Prediction of steady flows passing fixed cylinders using deep learning

Considerable attention has been given to deep-learning and machine-learning techniques in an effort to reduce the computational cost of computational fluid dynamics simulation. The present paper addresses the prediction of steady flows passing many fixed cylinders using a deep-learning model and investigates the accuracy of the predicted velocity field. The deep-learning model outputs the x- and y-components of the flow velocity field when the cylinder arrangement is input. The accuracy of the predicted velocity field is investigated, focusing on the velocity profile of the fluid flow and the fluid force acting on the cylinders. The present model accurately predicts the flow when the number of cylinders is equal to or close to that set in the training dataset. The extrapolation of the prediction to a smaller number of cylinders results in error, which can be interpreted as internal friction of the fluid. The results of the fluid force acting on the cylinders suggest that the present deep-learning model has good generalization performance for systems with a larger number of cylinders.

Serve Ball Trajectory Characteristics of Different Volleyballs and Their Causes

A floater serve in volleyball is a technique of serving a non-rotating or low-rotating ball, which is difficult to return because the flight path of the ball changes irregularly. On the other hand, the randomness of the trajectory makes it difficult for the ball to fall on the target. Players are required to serve taking into account the variability of the trajectory. In previous studies using wind tunnels, it was shown that aerodynamic characteristics such as drag force and lateral force applied to the ball vary depending on the type of ball and the orientation of the panel. Therefore, in order to control the flight trajectory, it is necessary to understand the aerodynamic characteristics of each ball. Since the velocity of the ball and the fluid force applied to the ball changes during flight, it is important to measure not only the fluid force at a steady state in the wind tunnel but also the actual flight distance of the ball. In this study, to provide valuable information for precise control of floater serves, we measured the drag force applied to the ball in a wind tunnel and the flight distance of the ball using an ejection machine, and clarified the effects of the type of ball and the panel face. In the drag force measurement, the drag force on three types of balls, V200W, MVA200, and FLISTATEC, was measured in the wind speed range of 4 m/s to 30 m/s. In the ejection measurement, the ball flight distances were measured while changing the orientation of the panel using an ejection machine. Basically, the FLISTATEC, MVA200, and V200W, in that order, were more likely to increase the distance and the variability, but it was shown that the drop point could be adjusted slightly by selecting the panel face. This result was also obtained when a human player actually served the ball, indicating the tactical importance of the player consciously controlling the direction of the panel. The tactical importance of the player’s conscious control of the direction of the panel was demonstrated. We also proposed receiver positions that would be effective based on the characteristics of each ball.

Motion Model of Spent Fuel Rack Considering Two-dimensional Gap Flow

A free-standing rack system, in which each rack is not fixed to the floor or the wall, is proposed and is in use in European countries and the US as a storage method for spent fuel from nuclear power stations. Although this system can reduce the influence of an earthquake's excitation force by using the frictional force between the rack's bottom surface and the floor surface, together with the fluid force excited by each rack's motion, design guidelines are not yet established. In this research project, to evaluate the fluid force more precisely, the gap between the racks is treated as a two-dimensional gap flow, and the pressure loss coefficient at the flow path junction and the top of the flow path were estimated based on the steady CFD calculation and incorporated into the motion model. Our primary concern in this paper is rocking motion. As a result, we concluded that increasing the pressure loss coefficient at the fuel rack's top suppressed rocking motion.

High-resolution flow simulation in Typhoon 21, 2018: massive loss of submerged macrophytes in Lake Biwa

The global activities of typhoons and hurricanes are gradually changing, and these storms can drastically affect lake ecosystems through the recession of submerged macrophytes that regulate the water quality in lakes. Using an echosounder, we captured the short-term, massive loss of submerged macrophytes attributed to the abnormal fluctuation of the water level induced by the approach of a catastrophic super typhoon in the southern basin of Lake Biwa, Japan. This paper investigates the physical processes responsible for the loss of vegetation using a high-resolution circulation model in Lake Biwa as a pilot study area. The circulation model was coupled with dynamical models of the fluid force and erosion acting on the vegetation. Our simulation successfully reproduced the water level fluctuation and high-speed current (torrent) generated by the typhoon gale. The simulated results demonstrated that the fluid force driven by the gale-induced torrent uprooted submerged macrophytes during the typhoon approach and that this fluid force (rather than erosion) caused the outflow of vegetation. As a result, this uprooting attributed to the fluid force induced the massive loss of submerged macrophytes in a large area of the southern basin, which might have increased primary production and reduced the stock of fish such as bluegill in the lake. Our model can estimate the reduction in the macrophyte height within the range of − 1.3 to − 0.4 m, suggesting that fluid forces greater than the time-averaged value (1.24 × 10−4 N) were available. Flow speeds of approximately 0.8 m/s might be the critical value that induces the fluid force acting on the uprooting of the submerged macrophytes. Our approach is practical for evaluating changes in lake environments attributed to the massive outflow of submerged macrophytes under various climate change scenarios.

