Journal Of Applied Fluid Mechanics
Volume:14 Issue: 3, May-Jun 2021

In this study, the round jet flow behavior arose by the confinement was investigated experimentally and numerically. The confinement characteristics based on multi-scale 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.

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 flat-bottomed 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.

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 in-between 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 pressure-gradient 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.

Suitable slot structure of the compressor blade can generate high-momentum jet flow through pressure difference between the pressure and suction surface, it has been proved that the slot jet flow can reenergize the local low-momentum 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 three-dimensional corner separation, a diffusion stator cascade with large camber angle is selected as the research object. Firstly, the Slotted_1 and Slotted_2 whole-span slotted schemes are set up, then the Slotted_3 scheme with whole-span slot and blade-end 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 whole-span 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 whole-span slotted schemes, Slotted_3 scheme has a better adaptability to wide range of incidence angles.

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

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.

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 Navier-Stokes 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 full-scale structure.

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 large-scale 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 anti-vortex film cooling. Compared to the baseline, the increase in the surface-averaged values is 15-22% 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 spanwise-averaged values.

A detailed numerical investigation of two different modes of shock wave-turbulent boundary layer interaction (SWBLI) is presented. Equivalence of ramp induced SWBLI (R-SWBLI), and impingement shock based SWBLI (I-SWBLI) is explored from the computational study using an in-house 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.

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.

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 Ansys-CFX. A two-equation eddy-viscosity turbulence model, shear stress transport (SST) was considered for all the computations along with gamma-theta (−) 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.

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 well-documented 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 k-ε 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 three-dimensional 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 high-speed 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.

The present numerical study deals with a mathematical model representing mass transfer in blood flow under stenotic condition. Streaming blood is considered as a non-Newtonian fluid characterized by Carreau fluid model and the vessel wall is taken to be flexible. The nonlinear pulsatile flow phenomenon is governed by the Navier-Stokes equations together with the continuity equation while that of mass transfer is governed by the convection-diffusion 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 flow-field and concentration-field 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 non-Newtonian fluids and also for rigid and flexible arteries.

Lighter weight, simpler structure and throat area controllable are the developing trends of aircraft engine exhaust system. To meet these challenges, a new concept of hybrid throat control (TC) nozzle was proposed to improve the control efficiency of throat area (η) by using a rotary valve with secondary injection. The flow mechanism of the hybrid TC nozzle and the effect of aerodynamic and geometric parameters on nozzle performance were investigated numerically. Then the approximate model characterizing the hybrid TC nozzle was established with design of experiment and response surface methodology. The approximate model was used to analysis the coupling effect between parameters and optimized the parameter combination. The results show that the flow area of the nozzle can be restricted effectively by the rotary valve and the secondary flow, and η is bigger than 5.24. Nozzle pressure ratio and secondary pressure ratio are the dominant factors for the nozzle throat area control performance. The optimization of the parameter combination was carried out with penalty function approach, with ratio of throat area control being 30 percent and corrected mass flow ratio of secondary flow being 5 percent The maximize error of the optimization result is 4.13 percent and it verifies the validity and feasibility of the approximate model.

A preheating exchanger is developed for improving acidic water degassing. Reasonable optimization of dualinlet swirl heating tubes is analyzed by computations of the flow and heat transfer. The comparisons of the swirl number and circumferential average Nusselt number between isobaric injection and isokinetic injection are performed. Inlet area ratios ranging from 0.1 to 0.9 exhibit an important influence on the flow phenomena and the heating performance. A lower value of inlet area ratio leads to the tendency for the fluid passing through inlet 2 to move upstream of inlet 2 and results in more vortex pairs between inlets 1 and 2. An inlet area ratio value of 0.5 exhibits the largest global average Nusselt number, normalized Nusselt number, and thermal performance factor. The optimized inlet area ratio is suitable for improving the degassing efficiency

This paper presents the stability characteristics of a high-speed aircraft in a possible emergency situation of a single horizontal tail failure during flight. The flight control system of the aircraft under study operates with a fail-safe mechanism where the malfunctioned horizontal tail is self-locked in neutral position, while the other tail can normally perform its operations. However, in such a scenario the aircraft is required to land at the nearest airfield on priority. Computational analysis is carried out to analyze the stability characteristics of the aircraft under this emergency where it is subjected to adverse pitching, rolling and yawing moments due to the locked horizontal tail. For computational analysis, a unique analysis technique is employed to isolate the horizontal tail geometry from aircraft and domain which helps in geometry/mesh consistency, even with different horizontal tail deflections. The results of baseline configuration are validated with literature and subsequently, the analysis is carried out at various flow conditions, horizontal tail deflections and ground clearances. A complete flight envelope is determined based on horizontal tail, ailerons and rudder deflection along with landing angle of attack for safe landing. The study can help in further improvement of the aircraft flight control computer to restrict the tail, aileron and rudder deflections up to the evaluated safe limits. Also, the designed methodology i2s applicable to all similar aircraft.

