Large Eddy Simulation Method Research Articles

Mechanism of low frequency high amplitude pressure fluctuation in a pump-turbine during the load rejection process

ABSTRACT Owing to the complexity of the load rejection process, the accompanied complex low frequency pressure fluctuations and their sources have not been determined. Herein, the load rejection transient process of a pump-turbine was simulated with a three-dimensional (3-D) large eddy simulation method and a dynamic mesh technology. The simulation results were validated against the experimental data. Through the joint time-frequency analysis of simulated pressure, the complex low frequency pressure fluctuation components were captured, which are generally lower than the rated rotational frequency of the runner. A comprehensive formation mechanism of these complex low frequency pressure fluctuation components was attributed to three transient hydraulic phenomena and their interactions: the water hammer in the pump-turbine, local reverse flow near the runner inlet, and 3-D blocking water ring in the vaneless space. These findings facilitate the elucidation and elimination of complex low frequency high amplitude pressure fluctuations during the load rejection transient process.

Critical angle of attack and the corresponding impact cavity for non-circuitous trajectory of water entry of circular cylinder

The critical initial angle of attack that the cylinder can move along the non-circuitous trajectory in water entry is investigated experimentally and numerically. A primary focus is on characterizing the relationship between the initial inclined angle α0 and horizontal velocity u0 in the parameters space of α0∼u0/v0, where u0/v0 is the horizontal velocity u0 to vertical velocity v0 ratio. To build a dense initial parameter space to capture the accurate critical curvature boundary (CCB), which is the critical parameters of the cylinder moving along a non-circuitous trajectory, a substantial water entry experiment is performed for the cylinder with different initial inclined angles and horizontal velocities. In this experiment, a high-speed camera is employed to record the position and the inclined angle of the cylinder. In addition, a large eddy simulation method is used to simulate some particular water entry cases that the cylinder initial parameters are near the CCB space. After the analysis, the initial parameter space for the oblique water entry almost locates in the left circuitous space. Based on the equation of the oblique water entry in the parameter space of α0∼u0/v0, an empirical equation of CCB is established by giving a correction item of deviation angle of attack. When the initial parameters of the cylinder satisfy the equation of CCB, the cylinder first moves along the initial line Lu and then along the offset line Ld. The offset distance doffset between these two lines first increases with α0 and u0/v0 and then gradually tends to a constant about doffset/D = 1, where D is the diameter of the cylinder. Moreover, the simulation results indicate that the pressure distribution on the cylinder surface is affected by the impact velocity, the wettability of the cylinder surface, the initial angle of attack and the distance between the cylinder surface and the pinch-off location.

Large Eddy simulation of ventilated cavitation with an insight on the correlation mechanism between ventilation and vortex evolutions

In this paper, the unsteady natural cavitation and ventilated cavitation are investigated with the large eddy simulation method. The predicted cavity morphology and evolution process before and after ventilation agree well with the available experimental data. The results indicate that in natural cavitation, the formation of a U-type cavity is closely related to the separation of a re-entrant jet and the effect of vortex evolution, while the local high pressure region plays an important role in shaping the appearance of the primary and secondary U-type cavity in the process of moving downstream. During ventilated cavitation, the deflation effect of the air drives the pair of vortices to shed periodically, accompanied by the downstream transportation of vorticity, and promotes the formation of arched air cavities. In the anticlockwise vortex shedding region, the vortex dilatation term has a concentrated distribution on the air-liquid interface which is due to the intense mixing of the phases. Further analysis demonstrates that, in the air cavity fluttering area, the magnitude of the velocity pulsation produces a significant downward tendency due to the gradual weakening of vortex intensity. Moreover, the interaction between vortices and turbulence shows a high degree of consistency in the wake flow.

