Omega Method Research Articles

Effects of different oil return pipe locations on the vortex characteristics of a cylindrical cyclone separator

The optimization of internal components in the oil-gas separator is crucial for enhancing the performance of the compressor system. In this reported study, the effects of three different oil return pipe locations on the vortex characteristics of a cylindrical cyclone separator are investigated by CFD simulation based on the omega method, and the relationship with the separation performance is analyzed. The vortex deformation and breakup near the oil return pipe are evident, with the degree following the order of C > B > A, which is conducive to reducing pressure loss. Cyclone C, with the inclined return pipe, exhibits the lowest pressure drop. The overall separation efficiency follows the sequence of A > B > C. For oil droplets of 5 ?m and larger, the separation efficiency is essentially the same, exceeding 97.5%. Cyclone A exhibits the best separation effect for oil droplets smaller than 5?m. This study provides some references for optimizing the internals of oil separators.

Cross section modeling using Monte Carlo tally derivatives for transient multiphysics prediction

Monte Carlo neutron transport is often seen as the standard for accurate neutronics simulation of nuclear reactors in steady-state. However, the time dependent equation includes time derivatives of flux and delayed neutron precursors which are difficult to model and tally. In this work, a high-order/low-order approach is adopted that uses the omega method to approximate the time derivatives as frequencies. While this scheme has been previously applied to prescribed transients, thermal feedback is now incorporated to provide a fully self-propagating Monte Carlo transient multiphysics solver which can be applied to transients of several seconds long. Such multiphysics problems are less straight-forward than prescribed transients that can be forced to be axially symmetric or otherwise artificially constrained. This work leverages tally derivatives, which are a Monte Carlo perturbation technique that can identify how a tally will change with respect to a small change in the system, to successfully predict thermal feedback and greatly improve accuracy in an axially non-symmetric, flow-initiated transient. Results show good agreement with a reference solution where very fine outer time steps are used for Monte Carlo calculations.

Effects of unstable flow structures on energy transfer mechanism in a centrifugal pump

To reveal the energy loss mechanism of the centrifugal pump, numerical simulation and experimental investigation are conducted to obtain the complex flow field of a single-stage centrifugal pump under various flow conditions. Particular emphasis is focused on the qualitative and quantitative analysis of the distribution and variation characteristics of irreversible loss in the pump model. The results show that the energy loss in the centrifugal pump mainly originates from the entropy generation caused by the turbulent dissipation and wall friction, which are typically generated in the volute and impeller domains. It is worth noticing that the energy loss in the volute is closely associated with non-uniform velocity distribution and the evolution of the shedding vortices from the impeller exit whilst the energy loss in the impeller are greatly affected by unstable flow phenomena such as flow separation, backflow, and jet-wake pattern. At the overload operating conditions, the wall entropy generation possesses a substantial influence on energy loss, which is mainly related to the wall shear stress. Meanwhile, the influence of the rotor-stator interaction and inflow impacting on the energy loss is enhanced with increasing flows. Finally, the omega method captured the vorticity structures near the tongue at partial flow conditions, thereby, revealing the relationship between the high magnitude of flow loss and the evolution of different scales of strong vorticity sheets.

Application of Vortex Identification Methods in Vertical Slit Fishways

The reproduction and survival of fish are often negatively affected by the construction of dams and other hydroelectric projects, which cut off their migratory routes. Building effective fish passage facilities that allow fish to pass through dams smoothly alleviates the negative impact of hydroelectric projects on the ecological environment, thus protecting the diversity of aquatic species and preventing the extinction of indigenous fish. Vertical slit fishways are highly effective, but turbulence inside the fishway pools directly affects fish passage. In this study, the large-eddy simulation framework is used to capture the vortex characteristics in the interior of vertical slit fishway pools, and the volume of fluid method is applied to simulate the free surface. The independence of the grid is assessed by the large-eddy simulation quality index, and the simulation results are compared with experimental acoustic Doppler velocimetry data. This work characterizes the vortex flow field inside the vertical slit fishway using the Q-criterion, Omega method, and Liutex vortex identification method. The results show that the vortex structure inside the fishway pool has obvious three-dimensional characteristics and vortex structure varies within the different fishway pool chambers. The analysis and comparison of the three different vortex identification methods show that the vortex structure captured by the Liutex method is more consistent with the actual motion pattern of the fishway water flow.

