Institute of Fluid Science

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Institute of Fluid Science

Science, Japan
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Palar P.S.,Tohoku University | Palar P.S.,Institute of Fluid Science | Shimoyama K.,Tohoku University | Shimoyama K.,Institute of Fluid Science
35th AIAA Applied Aerodynamics Conference, 2017 | Year: 2017

Multi-fidelity uncertainty analysis method is a potential technique to speed up the process of uncertainty quantification (UQ) when an expensive computational simulation is involved. In this paper, we investigate the capability of hierarchical Kriging (HK) method for multi-fidelity uncertainty analysis, especially for application in computational fluid dynamics (CFD). Besides the conventional HK, we also enhanced the HK method by utilizing polynomial chaos expansion (PCE) and the ensemble of Kriging and PCE as the low-fidelity surrogate model. We also extend the formulation of HK so that it could use polynomial scaling function to further enhance the approximation capability of HK. The MF framework was firstly demonstrated on the multi-fidelity Branin and Ishigami function. Computational study on two CFD test problems was performed and the result was compared with ordinary Kriging, PCE, and multi-fidelity PCE. In light of our investigation, we found that using the ensemble of PCE and Kriging as the low-fidelity surrogate model is a better and more robust way to deal with various problems in UQ instead of using just one type of surrogate model for HK. The HK framework is also more efficient than the multi-fidelity PCE which relies on a simple correction function. Results on common research model and RAE 2822 problems reveal the remarkable efficiency of HK for UQ in real-world CFD-based problems with multi-fidelity simulation. On the RAE 2822 problem, the use of first order polynomial scaling function is needed in order to make the HK more efficient than the single-fidelity surrogate. © 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

Shimoyama K.,Tohoku University | Shimoyama K.,Institute of Fluid Science | Kamisori K.,Tohoku University | Kamisori K.,Institute of Fluid Science
Journal of Aircraft | Year: 2017

This paper identifies exhaust heat characteristics depending on airfoil shapes at low Reynolds number to design a high-altitude long-endurance unmanned aerial vehicle with a wing-surface heat exchanger. In the first step, computational fluid dynamics simulations are conducted for different airfoils, each of which exhausts heat from different regions on the upper surface, at different angles of attack. This parametric study reveals that early transition near the leading edge is favorable to improve heat-exhaust characteristics, although it may increase skin friction. In addition, heat exhaust should be conducted only in the region of turbulent boundary layer behind the transition point. From these results, the airfoil shape significantly affects the Nusselt-number distribution along the upper wing surface due to the change in the location of laminar-turbulent transition and turbulent boundary-layer separation. In the next step, multi-objective optimization of an airfoil shape, which balances aerodynamic performance and heat-exhaust performance, is carried out. The obtained nondominated solutions show the tradeoffs between aerodynamic performance and heat-exhaust performance. It was confirmed that heat-exhaust performance can be controlled by the location of the laminar-turbulent transition, and improved without sacrificing aerodynamic performance at the cruising condition. © Copyright 2016 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Tokumasu T.,Institute of Fluid Science | Ogawa I.,Institute of Fluid Science | Koyama M.,Kyushu University | Ishimoto T.,Kyushu University | Miyamoto A.,Tohoku University
Journal of the Electrochemical Society | Year: 2011

The bond dissociation energy of a perfluorosulfonic acid (PFSA) molecule was investigated by density functional theory (DFT) to provide some dissociation trends of the PFSA molecule and the basic information for the design of more durable PFSA membranes. As the preliminary analysis, the chemical bond strengths of the PFSA molecule were analyzed exhaustively, and the vulnerable bond was identified by comparison of the results. The same calculations were performed for an ionized PFSA molecule to determine the influence of the ionization state on the bond strength. The C-S bond was the weakest among the side chain backbone in the neutral molecule; however, the C-S bond became stronger when ionized while it weakened the C-O bond connecting the side chain with the main chain. Analysis of the C-F bonds in the side chain showed that the dissociation energy decreases in the order of primary, secondary, and tertiary bonds, as also reported in the literature. Analysis of the main chain showed that the secondary C-F bonds neighboring the tertiary C-F bond connecting the side and main chains were the weakest. Ionization of the PFSA molecule weakens the average dissociation energy of C-F bonds. As the realistic analysis, the same calculations were performed considering the solvation effects to analyze the effects on the dissociation trend. Moreover, the possibility for improving the durability of PFSA membranes was investigated by partial substitution of H atoms or CH3 groups for F atoms. © 2010 The Electrochemical Society.

