Dynamic flow characteristics influenced by impinging shear layer mode in a rectangular tube with a closed side branch

There are many closed side branches in the gas conveying pipeline system. When the gas passes through the closed side branch, the shear layer will arouse the acoustic resonance in the closed side branch, which is harmful to the safe operation of the pipeline. The research work is insufficient about the influence of the cross-section shape of the closed side branch on acoustic resonance. Using the Detached-Eddy Simulation (DES) model, the acoustic resonance characteristics caused by the side branch pipe with different square cross-sections are simulated at the inlet boundary conditions of 25 m/s, 30 m/s and 35 m/s. The results show that in the center axis of the side branch, a 1/4 wavelength standing wave was formed, and the acoustic resonance occurs at a higher Strouhal number in circular branch. The cross-section shape of the side branch does not affect the acoustic resonance frequency, but it has a certain influence on the amplitude of pressure fluctuation and has a significant influence on the high-order frequency components.

Characteristics of Condensing Flows in Shear Layer of Axisymmetric Supersonic Jets

A numerical simulation technique was developed for investigating the oscillatory cold flow in a ramjet combustor configuration. Some simulations have been conducted, and the results are presented and analyzed here. The main objective of the simulations is to investigate the interaction between the vorticity component and the acoustic component of the flow field when the reduced frequency of the flow based on the speed of sound is of the order of unity. In constructing the numerical model, it was found that the interior of the combustor must be isolated from the external region by a choked nozzle. The resulting numerical simulations arc able to exclude the effects of the artificially imposed boundary conditions at the outflow boundary.Flow visualizations and frequency spectra of the vorticity and pressure fluctuations were analyzed to understand the flow field. It appears that the pressure fluctuation at the base of the backward-facing step, a signal taken as acoustic in nature, contains only low-frequency components. This is in contrast to the frequency spectrum of the vorticity fluctuation in the free shear layer near the separation point, which contains both high-frequency and low-frequency components. The vorticity fluctuation at the location where the shear layer impinges on the nozzle wall also contains only low-frequency components. The unsteady flow fields near the shear layer separation point and in the nozzle region are investigated. It appears that the boundary layer immediately upstream of the separation point is perturbed by the low-frequency pressure fluctuation at the base of the step. The perturbation is then amplified downstream by the shear layer instability. In the nozzle region, the Mach number in the subsonic region fluctuates at a high amplitude when a vortical structure impinges on the nozzle. However, the Mach number in the supersonic region downstream of the throat remains stationary.Mean flow quantities and higher moments were computed by running time averaging to evaluate the contribution of the coherent structures to the transport of momentum. The results were compared to available experimental data for a two-dimensional backward-facing step configuration. The good agreement over the majority of the flow region shows that the large-scale structures are faithfully simulated and are the main contributor to the transport of momentum.An attempt was made to extract acoustic information from the results of the numerical simulations by computing the instantaneous dilatation field, which is shown to drive the unsteady potential flow, i.c., acoustic disturbances. It turns out that the near-field dilatation contains mainly the characteristics of the sound sources. The quadrupole nature of the sound source around each large-scale vortex in the flow field is clearly demonstratcd. The complex distributed dilatation field near the impingement point of the shear layer on the nozzle wall is considered as a compact acoustic source and is analyzed by multipole expansion of the distributed field. The results show a strong axial acoustic dipole at the nozzle. The dipole fluctuation is 180 degrees out of phase with the impinging vorticity fluctuations. This result can be applied as the impedance for the vorticity-acoustic fluctuations at the nozzle. However, the propagation aspects of the sound field cannot easily be visualized in a near field. The spectra of the pressure and vorticity fluctuations at selective points are analyzed to reveal the existence of two types of fluctuations. One is the resonant acoustic mode, in which the vortical disturbances excite the acoustic free-modes. The other is the coupled mode, in which the acoustic disturbances and the vortical disturbances are coupled through the dipole radiation at the nozzle and the acoustic susceptibility of the separating shear layer at the dump plane. A simple model for the coupled mode is proposed to explain the latter mechanism and to provide an approximate method for estimating its frequency.KeywordsMach NumberAcoustic WaveShear LayerPressure FluctuationCombustion InstabilityThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The flow over a deep cavity at low subsonic velocity is considered in the present paper. The cavity length-to-depth aspect ratio is L/H = 0.2. Single hot-wire measurements characterized the incident turbulent boundary layer and show the influence of the cavity on the streamwise statistic components just downstream from the cavity. The streamwise mean and fluctuating velocity profiles are affected by the cavity. PIV measurements reveal the presence for ejection-like events responsible of local perturbations of the skewness and the flatness coefficients. Time-resolved PIV technic is also used to characterize phase properties of shear layer oscillating cycle. It is shown that for deep cavity with first Rossiter mode, only one vortical structure is formed at the cavity leading edge. Then, it grows while convecting downstream along the shear layer. A well-defined ejection process begins after the vortex impact near the cavity downstream corner. A cylinder device placed spanwisely near the cavity leading edge eliminates the resonance and highly modifies the behavior of the shear layer flow. In fact, the shear layer could be divided into upper and lower parts with different structure aspects.

