The present paper aims to analyze the propulsion of birds and fishes undergoing the ground effect as well as the lift of high-speed ground vehicle. Applying the analytical method which was developed for flutter of a soft plate placed at an arbitrary position in subsonic channel flows, calculations are carried out first for non-oscillatory case in compressible flow and then for oscillating cases of birds and fishes in incompressible flow. The results obtained show that the ground effect acts to increase not only the lift in steady flight but also the thrust and propulsive efficiency in oscillating modes. This method holds not only in the case of very close proximity to the ground but also in compressible flow case, so it would be applicable to the flutter analysis of high-speed ground vehicle with wings.
Recent researches on the dynamic Smagorinsky’s model of Germano et al. in the context of using finite difference method (FDM) on a turbulent channel flow have revealed the serious overestimation of mean velocity and streamwise turbulent intensity. The objective of our research here is to develop an isotropic eddy viscosity model that mitigates this problem, as an alternative to Smagorinsky’s model. The procedure proposed by Yoshizawa et al. was utilized and a more consistent eddy viscosity model for the dynamic procedure was obtained. The proposed model was tested in plane turbulent channel flows at Reτ=180 and 590 normalized by the friction velocity and the channel half-width, and predicted better statistics than Smagorinsky’s model. The new model was also found to be less sensitive to a discrete filtering operation than Smagorinsky’s. The organized structures near the wall were also investigated and it was found that the overestimation of the eddy structures reproduced by Smagorinsky’s model is clearly improved by the proposed model.
The original cubic interpolated pseudo-particle combined unified procedure method has been improved by introducing the state equation of Tait form for the first time so as to present the real pressure-density relationship of complex bubbly water. Computational results of the 1D shock tube problem and the 2D shear driven cavity water flow have demonstrated its accuracy and validity for compressible and incompressible flow simulations. The method has been applied to the simulation of flow of an impulsively started submerged water jet. The computational result agrees with the experimental one of Gharib et al. The result shows that vortex rings are formed at the head of the jet and move forward as the jet flow develops. The total circulation of vortices increases linearly with the vortex formation time during the jet injection. The pressure is a minimum at vortex centers and increases sharply with increasing vortex radii. The cavitation inception index takes a minimum value at vortex centers located around the jet periphery.
Direct numerical simulations have been performed to clarify the sound generation mechanism in two-dimensional temporally evolving compressible mixing layers. The sound generation in mixing layer is governed by the variations of vorticity which are induced by the Kelvin-Helmholtz instability. The pressure fluctuations with high frequency are observed in the period of vortex roll-up, and the amplitude of pressure fluctuations increase on the occasion of vortex pairing. The acoustic source term is governed by the Reynolds stress component and the viscous component is negligible. The effects of convective Mach number Mc on sound generation are also investigated. Both the pressure fluctuations and acoustic source term increase with the increase of convective Mach number. For Mc=0.6, shock wave called ‘eddy shocklet’ is produced by the vortex pairing, which dominates the sound generation from the mixing layer at high Mc. The far-field sound computed by DNS is compared with the predictions based on the acoustic analogies derived by Powell and Lighthill. The prediction by Powell’s analogy shows a good agreement with DNS, while the pressure fluctuations predicted by Lighthill’s analogy are low compared with the DNS results after the vortex pairing.
A new vortex scheme for simulating flows involving the natural convection and the interaction between the temperature and the vorticity is presented. The creation of vorticity due to the temperature is modeled either by creating a vortex pair from a temperature particle or by changing the strength of vortices according to the vorticity equation. The diffusion velocity method is used for simulating the diffusion of the vorticity and the temperature. The vortices of negative and positive strength are separately treated in the diffusion process to avoid an unreasonably large diffusion velocity. Our results indicate that these techniques successfully simulate creation of the vorticity due to the heat, the diffusion and the convection of the temperature and the vorticity, and the interaction of them.
A direct simulation of the falling motion of an immersed solid particle toward a wall is performed to investigate the mechanics of hydrodynamic collision of particles. The time-dependent boundary-fitted coordinate system is applied to the calculation of fluid flow around solid bodies. The fluid force acting on the particle is calculated by integrating the surface stress without any models. The obtained particle motion is compared with the corresponding experiment and theoretical analysis. The results of the numerical simulation, the experiment and the analysis using theoretical models agree quantitatively with respect to the falling motion of the particle. When the particle falls toward the wall, fluid force due to the squeeze of fluid in the gap is increased and prevents the particle from approaching the wall. As a result, the particle is decelerated and the fluid force caused by the unsteady motion of the particle is significantly increased. The numerical results reveal that the total fluid force acting on the particle can be explained by the sum of steady and unsteady fluid forces.
