scholarly journals Towards viable flow simulations of small-scale rotors and blade segments

2021 ◽  
pp. 8-8
Author(s):  
Jelena Svorcan ◽  
Aleksandar Kovacevic ◽  
Dragoljub Tanovic ◽  
Mohammad Hasan
Keyword(s):  
Turbulence Modeling ◽  
Turbulence Models ◽  
Operating Conditions ◽  
Flight Mechanics ◽  
Small Scale ◽  
Viscous Effects ◽  
Complex Flow ◽  
Thrust Coefficient ◽  
Spatial And Temporal Scales ◽  
Flow Models

The paper focuses on the possibilities of adequately simulating complex flow fields that appear around small-scale propellers of multicopter aircraft. Such unmanned air vehicles (UAVs) are steadily gaining popularity for their diverse applications (surveillance, communication, deliveries, etc.) and the need for a viable (i.e. usable, satisfactory, practical) computational tool is also surging. From an engineering standpoint, it is important to obtain sufficiently accurate predictions of flow field variables in a reasonable amount of time so that the design process can be fast and efficient, in particular the subsequent structural and flight mechanics analyses. That is why more or less standard fluid flow models, e.g. Reynolds-averaged Navier-Stokes (RANS) equations solved by the finite volume method (FVM), are constantly being employed and validated. On the other hand, special attention must be given to various flow peculiarities occurring around the blade segments shaped like airfoils since these flows are characterized by small chords (length-scales), low speeds and, therefore, low Reynolds numbers (Re) and pronounced viscous effects. The investigated low-Re flows include both transitional and turbulent zones, laminar separation bubbles (LSBs), flow separation, as well as rotating wakes, which require somewhat specific approaches to flow modeling (advanced turbulence models, fine spatial and temporal scales, etc). Here, the conducted computations (around stationary blade segments as well as rotating rotors), closed by different turbulence models, are presented and explained. Various qualitative and quantitative results are provided, compared and discussed. The main possibilities and obstacles of each computational approach are mentioned. Where possible, numerical results are validated against experimental data. The correspondence between the two sets of results can be considered satisfactory (relative differences for the thrust coefficient amount to 15%, while they are even lower for the torque coefficient). It can be concluded that the choice of turbulence modeling (and/or resolving) greatly affects the final output, even in design operating conditions (at medium angles-of-attack where laminar, attached flow dominates). Distinctive flow phenomena still exist, and in order to be adequately simulated, a comprehensive modeling approach should be adopted.

A Critical Evaluation of Turbulence Modeling in a Model Combustor

2012 ◽  
Author(s):  
Leiyong Jiang
Keyword(s):  
Turbulence Modeling ◽  
Critical Evaluation ◽  
Turbulence Models ◽  
Reynolds Stress Model ◽  
Operating Conditions ◽  
Moment Closure ◽  
Mean Velocity ◽  
Second Moment Closure ◽  
Heat Transfers ◽  
Air Diffusion

Based on the previous benchmark studies on combustion, scalar transfer and radiation models, a critical evaluation of turbulence models in a propane-air diffusion flame combustor with interior and exterior conjugate heat transfers has been performed. Results obtained from six turbulence models are presented and compared in detail with a comprehensive database obtained from a series of experimental measurements. It is found that the Reynolds stress model (RSM), a second moment closure, is superior over the five popular eddy-viscosity two-equation models. Although the main flow patterns are captured by all six turbulence models, only the RSM is able to successfully predict the lengths of both recirculation zones and give fairly accurate predictions for mean velocity, temperature, CO2 and CO mole fractions, as well as turbulence kinetic energy in the combustor chamber. In addition, the realizable k-ε (Rk-ε) model illustrates better performance than four other two-equation models and can provide comparable results to those from the RSM for the configuration and operating conditions considered in the present study.

scholarly journals A Formulation of the Thrust Coefficient for Representing Finite-Sized Farms of Tidal Energy Converters