Attention was focused on the interaction between vortex and cavitation as a basic study on modeling unsteady turbulent flows with cavitation. First, separated shear layer with cavitation were directly simulated. Cavitation caused by spanwise and streamwise vortices, which are typical features in high shear layer, was represented by a simple model. Experimentally observed tendencies in the location of vortex formation, the frequency of vortex shedding and the intensity of Reynolds stresses were reasonably reproduced. Influence of cavitation on a typical example of streamwise vortex was observed. It was found that the vorticity in the core of Burgers type vortex was significantly reduced. To establish a description of it, we used an artificially maintained Burgers vortex. As a result, an assumption of constant circulation along a closed circle expanding due to sudden development of cavity could represent the modification of the vortex. INTRODUCTION Flows in hydro-machineries are affected by various types of cavitation. Since 1990’s, several methods have been proposed for the numerical simulation of flow fields including unsteady cavitation. Attached (sheet) cavitation and cloud (bubble) cavitation have been reasonably reproduced. However, the influence of turbulence has not been taken into account completely in previous simulations although most of cavitating flows are turbulent. This causes inaccuracy in predicting the cavitation inception because the local minimum of pressure is thought to be corresponding to the core of turbulence vortices. On the other hand, the effect of cavitation on turbulence has usually been omitted. Therefore, to establish the computational method for turbulent cavitating flows, the interaction between cavitation and turbulence vortices should be correctly modeled. The aim of our study is to address the modeling strategies considering two questions. (1) How are fine-scale vortices in turbulence related to cavitation inception? (2) How does the cavitation modify turbulent vortices and turbulence statistics? Since both are interactive phenomena, they must be analyzed by two-way methodology. One of typical and appropriate examples for our objective is the separated flow in the wake of a thin fence in a two-dimensional channel. Iyer and Cessio (2002) visualized various types of vortex cavitation and they reported the turbulence statistics in the wake region. In the first half of this report, we show the results of the direct numerical simulation with cavitation model, which was developed by Okita and Kajishima (2002). Our result is compared with experimental observation by Iyer and Cessio (2002) qualitatively, because the flow configuration was simplified in our simulation. Then, we investigate the relationship between cavitation and vortical structure; namely, primary (spanwise) vortices and the secondary (streamwise) vortices. In the second half, the interaction between a Burgers vortex and cavitation is directly simulated in an idealized situation. One reason is that streamwise vortices in a shear layer, simulated in the former part, has a profile of Burgers vortex. Moreover, the finest scale eddies in fully developed turbulence has been found to be Burgers type. A Burgers vortex in a uniform stream is given at the inlet cross section of the computational domain for this purpose. Reducing the cavitation number, the structure of vortex decaying by cavitation inception is analyzed. Then we give a phenomenological model that represents the vortical structure during this process. OUTLINE OF COMPUTATION The procedure including cavitation model and numerical method should fit in with the spatio-temporal scale of unsteady motion of vortices in the turbulent shear layer. In