When gravitational mass flows hit water bodies, they create water waves, called tsunami. The slopes of the mountain flanks surrounding a glacial lake or the slopes of the side walls of artificially constructed reservoirs play important roles in the intensity of splash on landslide impact, amplitudes and propagation speeds of the resulting water waves and possible dam breaching or overspilling of water. The proper analyses of such dynamics are useful for the possible mitigation measures. Here, we apply a general two-phase mass flow model to perform several numerical experiments and present geometrically three-dimensional, high-resolution simulation results for rapidly moving two-phase landslide/debris flow down a plane with varying slopes at its different parts, impacting a fluid reservoir. First, the upstream slope is kept constant; later to make it closer to reality, sudden changes in slopes are imposed one after another at different parts of the topography. The results focus on the effects of the sudden slope changes in the formation and propagation of dynamically different solid- and fluid wave-structures in the reservoir. Results show that steeper upper part of the topography produces more highly intensified tsunami that propagates more longitudinally than the steeper lower part. Thus, steeper upper parts need stronger right coast and steeper lower parts demand stronger side walls in mountain reservoirs to withstand the wave impacts. The results may help for the proper modeling of landslide and debris induced mountain tsunamis in rapidly changing slopes, the dynamics of turbidity currents and sediment transports in fluid reservoirs in high mountain slopes.

The objective of this study is to investigate the effect of the air-to-liquid ratio (ALR) in a low range on the characteristics of the spray issuing from a pressure-swirl duplex nozzle. In this study, the pressure-swirl duplex nozzle was used as an atomizer with non-swirl shroud air. The shroud air was radially discharged inward across the nozzle face to avoid the contamination of the nozzle tip. Jet A-1 fuel was used as the working fluid. The analysis of the spray characteristics was carried out by using a phase Doppler anemometry (PDA) system and a laser based planer imaging system. The flow rate, discharge coefficient, spray structure, spray cone angle, velocity and drop size distributions were analyzed. The results show that the discharge coefficient of the pilot nozzle is higher than that of the main nozzle and the combined pilot and main nozzles. The spray angle tends to decrease almost linearly with increasing ALR. The shape of the spray gradually changes from a hollow cone to a full cone with increasing ALR, as revealed in the axial velocity distributions with an increase in the axial distance. The weighted mean SMD (WMSMD) increases by 1.2 as the ALR increases, but thereafter, it decreases again.

The aim of this work is to carry out an aerodynamic analysis to assess the power and identify the characteristics of the horizontal axis Magnus type wind turbine with spiral fins. A parametric study is achieved to analyze the effects of the different influence parameters such as the turbine tip speed ratio, the cylinder spinning ratio, the blade aspect ratio and the cylinder hub-tip ratio. The analysis approach adopted in this study is the Blade Element Momentum BEM using the experimental lift coefficient data for the configuration of spinning cylinders with spiral fins. In this analysis, losses are not taken in consideration. Both axial and angular interference coefficients are evaluated for this type of wind turbine. The former is assessed by solving a quadratic equation and the latter is calculated from a classic formulation including the term of spinning. An iterative process is followed to achieve this task. Concerning the obtained results, the aerodynamic characteristics of the Magnus wind turbine, analyzed in this study, provide some elucidation to lead a successful preliminary design of this novel type of machine.

The current paper reports on a new theory developed by modifying the basic Moore-Greitzer model, which can predict the performance of a compression system during the instabilities in more details. The general assumptions such as the compression system layout, the lags in the entrance and exit ducts, the compressor axisymmetric characteristic and the small disturbances are similar to those of Moore-Gereitzer model. However, a second order hysteresis is used in the current work for the pressure rise of the rotor and stator rows. As a result, some new parameters are added to the governing equations, such as the stall cell acceleration ( 2 2 d r / d ), second derivative of the mean axial flow coefficient ( 2 2 d  / d ), second derivative of the disturbance amplitude ( 2 2 d A / d ) and slope of the compressor characteristic curve. This gives the modified model new capabilities, like investigating the transient speed of the stall cell or the effect of the throttling rate on the instabilities, which are discussed in details in the current paper.