An Axial and Torsional Percussion Hammer with Considerable Potential to Increase the Drilling Speed of High-Temperature Geothermal Wells

Geothermal energy is a kind of green and sustainable energy and an essential alternative energy in the future. Improving the drilling speed of geothermal wells is helpful for the efficient exploitation of geothermal resources. Axial & Torsional Percussion Hammer (ATPH) was proposed in this paper for drilling enhancement. ATPH can convert the kinetic energy of the drilling fluid to percussion load, and improve the rock-breaking efficiency of the bit. Because of its mechanical seal design, it can operate efficiently at temperatures over 200 °C, which proves that it can be used in geothermal wells. The large eddy simulation (LES) method is chosen to calculate the percussion load characteristics of the ATPH oscillator, including its main frequency and amplitude. In the field drilling experiments of two high-temperature wells, ATPH showed better ROP enhancement ability and longer working life than PDM. In the well interval drilled by ATPH, the maximum temperature at the well bottom reached 202.54 °C.

On the genesis and evolution of barchan dunes: Hydrodynamics

Barchans are an important signature of turbulent atmospheric and aquatic flows on parts of the Earth’s surface where the supply of sediments is scarce. Khosronejad and Sotiropoulos [“On the genesis and evolution of barchan dunes: Morphodynamics,” J. Fluid Mech. 815, 117–148 (2017)] demonstrated that high-fidelity hydro-morphodynamic large-eddy simulation (LES) can replicate sub-aqueous barchan fields exhibiting the geometric features and morphodynamic interactions observed in nature. Herein, we first validate the ability of our LES method to simulate mean flow and turbulence statistics over a frozen barchan dune created in a control laboratory experiment. We then analyze our high-fidelity LES datasets to elucidate the hydrodynamic phenomena that drive the previously identified morphodynamic interactions at various stages of subaqueous barchan dune evolution, from flatbed to quasi-equilibrium. We uncover and quantify the hydrodynamic mechanisms that cause (i) the initiation of crescent microfeatures on the initial flatbed; (ii) the development of three-dimensional barchan dunes; and (iii) several other phenomena that occur during the barchan field maturation, such as the birth of new small barchans and the exposure of the underlying bedrock in the wake of the barchans. Defining a dimensionless timescale, i.e., the saturation timescale, as a function of the saturation length scale, we attempt to categorize for the first time various temporal stages of barchan field development. We show that the barchan field has specific characteristics corresponding to each of the barchan stage developments in terms of both the flow field and the bed topology. Comparing the simulation results of transverse and barchan dunes, we illustrate that the two dune types have similar topographic and hydrodynamics characteristics until bedrock exposure occurs.

Binary Neutron Star Merger Simulations with a Calibrated Turbulence Model

Magnetohydrodynamic (MHD) turbulence in neutron star (NS) merger remnants can impact their evolution and multi-messenger signatures, complicating the interpretation of present and future observations. Due to the high Reynolds numbers and the large computational costs of numerical relativity simulations, resolving all the relevant scales of the turbulence will be impossible for the foreseeable future. Here, we adopt a method to include subgrid-scale turbulence in moderate resolution simulations by extending the large-eddy simulation (LES) method to general relativity (GR). We calibrate our subgrid turbulence model with results from very-high-resolution GRMHD simulations, and we use it to perform NS merger simulations and study the impact of turbulence. We find that turbulence has a quantitative, but not qualitative, impact on the evolution of NS merger remnants, on their gravitational wave signatures, and on the outflows generated in binary NS mergers. Our approach provides a viable path to quantify uncertainties due to turbulence in NS mergers.

Assessment of Parshall flumes for discharge measurement of open-channel flows: A comparative numerical and field case study

This paper presents a series of numerical and field studies to examine the accuracy of two field-scale Parshall flumes, which are employed to measure the flow rate of a wastewater system in the city of Minneapolis, Minnesota. The numerical studies were carried out using the large-eddy simulation (LES) and level-set methods to compute the turbulent flow of sewers under two-phase flow (wastewater and air) conditions. Flow rate measurements in the field were conducted using the dye dilution approach and an in-site flow measuring tool. Using the combination of field measurements and numerical simulations, this research aims at quantifying (i) the margin of error of the filed-scale Parshall flumes and (ii) the water surface fluctuations within the Parshall flumes. The LES turbulent model, coupled with the level-set method, allowed for resolution of instantaneous water surface variations and uncertainty quantification of the flow rate measurements.

