Detached Eddy Simulation Model Research Articles

Wing design optimization and stall analysis with Co-flow Jet active control

Coupled with Co-flow Jet (CFJ) technology, the Non-dominated Sorting Genetic Algorithm II was utilized for the multi-objective combination optimization of an optimized Co-flow Jet wing, based on National Advisory Committee for Aeronautics (NACA) 6421. A high-precision numerical simulation using the delayed detached eddy simulation model was performed on the optimized wing to investigate the three-dimensional flow separation characteristics after static stall. The stall improvement was investigated by adjusting the momentum coefficient of the injection. The results show that the optimized wing exhibits significant improvements in aerodynamic performance and corrected aerodynamic efficiency. At an angle of attack of 10°, the average lift increased by 16.25% and the drag decreased by 27.23% compared to the CFJ6421 wing, while effectively addressing the problem of low modified aerodynamic efficiency of the CFJ wing at lower angles of attack. By utilizing higher momentum and improving the boundary layer control capability, flow separation is effectively suppressed, thus achieving the goal of stall recovery of the CFJ wing.

Application of a new DES model based on Wray-Agarwal turbulence model in flow simulation of a mixed-flow pump

In recent years, it has been demonstrated for many two- and three-dimensional external and internal turbulent flows that the one-equation Wray-Agarwal turbulence model can compute the complex turbulent flow fields with high computational accuracy, excellent computational convergence and efficiency. In this paper, Wray-Agarwal (WA) turbulence model is employed as part of a detached eddy simulation (DES) method to predict the performance of a mixed-flow pump. By comparing the computations with the experimental results, the differences and similarities between the WA-DES model and the Shear Stress Transfer (SST) k-ω model in predicting the internal and external flow characteristics of the pump are analyzed. The results show that both the SST k-ω model and the WA-DES model can reasonably predict the performance of the pump between 0.6 Q and 1.2 Q, where Q is the design flow rate; however, they have their own merits and deficiencies in predicting head and efficiency of the pump at low and high flow rates. For the velocity field in the rotor-stator interaction region, the WA-DES model shows better prediction accuracy since it can accurately predict the large-scale recirculating vortex structure at the inlet of the guide vane. The SST k-ω model over-predicts the separated flow region, which leads to the emergence of a small vortex structure before the backflow region of the pump. Although the turbulent eddy viscosity predicted by the WA-DES model is higher than that of the SST k-ω model and there is small difference in the results for the scale of the tip leakage vortex (TLV) between the two models, the overall simulation results of the WA-DES model for the high turbulent viscosity region and the pressure increase in the impeller are consistent with the SST k-ω model results. The results of this paper demonstrate the potential of WA-DES model for prediction of flows in pumps.

The comparative transient prediction on hydrodynamic characteristics and flow field properties of pump-jets with accelerating and decelerating ducts

Pump-jet holds a pivotal position in various marine applications, underscoring the need for comprehending their transient behavior for the purpose of design enhancement and performance refinement. This paper employs Reynolds-averaged Navier–Stokes equations method in conjunction with Detached Eddy Simulation model. The study delves into the ramifications of accelerating and decelerating ducts, distinguished by camber f and attack angles α, on transient hydrodynamic characteristics. The hydrodynamic characteristics are investigated numerically, after the validation of the numerical methodology by comparing simulation outcomes against experimental results. Subsequently, the study delves into propulsion characteristics, followed by an exploration of time-domain and frequency-domain data transformed through fast Fourier transform to analyze thrust fluctuations and pulsating pressures. Additionally, a detailed examination of pressure distribution and velocity field is provided, aiming to dissect the mechanisms through the variations in f and α influence the flow field. Findings suggest that the outlet velocity of accelerating ducts significantly surpasses the inlet velocity, a behavior contrasted by decelerating ducts. Notably, the patterns of accelerating and decelerating ducts resulting from alterations in f exhibit consistent characteristics with those brought about by changes in α. However, several opposite characteristics surface in transient flow field due to the distinct modifications in the duct profile. Furthermore, by considering vorticity magnitude distribution and vortices, a comparative analysis elucidates the effects of varying f and α on rotor and stator trailing vortices. This contributes to understanding the flow instability mechanism under differing duct configurations. It is evident that changes in f and α exert significant influence on both performance and flow field.

