Total Flow Field Research Articles

Experimental study on flow field characteristics of TBCC multibypass combustor

The turbine-based combined cycle (TBCC) engine, as a hypersonic air-breathing propulsion system, involves the transition between multiple bypasses during flight. During the transition process, the flow field of the combustor is affected. In this paper, the flow field of a TBCC multibypass combustor with typical structures is investigated experimentally. The effect of the combined structure on the mixing performance, total pressure loss and flow field distribution downstream of the TBCC multibypass combustor is investigated by changing the inlet aerodynamic parameters and structural parameters of the combustor. The results show that the diffusion ratio increased from 1.0 to 1.4, the thermal mixing efficiency decreased by 24.7%, and the total pressure recovery coefficient increased by 1.86%. Meanwhile, the recirculation zone is wider, and the velocity distribution behind the stabilizer is more uniform. The thermal mixing efficiency increases by 55.8% with the increase in the down inclination (35°∼45°) of the lobe mixer. The mixing characteristic of the triple-bypass combustor is greater than that of the double-bypass combustor. There is an obvious “phase difference” of the vortex shedding in the double-bypass combustor at different spanwise sections, while it does not exist in the triple-bypass combustor. The turbulence disturbance degree in the recirculation zone behind the stabilizer in the triple-bypass combustor is slightly higher than that in the double-bypass combustor, but the velocity distribution is more uniform. The increase in inlet Mach number increases the flow losses and decreases the mixing performance. However, the thermal mixing efficiency is positively impacted with respect to the inlet temperature. In addition, the turbulent kinetic energy of the shear layer behind the stabilizer is large, while it is relatively small in the recirculation zone. There is not only spanwise vortices but also radial vortices behind the stabilizer.

Design of Energy-Saving Duct for JBC to Reduce Ship Resistance by CFD Method

The present work is to modify the stern duct of Japan bulk carrier (JBC) to reduce the ship total resistance for Froude number 0.142. The research method is to use the CFD (computational fluid dynamics) tool OpenFOAM to predict the total resistance and viscous flow field around the ship hull considering the free surface. First, the V&V (verification and validation) analysis was conducted to ensure our CFD method is reliable, and to estimate its error and uncertainty range for the original JBC with and without the original stern duct. Second, the angle of attack (AoA) of the foil section of the stern duct was adjusted, thus forming a series of different ducts axisymmetric along the propeller shaft axis. The CFD simulations were performed for each duct appended behind the ship hull. By comparing the total resistance with and without the duct, the ship hull resistance reduction effect caused by the duct would be obtained. Finally, the duct with 7° AoA achieved the best resistance reduction of 2.49%. The original duct (with 20° AoA) could only provide a 0.6~0.7% reduction. The nominal wake was also analyzed with the ducts, in order to understand the detailed flow phenomena which had an essential influence on propeller inflow.

Oceanic Kármán Vortex Streets in the Luzon Strait in the Lee of Didicas Island from Multiple Satellite Missions

The oceanic Kármán vortex street is an important hydrokinetic phenomenon caused by the unsteady separation of sea currents in the wake of an obstacle. This study quantitatively analyzed the characteristics of the small-scale vortex street of Didicas Island in the Luzon Strait based on multimission satellite data. Five groups of vortex streets were captured, and each group was observed by two satellite images within one hour. Based on the displacement and change in the vortex street within one hour, we found that the formation time of a single vortex is longer than 40 min. The vortex propagation speed behind the island is approximately 0.98 m s−1 (ranging from 0.66–1.22 m s−1). The incoming velocity was calculated using the vortex propagation speed and the aspect ratio based on a vortex propagation model, which ranges from 0.72 to 1.47 m s−1 The incoming speed is 1.6–2.3 times the geostrophic speed extracted from AVISO data, implying that the contribution of non-geostrophic flow to the total flow field is comparable to that of geostrophic flow in this region.