Optimal excitation mechanism for combustion enhancement of supersonic shear layers with pulsed jets

Supersonic shear layers (convection Mach number = 0.6) with flow direction excitation are studied by a canonical Large Eddy Simulation (LES) method with quasi-laminar turbulent reaction model. By modulating velocity of central jet through ideal square wave under four excitation periods (τ = 20, 40, 60, and 80 μs), the present study aims to explore the optimal excitation mechanism for combustion enhancement. As the excitation period increases, combustion efficiency tends to increase and then decrease. The maximum combustion efficiency is achieved at τ=40μs, which is nearly four times that in the no excitation scenario. The analysis of mixing characteristics of the nonreacting cases reveals that mixing enhancement mainly explains combustion gain of different excitation modes. From the limiting formation of vortex, the size of the vortex core has fully developed to reach axial contact at τ=40μs. Any further increase in excitation intervals implies a larger vortex spacing, which leads to poor fluid mixing while lower excitation case forms continuous small vortices with less mixing region. The comparison of the circulation decomposition of the main vortex and trailing wakes indicates the pinch-off characteristic of the limiting formation of vortical structure. On the basis of these results, an optimal forcing criterion is proposed according to the limiting vortex formation with dimensionless excitation time scale τ∗≈1.64. This study provides a novel perspective for the optimal combustion enhancement of supersonic shear layers from the aspect of vorticity dynamics.

Methane emissions from a waste treatment site: Numerical analysis of aircraft-based data

Methane (CH4) is an important greenhouse gas. Emissions from landfills and waste treatment sites are regarded as the third largest anthropogenic source in the world. Precise estimates of CH4 emissions depend on a well-designed sampling procedure and detailed analyses. This study estimates CH4 emissions from a waste treatment site by analyzing the data from a flight, which was carried out in Eastern Ontario, Canada, on May 6, 2011. The CH4 dispersion plume at about 140 m above ground level downwind of the source is analyzed with data from 14 flight tracks. The observation period of about 72 min is characterized by unstable boundary layer conditions, strong wind, and shifting wind direction. To replicate the boundary layer turbulence and the CH4 dispersion during the measurement period, a combined large-eddy simulation (LES) and Lagrangian stochastic (LS) particle dispersion modeling method is used. A preliminary simulation run with mean meteorological variables during the experiment's duration is carried out. The results reproduced the shifts in wind direction well, in addition to the dispersion plume, but in a smaller magnitude in comparison to the measured data. Therefore, two sets of LES runs are carried out to represent the boundary layer turbulence before and after the wind shift. Another deficiency in the LES runs is that the simulated turbulent velocity variances are systematically smaller than the observed counterparts. In response to this discrepancy, additional terms are incorporated into the LS dispersion model so as to represent the turbulence dispersion process better. With these treatments, the LES-LS modeling results replicated the dispersion plume both in lateral width and in concentration distribution shape. Then, by using the simulated relationship between source and concentrations/vertical fluxes in space, the CH4 emission rate of the site is estimated as 2393 (kg CH4 hr−1) / 2088 (kg CH4 hr−1) respectively.

Flow separation control in a conical diffuser with a Karman-vortex generator

As a novel passive flow control device, a ring-shaped Karman-vortex generator (KVG) is employed to alleviate the severe flow separation of a conical diffuser with a total divergence angle of 36∘ at a Re=1.0×104. The baseline diffuser and 18 controlled schemes with different KVG sizes and location combinations are studied by an implicit large-eddy simulation method. The flow control mechanisms and design principles are discussed based on current simulations and relevant researches. The diameter of the KVG DK is suggested to be 3–4% of the length of the expansion section. The length to the throat and the distance from the wall are suggested to be 7.5–10.0DK and 2.0–3.0DK, respectively. For the optimal combinations, the alternately shedding Karman vortex street is seldom affected by the wall, and the unsteady wake can directly arrive downstream at the near-wall region and effectively enhance the flow mixing. The results demonstrate that a properly designed KVG can suppress massive flow separation in the expansion section and downstream region and promote pressure recovery performance.