Nonequilibrium Heating/Cooling Effects in a Mach Number 10 Hydrogen-Fueled Scramjet

The influence of thermal/chemical nonequilibrium on a hydrogen-fueled Ma 10 scramjet was analyzed by combining the JF-2 4 s hock tunnel test and improved delayed detached eddy simulation modeling. A remarkable change in flame stabilization mode when incorporating the two-temperature nonequilibrium model was observed. The nonequilibrium heating and cooling effects were analyzed for different sections of the scramjet. The nonequilibrium heating effect facilitates the upstream flame propagation by inducing an early ignition and thickening the inlet boundary layer. The nonequilibrium heating or cooling effect is generally weak in the nearly constant-area isolator and combustor, where the flow is mainly influenced by the chemical nonequilibrium with a variation of 10% in reaction rate. The nonequilibrium cooling effect mainly exists in the expanding nozzle, where Tv>Tt, but the energy replenishment from vibrational mode to trans-rotational mode is nearly frozen when Tt<1000 K. Under both conditions, the final mixing is nearly complete, and net thrust has been achieved. When considering the nonequilibrium effects, the final combustion efficiency increases from 87.68 to 99%, together with a 53.94% rise in peak pressure ratio and a 137.04% rise in specific impulse.

Numerical analysis of accelerating and decelerating ducts modified by camber on the unsteady flow field of pump-jet

Utilizing the Reynolds-averaged Navier-Stokes equations method in conjunction with Detached Eddy Simulation model, this paper investigates effects of duct camber (f) on transient hydrodynamic characteristics, particularly propulsion performance and flow field behavior. The research begins by validating the numerical methodology through experimental and numerical assessments of propeller VP1304 and a pump-jet operating under mooring conditions. The exploration commences with examining propulsion characteristics, followed by analyzing time-domain and frequency-domain data, wherein thrust fluctuations and pulsating pressures are scrutinized via fast Fourier transform (FFT). The pressure distribution and velocity field are subsequently presented to unveil the mechanisms triggered by variations in f. Comparative findings highlight that in cases of pump-jets with lower camber, the outlet velocity exceeds the inlet velocity, a trend contrary to scenarios involving higher f values. Additionally, by analyzing vorticity magnitude distribution and vortices, this study attains comparative insights into the effects of accelerating and decelerating ducts on rotor and stator trailing vortices, offering a window into the flow instability mechanism amidst diverse duct configurations. The results showcased to demonstrate the substantial impact of f variation on hydrodynamic properties. This comprehensive investigation yields practical guidance for the optimal design of pump-jets, potentially informing future design endeavours.

Investigation on the behavior of flow and aerodynamic noise generated around the tandem seal-vibrissa-shaped cylinder

Through comparing with the tandem circular and elliptic cylinders with the same characteristic dimensions, the behavior of flow and flow-induced noise generated around the tandem seal-vibrissa-shaped cylinder is studied based on delayed detached-eddy simulation model and acoustic analogy approach. The co-shedding pattern of flow developed around the tandem cylindrical-like bars is investigated. The spatial modes, mode energy, and mode coefficients of turbulent flow around the geometries are analyzed through spectral proper orthogonal decomposition. Results show that the lift fluctuations of downstream bar are stronger than those of upstream bar, and more aerodynamic noise is radiated from the downstream bar than from the upstream bar. The alternative arrangement of nodal and saddle planes of seal-vibrissa-shaped cylinder introduces three-dimensional flow separations and suppresses the shear layer interactions, inhibiting the regular vortex shedding of Kármán vortex street occurring in the tandem cylinder wake. The reversed vortex shedding generated by two adjacent saddle surfaces in the wake of seal-vibrissa-shaped cylinder balances the lateral force and mitigates the lift fluctuations greatly, thereafter reduces the aerodynamic noise generated by wall pressure fluctuations introduced by unsteady fluctuating forces exerting on the surfaces of geometries. Compared to the tandem circular and elliptic cylinders, the good noise reduction effect with sound pressure level reduced at main frequency range has been achieved from the tandem seal-vibrissa-shaped cylinder. The calculated spectra and amplitude levels of aerodynamic noise agree well with the experimental measurements from the anechoic wind tunnel, verifying the accuracy of the numerical simulations.