Influence of thermodynamic effects on rotor–stator cavity flow in liquid oxygen turbopump

Thermodynamic effects of the cryogenic medium have not been researched adequately for the accurate solution of the turbopump axial thrust, which is a key technique for the reusable rocket engine. In this paper, a liquid oxygen turbopump was chosen to reveal the influence of thermodynamic effects. Experimental tests using liquid nitrogen were carried out to verify the numerical model, and the numerical results under liquid oxygen were discussed to reveal the thermodynamic effects. The results show that the head coefficients and the efficiencies decrease under all operating conditions due to the alterations of the physical properties caused by the thermodynamic effects of the cryogenic medium. The total axial thrusts decrease in the range of 1.63% to 3.22%, and the maximum variations of the axial thrust acting on the impeller shroud and hub are 2.96% and 2.69%, separately, owing to the divergences of the cavity structure. The entropy generation rate was chosen to analyze the power loss, and the minimum deviation caused by the thermodynamic effects is 5.01% at the normal condition; the distributions of the entropy generation rate in the rotor–stator cavities are obviously changed due to the addition of the thermodynamic effects. The new omega method was selected to compare the vortex distribution. The vortex strength changes slightly, owing to the reduction of the medium viscosity caused by the temperature rise. It is critical to consider the thermodynamic effects of cryogenic media for accurately calculating the axial thrust of a high power-density turbopump.

Numerical and Experimental Analysis of Transient Flow Field and Pressure Pulsations of an Axial-Flow Pump Considering the Pump–Pipeline Interaction

The internal flow in a vertical axial-flow pump is a complex unsteady three-dimensional viscous flow. An unstable flow often produces complex flow phenomena such as flow separation, vortices, and secondary reflux, which reduces the operating efficiency of the pump and can endanger safety and stability. In this paper, computational fluid dynamics is used to calculate the flow characteristics in an axial-flow pump using the shear stress transport and curvature correction (SST-CC) model for turbulence modified to account for the rotational curvature. Furthermore, the dependability of the numerical results was confirmed by a test with an actual model of a pump. The transient deviation angle at the impeller inlet of the pump, the stream field attributes in various spanwise parts of the impeller and guide vane, and the velocity distributions at the impeller inlet and outlet were analyzed. The omega method was utilized to recognize the vortex structure inside the guide vane. Moreover, the development of the transient vortex structure inside the guide vane was studied. As the flow rate increased, the scale and turbulent kinetic energy of the vortex structure gradually decreased. The time-domain graph for the impeller inlet is clearly periodic, with three peaks and three troughs in an impeller rotational period. The dominant frequency in the spectrum at each monitoring point was basically the blade frequency, and the secondary dominant frequency was twice the blade frequency.

Analysis on measurement of hydrogen concentration in air mixture using 3 omega method

Analysis on measurement of hydrogen concentration in air mixture using 3 omega method

Response of black mulberry onto white mulberry rootstock to stenting (cutting-grafting) techniques and IBA concentrations