In finite-volume-based simulations with free-surface waves, it is usually desired to obtain a sharp interface between both fluid phases. A widely used approach for interface-capturing and sharpening is the combination of the volume-of-fluid method with the HRIC scheme. The HRIC scheme contains a user-defined parameter, the angle factor, which influences the magnitude of the interface-sharpening. The present work demonstrates that the optimum value for the angle factor is case dependent: too small values can cause substantial flow disturbances, such as vorticity production within the wave and the occurrence of parasitic wave components, whereas too large values can cause excessive dissipation of wave energy. The optimum value for the angle factor was found to depend on the wave steepness, the aspect ratio of the grid cells, on the cell size and to a lesser degree on the time step size. Results from an extensive parameter study are presented, which can provide guidance for optimizing the angle factor for flow simulations of free-surface wave propagation. Further, two methods are presented which can be used to determine the optimum value of the angle factor. The magnitude of errors that can occur due to improper choices of the angle factor are discussed and recommendations are given to increase the accuracy of flow simulations with free-surface waves.

The aim of the present paper is the study of interaction of the abrupt wave with vertical and inclined rectangular obstacles. For this purpose, in the first step, two experiments have been done. The tests were performed with smooth rectangular cross-section channels, and related data were extracted using digital image processing. Flow behavior was recorded with one adjacent CCD camera through the glass walls of the entire downstream channel. In the second step, the numerical study has been done by a mesh-free particle Lagrangian method (Incompressible Smoothed Particle Hydrodynamics, ISPH) and a mesh-based Eulerian method (Finite Volume Method with Volume of Fluid surface tracking approach, FV-VOF). The capabilities of the numerical methods in simulation of the sudden variations free surface flows have been assessed. A comparison between the computed results and the experimental data shows that both numerical models simulate the mentioned flows with reasonable accuracy.

Miniaturized electronic components require effective heat transfer mechanism to dissipate heat with less surface area available for convective heat dissipation. Liquid cooling system with in- built microchannel is one of the feasible options. The new idea proposed in the present work is incorporation of waviness at selective locations in the microchannel. This method enhances heat transfer as well as maintains uniform surface temperature. Three-dimensional numerical simulations are carried out using ANSYS Fluent 15. Water is taken as the working fluid. Present numerical results of base case with plane wall are validated using published experimental and numerical results available in literature. Systematic study has been conducted by varying the flow Reynolds number and design parameters viz., wave amplitude and wavelength of the waviness on bottom wall. The computational results are presented in the form of Nusselt number, pressure drop and friction factor. Performance of the wavy wall microchannel is better with short wavelengths. In the present configuration of rectangular microchannel, wave amplitude of 0.2Dh with wavelength of 3Dh shows optimum performance. Moreover, selective waviness on bottom wall shows better performance with uniform surface temperature.

An annular venturi injector (AVI) was proposed to form an intermittent flow structure in airlift pump for a good pump performance. Experiments were conducted to investigate the performance of the airlift pump with this AVI by comparing with pump performance with traditional injectors, at a series of air flow rates . It was found that airlift pump with AVI had higher flow rates of output liquid and particle, than those with the traditional injectors. This AVI promoted the gas core to collapse and formed an intermittent flow structure in rising pipe. For this intermittent structure, its slug length, firstly increased to a maximal value with increasing gas flow rate and then remained stable even under a high gas flow rate, while its slug frequency decreased with gas flow rate and then remained to a minimal value under a high gas flow rate.

In order to improve the performance of a high-load transonic axial compressor, this paper proposes a method of applying endwall synthetic jet to the casing for active flow control. Taking NASA Rotor35 as the research object, the aerodynamic performance of the compressor is numerically calculated by applying three sets of synthetic jets with different excitation parameters at five different axial positions of 0%Ca, 25%Ca, 50%Ca, 75%Ca and 96.15%Ca. The results show that the three parameters of excitation position, jet peak velocity and jet frequency all have an effect on the performance of the compressor. The excitation position has the greatest influence on the flow margin of the compressor, and the best position is 25%Ca. After the jet peak velocity is increased from 100m/s to 150m/s, the flow margin, total pressure ratio and efficiency of the compressor are not greatly improved, which shows that the impact of the jet peak velocity is not as good as the excitation position. After continuing to increase the excitation frequency of the synthetic jet from 600Hz to 1200Hz, although the flow margin of the compressor is slightly reduced, the total pressure ratio and efficiency are further improved. This shows that there may be a threshold for the jet frequency, and only when the jet frequency is greater than the threshold can the overall aerodynamic performance of the compressor be improved