Flow-induced noise from a seal-vibrissa-shaped cylinder

Inspired by the special structure of seal vibrissae and their ability to effectively suppress vortex-induced vibrations, this paper studies the characteristics of flow and aerodynamic noise generated around a seal-vibrissa-shaped cylinder at a Reynolds number (based on the hydrodynamic diameter and freestream velocity) of 6 × 104 using the delayed detached-eddy simulation model combined with the acoustic analogy approach. The far-field aerodynamic noise has also been measured at Reynolds numbers of 6 × 104∼1.2 × 105 in an anechoic wind tunnel to compare the seal-vibrissa-shaped cylinder with cylindrical and elliptical bars of the same characteristic dimensions. The calculated spectra of aerodynamic noise agree well with the wind tunnel measurements, verifying the accuracy of the numerical simulations. It is found that the three-dimensional flow separations introduced by the alternative saddle and nodal planes of the seal-vibrissa-shaped cylinder inhibit shear layer interactions and reduce the spanwise coherence, resulting in a wake with no dominant coherent structures and thus suppressing pressure fluctuations on the solid surfaces. The configuration of seal-vibrissa-shaped cylinder eliminates the regular Kármán vortex street which generally occurs in a cylinder wake, and consequently inhibits the tonal peak in the aerodynamic noise. The sound pressure level is also reduced in most frequencies. The consistency of the normalized spectra of the far-field aerodynamic noise and the vortex shedding frequency at different inflow speeds demonstrates the high accuracy of the anechoic wind tunnel measurements. Compared with cylinders with circular or elliptical cross sections, the seal-vibrissa-shaped cylinder effectively reduces the flow-induced noise by, respectively, 13 dB or 11 dB for a far-field receiver of 90° at Reynolds number of 6 × 104, showing great potential in reducing aerodynamic noise.

Aerodynamic stability of vehicle passing through a bridge tower at high speed under crosswind conditions with different road adhesion coefficients

In a complex environment, large bridges will present different adhesion coefficient, often accompanied by large crosswind, which makes the flow field around the vehicle complex and changeable. It is easy to cause unnecessary lane change and sideslip of the vehicle, which will affect the driving stability and lead to serious traffic accidents. Therefore, this paper studies the aerodynamic stability of cars passing through the bridge tower at high speed under different road adhesion and crosswind conditions. The detached-eddy simulation (DES) model was employed and the reliability of the DES model was verified by wind tunnel tests. An overlapping mesh technique was adopted to realize the motion of the vehicle. A multibody dynamic (MBD) model of the vehicle was established, and its robustness was verified. A two-way coupling model was then established based on the aerodynamic and MBD models. Subsequently, the aerodynamic characteristics and dynamic response of the vehicle passing through the bridge tower at a high speed were compared and analyzed using one-way and two-way coupling methods, with road adhesion coefficients of 1.0, 0.6, and 0.4 under crosswind conditions. The results show that the aerodynamic characteristics of the vehicle passing the bridge tower at a low adhesion coefficient under two-way coupling change evidently, and the trajectory and body attitude of the vehicle change significantly. As the adhesion coefficient of the road surface decreases, the vehicle passes through the bridge tower with a large lateral displacement and yaw angle. The maximum lateral force of −1406 N and the maximum yaw moment of 803 N∙m are generated when the car passes through the bridge tower under the two-way coupling with the adhesion coefficient of 0.4. Under two-way coupling, the lateral displacement and yaw angle caused by the bridge deck with an adhesion coefficient of 0.4 are 0.265 m and 0.0205 rad, respectively, which is larger than those of the bridge deck with adhesion coefficients of 1.0 and 0.6. Because the coupling effect of aerodynamic and vehicle motion is not considered in the one-way coupling, the one-way and two-way coupling simulation results differ significantly. The results indicate that it is necessary to use a two-way coupling method to study the aerodynamic stability of vehicles passing through a bridge tower at a high speed under crosswind conditions.