Abstract Ornamental black mulberry (Morus nigra L.) is used in landscape. Ornamental black mulberry which has high shade, tolerates air pollution and wind. White mulberry (Morus alba L.) is proper for kind of soils conditions. Therefore, this research was conducted to investigate the effects of stenting methods and IBA levels on some characters of black mulberry scions onto white mulberry rootstock for the first time. For this purpose, an experiment was carried out as factorial in completely randomized design with 10 replications. Two factors were including two stenting method (splice and omega) and three levels of Indole-3-butyric acid (0, 500 and 1000 mg L-1). Three months after grafting, percentage of rootstock callus formation, percentage of rooting, percentage of leaf formation, number of leaf formation, the longest shoot and the longest root formation were measured. Results showed that the effect of stenting method was significant on all of evaluated parameters and splice method was superior to omega method. Also, the effects of different levels of IBA were significant on all of measured traits and they are increased by increasing the levels of IBA and the best was with 1000 mg L-1. The interaction effects of these factors are significant on all of measured characteristics except for percentage of rootstock callus formation and rooting percentage. In conclusion, the stenting via splice method had higher success in comparison with omega method and the morphological traits increased by increasing the application of higher levels of IBA. The application of stenting method by splice plus 1000 mg L-1 IBA which was carried out for the first time is recommended for reproduction of ornamental black mulberry onto white mulberry.

Effect of precipitation of silicone oil-based nanofluid on thermal conductivity

This study investigated the effect of the precipitation of nanofluid additives on thermal conductivity in various environments using a bidirectional 3 omega (3ω) method. Silicone oils with viscosities of 50, 500, and 1000 CS were used as the base fluid. Nanoparticles containing aluminium spheres of 100 nm diameter were added to the base fluid at a mass ratio of 0.5%. The experimental set-up was designed and fabricated, and the bidirectional 3ω method was verified by measuring the thermal conductivity of the well-known liquid. Using the nanofluids, thermal conductivities were measured at various temperatures and viscosities. The measurement results are discussed with respect to the elapsed time of precipitation. In addition, the effect of penetration depth on the thermal conductivity is investigated.

Direct Comparison of High-Order/Low-Order Transient Methods on the 2D-LRA Benchmark Problem

Transient simulations of nuclear systems face the computational challenge of resolving both space and time during reactivity changes. A common strategy for tackling this issue is to split the neutron flux into shape and amplitude functions. This split can be solved with high-order/low-order methods. In this paper, a direct comparison of commonly used approximations (e.g., adiabatic, omega, alpha eigenvalue, frequency transform, quasi-static) is performed on the two-dimensional Laboratorium für Reaktorregelung und Anlagensicherung (2D-LRA) benchmark problem using a diffusion solver as the high-order solver and point kinetics as the low-order solver. Additionally, a novel hybrid omega/alpha-eigenvalue solver that incorporates frequencies to model delayed neutrons is introduced. The goal of the comparison is to quantify the performance of each method on a common problem to help inform promising pathways for costly high-fidelity solvers. Overall, we show that exponential frequency approximations are an effective strategy for increasing the accuracy of transient simulations with no added cost. Root-mean-square error of the power distribution at the peak of the transient was consistently decreased by 20% by including frequencies. In particular, the hybrid omega/alpha-eigenvalue method shows improvement over existing eigenvalue solvers as a high-order method. However, in our implementation, the cost of solving for the alpha eigenmode is too costly to recommend over the omega method. While time-differencing schemes are more accurate, we believe the eigenvalue methods are more adaptable to further applications in Monte Carlo transients. Furthermore, they required fewer outer time steps, significantly reducing the computational cost.

Aerosol jet printed 3 omega sensors for thermal conductivity measurement.

The 3 omega (3ω) method is a trusted technique for measuring thermal conductivity-a fundamental material property of critical importance in a broad range of applications. However, traditional 3ω sensor processing requires some form of physical vapor deposition, such as metal evaporation or sputtering. These 3ω sensor deposition techniques limit the materials and sample sizes applicable to the 3ω method. This work demonstrates an aerosol jet printing method to directly printsilver 3ω sensors that yield accurate temperature-dependent measurement up to 300°C on materials with thermal conductivity ranging from 1 to 150 W/m K. The interrelationship between printed sensor geometry, sensor thermal stability, and applicability to the 3ω method is examined. Thermal conductivity measurement with 3ω sensors conventionally sintered at 300°C agrees to independent laser flash measurement within 4% from room temperature to 150°C. An unconventional rapid high-temperature sintering method is shown to produce sensors that agree within 3% of the laser flash measurements from room temperature to 300°C. The rapid sintering profiles also reduced the sensor-substrate thermal boundary resistance of the printed sensors by as much as 88%. The direct printing of 3ω sensors creates opportunities for measurement of thermal transport properties in applications previously inapplicable to the 3ω method.

