Numerical and experimental investigations on the mach 2 pseudo-shock wave in a square duct

This paper describes numerical and experimental investigations for the multiple shock wave/turbulent boundary layer interaction in a Mach 2 supersonic square duct. The numerical simulation is carried out with the Harten-Yee second-order accuracy TVD scheme and the Baldwin-Lomax turbulence model. The flow conditions are a free-stream Mach number ofM _∞≈=2.0 and a Reynolds number ofRe _∞;=2.5×10⁷ and the flow confinements are δ_∞/h=0.15 (case A) and δ_∞/h=0.25 (case B), respectively. The computational results for both cases show good agreement with the experimental results. Based on these agreements, the flow quantities, which are very difficult to obtain experimentally, are analyzed by numerical simulation. Moreover, the effect of flow confinement on the pseudo-shock wave characteristics is also presented.     

Investigation on the flameholding mechanisms in supersonic flows: backward-facing step and cavity flameholder

Abstract  

As effective devices to extend the fuel residence time in supersonic flow and prolong the duration time for hypersonic vehicles cruising in the near-space with power, the backward-facing step and the cavity are widely employed in hypersonic airbreathing propulsive systems as flameholders. The two-dimensional coupled implicit RANS equations, the standard k-ε turbulence model, and the finite-rate/eddy-dissipation reaction model have been used to generate the flow field structures in the scramjet combustors with the backward-facing step and the cavity flameholders. The flameholding mechanism in the combustor has been investigated by comparing the flow field in the corner region of the backward-facing step with that around the cavity flameholder. The obtained results show that the numerical simulation results are in good agreement with the experimental data, and the different grid scales make only a slight difference to the numerical results. The vortices formed in the corner region of the backward-facing step, in the cavity and upstream of the fuel injector make a large difference to the enhancement of the mixing between the fuel and the free airstream, and they can prolong the residence time of the mixture and improve the combustion efficiency in the supersonic flow. The size of the recirculation zone in the scramjet combustor partially depends on the distance between the injection and the leading edge of the cavity. Further, the shock waves in the scramjet combustor with the cavity flameholder are much stronger than those that occur in the scramjet combustor with the backward-facing step, and this causes a large increase in the static pressure along the walls of the combustor.     

Flow turbulization in a pseudo-shock forming in an axisymmetric duct with a frontal inlet

The results of the numerical modeling of the supersonic flow in an axisymmetric duct in which a pseudo-shock arises are presented. The duct includes the frontal inlet with a funnel-shaped part of initial compression of the supersonic flow and with a cylindrical throat part as well as the subsequent (cylindrical or diverging) diffuser where the flow slows down to a subsonic velocity. The flow conditions at the freestream Mach number M = 6 have been considered. Numerical computations of the flow have been done using a Navier–Stokes equations code and the k-ω SST turbulence model. As a result of computations, such flow parameters have been determined as the location of the pseudo-shock beginning, the length of the pseudo-shock supersonic part, the pressure distribution on the duct wall, the total pressure losses as well as the characteristics of flow turbulence. In particular, the variation of the turbulence intensity and turbulent viscosity along the pseudo-shock length have been examined and, based on these characteristics, the possibility of determining the location of a cross section, in which the pseudo-shock can be treated as completed, have been considered.     

Experimental and numerical investigation of flow separation in a supersonic nozzle

The flow in a two-dimensional symmetric convergent-divergent supersonic nozzle with and without a deflector at the exit plane was tested experimentally and numerically. Combined optoelectronic devices were used to demonstrate the possibilities of the shadow and schlieren methods and holographic interferometry for the flow visualization especially in the separation region. A series of experiments was performed in a trisonic wind tunnel (VTI, Beograd). The experimental results obtained using optical methods are compared with the pressure measurements and numerical calculations. Numerical results were obtained using a code for solving the average Navier-Stokes equations. Five structured meshes with different resolutions and two turbulent models were employed. The coupled influence of the mesh resolution and the order of accuracy of the used numerical scheme is analyzed.     