Evaluation and analysis of evaporation cooling on thermodynamic and pressure characteristics of intake air in a precooled turbine engine

The precooled turbine engine is applied to overcome the limitation of Mach number due to high temperature inlet air. This paper aims to investigate the effect of water injection cooling on the high-temperature intake air. Then, the theory evaluation and Eulerian-Lagrangian multiphase flow method are conducted to explore the thermodynamic process and resistance characteristics of the pre-cooling section built-in even spray apparatus with a drag reduction. Results show that larger amount of the injection flow rate at higher Mach number will deteriorate total pressure loss and flow field uniformity. Evaporation cooling can decrease flow loss by 9.4%–60.7%. Within 27 ms, total-temperature drop is in 14–144 K range with a low total-pressure drop coefficient of 0.56%–1.29%. Especially, mass flow will increase by 1.15%–18.50%. Thus, water injection cooling is conducive to a higher acceleration, as well as for improving the thermodynamics characteristics of inlet air for a turbine engine at a high Mach number.

Numerical Simulation of Viscous Flows Around a Surface Combatant Model at Different Drift Angles Using Overset Grids

Viscous flows around ship hull is of great complexity, and when the ship is advancing with drift angles, the flow field can be more complicated. In this paper, the viscous flow field around an obliquely towed surface combatant DTMB 5512 is computed using the unsteady Reynolds-averaged Navier–Stokes (URANS) method. The numerical simulations are carried out by the in-house CFD solver naoe-FOAM-SJTU, which is developed on the open source platform OpenFOAM. To refine local grids and simulate dynamic ship motions, the overset grid approach is applied. Grid convergence study is first performed at straight-ahead towed condition with three sets of grid number and the results show monotonically convergence. Six different drift angles for the DTMB 5512 model at Froude number of 0.28 are simulated and the predicted hydrodynamic forces and flow field are presented. During the simulation, the ship hull is free to trim and sinkage at a free surface environment. As for straight-ahead towed condition, the total resistance coefficient and flow field, i.e., wave pattern and wake region, between this work and the experiment are compared. Both force coefficients and flow field show good agreement with the available experimental data. For oblique towed conditions, the lateral force, yaw moment and the derived hydrodynamic derivatives are also presented and compared with the experimental results. The variation of wave pattern and wake region at different drift angles are presented and analyzed. The results show that the current approach can be an effective tool to predict the viscous flows and hydrodynamic loads for oblique towed ships.

Aerodynamical behavior of spherical debris in the supersonic and rarefied wind tunnel MARHy

The atmospheric re-entry of space debris poses a significant risk to humans in the event of a ground impact. Predicting the trajectory of space debris is still a very complex task. The debris passes through different flow regimes and this makes it difficult to correctly estimate its aerodynamic behavior. This work responds to the need of setting up experimental databases in the transition and the slip flow regimes that will enable to validate the numerical codes. This paper presents the experimental results based on the study of the influence of rarefied effects on the shock shapes around the spheres. The experiments were conducted in the MARHy super-hypersonic rarefied wind tunnel, with flows at Mach 4 and Knudsen numbers ranging from 3×10−3 to 6×10−2. The shape of the shock waves and the standoff were determined by CMOS camera visualization. The total pressure flow field around the spheres was measured by a Pitot tube. Numerical modelling for a test case was performed using the Monte Carlo code DS2V, and validated with the experimental measurements.