Relations between shear flow and vortices in the near wake of a high speed train

Two flow cases for high speed train models with different lengths have been numerically computed by performing the improved delayed detached-eddy simulation. Based on the Omega method and turbulence production (TP) distribution, the relations between the shear flow and vortices in the near turbulent wake of a high speed train have been comparatively analyzed. First, in the wake region immediately close to the tail, the boundary layer separation plays significant roles. And the mechanism makes shear deformation prominent in the region with the formed vortices. Moreover, the shear layers are pertinent to the boundary-layer thicknesses and help to the TP distribution. However, the shear-dominated region is very limited due to high dissipation. One the other hand, a vast majority of the vortices captured with the parameter Omega increasing in the downstream region away from the tail. And the TP distributions are stable at different streamwise positions, though obviously decreased. They are greatly attributed to the mean strain rate in the horizontal plane. Meanwhile, the vortical vorticity is thought to be the dominant component inside the vortex cores, although the shear becomes weaker. And thus the turbulence itself can be spatially sustained due to the relatively stable vortex structure. Moreover, the weak shear is believed to depend upon the interaction between the vortices and the ground.

Designing a highly immunogenic multi epitope based subunit vaccine against Bacillus cereus.

ObjectiveSerious non-gastrointestinal-tract infections and food poisoning are caused by Bacillus cereus. Vaccination against B. cereus is very important. The aim of this study was to identify and analyze B and T cell epitopes for chromate transporter protein of the bacteria.MethodsMultiple sequence alignment with the Clustal Omega method was used to identify conserved regions and Geneious Prime was used to produce a consensus sequence. T and B cell epitopes were predicted by various computational tools from the NetCTL and Immune Epitope Database (IEDB), respectively.ResultsAltogether, 6 HTL cells and 11 CTL epitopes were predicted. This vaccine's molecular docking is done with Patch Dock and LigPlot to verify interactions. The immune server (C-IMMSIM) was used to develop In silico immune response in order to assess the multi-epitope vaccine's immunogenic profile.ConclusionWe designed universal vaccine against B. cereus responsible for food poisoning. The disease may be avoided with the aid of the proposed epitope-based vaccine.

Numerical study on the aerodynamics of an experimental wind turbine: Influence of nacelle and tower on the blades and near-wake

The nacelle and tower affect the flow through the rotor and near-wake of horizontal-axis wind turbines. For this reason, aerodynamic models of the full geometry of wind turbines are needed to gain insight to improve turbine designs. In this work, the unsteady flow around an experimental horizontal-axis wind turbine is studied using computational fluid dynamics simulations. The emphasis is put on the effects of the nacelle, tower and rotation of the blades on the induction region and near-wake. The simulations are performed using unsteady Reynolds-Averaged Navier-Stokes equations in conjunction with the shear-stress transport k-ω model. The sliding mesh method is employed to enable the rotation of the blades. The simulations were performed at the design tip speed ratio (λ=6.67) and two off-design conditions (λ = 4.17 and 10) to assess the general validity of the numerical model. The obtained results of global loads, force distribution on the blades and tower, velocity components and angle of attack are presented in detail. It is found that the tower has little effect on the rotor loads (circular pattern), but the blades have an important effect on the tower (three-leaved rose pattern). The force distribution reveals only significant fluctuations on the blades (r/R<0.75) at λ = 4.17, whereas the tower is affected in the zone exposed to the wake behind the rotor for all three wind speeds. Furthermore, the interaction between blades is evaluated in terms of velocity and angle of attack. The correspondence between the results for different rotor blade positions reveals the effect of the ground on the rotor is negligible. The Omega method is used to visualize the behaviour of the tip and root vortices, wake expansion, coherent structures and development of the near-wake. The numerical results presented here contribute to a deeper understanding of the unsteady flow around horizontal-axis wind turbines.

