Water Channel Tests Research Articles

Experimental and numerical studies on a passively deformed coupled-pitching hydrofoil under the semi-activated mode

Flexibility is expected to positively affect the hydrodynamic and energy-harvesting performance of deformable hydrofoils, which have a simple structure and can be easily implemented in practical engineering applications. However, the characteristics of the fluid-structure interaction and power capturing are complicated. In this study, to reveal the fluid-structure interaction (FSI) mechanism and the effects of the passive deformation on the energy-harvesting performance, a flexible hydrofoil to conduct a coupled-pitching motion under the semi-activated mode was proposed and investigated experimentally and numerically. In the experimental study, a deformation-measuring technology based on a digital imaging algorithm was developed and employed for water channel tests. The effects of the hydrofoil profile and activated pitching amplitude on the deformation status, torque output, and energy-harvesting performance were studied experimentally. In the numerical study, a three-dimensional unsteady two-way FSI model was established and validated using experimental results. The influence mechanisms of the damping torque coefficient, activated pitching amplitude, and elasticity modulus on the passive deformation, its phase difference with the activated pitching angle, and the hydrodynamic and energy-harvesting performances were studied. Compared with the rigid hydrofoil, the energy-harvesting efficiency and power coefficient of the passively deformable hydrofoil could be increased by 4.8 % and 8.0 %, respectively, as the phase difference zone was close to 3π/4. In addition, positive and negative phase difference zones for performance enhancement were identified, which provides a future direction for optimizing flexible hydrofoils and control strategies.

Research on flow field quality measurement of super high speed circulating water channel based on LDV

The flow field quality of the super high speed circulating water channel (SHSCWC) test section in the open (with a free liquid surface state) and closed (with a cavitation tunnel state) was studied. An array of flow field measuring points was established to measure the axial velocity using a laser doppler velocimeter (LDV) for both states. Meanwhile, ultrasonic sensors were used to measure the levelness of the free liquid surface in the test section. The measurement results indicated that in the open state, the non-uniformity and instability of 8 kinds of flow velocity in the flow field section are less than 1.0%, respectively, while the free liquid surface level is less than 25mm at the flow rate of 8m/s. In the closed state, the non-uniformity and instability of eight types of flow velocity in the flow field section are all below 1.0%, with turbulence being less than 0.5%. The measurement results in both states meet the requirements of technical indicators, and provide data support for the experimental research and optimal design of the SHSCWC.

Characteristics of Twin-Runner Darreus Water-Turbines for Tidal Current Power Generation

In order to make the electrical-power generation feasible under the lower tidal-current condition the twin-runner Darreus water-turbine was developed. The counter rotations of twin runners are expected to increase the power output. In order to find out the best distance between the runners the wind-tunnel tests using the subscale model were carried out. At the dimensionless distance of 0.18 the rotational numbers were increased up to 156 percent of those at the single-runner configuration. With this distance the water-channel test was also conducted. It was confirmed that the revolution numbers of the twin runners were increased by 20 percents of those in the single-runner configuration. The increasing effect of speed by a diffuser was investigated. At the stand-alone test of a diffuser in the wind tunnel the velocity of the current passing through the diffuser was accelerated by 1.1 of the free-stream velocity, however, in the water-channel test the subscale Darreus model with the shorter diffuser showed no increasing effect. This was primarily because of the short length of the diffuser to the mouth distance, which was determined from the standpoint of the structural efficiency. The demonstrative power generation test is planned at the Kanmon strait in March, 2012.

Impact of solid and hollow bluff bodies on the performance and dynamics of flag-based energy harvester

This study investigates the energy harvesting potential of the wake of different bluff bodies using a piezoelectric membrane. Both hollow and solid C-shaped cylinders with similar dimensions in normal and inverted orientations are placed upstream of the membrane as a kinetic source of energy. A series of water channel tests are performed by varying stream-wise gap (x*) 0.5 ≤ x* ≤ 4.0 between the cylinder and membrane and flow velocity (V) 0.12 ≤ V ≤ 0.26 m/s. The instability region is expanded remarkably by using different cross-sections. Different flapping modes are observed, including biased, intermittent, and continuous. It is shown that the time-mean wake fluctuates with changes in the cross-section of the bluff body and has a substantial influence on the energy harvesting efficiency and dynamical behaviors of the membrane in terms of peak-to-peak oscillation amplitude and flapping frequency. A threshold flow velocity of 0.2 m/s for continuous energy harvesting is defined for all bluff bodies. The highest electrical energy is obtained at a flow velocity of 0.26 m/s and 2.5 diameters downstream from the hollow C-shaped bluff body when placed in an inverted orientation. A comparison of all the configurations of the bluff bodies indicates that the hollow C-shaped bluff body in inverted orientation produces the strongest wake, which results in a 36 % increase in flapping amplitude and a 35 % improvement in harvested power for the piezo-membrane based energy harvester compared to the baseline case (a solid C-shape).

