Self-induced Flow Research Articles

Effect of plate position and size for self-induced flow oscillation of underexpanded supersonic jet impinging on perpendicular plate

When the underexpanded supersonic jet impinges on the obstacle, it is well known that the self-induced flow oscillation occurs at the specific condition of the pressure ratio in the flowfield, the position of an obstacle and so on. This oscillation is related with the noise problems of aeronautical and other industrial engineering so that the characteristic and the mechanism of self-induced flow oscillation have to be cleared to control the various noise problems. But, it seems that the characteristics of the oscillated flowfield and the mechanism of oscillation have to be more clear to control the oscillation. This paper aims to clarify the effect of the plate position and the width for the self-induced flow oscillation of an underexpanded supersonic jet impinging on the perpendicular plate by the experiment and the numerical analysis. From the results, it is clear that the occurring domain of the self-induced flow oscillation and its dimension strongly depend on the plate position and the width.

Propagation of magnetic island due to self-induced zonal flow

Propagation of magnetic island caused by drift-tearing instability is numerically investigated based on a reduced set of two-fluid equations. It is found that the island propagates into the ion diamagnetic direction when the island growth is saturated, and the propagation velocity becomes small as the viscosity increases. The island propagates with zonal flow generated by drift-tearing instability, because flattening of pressure inside the island is enhanced by parallel ion velocity. The mechanism of zonal flow generation depends on the viscosity. When the viscosity is small, the flow driven by the Reynolds stress is diminished by the Maxwell stress, and the small difference between them is overcome by the ion diamagnetic stress, and thus the zonal flow directs toward the ion diamagnetic direction. When the viscosity is large, the viscous stress is counteracted by the Reynolds and the Maxwell stresses, and the small difference between them is overcome by the ion diamagnetic stress, and thus zonal flow directs toward the ion diamagnetic direction.

Effect of Pressure Ratio for Self-Induced Flow Oscillation of Underexpanded Supersonic Jet Impinging on Cylindrical Body

Effect of Pressure Ratio for Self-Induced Flow Oscillation of Underexpanded Supersonic Jet Impinging on Cylindrical Body

Modification of Axial Fan Flow by Trailing Edge Self-Induced Blowing

Axial fans often show adverse flow conditions at the fan hub and at the tip of the blade. The modification of conventional axial fan blade is presented. Hollow blade was manufactured from the hub to the tip. It enables the formation of self-induced internal flow through internal passages. The internal flow enters the passage of the hollow blade through the opening near the fan hub and exits through the trailing edge slots at the tip of the hollow blade. The study of the influence of internal flow on the flow field of axial fan and modifications of axial fan aerodynamic characteristics is presented. The characteristics of the axial fan with the internal flow were compared to characteristics of a geometrically equivalent fan without internal flow. The results show integral measurements of performance testing using standardized test rig and the measurements of local characteristics. The measurements of local characteristics were performed with a hot-wire anemometry and a five-hole probe. Reduction in adverse flow conditions near the trailing edge at the tip of the hollow blade, boundary-layer reduction in the hollow blade suction side, and reduction in flow separation were attained. The introduction of the self-induced blowing led to the preservation of external flow direction defined by the blade geometry, which enabled maximal local energy conversion. The integral characteristic reached a higher degree of efficiency.

Numerical study of self-induced transonic flow oscillations behind a sudden duct enlargement

A sonic flow in a plane duct passing an abrupt increase in cross section is studied using compressible large-eddy simulations. Different flow patterns are likely to appear in this configuration according to the ratio between the downstream ambient pressure and the upstream reservoir pressure. For low pressure ratios, the flow is entirely supersonic in the channel and a steady symmetrical shock pattern is observed. For higher pressure ratios, the flow can be attached to one side of the channel with a jet-like shock cell structure, or can be characterized by strong oscillations of a single normal shock located near the sudden expansion, known as base-pressure oscillations in literature. A hysteresis phenomenon is found experimentally and the state reached by the transonic flow depends on the path followed by the pressure ratio. Moreover, a coupling of these base-pressure oscillations with the quarter-wavelength resonance of the duct can occur. All these regimes are numerically investigated and the results are favorably compared to available experimental data. A case of frequency locking of this self-excited mechanism is also reproduced, in agreement with a modeling of the resonator. The governing equations are solved using high-order central finite differences combined with an overset grid approach. The large-eddy simulations are based on a relaxation filtering and a nonlinear shock-capturing scheme is also implemented for shock waves.