Application of Omega vortex identification method in cavity buffeting noise

The cavity buffeting noise is related to the free shear layer oscillation and the periodic vortex shedding, where weak vortices coexist with strong vortices and the strong shear phenomenon also exists at the opening of the cavity. Therefore, it is of great significance to accurately capture vortices at the opening for the control of the cavity buffeting noise. This paper first compares the Omega vortex identification method with the Q and λ2 criteria based on the large eddy simulation (LES) of the backward-facing step flow, and it is found that the Omega method enjoys the following advantages: it is not sensitive to a moderate threshold change and Ω = 0.52 can be used as a fixed threshold, it can capture both the strong and weak vortices at the same time; and it will not be contaminated by the shear. Then the Omega (Ω) method is applied to the LES of the cavity buffeting noise: the mechanism of the cavity buffeting noise is studied based on a simple cavity model firstly, and then the effects of the incoming boundary layer thicknesses and the incoming boundary layer shapes on the cavity buffeting noise are analyzed. The results show that: the Ω method clearly captures the processes of the vortex generation, development, collision and fragmentation, verifying that the generation of the cavity buffeting noise is related to the free shear layer oscillation and the periodic vortex shedding; as the thickness of the incoming boundary layer increases, the free shear layer becomes more stable and the Helmholtz resonance is avoided effectively, thereby the cavity buffeting noise is reduced effectively, adding a convexity upstream of the cavity opening to interfere the shape of the incoming boundary layer to reduce the acoustic feedback effect can reduce the cavity buffeting noise effectively.

Evaluation of residual creep life of steels using macro-pillar creep testing and Omega method

Residual creep life evaluation is an essential part of Fitness-For-Service (FFS) assessment for engineering components operating at elevated temperatures. This paper elaborates a new evaluation approach based on macro-pillar compression testing (MPCT) and Omega method. The MPCT creep test was performed at the same stress level as the tensile creep test by adjusting the testing load using an algorithm, which is also provided in this paper. The barreling effect of MPCT creep test was corrected by two proposed parameters, which were calibrated using finite element method with 6 types of Cr-Mo alloys from API 579. Finally, a 1.25Cr-0.5Mo-Si steel was prepared for both MPCT and round bar tensile creep tests at 550 ℃ and different initial stress levels. The predicted residual creep life by MPCT has an error of 2.2% at 140 MPa, 6.4% at 160 MPa and 13.5% at 180 MPa.

Thermal conductivity measurement of VO2 nanofluid using bidirectional 3ω method

Nanofluids containing vanadium dioxide (VO2) are used in applications such as actuators, smart windows, and gastrointestinal tracts. Therefore, measuring the thermal conductivity of nanofluids with VO2 characteristics is important in various environmental industries. In this study, the bidirectional 3 omega (3ω) method was used to measure the thermal conductivity of nanofluids. Experimental equipment for measuring VO2 nanofluids in various environments was designed and fabricated. The effectiveness of the bidirectional 3ω equipment was verified by measuring the thermal conductivity of ethylene glycol, which has been extensively reported in the literature. The effects of elapsed time, specimen thickness, and operation temperature on the thermal conductivity are discussed in this paper. In addition, the measuring error was investigated with regard to the precipitation of particles in the suspension.

