Showing papers by "Kozo Fujii published in 2015"

Mechanisms for laminar separated-flow control using dielectric-barrier-discharge plasma actuator at low Reynolds number

Abstract: Large-eddy simulations have been conducted to investigate the mechanisms of separated-flow control using a dielectric barrier discharge plasma actuator at a low Reynolds number. In the present study, the mechanisms are classified according to the means of momentum injection to the boundary layer. The separated flow around the NACA 0015 airfoil at a Reynolds number of 63 000 is used as the base flow for separation control. Both normal and burst mode actuations are adopted in separation control. The burst frequency non-dimensionalized by the freestream velocity and the chord length (F+) is varied from 0.25 to 25, and we discuss the control mechanism through the comparison of the aerodynamic performance and controlled flow-fields in each normal and burst case. Lift and drag coefficients are significantly improved for the cases of F+ = 1, 5, and 15 due to flow reattachment associated with a laminar-separation bubble. Frequency and linear stability analyses indicate that the F+ = 5 and 15 cases effectively exc...

Multifactorial Effects of Operating Conditions of Dielectric-Barrier-Discharge Plasma Actuator on Laminar-Separated-Flow Control

Abstract: A substantial number of large-eddy simulations are conducted on separated flow controlled by a dielectric barrier discharge plasma actuator at a Reynolds number of 63,000. In the present paper, the separated flow over a NACA 0015 airfoil at an angle of attack of 12 deg, which is just poststall, is used as the base flow for separation control. The effects of the location and operating conditions of the plasma actuator on the separation control are investigated by a parametric study. The control effect is evaluated based on the improvement of not only the lift coefficient but also the drag coefficient over an airfoil. The most effective location of the plasma actuator for both lift and drag improvement is precisely confirmed to be upstream of the natural separation point. Even a low burst ratio is found to be sufficient to obtain the same improvements as the cases with a high burst ratio. The effective nondimensional burst frequency F+ is observed at 4≤F+≤6 for the improvement in the lift coefficient and at...

Mechanisms of surface pressure distribution within a laminar separation bubble at different Reynolds numbers

Abstract: Mechanisms behind the pressure distribution and skin friction within a laminar separation bubble (LSB) are investigated by large-eddy simulations around a 5% thickness blunt flat plate at the chord length based Reynolds number 5.0 × 103, 6.1 × 103, 1.1 × 104, and 2.0 × 104. The characteristics inside the LSB change with the Reynolds number; a steady laminar separation bubble (LSB_S) at the Reynolds number 5.0 × 103 and 6.1 × 103, and a steady-fluctuating laminar separation bubble (LSB_SF) at the Reynolds number 1.1 × 104, and 2.0 × 104. Different characteristics of pressure and skin friction distributions are observed by increasing the Reynolds number, such that a gradual monotonous pressure recovery in the LSB_S and a plateau pressure distribution followed by a rapid pressure recovery region in the LSB_SF. The reasons behind the different characteristics of pressure distributions at different Reynolds numbers are discussed by deriving the Reynolds averaged pressure gradient equation. It is confirmed that the viscous stress distributions near the surface play an important role in determining the formation of different pressure distributions. Depending on the Reynolds numbers, the viscous stress distributions near the surface are affected by the development of a separated laminar shear layer or the Reynolds shear stress. In addition, we show that the same analyses can be applied to the flows around a NACA0012 airfoil.

LES of transient flows controlled by DBD plasma actuator over a stalled airfoil

Abstract: Large-eddy simulations LES are employed to understand the flow field over a NACA 0015 airfoil controlled by a dielectric barrier discharge DBD plasma actuator. The Suzen body force model is utilised to introduce the effect of the DBD plasma actuator. The Reynolds number is fixed at 63,000. Transient processes arising due to non-dimensional excitation frequencies of one and six are discussed. The time required to establish flow authority is between four and six characteristic times, independent of the excitation frequency. If the separation is suppressed, the initial flow conditions do not affect the quasi-steady state, and the lift coefficient of the higher frequency case converges very quickly. The transient states can be categorised into following three stages: 1 the lift and drag decreasing stage, 2 the lift recovery stage, and 3 the lift and drag converging stage. The development of vortices and their influence on control is delineated. The simulations show that in the initial transient state, separation of flow suppression is closely related to the development spanwise vortices while during the later, quasi-steady state, three-dimensional vortices become more important.

