Detached Eddy Simulation Method Research Articles

Numerical analysis of the wake dynamics of a propeller

This paper investigates the inception mechanism of propeller wake instability based on an improved detached eddy simulation method at a moderate advance coefficient of J = 0.65. Computational fluid dynamics simulations involving a rotating propeller using a dynamic overset technique are performed at J = 0.38 and J = 0.88 to validate the numerical approach, and these results are compared against experimental data of thrust and torque coefficients and phase-averaged axial velocity from the literature. The results indicate that propeller wake instability results from interactions among vortex structures behind the propeller and the high-speed shear layer. In addition, the diffusion of azimuthal velocity plays an important role in the mutual induction process. Finally, we propose a model that includes the main physical processes leading to tip vortex instability and can predict the time and location of vortex pairing. The present study provides deeper insight into the flow physics driving the tip vortex pairing process.

Entropy production analysis for vortex rope of a Francis turbine using hybrid RANS/LES method

The precessing vortex core in the draft tube of Francis turbine is investigated using entropy production method. The high-fidelity Delayed Detached Eddy Simulation (DDES) method with SST k-ω turbulence model is used to acquire the entropy dissipative components of the flow, and the Q-criterion is selected to evaluate the vortex cores inside the draft tube under different operating conditions, separately. The results show that the numerical simulation is in good agreement with the experimental data obtained from the model test. It is found that the vortex motion, separation flow and shock phenomenon are considered to be hydrodynamic factors for the formation of entropy. Furthermore, draft tube accounts for the greatest proportion of entropy production, which is ascribed to the vortex rope. Vortex rope will rotate in the same direction as the runner, but the rotational speed is lower, resulting in low frequency pressure pulsation with large amplitude in the draft tube. It is concluded that the key reason why great energy dissipation is caused in the draft tube is that the vortex rope is transporting continuously towards the downstream, especially when the turbine deviating from optimum operating point.

Study of wind loading and flow characteristics on an array of parabolic trough collectors

Wind loads and structure of the flow field around an array of parabolic trough collectors (PTCs) were investigated using Delayed Detached Eddy Simulation (DDES) method. A scaled three-dimensional (3D) model was adopted and simulated using the ANSYS-Fluent software. Several pitch angles, including θ = 0°, 45°, 135°, and 180° and the stow mode with the pitch angle, θ = 90°, and flow Reynolds number of 2×105, are considered for numerical solutions. For validation of the numerical model, the mean wind load coefficient on a stand-alone PTC and an array model of PTCs were simulated and compared with the wind tunnel data. Detailed analysis of wind characteristics and wind loading on an array for five-pitch angles was provided. Results demonstrated that except for the stow mode, the first PTC in the array experienced the maximum mean pressure in the upwind side of the PTCs model and corresponded to the maximum mean drag coefficient. Besides, the minimum mean drag coefficient belongs to the second row in the array except for the stow mode. A comparison between flow patterns around the PTCs revealed the high vorticity concentration in front of the collectors for pitch angles θ = 0° and θ = 180°.

Assessing IDDES-Based Wall-Modeled Large-Eddy Simulation (WMLES) for Separated Flows with Heat Transfer

The Wall-modeled Large-eddy Simulation (WMLES) methods are commonly accompanied with an underprediction of the skin friction and a deviation of the velocity profile. The widely-used Improved Delayed Detached Eddy Simulation (IDDES) method is suggested to improve the prediction of the mean skin friction when it acts as WMLES, as claimed by the original authors. However, the model tested only on flow configurations with no heat transfer. This study takes a systematic approach to assess the performance of the IDDES model for separated flows with heat transfer. Separated flows on an isothermal wall and walls with mild and intense heat fluxes are considered. For the case of the wall with heat flux, the skin friction and Stanton number are underpredicted by the IDDES model however, the underprediction is less significant for the isothermal wall case. The simulations of the cases with intense wall heat transfer reveal an interesting dependence on the heat flux level supplied; as the heat flux increases, the IDDES model declines to predict the accurate skin friction.