An IDDES study of the near-wake flow topology of a simplified heavy vehicle

Abstract The complex wake flow of a GTS (ground transportation system) model contributes to large percentage of the aerodynamic drag force. Therefore, predicting accurate wake flow will help carry out the drag reduction strategies. In this paper, the near-wake flow topology of the GTS was studied at Re = 2.7×104 to assess the capability of a hybrid RANS/LES (Reynolds-averaged Navier–Stokes/large eddy simulation) approach, known as IDDES (improved delayed detached eddy simulation). The current study also aims to understand the effects of different computational parameters, e.g. the spatial resolution, time step, residual level, discretization scheme and turbulence model, on this asymmetrical wake flow configuration. A comparison of IDDES with previous water channel tests, well-resolved LES, partially averaged Navier–Stokes and URANS (unsteady RANS) was included to better understand the benefits of this hybrid RANS/LES approach. The results show that on the medium and fine grids, the IDDES produces an asymmetrical flow topology (known as flow state I) in the near-wake of the vertical midplane, as reported in previous studies. The recommended parameters for the time step (1×10–4 s) and residual level (1×10–4) provide sufficient accuracy of wake predictions to show good agreement with experiments. For the convective term of the momentum equation in IDDES, the bounded central difference discretization scheme is proposed to be adopted for discretization. Additionally, URANS cannot accurately capture this asymmetrical flow field. IDDES proves to be capable of predicting the wake flow field of this simplified heavy vehicle with high accuracy. All obtained conclusions can provide references for the aerodynamic drag reduction of the GTS.

Wind–Water Experimental Analysis of Small SC-Darrieus Turbine: An Approach for Energy Production in Urban Systems

Smart cities have a significant impact on the future of renewable energies as terms such as sustainability and energy saving steadily become more common. In this regard, both wind and hydrokinetic compact-size turbines can play important roles in urban communities by providing energy to nearby consumption points in an environmentally suitable manner. To evaluate the operation of a Darrieus turbine rotor as a wind or hydro microgenerator, a series of wind tunnel and water current flume tests were performed. Power and characteristic curves were obtained for all test conditions. In the wind tests, all curves seemed to be identical, which means that the turbine rotor works properly under open-field conditions. Two blockage correction equations were applied to the water channel tests that were performed under blockage values ranging from 0.2 to 0.35 to estimate the operational behavior in open water. Finally, it has been demonstrated that, with the condition of maintaining the Reynolds number between experiments in the wind tunnel and water flume, the turbine wind characteristics represents the its operation in open-water conditions.

Sea stars generate downforce to stay attached to surfaces

Intertidal sea stars often function in environments with extreme hydrodynamic loads that can compromise their ability to remain attached to surfaces. While behavioral responses such as burrowing into sand or sheltering in rock crevices can help minimize hydrodynamic loads, previous work shows that sea stars also alter body shape in response to flow conditions. This morphological plasticity suggests that sea star body shape may play an important hydrodynamic role. In this study, we measured the fluid forces acting on surface-mounted sea star and spherical dome models in water channel tests. All sea star models created downforce, i.e., the fluid pushed the body towards the surface. In contrast, the spherical dome generated lift. We also used Particle Image Velocimetry (PIV) to measure the midplane flow field around the models. Control volume analyses based on the PIV data show that downforce arises because the sea star bodies serve as ramps that divert fluid away from the surface. These observations are further rationalized using force predictions and flow visualizations from numerical simulations. The discovery of downforce generation could explain why sea stars are shaped as they are: the pentaradial geometry aids attachment to surfaces in the presence of high hydrodynamic loads.