Turbulence modulation in particle-laden flow, especially the influence of vortex shedding, was investigated by means of the direct numerical simulation. To this end, we developed a finite-difference scheme to resolve the flow around each particle moving in turbulence. The method was applied to the flow around a sphere and the accuracy was confirmed up to the Reynolds number range with vortex shedding. The agreement between our 4th-order central finite-difference method and spectral method for turbulent channel flow without particles was also fine. Then, we simulated upward flow in a vertical channel including solid particles. The velocity and vorticity fluctuations as well as Reynolds shear stress were strongly affected by wakes from particles. The shed vortices were elongated in the mainstream direction by the velocity gradient and resulted in the hairpin vortices. They increased the energy production rate in couple with production due to particle-turbulence correlation.
A computer-based investigation has been conducted, regarding the interaction between a vortex ring, generated by starting a shock wave of Mach number 1.4, and plane shock waves of various shock Mach numbers. It is shown that a self-intensification of shock waves takes place due to the mechanism of wave convergence by the interaction of shock wave and vortex. In order to evaluate the strength of intensification, the mean values of the maximum pressure and density for the time of interaction are calculated for the various shock Mach numbers. A remarkable feature observed during the computation is that the strength increases, then achieves a constant value, and finally decreases as the shock Mach number increases. These results show the typical features of interactions, such as the weak shock-strong vortex ring, a similar strength interaction of both shock and vortex, and a strong shock-weak vortex ring.
An extensive experimental investigation was carried out with the aim of understanding the hydrodynamics involved in accelerating water column contained in a circular tube and generating large flow rate water spray by shock tube. Water column in tube was accelerated by two means, one is by compressed gas directly, this facility operates like as a shock tube but the flow in the tube is gas/liquid two phase flow, the other is by a gas/gas shock tube. Evolution of the two-phase flow in tube and the generated water spray was visually observed. Velocity measurement by laser cutting technology was made to understand the motion of the water column and the bubble in tube. In addition, wave motion in tube measured by PVDF pressure transducers shows that the pressure wave in a gas/water shock tube was significantly different from that in a gas/gas one in many aspects.
The non-equilibrium flow near a solid surface is simulated using the molecular dynamics method. The fluid is set to be a Lennard-Jones fluid where the interacting potential between the fluid molecules is the Lennard-Jones potential. The fluid molecule is given the parameters of Xe and the solid molecule is that of Pt. First, the system is maintained at thermal equilibrium and the density distribution, radial distribution, surface coverage, pressure distribution, interface tension, and adsorption coefficient of the fluid molecule is analyzed. Next, shear velocity was given to the solid molecules to drive the fluid molecules. The density distribution, velocity distribution, and temperature distribution of the fluid molecule is analyzed and results are shown.
In the process of growing crystals in solutions, double diffusive convection due to thermal and solutal gradients occurs. A measurement system comprising a rapid heat-transfer control system and an accurate real-time phase-shifting interferometer has been developed to investigate the transient double diffusive convection. The heat-transfer control system has a small test-cell in which the liquid temperature is controlled by non-equilibrium thermoelectric devices. In-situ measurement of the transient double diffusive convection and crystal growth rates of NaClO3 are carried out by using this system. Transient double diffusive fields are observed directly and clearly. The crystal growth rates in the present experiments show similar tendencies to the data of microgravity experiments.
Two-dimensional magneto-hydrodynamic surface waves generated by a moving oscillatory pressure distribution in an inviscid, incompressible and electrically conducting fluid of finite uniform depth are investigated. In the ultimate steady state, there exist six progressive waves, four original gravity waves and two other magnetic waves, all of which are mainly due to magnetic force. The effect of this force on the waves produced is studied in detail. It is found that, due to this force, disturbance in the wake is increased.
To control the flow around a square prism, a rod was set upstream of the prism. The side length of the prism was 20 mm and the diameter of the rod was changed from 2 to 10 mm. The spacing between the axes of the prism and the rod was 40 to 100 mm. The Reynolds number ranged from 5.3×103 to 3.2×104. The relationship between the local heat transfer and the fluid flow around the square prism was investigated. Three flow patterns were observed. The best flow pattern for heat transfer enhancement and drag reduction is the pattern without the vortex shedding from a rod. Changes in the flow pattern depended on the rod diameter, its position and its Reynolds number. The average heat transfer of the square prism increased by 35% compared with the case of the square prism without a rod. The drag coefficient was reduced by about 80%.