Numerical study on the supersonic near-wake flow structures of a cylinder at an angle of attack

The supersonic near-wake flow experiment conducted by Dutton on a cylinder with 10 degrees angle of attack was studied numerically with Delayed Detached Eddy Simulation (DDES) method. The wind tunnel test section were included in the geometry model to examine if its omission led to the unsuccessful prediction of several important flow features in previous studies. Flow structures were compared to experiment measurements, Gaitonde's k-ε model results and our previous Monotonically Integrated Large Eddy Simulation (MILES) results. It was revealed that with present geometry model and numerical method, good agreements with wind tunnel experiment result were obtained, including the wave structures, density fields and velocity fields. In particular, the inclusion of wind tunnel test section wall in geometry model reproduces the finite area nozzle exit flow and the expansion wave behind the recirculation zone in the actual experiment setup, which lead to correct prediction of the position of maximum streamwise velocity point. The present simulation also reproduced the uniform pressure profile and value observed in the experiment and the deviation between computation and experiment was within 10%. The flow field data obtained was further analyzed to investigate the vortex surface shape and recirculation region shape in near wake flow with non-zero angle of attack and significant differences were found compared to the zero angle of attack case. The vortex surfaces show a non-axisymmetric slitting movement in the near wake region, which gives an explanation to the non-circular density contour pattern on streamwise flow field slices. The recirculation region shape was identified with zero isosurface of streamwise velocity and was found to be of complex three-dimensional shape with maximum length beside the symmetry plane and a local minimum at windward side of the symmetry plane, which is significantly different from that of zero angle of attack case. The observation that the length of the recirculation zone was greatly reduced on the symmetry plane at non-zero angle of attack in previous study is due to the aforementioned local minimum on the symmetry plane. The reduction of recirculation length is much less if we consider the three-dimensional shape of the region.

Predictions of Conjugate Heat Transfer in Turbulent Channel Flow Using Advanced Wall-Modeled Large Eddy Simulation Techniques.

In this paper, advanced wall-modeled large eddy simulation (LES) techniques are used to predict conjugate heat transfer processes in turbulent channel flow. Thereby, the thermal energy transfer process involves an interaction of conduction within a solid body and convection from the solid surface by fluid motion. The approaches comprise a two-layer RANS–LES approach (zonal LES), a hybrid RANS–LES representative, the so-called improved delayed detached eddy simulation method (IDDES) and a non-equilibrium wall function model (WFLES), respectively. The results obtained are evaluated in comparison with direct numerical simulation (DNS) data and wall-resolved LES including thermal cases of large Reynolds numbers where DNS data are not available in the literature. It turns out that zonal LES, IDDES and WFLES are able to predict heat and fluid flow statistics along with wall shear stresses and Nusselt numbers accurately and that are physically consistent. Furthermore, it is found that IDDES, WFLES and zonal LES exhibit significantly lower computational costs than wall-resolved LES. Since IDDES and especially zonal LES require considerable extra work to generate numerical grids, this study indicates in particular that WFLES offers a promising near-wall modeling strategy for LES of conjugated heat transfer problems. Finally, an entropy generation analysis using the various models showed that the viscous entropy production is zero inside the solid region, peaks at the solid–fluid interface and decreases rapidly with increasing wall distance within the fluid region. Except inside the solid region, where steep temperature gradients lead to high (thermal) entropy generation rates, a similar behavior is monitored for the entropy generation by heat transfer process.

Performance Prediction of a Hard-Chine Planing Hull by Employing Different CFD Models