Large Eddy Simulation in the Optimization of Laidback Fan-Shaped Hole Geometry to Enhance Film-Cooling Performance

Large eddy simulation (LES) was applied to optimize the geometry configuration of a laidback fan-shaped hole in order to maximize overall averaged film-cooling performance. The cooling hole is located on a flat plate surface with a 30-degree injection angle with respect to the main flow stream at a constant film density ratio of 2, and a blowing ratio of 2. The computational results achieved by our method were validated by previously gathered experimental data in terms of laterally and time-averaged film-cooling effectiveness. Three geometric parameters, the metering length, forward expansion angle, and lateral expansion angle of the fan-shaped hole were selected as design variables. Thirteen different design cases were selected using the Box-Behnken approach and were numerically modeled. The response surface methodology (RSM) was used to maximize the overall averaged film-cooling effectiveness as an objective function. The film-cooling performance of the numerically optimized cooling hole is significantly improved by 49.55% compared to that of the reference cooling hole. It is confirmed that the numerically optimized cooling hole found using the LES method is in a good agreement with the experimentally optimized cooling hole in terms of optimal shape configuration. In addition, the overall averaged film-cooling effectiveness found by both approaches was very similar. This study demonstrates that the LES approach combined with the RSM optimization algorithm is an effective method for the optimization of laidback fan-shaped cooling holes.

Computational modelling of nasal respiratory flow

CFD has emerged as a promising diagnostic tool for clinical trials, with tremendous potential. However, for real clinical applications to be useful, overall statistical findings from large population samples (e.g., multiple cases and models) are needed. Fully resolved solutions are not a priority, but rather rapid solutions with fast turn-around times are desired. This leads to the issue of what are the minimum modelling criteria for achieving adequate accuracy in respiratory flows for large-scale clinical applications, with a view to rapid turnaround times. This study simulated a highly-resolved solution using the large eddy simulation (LES) method as a reference case for comparison with lower resolution models that included larger time steps and no turbulence modelling. Differences in solutions were quantified by pressure loss, flow resistance, unsteadiness, turbulence intensity, and hysteresis effects from multiple cycles. The results demonstrated that sufficient accuracy could be achieved with lower resolution models if the mean flow was considered. Furthermore, to achieve an established transient result unaffected by the initial start-up quiescent effects, the results need to be taken from at least the second respiration cycle. It was also found that the exhalation phase exhibited strong turbulence. The results are expected to provide guidance for future modelling efforts for clinical and engineering applications requiring large numbers of cases using simplified modelling approaches.

Investigation of airflow field in the upper airway under unsteady respiration pattern using large eddy simulation method

In this paper, the airflow field in the upper airway under unsteady respiration process is predicted using large eddy simulation. The geometrical model is created by combining a popular cast-based mouth–throat model with tracheo-bronchial airways modeled with a trumpet-shaped conduit. The respiration process is simulated by sinusoidal displacing the bottom surface of the geometrical model. Large eddy simulation with dynamic sub-grid scale model is adopted for modeling the turbulent flow via a commercial CFD software, Converge. This study has found that (1) the secondary vortices in the mouth cavity are much more complex considering the lung expansion than setting the quasi-steady inspiration flow at the mouth-inlet; (2) the properties of secondary vortices in the trachea are not evidently different at the same Reynolds number at the accelerating and decelerating inspiration phases; (3) the reversed pharynx jet as well as recirculation zone is much unsteadier at the accelerating expiration phase than decelerating expiration phase for the same Reynolds number. We conclude that the properties of airflow structures are highly impacted by the respiration pattern and more investigations should be conducted, particularly, on the airflow structures during expiration phase for further understanding the properties of flow field.

