| 1. |
- Angioni, C., et al.
(författare)
-
Dependence of the turbulent particle flux on hydrogen isotopes induced by collisionality
- 2018
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674 .- 1070-6631 .- 1089-7666. ; 25:8
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Tidskriftsartikel (refereegranskat)abstract
- The impact of the change of the mass of hydrogen isotopes on the turbulent particle flux is studied. The trapped electron component of the turbulent particle convection induced by collisionality, which is outward in ion temperature gradient turbulence, increases with decreasing thermal velocity of the isotope. Thereby, the lighter is the isotope, the stronger is the turbulent pinch, and the larger is the predicted density gradient at the null of the particle flux. The passing particle component of the flux increases with decreasing mass of the isotope and can also affect the predicted density gradient. This effect is however subdominant for usual core plasma parameters. The analytical results are confirmed by means of both quasi-linear and nonlinear gyrokinetic simulations, and an estimate of the difference in local density gradient produced by this effect as a function of collisionality has been obtained for typical plasma parameters at mid-radius. Analysis of currently available experimental data from the JET and the ASDEX Upgrade tokamaks does not show any clear and general evidence of inconsistency with this theoretically predicted effect outside the errorbars and also allows the identification of cases providing weak evidence of qualitative consistency.
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| 2. |
- Saarelma, S., et al.
(författare)
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Self-consistent pedestal prediction for JET-ILW in preparation of the DT campaign
- 2019
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666 .- 1070-664X .- 1089-7674. ; 26:7
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Tidskriftsartikel (refereegranskat)abstract
- The self-consistent core-pedestal prediction model of a combination of EPED1 type pedestal prediction and a simple stiff core transport model is able to predict Type I ELMy (edge localized mode) pedestals of a large JET-ILW (ITER-like wall) database at the similar accuracy as is obtained when the experimental global plasma beta is used as input. The neutral penetration model [R. J. Groebner et al., Phys. Plasmas 9, 2134 (2002)] with corrections that take into account variations due to gas fueling and plasma triangularity is able to predict the pedestal density with an average error of 15%. The prediction of the pedestal pressure in hydrogen plasma that has higher core heat diffusivity compared to a deuterium plasma with similar heating and fueling agrees with the experiment when the isotope effect on the stability, the increased diffusivity, and outward radial shift of the pedestal are included in the prediction. However, the neutral penetration model that successfully predicts the deuterium pedestal densities fails to predict the isotope effect on the pedestal density in hydrogen plasmas.
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| 3. |
- Abidakun, Olatunde, et al.
(författare)
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Impacts of fuel nonequidiffusivity on premixed flame propagation in channels with open ends
- 2021
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 33
-
Tidskriftsartikel (refereegranskat)abstract
- The present study scrutinizes premixed flame dynamics in micro-channels, thereby shedding light on advanced miniature micro-combustion technologies. While equidiffusive burning (when the Lewis number Le = 1) is a conventional approach adopted in numerous theoretical studies, real premixed flames are typically non-equidiffusive (Le ≠ 1), which leads to intriguing effects, such as diffusional-thermal instability. An equidiffusive computational study [V. Akkerman et al., Combust. Flame 145, 675–687 (2006)] reported regular oscillations of premixed flames spreading in channels having nonslip walls and open extremes. Here, this investigation is extended to nonequidiffusive combustion in order to systematically study the impact of the Lewis number on the flame in this geometry. The analysis is performed by means of computational simulations of the reacting flow equations with fully-compressible hydrodynamics and onestep Arrhenius chemical kinetics in channels with adiabatic and isothermal walls. In the adiabatic channels, which are the main case of study, it is found that the flames oscillate at low Lewis numbers, with the oscillation frequency decreasing with Le, while for the Le > 1 flames, a tendency to steady flame propagation is observed. The oscillation parameters also depend on the thermal expansion ratio and the channel width, although the impacts are rather quantitative than qualitative. The analysis is subsequently extended to the isothermal channels. It is shown that the role of heat losses to the walls is important and may potentially dominate over that of the Lewis number. At the same time, the impact of Le on burning in the isothermal channels is qualitatively weaker than that in the adiabatic channels.