Flow past open cavities is well known to give rise to highly coherent and self-sustained oscillations, leading to undesirable aeroacoustic resonance. Cavity flows are encountered not only in engineering applications but also our daily life, for example, the weapon bays, landing gears and wheel wells of aircrafts, in the depressions of submarine and ship hulls, in the pantograph recess of high-speed train, in the sunroof of cars or in the closed side branches of pipelines. Periodic and intense aeroacoustic vibrations deriving from the self-sustained oscillations of cavity flows can give rise to structural fatigue, optical distortion and store separation problems, especially for high-speed aircrafts. For a typical open-cavity flow, the induced acoustic level exceeds 160dB at transonic Mach numbers (MacManus & Doran, 2008) and it still reaches approximately 130dB at around 100 ∼110km/h for passenger vehicles because the passenger compartment acts as a Helmholtz resonator (Gloerfelt, 2009). Cavity-like geometries are also observed in places such as urban canyons, rivers and lakes. For these environmental fields, cavity flows affect the mass transfer processes of various pollutants and chemical toxic substances that occur between the cavity and the main flow (Chang et al., 2006). In the last decade, open cavities have attracted many researchers engaged in scramjet engines with regard to mixing and flame-holding enhancement for supersonic combustion (Asai & Nishioka, 2003; Kim et al., 2004). Because of these issues across a wide range of applications, cavity flows have been of practical and academic interests for more than a half-century. The flow-induced oscillations in an open cavity arise from a feedback loop formed as a result of successive events that take place in sequence. Figure 1 illustrates the schematic of cavity flows with an acoustic resonance. A boundary layer of thickness δ separates at the upstream edge of the cavity of length L and depthD. The resulting separating shear layer is convectively unstable due to Kelvin-Helmholtz instability, and it soon rolls up into vortices. Every time the organized vortical structures collide the downstream corner, the expansion waves are radiated from the corner owing to the vorticity distortion at lowMach numbers (Yokoyama & Kato, 2009), while as Mach number increases, the compression waves are generated near the downstream corner, especially for supersonic flows (Nishioka et al., 2002). It should be noted that the hypersonic shear layers do not always roll up into isolated vortices, just forming wavy patterns. The strength of these induced waves is determined by the relative position of the traveling vortices and the downstream corner. Rockwell & Knisely (1978) classified the vortex-corner interactions into four possible events on the basis of flow visualizations: Complete Escape (CE), Partial Escape (PE), Partial Clipping (PC) and Complete Clipping (CP). The incident acoustic waves propagate inside the cavity towards the upstream corner and 17

The low swirl flow is a novel method for stabilizing lean premixed combustion to achieve low emissions of nitrogen oxides. Understanding the characteristics of low swirl flows is of both practical and fundamental interest. In this paper, in order to gain better insight into low swirl stabilized combustion, large eddy simulation and dynamically thickened flame combustion modeling are used to characterize various features of non-reacting and reacting low swirl flows including vortex breakdown, shear layers’ instability, and coherent structures. Furthermore, four test cases with different equivalence ratios are studied to evaluate the effects of equivalence ratio on the flame and flow characteristics. A finite volume scheme on a Cartesian grid with a dynamic one equation eddy viscosity subgrid model is used for large eddy simulations. The obtained results show that the combustion heat release and increase in equivalence ratio toward the stoichiometric value decrease the local swirl number of the flow field, while increasing the flow spreading at the burner outlet. Results show that the flame becomes W shaped as the equivalence ratio increases. Moreover, the combination of the swirling motion and combustion heat release temporally imposes a vortex breakdown in the post-flame region, which leads to occurrence of a transient recirculation zone. The temporal recirculation zone disappears downstream of the burner outlet due to merging of the inner shear layer from all sides at the centerline. Also, various analyses of shear layers’ wavy and vortical structures show that combustion heat release has the effect of decreasing the instability amplitude and vortex shedding frequency.