Mechanism of runner high-pressure side on stall characteristics at typical unsteady operating points in both modes of a pump turbine

Stall phenomenon, a classical physical phenomenon which is located in the vaneless region of a pump–turbine and accompanied by a complex vortex evolution process, is strongly related to the formation of hump unsteady region at the pump mode and S unsteady region at the turbine mode. In the present paper, a detached eddy simulation model is employed to numerically investigate the impact of runner high-pressure side (HPS) on stall characteristics at typical unsteady operating points, namely, a valley point in the hump region at the pump mode and a runaway point in the S region at the turbine mode. It is found that the stall characteristics at both investigated points are obviously changed: For the valley point, only three fixed stall cells exist in the original plan, while four additional rotating stall cells appear and rotate at the speed of 0.02nr (nr, runner rotation speed) in the optimized plan (OPT). The distinctive coexistence phenomenon of both fixed stall and rotating stall is reported for the first time and is attributed to the complex vortex evolution controlled by optimized HPS; for the runaway point, both the intensity and frequency of the stall characteristic are slightly increased in OPT. Moreover, for both operating points, the optimized HPS can effectively decrease the backflow at shroud, resulting in a significant decrease in the relative backflow rate within a complete flow period, of which 17.3% is for the valley point and 4.8% is for the runaway point. Finally, a local hydraulic loss rate (LHLR) method is adopted to investigate the hydraulic loss evolution process, and it is found that the high LHLR region in OPT is more concentrated in both circumferential direction and radial direction in the vanless region at both operating points. Based on the runner with optimized HPS proposed in the present paper, many unsteady hydraulic characteristics that is related to the stall phenomenon might be eliminated to some extent.

A Computational Examination of Side-by-Side Rotors in Ground Effect

This study investigates the interactional aerodynamics of hovering side-by-side rotors in ground effect. The 5.5-ft diameter, three-bladed fixed-pitched rotors are simulated using computational fluid dynamics at a targeted 5 lb/ft2 disk loading. Simulations are performed using the commercial Navier–Stokes solver, AcuSolve ®, with a delayed detached eddy simulation model. Side-by-side rotors are simulated at two heights above the ground (H/D = 0.5 and H/D = 1), and with two hub–hub separation distances (3R and 2.5R). The performance of side-by-side rotors in ground effect (IGE) is compared to isolated rotors out of ground effect. Between the side-by-side rotors IGE, a highly turbulent mixing region is identified where the wakes of each rotor collide. The flow fountains upwards, as well as exits outwards (along a direction normal to a plane connecting the two rotor hubs) with fountaining between the rotors reaching up to 0.75D above the ground. As blades at H/D = 0.5 traverse the highly turbulent flow, strong vibratory loading is induced and a thrust loss is observed over the outboard sections between the rotors that is large enough to negate any nominal ground effect benefits inboard. Side-by-side rotors at H/D = 0.5 with 2.5R hub–hub spacing produce peak-to-peak thrust oscillations up to 16% of the steady thrust. Rotors placed higher, at H/D = 1 are positioned above the turbulent mixing flow and produce significantly lower vibratory loads. The spacing between rotors at H/D = 0.5 and 3R hub–hub separation allows strong vortical structures to develop between the rotors which move from side to side over multiple revolutions. When the vorticity moves closer to one of the rotors, it produces a greater lift deficit over the outboard region and a stronger vibratory loading. For rotors closer together, at H/D = 0.5 and 2.5R separation, the vortical structures between rotors are constrained to a more concentrated area and show less side-to-side drift.