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| 4. |
- Ahlman, Daniel, et al.
(författare)
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Direct numerical simulation of a plane turbulent wall-jet including scalar mixing
- 2007
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 19:6, s. 065102-
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Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulation is used to study a turbulent plane wall-jet including the mixing of a passive scalar. The Reynolds and Mach numbers at the inlet are Re=2000 and M=0.5, respectively, and a constant coflow of 10% of the inlet jet velocity is used. The passive scalar is added at the inlet enabling an investigation of the wall-jet mixing. The self-similarity of the inner and outer shear layers is studied by applying inner and outer scaling. The characteristics of the wall-jet are compared to what is reported for other canonical shear flows. In the inner part, the wall-jet is found to closely resemble a zero pressure gradient boundary layer, and the outer layer is found to resemble a free plane jet. The downstream growth rate of the scalar is approximately equal to that of the streamwise velocity in terms of the growth rate of the half-widths. The scalar fluxes in the streamwise and wall-normal direction are found to be of comparable magnitude. The scalar mixing situation is further studied by evaluating the scalar dissipation rate and the mechanical to mixing time scale ratio.
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| 5. |
- Ahlman, Daniel, et al.
(författare)
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Direct numerical simulation of non-isothermal turbulent wall-jets
- 2009
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 21:3
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Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulations of plane turbulent nonisothermal wall jets are performed and compared to the isothermal case. This study concerns a cold jet in a warm coflow with an ambient to jet density ratio of ρa/ρj = 0.4, and a warm jet in a cold coflow with a density ratio of ρa/ρj = 1.7. The coflow and wall temperature are equal and a temperature dependent viscosity according to Sutherland’s law is used. The inlet Reynolds and Mach numbers are equal in all these cases. The influence of the varying temperature on the development and jet growth is studied as well as turbulence and scalar statistics. The varying density affects the turbulence structures of the jets. Smaller turbulence scales are present in the warm jet than in the isothermal and cold jet and consequently the scale separation between the inner and outer shear layer is larger. In addition, a cold jet in a warm coflow at a higher inlet Reynolds number was also simulated. Although the domain length is somewhat limited, the growth rate and the turbulence statistics indicate approximate self-similarity in the fully turbulent region. The use of van Driest scaling leads to a collapse of all mean velocity profiles in the near-wall region. Taking into account the varying density by using semilocal scaling of turbulent stresses and fluctuations does not completely eliminate differences, indicating the influence of mean density variations on normalized turbulence statistics. Temperature and passive scalar dissipation rates and time scales have been computed since these are important for combustion models. Except for very near the wall, the dissipation time scales are rather similar in all cases and fairly constant in the outer region.
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| 6. |
- Ahn, MyeongHwan, et al.
(författare)
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Large-eddy simulations of flow and aeroacoustics of twin square jets including turbulence tripping
- 2023
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 35:6
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we investigate the flow and aeroacoustics of twin square (i.e., aspect ratio of 1.0) jets by implicit large-eddy simulations (LESs) under a nozzle pressure ratio of 3.0 and a temperature ratio of 1.0 conditions. A second-order central scheme coupled with a modified Jameson's artificial dissipation is used to resolve acoustics as well as to capture discontinuous solutions, e.g., shock waves. The flow boundary layer inside of the nozzle is tripped, using a small step in the convergent section of the nozzle. The time-averaged axial velocity and turbulent kinetic energy of LES with boundary layer tripping approaches better to particle image velocimetry experimental data than the LES without turbulence tripping case. A two-point space–time cross-correlation analysis suggests that the twin jets are screeching and are coupled to each other in a symmetrical flapping mode. Intense pressure fluctuations and standing waves are observed between the jets. Spectral proper orthogonal decomposition (SPOD) confirms the determined mode and the relevant wave propagation. The upstream propagating mode associated with the shock-cell structures is confined inside jets. Far-field noise obtained by solving Ffowcs Williams and Hawkings equation is in good agreement with the measured acoustic data. The symmetrical flapping mode of twin jets yields different levels of the screech tone depending on observation planes. The tonalities—the fundamental tone, second and third harmonics—appear clearly in the far-field, showing different contributions at angles corresponding to the directivities revealed by SPOD.