Experimental studies were performed to investigate the fluid flow and local heat transfer around two cubes arranged in tandem on a flat plate turbulent boundary layer. The Reynolds number, based on the cube height d, ranged from 4.0×103 to 3.2×104. The thickness of the turbulent boundary layer δ was δ/d≤2.17. The surface temperature distribution around the cubes was measured with thermocouples under conditions of constant heat flux. The effect of spacing b between two cubes was clarified in the range of 0.33≤b/d≤1.33. In the case of b/d≥0.67, the horseshoe vortex is formed in front of each cube. However, at b/d=0.33, it does not form in front of the 2nd cube. The local heat transfer is high in the horseshoe vortex regions and the flow reattaching regions. The average Nusselt numbers of 1st cube, ¯Nu, and 2nd cube, ¯Nu, are expressed as ¯Nu=0.24Re0.66 and ¯Nu=0.27Re0.64, respectively. They are virtually unaffected by the spacing. However, that of base plate between two cubes, ¯Nu, has a maximum value of ¯Nu=0.47Re0.60 at about b/d=0.67.
In order to investigate the performance of the micro channel heat exchanger, three-dimensional numerical simulations and experiments on heat transfer behavior and pressure loss were carried out. So far as the heat transfer phenomena is concerned, results obtained using a silicon chip micro channel model showed a very small thermal resistance, about 0.1 (Kcm2/W). And, measured pressure loss showed good agreement with that of analytical result obtained on the basis of fully developed laminar pipe flow assumption. Furthermore, a practical setup was made with a micro channel heat exchanger to clarify the possibility of using the micro channel heat exchanger in electrical equipment. As a result, it was confirmed that the performance of the micro channel heat exchanger system is sufficient to cool a silicon chip which generates a large amount of heat, and the scale of the system is compact compared to that of the whole setup of electrical equipment.
The effect of a single row of oblique pin fins on the endwall heat transfer is investigated experimentally in the two types of pin fin arrangements. One is the “single row” arrangement, in which all of the pin fins are inclined toward the same direction and the other is the “cross row” arrangement, in which the pin fins are alternately inclined toward the upstream and downstream directions. Pin fins are inclined toward the streamwise direction from θ=−60 degrees (upstreamwise) to +60 degrees (downstreamwise), where θ is an angle between the axis of the pin fin and the normal of the endwall surface. The averaged Nusselt number of the oblique pin fins for the single row arrangement is smaller than that of the perpendicular pin fins, because the endwall heat transfer in the downstream region of the pin fins is not enhanced. In the case of the cross row arrangement of −30≤θ≤+30, the heat transfer shows almost the same value at the perpendicular pin fins. The cross row arrangement of θ=±30 degrees shows about 25% less pressure loss than that of the perpendicular pin fins in spite of having almost the same averaged Nusselt number.
The determination of the turbulence parameters is a difficult and important problem for combustion analysis. In this study, we propose a newly developed method of defining unsteady turbulence intensity in order to examine the relation between the turbulence characteristics and the burning velocity of turbulent flame propagating in homogeneous fuel-air mixture. We use the discrete wavelet transform, which is a time-frequency analysis method for calculating unsteady turbulence intensity and use a complex-type mother wavelet, the RI-Spline wavelet as the mother wavelet. Our experiments demonstrate the merits of our approach.
In an attempt to elucidate formation and disappearance processes of droplet clusters in spray flames, simultaneous measurements consisting of laser tomography and flame chemiluminescence detection are applied to a premixed-spay burner. The smart combination of measurements provides time-series data-set serving for better understanding of spray flames, which essentially contains inhomogeneity in space and time. It is revealed that preferential flame propagation through a premixed-spray stream plays a significant role in creating droplet clusters and that droplet clusters formed in this manner evanesces from their outer boundaries. Those observation confirms that the premixed-spray flame comprises both premixed-mode flame in upstream region and diffusion-mode flame in downstream region, respectively, i.e., two-stage flame structure previously reported for spray flames stabilized in either counter or stagnation flows.