This paper presents CFD (Computational Fluid Dynamics) simulations of the performance of a planing hull in a calm-water condition, aiming to evaluate similarities and differences between results of different CFD models. The key differences between these models are the ways they use to compute the turbulent flow and simulate the motion of the vessel. The planing motion of a vessel on water leads to a strong turbulent fluid flow motion, and the movement of the vessel from its initial position can be relatively significant, which makes the simulation of the problem challenging. Two different frameworks including k-ε and DES (Detached Eddy Simulation) methods are employed to model the turbulence behavior of the fluid motion of the air–water flow around the boat. Vertical motions of the rigid solid body in the fluid domain, which eventually converge to steady linear and angular displacements, are numerically modeled by using two approaches, including morphing and overset techniques. All simulations are performed with a similar mesh structure which allows us to evaluate the differences between results of the applied mesh motions in terms of computation of turbulent air–water flow around the vessel. Through quantitative comparisons, the morphing technique has been seen to result in smaller errors in the prediction of the running trim angle at high speeds. Numerical observations suggest that a DES model can modify the accuracy of the morphing mesh simulations in the prediction of the trim angle, especially at high-speeds. The DES model has been seen to increase the accuracy of the model in the computation of the resistance of the vessel in a high-speed operation, as well. This better level of accuracy in the prediction of resistance is a result of the calculation of the turbulent eddies emerging in the water flow in the downstream zone, which are not captured when a k-ε framework is employed. The morphing approach itself can also increase the accuracy of the resistance prediction. The overset method, however, overpredicts the resistance force. This overprediction is caused by the larger vorticity, computed in the direction of the waves, generated under the bow of the vessel. Furthermore, the overset technique is observed to result in larger hydrodynamic pressure on the stagnation line, which is linked to the greater trim angle, predicted by this approach. The DES model is seen to result in extra-damping of the second and third crests of transom waves as it calculates the stronger eddies in the wake of the boat. Overall, a combination of the morphing and DES models is recommended to be used for CFD modeling of a planing hull at high-speeds. This combined CFD model might be relatively slower in terms of computational time, but it provides a greater level of accuracy in the performance prediction, and can predict the energy damping, developed in the surrounding water. Finally, the results of the present paper demonstrate that a better level of accuracy in the performance prediction of the vessel might also be achieved when an overset mesh motion is used. This can be attained in future by modifying the mesh structure in such a way that vorticity is not overpredicted and the generated eddies, emerging when a DES model is employed, are captured properly.

A Numerical Investigation on the Rapid Turbulent Filtration Combustion under High Background Pressure

ABSTRACT A numerical investigation of the rapid turbulent filtration combustion within a packed pebble bed is conducted at a fixed superficial velocity of 0.5 m/s, equivalence ratio of 1.0 (methane/air) under two background pressures of 1.0 and 0.5 MPa. Turbulence is modeled with the detached eddy simulation (DES) method. The interaction between turbulence and chemical reaction is considered through the fast reaction method, i.e., burning velocity model (BVM). To validate the DES method, the relative turbulence intensities in a 3D staggered cylinders are verified previously which are in excellent agreement with the experimental data (no reaction). With the confidence in the DES method, the turbulent combustion characteristics in a 3D packed pebble bed are studied in detail, including pressure fluctuation, spatial inhomogeneities of velocity and temperature fields, and evolution of the flame surface. Numerical results show that a strong pressure fluctuation arises after ignition due to thermal expansion contributing significantly to the velocity and temperature fluctuations for the case of 1.0 MPa, whereas it is relatively moderate for the case of 0.5 MPa. Though the evolution of the flame surface both can be divided into two stages for the two cases, i.e., initial-rapid spreading stage, and stable propagating stage, there is a slight difference between them. Specifically, it closely resembles to an atomic blast for the former, while it is similar to a volcanic eruption for the later. Undergoing this stage, about 20 ms, the wrinkle and fragmented flames or flamelets creep over the pebbles through the thin layers close to the walls. Statistical stability is identified in the process of flame propagating upstream. Turbulent flame speeds obtained in this study agree well with the experimental data, especially for the case of 1.0 MPa.

Prediction of cavitating propeller underwater radiated noise using RANS & DES-based hybrid method

ABSTRACT This study focuses on the prediction of the hydrodynamic and hydroacoustic performance of a cavitating marine propeller in open water condition using Reynolds-averaged Navier-Stokes (RANS) and Detached Eddy Simulation (DES) solvers. The effectiveness of the methods is investigated for the recently introduced benchmark propeller that belongs to the research vessel ‘The Princess Royal’. The main emphasis of the study is to examine the capabilities of the RANS and DES solvers for predicting the hydrodynamic performance of a propeller in the presence of sheet and tip vortex cavitation (TVC). In the numerical simulations of the cavitating propeller flow, the Schnerr-Sauer cavitation model based on a reduced Rayleigh-Plesset equation was used to model the sheet and tip vortex cavitation. An alternative Vorticity-based Adaptive Mesh Refinement (V-AMR) technique was employed for the accurate realisation of the TVC in the propeller’s slipstream. In the hydroacoustic calculations, a porous Ffowcs Williams Hawkings equation (P-FWH) was employed together with the DES solver. The numerical hydrodynamic and hydroacoustic results are compared with those of experimental data for the benchmark propeller available from the University of Genova Cavitation Tunnel. The results show that both the RANS and DES solvers are successful for modelling of the sheet cavitation on the propeller blades. However, the prediction of the TVC extension using the RANS solver is found to be insufficient in comparison to the TVC prediction when using the DES method. This is due to the inherent modelling limitations of the RANS solver. In addition to hydrodynamic performance predictions, the overall noise spectrums were found in an agreement with the experimental data with discrepancies between the low and high-frequency region.

