

Investigation on the Gas Jet Flow Performance Confined in Round Pipe


Pages :
669680


Authors :
H. Zhang,
L. Jia,
L. S. Cui,
C. H. Li,
In this study, the round jet flow behavior arose by the confinement was investigated experimentally and numerically. The confinement characteristics based on multiscale characterizations in the atmospheric gas jet flow, which generated from a circular symmetrical subsonic nozzle and flowed into a confined round pipe, was used to studied. The studied region near the nozzle possess really short axial length (within 12d), providing initial conditions and boundaries that affected the flow behaviors in this region. A wide range of inlet velocities (0.98 m/s~84.72 m/s) and confined space sizes (1~20) were involved for simulated and quantitative discussions. The velocity profile evolution was recorded by simulations and characterized by a laser Doppler anemometer (LDA) with a resolution of 0.01 m/s. The confinement characteristics were systematically presented to elucidate the performance under the studied confined conditions with different metrics, such as centerline velocity decay (VR), entrainment rate (MR), pressure coefficient (Cp) and length of recirculation region (LR). The results indicated that the recirculation fluid action mainly contributed to the promoted initial velocity profile evolution with the introduction of the confined space. Theoretically, the confined space could reduce the maximum absolute entrainment mass flux, initiating a peak at the center of the recirculation region, which was controlled by the inlet velocity and the confined space size collectively. The remarkable effect of confined space size further contributes to the confinement characteristics of gas jet flow, representing a shortened flow distance despite similar process as that by reducing the diameter ratio (dR). In some cases, because of the miniaturization of confined space size, the dispersion of initial velocity profile evolution was not significant displayed throughout the variable inlet velocity range, especially when the dR was less than 2. This study gives a new insight in the performance of jet flow confined in round pipe, and such knowledge will be helpful to provide great potential and reference to applied fluid mechanics.





Stability of Solids in Stepped Flume Nappe Flows: Subsidies for Human Stability in Flows


Pages :
681690


Authors :
H. de B. Ribeiro,
A. L. A. Simões,
L. D. da Luz,
L. S. G. Mangieri,
H. E. Schulz,
Knowing the details of the interaction between people and runoff flows caused by heavy rainfall or by floods due for example by the rupture of reservoirs or dams is essential to prevent accidents with humans. There are information in the literature on the equilibrium capacity of individuals partially immersed in flows occurring in flatbottomed channels, but there are many gaps regarding the use of urban draining staircases during the occurrence of rainfalls that generate runoff over their steps, and their impact on people. This study considered the effect of the flow on the stability of five obstacles positioned on one of the steps of a reduced model of a draining staircase. The results were used to calculate dimensionless parameters which involve the mass and height of the obstacle, the water density, critical depth of the flow and step height. These parameters were justified by a fundamental toppling and drag formulation, and good correlations between the obtained dimensionless parameters were obtained following adequate power laws. Comparisons between the data obtained in the present reduced model of staircase and literature data of flat bottom channels showed similar behaviors. Finally, a scaling procedure to compare results of different scales and situations was also presented. Excellent correlations using different literature data and those of the present study were obtained.





Propagation of Shock Waves of Varying Curvature


Pages :
691701


Authors :
S. Lewin,
B. Skews,
When a shock wave having variable concave curvature propagates, it can develop a kink followed by the development of a reflected shock. A typical example is a plane incident shock encountering a surface with concave curvature, the part of the shock adjacent to the surface curves forward and subsequently develops into a Mach reflection with a Mach stem, shear layer and reflected shock. The physical mechanisms associated with the evolution of the shock profile was evaluated for shock waves with initial profiles comprising a cylindrical arc, placed inbetween two straight segments, propagating in a converging channel. The temporal variation of the pressure distribution immediately behind the shock wave was studied using CFD. This revealed a pressure imbalance in the region where the curved (which was initially cylindrical) and straight shock segments meet. This imbalance occurs due to the difference in the propagation behaviour of curved and planar shock waves, and results in the development of reflected shocks on the shock front. The angle at which the channel walls converge, the initial curvature radius, and the shock Mach number, was varied between 40 and 60 degrees, 130 and 190 mm and 1.1 to 1.4, respectively. The variation with time of the pressuregradient distribution and the maximum pressure gradient behind theshock wave was evaluated. From this, the trajectory angle of the triple points, and the rate at which the reflected shocks develop, was deduced. It was found that when shock waves with larger curvature radii propagate in channels with lower wall angles, the reflected shocks develop at a slower rate, and the triple points follow a steeper trajectory. Consequently, the likelihood of reflected shocks emerging on the shock front, within the duration of the shock propagation, is reduced. This is due to the triple points intersecting the walls, before reflected shocks can fully develop. Similarly, when the shock Mach number is higher, the trajectory angle of the triple points is greater, and they intersect the walls before the reflected shocks can emerge.