The impact of a vortex ring on a porous screen

In this paper, we re-examine the issue raised by Morton (2000) regarding the impact of a vortex ring on a porous screen. We show, through a systematic experiment, that Morton's observation can only occur at very low Reynolds number or above a certain critical Reynolds number, which is dependent on the porosity of the screen. In particular, we found that a primary vortex ring of moderate Reynolds number impacting a porous screen can also undergo vortex rebound similar to that observed in vortex ring–solid wall interaction. At a higher Reynolds number, the self-induced flow of the primary vortex ring around the axis of symmetry passes through the screen as a jet-like flow, which later regenerates into a new vortex ring of the same sign but weaker circulation. At an even higher Reynolds number, the primary vortex ring has sufficient energy to pass through the screen via a possible mechanism of vortex stretching and reconnection, and continues as a modified ring in its lee.

Circumferential propagation of tip leakage flow unsteadiness for a low-speed axial compressor

Full-annulus three-dimensional unsteady numerical simulations were conducted for a low-speed isolated axial compressor rotor, intending to identify the behavior of self-induced unsteady tip leakage flow within multi-blade passages. There is a critical mass flow rate near stall point, below it, the self-induced unsteadiness of tip leakage flow can propagate circumferentially and thus initiates two circumferential waves. Otherwise, the self-induced unsteady tip leakage flow oscillates synchronously in each single blade passage. The major findings are: 1) while the self-induced unsteadiness of tip leakage flow is a single-passage phenomenon, there exist phase shifts among blade passages in multi-passage environments then evolving into the first short length wave propagating at about two times of rotor rotation speed after the transient period ends; and 2) the time traces of the pseudo sensors located on the rotor blade tips reveal another much longer length-scale wave modulated with the first wave due to phase shift propagating at about half of rotor rotation speed. Features of the short and long length-scale circumferential waves are similar to those of rotating instability and modal wave, respectively.

Numerical and experimental investigation of axial fan with trailing edge self-induced blowing

Axial fans often show adverse flow conditions at the fan hub and at the tip of the blades. In the present paper, a modification of conventional axial fan blades with numerical and experimental investigation is presented. Hollow blades were manufactured from the hub to the trailing edge at the tip of the blades. They enable the formation of self-induced internal flow through internal passages. The internal flow enters the internal radial flow passages of the hollow blades through the openings near the fan hub and exits through the trailing edge slots at the tip of the blade. The study of the influence of internal flow on the flow field of axial fan and the modifications of aerodynamic characteristics of the axial fan have been made. The numerical and experimental results show a comparison of integral and local characteristics of the axial fan with the internal flow, compared to characteristics of a geometrically equivalent fan without internal flow. The experimental results of local characteristics were performed with a five-hole probe and computer-aided visualization. A reduction of adverse flow conditions near the trailing edge at the tip of the blade was achieved, as well as boundary layer reduction on the blade suction side and the reduction of flow separation. The introduction of self-induced blowing led to the preservation of the direction of external flow, defined by blade geometry, and enabled maximal local energy conversion. The integral characteristic reached higher degree of efficiency.

Patented Hollow Blades of the Axial Fan with Trailing Edge Self-Induced Blowing

Axial fans often show adverse flow conditions at the fan hub and at the tip of the blades. In the present paper, a modification of conventional axial fan blades with experimental investigation is presented. Patented hollow blades were manufactured from the hub to the trailing edge at the tip of the blades. They enable the formation of self-induced internal flow through internal passages. The internal flow enters the internal radial flow passages of the hollow blades through the openings near the fan hub and exits through the trailing edge slots at the tip of the blade. The influence of internal flow on the flow field of axial fan and the modifications of aerodynamic characteristics of the axial fan have been studied along with recent patents. An experimental investigation of the interaction of two flows with the flow around hollow airfoil NACA series in a low-speed wind tunnel was studied. The determination of flow structure on the hollow airfoil was performed with computer-aided visualization. With the introduction of the internal flow and interaction with the external flow, a reduction of circulation effects on the fan hollow blade was achieved. The introduction of self-induced blowing led to the preservation of the direction of external flow, defined by blade geometry, and enabled maximal local energy conversion. Aerodynamic characteristic of the axial fan reached higher degree of pressure difference and efficiency through the entire fan working conditions. Keywords: Axial fan, hollow blade, aerodynamic characteristic, wind tunnel and computer-aided visualization

0609 超音速衝突噴流の自励振動の周波数と衝撃波構造との関係(OS6-3 超音速流と衝撃波,オーガナイズドセッション)