Comparison of Numerical Methods Evaluating Airfoil Aerodynamic Characteristics at Low Reynolds Number

Abstract: The flowfields around the NACA 0012 airfoil at Reynolds numbers 1×104, 3×104, and 5×104 are studied, and the predictability of aerodynamic characteristics derived from various numerical methods is examined. Two-dimensional laminar simulation, two-dimensional Reynolds-averaged Navier–Stokes simulation using the Baldwin–Lomax turbulence model, and three-dimensional implicit large-eddy simulation are employed in this study. The two-dimensional laminar and three-dimensional implicit large-eddy simulations accurately predict the separation point, and capture the characteristics of a separation bubble for each Reynolds number and each angle of attack. Nonlinearity in the lift curve is also captured in the results of the two-dimensional laminar and three-dimensional implicit large-eddy simulations. The two-dimensional Reynolds-averaged Navier–Stokes simulation using the Baldwin–Lomax turbulence model predicts the separation point nearer the trailing edge than does the two-dimensional laminar and three-dimensiona...

Phase Behavior of a Piperidinium-Based Room-Temperature Ionic Liquid Exhibiting Scanning Rate Dependence

Abstract: The structural flexibility and conformational variety of the ions in room-temperature ionic liquids (RTILs) have significant effects on their physicochemical properties To begin a systematic study of the thermodynamic properties of nonaromatic RTILs, 1-methyl-1-butylpiperidinium bis(fluorosulfonyl)amide ([Pip1,4][FSA]) was selected as the first sample In addition to the rotational flexibility of the alkyl group, the [Pip1,4](+) cation has characteristic ring-flipping flexibility, which is very different from the behavior of the well-studied imidazolium-based cations Calorimetry investigations using laboratory-made high-sensitivity calorimeters and Raman spectroscopy revealed that [Pip1,4][FSA] has two crystalline phases, Cryst-α and Cryst-β, and that every phase change is linked to conformational changes of both the cation and anion Each phase change is also governed by very slow dynamics The phase changes from supercooled liquid to Cryst-α and from Cryst-α to Cryst-β, which were observed only during heating, are not in fact phase transitions but structural relaxations Notably, the temperatures of these structural relaxations exhibited heating rate dependences, from which the activation energy of the ring-flipping was estimated to be 388 kJ/mol It is thought that this phenomenon is due to the associated conformational changes of the constituent ions in viscous surroundings

Anion and cation dynamics of sulfonylamide-based ionic liquids and the solid–liquid transitions

Abstract: Some of the important factors that characterise room-temperature ionic liquids (RTILs) are the variety of conformations adopted by the constituent ions and their flexibility. Using 1,3-dimethylimidazolium bis(fluorosulfonyl)amide ([C1mim][FSA]) and 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)amide ([C1mim][NTf2]) as samples, the longitudinal and transverse relaxation times (T1 and T2) for 19F and 1H were determined as a function of temperature and were correlated with the dynamics of the phase behaviours of the two RTILs. Because the anions and cations in the two compounds have 19F and 1H nuclei, respectively, their dynamics can be independently investigated and the relationships between them can be discussed. For [C1mim][FSA], the only observed phase changes included melting and crystallisation. The temperature dependences of T1 and T2 for 19F were similar to those of T1 and T2 for 1H, indicating similar dynamics due to the formation of strong anion–cation interactions. For [C1mim][NTf2], the T1 and T2 values for both 19F and 1H discontinuously changed at same temperatures, which were assigned to the crystallisation and melting points. However, the T1 curves for 19F and 1H were different in the crystalline region, suggesting independent dynamics for the anions and cations in [C1mim][NTf2]. In the crystalline state for each salt, the cation dynamics was distinctly separated into the framework movement of the imidazolium ring and the movement of the methyl groups, while the anion dynamics was characterised by the movement of the entire anion. The influence of the crystal structure on the dynamics of each salt was also considered.

