Three-dimensional Detached Eddy Simulation Research Articles

Effect of parallel-jet addition on the shock train characteristics in a central-strut isolator by detached eddy simulation

For the Rocket-Based Combined-Cycle (RBCC) engine, the addition of primary rocket jets makes both the flow field and the shock train structure more complicated. In this study, a three-dimensional Detached Eddy Simulation (DES) modeling was employed for the numerical analysis of a full-scale central-strut isolator. The characteristics of shock train for the flight Mach 3 were studied. As a result, under the effect of central high-velocity central jets and different back pressures, the structure of shock train could be changed. At the low back pressure, the expansion wave originated from the trailing edge of the central strut still existed, and one X-type shock wave was formed at downstream. Then the strength of the subsequent shock waves between the parallel-jets and the wall in shock train gradually decays. As the back pressure increased, the leading edge of the shock train moved upstream. The shock train was then transformed into two parts: one part was the oblique shock wave generated in the strut section; another was the quasi-normal shock waves formed in the mixing section within the region between the shear layer and the parallel jets. The origination of the first shock wave could cause the generation of the vortexes in the isolator. With the parallel-jet addition, as the vortexes were transported downstream, they broke up into small scales and in more random orientation at the back wall of the central-strut.

DES study of blade trailing edge cutback cooling performance with various lip thicknesses

Three-dimensional detached-eddy simulation (DES) study has been carried out to evaluate the cooling performance of a trailing-edge cutback turbine blade with various lip thickness to slot height ratios (t/H). By adopting the shear-stress transport (SST) k-ω turbulence model, the numerical investigations were performed at two successive steps: first, to validate simulation results from an existing cutback turbine blade model with staggered circular pin-fins arrays inside the cooling passage against experimental measurements and other available numerical predictions; second, to understand the effects of the lip thickness to the slot height ratio on the blade trailing-edge cooling performance. It was found from the model validations that at two moderate blowing ratios of 0.5 and 1.1, DES predicted film cooling effectiveness are in very good agreement with experimental data. Further comparisons of four various t/H ratios (t/H = 0.25, 0.5, 1.0, 1.5) have revealed that the thermal mixing process between the ‘cold’ coolant gas and the ‘hot’ mainstream flow in the near wake region of the exit slot has been greatly intensified with the increase of the t/H ratio. As a result, it causes a rapid decay of the adiabatic film cooling effectiveness downstream of the blade trailing-edge. The observed vortex shedding and its characteristics in the near wake region are found to play an important role in determining the dynamic process of the ‘cold’ and the ‘warm’ airflow mixing, which in turn have significant influences on the prediction accuracy of the near-wall heat transfer performance. As the four t/H ratio increases from 0.25 to 1.5, DES predicts the decrease of main shedding frequencies as fs = 3.69, 3.2, 2.21, and 1.49 kHz, corresponding to Strouhal numbers St = 0.15, 0.20, 0.23, and 0.22, respectively. These results are in good agreement with available experimental measurements.

Aerodynamic forces induced by vertically oscillating incoming flow on a yawed horizontal circular cylinder

Large-amplitude vibrations of stay cables in cable-stayed bridges are an aeroelastic phenomenon caused by an interaction between wind and motion of the cables that is not yet fully explained. Understanding how flow around and the associated forces on an oscillating cable in the across-flow direction are developed interactively is essential to describe the mechanism of cable vibrations. This can ultimately lead to effective mitigation strategies for the large-amplitude vibrations. In this numerical study, using three-dimensional detached eddy simulation (DES), we considered vertically oscillating incoming flow over a stationary, yawed horizontal circular cylinder to investigate the effects of oscillation frequency on flow around and forces on the cylinder. The flow patterns and forces indicate that the incoming and around-cylinder oscillating flows interact significantly. Results showed that the flow around a yawed cylinder develops in different ways depending on the frequency of the incoming flow oscillation, resulting in changed characteristics of flow-induced forces. When incoming flow oscillated at frequencies similar to those of forces generated on the cylinder under uniform incoming flow, along-span flow that generated the low-frequency forces was intensified, i.e., the associated forces were strengthened while preserving their low frequency. However, when the incoming flow oscillated at a frequency close to that of Kármán vortex shedding, the along-span flow was suppressed while the Kármán vortex shedding phenomenon was strengthened over the whole cylinder length. This study suggests that when an oblique cable oscillates at a low frequency due to oblique wind-induced aerodynamic forces, those forces play an important role in initiating and increasing the vibration at the low frequency. A fluid-structure interaction analysis is planned that will shed more light on the phenomenon studied in this work by considering only the fluid motion.