The effect of doors and cavity on the aerodynamic noise of fuselage nose landing gear

The aerodynamic noise of landing gears have been widely studied as an important component of the airframe noise. During take-off and landing, there are doors, cavity and fuselage around the landing gear. The noise caused by these aircraft components will interfere with aerodynamic noise generated by the landing gear itself. Hence, paper proposes an Improved Delayed Detached Eddy Simulation (IDDES) method for the investigation of the flow field around a single fuselage nose landing gear (NLG) model and a fuselage nose landing gear model with doors, cavity and fuselage nose (NLG-DCN) respectively. The difference between the two flow fields were analyzed in detail to better understand the influence of these components around the aircraft’s landing gear, and it was found that there is a serious mixing phenomenon among the separated flow from the front doors, the unstable shear layer falling off the leading edge of the cavity and the wake of the main strut which directly leads to the enhancement of the noise levels. Furthermore, after the noise sound waves are reflected by the doors several times, an interference phenomenon is generated between the doors. This interference may be a reason why the tone excited in the cavity is suppressed.

Coupled Computational Fluid Dynamics–Discrete Element Method Study of Bypass Flows in a Pebble Bed Reactor

In the high-temperature reactor design, it is common practice to leave gaps between the graphite blocks of the reflectors to accommodate thermal dilatation and material swelling, as well as to provide an additional cooling source during operation. These gaps give rise to bypass flows entering the reactor core. The bypass flows can change friction factors and heat exchange coefficients obtained in the bulk of the pebble bed. In this paper, a coupled computational fluid dynamics–discrete element method (CFD-DEM) model is proposed. In this model, the pebbles are resolved by the CFD grid and the turbulent field is partially captured using a detached eddy simulation method. The DEM model is first validated against empirical correlations for the packing of the pebbles, and the coupled model is then tested against thermal measurements in the SANA experiment. Then the model is used to perform three-dimensional studies of the effects of the bypass flows in a representative pebble bed configuration. It is determined that the effect of cross flows can be approximately bounded to the first two layers of pebbles next to the reflector wall. Additionally, an increase of ~12% in the Nusselt number in the pebbles next to the reflector is predicted, with a maximum local increase in the pebble of ~100%.

Stall characteristics analyses and stall lift robustness inverse design for high-lift devices of a wide-body commercial aircraft

Stall is an important cause closely associated with the flight accidents of commercial aircrafts, and stall lift robustness of HLD (high-lift devices) plays a significant role in flight safety issues. In this study, stall characteristics of the HLD of a wide-body commercial aircraft are simulated and analyzed via DES (detached eddy simulation) method, and the impacts of geometric parameters of the HLD on stall lift characteristics are investigated. Afterwards, stall lift robustness design for the HLD is performed via a proposed inverse method which combines the GRNN (generalized regression neural network) method and PCA (principal component analysis) method. Then the inverse design model is established with the aerodynamic parameters as input and the geometric parameters as output. The PCA method is introduced to reduce the dimension of the input data, while the GRNN is employed to predict the geometric parameters. The design process is iterated with new sample points added in. The validated results via DES method indicate that the finally designed HLD configuration conforms with the design targets very well, and thus the stall lift robustness is improved obviously. More specifically, the average lift coefficient near the stall AoA (angle of attack) is improved by 1.4%, and the variance of the lift coefficients near the stall AoA is reduced by 79%, while the stall AoA remains unchanged.

The effect of wall temperature on three-dimensional rotating detonation wave

The effect of wall temperature on the three-dimensional rotating detonation wave in an annular chamber is investigated by utilizing premixed hydrogen/air as the reactant and the improved delayed detached-eddy simulation method to evaluate the turbulent transportation. Four cases are set with different wall temperature, i.e. adiabatic wall, isothermal wall with 400K, 800K and 1200K. When using isothermal wall, there is no reactant deficit zone which appearing in the case with adiabatic wall. As the wall temperature increasing, the leading oblique shock will form at both side of the detonation front. When comparing the specific impulse with the adiabatic case, all the three isothermal cases have a better performance. And the lowest wall temperature case shows the highest specific impulse.