Flow Hydrodynamic Characteristics in the Dam-Break Induced Wave Tip Region

Traditional models assuming a vertically uniform structure of the horizontal flow velocity and neglecting the vertical velocity distribution cannot accurately describe the complex boundary layer flow features in the dam-break induced wave tip region. Based on the assumption that the horizontal velocity profile in the wave tip region follows a vertically parabolic distribution with shear extending to the water surface, new solutions for the hydrodynamic characteristics in the wave tip region were derived with respect to the simplified force balance or the steady momentum equation, respectively. The force balance-based models show the relation between water depth [Formula: see text] and distance from the tip [Formula: see text] in the wave tip region as [Formula: see text], whereas a complex and implicit [Formula: see text] relation is confirmed after applying the momentum equation. Comparing with other models, the present momentum equation-based model gives the best agreements with experiments, which can illustrate the general spatial distribution of the wave profile in the wave tip region. Model predicted characteristics of surface particles’ motion are consistent with the experimental observation of Baldock et al. ([ 2014 ] “Flow convergence at the tip and edges of a viscous swash front — experimental and analytical modeling,” Coast Eng. 88, 123–130). As for the vertical velocity distribution in the wave tip region (being downward direction), it increases monotonically from the bed to the water surface and its magnitude increases when approaching the wave tip. Taking into account the relative streamline distribution and total flow field feature in the wave tip region, the present model can reproduce the uniform flow convergence pattern at the wave tip front, as experimentally observed in Baldock et al. [ 2014 ].

Radar/infrared integrated stealth optimization design of helicopter engine intake and exhaust system

Along with the diversification and sophisticated development of detection methods in modern warfare, helicopters are increasingly subject to unilateral or simultaneous threats from radar and infrared detectors. In order to improve the survivability and operational effectiveness of the helicopter, a comprehensive stealth approach based on Pareto solution is presented. Considering the geometric constraints and aerodynamic characteristics of the engine intake and exhaust system, the model of the system is established by the full factorial design, the internal, central and external flow fields are constructed, then the high-precision computational fluid dynamics method is used to simulate the total flow field under the rotor downwash airflow in hovering state. The radar cross section of the system is evaluated by the physical optics and physical theory of diffraction. Based on the Monte Carlo and ray tracking method, the infrared signature of the system is calculated and analyzed in detail. Under the comprehensive evaluation and selection of comprehensive stealth approach, the optimization model of the system is continuously established and updated. The ultimate design has achieved good results in both radar cross section reduction and infrared radiation suppression and the proposed method is effective and efficient for radar/infrared integrated stealth of helicopter engine intake and exhaust systems.

The Study on Internal Flow Characteristics of Magnetic Drive Pump

In order to study the inter flow of magnetic drive pump, the total flow field of magnetic drive pump was simulated with CFD software based on SST $k$ - $\omega $ turbulence model, and the flow fields distribution rules of cooling channel were analyzed under different working conditions. The numerical results show that the internal flow of magnetic drive pump was disorganized under small flow condition, then the inter flow becomes gradually smooth with the increase in flux, at the same time, the area with low speed near the outlet of reflow hole and the pressure of the cool circulation channel become smaller. The pressure of cooling channel gradually decreases in flow process from deflector hole to reflow hole, and the area of low pressure is easily formed near the outlet of reflow hole.

The total flow control equations and the characteristics of unsteady gradually varied flow in open channels

Most unsteady channel flows in nature and practical engineering appear as gradually varied ones, and in the free surface, the deformation conforms to the long wave hypothesis. One-dimensional total flow models were usually used to for the numerical simulation of long-term and long-distance reaches to describe the water movements, however, the models lack a clear relationship between the three-dimensional flow field and the total flow field. Moreover, few studies of the variations of the roughness coefficient against the time in unsteady flows were conducted. The following results are obtained through the theoretical analysis and the numerical simulations in this paper. (1) One-dimensional total flow control equations of the unsteady gradually varied flow in open channels are obtained directly from the mathematical model of the viscous fluid motion, and can both reflect the influence of the turbulence and provide an explicit expression of the energy slope term. These equations establish a direct connection between the descriptions of the three-dimensional flow fields and the one-dimensional total flows. (2) Synchronous prototype observation data and planar two-dimensional numerical simulation results are used to extract the one-dimensional total flow information and discuss the total flow characteristics. (3)The orders of magnitude for terms in the total flow motion equation are compared, and the variation of the roughness coefficient against the time is analyzed.