Energies ◽  
2019 ◽  
Vol 12 (20) ◽  
pp. 3861
Author(s):  
Karina Soto-Rivas ◽  
David Richter ◽  
Cristian Escauriaza
Keyword(s):  
Ocean Circulation ◽  
Spatial Organization ◽  
Local Environment ◽  
Tidal Energy ◽  
Tidal Channels ◽  
Complex Flow ◽  
Thrust Coefficient ◽  
Spatial And Temporal Scales ◽  
Flow Phenomena ◽  
Streamwise Distance

Tidal energy converter (TEC) arrays in tidal channels generate complex flow phenomena due to interactions with the local environment and among devices. Models with different resolutions are thus employed to study flows past TEC farms, which consider multiple spatial and temporal scales. Simulations over tidal cycles use mesoscale ocean circulation models, incorporating a thrust coefficient to model the momentum sink that represents the effects of the array. In this work, we propose an expression for a thrust coefficient to represent finite-sized farms of TEC turbines at larger scales, C t F a r m , which depends on the spatial organization of the devices. We use a coherent-structure resolving turbulence model coupled with the actuator disk approach to simulate staggered turbine configurations in more detail, varying the separation among devices and the ratios between the channel depths and hub heights. Based on these simulations, we calculate the resultant force for various subsets of devices within the farm, and their corresponding effective thrust coefficient, C t F a r m . We conclude that the thrust coefficient depends solely on the lateral separation of the devices, S y , for farms with only two rows. For farms with more than two rows, the streamwise distance, S x , must be considered as well. With the proposed expression, it is possible to calculate efficiently the effects of finite-sized TEC farms and incorporate a momentum sink into ocean circulation models, without assuming a constant coefficient derived from an infinite farm approximation.

Turbulent Scalar Flux Models for the Mixing of Hot Gases in the Swirling Flow of High-Temperature Reactor Lower Plenums

2008 ◽  
Author(s):  
Dominik von Lavante ◽  
Eckart Laurien
Keyword(s):  
High Temperature ◽  
Reynolds Stress ◽  
Turbulence Modeling ◽  
Swirling Flow ◽  
Turbulence Models ◽  
Operating Conditions ◽  
Vortical Flow ◽  
Mixing Efficiency ◽  
Scalar Flux ◽  
Flux Transport

With recent progress in high-temperature pebble-bed reactor programs research focus has started to include more ancillary engineering issues. One very important aspect for the realisability is the mixing of hot and colder helium in the reactor lower plenum. Under nominal operating conditions, depending on core design, the temperature of hot gas leaving the core can locally differ up to 210° C. Due to material limitations, these temperature differences have to be reduced to at least ±15° C. Several reduced-size air experiments have been performed on this problem, but their applicability to modern commercially sized reactors is not certain. With the rise in computing power CFD simulations can be performed in addition, but advanced turbulence modeling is necessary due to the highly swirling and turbulent nature of this flow. The presented work uses the geometry of the German HTR-Modul which consists of an annular mixing channel and radially arranged ribs. Using the commercial CFD code ANSYS CFX, we have made detailed analyses of the complex 3D vortical flow phenomena within this geometry. Several momentum transport turbulence models, e.g. the classical k-e model, advanced two-equation models and Reynolds-Stress Models were compared with respect to their accuracy for this particular flow. In addition, the full set of turbulent scalar flux transport equations was implemented for modeling the three components of turbulent transport of enthalpy seperately and were compared with the standard turbulent Prandtl number approach. As expected from previous work in related fields of turbulence modeling, the differences in predicting the mixing performance between models were significant. Only the full Reynolds-Stress model coupled with the scalar flux equations was able to reproduce the experimentally observed reduction of mixing efficiency with increasing Reynolds number. The correct scaling of mixing efficiencies demonstrates that the utilized turbulence models are able to reproduce the physics of the underlying flow. Hence they could be employed for the scaling and optimization of the lower plenum geometry. The results also showed that the original geometry used for the HTR-Modul is insufficient to provide adequate mixing, and that hence a not sufficiently mixed coolant for future reactor designs might be an issue. Based on this work, an optimization for future lower plenum geometries has become feasible.

