Detached Eddy Simulation Method Research Articles

Impact of Underlying RANS Turbulence Models in Zonal Detached Eddy Simulation: Application to a Compressor Rotor

The present study focuses on the impact of the underlying RANS turbulence model in the Zonal Detached Eddy Simulation (ZDES) method when used for secondary flow prediction. This is carried out in light of three issues commonly investigated for hybrid RANS/LES methods (detection and protection of attached boundary layer, emergence, and growth of resolved turbulent fluctuations and accurate prediction of separation front due to progressive adverse pressure gradient). The studied configuration is the first rotor of a high pressure compressor. Three different turbulence modelings (Spalart and Allmaras model (SA), Menter model with (SST) and without (BSL) shear stress correction) are assessed as ZDES underlying turbulence model and also as turbulence model of unsteady RANS simulations. Whatever the underlying turbulence model, the ZDES behaves well with respect to the first two issues as the boundary layers appear effectively shielded and the RANS-to-LES switch is close downstream of trailing edges and separation fronts leading to a quick LES treatment of wakes and shear layers. Both tip leakage and corner flows are strongly influenced by the Navier–Stokes resolution approach (unsteady RANS vs. ZDES) but the underlying turbulence modelling (SA vs. SST vs. BSL) impacts mainly the junction flow near the hub for both approaches. ZDES underlying turbulence model choice appear essential since it leads to quite different corner flow separation topologies and so to inversion of the downstream stagnation pressure radial gradient.

Performance and Aeroacoustic Noise Prediction for an Array of Small-Scale Vertical Axis Wind Turbines

Small-scale vertical axis wind turbines are good candidates for urban area use where, due to various obstacles, the airflow is turbulent. In urban areas, air velocity is mostly low and site specific, so just in a few places, like tall buildings, there are suitable airflow situation in order to use turbine, so it is a good idea to install more than one turbine in such locations. On the other hand, acoustic noise that is emitted from turbines may bother people living in that area. In the current study, power performance of small-scale vertical axis wind turbines in a V-form array configuration is investigated using detached eddy simulation method. Also, Ffowcs Williams and Hawkings acoustic analogy formulation is conducted in order to predict the noise radiation level of turbines. Results showed that power performance of most of the turbines increased due to venturi effect that is made by low speed regions behind turbines. Acoustic noise analysis showed that there is a strong enhancement in sound pressure level in receivers compared with the isolated turbine.

A DDES study of the flow past a prolate spheroid using a high-order U-MUSCL scheme

Flow past a prolate spheroid, which is a representative simplified configuration for vehicles such as maneuvering ships, submarines and missiles, comprises a series of complex flow phenomena including pressure-induced flow separation, which results in unsteady forces and movements that may be detrimental to vehicles’ performance. In this paper, a Delayed Detached Eddy Simulation (DDES) method combined with a new high-order U-MUSCL scheme is proposed to more precisely and accurately capture the flow separation and vortex structure. This method is applied to simulate the aerodynamic performance of the 6:1 prolate spheroid at an AOA of [Formula: see text] with the Reynolds number of [Formula: see text]. Axial pressure distribution of five individual chord wise sections and flow field structure of the aft body are analyzed. Numerical results agree well with the experimental data. It can be concluded that DDES combined with three-order U-MUSCL scheme demonstrates reliable performance since it captures the vortex structure of aft body distinctly and predicts the separation and reattachment points of the secondary vortex precisely.

Influences of boattail structures on aerodynamic characteristics of supersonic spinning projectiles

Previous research studies indicate that the proportion of Magnus force at the spinning projectile tail position is very high. Meanwhile, the large mutations of aerodynamic characteristics are found after adding boattail structures. In order to study the influences of boattail structures on aerodynamics of a spinning projectile, a 6.37-diameter long tangential-ogive-cylinder projectile is selected as the original model. Moreover, several boattail configurations are adopted to investigate the effects of boattail structures on the aerodynamics. Numerical simulations with the use of detached eddy simulation method have been performed to study the modification of boattail structure on the surface pressure of the projectile to expose the effects on the aerodynamic characteristics. The reliability of the numerical method has been validated through comparison with experimental data. The numerical results show that the variations of the wake vortex field due to the boattail structures have important influences on the surface pressure and the aerodynamic characteristics. The results provide important observations which can be valuable for design optimization of supersonic spinning projectile.