0609 超音速衝突噴流の自励振動の周波数と衝撃波構造との関係(OS6-3 超音速流と衝撃波,オーガナイズドセッション)

Detailed Evaluation of the Natural Circulation Mass Flow Rate of Water Propelled by Using an Air Injection

One-dimensional (1D) air-water two-phase natural circulation flow in the “thermohydraulic evaluation of reactor cooling mechanism by external self-induced flow—one-dimensional” (THERMES-1D) experiment has been verified and evaluated by using the RELAP5/MOD3 computer code. Experimental results on the 1D natural circulation mass flow rate of water propelled by using an air injection have been evaluated in detail. The RELAP5 results have shown that an increase in the air injection rate to 50% of the total heat flux leads to an increase in the water circulation mass flow rate. However, an increase in the air injection rate from 50 to 100% does not affect the water circulation mass flow rate, because of the inlet area condition. As the height increases in the air injection part, the void fraction increases. However, the void fraction in the upper part of the air injector maintains a constant value. An increase in the air injection mass flow rate leads to an increase in the local void fraction, but it has no influence on the local pressure. An increase in the coolant inlet area leads to an increase in the water circulation mass flow rate. However, the water outlet area does not have an influence on the water circulation mass flow rate. As the coolant outlet moves to a lower position, the water circulation mass flow rate decreases.

Detailed Evaluation of the Natural Circulation Mass Flow Rate of Water Propelled by Using an Air Injection

One-dimensional (1D) air-water two-phase natural circulation flow in the “thermohydraulic evaluation of reactor cooling mechanism by external self-induced flow—one-dimensional” (THERMES-1D) experiment has been verified and evaluated by using the RELAP5/MOD3 computer code. Experimental results on the 1D natural circulation mass flow rate of water propelled by using an air injection have been evaluated in detail. The RELAP5 results have shown that an increase in the air injection rate to 50% of the total heat flux leads to an increase in the water circulation mass flow rate. However, an increase in the air injection rate from 50 to 100% does not affect the water circulation mass flow rate, because of the inlet area condition. As the height increases in the air injection part, the void fraction increases. However, the void fraction in the upper part of the air injector maintains a constant value. An increase in the air injection mass flow rate leads to an increase in the local void fraction, but it has no influence on the local pressure. An increase in the coolant inlet area leads to an increase in the water circulation mass flow rate. However, the water outlet area does not have an influence on the water circulation mass flow rate. As the coolant outlet moves to a lower position, the water circulation mass flow rate decreases.

Unsteady tip clearance flow pattern in an isolated axial compressor rotor with micro tip injection

A numerical study of the effect of discrete micro tip injection on unsteady tip clearance flow pattern in an isolated axial compressor rotor is presented, intending to better understand the flow mechanism behind stall control measures that act on tip clearance flow. Under the influence of injection the unsteadiness of self-induced tip clearance flow could be weakened. Also the radial migration of tip clearance vortex is confined to a smaller radial extent near the rotor tip and the trajectory of tip clearance flow is pushed more downstream. So the injection is beneficial to improve compressor stability and increase static pressure rise near rotor tip region. The results of injection with different injected mass flow rates show that for the special type of injector adopted in the paper the effect of injection on tip clearance flow may be different according to the relative strength between these two streams of flow. For a fixed injected mass flow rate, reducing the injector area to increase injection velocity can improve the effect of injection on tip clearance flow and thus the compressor stability. A comparison of calculations between single blade passage and multiple blade passages validates the utility of single passage computations to investigate the tip clearance flow for the case without injection and its interaction with injected flow for the case with tip injection.

Superfluid neutron star turbulence

We analyse the implications of superfluid turbulence for neutron star physics. We begin by extending our previous results for the mutual friction force for a straight vortex array to account for the self-induced flow which arises when the vortices are curved. We then discuss Vinen's phenomenological model for isotropic turbulence, and derive the associated (Gorter-Mellink) form for the mutual friction. We compare this derivation to a more recent analysis of Schwarz, which sheds light on various involved issues. Having discussed isotropic turbulence, we argue that this case is unlikely to be relevant for neutron stars. Instead we expect a rotating neutron star to exhibit polarised turbulence, where relative flow drives the turbulence and rotation counteracts it. Based on recent results for superfluid Helium, we construct a phenomenological model that should have the key features of such a polarised turbulent system.