Investigation of turbulence-induced quadrupole source acoustic characteristics of a three-dimensional hydrofoil

In order to investigate the turbulence-induced acoustic characteristics of hydrofoils, the flow and sound field for a model NH-15-18-1 asymmetric hydrofoil were calculated based on the mixed method of large eddy simulation (LES) with Lighthill analogy theory. Unsteady fluid turbulent stress source around the hydrofoil were selected as the inducements of quadrupole sound. The average velocity along the mainstream direction was calculated for different Reynolds numbers [Formula: see text]. Compared to experimental measurements, good agreement was seen over a range of [Formula: see text]. The results showed that the larger the [Formula: see text], the larger the vortex intensity, the shorter the vortex initial shedding position to the leading edge of the hydrofoil, and the higher the vortex shedding frequency [Formula: see text]. The maximum sound pressure level (SPL) of the hydrofoil was located at the trailing edge and wake of the hydrofoil, which coincided with the velocity curl [Formula: see text] distribution of the flow field. The maximum SPL of the sound field was consistent with the location of the vortex shedding. There were quadratic positive correlations between the total sound pressure level (TSPL) and the maximum value of the vortex intensity [Formula: see text] and velocity curl, which verified that shedding and diffusion of vortices are the fundamental cause of the generation of the quadrupole source noise.

Analysis of the Effect Made by the Variable Heat Capacity on the Characteristics of Pressure Pulsations in a High-Speed Air Intake Using the RANS/ILES Method

The Reynolds-averaged Navier–Stokes (RANS)/Implicit Large Eddy Simulation (ILES) method is applied to analyze the effect of variable heat capacity on the flow characteristics and pressure pulsation spectra for various operation modes of a model high-speed air intake. The dependence of the heat capacity on temperature is described by a fourth-degree polynomial. The calculation method and the technique for calculating the high-order heat capacity on the faces of the computational cells consistent with the technique for calculating flow parameters are briefly outlined. The calculations are performed under the incoming flow temperature corresponding to real flight conditions.

Flow dynamics and heat transfer characteristics analysis for floatation nozzle using large eddy simulation and proper orthogonal decomposition method

Floatation nozzle is a key component for manufacturing high performance substrates in the roll to roll processes because it can support the substrate without contact. The heat transfer and pressure distribution of the floatation nozzle, which is highly associated with the flow dynamics, greatly affect drying/heating/cooling efficiency and operating costs of the nozzle. However, the flow dynamics and heat transfer of the floatation nozzle have not well been understood. In this study, a large eddy simulation model validated against experimental result and proper orthogonal decomposition were employed to study the flow dynamics and heat transfer of floatation nozzle. In addition, the large eddy simulation results of confined slot jet and unconfined slot jet impinging on a vertical surface were provided to get an in-depth understanding of the floatation nozzle. We found that a secondary stagnation region was existed in the floatation nozzle which did not appear in a single slot jet and was quietly different than that of multiple jets. Moreover, the primary and secondary vortices of the floatation nozzle and slot jets were identified by proper orthogonal decomposition method and instantaneous flow field. Furthermore, heat transfer mechanisms in different regions were illustrated, and the reasons of primary and secondary Nusselt number peaks were explained. Finally, it was found that an air cushion is generated for floatation nozzle, which leaded to the floatation ability of the floatation nozzle is about 20 times as much as that of the unconfined slot jet.