Effects of column shape and configuration on the vortex-induced motions of semi-submersibles

The vortex-induced motions (VIM) of offshore floaters occur when the frequency of the vortex shedding is close to the natural frequency, and this phenomenon can severely affect the fatigue life of risers and moorings. During the design and operation stage of the semi-submersible (SS), VIM behavior has emerged as an important issue in offshore engineering. The shape of the columns, circular or square, and the wake interference, characterize the VIM of the SS. Numerical studies using the validated Detached Eddy Simulation (DES) method are carried out for parametric analysis of the VIM performance of various SS models consisting of different column shapes: with four rounded square columns, four circular columns and the mixed square-circular columns. The results show that the most significant transverse responses at 0∘ current incidence are observed for the SS with four circular columns and the one with two circular upstream columns. At a diamond configuration (i.e., 45∘ and 135∘), the SS models with a square upstream column all present the most pronounced VIM. The analysis shows that the vortex shedding characteristics of each column at different current incidence govern the VIM of the SS models. The square and circular columns both do large positive work on the transverse motions, and thus improve the VIM response at a certain current incidence. Therefore, the design of SS with square, circular and multi-shape columns can not mitigate the VIM behaviors efficiently. Designing a SS model with irregular polygon-shaped columns or twisted columns to avoid the strong vortex shedding may work.

An improved delayed detached eddy simulation study of the bogie cavity length effects on the aerodynamic performance of a high-speed train

This paper uses an improved delayed detached eddy simulation method to investigate the unsteady flow features of the high-speed trains with various cavity lengths at Re = 1.85×106. The improved delayed detached eddy simulation results are validated against the experimental data obtained during previous wind tunnel tests. The effects of cavity length on the resistance force, flow structures beneath the high-speed train and in the wake are analyzed. The results show that a longer cavity significantly increases the streamwise velocity level near the rear plates and forms a stronger impinging flow on the rear plates, and thus contributes to a higher value of resistance. Furthermore, a longer cavity decreases the pressure coefficients around the near wake region from the top of the ballast to the tail nose in the vertical direction and thereby increases the pressure drag of the high-speed train. Additionally, a longer bogie cavity is found to increase the longitudinal vortex scales in the near wake region. All these changes on the flow field bring to 5.8% and 11.5% drag increase when the bogie cavities are elongated by 20% and 40%, respectively, of the wheelbase.

Numerical study of scale effect on the wake dynamics of a propeller

In this study, the effects of the scale on the evolution of propeller wake is simulated and analyzed under open water conditions. The simulation is conducted using numerical techniques based on the detached eddy simulation method. The open water performance such as thrust and torque are analyzed for validation, and the instantaneous flow field, vortical structure and the kinetic energy spectra are investigated for the scale effect on the wake dynamics of a propeller. The results indicated that the influence of scale effect on load and wake dynamics of a propeller can differ significantly under different conditions. Under lighter loading condition, the average spanwise circulation at full-scale are considerably larger than those at model-scale at the range of inner radius, and the full-scale propeller exhibits a stronger blade shed vortex. Analysis of vortical structure shows that the transition-to-instability position of vortices vary under different conditions. Power spectral density analysis of the kinetic energy demonstrates that the kinetic energy of full-scale propellers is generally proportionally larger than that of model-scale propellers.