The head-on quenching of premixed flames on the single wall surface has been analyzed based on thermal theory and as results, the flame temperature, nondimensional flame thickness and a Peclet number, PQe, have been obtained at the time of quenching. Then, to confirm the adequacy of the analytical results, experiments using a combustion vessel were performed, and photographs were taken at the moment of head-on quenching of methane/air flames on a single wall surface using the Schlieren method, to measure the flame thickness at the time of quenching. As a result, the following were confirmed: regarding the nondimensional flame thickness at the time of quenching, although quantitative differences are observed between experimental values and theoretical values, it is always larger than 1 under all experimental conditions; PQe obtained from the experiments agrees qualitatively with that determined by theoretical analysis.
The paper presents the application of a three component model to the theoretical study of the combustion process of a Diesel engine fueled with sunflower oil and sunflower oil-Diesel fuel mixtures. The model assumes that the working fluid consists of three components: the fresh air, the flame and the burned gases. The combustion model uses the energy conservation equation: vc·Qc·dξα=dUα+dLα+dQwα,  where vc is the fuel cyclic dose, Qc is the fuel heating value, ξα=vcα/vc, vcα is the quantity of burned fuel up to the moment α, Uα is the internal energy of the working fluid, Qwα is the heat exchanged through the cylinder walls and Lα is the mechanical work. The heat release law was assumed to be a Vibe type one: ξα=Rc·[1−exp(−6.9·AmPp+1)]+(1−Rc)·[1−exp(−6.9·Amd+1)],  where: ·Ap=(α−αd)/(αP−αd) and A=(α−αd)/(αF−αd); ·αd-start of combustion angle; ·αf-end of combustion angle; ·αP-end of rapid combustion angle. Using Eqs.  and  we have obtained the cylinder pressure during combustion, for the vegetable fuels taken into account; the peak values were confirmed during the experiments.
In order to establish the optimum fueling in a natural gas fueled dual fuel engine, experiments were done for some operational parameters on the engine performances and the exhaust emissions. The results show that the pilot fuel quantity should be increased and its injection timing should be advanced to suppress unburned hydrocarbon emission in the middle and low output range, while the quantity should be reduced and the timing retarded to avoid onset of knock at high loads. Unburned hydrocarbon emission and thermal efficiency are improved by avoiding too lean natural gas mixture by restricting intake charge air. However, the improvement is limited because the ignition of pilot fuel deteriorates with excessive throttling. It is concluded that an adequate combination of throttle control and equivalence ratio ensures low hydrocarbon emission and the thermal efficiency comparable to diesel operation.
A new gas turbine cycle was proposed as a method for further improvement of thermal efficiency of a steam injection gas turbine. This cycle has an exhaust heat recovery process which preheats a part of the compressor discharge air and generates steam for injection. In this paper are reported the results of parametric performance analysis of the new cycle gas turbine, and comparisons of thermal efficiency between the new gas turbine cycle and conventional steam injection gas turbine systems. The results of the analysis show that the new cycle gas turbine can realize higher efficiency than the three conventional systems, in the usual pressure ratio range as that applied in actual gas turbines. The effect of the injection steam pressure on thermal efficiency of the new cycle gas turbine is also evaluated.
The kinetic power and efficiency equations for general jet propulsion systems are classically given in a much cursory, incomplete, and ununified format. This situation prohibits the propulsion designer from seeing the panorama of interrelated propulsion parameters and effects. And in some cases, it may lead to an energy-inefficient propulsion system design, or induce significant offset in propulsion performance as demonstrated in this study. Thus, herein we attempt to clarify some related concepts and to rigorously derive the associated generalized equations with a complete spectrum of physical parameters to be manipulated in quest of better performance. By a highly efficient interweaved transport scheme, we have derived the following equations for general jet propulsion systems: i.e., generalized total kinetic power, generalized kinetic power delivered to the jet propulsion system, generalized thrust power, generalized available propulsion power, and relevant generalized propulsive, thermal, and overall efficiency equations. Further, the variants of these equations under special conditions are also considered. For taking advantage of the above propulsion theories, we also illustrate some novel propulsion strategies in the final discussion, such as the dive-before-climb launch of rocket from highland mountain on eastbound rail, with perhaps minisatellites as the payloads.