A ranking method based on two preference criteria: Chebyshev function and ∈-indicator

Abstract: Previously a preference relation based on the Chebyshev achievement function to solve many-objective optimization problems was proposed. Although using this preference relation improved the performance of NSGA-II, in this paper we present a new ranking method based on the ∈-indicator and the Chebyshev achievement function. The goal of this new method is two fold: i) to improve the performance of the original algorithm, and ii) to design a parallel sorting method in order to use it with large populations (≫ 104 individuals). To do so, unlike the original approach, we have completely replaced the nondominated sorting by a method that ranks the population based on these two preference criteria. As the experiments show, the resulting algorithm outperforms both the standard NSGA-II and our previous approach in selected DTLZ problems. We also present a parallel implementation of the new sorting method. The running time analysis shows that the communication overhead is low enough to allow the speedup reach its peak for a large number of processors.

The effects of actuation frequency on the separation control over an airfoil using a synthetic jet

Abstract: The simulation of separation control using a synthetic jet (SJ) is conducted around an NACA (National Advisory Committee for Aeronautics) 0015 airfoil by large-eddy simulation (LES) with a compact difference scheme. The synthetic jet is installed at the leading edge of the airfoil and the effects of an actuation frequency F + (normalized by chord length and velocity of freestream) are observed. The lift-drag coefficient is recovered the most for F + = 6. The relationship between momentum addition by turbulent mixing and large vortex structures is investigated using a phase-averaging procedure based on F + . The Reynolds shear stress is decomposed into periodic and turbulent components where the turbulent components are found to be dominant on the airfoil. The strong turbulent components appear near the large vortex structures that are observed in phase- and span-averaged flow fields.

LES of separated-flow controlled by DBD plasma actuator around NACA 0015 over reynolds number range of 104 - 106

Abstract: We have conducted high-fidelity large-eddy simulations on the separated flow around an airfoil with control by the DBD plasma actuator over a wide Reynolds number range. The Reynolds numbers based on a chord length were set to 63,000, 260,000 and 1,600,000. For the no control cases, the flow separates near the leading edge in laminar state at Reynolds numbers of 63,000 and 260,000, and massive turbulent separation occurs at Reynolds number of 1,600,000. The separation control with the burst actuation can achieve the flow reattachment through the promotion of the turbulent transition for the Reynolds numbers of 63,000 and 260,000, resulting in the improvement in both the lift and drag. On the other hand, the lift coefficient can be mainly increased over 45 % through the large-scale vortex paring induced by the burst plasma actuation for the Reynolds number of 1,600,000. The effects of the burst frequency on the separation control are evaluated based on the improvement of the aerodynamic performance. In this evaluation, the effective burst frequency non-dimensionalized by a chord length and freestream velocity (F = fc/u∞) comes to change with the Reynolds number. While relatively high burst frequencies (F ≈ 5) show the good improvement in the lift-drag ratio at Reynolds number of 63,000, the lower burst frequency (F ≈ 1) shows the highest improvement at Reynolds number of 1,600,000. On the other hand, when the non-dimensional burst frequency based on the momentum thickness and edge velocity of the separation shear-layer (Fθs) is considered, the high liftdrag ratio can be recognized at Fθs ≈ 10−2 for all the Reynolds number conditions. ∗Postdoctoral Fellow, Department of Space Flight Engineering and AIAA Member. †Engineer,Engineering Solution Division. ‡Postdoctoral Fellow, Department of Space Flight Engineering and AIAA Member. §Graduate Student, Department of Aeronautics and Astronautics and AIAA Member. ¶Postdoctoral Fellow, Department of Space Flight Engineering and AIAA Member. ∥Assistant Professor, Department of Space Flight Engineering and AIAA Member. ∗∗Professor, Department of Space Flight Engineering and AIAA Fellow.