Investigation of flow separation in a centrifugal pump impeller based on improved delayed detached eddy simulation method

The mechanism of flow separation in the impeller of a centrifugal pump with a low specific speed was explored by experimental, numerical, and theoretical methods. A novel delayed Reynolds-averaged Navier–Stokes/large eddy simulation hybrid algorithm combined with a rotation and curvature correction method was developed to calculate the inner flow field of the original pump for the large friction loss in the centrifugal impeller, high adverse pressure gradient, and large blade curvature. Boundary vorticity flux theory was introduced for internal flow diagnosis, and the relative velocity vector near the surface of the blade and the distribution of the dimensionless pressure coefficient was analyzed. The validity of the numerical method was verified, and the location of the backflow area and its flow features were determined. Finally, based on flow diagnosis, the geometric parameters influencing the flow state of the impeller were specifically adjusted to obtain a new design impeller. The results showed that the distribution of the boundary vorticity flux peak values, the skin friction streamline, and near-wall relative velocities improved significantly after the design change. In addition, the flow separation was delayed, the force applied on the blade was improved, the head under the part-load condition was improved, and the hydraulic efficiency was improved over the global flow ranges. It was demonstrated that the delayed Reynolds-averaged Navier–Stokes/large eddy simulation hybrid algorithm was capable to capture the separation flow in a centrifugal pump, and the boundary vorticity flux theory was suitable for the internal flow diagnosis of centrifugal pump.

CFD evaluation and experimental comparison on the flow around fixed multi-column configurations

The flow-induced motion (FIM) of platforms is a vital phenomenon issue to be investigated to help the designers and to decrease the costs related to the mooring and/or riser systems. On this matter, when talking about multi-column configurations, captive tests (also called fixed or stationary tests) can be a methodology worth of examining the vortex detachments, the source for the oscillatory lift, the drag force, and the interference of the wake of the upstream column on the downstream ones. Computational fluid dynamics (CFD) evaluations on the flow around a fixed array of one, three, and four columns with low aspect ratio, $${\text{AR}} = H/L = 1.5$$ (the ratio between the column draft, $$H$$, and the column face dimension, $$L$$) were conducted. Three different shapes of the column sections were tested, namely, circular, square, and diamond. For the multi-column configurations, the spacing ratio tested was $$S/L$$ (where $$S$$ is the distance from center to center of each column). The Reynolds number tested was 100,000. Lift and drag forces were monitored in each column aiming at the integral quantities of the hydrodynamic to compare with the experimental results. Time averages of velocities and vorticity fields were also observed. The calculations were performed with the flow solver StarCCM+; the code solves the multi-phase incompressible Unsteady Reynolds-Averaged Navier–Stokes (URANS) equations. Simulations were carried out with the Improved Delayed Detached Eddy Simulation (IDDES) method. The IDDES estimates the small turbulence scales near the body boundary with the Reynolds-Averaged Navier–Stokes (RANS) model and the large scales with the Large Eddy Simulation (LES) model. For the volume of fluid (VoF), an interface-capturing approach was used. The multi-column arrays presented no regular vortex shedding pattern when compared with the single-column case. The vortex shedding behavior was mainly determined by the column section. For the multi-column configurations, the number of columns, 3 or 4, was not the main issue. CFD calculations compared with the experiments showed good agreement for the circular and square cases and a significant difference for the diamond cases, which need to be further investigated.

Nonlinear Combustion Instability in a Multi-Injector Rocket Engine

A computational study is presented of the nonlinear combustion instability of a multi-injector rocket engine. The study addresses a choked nozzle and a combustion chamber with 10 and 19 coaxial injector ports. Computations using a three-dimensional unsteady shear-stress transport delayed detached-eddy simulation method provide detailed time-resolved information about the combustion instability. The spontaneous longitudinal-mode instability is observed for the 10- and 19-injector geometries with a combustion-chamber diameter of 28 cm. The triggered tangential and longitudinal instability modes are obtained for the 19-injector geometry with a combustion-chamber diameter of 43 cm by pulsing the injector mass flux. It is shown that an oscillating combustion-chamber flow can be triggered to a new mode with a larger disturbance amplitude. The streamwise vorticity created by the instability impacts the heat release in a manner that supports the instability. A preliminary study of injector-size scaling effects is performed by comparison of the 10- and 19-injector combustors.