Energy-harvesting performance of a coupled-pitching hydrofoil under the semi-passive mode

The semi-passive flapping hydrofoil is a promising device for harvesting tidal stream energy in shallow waters with significant practical advantages compared to traditional rotor turbines. The energy-harvesting performance of a coupled-pitching hydrofoil under the semi-passive mode was studied experimentally and numerically. In the water channel test, a magnetic damper and an electric generator was employed to provide the constant and varying loads, respectively. The hydrofoil model performed better in energy-harvesting for the reduced frequency of 0.10 and the activated pitching amplitude of 70–90°. The two-dimensional and three-dimensional numerical models were established and the dual-sliding-overlapped-mesh technology was used to realize the complicated coupled-pitching motions and to predict the fluid-structure interaction of the hydrofoil. The vortex structures and corresponding pressure distribution characteristics over the hydrofoil surface were presented for analysis of the operating performance of the semi-passive system. The optimal moment of inertia and location of the activated pitching pivot were obtained. In the numerical study, the peak power coefficient and efficiency were achieved at 0.75 and 0.33, respectively.

Comparing Models of Lateral Station-Keeping for Pitching Hydrofoils.

Fish must maneuver laterally to maintain their position in schools or near solid boundaries. Unsteady hydrodynamic models, such as the Theodorsen and Garrick models, predict forces on tethered oscillating hydrofoils aligned with the incoming flow. How well these models predict forces when bio-inspired hydrofoils are free to move laterally or when angled relative to the incoming flow is unclear. We tested the ability of five linear models to predict a small lateral adjustment made by a hydrofoil undergoing biased pitch oscillations. We compared the models to water channel tests in which air bushings gave a rigid pitching hydrofoil lateral freedom. What we found is that even with no fitted coefficients, linear models predict some features of the lateral response, particularly high frequency features like the amplitude and phase of passive heave oscillations. To predict low frequency features of the response, such as overshoot and settling time, we needed a semiempirical model based on tethered force measurements. Our results suggest that fish and fish-inspired vehicles could use linear models for some aspects of lateral station-keeping, but would need nonlinear or semiempirical wake models for more advanced maneuvers.

Natural ventilation of an isolated generic building with a windward window and different windexchangers: CFD validation, sensitivity study and performance analysis

Windexchangers are relatively small structures located on the building rooftop to promote natural ventilation. This paper presents a Computational Fluid Dynamics (CFD) validation study, sensitivity analysis and performance comparison of three windexchanger (WE) configurations applied to a generic isolated building with a windward window. The study is limited to wind-driven (isothermal) ventilation, for wind perpendicular to the windward facade. The CFD simulations are based on the 3D-steady Reynolds-Averaged Navier–Stokes (RANS) equations. The validation study is performed with experimental results from a previously published water channel test. The sensitivity analysis focuses on the domain size, grid resolution and turbulence model. The performance evaluation of the three WE configurations is based on the mean velocity and mean static pressure coefficients in the vertical centerplane, the ventilation volume flow rate and the volume percentage of the living zone with air speed ratio equal to or above 0.10. The WE configuration with four openings and one duct shows the highest ventilation flow rate (0.232 m3/s) and the highest volume percentage (21%). This study shows that the assessment and selection of WE configurations should not only be based on volume flow rate or ACH but should consider the living zone air speed ratio as well, specifically concerning the flow distribution in the living zone.

Empirical model for probabilistic rock stability on flat beds under waves with or without currents

An empirical formula was developed for predicting the stability of isolated quarry rocks on relatively flat beds under waves with or without current in terms of the damage ratio. The damage ratio was examined using 100 crushed stones (median mass M50 = 293 g and mass density ρs = 2.62 g/cm3) as scale models of quarry rocks and replicas with a lower density (ρs = 1.38 g/cm3) by placing them on beds of different roughness in a wave flume, not only under periodic waves with periods of 2–3.5 s, but also in symmetric and asymmetric oscillatory flows (periods: 8–12 s), simulating waves without and with current in a circulating water channel. Comparison between hydrodynamic forces expressed in terms of three non-dimensional mobility indices: the maximum velocity, maximum semi-velocity amplitude (Ua), and maximum acceleration relative to the friction force (expressed in terms of the median coefficient of friction) suggested that the damage ratio was most closely related to the Ua-based mobility index (ψ2). Nevertheless, significant differences remained between data from the wave flume and circulating water channel tests. The variation in the damage ratio, which included the effects of the oscillatory-velocity asymmetry, oscillation period, superimposed steady current, mass density of stones, and bottom friction, was reasonably well described via the product of ψ2 and a function of a Keulegan–Carpenter number. The results of the field tests on quarry rocks (with M50 = 2.04 t) placed on a thin sand layer overlaying hard substrate show that the minimum stable mass is consistent with the prediction.