Constrained large-eddy simulation of supersonic turbulent boundary layer over a compression ramp

A supersonic turbulent boundary layer over a compression ramp is numerically investigated using the constrained large-eddy simulation (CLES) method. The compression corner is characterised by a deflection angle of 24°. The free-stream Mach number is Ma_∞ = 2.9, and the Reynolds number based on the momentum thickness of inlet boundary layer is Re_θ = 2300. The mean and statistical quantities, such as mean velocity, wall pressure and Reynolds stresses, are thoroughly analysed and compared with those from traditional large-eddy simulation (LES), experimental measurement and direct numerical simulation (DNS). It turns out that CLES can predict the friction coefficient, wall-pressure distribution, size of separation bubble, Reynolds stresses, etc. more accurately than traditional LES, and the results are in reasonable agreement with the experimental and/or DNS data. Also discussed are the effects of specific parameterisations of the Reynolds constraint and interfacial positions separating the constrained and unconstrained regions on the performance of the CLES method.     

Numerical simulation and flow visualization using soap film of the self-organized vortex structure in the wake of an array of cylinders

Abstract  

With the aid of computational fluid dynamics (CFD) and simple flow visualization technique using flowing soap-film, we present here the wake structures behind an array of cylinders for Reynolds numbers corresponding to both laminar and turbulent flow regimes. The image results illustrate interesting vortex interactions past these equally spaced cylinders; for low Reynolds number flow, well-organized wake pattern persists and manifests unsteadily to different symmetry states. An increase of free stream flow velocity causes the wake transition, resulting in the formation of asymmetric flow wake with chaotic mixing at the far wake. Observations from both the numerical simulations and soap-film are in good agreement at least qualitatively.     

Detection of separation in S-duct diffusers using shear sensitive liquid crystals

In the present experimental investigation, shear sensitive liquid crystals have been successfully used to study the flow characteristics and detect separation in two-dimensional Sduct diffusers of different curvatures. Tapered-fin vortex generators in two different orientations were used to control flow separation that was observed on one of the curved walls of the diffuser. The results were verified by conventional oil flow visualization technique and excellent agreement was observed. In addition to visualization, detailed measurements that included wall static pressure, skin friction, diffuser exit total pressure and velocity distributions were taken in a uniform inlet flow with Reynolds number of 3.49 × 10⁵. These results are presented here in terms of skin friction distribution, distortion and total pressure loss coefficients. The extent of the separation zone (in terms of intensity of red distribution) in the diffuser with and without vortex generators (in both configurations) compared well with the Preston tube measurements. The present study demonstrates that shear sensitive liquid crystals can be efficiently used to study the flow physics in complex internal flows. In addition, the results also indicate that shear sensitive liquid crystals can be effectively used not only as flow visualization tool but also to gain quantitative information about the flow field in internal flows.     

Direct numerical simulation of the flow around a wall-mounted square cylinder under various inflow conditions

The flow around a wall-mounted square cylinder of side d is investigated by means of direct numerical simulation (DNS). The effect of inflow conditions is assessed by considering two different cases with matching momentum-thickness Reynolds numbers Re_θ ? 1000 at the obstacle: the first case is a fullyturbulent zero pressure gradient boundary layer, and the second one is a laminar boundary layer with prescribed Blasius inflow profile further upstream. An auxiliary simulation carried out with the pseudo-spectral Fourier–Chebyshev code SIMSON is used to obtain the turbulent time-dependent inflow conditions which are then fed into the main simulation where the actual flow around the cylinder is computed. This main simulation is performed, for both laminar and turbulent-inflows, with the spectral-element method code Nek5000. In both cases the wake is completely turbulent, and we find the same Strouhal number St ? 0.1, although the two wakes exhibit structural differences for x > 3d downstream of the cylinder. Transition to turbulence is observed in the laminar-inflow case, induced by the recirculation bubble produced upstream of the obstacle, and in the turbulent-inflow simulation the streamwise fluctuations modulate the horseshoe vortex. The wake obtained in our laminar-inflow case is in closer agreement with reference particle image velocimetry measurements of the same geometry, revealing that the experimental boundary layer was not fully turbulent in that dataset, and highlighting the usefulness of DNS to assess the quality of experimental inflow conditions.     