Numerical Investigation on Unsteady Separation Flow Control in an Axial Compressor Using Detached-Eddy Simulation

Unsteady excitation has proved its effectiveness in separation flow control and has been extensively studied. It is observed that disordered shedding vortices in compressors can be controlled by unsteady excitation, especially when the excitation frequency coincides with the frequency of the shedding vortex. Furthermore, former experimental results indicated that unsteady excitation at other frequencies also had an impact on the structure of shedding vortices. To investigate the impact of excitation frequency on vortex shedding structure, the Detached-Eddy Simulation (DES) method was applied in the simulation of shedding vortex structure under unsteady excitations at different frequencies in an axial compressor. Effectiveness of the DES method was proved by comparison with URANS results. The simulation results showed a good agreement with the former experiment. The numerical results indicated that the separation flow can be partly controlled when the excitation frequency coincided with the unsteady flow inherent frequency. It showed an increase in stage performance under the less-studied separation flow control by excitation at a certain frequency of pressure side shedding vortex. Compared with other frequencies of shedding vortices, the frequency of pressure side shedding vortex was less sensitive to mass-flow variation. Therefore, it has potential for easier application on flow control in industrial compressors.

Experimental and numerical analysis of a novel Darrieus rotor with variable pitch mechanism at low TSR

The Darrieus vertical axis wind-turbine (VAWT) has been the subject of numerous recent studies aimed at improving its aerodynamic performance in order to locate it in urban areas. This article is devoted to the study of an original VAWT with variable-pitch and low tip speed ratio TSR which is favorable to noise reduction and can operate at low velocity wind. The aerodynamic performance of this turbine is studied experimentally in wind tunnel as well as by CFD. In order to obtain the 3D-flow field around the wind turbine rotor, the numerical simulations are performed by means of detached eddy simulation method (DES). The simulation of pitch variation is made possible by using sliding-mesh method. Thus a specially created program adapts the pitch depending on the blade azimuthal position during rotation. The obtained results show that adapted pitch blades are preferable because they permit to obtain a power coefficient Cp that rivals other VAWT in the case of TSR<1. The maximum torque fluctuation during rotation is lower in the case of adapted variable-pitch compared to fixed-pitch and thus the maximum aerodynamic loads on the structure can be reduced. Moreover, the pitch adaptation leads to lower interaction effects between the upstream-blade wake and down-stream blades.

Effect of wall proximity on the flow over a cube and the implications for the noise emitted

The flow over an object such as a cube and the resulting aerodynamic noise are affected by its proximity to a wall. To evaluate the effect of wall proximity on the aerodynamics induced by a cube, numerical investigations have been performed for the flow past the cube elevated to different heights above a solid surface, using the delayed detached eddy simulation method. A benchmark case of a wall-mounted cube in uniform flow is first studied, which gives commendable agreement with the available measurement results, validating the numerical methodology adopted. Subsequently, the cube is elevated to different heights above the ground. Detailed flow topologies around the cube affected by the elevated height are investigated. In addition, the effect of wall proximity on near-wall flow patterns and distributions of the surface pressure are also analyzed. After examining the flow features, the far-field noise emitted from the cube at different elevated heights is predicted by using the Ffowcs Williams-Hawkings acoustic analogy and some implications of the effect of wall proximity on the emitted noise are summarized. For the wall-mounted cube, the noise is greatest along the lateral direction. As the cube is lifted, the radiated sound in the vertical direction increases rapidly and peaks at one quarter of its side length above the ground. The noise induced by the cube tends to be broadband although broad peaks at a Strouhal number of around 0.1 are observed in the vertical and the lateral directions.