Evaluation for VOF Simulation of Flow Field in Asphalt Foaming Chamber Based on Volume Fraction

Abstract Asphalt with high viscosity, air, and water constitutes multi-phase turbulent flow accompanied by complex dynamic phenomena such as heat transfer and phase transition in the foaming chamber during the asphalt foaming process. The foaming process is difficult to fully describe by mathematical model. In order to establish the correspondence between the approximate simulation flow field in the foaming chamber and the physical experiment, it is necessary to propose a reasonable evaluation method for the flow field simulation in the foaming chamber. Approximate numerical expression of asphalt foaming process is established by the volume of fluid (VOF) multiphase flow calculation method combined with self-defined phase and mass transfer functions. Based on the principle of asphalt foaming, effective foaming area is defined as the area where temperature is ≥100°C, volume fraction of asphalt is ≥0.5, and volume fraction of water vapor is between 0.1 and 0.4. The ratio of the effective foaming area to the total flow field is taken as the evaluation index. The proportion of effective foaming area obtained by asphalt foaming numerical simulation is compared with the expansion rate and half-life obtained by asphalt foaming experiments with three foaming chambers of different sizes. The comparison shows that ratio of effective foaming area has the same tendency as expansion rate and half-life. And the simulation evaluation index is verified. Suggestions are put forward on the structure design of asphalt foaming chamber based on the distribution of effective foaming area. It shows that this method achieves rapid theoretical evaluation of foaming chamber structure and could be used for the optimal design of foaming chamber.

Enhanced gas flow uniformity across parallel channel cathode flow field of Proton Exchange Membrane fuel cells

Distributing reactant gases uniformly within a cell to ensure proper delivery of reactants is one the crucial requirements for better performance of a Proton Exchange Membrane fuel cell (PEMFC). This investigation was focused on enhancing flow uniformity and distribution of cathode air in parallel channel flow field of a fuel cell. 2D Computational Fluid Dynamic (CFD) simulations (i.e. using ANSYS) were performed to solve continuity and momentum equations across the cathode flow field. Two main designs with modified headers that provide (i) Z-type, and (ii) combination of Z and U-types parallel channel configurations, were investigated for uniform distribution of gases. Our presented designs were able to allow 60–70% of total flow through frontal 80% of total gas flow field area which was just reversed case compared to conventional design where nearly 80% of flow passes through 20% of total area at the end portion of the gas flow field. Further improvement in flow distributions was observed by segmenting the flow field.

Turbulence dependence on winds and stability in a weak‐wind canopy sublayer over complex terrain

AbstractThe daytime and nighttime turbulence profiles within a weak‐wind forest canopy were investigated by using data collected within a temperate mixed conifer canopy in northern Idaho, USA. Turbulence measurements made at three heights on a single tower within a Douglas fir canopy were compared. Data were split between the daytime and nighttime to determine the relationships among the local temperature gradient, wind direction, wind speed, and turbulence levels. The total flow field distributions and vertical statistical profiles were determined for the overnight and daytime periods to observe how the overall flow changed with time of day. During the day, the wind probability distribution function was consistent between heights but depended on the canopy depth overnight. The skewness changed with the dominant wind direction. The kurtosis increased with depth into the canopy and from during the day to overnight. The range of wind speeds observed was higher under unstable conditions than stable conditions. Daytime turbulence had no dependence on wind direction. Overnight, the relationship between turbulence and wind speed changed with wind direction and canopy depth. The highest turbulence values were associated with downslope winds near the canopy top, but the wind direction for the highest turbulence was variable within the trunk space.