A potential zero emission combined power generation plant fired by liquefied natural gas (LNG) has been investigated. A mixture of carbon dioxide (CO2)-steam is used as the working fluid of a gas turbine cycle, which replaces the normal combustion-in-air products and air, notably as the thermal ballast for the control of flame temperature. Oxygen (O2) is used as the fuel oxidant and is obtained from an air separation unit (ASU). The excess CO2 due to combustion is extracted by a simple flow separator and liquefied ready to be reused and/or sequestered. The plant configuration and thermodynamics of the cycle are discussed first and then the optimised overall efficiency of the plant is calculated with a comparison of 100% and 120% stoichiometric combustion. The overall net efficiency, optimised to pressure and temperature levels complying with the material and cooling techniques currently available, is around 56% (LHV basis), including the energy penalty of the ASU and the CO2 separation.
A system that uses magnetic force can be effective for treating sewage water because it can rapidly separate solids from liquid. We are developing such a system that uses a superconducting magnet that consumes little power but produces a strong magnetic field. The system also uses rotating magnetic filters and can be operated continuously. We constructed a prototype and tested it using sewage water. The system successfully removed 90% of BOD (biochemical oxygen demand; the BOD in the effluent for this system was less than 0.22 μm) and 97% of SS (suspended solids). This means that the total space required for a sewage treatment facility might be reduced when such a system is used.
Water flow simulation tests were performed on the flow-induced oscillations of the thermowell in the prototype fast breeder reactor (FBR), MONJU. The displacements of the target cylinder were measured, and the oscillation amplitudes, the frequency characteristics, and the phase relationships were estimated. The estimations showed that the oscillations of the target cylinder had a one-dimensional oscillation region in the in-line direction with symmetric vortices shedding and a two-dimensional oscillation region induced by alternative vortices. The phase estimation, carried out by a methodology using wavelet analysis and statistical analysis, indicated that the effect of the alternative vortices on the in-line oscillation was changed with the flow velocity.
Flow-induced in-line oscillation of a circular cylinder has been experimentally studied by free-oscillation tests in a water tunnel. Response amplitudes of a circular cylinder have been measured for determining the values of the reduced mass-damping parameter of less than 1.0. In the free-oscillation tests, the cylinder models were spring-mounted so as to oscillate as a two-dimensional rigid cylinder in the water tunnel. Two types of excitation phenomena appear at approximetaly half of the resonance flow velocity. The response amplitudes are sensitive to the reduced mass-damping parameter during the in-line oscillation of the first excitation region with a symmetric vortex street, and the alternate vortices are periodically shed, locking-in with the vibration of the cylinder in the second excitation region. A hysteresis phenomenon is observed to appear in the in-line oscillation of the latter region. A cantilevered circular cylinder with a finite length aspect ratio of 10 was tested for fluidelastic characteristics of the cylinder, and these characteristics are found to be quite different from those of the two-dimensional cylinder, having only one wide velocity region of excitation. The results of this study are providing important supporting data for the recent publication “Guidline for Evaluation of Flow-Induced Vibration of a Cylindrical Structure in a Pipe” by the Japan Society of Mechanical Engineers, Standard JSME S012-1998.
The flow-induced in-line oscillation of a cantilevered circular cylinder was experimentally studied through free-oscillation tests in a water tunnel. The response displacement amplitude at a circular cylinder tip was measured at reduced velocity from 1.0 to 4.0. A cantilevered cylinder was supported by a plate spring mounted on the water tunnel wall. The cylinder aspect ratio was varied from 5 to 21 to investigate the effect of aspect ratio on the response displacement. It is found that cylinders with aspect ratios of 5 and 10 have one excitation region, while cylinders with aspect ratios of 14 and 21 have two excitation regions. The aspect ratio, therefore, affects the amplitude of the excitation regions. The influence of end-effect was also investigated using cylinders with an end plate attached to the free end. Since the cylinders with an end plate show two excitation regions, even at an aspect ratio of 5, the flow around the free end of a cantilevered cylinder causes the end-effect. The mechanism of vibration was investigated using a cylinder with a splitter plate in wake to prevent alternate vortices. The amplitude is greater than those of a normal cylinder without a splitter plate, especially at Vr=2.3 to 3.0, where a cylinder with an end plate shows the second excitation region. In other words, the alternate vortices suppress the amplitude in this range. The maximum amplitude of each excitation region decreases in proportion to Cn and the amplitude of the first excitation is more sensitive to Cn.