Numerical Investigation of a Banki Turbine in Transient State: Reaction Ratio Determination

2012 ◽  
Author(s):  
Sergio D. Croquer ◽  
Jesus de Andrade ◽  
Jorge Clarembaux ◽  
Freddy Jeanty ◽  
Miguel Asuaje
Keyword(s):  
Numerical Models ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Turbulence Models ◽  
Transient State ◽  
Major Influence ◽  
Small Scale ◽  
Complex Flow ◽  
Impulse Turbine ◽  
Reaction Ratio

Cross-Flow Turbines (CFT) also known as Banki Turbines, are often considered for small scale hydroelectric generation. They are known for their simple construction, maintenance and operation, which means they incur in lower CAPEX and OPEX when compared to other types of turbines. However, they also tend to have a modest efficiency (82% [1–3]), hence they are not considered for big scale operations. Little is known about the flow characteristics inside the runner of the CFT. The objective of this investigation is to better understand the flow inside CFTs using Computational Fluid Dynamics (CFD) tools. Steady and Transient State simulations were performed for a CFT at an specific speed NS = 45. SST and κ–ε turbulence models were compared in terms of simulation requirements and obtained results. A proposed runner-nozzle interface, considering real CFT existent gap between these two components (free space) was evaluated as well. Results were compared to available experimental data. Maximum, numerically calculated efficiency deviation from reported experimental global efficiency was 15%. Pressure and velocity profiles along nozzle outlet, energy transfer stages location and CFT reaction ratio values were addressed. Results were compared in terms of runner-nozzle interface (gap vs no-gap), turbulence model (SST vs κ–ε) and calculation regime (steady vs transient regime). Only calculation state (steady vs transient) was found to have major influence over results. Transient state calculations better representing complex flow inside the CFT. Obtained degrees of reaction (no runner-nozzle gap, SST, transient state) were 0.12 and 0.08, for 1st and 2nd stages respectively. Hence the CFT is defined, according to this numerical models, as an impulse turbine.

Turbulence Modeling for Secondary Flow Prediction in a Turbine Cascade

1992 ◽  
Vol 114 (3) ◽  
pp. 590-598 ◽  
Author(s):  
J. G. E. Cleak ◽  
D. G. Gregory-Smith
Keyword(s):  
Secondary Flow ◽  
Turbulence Modeling ◽  
Turbulence Models ◽  
Equation Model ◽  
Turbulent Shear ◽  
Shear Stresses ◽  
Mixing Length ◽  
Turbine Cascade ◽  
Mean Flow ◽  
Complex Flow

Predictions of secondary flow in an axial turbine cascade have been made using three different turbulence models: mixing length, a one-equation model and a k–ε mixing length hybrid model. The results are compared with results from detailed measurements, not only by looking at mean flow velocities and total pressure loss, but also by assessing how well turbulence quantities are predicted. It is found that the turbulence model can have a big influence on the mean flow results, with the mixing length model giving generally the best mean flow. None of the models give good predictions of the turbulent shear stresses in the vortex region, although the k–ε model gives quite good turbulent kinetic energy values. The one-equation model is the only one to contain a transition criterion. The importance of such a criterion is illustrated, but the present one needs development to give reliable predictions in the complex flow within a blade passage.

scholarly journals Application of different turbulence models for improving construction of small-scale boiler fired by solid fuel

2017 ◽  
Vol 21 (suppl. 3) ◽  
pp. 809-823
Author(s):  
Nebojsa Manic ◽  
Vladimir Jovanovic ◽  
Dragoslava Stojiljkovic ◽  
Zagorka Brat
Keyword(s):  
Turbulence Model ◽  
Solid Fuel ◽  
Turbulence Modeling ◽  
Turbulence Models ◽  
Experimental Tests ◽  
Computer Hardware ◽  
Small Scale ◽  
Heat Output ◽  
Ansys Fluent ◽  
Model Fluid