Drivers of Barotropic and Baroclinic Exchange through an Estuarine Navigation Channel in the Mississippi River Delta Plain

Estuarine navigation channels have long been recognized as conduits for saltwater intrusion into coastal wetlands. Salt flux decomposition and time series measurements of velocity and salinity were used to examine salt flux components and drivers of baroclinic and barotropic exchange in the Houma Navigation Channel, an estuarine channel located in the Mississippi River delta plain that receives substantial freshwater inputs from the Mississippi-Atchafalaya River system at its inland extent. Two modes of vertical current structure were identified from the time series data. The first mode, accounting for 90% of the total flow field variability, strongly resembled a barotropic current structure and was coherent with alongshelf wind stress over the coastal Gulf of Mexico. The second mode was indicative of gravitational circulation and was linked to variability in tidal stirring and the horizontal salinity gradient along the channel’s length. Tidal oscillatory salt flux was more important than gravitational circulation in transporting salt upestuary, except over equatorial phases of the fortnightly tidal cycle during times when river inflows were minimal. During all tidal cycles sampled, the advective flux, driven by a combination of freshwater discharge and wind-driven changes in storage, was the dominant transport term, and net flux of salt was always out of the estuary. These findings indicate that although human-made channels can effectively facilitate inland intrusion of saline water, this intrusion can be minimized or even reversed when they are subject to significant freshwater inputs.

Numerical investigation on mixing and combustion of transverse hydrogen jet in a high-enthalpy supersonic crossflow

Mixing and combustion characteristics of a three-dimensional unsteady reacting flowfield generated by transverse hydrogen jet into high-enthalpy supersonic crossflow are investigated numerically. A hybrid RANS/LES (Reynolds-Averaged /Large Eddy Simulation) method acting as wall-modeled LES is adopted, where the ninth-order WENO (Weighted Essentially Non-oscillatory) scheme is introduced to discretize the inviscid fluxes. The validations of numerical results are performed for the jet penetration height, time-averaged and instantaneous structures of reacting flowfield. It is found that the Kelvin–Helmholtz instability of jet shear layer on the windward side of jet plume discontinuously induces large scale coherent structures, which promote the combustion by enhancing the fuel mixing and enlarging the reacting area. The chain reactions creating OH radical mainly occur in the lean-fuel region and the heat-releasing chain reactions consuming OH radical are in the rich-fuel region. In the boundary layer and windward shear layer where supersonic crossflow stagnates, a diffusion flame with autoignition first occurs and then propagates to the downstream of jet plume, which holds the flame stabilization of total flowfield.

Effect of impeller reflux balance holes on pressure and axial force of centrifugal pump

The size of impeller reflux holes for centrifugal pump has influence on the pressure distribution of front and rear shrouds and rear pump chamber, as well as energy characteristics of whole pump and axial force. Low specific-speed centrifugal pump with Q=12.5 m3/h, H=60 m, n=2950 r/min was selected to be designed with eight axial reflux balance holes with 4.5 mm in diameter. The simulated Q-H curve and net positive suction head (NPSH) were in good agreement with experimental results, which illustrated that centrifugal pump with axial reflux balance holes was superior in the cavitation characteristic; however, it showed to little superiority in head and efficiency. The pressure in rear pump chamber at 0.6 times rate flow is 29.36% of pressure difference between outlet and inlet, which reduces to 29.10% at rate flow and 28.33% at 1.4 times rate flow. As the whole, the pressure distribution on front and rear shrouds from simulation results is not a standard parabola, and axial force decreases as flow rate increases. Radical reflux balance holes chosen to be 5.2 mm and 5.9 mm in diameter were further designed with other hydraulic parts unchanged. With structural grids adopted for total flow field, contrast numerical simulation on internal flow characteristics was conducted based on momentum equations and standard turbulence model (κ-ɛ). It is found that axial force of pump with radical reflux balance holes of 5.2 mm and 5.9 mm in diameter is significantly less than that with radical reflux balance holes of 4.5 mm in diameter. Better axial force balance is obtained as the ratio of area of reflux balance holes and area of sealing ring exceeds 6.