One of the useful ways to measure the effect of the flow control devise is to use the wall-shear stress sensor to measure the wall-shear stress directly. The sensor used in this paper measures the wall-shear stress, which is reduced by the flow control devise. In this paper, the wall-shear stress of the plane shear layer with the plasma synthetic jet actuator (PSJA) is investigated. PSJA is a flow control device composed of electrodes with A.C. signal. The actuator uses electrohydrodynamic (EHD) effect and induces flow around the electrodes. PSJA has great advantage such as miniaturization, maintenance free, and easy to control compared to other actuators. In this paper, the wall-shear stress of plane shear layer in a low-speed turbulent wind tunnel is observed to measure the effect of the PSJA. The results show that the PSJA changes the flow condition of shear layer by accelerating the flow in shear layer. The wall-shear stress reduces and increases according to the displacement of the wall-shear stress sensor and the actuator.

In this study experiments are conducted on the flow within a horizontal pipe with two closed side branches of equal length. This is referred to as quasi-cross configuration as the distance between the two closed side branches is much smaller than the acoustic wavelength of the first acoustic mode. Focus is placed upon investigating of the effect of high amounts of injected water on Flow-Induced Pulsations (FIPs). A first test with only dry air is performed and pressure fluctuations are measured at the closed end of each side branch to investigate FIPs. Several tests are conducted with increasing amounts of water, in droplet form, which is injected with a nozzle positioned upstream the branches. To understand the effect of the multiphase flow pattern, tests are performed with the injector located far from and close to the side branches. With far injector and at low injection rates, no decrease of pulsations is observed while the amount of injected water increases. At high injection rates, a shift of the pulsation peak to lower velocities and an increase of the Strouhal number calculated by using the diameter of the side branch and the superficial gas velocity are reported. With close injector, pulsations are eliminated as the injection rate increases. At low injection rates, an increase of the Strouhal number is also observed.

To characterize the turbulence of boundary layers in the energy-bearing interval of wave numbers several turbulence scales are sometimes used (for example, [1, 2]). In particular, the universality of the semiempirical model of turbulence [2] can be extended in this way. A turbulence model with one equation (energy balance of the turbulence) has been constructed and used [3–6] and it has been established that the number of problems that can be solved for a universal choice of the values of the empirical coefficients increases appreciably if not one but two turbulent scales are used. In the present paper, it is shown that the introduction of a second scale makes it possible to take into account the interaction of shear layers in flows with two shear layers (for example, a channel or jet), and also to take into account the influence of turbulence of an external flow on a boundary layer. The interaction of shear layers is taken into account in theories containing a transport equation for the turbulent frictional stress τt (for example, [7]), in which the essence of the interaction reduces to diffusion of τt from layer to layer. In the present paper, a predominant volume interaction effect is assumed. It takes the form of a difference between the interaction of large-scale vortices with a shear deformation motion in flows with one and two shear layers, and also in the presence of turbulence in an external flow.

Flow-induced acoustic resonances in piping with closed side branches or T-junctions are one of the phenomena causing severe structural vibration and fatigue damage of the piping and components in many engineering applications such as power plants. Practical piping systems of power plants often have a steam flow, and moreover, the steam state can be not only dry steam but also wet steam. From our previous experiments under low-pressure dry and wet steam flows using a single side branch, higher acoustical damping was confirmed under wet steam than that under dry steam, which is considered to be caused by the existing liquid phase. Although the static pressure in practical steam piping is often higher than that in our previous experiments, the effects of the static pressure on acoustical damping under a wet steam flow have not been clarified. Thus, we constructed a new test facility that can be used to perform continuous flow test under dry and wet steam flows with higher pressures than our previous test facility. In this paper, we give an overview of the new steam test facility and some experimental results for the acoustic resonance in a single side branch under higher-pressure dry and wet steam flows than those in our previous studies, using the new facility to investigate and evaluate the effects of the static pressure.