Passive Control with BladeEnd Slots and WholeSpan Slot in a Large Camber Compressor Cascade


Pages :
703714


Authors :
H. Wang,
B. Liu,
B. Zhang,
Suitable slot structure of the compressor blade can generate highmomentum jet flow through pressure difference between the pressure and suction surface, it has been proved that the slot jet flow can reenergize the local lowmomentum fluid to effectively suppress the flow separation on the suction surface. In order to explore a slotted method for better comprehensive suppressing effects on the boundary layer separation near blade midspan and the threedimensional corner separation, a diffusion stator cascade with large camber angle is selected as the research object. Firstly, the Slotted_1 and Slotted_2 wholespan slotted schemes are set up, then the Slotted_3 scheme with wholespan slot and bladeend slots is proposed, finally the performance of original cascade and slotted cascades is computed under a wide range of incidence angles at the Mach number of 0.7. The results show that: in the full range of incidence angles, compared with the wholespan slotted cascades, the development of the endwall secondary flow on the suction surface of Slotted_3 cascade is effectively suppressed, the degree of the mutual interference between the secondary flow and the main flow is reduced. Besides, on the suction surface of Slotted_3 cascade, the boundary layer separation near blade midspan and the corner separation are basically eliminated. As a result, compared with those of original cascade, the total pressure losses of Slotted_3 cascade are reduced in the full range of incidence angles, and its operating range of incidence angles is broadened. Moreover, compared with the wholespan slotted schemes, Slotted_3 scheme has a better adaptability to wide range of incidence angles.





A Simple Nonlinear Eddy Viscosity Model for Geophysical Turbulent Flows


Pages :
715722


Authors :
J. P. Panda,
K. Sasmal,
S. Maity,
H. V. Warrior,
Eddy viscosity models in turbulence modeling can be mainly classified as linear and nonlinear models. Linear formulations are simple and require less computational resources but have the disadvantage that, those can’t predict actual flow pattern in complex geophysical flows where streamline curvature and swirling motion are predominant. A constitutive equation of Reynolds stress anisotropy is adopted for the formulation of eddy viscosity including all the possible higher order terms quadratic in the mean velocity gradients and a simplified model is developed for actual oceanic flows where only the vertical velocity gradients are important. The simplified formulation is used for the study of natural convection flow in a vertical water column and the results are compared with the observational data and predictions of other existing turbulence models. The developed formulation can be incorporated in other computational fluid dynamics codes for the flow analysis in various engineering applications. The model predictions of marine turbulence and other related data (e.g. sea surface temperature, surface heat flux and vertical temperature profile) can be utilized in determining the effective siting for the Ocean Thermal Energy Conversion (OTEC) plants and in particular for the development of tidal energy projects.





Numerical Simulation of Flow and Heat Transfer Characteristics of a Liquid Jet Impinging on a Cylindrical Cavity Heat Sink


Pages :
723732


Authors :
Z. G. Tang,
F. Deng,
S. C. Wang,
J. P. cheng,
In this study, the flow and heat transfer characteristics of a liquid jet impinging on cylindrical cavity heat sinks with a local body heat source were numerically investigated. The Transition SST turbulence model was validated and adopted. The parameters of structure and flow, including d/D = 2, 3, and 4, h/D = 0.05, 0.10, 0.15, and 0.20, and Re = 5,000, 10,000, and 23,000, were investigated. The results revealed that the adoption of a cylindrical cavity structure can improve the heat transfer capacity of the heat sink. A horseshoe vortex introduced by an inclined jet near the cavity edge region improved the heat transfer performance. The maximum enhancement of the cylindrical cavity heat sink was 11.8% compared with the flat plate heat sink when d/D = 3, h/D = 0.15, and Re = 23,000.