Ambient air particle transport into the effluent of a cold atmospheric-pressure argon plasma jet investigated by molecular beam mass spectrometry

Ambient air species, which are transported into the active effluent of an atmospheric-pressure plasma jet result in highly reactive oxygen and nitrogen species (RONS). Especially for the envisaged application field of plasma medicine, these RONS are responsible for strong biological responses. In this work, the effect of ambient air transport into the effluent of an atmospheric-pressure plasma argon jet on the on-axis densities of nitrogen, oxygen and argon was investigated by means of absolutely calibrated molecular beam mass spectrometry (MBMS). According to biomedical experiments a (bottomless) Petri dish was installed in front of the MBMS. In the following, the near flow field is referring to the region close to the nozzle exit and the far flow field is referring to the region beyond that. The absolute on-axis densities were obtained by three different methods, for the near flow field with VUV-absorption technique, for the far flow field with the MBMS and the total flow field was calculated with a computational fluid dynamics (CFD) simulation. The results of the ambient air particle densities of all independent methods were compared and showed an excellent agreement. Therefore the transport processes of ambient air species can be measured for the whole effluent of an atmospheric-pressure plasma jet. Additionally, with the validation of the simulation it is possible in future to calculate the ambient species transport for various gas fluxes in the same turbulent flow regime. Comparing the on-axis densities obtained with an ignited and with a non-ignited plasma jet shows that for the investigated parameters, the main influence on the ambient air species transport is due to the increased temperature in the case when the jet is switched on. Moreover, the presence of positive ions (e.g. ) formed due to the interaction of plasma-produced particles and ambient air species, which are transported into the effluent, is shown.

Unsteady Reversed Stagnation-Point Flow over a Flat Plate

This paper investigates the nature of the development of two-dimensional laminar flow of an incompressible fluid at the reversed stagnation-point. Proudman and Johnson 1962 first studied the flow and obtained an asymptotic solution by neglecting the viscous terms. Robins and Howarth 1972 stated that this is not true in neglecting the viscous terms within the total flow field. Viscous terms in this analysis are now included, and a similarity solution of two-dimensional reversed stagnation-point flow is investigated by solving the full Navier-Stokes equations.

Unsteady Reversed Stagnation-Point Flow over a Flat Plate

This paper investigates the nature of the development of two-dimensional laminar flow of an incompressible fluid at the reversed stagnation-point. Proudman and Johnson (1962) first studied the flow and obtained an asymptotic solution by neglecting the viscous terms. Robins and Howarth (1972) stated that this is not true in neglecting the viscous terms within the total flow field. Viscous terms in this analysis are now included, and a similarity solution of two-dimensional reversed stagnation-point flow is investigated by solving the full Navier-Stokes equations.

A utilização de soluções exactas baseadas em análise espectral na caracterização do escoamento subterrâneo controlado pela topografia

Spectral analysis enhances the ability to analyze groundwater flow at a steady state by separating the top boundary condition into its periodic forms. Specifically, spectral analysis enables comparisons of the impact of individual spatial scales on the total flow field. New exact spectral solutions are presented for analyzing 3D groundwater flow with an arbitrarily shaped top boundary. These solutions account for depth-decaying, anisotropic and layered permeability while utilizing groundwater flux or the phreatic surface as a top boundary condition. Under certain conditions, groundwater flow is controlled by topography. In areas where the groundwater flow is controlled by the topography, the unknown water table is often approximated by the topography. This approximation induces a systematic error. Here, the optimal resolution of digital elevation models (DEMs) is assessed for use as a top boundary in groundwater flow models. According to the analysis, the water-table undulation is smoother than the topography; therefore, there is an upper limit to the resolution of DEMs that should be used to represent the groundwater surface. The ability to represent DEMs of various spectral solutions was compared and the results indicate that the fit is strongly dependent on the number of harmonics in the spectral solution.