Cavity flows find applications in weapon bays, car sunroofs, urban street canyons, ship hulls, high-speed pantograph recess etc. The objective of the present work is to provide an insight on the flow behavior in and around a deep round cavity with varying aspect ratios. Cavities of aspect ratios 0.2 and 0.5 are considered for an inflow Mach number 1.5. Numerically investigation was done using ANSYS FLUENT. Cavity flows consist an intrinsic flow mechanism that makes an extensive choice for most flow studies. Although flow past an open cavities has been numerically studied in the past, very few studies done on flow over deep cavities till date. Result reveals that the cavity geometry of reduced aspect ratio eliminates the corner vortices within the cavity and thus supplements less energy to the shear layer. The shear layer formation, its convection, impingement on trailing edge, secondary and primary eddies interaction etc are resolved. The flow oscillation characteristics changes with varying aspect ratio.

Self-sustained oscillations in a shear layer were studied in a confined chamber with injecting walls. Experiments were carried out over a wide range of Reynolds numbers (0.86 x 10 5 ≤ Re ≤ 1.94 x 10 5 ) and of Mach numbers (0.05 ≤ M ≤ 0.227) in order to underline the effect of dynamic conditions on the pressure amplifications and on the excited acoustic modes. The presence of an obstacle generated a highly sheared flow in which vortex-shedding frequency was set by an acoustic-hydrodynamic coupling. The Mach number controlled the selectivity process of the vortex-shedding frequency, but turbulence dissipation weakened this coupling by a loss of structure coherence. As this did not affect the pressure fluctuation level, a second noise source was detected; vortices near the injecting wall, resulting from a mixing effect between shear-layer oscillations and injection flow through the porous wall, were identified

As an effective drag reduction control method, porous media have attracted extensive attention in various engineering fields. In this paper, the large eddy simulation method is used to study the flow problem around a cylinder with and without porous media covering at subcritical Reynolds numbers. The drag reduction characteristics and the mechanism of a cylinder covered with porous media were studied. The results of the aerodynamic drag coefficient show that when the Reynolds number is 5.6 × 104 and when the porous media are arranged on the leeward side of the cylinder at an azimuth angle of 270°, the drag reduction rate is the largest, reaching 8.53%. The flow field results indicate that after porous media covering, the large-scale wake vortex shedding was effectively suppressed, the vortex shedding frequency was reduced, the vortexes in the wake area became thinner and longer, and the vorticity decreased. It is confirmed that porous media can stabilize the wake shear layer and suppress the unsteady vortex motion in the wake. Meanwhile, airflow can penetrate the porous media, leading to velocity slip at the interface between the porous media and the airflow, making the free shear layer more stable, and strengthen the wake vortex.

A direct numerical simulation has been performed to study instantaneous behavior in lead-bismuth eutectic flows past a vertical, backward-facing step. A turbulent forced convection case and two cases of mixed convection, the first buoyancy-aided flow at a Richardson number Ri of 0.1 and the second buoyancy-opposed flow at $$Ri=0.02$$ , are simulated and discussed. The Reynolds number based on the bulk velocity and step height is 4805. A uniform heat flux is imposed on the expansion wall behind the step. In the forced convection case, the numerical results reveal two characteristic unsteady flow phenomena. The first is vortex-shedding motion along the separating shear layer, while the second is wall-normal flapping of the shear layer. These unsteady motions have significant influences on the thermal field. The vortex-shedding motion induces some streak-like low-temperature structures on the heated wall, while the flapping motion induces oscillation of the maximum temperature on the wall. In the mixed convection cases, buoyancy alters the flow field substantially. The two unsteady flow phenomena noted above constitute motions inherent to backward-facing step flow. Buoyancy plays a material role in vortex development, affecting vortex ranges and time-scales. While the vortex shedding frequency is insensitive to buoyancy, the frequency of the flapping motion increases with the buoyancy. These results contribute to an improved understanding of separating and reattaching flows, especially in association with buoyancy and temperature fluctuations. The data serve to aid future development and validation of improved heat-flux modeling of low-Prandtl-number fluids.