Study on the optimum mixing throat length for drive nozzle position of the central jet pump

The present paper describes a numerical prediction of optimum mixing throat length for various drive nozzle positions of the central jet pump. The flow pattern and pressure distribution in the pump for various positions of the drive nozzle are investigated by three-dimensional numerical analysis using the RNG k-ε turbulent flow model. Numerical analysis was carried out for values of the nozzle throat ratiod/D of the jet pump of 0.5, 0.6 and 0.7, respectively. The static pressure in the flow field of the jet pump is calculated for the following conditions: (1) drive nozzle position from the entrance of the throatl /D=0 ∼ 2.0, (2) flow rate ratioM=0∼ 1.2, and (3) Reynolds numberRe=3.6×10⁵. These calculations revealed that (1) the optimum length of the mixing throat forl/D=0∼ 1.0 isLm/D=2.0 ∼ 3.5, (2) the length of the mixing throat forl/D=0 andM=0 (suction flow rate ratio=0) is approximatelyLm/D=3.5, and (3) the maximum efficiency is obtained ford/D=0.6 atl/D=0.5. Moreover, the flow pattern in the mixing throat is clarified through a spark tracing experiment. The results obtained in the visualization experiment and the numerically obtained mixing length agreed well.     

Design of supersonic three-dimensional inlets using two-dimensional isentropic compression flow

The design of supersonic three-dimensional inlets using the V-shaped body forming a two-dimensional flow including an initial oblique shock wave and a subsequent isentropic compression wave is considered. Such a flow appears attractive for inlets design due to a possibility of obtaining high compression levels of external flow over the inlet ramp with small losses of the total pressure. Numerical computations of the flows around the designed configurations were carried out in design and off-design regimes using Euler code. The flow structure was identified, the aerodynamic characteristics of the inlets were determined. The investigation covers the range of supersonic speeds corresponding to the freestream Mach numbers M_∞= 1.8−2.5.     

Experimental and numerical study on onset of inflow in near field of buoyant jet at low Froude number

Abstract  

In the present paper, the onset of inflow in the near field of a vertical buoyant jet issuing from a square duct is studied by experimental flow visualization and numerical simulation. The experimental critical condition for the onset of inflow is obtained from the scanning LIF visualization in the near field of the buoyant jet at various combinations of Froude numbers and Reynolds numbers. The experimental result shows that the critical Froude number increases with an increase in Reynolds number of the buoyant jet. The critical condition is also examined by numerical simulation based on the Navier-Stokes equation and energy conservation equation, under the assumption that the flow separation occurs at the duct exit. The main feature of the inflow observed by experiment is well reproduced in the numerical results.     

Flow characteristics around a square cylinder with changing chamfer dimensions

Abstract  

It is known that for a square cylinder subjected to uniform flow, the drag force changes with the angle of attack. To clarify the flow characteristics around a square cylinder with corner cutoffs, we measured the drag coefficient and the Strouhal number for changing chamfer dimensions. We analyzed the flow around a square cylinder with corner cutoffs by applying the RNG k–ε turbulent model, and investigated the surface flow pattern using visualization by means of the oil film and mist flow method. From these results, we obtained the surface flow patterns by the oil film method and numerical analysis. The numerical results agreed well with the experimental values. The drag coefficient of the square cylinder with corner cutoffs decreased suddenly at an angle of attack of about α = 0°– 10° when compared with the drag coefficient for a square cylinder. The minimum value of the drag coefficient for the square cylinder with corner cutoffs decreased by about 30% compared with that for the square cylinder. The drag coefficient of the square cylinder with 10% corner cutoffs was found to be smallest, since the wake area of this square cylinder was smaller compared with that of the other square cylinder.     