Experimental Investigation and CFD Modelling of Settling Efficiency in a Cylindrical Tank


Pages :
733740


Authors :
G. Isenmann,
M. Dufresne,
J. Vazquez,
R. Mosé,
C. Fagot,
Evaluating the performance of a settling tank is an important issue for wastewater system managers. The relevance of a CFD approach for determining the settling efficiency of a tank has already been demonstrated from experimental data obtained from scale models of basins. The CFD modelling strategy is based on the resolution of NavierStokes equations to calculate the flow (Eulerian approach), and then on a Newton equation to calculate particle trajectories (Lagrangian approach). In this study, experimental data on settling efficiency are collected in a 1 scale cylindrical settling tank constructed in the laboratory. Eighteen experiments were carried out to collect data (settling efficiency) for a range of flow rates (between 5 and 30 l/s) and three materials representative of the sediments encountered in sewage networks. These data were then compared with the results obtained by CFD modelling to assess the relevance of the numerical approach for a fullscale structure.





Film Cooling Performance of the Staggered Arrangement of Auxiliary Holes and Main Holes on a Flat Plate


Pages :
741752


Authors :
W. Zhang,
H. R. Zhu,
The film cooling effectiveness of a staggered arrangement of small and large holes was investigated. The small auxiliary holes were located normal to the cooled surface, whereas the large main holes had an inclination angle of 30°. The center points of the small holes were located upstream, downstream, and in the same position as the main holes. A hole pitch (P/D) of 3 and a thickness (t/D) of 3 were considered. The film cooling performance of the hole in trench structure and the cylindrical hole was also determined. The numerical results show that largescale vortices caused by the auxiliary hole injection inhibit the development of vortices caused by the main hole in the streamwise direction. This result differs from that of antivortex film cooling. Compared to the baseline, the increase in the surfaceaveraged values is 1522% for the staggered arrangement, depending on the blowing ratio (0.5 to 2.0). The positions of the auxiliary holes have an effect on the spanwiseaveraged values.





A Numerical Investigation on the Equivalence of Shock−Wave/ BoundaryLayer Interactions using a Two Equations RANS Model


Pages :
753767


Authors :
B. John,
P. Vivekkumar,
A detailed numerical investigation of two different modes of shock waveturbulent boundary layer interaction (SWBLI) is presented. Equivalence of ramp induced SWBLI (RSWBLI), and impingement shock based SWBLI (ISWBLI) is explored from the computational study using an inhouse developed compressible flow solver. Multiple flow deflection angles and ramp angles are employed for this study. For all the investigated cases, a freestream Mach number of 2.96 and Reynolds number of 3.47×107m−1 are considered. The k−ε model with the improved wall function of present solver predicted wall pressure distributions and separation bubble sizes very close to the experimental measurements. However, the separation bubble size is slightly over overpredicted by the k−ω model in most of the cases. The effect of overall flow deflection angle and upstream boundary layer thickness on the SWBLI phenomenon is also studied. A nearly linear variation in separation bubble size is observed with changes in overall flow deflection angle and upstream boundary layer thickness. However, the equivalence of SWBLI is noted to be independent of these two parameters. The undisturbed boundary thickness at the beginning of the interaction is identified as the most adequate scaling parameter for the length of the separated region.





On the Stabilization of a Viscoelastic Jeffreys Fluid Layer Heated from Below


Pages :
769778


Authors :
I. PérezReyes,
A. S. OrtizPérez,
Feedback control is applied to the problem of a viscoelastic Jeffreys fluid layer heated from below to investigate conditions for delay of the onset of convection. Interesting results for fixed Prandtl number 1 and 10 were found showing that for some conditions proportional control may not work as expected. Also, some limits of the feedback control in terms of the parameters of the system through an analytical approach by mean of the Galerkin method are discussed. In order to complete the study a numerical analysis was also performed to map the space of physical parameters. The results of this work are discussed and compared with results of previous authors while attention to small control adjustments is paid.