Due to the rapid progress in computer hardware and software, CFD became a powerful and effective tool for implementation turbulence modeling in defined combustion mathematical models in the complex boiler geometries. In this paper the commercial CFD package, ANSYS FLUENT was used to model fluid flow through the boiler, in order to define velocity field and predict pressure drop. Mathematical modeling was carried out with application of Standard, RNG, and Realizable k-? turbulence model using the constants presented in literature. Three boilers geometry were examined with application of three different turbulence models with variants, which means consideration of 7 turbulence model arrangements in FLUENT. The obtained model results are presented and compared with data collected from experimental tests. All experimental tests were performed according to procedures defined in the standard SRPS EN 303-5 and obtained results are presented in this paper for all three examined geometries. This approach was used for improving construction of boiler fired by solid fuel with heat output up to 35 kW and for selection of the most convenient construction.

Evaluation of Turbulence Modeling Approaches for Cross-Flow in a Helical Tube Bundle

2018 ◽  
Author(s):  
Adam R. Kraus ◽  
Haomin Yuan ◽  
Elia Merzari
Keyword(s):  
Pressure Drop ◽  
Turbulence Modeling ◽  
Tube Bundle ◽  
Cross Flow ◽  
Turbulence Models ◽  
Computational Time ◽  
High Fidelity ◽  
Complex Flow ◽  
Helical Tube ◽  
Flow Phenomena

Helical steam generators are proposed for use in a number of advanced nuclear reactor designs. The cross-flow around the helical tubes is a complex flow-field, and accurate knowledge of this flow is necessary for estimating pressure drop, heat transfer, and risk of flow-induced vibration. However, legacy data for helical tube cross-flow are scarce, and building new large-scale experiments that investigate relevant phenomena can be costly. Thus large uncertainties must currently be taken into account in the design of these systems. Numerical modeling with CFD can provide improved insight into the flow phenomena to reduce this uncertainty, but choosing a methodology can prove difficult. LES methods provide high-fidelity data, but require immense computational time to perform even an investigatory calculation for a moderate-sized sector, let alone for many design iterations. URANS methods offer significantly lower computational time, but it can be difficult to confidently justify the accuracy of a particular model without validation, particularly given the highly three-dimensional and complex flow-field present here. To better establish a basis for URANS turbulence modeling, an LES simulation was performed using Nek5000, a massively-parallel spectral element code developed at Argonne National Laboratory, for the geometry of a legacy helical tube bundle experiment. Data from this high-fidelity LES simulation were compared with URANS simulations using a number of turbulence models with the commercial code STAR-CCM+. Turbulent kinetic energy in the flow channels as well as bundle pressure drop were compared. The values of these key parameters were found to vary significantly between different turbulence models, with some models predicting pressure drops and kinetic energies well below those seen in LES. Some models were identified that showed good potential for predicting helical tube bundle flow phenomena. Further work, at a wider range of flow velocities, will be useful to further solidify the range of applicability of these models.

scholarly journals Turbulence Modeling in Side-Entry Stirred Tank Mixing Time Determination

2021 ◽  
Vol 333 ◽  
pp. 02003
Author(s):  
Suci Madhania ◽  
Ni’am Nisbatul Fathonah ◽  
Kusdianto ◽  
Tantular Nurtono ◽  
Sugeng Winardi
Keyword(s):  
Turbulence Model ◽  
Turbulence Modeling ◽  
Mixing Time ◽  
Paper Industry ◽  
Turbulence Models ◽  
Computational Prediction ◽  
Operating Conditions ◽  
Stirred Tank ◽  
Critical Parameters ◽  
Working Fluid