Experimental measurements of sediment incipient velocity by using B-scan ultrasound imaging device in the water channel

Abstract Sediment incipient velocity (SIV) is an important parameter for the rule of sediment movement and fluvial processes in the river sediment engineering. The field measurement of the SIV during the process of sediment incipient motion near the riverbed is a difficult problem. For this problem, this paper puts forward a novel method of measuring SIV by using B-scan ultrasound imaging device and current meter in the water channel of river model experiment. In this method, we use a B-scan ultrasound imaging device to get the ultrasonic images of moving particles and riverbed under the water, and use a current meter to obtain the flow velocities of observed points near the bed at the same time. By statistical analysis of the images, relationships between the imaging spot number/area of particles and the water flow velocity are obtained, which have shown the whole process of sediment incipient motion clearly. Results show there are some suddenly changed positions in these relationships. These suddenly changed positions correspond to the states of incipient motion, which can be used to analyse and measure the SIV. After kinds of model experiments, the measured SIV is within an acceptable range and can be verified by the change of riverbed boundary lines. This method is very suitable for river model experiments, especially for water channel test under muddy water or sediment-laden flow. This paper provides a new approach to the research and analysis of sediment movement with the advantages of direct observation and real-time research.

On the optimization of 3D-flow and heat transfer by using the Ice Formation Method: Vane endwall heat transfer

Technical flow devices are usually optimized by using numerical algorithms that analyze a multitude of design variants for the flow restraining geometry until an optimum solution is found. In this paper, we present an alternative approach: the Ice Formation Method (IFM). We show the application of this method to a turbine vane endwall in order to optimize the endwall with respect to heat transfer. On the basis of a flat endwall, we first created ice-contoured endwalls in our water channel test facility. We then analyzed these contours using numerical simulations with air to approve them for the working medium of gas turbines. Results show that the IFM reliably creates contours with reduced heat transfer coefficients (HTC). In this vein, we created one contour that reduces the average HTC by 5.2% compared to the baseline. Analyzing this contour revealed that it adapts to the flow field by breaking up the near endwall vortices and thus reducing secondary flows. We found that this reduction in HTC results from the natural character of the IFM to reduce entropy production rates due to heat conduction.

Study of Characteristics of Darrieus-type Straight-bladed Vertical Axis Wind Turbine by Use of Ailerons

This paper presents a new starting assist method for Darrieus turbines using movable ailerons. To confirm the effect of the ailerons, we conducted wind tunnel tests to find the starting torque, starting wind speed, and load torque characteristics. The tested turbine has four blades, and the tested ailerons are fixed-angle and movable types. The movable ailerons increase the starting torque, thereby reducing the wind speed for startup. When a movable aileron is used, the load torque at low rotational speed is increased. Furthermore, the load torque of the turbine is roughly equal to the bare blade turbine at high rotational speed. 2007), and were found to provide more efficiency than symmetrical blades in water channel tests (Kihoh and Shiono, 1992). To investigate how the WT characteristics were changed by the ailerons, the ailerons were initially mounted at a fixed angle (fixed- aileron blades), and this arrangement was observed for the effect of the ailerons on the starting torque of the WT. The fixed-aileron blade experiment showed that the starting torque is increased in some of the position angles and decreased in other position angles, but the mean starting torque is not greatly increased. Therefore, we propose movable ailerons that are equipped with an extension- retraction mechanism. A wind tunnel experiment showed that movable ailerons increase the starting torque of the WT and that the wind speed needed for startup is decreased. Self-starting is not the only important feature of a WT; load characteristics are just as important. Therefore, the influence of the movable ailerons on load characteristics is also examined. Due to the effects of the fittings for controlling the angle of the movable aileron in the model constructed here, it was noted that at high rotational speed the ailerons reduce the WT output to below the power obtained in a model whose blades have no ailerons (bare blades). Therefore, a method is demonstrated for overcoming this.