Spatial growth of the spanwise disturbance induced by a synthetic jet on separation control over an airfoil

Abstract: The spatial growth of the spanwise disturbance induced by a synthetic jet is investigated on separated-flow control around NACA0015 (Re=63, 000 and AOA = 12.0◦) using a largeeddy simulation. The synthetic jet is installed at a leading edge which is numerically modeled by a three-dimensional deforming cavity: “Cavity model”; and an artificial jet profile for a boundary condition: “Bc model”. The jet profile of the Bc model is assumed to be sinusoidally oscillated in a spanwise direction with a wave number from kyin/2π = 0 to 95. In the Cavity model case, the modes around ky/2π = 20 to 30 are selectively amplified near the synthetic jet, which remains also in the turbulent boundary layer. In the Bc model cases, the most quick turbulent transition occurs in the case with kyin/2π = 30, where the coherent spanwise mode strongly remains in the turbulent boundary layer although its aerodynamic performance is not best. This result indicates that in the present condition, the spanwise disturbance of the jet profile does not always contribute to the higher aerodynamic performance even if it provides smooth and quick turbulent transition.

Design Exploration of a DBD Plasma Actuator for Massive Separation Control

Abstract: The main aim of this paper is to elucidate the mechanism of massive separation control by using a dielectric barrier discharge plasma actuator (DBDPA). A technique of design exploration is applied to find good operating-parameter combinations for the DBDPA. We consider a NACA 0015 airfoil with 16◦ angle of attack and Reynolds number Re = 63000. The flow without the control is massively separated, however we can suppress the separation using the DBDPA with the relevant operating parameters. Using good parameter combinations obtained by design exploration technique, the nature of the flow around the airfoil with and without control is explored in detail.

Contribution of large-scale vortex and fine-scale turbulent structure in separated flow control using DBD plasma actuator

Abstract: ○ 浅野兼人, 東大•院, 〒252-5210 神奈川県相模原市中央区由野台 3-1-1, asano@flab.isas.jaxa.jp 浅田健吾, 東大•院, 〒252-5210 神奈川県相模原市中央区由野台 3-1-1, asada@flab.isas.jaxa.jp 加藤宏基, 東大•院, 〒252-5210 神奈川県相模原市中央区由野台 3-1-1, kato@flab.isas.jaxa.jp 佐藤允, ISAS/JAXA, 〒252-5210 神奈川県相模原市中央区由野台 3-1-1, sato@flab.isas.jaxa.jp 焼野藍子, ISAS/JAXA, 〒252-5210 神奈川県相模原市中央区由野台 3-1-1, yakeno@flab.isas.jaxa.jp 青野光, ISAS/JAXA, 〒252-5210 神奈川県相模原市中央区由野台 3-1-1, aono@flab.isas.jaxa.jp 野々村拓, ISAS/JAXA, 〒252-5210 神奈川県相模原市中央区由野台 3-1-1, nonomura@flab.isas.jaxa.jp 藤井孝藏, ISAS/JAXA, 〒252-5210 神奈川県相模原市中央区由野台 3-1-1, fujii@flab.isas.jaxa.jp Kento Asano, The University of Tokyo, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan Kengo Asada, The University of Tokyo, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan Hiroki Kato, The University of Tokyo, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan Makoto Sato, ISAS/JAXA, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan Aiko Yakeno, ISAS/JAXA, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan Hikaru Aono, ISAS/JAXA, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan Taku Nonomura, ISAS/JAXA, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan Kozo Fujii, ISAS/JAXA, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan

Validation of Numerical Analysis to Estimate Airfoil Aerodynamic Characteristics at Low Reynolds Number Region

Abstract: In this study, two-dimensional laminar simulation (2-D Lam), two-dimensional Reynolds Averaged Navier-Stokes simulation with the Spalart-Allmaras turbulence model (2-D RANS(SA)), and implicit three-dimensional large-eddy simulation (3-D LES) are performed for NACA0012, NACA0006, and Ishii airfoils at Rec = 3.0 × 104. The relation between a predictability of airfoil aerodynamic characteristics and a dependence of airfoil geometry shape of each numerical method is evaluated at the low Reynolds number. Although little discrepancy is observed for the lift coefficient predictability, significant differences are presented in terms of the separation and reattachment points predictability depending on the numerical methods. The 2-D Lam simulation can predict the lift coefficients as well as the separation and reattachment points qualitatively as similar to the 3-D LES results except for the high angle of attack which is accompanied by the massive separation. The 2-D RANS(SA), the weak nonlinearity and stall phenomena for the lift coefficients are observed. A good predictability of the separation point are shown, however, it cannot be estimated the reattachment points due to the trend to predict widely for the separation region. The predictabilities of each numerical method appear regardless of the airfoil shapes.Copyright © 2015 by JSME