Investigations on the accurate prediction of supersonic shear layers for detached eddy simulation

One of the current problems of the detached eddy simulation (DES) and delayed DES (DDES) methods is the inaccuracy in prediction of the separated shear layer flows, which mainly reflects in the delayed development of the shear layer. To correctly simulate the separated shear flow, various modified definitions of the DDES length scale are investigated and the improved definition of the subgrid eddy viscosity is introduced to deal with the delayed development problem for the first time. In this paper, the shear layer without the upstream boundary layer is studied in order to assess the subgrid model of the DDES in the simulation of shear layers. Large eddy simulation (LES) and experiment results are introduced to test the performance of different definitions of the DDES parameters. According to the results, both the definitions of the length scale and subgrid eddy viscosity coefficient should be improved to get the best prediction. DDES with the LES length scale and vorticity-correlated length scale as well as the modified definition of the subgrid eddy viscosity coefficient predict a more accurate development of the shear layer than DDES with the original definitions, which provides a possible solution to the delay development problem of the shear layer. Furthermore, the LES mode of DDES performs considerably differently from the Smagorinsky-LES, which shows the former subgrid model is essentially different from that of Smagorinsky-LES. Theoretical analyses are made in order to find out the quantitative difference of the subgrid eddy viscosity between the LES mode of DDES and Smagorinsky-LES.

Study on vortex shedding mode on the wake of horn/ridge ice contamination under high-Reynolds conditions

This paper studied the unsteadiness of vortex motion produced by a three-dimensional wing section with horn/ridge ice contamination. Using improved delayed detached eddy simulation method, multi-scale vortex and their associated flow structures were successfully captured. Results have shown a diversity of unsteadiness scales at different time series, including shear layer instability, vortex pairing, co-rotating and breaking up. Proper orthogonal decomposition was then introduced to extract the characteristic vortex shedding modes with scheduling the eigenvalues λi from large to small. The dominate and secondary proper orthogonal decomposition modes under horn ice condition were displayed, which could be illustrated as fluctuations near recirculation zone, and large-scale vortex shedding/reattaching motion, respectively. The proper orthogonal decomposition modal characteristics for ridge ice showed that vortex scales varied from large to small. The trajectory of large-scale vortex reattaching and co-rotating exist simultaneously with the pressure peak and recover, which also verified the association of proper orthogonal decomposition modes with different scales of vortices. Future works would be presented on demonstration of the complex structures and the dynamic features in such flow.

Unsteadiness of Tip Leakage Flow in the Detached-Eddy Simulation on a Transonic Rotor with Vortex Breakdown Phenomenon

Tip leakage vortex (TLV) in a transonic compressor rotor was investigated numerically using detached-eddy simulation (DES) method at different working conditions. Strong unsteadiness was found at the tip region, causing a considerable fluctuation in total pressure distribution and flow angle distribution above 80% span. The unsteadiness at near choke point and peak efficiency point is not obvious. DES method can resolve more detailed flow patterns than RANS (Reynolds-averaged Navier–Stokes) results, and detailed structures of the tip leakage flow were captured. A spiral-type breakdown structure of the TLV was successfully observed at the near stall point when the TLV passed through the bow shock. The breakdown of TLV contributed to the unsteadiness and the blockage effect at the tip region.

Dynamic analysis of the flow fields around single- and double-unit trains

With the increasing need for passenger transportation, the double-unit formation of high-speed trains (HSTs) has been widely employed. However, the dynamic characteristics of the flow around double-unit HSTs have rarely been studied. In this study, the Improved Delayed Detached Eddy Simulation (IDDES) method is employed to analyse the differences in flow fields around single- and double-unit trains. The numerical case is validated through wind tunnel experiments, mesh independence tests and IDDES assessments. The results show that the existence of the double region concentrates the Strouhal number of the aerodynamic force of the wake at 0.15. For the flow downstream of the double region, the slipstream velocity detected until the far-wake region is higher than that in the single-unit case. The standard deviation of the velocity, which reflects the intensity of the turbulence, is also higher for the double-unit case. For the near wake, the vortex dominate region is dominated by flow structure shedding from the snow plough thus similar for two cases; for the downwash dominate region, the double-unit wake exhibits more turbulent flow structures and lower dominant frequency for velocity. The proper orthogonal decomposition results also indicate more turbulent wakes within the downwash dominate region in the double-unit case.