The Effect of Bogie Positions on the Aerodynamic Behavior of a High-Speed Train: An IDDES Study

In this study, an improved delayed detached-eddy simulation method has been used to investigate the aerodynamic behavior of the CRH2 high-speed trains (HST) with different first and last bogie positions. The results of the numerical simulations have been validated against experimental data obtained from a previous wind tunnel test, a full-scale field test and a reduced-scale moving model test. The results of the flow prediction are used to explore the effects of the bogie positions on the slipstream, wake flow, underbody flow and aerodynamic drag. Compared with the original HST model, the downstream movement of the first bogie, has a great effect on decreasing the slipstream velocity and pressure fluctuation aside the HST, especially around the lower part of the HST. Furthermore, the size of the longitudinal vortex structure and slipstream velocity in the near wake region also decrease significantly by moving the last bogie upstream. Additionally, the movement of the first and last bogies toward the HST center, effectively decreases the drag values of the head and tail car, while a lower effect is observed on the intermediate cars.

Dynamic characteristics analysis of hammer shock in serpentine inlet

To analyze the dynamic characteristics of hammer shocks caused by engine surge, a serpentine inlet with a front fuselage is simulated under conditions of subsonic inflow and three flight angles based on the improved delayed detached eddy simulation method. An unsteady back pressure boundary condition in an aerodynamic interface plane is used to simulate the overpressure during engine surge. There is a certain angle between the normal line of the hammer shock and the centerline, which is approximately equal to the corresponding flight angle. The inlet wall pressures are above 2.3 times of the free-stream static pressure, and even local transient pressures reach more than 3 times. Complex flow field structures are generated behind the shock, which are affected by the centrifugal force and lateral pressure gradient. The velocities and intensities of the hammer shocks and the pressure distributions on the wall and the shapes of the flow field structures are greatly affected by the flight angle. In a large yaw angle, the hammer shock velocity is the fastest, and the internal airflow and load are the most severe. In particular, it is necessary to consider the influence of the ultra-high dynamic loads in the opposite direction at the two bends in a short period of time.

Combustion Dynamics Simulation of a 30-Injector Rocket Engine

ABSTRACT A computational study of the nonlinear rocket-engine combustion instability is presented for an engine with 30 coaxial methane-oxygen injector ports, a choked nozzle, and a combustion chamber. Three combustion chamber lengths of 18 cm, 24 cm, and 30 cm are simulated with differing instability modes depending on length. Computations using a three-dimensional unsteady k − ω shear-stress transport delayed detached-eddy simulation method provide detailed time-resolved information about the combustion instability. Our flame-index analysis explains why the premixed-flame structures dominate over diffusion flame-structures in the driving mechanism for the instability. This causes the engine to become more unstable as the mixture becomes more fuel rich. Spontaneous longitudinal-mode and tangential-mode instabilities are observed. The oscillation amplitude of the two instability modes vary in alternating fashion during the simulation time. The alternating behavior of the two instability modes can be modified by pulsing the injector mass-flux on the mixture ratio at a particular frequency.

Energy consumption analysis and multiple-criteria evaluation of high-speed trains with different marshaled forms in China

Because of the current state of global resource shortages and environmental degradation, energy-saving and environmentally friendly national policies and low-carbon concepts have attracted both individual and societal attention, especially for rail transit. The rapid development of high-speed railways and urban rail transit in China has presented increasing transport energy demands and environmental pressures. This paper conducts an energy consumption comparative analysis of high-speed trains with different marshaled forms, including 3, 5, 8, 10, 12, 16, 17 and 18 cars, using the delayed detached eddy simulation method. A wind tunnel test with a 3.6% difference is applied to verify the obtained numerical results. A multiple-criteria evaluation methodology is presented to select the optimal alternatives for the marshaled forms of high-speed trains, which considers economic, social and environmental factors. The 1318-km (820-mile) Beijing-Shanghai line and a 350-kph high-speed train are employed in this research. The results can effectively guide the engineering design of high-speed trains and provide an evaluation tool in the decision-making process for designers.

