Gating in Nanofluidic Channels

Nanofluidic field effect devices feature a gate electrode embedded in the nanochannel wall. The gate electrode creates local variation in the electric field allowing active, tunable control of ionic transport. Tunable control over ionic transport through nanofluidic networks is essential for applications including artificial ion channels, ion pumps, ion separation, and biosensing. Using DC excitation at the gate, experiments have demonstrated multiple current states in the nanochannel, including the ability to switch off the measured current; however, experimental evaluation of transient signals at the gate electrode has not been explored. Modeling results have shown ion transport at the nanoscale has known time scales for diffusion, electromigration, and convection. This supports the evidence detailed here that use of a time-dependent signal to create local perturbation in the electric field can be used for systematic manipulation of ionic transport in nanochannels. In this report, sinusoidal waveforms of various frequencies were compared against DC excitation on the gate electrode. The ionic transport was quantified by measuring the current through the nanochannels as a function of applied axial and gate potentials. It was found that time varying signals have a higher degree of modulation than a VRMS matched DC signal.

Computational Study of Multiple Hydrokinetic Turbines: The Effect of Wake

Computational Fluid Dynamics (CFD) simulations have been conducted to investigate the performance of a predetermined propeller-based hydrokinetic turbine design in staggered and non-staggered placements for river applications. Actual turbine models were used instead of low fidelity actuator line or actuator disks for CFD simulations to achieve more reliable results. The k-ω Shear Stress Transport (SST) turbulence model was employed to resolve wall effects on turbine surface and to determine wake interactions behind the turbines. The wake interaction behind the upstream turbine causes significant drop on downstream turbine performance within non-staggered configuration. The upstream turbines in both staggered and non-staggered placement offers the same relative power of 0.96, while the relative power for downstream turbine is 0.98 for staggered installment and 0.16 for inline placement.

Analysis of Turbulence Induced Thermal Mixing Effects on T-Junction Fluid-Structure Degradation

Thermal striping generally is recognized as a significant long-term degradation mechanism in the primary cooling water circuit of nuclear power plants (NPPs). This phenomenon occurs by mixing of hot and cold water streams in the primary coolant loop. Depending on the flow configuration, the turbulent mixing process can lead to thermal striping, temperature fluctuations in the T-junction region, thermal fatigue, and crack generation in the associated structure. The objective of this study is to provide an in-depth look into the underlying physics for thermal fatigue to determine appropriate screening criteria and risk significance for the regulatory safety evaluation process. In addition, the structure of turbulence in the T-junction also is investigated. The computational method comprised of Large Eddy Simulation (LES) modeling to simulate turbulence and Proper Orthogonal Decomposition (POD) analysis to capture the coherent structures and turbulence scales. In addition, Conjugate Heat Transfer (CHT) analyses have been performed to predict the thermal field and temperature distribution in the solid piping material of the T-junction. Finally, the corresponding thermal stress in the solid pipe is estimated based on a simplified one-dimensional model to assess the thermal-structure degradation.

Experimental and Computational Studies of Gravity-Driven Dense Granular Flows

The current paper focusses on the characterization of gravity-driven dry granular flows in cylindrical tubes. With a motive of using dense particulate media as heat transfer fluids (HTF), the study was primarily focused to address the characteristics of flow regimes with a packing fraction of ∼60%. Experiments were conducted to understand the effects of different flow parameters, including: tube radius, tube inclination, tube length and exit diameter. These studies were conducted on two types of spherical particles — glass and ceramic — with mean diameters of 150 μm and 300 μm respectively. The experimental data was correlated with the semi-empirical equation based on Beverloo’s law. In addition, the same flow configuration was studied through three-dimensional computer simulations by implementing the Discrete Element Method for the Lagrangian modelling of particles. A soft-particle formulation was used with Hertz-Mindilin contact models to resolve the interaction forces between particles. The simulation results were used to examine the velocity, shear rate and packing fraction profiles to study the detailed flow dynamics. Curve-fits were developed for the mean velocity profiles which could be used in developing hydrodynamic analogies for granular flows. The current work thus identifies the basic features of gravity driven dense granular flows that could form a basis for defining their rheology.

Using LES to Understand Wake Evolution During the Diurnal Cycle

Numerical study using actuator-line method based Large Eddy Simulation (LES) has been performed to understand the role of atmospheric stability on the wake effects of horizontal-axis full-scale 5-MW wind turbine (WT). The paper will specifically focus on using specific instances in the diurnal cycle corresponding to stable, neutral and unstable ABL state to gain understanding on the transient aerodynamics of a wind turbine throughout the diurnal cycle. Capturing accurate Atmospheric Boundary Layer (ABL) characteristics is key factor in improving the accuracy of WT model predictions as turbulence developed in the ABL has potential to adversely affect the fatigue lifetime and performance of wind turbines. ABL simulations for the diurnal cycle are performed to isolate the key ABL metrics such as surface momentum flux, boundary layer height, surface temperature flux, wind shear, and temperature gradient that influence the wake evolution of WT. Precursor ABL inflow is generated for the WT simulations. The positive heat flux on the surface causes high vertical velocity fluctuations described with streaks and updraft motions during the day while surface cooling rates result in increased shear and strong temperature gradients during the night. The surface temperature, geostrophic wind velocity, heating/cooling rates, and period of the diurnal cycle are varied in different simulations to compare turbulent statistics and the helical vortices of the wind turbine wake. The results have revealed surface temperature and surface flux are the important ABL metrics that have a strong effect on altering the turbulence in the WT wake. In addition, instabilities related to WT blade rotation exhibit sensitivity to ABL metrics. The positive heat flux shows higher mixing and causes large wake movement in the day-time conditions. The results aid in quantifying the movement of the wake at different times of the diurnal cycle. During night-time conditions mixing is low, causing slower wake recovery times. This is the first study to clearly isolate the key ABL metrics that influence the full-scale WT near-wake effects The study has implications in improving the predictions of WT power loss due to wake deficits. Further, this study sets an important direction on future modeling studies in identifying the ABL conditions in a diurnal cycle that influence the WT wake evolution.