Examination of air flow characteristics over an open rectangular cavity between the plates

Air flow characteristics of an open cavity have been numerically examined by using different turbulence models of Detached Eddy Simulation (DES), k-ε Realizable, k-ω Shear Stress Transport (SST) and Large Eddy Simulation (LES) based on an experimental study of open literature. Numerical results of transient analyses have been compared to experimental results at cavity length-based Reynolds number of Re = 4600. Pressure distributions, streamline patterns, streamwise velocity components, mean velocity values and their vectors have been given in terms of contour graphics. Moreover, velocity profiles have been presented. Pressure fluctuations have been triggered by flow separation and its reattachment. Due to upstream separation of boundary layer, there was curved boundary layer obtained between the outer potential-flow-like and the recirculation zones. As a result, negative velocity values are evidence for rotational flows affected by formation of secondary flows in the cavity. Furthermore, lower pressure region has been observed as a result of rotational flow which was powerful in the open cavity. Numerical results of DES and LES turbulence models are in good agreement with the results of reference study. As the numerical results obtained by LES turbulence model are approximately same with those of experimental reference study, LES turbulence model is mostly recommended. As an option to these turbulence models, k-ω SST model could be utilized for limited computer capacity. However, k-ε Realizable model is not sufficient for capturing rotational flows which are very effective in terms of present case.

Research on tip leakage vortex characteristics of axial-flow circulating pumps under unpowered driven conditions

To explore the flow characteristics around the tip clearance and the evolution of tip leakage vortex, this paper adopted the delayed detached eddy simulation model to calculate the blade tip region and employed the vorticity transport equation to analyze the forms and strength of tip leakage vortex. It is found that under the effect of tip leakage vortex, the unsteady flow at blade edges are serious under unpowered driven condition. Tip leakage vortex is generated from the leading edge of the blade. From the leading edge to the trailing edge of the blade, the intensity of tip leakage vortex gradually decreases and the generation frequency gradually increases. The core intensity of tip leakage vortex at L1 to L5 points decreases gradually, with a decreased range of about 24.8%, 34.4%, 55.03% and 60.48%.

INVESTIGATION OF FLOW OVER NACA0021 AIRFOIL WITH LEADING-EDGE TUBERCLES USING TRANSITION-BASED HYBRID RANS/LES MODEL

The present paper numerically investigates flow control over NACA0021 (National Advisory Committee for Aeronautics) airfoil by applying a leading-edge tubercle as a passive flow control device at transitional Reynolds number (Re) regime. The study includes simulations over the unmodified airfoil and modified airfoil operating at Re=120000 using kT-kL-{\omega} based Delayed Detached Eddy Simulation (DDES) model. The calculations include both the pre-stall and post-stall regions, while the performance of the mean flow aerodynamic properties like pressure, lift, skin-friction, and drag coefficients are also investigated in the presence of tubercles. The spanwise flow over the modified airfoil interacts with the primary flow. The turbulent separation gets delayed compared to the unmodified airfoil, thereby significantly improving the stall behavior and flow behavior in the post-stall region. The modified airfoil provides the gradual stall behavior against the steep stall behavior for the unmodified airfoil. To optimize tubercle configuration, the study considers two modified airfoils with different combinations of amplitude and wavelength. The unsteady analysis, including the reduced-order modeling, i.e. 3D Proper orthogonal decomposition (POD), characterizes the dominant vortical structure of the flow to evaluate the improvement in the aerodynamic performance of the modified airfoil. The model decomposition for flow over airfoil helps to provide a deeper understanding of the flow control phenomenon.

Assessment of nonlinear-low Reynolds number/detached eddy simulation turbulence model for wake flow field simulation of a realistic automotive model

Since the results of wake flow simulation with commonly used turbulence models are unsatisfactory, by introducing a nonlinear Reynolds stress term and combining the DES (Detached Eddy Simulation) model, this paper further validates the nonlinear-LRN (Low Reynolds Number)/DES turbulence model which can predict the flow separation and the reattachment phenomenon more accurately. This model was verified by a wall-mounted hump flow case and was applied to the time-averaged and transient flow field structure analysis of a realistic automotive model with several widely used turbulence models. These simulation results were compared with experimental data, indicating that the nonlinear-LRN/DES model gives better agreement with the experiment and can predict the automobile wake flow structures and aerodynamic characteristics more accurately. Furthermore, the performance of the nonlinear-LRN/DES model in mesh with different refinements is compared, concluding that the new proposed model can obtain high accuracy in the coarse mesh.