Prediction of the NOx Emissions of a Swirl Burner in Partially and Fully Premixed Mode on the Basis of Water Channel Laser Induced Fluorescence and Particle Image Velocimetry Measurements

The paper describes the development and validation of an efficient and cost effective method for the prediction of the NOx emissions of turbulent gas turbine burners in the early burner design phases, which are usually focused on the optimization of the swirler aerodynamics and the fuel-air mixing. Since the method solely relies on nonreacting tests of burner models in the water channel, it can be applied before any test equipment for combustion experiments exists. In order to achieve optimum similarity of fuel-air mixing in the water channel tests with engine operation the model is operated at the engine momentum ratio. During the laser induced fluorescence (LIF) measurements the water flow representing the fuel is doped with fluorescent dye, a plane perpendicular to the length axis near the burner exit plane is illuminated with a 5W Ar-ion laser, and the fluorescence is recorded with a video camera from downstream. From the video sequence,s the local probability density functions (PDF) of the dye concentration fluctuations are calculated from the data. Furthermore, the time mean velocity fields are measured with particle image velocimetry (PIV). The PDFs of the local equivalence ratio are derived from the LIF data. Assuming flamelets, the NOx generation in the entire equivalence ratio range observed in the water channel tests is computed using the unstrained freely propagating one-dimensional flame model in Cantera and the GRI3.0 reaction scheme. Although neither flame stretch nor post flame NOx generation were considered, the computed NOx values were in excellent agreement with the experimental data from perfectly premixed combustion experiments. The local time averaged NOx mole fraction is obtained by integrating the flamelet NOx over the mixture PDF. Finally the global NOx emission of the burner at the considered operating point is obtained by spatial integration, considering the measured velocity field. The method was validated using a conical swirl burner with two fuel injection stages, allowing the degree of premixedness to be adjusted over a wide range, depending on the specific fuel injection scenario. For the case with fuel injection along the air inlet slots NOx values slightly above the minimum NOx limit for perfectly premixed combustion were computed. This is consistent with the emission measurements and indicates the finite mixing quality of this injection method. In the partially premixed regime the configurations with potential for low NOx emissions were reliably identified with the LIF and PIV based water channel method. The method also shows the steep increase of the NOx emissions with the decreasing degree of premixing observed in the experiments, however, quantitative predictions would have required a postprocessing of the data from the LIF mixing study with a higher spatial resolution than available.

Comparison of CFD and operational dispersion models in an urban-like environment

Chemical plants, refineries, transportation of hazardous materials are some of the most attractive facilities for external attacks aimed at the release of toxic substances. Dispersion of these substances into the atmosphere forms a concentration distribution of airborne pollutants with severe consequences for exposed individuals. For emergency preparedness and management, the availability of assessed/validated dispersion models, which can be able to predict concentration distribution and thus dangerous zones for exposed individuals, is of primary importance. Air quality models, integral models and analytical models predict the transport and the turbulent dispersion of gases or aerosols after their release without taking into account in detail the presence of obstacles. Obstacles can modify the velocity field and in turn the concentration field. The Computational Fluid Dynamics (CFD) models on the other hand are able to describe such phenomena, but they need to be correctly set up, tested and validated in order to obtain reliable results. Within the project Europa-ERG1 TA 113.034 NBC Modelling and Simulation several different approaches in CFD modelling of turbulent dispersion in closed, semi-confined and urban-like environment were adopted and compared with experimental data and with operational models. In this paper the results of a comparison between models describing the dispersion of a neutral gas in an idealized urban-like environment are presented and discussed. Experimental data available in the literature have been used as a benchmark for assessing statistical performance for each model. Selected experimental trials include some water channel tests, that were performed by Coanda at 1:205 scale, and one full-scale case that was tested in the fall of 2001 at the Dugway Proving Grounds in Utah, using an array of shipping containers. The paper also suggests the adoption of improved statistical parameters in order to better address differences between models, and to have a more straightforward method for comparing models suitable for emergency preparedness aims. © 2011 Elsevier Ltd.