Experimental Study on the End Effect and the Effect Due to the Difference in End Shape of the Fluid Force Acting on a Rotating Cylinder in a Uniform Flow

When investigating the characteristics of the fluid force acting on a rotating cylinder in a flow, three-dimensional effects such as edge shape and walls become a problem. In this paper, we investigated the fluid force characteristics of the rotating cylinder, as well as the effect of the flow under the end of the cylinder and the shape of the cylinder end in the water tank experiment. There are three types of experimental conditions: No end flow condition, end flow condition, and condition with end plate. There are five disk-shaped end plates with different diameters, and the changes in fluid force are quantitatively summarized for each ratio between the diameter of the cylinder model and the end plate. By generating a flow under the end of the cylinder, the fluid force is significantly reduced compared to the no end flow condition. By attaching the end plate, the fluid force does not decrease even when a flow is generated under the end of the cylinder. The fluid force increases until the ratio DE/D of the diameter D of the cylinder model and the diameter DE of the end plate is around 3, but after that, it shows a constant value.

Forced Response Excitation of a Compressor Stator Owing to Shock Wave Induced by Adjacent Rotor Blade

The accurate prediction of high cycle fatigue (HCF) is becoming one of the key technologies in the design process of state-of-the-art axial compressors. If they are not properly designed, both rotor blades and stator vanes can be damaged. There are two main factors to cause HCF. One is low engine order (LEO) and the other is high engine order (HEO) excitation by fluid force associated with adjacent rotor-stator interaction. For the front stages of axial compressors for power generations and aero engines, the inlet Mach number of a rotor tip typically exceeds the speed of sound and strong shock waves tend to be induced. This can be the source of HEO excitation fluid force, and adjacent stator vanes are sometimes severely damaged. Thus, the aim of this study is to establish an efficient method for predicting the vibration response in this type of problem with high accuracy. To achieve this, numerical investigations are carried out by one-way fluid structure interaction (FSI) simulation. To validate the accuracy of FSI simulation, experiments are also conducted using a gas turbine engine for power generation. In the experiment, the vibration level is measured with strain gauges mounted on the surface of stator vanes and the data are compared with the predicted results. In the first part of the study, efficient prediction methods of excitation fluid force on the stator vane are investigated by time transformation (TT) and harmonic balance (HB) methods. Their accuracies are evaluated by comparing the results with those calculated by transient rotor stator (TRS) simulation whose pitch ratio is one between rotor and stator computational domains. It is found that the TT method can accurately predict the excitation fluid force with lower computation load even when there are pitch differences between rotor and stator regions. In the second part of the study, forced response analyses are carried out using the excitation fluid force obtained in the unsteady flow simulation. To obtain the total damping of the system, both hammering test and flutter simulations are carried out. Computed results are validated with experimental data and it is found that the predicted vibration level is in good agreement with experimental results. Through this study, the effectiveness of one-way FSI simulation is confirmed for this type of forced response prediction. By utilizing the combination of efficient unsteady computational fluid dynamics (CFD) methods and harmonic response analysis, vibration amplitude can be predicted accurately and efficiently.

Visualization of Destabilization Force of Labyrinth Gas Seal Using Fast-Responding Pressure Sensitive Paint

Labyrinth seals are used in the clearance between rotor and stator such as steam turbines and centrifugal compressors in order to suppress loss of leakage. The leakage flow is induced in labyrinth seal by pressure difference between the inlet and outlet of the seal. This leakage flow produces destabilization fluid force on the rotor in association with the swirl flow. Destabilization fluid force can lead the rotor system unstable by its phase delay from subsynchronous whirling vibration of the rotor. Recently, this kind of problems have been reported sometimes with increasing power output and improving performance by decreasing leakage flow. In order to understand detailed characteristics of this destabilization fluid force, so many CFD calculations have been performed and unsteady pressure measurements have been conducted using rotational test rig. However, CFD calculation results often do not agree with test result especially for complex seal configurations. Although unsteady pressure was also measured by pressure sensors, measurement results were not enough for validation of CFD results and detailed understand of the fluid force mechanisms because of insufficient spatial resolution. In this study, unsteady pressure filed in labyrinth seal cavity of rotational test rig was visualized using fast-responding polymer-ceramic pressure sensitive paint (PC-PSP). The unsteady pressure distribution on the surface of seal cavity was measured by high-speed camera. For the experiment, a 500mm diameter rotor was used so that size of the labyrinth seal and flow pattern in the cavity can be close to the large-scale steam turbine. And the rotor was excited in a circular whirl orbit by an electromagnetic actuator. The stator side cavity wall was made by transparent material in order to obtain optical access. In the case of existing PSP measurements, only rotor side pressure distribution could be measured in this set up. However, measuring rotor side surface by PSP is extremely challenging because of its high peripheral speed. The back side illumination measurement technique has been developed and applied in this study. PSP is coated on the inner surface of transparent stator wall. Excitation light sources were delivered from outside of the transparent wall and highspeed camera was also installed outside. This procedure allows PSP measurement only by processing motionless image of the stator wall. Furthermore, composition of the PC-PSP has been optimized for this measurement because back illumination procedure decreases response speed of the PSP. As a result, high-accuracy, high-definition pressure distribution data was obtained. Pressure data measured by PSP were compared with pressure sensor data and CFD calculation results. Unsteady pressure levels measured by PSP agreed well with pressure data measured by pressure sensors. Unsteady pressure distributions measured by PSP agreed well with pressure distribution calculated by CFD. From these results, detailed characteristics of destabilization fluid force were understood. And these data can be used for brush-up of CFD calculation model and procedure.