A study on the performance of the cavitating flow structure and load characteristics of the vehicle launched underwater

This paper analyzes cavitating flow structure and load characteristics of vehicles launched underwater for different cavitation numbers and different angles of attack. The improved delayed detached-eddy simulation model and volume of fluid, as well as overlapping mesh technique, are adopted. Additionally, a verification of the underwater launch simulation method and cavitation model is presented. Cavitating flow structure, wall vortex structures, and load characteristics are studied with a focus on the evolution mechanism of the cavitation flow field during the water-exit process. The results show that the attached cavitation rapidly collapses from top to bottom under the combined effect of large–medium density difference and reentry jet. Due to the presence of attachment cavitation, the development of the wall vortex structure represented by the hairpin vortex is inhibited. Considering the compressibility of the vapor phase, the peak of the synchronous collapse pressure is much larger than the collapse pressure with incompressibility. The pressure appears to be characterized by short widths and high peaks during the collapse of the water-exit. As the vehicle exits the water with a certain angle of attack, the range and peak of the cavitation collapse pressure rapidly reduce. In particular, the pressure side cavitation shedding and collapse behavior at the initial moment may lead to a larger pressure peak.

Deterioration of aerodynamic performance of a train driving through noise barriers under crosswinds

Unilateral vertical noise barrier (UVNB), bilateral vertical noise barrier (BVNB) and fully enclosed noise barrier (FENB) are widely used along high-speed railways. The running safety of a high-speed train (HST) faces challenges when entering and exiting a noise barrier in crosswind. A series of computational fluid dynamics numerical simulations of train-noise barrier-crosswind based on the improved delayed detached eddy simulation model and ‘mosaic’ mesh technology are conducted. The influence of different noise barrier types on the aerodynamic load of the carriage is studied. Three buffer structures with different lengths are designed to alleviate the deterioration of aerodynamic performance of HST. Results shows that: The amplitudes of the lift force of the tail car, the lateral force and pitching moment of the head car in the BVNB are the largest, and the amplitude of the yawing moment of the head car in the FENB is the largest. Considering the engineering effects and economic benefits, a buffer structure with a length of six times head car length with a gradual ventilation rate is recommended for project, and the reduction rates of the change rates of the lift and yawing moment are 62.8% and 76.4%, respectively.

RANS/LES investigation on the performance of air film fusion around a vertically launched underwater vehicle

The Improved Delayed Detached Eddy Simulation (IDDES) model, energy equation, Volume of Fluid (VOF), and overlapping grid technique are adopted in this paper. Analyzing the performance of Froude numbers, and row-spacing-to-diameter ratios on the air film development process. The evolution of air film around a vertically launched underwater vehicle technology is the passive exhaust process, in which a high-pressure cavity of the limited volume is formed by crossflow shearing, environmental depressurization, and other factors. With the increase of the Froude number, the maximum diameter and length of the air film at the same feature position are increased, which is beneficial to air film fusion to a certain extent. However, research results indicate air film fusion does not occur at the large Froude numbers. In the establishment of the air film fusion model, there is a non-linear relationship between the Froude number and the pressure ratio inside and outside the cavity. As the Froude number increases, the pressure ratio becomes larger. However, the largest cross-sectional area of air film is not the largest. In the larger range of Froude numbers, the relationship between gas film fusion characteristics and Froude number is more complicated, and further research is studied. Finally, the effects of row-spacing-to-diameter ratios on the air film development process were discussed.