Computational and Experimental Investigations of Separation Control of LP Turbine Cascade Blades using Gurney Flaps


Pages :
779792


Authors :
G. Tatpatti,
N. Sitaram,
K. Viswanath,
The paper reports computational and experimental investigations carried out to control of laminar flow separation in LP turbine cascade blades at low Reynolds numbers. T106 LP turbine blade profile with a chord of 60 mm and blade spacing of 48 mm was used. The blade Zweifel loading factor was 1.03. Passive separation control device of Gurney flaps (GFs) of different shapes and sizes were used. Computations were carried out in AnsysCFX. A twoequation eddyviscosity turbulence model, shear stress transport (SST) was considered for all the computations along with gammatheta (transition model. Computations were carried out for five different Reynolds numbers. Lift coefficient, total pressure loss coefficient, overall integrated loss coefficient and ratio of lift coefficient to overall integrated loss coefficient were used as a measure of aerodynamic performance for the cascade. From the computations, Flat and Quarter Round GFs of heights of 1.33% of chord were identified as the best configurations. Experiments in a seven bladed cascade were carried out for these configurations along with the basic configuration without GF at five Reynolds numbers. Experimental results agreed well with the computational results for these three cases at the five Reynolds numbers.





Compressibility Modified RANS Simulations for Noise Prediction of Jet Exhausts with Chevron


Pages :
793804


Authors :
Y. Jin,
X. Han,
P. Fan,
The impact of compressibility modified RANS turbulence closures is investigated for high subsonic round and chevron jet flows with Mach = 0.9 and Re = 1.03×106, including the predicted acoustic noise generation. The welldocumented chevron jet flow and noise cases, namely NASA SMC000 and SMC006 are selected as the simulation configurations. Two compressibility RANS closures are considered, which are based on the kε turbulence model. The first type only considers the compressibility dissipation rate, and the second type accounts for three modifications of compressibility dissipation rate, pressure dilation and production limiter. The acoustic noise is calculated employing the SNGR (Stochastic Noise Generation and Radiation) method using the flow prediction of the threedimensional RANS simulations. The results show that both of the two types of compressibility modified RANS models improve the accuracy of the mean flow and turbulence quantities. This results in more accurate jet noise predictions than with the standard RANS model. The first type modification is found to be moderate and the second type is remarkable. The noise results by the second type model, i.e. Sarkar2 model, agree with the experimental data quite well. For the mean flow field, the compressibility modified model (Sarkar2 model) estimates a shorter potential jet core, and improved predictions of the velocity in the downstream region are observed. The study demonstrates the importance of considering the compressibility modified RANS closure for the noise prediction of highspeed jets via the comparison to experimental data. Hence, the SNGR method is found to be cost effective for jet noise prediction, when compared to other approaches.





Numerical Simulation of Mass Transfer in Pulsatile Flow of Blood Characterized by Carreau Model under Stenotic Condition


Pages :
805817


Authors :
Su Mukhopadhyay,
M. Shankar Mandal,
Sw. Mukhopadhyay,
The present numerical study deals with a mathematical model representing mass transfer in blood flow under stenotic condition. Streaming blood is considered as a nonNewtonian fluid characterized by Carreau fluid model and the vessel wall is taken to be flexible. The nonlinear pulsatile flow phenomenon is governed by the NavierStokes equations together with the continuity equation while that of mass transfer is governed by the convectiondiffusion equation coupled with the velocity field. A finite difference scheme is developed to solve these equations accompanied bysuitable initial and boundary conditions. Results obtained are examined for numerical stability up to wanted degree of correctness. Various significant hemodynamic parameters are examined for additional qualitative insight of the flowfield and concentrationfield over the entire arterial segment with the help of the obtained numerical results. Comparisons are made with the available results in open literature and good agreement has been achieved between these two results. Comparisons have been made to understand the effects of viscosity models for Newtonian and nonNewtonian fluids and also for rigid and flexible arteries.