Characterization of flow oblique to a circular cylinder with low aspect ratio using 3-D detached eddy simulation

Large-amplitude vibrations of stay cables in cable-stayed bridges can threaten the safety and serviceability of the structures. Understanding of the excitation mechanism is necessary to mitigate such vibrations effectively and efficiently. Experimental research has investigated the mechanism of oscillation using flow oblique to a cylinder; however, since the aspect ratio of the cylinder is much lower than that of a real stay cable in bridges, the results might not reliably describe the real phenomenon. The aim of this study is (1) to provide better understanding of aspect ratio effects of a circular cylinder on aerodynamic characteristics of the cylinder oblique to flow associated with the large-amplitude cable vibrations and (2) to provide the experimentalists suggestions of a requisite aspect ratio and appropriate pressure-measuring positions of a cylinder for fully developed flow around the cylinder. To this end, the current work applied three-dimensional detached eddy simulations (DES) to flow around a yawed and inclined cylinder to investigate the importance of the aspect ratio of the cylinder when flow oblique to the cylinder develops fully along its spanwise axis. The Reynolds number is 1.4 × 10 5 based on the incoming flow velocity and the diameter of the cylinder. Three aspect ratios ( L/ D=10, 20, and 30; L: a cylinder length; D: a cylinder diameter) and two numerical conditions (slip and periodic) on spanwise boundaries were employed. Results showed that three-dimensional flow and the associated forces on a yawed and inclined cylinder are significantly influenced by the spanwise aspect ratios and spanwise boundary conditions. This study suggests that when a wind tunnel experiment investigates flow oblique to a very slender cylinder, such as attempting to model a stay cable, experimentalists should use a sufficiently high spanwise aspect ratio of the cylinder. For the case of the 30° yaw and 45° inclined cylinder, the requisite ratio would be approximately 60 or higher and appropriate pressure-measuring positions of a half to two-thirds of the cylinder length from its upper/upstream end in order to accurately model inherently three-dimensional characteristics of the flow.

Computational study on aerodynamic mitigation of wind-induced, large-amplitude vibrations of stay cables with strakes

Modifications of circular cylinder surfaces, such as strakes and helical wires, effectively mitigate Kármán vortex-induced vibrations normal to flow and have been applied to the reduction of large-amplitude vibrations of stay cables in bridges, which occur under wind oblique to a cable with or without rainfall. This aerodynamic control method cannot be fully effective without understanding the behavior of the flow around and the associated forces on oblique cables. To address this issue, flow around a yawed cylinder with various strake patterns was studied using three-dimensional detached eddy simulation (DES) at Reynolds number of 1.4×10 5. Results demonstrated that strake patterns strongly influence the development of flow structures around a yawed cylinder and therefore the associated forces on the cylinder. The results suggest that particular strake patterns can mitigate large-amplitude and low-frequency vibrations of stay cables induced by oblique wind.

Predicting Wall Pressure Fluctuationover a Backward-Facing Step Using Detached Eddy Simulation

Numerical simulations of a turbulent flow over a backward-facing step are performed. The governing equations are solved by the finite volume code Cobalt. Unsteady three-dimensional detached eddy simulations are carried out with Menter's shear stress transport turbulence model acting as a subgrid-scale model. Mean flow quantities such as pressure, velocity, and skin-friction coefficients are accurately predicted. Velocity and pressure fluctuations are resolved by the three-dimensional computations. The dominant frequency is in good agreement with existing experimental data, and power spectral analysis of wall pressure fluctuation is consistent with empirical relations found in the literature for several operating conditions.

Investigation on 3-D characteristics of flow around a yawed and inclined circular cylinder

A three-dimensional detached eddy simulation (DES) study was conducted to investigate the three-dimensional characteristics of the fully developed flow past a yawed and inclined circular cylinder. Flow at Reynolds number ( Re) of 1.4×10 5 (based on freestream velocity U and the diameter of the cylinder D) over a cylinder with axial lengths of 10 D, 20 D and 30 D was simulated. Spanwise wall boundary conditions were also studied to examine their effect on the flow field. The simulation shows that the flow is highly three-dimensional. The swirling flow with low pressure on the cylinder plays an important role in generating multiple moving forces, which can produce low-frequency loadings. The mechanism of generation of forces on the cylinder provides fundamental information necessary for the understanding of the oscillation of a yawed and inclined circular cylinder.

G704 ターボポンプ用インデューサ内の非定常キャビテーション流れの数値計算(2)(GS-7 キャビテーション,一般セッション)

The unsteady cavitating flow in a turbopump inducer has been calculated using an accurate three-dimensional detached eddy simulation (DES) code, which took into account the cavitating phenomena. The three-dimensional code is based on the numerical method for compressible flow using low Mach number flow approximation and a cavitation model. Qualitative agreement between our computational result and experimental data was satisfactory. The magnitude of the fluctuation in cavity volume increased as the cavities extend and close to the throat of the blade passage. The effect of the tip-gap on the cavity volume was shown; the time-averaged volume of cavities for the inducer with the tip-gap was about five times larger than that for the inducer without the tip-gap at an operating condition. Vorticity generation in cavities due to baroclinic torque was found. Multiple blade passages calculations are under way to find unsteady phenomena such as rotating cavitation.