Numerical Investigation of the Clocking Effect Between Inducer and Impeller on Pressure Pulsations in a Liquid Rocket Engine Oxygen Turbopump

The clocking positions between the inducer and the impeller have a certain impact on the performance of the high-speed centrifugal pump, which however, is often ignored by designers. In the present study, three-dimensional numerical simulation based on detached eddy simulation method is adopted to evaluate the influence of this clocking effect on unsteady pressure pulsations in a full-scale liquid rocket engine oxygen turbopump. A new omega vortex identification method is introduced to clarify the internal correlation between unsteady flow structures and pressure pulsations and to shed comprehensive light on the formation mechanism of this clocking effect. Results show that the clocking effect has little influence on the unsteady pressure field in inducer passages while it significantly affects the rotor–stator interaction (RSI) effect leading to the alteration of the pressure spectra in RSI region, diffuser and volute diffuser pipe. The components at the inducer blade passing frequency in the pressure spectra are remarkably suppressed and the total pressure pulsation energy in these regions is decreased by an average of 13.94%, 12.94%, and 34.65%, respectively, when the inducer blade trailing edges are located in the middle of two adjacent impeller blades. The vortex analysis in the specific region reveals that the pressure pulsations in RSI region and the downstream regions are closely associated with the unsteady vortex shedding from the diffuser blades and the formation of the clocking effect is precisely due to different processes of the periodic vortex shedding from the diffuser blade pressure surfaces.

Numerical study of snow accumulation on the bogies of a high-speed train using URANS coupled with discrete phase model

The snow accumulation on bogies of a three-car grouping high-speed train has been investigated using unsteady Reynolds-Averaged Navier-Stokes simulations (URANS) coupled with Discrete Phase Model (DPM). The accuracy of mesh resolution and methodology of computational fluid dynamics (CFD) are validated against experimental results. The characteristics of flow fields around bogie regions, the snow particles' motion and snow accumulation are analysed herein. Also the computational costs of numerical simulations using URANS and detached eddy simulation (DES) in the present study are compared. The results show that snow particles begin to flow out from the high-speed train bottom as they come to the first bogie region. After that they whirl around, forming a wind-snow flow in the wake region. Bogie 2 experiences the most snow accumulation. Subsequently, the snow covering on the bogie regions decreases along the train length. A comparison of snow accumulation between URANS and DES methods is made, showing the URANS is sufficient to predict this snow accumulation, which only takes up 2/3 computational cost of the DES. The geometry model consisting of a leading vehicle and a 1/3 s vehicle is recommended to investigate the snow accumulation issue on the bogies of the train.

Analysis of the turbulent flow and trailing vortices induced by new design grooved blade impellers in a baffled tank

U or V shaped grooves have shown reduction in energy and power consumption in blunt bodies immersed in turbulent streams. In this work, the performance of two new design turbines with four blades rotated 45°, one with U and the other with V grooved shape blades, were evaluated in a baffled stirred tank reactor and they were compared with the one of the regular 4PBT. The power consumption, mean velocities, turbulent kinetic energy production (TKE), energy dissipation rate (ε) and trailing vortices induced by new design impellers with grooved walls and 4PBT were evaluated experimentally and modeled by detached eddy simulation (DES) approach, which has not been tested in blade walls with small perturbations. Validation of the results was carried out by PIV (Particle image velocimetry) experimental tests. The evolution of new and distinctive trailing vortices located in the upper and lower faces of the grooved new design impellers is presented. They were measured by angled resolved PIV technique, and visualized in three dimensions by the DES method. The V-grooves shaped impeller generated a larger region of influence of the turbulent kinetic energy along the discharge region than the other impellers. Moreover, presented higher mean dissipation rate levels for the most of the measured angles and induced higher number of trailing vortices in the blades vicinity than the other turbines. It was found that in a wide range of the Reynolds number (Re) (40 × 103 < Re < 125 × 103) the dissipated power measured experimentally from shaft, presented reductions of the order of 6% and 4% for the U and V grooved new models, respectively, compared with that of the regular 4PBT impeller.