Dynamic Adaptation of Aerodynamic Flame Stabilization of a Premix Swirl Burner to Fuel Reactivity Using Fuel Momentum

Due to the expected increase in available fuel gas variants in the future and the interest in independence from a specific fuel, fuel flexible combustion systems are required for future gas turbine applications. Changing the fuel used for lean premixed combustion can lead to serious reliability problems in gas turbine engines caused by the different physical and chemical properties of these gases. A new innovative approach to reach efficient, safe, and low-emission operation for fuels such as natural gas, syntheses gas, and hydrogen with the same burner is presented in this paper. The basic idea is to use the additionally available fuel momentum of highly reactive gases stemming from their lower Wobbe index (lower volumetric heating value and density) compared with lowly reactive fuels. Using fuel momentum opens the opportunity to influence the vortex dynamics of swirl burners designed for lowly reactive gases in a favorable way for proper flame stabilization of highly reactive fuels without changing the hardware geometry. The investigations presented in this paper cover the development of the optimum basic aerodynamics of the burner and the determination of the potential of the fuel momentum in water channel experiments using particle image velocimetry. The results show that proper usage of the fuel momentum has enough potential to adjust the flow field to different fuels and their corresponding flame behavior. As the main challenge is to reach flashback safe fuel flexible burner operation, the main focus of the study lies on avoiding combustion induced vortex breakdown. The mixing quality of the resulting injection strategy is determined by applying laser induced fluorescence in water channel tests. Additional OH∗ chemiluminescence and flashback measurements in an atmospheric combustion test rig confirm the water channel results for CH4, CH4/H2 mixtures, H2 with N2 dilution, and pure H2 combustion. They also indicate a large operating window between flashback and lean blow out and show expected NOx emission levels. In summary, it is shown for a conical four slot swirl generator geometry that the proposed concept of using the fuel momentum for tuning of the vortex dynamics allows aerodynamic flame stabilization for different fuels in the same burner.

Study on a Flapping Wing Hydroelectric Power Generation System

In this paper, a hydroelectric power generation system able to extract the water flow energy from the hydroelastic response of an elastically supported rectangular wing is experimentally investigated. Except for a vertical wing, supported in the manner of a cantilever, the generator has no other parts submerged in water. An electric motor is used to excite pitching oscillations of the wing. Both the wing and the electric motor are supported by leaf springs which are designed to work both as a linear guide for the sway oscillations and as elastic elements. The wing mass in sway direction necessary to achieve a hydroelastic response is obtained by using a mechanical snubber mechanism. The appropriate load to generate electricity is provided by magnetic dampers. By employing the linear potential hydrodynamic theory, a theoretical analysis was performed to model the water channel tests on the power generation system and to determine its structural and hydrodynamic features. Tests revealed that the proposed power generation system is feasible and able to work with an efficiency of 32-37%.

フラッタ水力発電システムの研究(流体工学,流体機械)

In this paper, a hydroelectric power generation system able to extract the water flow energy from the hydroelastic response of an elastically supported rectangular wing is experimentally investigated. Except for a vertical wing, supported in the manner of a cantilever, the generator has no other parts submerged in water. An electric motor is used to excite pitching oscillations of the wing. Both the wing and the electric motor are supported by leaf springs which are designed to work both as a linear guide for the sway oscillations and as elastic elements. The wing mass in sway direction necessary to achieve a hydroelastic response is obtained by using a mechanical snubber mechanism. The appropriate load to generate electricity is provided by magnetic dampers. By employing the linear potential hydrodynamic theory, a theoretical analysis was performed to model the water channel tests on the power generation system and to determine its structural and hydrodynamic features. Tests revealed that the proposed power generation system is feasible and able to work with an efficiency of 32-37%.

Vortex Shedding from a Pair of In-Line Forced Oscillating Parallel Arranged Circular Cylinders (In the Case of Equal Diameter)

In this study, the flow features of vortex shedding from a pair of parallel arranged circular cylinders oscillating along the direction of the flow were observed by visualizing water flow experiment at the ranges of the frequency ratio f/fk=0-7, amplitude ratio 2a/d=0.125, 0.25, 0.5 and 0.75, gap ratio G/d=0.25, 0.75 and 1.75, and Reynolds numbers Re=470-670 (towing water tank test) and 700-750 (closed circuit water channel test). The variations of mean vortex shedding frequency from each oscillating cylinder were investigated. As a result of the experiments, typical flow patterns of lock-in state or un-lock-in state were shown every gap ratio G/d. Although the cylinder oscillation frequency f is lower than the natural Karman vortex's frequency fk, the lock-in phenomenon can be seen. It was found that the measurement result depended on the observation point because the rearrangement of vortex formation had changed the characteristics of the flow. When the cylinders were oscillated to the direction of flow, the flow pattern of bias gap flow became not seen. The map of lock-in state in the lock-in range was obtained. It was found that the lock-in range and the lock-in form were more different from the case of single oscillating cylinder.