A finite difference WENO scheme maintaining velocity, pressure and temperature equilibrium in multicomponent compressible fluid analysis

Abstract: In this paper, the implementation of the finite difference WENO scheme maintaining velocity, pressure and temperature equilibrium in the multicomponent compressible fluid analysis is discussed. First, the new finite difference WENO scheme is proposed for the quasi-conservative form which is often used in the finite volume formulation in the previous study. This new WENO scheme adopts the split form which separates the consistent and dissipation parts, and the dissipation part is formulated in a conservative form, keeping the conservation for conservative variables. The proposed scheme can keep the velocity, pressure and temperature equilibriums for the various problems. Based on the results here and previous studies, the vector form of numerical dissipation is reconsidered.

Fully-conservative high-order FR scheme on moving and deforming grids

Abstract: An appropriate procedure to construct the symmetric conservative metrics is presented in the high-order conservative flux-reconstruction scheme. The present framework enables a direct discretization of the strong conservation form of governing equations without any errors in the freestream preservation and global conservation properties on threedimensionally moving and deforming grids. We show that a straightforward implementation of the symmetric conservative metrics often fails to construct metric polynomials with the same order of a solution polynomial, which severely degrades a numerical accuracy. On the other hand, the symmetric conservative metrics constructed by the appropriate procedure can preserve the freestream solution regardless of the order of shape functions; in addition, a convecting vortex is more accurately computed on deforming grids. The global conservation property is also demonstrated numerically for the convecting vortex on deforming grids.

Generation of Acoustic Disturbances in Supersonic Laminar Cavity Flows

Abstract: The generation of acoustic disturbances in supersonic laminar cavity flows is investigated by large-eddy simulations of supersonic laminar flow (M = 1.2, 2.0, and 3.0) past a rectangular cavity with a length-to-depth ratio of 2. Results suggest that well-originated large-scale vortical structures with strong spanwise coherence are present in the shear layer. Compressibility effects have significant impacts on the shear-layer development and the fluctuation properties. The dominant mechanism for the acoustic radiation in supersonic laminar cavity flows is shown to be associated with the successive passage of large-scale vortices over the cavity trailing edge. It is found that Mach waves radiated from the cavity shear layer may have significant contributions for the noiseradiation in terms of enhancing the strength of the feedback compression waves.

Phase Behavior of a Piperidinium-Based Room-Temperature Ionic Liquid Exhibiting Scanning Rate Dependence B

Abstract: The structural flexibility and conformational variety of the ions in room-temperature ionic liquids (RTILs) have significant effects on their physicochemical properties. To begin a systematic study of the thermodynamic properties of nonaromatic RTILs, 1-methyl-1-butylpiperidinium bis(fluorosulfonyl)amide ([Pip₁,₄][FSA]) was selected as the first sample. In addition to the rotational flexibility of the alkyl group, the [Pip₁,₄]⁺ cation has characteristic ring-flipping flexibility, which is very different from the behavior of the well-studied imidazolium-based cations. Calorimetry investigations using laboratory-made high-sensitivity calorimeters and Raman spectroscopy revealed that [Pip₁,₄][FSA] has two crystalline phases, Cryst-α and Cryst-β, and that every phase change is linked to conformational changes of both the cation and anion. Each phase change is also governed by very slow dynamics. The phase changes from supercooled liquid to Cryst-α and from Cryst-α to Cryst-β, which were observed only during heating, are not in fact phase transitions but structural relaxations. Notably, the temperatures of these structural relaxations exhibited heating rate dependences, from which the activation energy of the ring-flipping was estimated to be 38.8 kJ/mol. It is thought that this phenomenon is due to the associated conformational changes of the constituent ions in viscous surroundings.