Okajima J.,Institute of Fluid Science | Maruyama S.,Institute of Fluid Science
Environmental Progress and Sustainable Energy | Year: 2015

A nontracking, nonimaging solar concentrator with a low-heat-loss configuration was proposed, and its optical and thermal performance was investigated. The reflector has a compound parabolic and involute shape so that the absorber is heated as uniformly as possible. To eliminate heat loss by conduction and convection, the mirror and absorber are enclosed in an evacuated glass tube. The concentrator presented here was simulated for the location of Sendai, Japan, and its acceptance angle was estimated as 23.44°. A ray-tracing model was developed to evaluate its optical and thermal performance. The average optical efficiency was evaluated by a 2-D ray-tracing model, and a value of 72.7% was obtained. A thermal analysis of the absorber was conducted to evaluate the temperature uniformity. The results indicate that the temperature distribution of the absorber can be considered uniform. The thermal efficiency of the proposed solar concentrator was calculated as a function of the incidence angle and absorber temperature. With a concentration ratio of 2.51, the concentrator, even at an absorber temperature of 373 K, operates with an average efficiency of 47.8%, although the absorber was assumed to have a gray surface. The concentrator's thermal efficiency was compared with that of other solar collectors, and found to be higher than that of conventional solar collectors. © 2015 American Institute of Chemical Engineers Environ Prog.

Sun M.,Tohoku University | Sun M.,Institute of Fluid Science | Hirao K.,Tohoku University
51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 2013 | Year: 2013

A simple method is proposed to evaluate the curvature of interfaces. The essence of the method is to treat the surface normal (SN) vectors as independent variables, and to integrate them separately. The curvature is then evaluated from the SN vectors. It is found that the curvature can be evaluated with uniform second-order accuracy for circles as small as two grid cells in diameter, which has never been reported before. In addition, the method is based on the numerical solution of the partial differential equations, so it can be straightforwardly extended to unstructured grid systems. Three possible methods to evaluate the curvature from the SN vectors have been compared. It turns out that only the curvature interpolated from that of the centroid to the center of an interface segment achieves the second-order accuracy. © 2013 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Iga Y.,Tohoku University | Iga Y.,Institute of Fluid Science | Yoshida Y.,Japan Aerospace Exploration Agency | Yoshida Y.,Kakuda Space Center
Journal of Propulsion and Power | Year: 2011

The propagation mechanism of rotating cavitations, especially that of supersynchronous rotating cavitation, is still unclear. The direction of propagation of supersynchronous rotating cavitation is in the direction of rotation of an impeller, which is opposite of the direction of rotating stall in general pumps and fans. Also, neither the mechanism of the order of appearance of rotating cavitations (supersynchronous, synchronous, and subsynchronous propagation) nor the mechanism of the discontinuity of the propagation speed during the transition between different types of rotating cavitations has been determined thus far. The present study indicates that they can be explained by the existence of latent rotating stall and a specific feature of cavitation: A decrease in the break-off frequency of a cavitation according to its development. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc.