Application and assessment of the improved delayed detached eddy simulation model to the cold spray process: Toward high fidelity computation fluid dynamics simulations

Despite many years of research about the comprehension of fluid-dynamic related phenomena and improvements in the performance of cold spray systems, ensuring high fidelity simulations of gas and particle flow remains a challenge. In this work, a detailed high fidelity modeling, namely, improved delayed detached eddy simulation, in axisymmetric geometries was proposed and compared with a more usual numerical framework, the Reynolds averaged Navier–Stokes model. After its validation against literature data in three different nozzle configurations, the new model could demonstrate more accurate predictions of phenomena such as oblique shocks, bow shock, and particle–gas flow coupling. Finally, thanks to the high fidelity numerical framework, a new nozzle geometry with a narrower extension of the particle jet was proposed and assessed.

Numerical investigation of flow around two cylinders in tandem above a scoured bed

Flow mechanisms around two cylinders in tandem arrangement above a scoured bed have been investigated using the three-dimensional unsteady Navier–Stokes equations with the Spalart–Allmaras improved delayed detached-eddy simulation model. A turbulent inlet boundary layer generation method is adopted to obtain more realistic inlet boundary conditions. First, uniform flow over a single cylinder at Re = 3900 and flow over a single cylinder above scoured beds at Re = 6000 were simulated to validate the numerical model, boundary layer generation method, and mesh density effect. Second, two cylinders in the tandem arrangement above scoured beds with six different pitch ratios L/D are investigated numerically in terms of instantaneous vortex characteristics, the hydrodynamic force, and time-averaged flow fields. The simulation results in scoured beds are compared with simulations under the near flat wall and wall-free conditions. The major findings can be summarized as follows. (1) When L/D≤2.0, the wake of two tandem cylinders is dominated by the intermittent shedding, and the downstream sand dune in the scoured bed hinders the Kármán vortex formation at the rear of the downstream cylinder. Lift force fluctuations of the two cylinders have small amplitudes, and their spectra show a multi-peak distribution and no dominant peak frequency in the spectrum of the upstream cylinder. A squarish cavity-like recirculation zone is formed between two cylinders at L/D=2.0. (2) When L/D≥3.0, the periodic vortex shedding is evident in the wake of the upstream cylinder, and the small sand berm between two cylinders has an impact on the bottom shear layer of the upstream cylinder. The downstream cylinder is periodically impacted by the vortices shed from the upstream cylinder. Lift force spectra of the upstream and downstream cylinders have the same peak frequency. (3) Due to the influence of scoured bed and the inlet boundary layer, the time-averaged lift coefficient of the upstream cylinder remains negative when L/D≥1.5, and the critical spacing for drag inversion is relatively smaller compared with under wall-free conditions. The negative pressure coefficient values of the upstream cylinder are smaller than the values in near flat wall and wall-free conditions.

Assessing Reliable Turbulent Model in Simulating Flip Bucket Flow

The main aim of the study is to investigate and define the turbulent model to be applied while numerically modeling ogee spillway associated with flip bucket to dissipate the energy. A previous experimental investigation was replicated numerically by ANSYS FLUENT software, where four different methods to model turbulence: standard k-ε model (SKE), realizable k-ε model (RKE), renormalization group k-ε model (RNG) and detached eddy simulation model (either DES-KW or DES-RKE) were applied. Two different previous sets of experimental measurements of two different dams (Case A, Case B) were considered to reinforce the study's decision about a favorable model in simulating the experimental results. This research was initiated to assess their behavior by numerical turbulence models. At first, numerical simulation is carried out to the flip bucket (case A) to determine the model that provides reasonable results and contrasts them against experimental results. In this part of research, the simulations were conducted under three discharges (Q=0.22, 0.13 and 0.07m³). The previous experimental results and previous empirical derived equations were used to compare with the numerical simulation results. For Case A, the results show that RKE precisely described the jet trajectory length, at higher discharge and jet velocity values, while DES specifically designated the jet trajectory length generally but in relatively longer calculation duration. In addition, Case B, the results of the throw length are calculated for each model and compared with the measured length. This comparison showed that DES was the most reliable turbulent model. The outcome of this study agrees with the outcome of Part I study as both studies show that DES method is the most appropriate turbulent models to be used in studying flow over ogee spillway equipped with flip bucket energy dissipater. This simulation is very important in order to investigate the damages occurring at impingement location in other designs.