Computational fluid dynamics for ameliorating oil recovery using silicon-based nanofluids and ethanol in oil-wet reservoirs

Impulsive emulsion formation, porous media wettability alteration, and interfacial tension (IFT) reduction are a list of advantages gained from the application of nanoparticles for enhanced oil recovery (EOR). However, low displacement efficiency (DE) coupled with in-effective mixing of the injected nanofluid to redeem the immobile volume of crude oil subsurface present a major challenge to the petroleum industry. The molecular chemistry of ethyl alcohol (ethanol) solvent enables the genesis of strong covalent bonds/mixing with the dense oil. As a result, the fastening converts the aromatic state of the crude oil to a lighter component to ease the flow. In this paper, we numerically investigated the potential for improving dead oil recovery in a heterogeneous rock setting using a blend of ethanol and silicon-based nanofluids to describe the EOR fluid. Herein, silica, silane and silicon carbide nanofluids were studied for their DE with ethanol as the co-solvent. A 2D heterogeneous pore-model was created and discretized to define the computational domain. Computational fluid dynamics code (ANSYS Fluent) facilitated the induction and analysis of interfacial property dynamics within the modelling space. The simulation was performed using the improved delayed detached-eddy simulation method, whereas the continuum surface-force equation with the Euler–Euler mixture multiphase method was used to model the fluid–wall and fluid–fluid adhesions at interfaces. Findings revealed that, silica nanofluid performed optimally compared to its counterpart. Furthermore, approximately 55.34% of the immobile oil was recovered using the optimal blend formulation comprising of equal proportion of silica nanofluid and ethanol. The increase in disjoining pressure and water molecules concentration at the fluid–wall​ contact, and the reduction in interfacial tension due to the evolution of in-situ surface acting agents (surfactants) from the chemical reaction kenetics accounts for this increase. In addition, the blend demonstrated good thermal stability for typical reservoir temperatures of around 400 K due to high intrinsic thermal resilence of silica nanoparticles present in the mixture.

Analysis of the Pressure Pulsation and Vibration in a Low-Specific-Speed Centrifugal Pump

Abstract Unsteady pressure pulsation and fluid force induced by flow instabilities in the centrifugal pump is an important cause of vibration, which is detrimental to the safe operation of the pump. In this study, we numerically investigated the pressure pulsation and radial force in a low-specific-speed centrifugal pump by using the detached-eddy simulation method. We also performed a vibration displacement experiment on the shaft of the centrifugal pump. The vortex identification method was introduced to clarify the internal correlation between unsteady flow structures with pressure pulsations. The results showed that the pressure pulsations at the impeller outlet were closely associated with the periodic vortex shedding from the blade pressure surface. The rotor–stator interaction between a relatively big trailing vortex core and volute tongue generated larger pressure pulsation and radial force in the pump at a low flow rate. Under a large flow rate, the trailing vortex core was easily broken and dispersed, and this resulted in smaller pressure pulsation and radial force compared with that at a low flow rate. Under the design flow rate, the pressure pulsation intensity and the radial force in the impeller were smaller than that under the off-design flow rate. Compared with the spectra between the radial force on the impeller and radial displacement on the shaft, they both presented higher amplitude at the shaft frequency. The vibration of the pump shaft was closely related to the radial force on the impeller.

Correlations of unsteady vortex burst point and dynamic stability over a pitching double-delta wing

Pitching dynamic stability is significant for the aerodynamic performance of an aircraft at a large angle of attack (AoA). Commonly, the dynamic behaviour of vortex breakdown flows over an aircraft with a delta or double-delta wing (DW or DDW) configuration is closely related to the pitching dynamic stability. An 80°/65° DDW subjected to sinusoidal pitching motions around the stall AoA was numerically investigated using the delayed detached eddy simulation method [1]. This study is focused on the correlation between the unsteady behaviour of the vortex burst point (BP) and pitching dynamic stability. The reverse of the pitching dynamic stability is closely related to the inconsistent movement of the BP during the upstroke and downstroke in one pitching period. When the upstream speed of the BP is higher than the downstream speed, the variation in the pitching moment is ahead of the AoA, which leads to dynamic instability. Conversely, when the downstream speed is higher than the upstream speed, the DDW is dynamically stable. This correlation can be interpreted through the energy transfer relationship between the vortex breakdown flows and DDW model.