Computation of streamwise vorticity in a compressible flow of a winglet nozzle-based COIL device

Chemical oxygen iodine laser (COIL) is a high-power laser with potential applications in both military as well as in the industry. COIL is the only chemical laser based on electronic transition with a wavelength of 1.315 μm, which falls in the near-infrared (IR) range. Thus, COIL beam can also be transported via optical fibers for remote applications such as dismantling of nuclear reactors. The efficiency of a supersonic COIL is essentially a function of mixing specially in systems employing cross-stream injection of the secondary lasing (I₂) flow in supersonic regime into the primary pumping (O₂¹Δ_g) flow. Streamwise vorticity has been proven to be among the most effective manner of enhancing mixing and has been utilized in jet engines for thrust augmentation, noise reduction, supersonic combustion, etc. Therefore, a computational study of the generation of streamwise vorticity in the supersonic flow field of a COIL device employing a winglet nozzle with various delta wing angles of 5°, 10°, and 22.5° has been carried out. The study predicts a typical Mach number of approximately 1.75 for all the winglet geometries. The analysis also confirms that the winglet geometry doubles up both as a nozzle and as a vortex generator. The region of maximum turbulence and fully developed streamwise vortices is observed to occur close to the exit, at x/λ of 0.5, of the winglets making it the most suitable region for secondary flow injection for achieving efficient mixing. The predicted length scale of the scalloped mixer formed by the winglet nozzle is 4λ. Also, the winglet nozzle with 10° lobe angle is most suitable from the point of view of mixing developing cross-stream velocity of 120 m/s with acceptable pressure drop of 0.7 Torr. The winglet geometry with 5° lobe angle is associated with a low cross-stream velocity of 60 m/s, whereas the one with 22.5° lobe angle is associated with a large static and total pressure drop of 1.87 and 9.37 Torr, respectively, making both the geometries unsuitable for COIL systems. The experimental validation shows a close agreement with the computationally predicted values. The studies for the most suitable 10° lobe angle geometry show an observed Mach number of 1.72 with an improved mixing efficiency of 74% due to the occurrence of predicted streamwise vortices in the flow.     

On flow unsteadiness induced by structural vibration

In this paper, the effects of structural vibration on flow unsteadiness are investigated numerically. A fully coupled model, that solves the unsteady flow equations as well as the dynamic equations of the structure, is used. Numerical experiments are carried-out for flow over a backward-facing step, where a large number of numerical and experimental data exist for comparisons. The flexible structure is upstream of the step and is excited by a plane acoustic wave from the side opposite to the flow. Three Reynolds number cases are studied: 300 for a laminar flow, 3000 for a transitional flow and 15 000 for a turbulent flow. The results obtained are in good agreement with experimental observations and show the strong coupling between structural vibration and the resulting flow unsteadiness.     

Characteristics of the plume particles removed by a swirling flow nozzle in laser ablation

The plume particles removed by a swirling flow nozzle in laser ablation have been characterized with numerical and experimental approaches in this paper. The swirling flow was simulated by a computational fluid dynamic (CFD) software with RNG k–ε turbulent model. The air flow passed through a specifically designed swirling flow nozzle and impinged on the substrate with various inlet velocities. The trajectories of the plume particles with various diameters in the flow field were calculated and compared with the flow visualization in the experiments. The results show that the velocity distribution of the swirling flow on the substrate was significantly affected by the swirling strength of the flow. It shows that the plasma plume was removed efficiently and the surface roughness was significantly reduced by the implementation of swirling flow in laser ablation.     

Snapshot POD analysis of transient flow in the pilot stage of a jet pipe servo valve

The spatial evolution of a turbulent flow in the pilot stage of a jet pipe servo valve at the inlet pressure and deflection angle of the jet pipe is investigated using a large eddy simulation (LES). The pressure of the same flow field is measured by a high frequency dynamic pressure sensor in the experiments and is compared with the results of the LES, as well as their root-mean-square (RMS) and fast Fourier transform (FFT) results. The results of experiments and LES are in good agreement, indicating that LES is able to predict the flow dynamics. Velocity datasets based on LES are utilised to conduct the snapshot proper orthogonal decomposition (snapshot POD) technique. The snapshot POD analysis results of the first 4 modes show a full ability to directly visualise details of the coherent structures. The influences of the inlet pressure and deflection angle of the jet pipe are also discussed. Under different inlet pressures, the velocity eigenfunctions of the first mode are similar, while the locations and strengths of the vortices in high modes are different. The Lamb-Oseen vortices that affect the trajectory of jet streams are observed in the vicinity of the entrances of receiver channels only in the first mode, and several spindly vortices appear in the region of ?5?y/n?相似文献   