Yonezawa M.,Tohoku University | Yonezawa M.,Institute of Fluid Science | Obayashi S.,Tohoku University | Obayashi S.,Institute of Fluid Science
Journal of Aircraft | Year: 2010

This paper investigates the aerodynamic performance of the three-dimensional lifting supersonic biplane and its sonic boom. Although the Busemann biplane is known to cancel the wave drag, it does not produce lift, either. A few decades later, the supersonic biplane airfoils with lift were reported. This paper extends their ideas to the three-dimensional biplane. The aerodynamic performance was revealed by using computational fluid dynamics. The possibility of sonic boom mitigation due to shock wave interaction was demonstrated. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc.

News Article | October 20, 2015

Spoilers on the back of cars aren't anything new. However, spoilers on tires are cutting-edge technology if you ask Yokohama. The Japanese tire manufacturer has fastened small, subtle spoilers onto its tires' outer walls, alleging the aerodynamic advance will increase fuel efficiency. Yokohama will roll out its concept tire at the 2015 Tokyo Motor Show next week on Oct. 28. Yokohama added spoilers onto tires by installing the thin fins at distinct angles on their tires' outer sidewalls. According to CNET, the spoilers work in two ways — fighting aerodynamic drag, allowing tires to work with more fuel efficiency, and lowering aerodynamic lift, positively affecting traction. The tire manufacturer thought of the idea after gathering data from the Tohoku University's Institute of Fluid Science and learning the effects of tiny fins attached to the sidewalls of tires. Yokohama says the study used a Supercomputer "K" to run aerodynamic simulations at different parameter values. From the simulations, they devised the idea to add spoilers to their tires. Overall, this development seeks to control aerodynamic flow throughout the vehicle's body by using the spoilers. If this concept hits the market, it will be interesting to see if the average driver invests in these tires and overall technology they boast or whether the advance will be relegated to car enthusiasts, ready to do anything to improve their vehicle's performance. Whether you fall in the former or latter category, anything designed to promote fuel efficiency can be deemed a positive.

Takagi Y.,Institute of Fluid Science | Sawada H.,Institute of Fluid Science | Obayashi S.,Institute of Fluid Science
AIAA Journal | Year: 2016

The magnetic suspension and balance system for the θ85 mm suction-type supersonic wind tunnel at Tohoku University, which is capable of magnetically suspending the model against the starting and stopping loads without mechanical support, has been successfully developed. To implement fast control system response, a new optical position sensor was built and calibrated. The 1.24 kHz synchronous control system, composed of a proportionalintegral controller plus a double-phase advancer, made the rise time of a step response 30% faster than the Japan Aerospace Exploration Agencyfs existing magnetic suspension and balance system. The frequency characteristics of the magnetic field intensity formed in the test section were measured and improved to achieve faster performance of the control system. The control simulation of the magnetic suspension and balance system for the θ85 mm suctiontype supersonic wind tunnel at Tohoku University indicated the appropriate conditions, such as increasing the moment of inertia of the model and setting a cutoff frequency of 100 Hz, and led to successful magnetic suspension tests. © 2015 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Konishi Y.,Institute of Fluid Science | Okuizumi H.,Institute of Fluid Science | Ohno T.,Institute of Fluid Science
Procedia Engineering | Year: 2016

There are some reports that the Magnus force becomes negative at some situation in wind tunnel test. If so, there is possibility of a variety of curves using change of the direction of the Magnus force. PIV measurements of a flying table tennis ball were conducted to confirm whether a similar phenomenon was observed in real flight. A high-speed camera with a frame rate of 10k fps was used to capture the instantaneous flow field of the flying ball. The imaging region was 210 mm × 210 mm. The Reynolds number was approximately 6.5 × 104, which corresponds to a smash in table tennis. A coordinate transformation of the ball's fixed coordinate system captured the wake motion of non-rotating and rotating balls. In the non-rotating condition, the averaged velocity field of the ball was observed to be symmetric, whereas, in the rotating condition, it was asymmetric, which shows the Magnus effect. At spin parameter is 0.65, the Magnus force becomes zero to indicate the appearance of the negative Magnus force. These observations quantitatively agree with the wind tunnel test. © 2016 The Authors. Published by Elsevier Ltd.

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