Study of the vortex structure of a subsonic jet in an axisymmetric transonic nozzle

The jet interaction flow field generated by a subsonic circular jet exhausting into a transonic cross-flow over a convergent–divergent nozzle is investigated using numerical simulations. The simulations use the three-dimensional large eddy simulation and Reynolds-averaged Navier–Stokes equations coupled with the standard k-ε turbulence model. The numerical method is verified via cold-flow and schlieren experiments. The vortex structures are identified via the Liutex–Omega method, and the flow details of various pressure ratios and injection angles are studied. The numerical results capture the main vortex structures of a jet in cross-flow, such as the trailing upper vortex and trailing major vortex. The trailing top vortex, which is difficult to capture, and a new vortex structure, named the longitudinal shear vortex, are both observed when the momentum flux is sufficiently large. This study identifies the longitudinal shear vortex for restricted flow, which to some extent facilitates the mixing of the cross-flow and the jet. The results presented in this paper indicate that the vortex structure distribution of a subsonic jet and transonic cross-flow in the restricted region can be optimized. The main vortex structures are analyzed in detail.

Cavitation vortex dynamics of unsteady sheet/cloud cavitating flows with shock wave using different vortex identification methods

Cavitation is a complex multiphase flow phenomenon with an abrupt transient phase change between the liquid and the vapor, including multiscale vortical motions. The transient cavitation dynamics is closely associated with the evolution of the cavitation vortex structures. The present paper investigates the cavitation vortex dynamics using different vortex identification methods, including the vorticity method, the Q criterion method, the Omega method (Ω), the λ2 method and the Rortex method. The Q criterion is an eigenvalue-based criterion, and in the Omega method, the parameter is normalized, is independent of the threshold value and in most conditions Ω=0.52. The Rortex method is based on an eigenvector-based criterion. Numerical simulations are conducted using the implemented compressible cavitation solver in the open source software OpenFOAM for the sheet/cloud cavitating flows around a NACA66 (mod) hydrofoil fixed at α=6°, σ=1.25 and Re=7.96×105. The flow is characterized by the alternate interactions of the re-entrant flow and the collapse induced shock wave. Results include the vapor structures and the vortex dynamics in the unsteady sheet/cloud cavitating flows, with emphasis on the vortex structures in the cavitation region, the cavity interface, the cavity closure, the cavity wakes, and the foil wakes with the shedding cavity. The comparisons of the various methods, including that the vorticity method, the Q criterion method, the Omega method, the λ2 method and the Rortex method, show the performances of different methods in identifying the cavitation vortex structures. Generally, during the attached cavity growth stage, the Q criteria can well predict the vortex structures in the cavitation region and at the foil trailing edge in the pure liquid region, while with the Omega method and the Rortex method, the vortex structures outside the attached cavity and on the foil pressure side can also be predicted. The λ2 method can well predict the vortex structures in the cavity closure region. During the re-entrant jet development stage, the vortex structures in the re-entrant jet region is weak. During the cavity cloud shedding stage, the vortex dynamics at the foil leading edge covered by newly grown cavity sheet is different from that during the attached cavity sheet growth stage. During the shock wave formation and propagation stage, strong vortex structures with both the size and the strength are observed owing to the cavity cloud shedding and collapse behavior. The influence of the small parameter e in the Omega method on the cavitation vortex identification is discussed.

The visualization of turbulent coherent structure in open channel flow

Due to its multiscale and multi-layer natures, the coherent structures of turbulent in the open channel flow is complex and difficult to be visualized for understanding its evolution. In this paper, five types of methods for the vortical structure in the fluids, namely the Q - criterion, the vorticity, the Omega method, the velocity-vorticity correlation structures (VVCS) as well as the most recent Rortex method, are adopted to visualize the turbulent flow in the open channel. With the free surface modelled as a free slip boundary, a direct numerical simulation (DNS) is carried out to study the multi-layered flow structure characteristics under the free surface. The visualization results by the Q - criterion, the vorticity, the Omega method and the Rortex are firstly analyzed. Then the turbulent flow layers near the free surface are identified with corresponding anisotropy indices. Afterwards, the VVCS within various turbulence layers are visualized accordingly. This research indicates that the VVCS can straightforwardly show the geometry information of the coherent structures of turbulent in different layers for the open channel flow.