CFD在双吸式离心泵优化设计中的应用

针对某双吸式离心泵流量和扬程达不到设计要求,效率偏低的情况,对该泵内部三维湍流进行了数值模拟,通过对泵内流场和总压变化过程的分析,找出了该泵达不到设计要求的原因,提出了切割叶轮进口以扩大进口面积的改进方案.对改进后的泵内部流场进行了数值模拟,并与改进前泵内流场数值计算结果进行对比,性能预测表明改进后的泵基本达到了设计要求.在此基础上,对改进后的泵进行了实验测试,结果表明上述改进措施是有效的,数值模拟方法为水泵的优化设计提供了有力工具.     

A numerical and experimental study of the high-enthalpy high-speed cavity flow

Results of an experimental and numerical study of supersonic turbulent high-enthalpy flow in a channel with cavity are reported. On the basis of wind-tunnel tests performed in the IT-302M short duration wind tunnel, data on the flow structure and on the distribution of static pressure along the model walls were obtained. These data were subsequently used to verify the numerical algorithm. In the calculations, a parametric study of the effects of Mach number, cavity configuration, and temperature factor on flow quantities was performed. It was numerically shown that variation of the above parameters leads to a transition of the flow regimes in the vicinity of the cavity.     

A pressure-based algorithm for multi-phase flow at all speeds

A new finite volume-based numerical algorithm for predicting incompressible and compressible multi-phase flow phenomena is presented. The technique is equally applicable in the subsonic, transonic, and supersonic regimes. The method is formulated on a non-orthogonal coordinate system in collocated primitive variables. Pressure is selected as a dependent variable in preference to density because changes in pressure are significant at all speeds as opposed to variations in density, which become very small at low Mach numbers. The pressure equation is derived from overall mass conservation. The performance of the new method is assessed by solving the following two-dimensional two-phase flow problems: (i) incompressible turbulent bubbly flow in a pipe, (ii) incompressible turbulent air–particle flow in a pipe, (iii) compressible dilute gas–solid flow over a flat plate, and (iv) compressible dusty flow in a converging diverging nozzle. Predictions are shown to be in excellent agreement with published numerical and/or experimental data.     

Numerical study on combustion of H2—O2 in a supersonic reactive expansion in a nozzle

A study on expansion flow inside a nozzle considering full mechanism chemistry of hydrogen and oxygen was carried out. In this study, a full implicit scheme for turbulent reactive flow was obtained by combining the second order TVD scheme of Yee and Harten (1987, Implicit TVD schemes for hyperbolic conservation laws in curvilinear coordinates. American Institute of Aeronautics and Astronautics Journal, 25(2), 266–274) with the efficient implicit lower-upper scheme of Shuen and Yoon (1989, Numerical study of chemically reacting flows using a lower-upper symmetric successive overrelaxation scheme. American Institute of Aeronautics and Astronautics Journal, 27(12), 1752–1756). The species equations, Navier–Stokes equations and turbulence model were implemented in the numerical scheme and solved in conjunction with full detailed finite rate chemistry. The numerical scheme is verified by comparison with experimental results of a converging–diverging nozzle. Effects of inlet pressure, temperature and fuel-oxidant mass ratio on nozzle flow field were studied. Variation of chemical species under different conditions was investigated by considering a chemical mechanism. Results show that increasing inlet pressure increases the rate of reactions due to increasing the concentration of reactants. For lower inlet pressure the radical H increases slightly in the diverging part of the nozzle, while for higher pressures it decreases along the nozzle. Inlet fuel–oxidant mass ratio affects the variation of all species with a greater effect for a near stoichiometric ratio. It was also shown that a higher inlet temperature provides a more enhanced reaction zone in the diverging part of the nozzle.