Numerical and experimental investigation of the geometrical scale effect on a confined subsonic-supersonic shear layer

The evolution characteristics of confined subsonic-supersonic shear layers have been investigated experimentally employing the schlieren system and numerically using the large eddy simulation (LES) method under different geometrical scales, including the height of combustion chamber, the thickness of splitter plate, and the exit height of nozzle for the supersonic stream. The LES results are in good agreements with the experimental data for the distributions of velocity and turbulent intensity profiles as well as the schlieren image. From the experiments, the “shock train” structures are found in the middle supersonic flow, which would interact with the shear layers, to thicken the thickness locally and generate the “wave” structures in the internal edges of shear layers. Besides, the LES results indicate that the normalized confined height (h/H) would chiefly affect the development of shear layer, and it will grow better for the h/H being less than 0.133. Under the same h/H condition, the higher height of combustion chamber could promote the growth of shear layer. And the thicker splitter plate could primarily intensify the transverse fluctuations of shear layer to enhance the three-dimensional features. Whereas, the height increase of nozzle exit for the supersonic stream has little improving on the development of shear layer.

Numerical investigation of unsteady cloud cavitating flow around the Clark-Y hydrofoil with adaptive mesh refinement using OpenFOAM

In this paper, the cavitating flow around a three-dimension Clark-Y hydrofoil is investigated by the large eddy simulation (LES) method coupled with the volume of fluid (VOF) model in OpenFOAM. The simulated results are compared with the experimental data and a reasonable agreement is obtained. The cavitation-vortex interaction is studied with the vorticity transport equation and much stronger baroclinic torque term is observed. The adaptive mesh refinement (AMR) method is adopted for improving the accuracy and efficiency of numerical simulation. Comparing with the results presented by the mesh with fine resolution, even the coarse resolution mesh, with AMR method, can precisely characterize identical complex cavity structures and flow details. The local high pressure induced by the reverse flow and small residual vortex structures are obtained by the coarse resolution mesh with the AMR method. Further analyzes of the variation trend of total grid numbers using the AMR in one typical cycle show that the rates of the cloud cavitation shedding and collapse are faster than that of the attached cavity growth. This research illustrates that the AMR is a potential approach to accurately model multi-scale cavitating flows and to better understand cavitation mechanism on the premise of limited computational costs.

Numerical simulation of the transient flow in a pump-turbine during load rejection process with special emphasis on hydraulic acoustic effect

In this study, a load rejection process of a pump-turbine was simulated using the large eddy simulation method with and without considering the hydraulic acoustic effect. In the simulation, the hydraulic acoustic effect was considered via a revised density function which is called weak water compressibility model. Analysis showed that numerical results considering water compressibility and without the consideration both agree well with the experimental data except some local regions. Compared with the experiments, the simulation deviations of the rotational speed were less than 5%, and the local maximum deviations of the fluctuating pressure were no more than 30% during the whole load rejection process. However, the simulation considering the water compressibility has a better behavior than that without the consideration in the predictions of pressure fluctuation. And the performance characteristics with the consideration are systematically lower than those without the consideration. In some operating conditions, the performance characteristics with the consideration are even 30%–50% lower than those obtained without the consideration. Additionally, it is noted that the fluctuation of the axial hydraulic thrust on the runner with consideration is more severe than that without the consideration. Hence, the water compressibility has an obvious influence on the simulation of the load rejection of pump-turbines.

Numerical and experimental study on the generation and propagation of negative wave in high-pressure gas pipeline leakage

Negative-wave-based leakage detection and localization technology has been widely used in the pipeline system to diminish leak loss and enhance environmental protection from hazardous leak events. However, the fluid mechanics behind the negative wave method has yet been disclosed. The objective of this paper is to investigate the generation and propagation of negative wave in high-pressure pipeline leakage. A three-dimensional computational fluid dynamic (CFD) study on the negative wave was carried out with large eddy simulation (LES) method. Experimentally validated simulation presented the transient wave generation at the leak onset and the comprehensive wave evolution afterwards. Negative wave was proven to be a kind of rarefaction acoustic waves induced by transient mass loss at the onset of leakage. Diffusion due to the density difference at wave fronts drives the negative wave propagation. Propagation of negative wave can be categorized into three states – semi-spherical wave, wave superposition and plane wave, based on different wave forms. The wave characteristics at different states were elucidated and the attenuation effects were discussed respectively. Finally, a non-dimensional correlation was proposed to predict the negative wave amplitude based on pipeline pressure and leak diameter.