Mixing is one of the critical processes in the industry. The stirred tank is one of the operating units commonly used in the mixing process. Several factors greatly influence the efficiency of the stirred tank, including the stirred-tank design, operating conditions, and working fluid properties. The side-entry stirred tank is widely applied in industry, among others; the processing of crude oil in the refinery industry, water-molasses mixing in the bioethanol industry, pulp stock chest in the pulp and paper industry, and anaerobic digester for biogas reactors. Mixing time is one of the critical parameters used in the design of the stirred tank. This research will model mixing time in a flat bottomed-cylindrical side-entry stirred tank with dimensions D = 40 cm and T = 40 cm using CFD ANSYS 18.2 by applying the Standard κ − ε (SKE) and Realizable κ − ε (RKE) turbulence models. The stirrer used is a three-blade marine propeller d = 4 cm which is an axial type impeller. The phenomenon of mixing in the side-entry stirred tank, qualitatively described through computational prediction results in the form of flow profiles and tracer density change contours locally. Moreover, quantitatively indicated by mixing time validated using experimental data carried out by the conductometry method. The computational prediction shows that the mixing time modeled using the SKE turbulence model shows a similarity level of 68.16%, while the RKE turbulence model shows 31.94%.

Numerical Investigation of Flow Around a Multi-Element Airfoil with Hybrid RANS-LES Approaches Based on SST Model

2017 ◽  
Vol 34 (2) ◽  
pp. 123-134 ◽  
Author(s):  
L. Zhang ◽  
J. Li ◽  
Y. F. Mou ◽  
H. Zhang ◽  
W. B. Shi ◽  
...  
Keyword(s):  
Turbulence Modeling ◽  
Experimental Result ◽  
Numerical Dissipation ◽  
Small Scale ◽  
Detached Eddy Simulation ◽  
Complex Flow ◽  
Third Order ◽  
Mean Velocity ◽  
Eddy Simulation ◽  
Delayed Detached Eddy Simulation

AbstractAccurate prediction of the flow around multi-element airfoil is a prerequisite for improving aerodynamic performance, but its complex flow features impose high demands on turbulence modeling. In this work, delayed detached-eddy-simulation (DDES) and zonal detached-eddy-simulation (ZDES) was applied to simulate the flow past a three-element airfoil. To investigate the effects of numerical dissipation of spatial schemes, the third-order MUSCL and the fifth-order interpolation based on modified Roe scheme were implemented. From the comparisons between the calculations and the available experimental result, third-order MUSCL-Roe can provide satisfactory mean velocity profiles, but the excessive dissipation suppresses the velocity fluctuations level and eliminates the small-scale structures; DDES cannot reproduce the separation near the trailing edge of the flap which lead to the discrepancy in mean pressure coefficients, while ZDES result has better tally with the experiment.

A RANS Based Method for the Estimation of Ship Roll Damping With Forward Speed

2004 ◽  
Author(s):  
Kumar B. Salui ◽  
Vladimir Shigunov ◽  
Dracos Vassalos
Keyword(s):  
High Speed ◽  
Turbulence Modeling ◽  
Finite Difference Methods ◽  
Computer Hardware ◽  
Navier Stokes ◽  
Roll Motion ◽  
Ship Motions ◽  
Viscous Effects ◽  
Flow Models ◽  
Ship Roll

For the prediction of ship roll motion, viscous effects must be taken into account. Several methods, experimental and theoretical, have previously been used to calculate hydrodynamic forces in roll motion. Theoretical methods applied so far to this problem have been based mainly on potential flow models, which cannot account for viscous effects adequately or need pre-defined flow separation like vortex methods. Recent development of computer hardware enables application of methods based on flow field discretisation such as finite-difference methods to solution of problems of practical ship design such as ship motions and control. In the present study, a Reynolds-Averaged Navier-Stokes solver is used to calculate hydrodynamic loads during forced roll motion at different Froude numbers. The solution method adopted is based on unstructured finite-volume discretisation with collocated arrangement of flow variables. A pressure-correction algorithm (SIMPLE) is used for the pressure-velocity coupling. A standard k–ε model is used for the turbulence modeling. An advanced differencing scheme called high-resolution interface capturing (HRIC) is used for accurate resolution of the free surface in the scope of a multiphase-type description. A high-speed hard chine vessel with and without skeg is studied. Close agreement is found between the present calculations and experimental results.

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