Experimental validation of the design criteria for preventing the failure of a thermometer well by vortex-induced vibration is presented, clarifying the effects of Reynolds number and structure damping. An existing avoidance criterion for the vortex-induced vibration in the flow direction, which is given in terms of the reduced velocity Vr as Vr<1.0, has been developed based on the experimental data mainly in the sub-critical range. The applicability of this criterion in the super-critical range is examined in this paper based on the results of experiments performed in the Reynolds number region from 7.8×104 up to 1.1×106 at Vr=1.0. On the other hand, an existing suppression criterion of vortex-induced vibration in the flow direction is given in terms of the reduced damping Cn as Cn>1.2 (or 2.5) under the condition of Vr<3.3 in design guidelines. This criterion, however, has been proposed based on very limited available data The appropriateness of the suppression criterion is also examined in this paper based on the results of experiments in realistic thermometer well conditions, i.e. cantilever cylinders with varied structure damping in a pipe water flow. As a result, it becomes clear that Vr<1.0 is applicable in the super-critical range up to 1.1×106 and that the criteria of Vr<3.3 and Cn>1.2 are reasonably applicable to a cantilever cylinder in a pipe water flow.
Experiments have been conducted on turbulence-induced vibration of a circular cylinder in water flow with supercritical Reynolds number ranging from 3×105 to 3×106. Based on the power spectral density of cylinder vibrations measured at several Reynolds numbers, fluctuating force coefficients, Strouhal number and correlation length were evaluated. As a result, it was clarified that the prediction method based on the random vibration theory introducing the correlation length has sufficient margin for actual turbulence-induced response.
Synchronized vibrations were measured for a circular cylinder in a water cross flow at subcritical Reynolds numbers to compare the synchronization range between the subcritical and supercritical regions and clarify the effect of the Strouhal number on the range. A small vibration in the lift direction was found in only the subcritical region when the Karman vortex shedding frequency was about 1/5 of the cylinder natural frequency. The ratio of the Karman vortex shedding frequency to the natural frequency where the self-excited vibration in the drag direction by the symmetrical vortex shedding began was about 1/4 in the subcritical region, and increased to 0.32 at the Strouhal number of 0.29 in the supercritical region. The frequency ratio at the beginning of the lock-in vibration in the drag direction by the Karman vortex shedding was about 1/2, and that in the lift direction decreased from 1 to 0.8 with decreasing Strouhal number.
In this paper a low dimensional dynamical system is used to model the local evolution of a disturbance superimposed on a basic shear flow. In particular, nonlinear interactions between planar waves and oblique waves which travel phase-locked in the far wake of a cylinder are simulated. At this aim a low-dimensional dynamical system is derived by imposing the respect of a nonlinear truncation condition together with the phase-locked condition. The numerical results are compared with some recent experimental results obtained analysing the dynamics of a cylinder far wake. Different experimental conditions were modelled by dynamical systems defined by one or more coupled triadic systems. The energy spectral distribution of the numerical time histories of the streamwise velocity component well reproduce the experimental dynamics of the cylider far-wake. Also the numerical reconstruction of the streaklines shows a good agreement with the visualized flow field.
Two types of shroud for propeller fans were introduced to reduce the fan noise by controlling the tip vortex of a fan rotor. Three-dimensional vortical flow structures and velocity fluctuation near the fan rotor tip have been investigated by experimental analyses using laser Doppler velocimetry (LDV). The tip vortex formed near the midchord of the rotor tip develops in the tangential direction. This tangential structure of the tip vortex induces an inward radial flow near the leading edge of the rotor tip and a reverse flow between the rotor tip and the shroud, causing a large blockage effect on the through flow (main flow) in the rotor. High velocity fluctuations are observed in the interference region between the tip vortex and the through flow. The shroud designed so as to diminish the reverse flow between the rotor tip and the shroud can decrease the blockage effect due to the tip vortex, which decelerates the through flow in the rotor, thus leading to the reduction in the fan noise.
The necklace vortex excitation, one of the longitudinal vortex excitations, appeared on the upstream cylinder in a crisscross circular cylinder system in uniform flow. The influence of the diameter ratio, gap-to-diameter ratio s/d and the logarithmic damping factor on the oscillation behavior caused by the necklace vortex excitation was investigated using a wind tunnel. The amplitude of the oscillation increases with increasing diameter ratio. The substantial fluctuating lift coefficient of the necklace vortex was introduced based on the spanwise dimension of the vortex, (CLR)'rms, and it was evaluated using the measurement of the oscillation amplitude. In the necklace vortex excitation region, the relationship between (CLR)'rms and the gap-to-diameter ratio collapses on a single curve irrespective of the diameter ratio of the two cylinders, and (CLR)'rms decreases with increasing gap-to-diameter ratio.