Vortex Rings in Superfluid 3He–B at Low Temperatures

Turbulence in classical fluids has far-reaching technological implications but is poorly understood. A better understanding might be gained from studying turbulence in quantum systems. In a pure superfluid (at low temperatures), there is no viscosity and vortex lines are quantised. Quantum turbulence consists of a tangle of quantised vortex lines which interact via their self-induced flow. We have recently developed techniques for detecting vortices in superfluid 3He–B in the low temperature limit. We find that the transition to turbulence from a moving grid occurs by the entanglement of emitted vortex rings. Here, we discuss the propagation of the ballistic vortex rings emitted at low grid velocities. We have measured the temperature at which the rings decay before reaching the detectors. Our results, at two different pressures, confirm that the vortex rings decay in accordance with mutual friction.

Efficient Immersed Boundary Method for Strong Interaction Problem of Arbitrary Shape Object with the Self-Induced Flow

An immersed boundary method is improved and applied to 2-D flow fields involving a single stationary/moving object. The present immersed boundary method employs a body force proportional to a solid volume fraction for coupling the solid and fluid motions at the interface. A hyperbolic-tangent function is newly introduced as a surface digitiser for computing the volume fractions at the interface cells, and this improvement is proved to be efficient for problems involving arbitrary shape object. The present method is applied to a uniform flow field around a circular cylinder and an interaction problem of fluid and free-falling object. The computational results are found to agree with the results by the different methods of the present authors and the results reported in the literatures. Also the computation time is considerably cut down compared to the other methods by the series of improvements to the immersed boundary method.

Occurrence of Flip-Flop Flows in Diamond-Shaped Cylinder Bundles

A self-induced flip-flop flow phenomenon in diamond-shaped cylinder bundles has been disclosed in previous studies by the authors. The flip-flop flow oscillation promotes fluid mixing and thus contributes to an enhancement in convective heat and mass transfer when the cylinder bundles are used as heat and mass exchanger equipment. An experimental study was performed to disclose the occurrence of flip-flop flows in diamond-shaped cylinder bundles. Velocity and pressure fluctuations in divergent-flow region inside diamond-shaped cylinder bundles were measured by LDV and piezo-meter, respectively. The difference in the oscillations of flip-flop flows and von Karman vortex streets is disclosed quantitatively. The mechanisms that cause the reciprocating bubble migration in network ducts of diamond-shaped cylinders are explained.

Numerical Parametric Study of Inlet Distortion Propagation in Compressor Using Integral Approach with Taguchi Method

Due to the advancement in aircraft propulsion, there is a need to understand compressor surge and stall from the engine operational requirement. Distorted inlet conditions in axial compressors may create large oscillations of the mass flow rate, which is known as “surge,” and self-induced circumferential flow distortions rotating around the annulus, which is called “rotating stall,” or a combination of both phenomenons may exist together. Therefore, it is important to carry out analysis on the parameters that may result in either surge or stall. This study looks into the research that has been done by Kim et al. [9] (on the distorted inlet flow propagation in the axial compressors). Integral method is applied to the problems of distorted inlet flow, investigating on the relationships and the effects that some of the key parameters would have on the propagation of inlet distortion flow. The objective of this study is to justify the findings from Kim et al.'s research by using Taguchi's quality control method. The results indicate that major parameters on the inlet distortion propagation are the ratio of drag-to-lift coefficient, the inlet distorted velocity coefficient, and inlet incidence angle in influence order. This conclusion is different from that in Kim et al.'s research. Their research was carried out using only a few test cases with integral method. When integral method with Taguchi quality control method is used, the prediction of degrees of influence by the parameters on the distortion propagation becomes more reasonable and accurate.

Occurrence of Flip-Flop Flows in Diamond-Shaped Cylinder Bundles

A self-induced flip-flop flow phenomenon in diamond-shaped cylinder bundles has been disclosed in previous studies by the authors. The flip-flop flow oscillation promotes fluid mixing and thus contributes to an enhancement in convective heat and mass transfer when the cylinder bundles are used as heat and mass exchanger equipment. An experimental study was performed to disclose the occurrence of flip-flop flows in diamond-shaped cylinder bundles. Velocity and pressure fluctuations in divergent-flow region inside diamond-shaped cylinder bundles were measured by LDV and piezo-meter, respectively. The difference in the oscillations of flip-flop flows and von Karman vortex streets is disclosed quantitatively. The mechanisms that cause the reciprocating bubble migration in network ducts of diamond-shaped cylinders are explained.

507 円柱体に干渉する超音速不足膨張噴流の自励振動 : 圧力比と位置が振動周波数に及ぼす影響(流体工学VI)

507 円柱体に干渉する超音速不足膨張噴流の自励振動 : 圧力比と位置が振動周波数に及ぼす影響(流体工学VI)