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| 7. |
- Akkerman, V., et al.
(författare)
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Analysis of flame acceleration induced by wall friction in open tubes
- 2010
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 22:5, s. 1-14
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Tidskriftsartikel (refereegranskat)abstract
- Spontaneous flame acceleration leading to explosion triggering in open tubes/channels due to wall friction was analytically and computationally studied. It was first demonstrated that the acceleration is affected when the thermal expansion across the flame exceeds a critical value depending on the combustion configuration. For the axisymmetric flame propagation in cylindrical tubes with both ends open, a theory of the initial (exponential) stage of flame acceleration in the quasi-isobaric limit was developed and substantiated by extensive numerical simulation of the hydrodynamics and combustion with an Arrhenius reaction. The dynamics of the flame shape, velocity, and acceleration rate, as well as the velocity profile ahead and behind the flame, have been determined. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3425646]
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| 8. |
- Akkerman, V., et al.
(författare)
-
Flow-flame interaction in a closed chamber
- 2008
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 20:5, s. 21-
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Tidskriftsartikel (refereegranskat)abstract
- Numerous studies of flame interaction with a single vortex and recent simulations of burning in vortex arrays in open tubes demonstrated the same tendency for the turbulent burning rate proportional to U-rms lambda(2/3), where U-rms is the root-mean-square velocity and lambda is the vortex size. Here, it is demonstrated that this tendency is not universal for turbulent burning. Flame interaction with vortex arrays is investigated for the geometry of a closed burning chamber by using direct numerical simulations of the complete set of gas-dynamic combustion equations. Various initial conditions in the chamber are considered, including gas at rest and several systems of vortices of different intensities and sizes. It is found that the burning rate in a closed chamber (inverse burning time) depends strongly on the vortex intensity; at sufficiently high intensities it increases with U-rms approximately linearly in agreement with the above tendency. On the contrary, dependence of the burning rate on the vortex size is nonmonotonic and qualitatively different from the law lambda(2/3). It is shown that there is an optimal vortex size in a closed chamber, which provides the fastest total burning rate. In the present work, the optimal size is six times smaller than the chamber height.
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| 9. |
- Albernaz, Daniel L., 1984-, et al.
(författare)
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Real fluids near the critical point in isotropic turbulence
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Ingår i: Physics of fluids. - 1070-6631 .- 1089-7666.
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the behavior of a uid near the critical point by using numerical simulations of weakly compressible three-dimensional isotropic turbulence. Much has been done for a turbulent ow with an ideal gas. The primary focus of this work is to analyze uctuations of thermodynamic variables (pressure, density and temperature) when a non-ideal Equation Of State (EOS) is considered. In order to do so, a hybrid lattice Boltzmann scheme is applied to solve the momentum and energy equations. Previously unreported phenomena are revealed as the temperature approaches the critical point. These phenomena include increased uctuations in pressure, density and temperature, followed by changes in their respective probability density functions (PDFs). Unlike the ideal EOS case, signicant dierences in the thermodynamic properties are also observed when the Reynolds number is increased. We also address issues related to the spectral behavior and scaling of density, pressure, temperature and kinetic energy.
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| 10. |
- Albernaz, Daniel L., et al.
(författare)
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Thermodynamics of a real fluid near the critical point in numerical simulations of isotropic turbulence
- 2016
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 28:12
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the behavior of a fluid near the critical point by using numerical simulations of weakly compressible three-dimensional isotropic turbulence. Much has been done for a turbulent flow with an ideal gas. The primary focus of this work is to analyze fluctuations of thermodynamic variables (pressure, density, and temperature) when a non-ideal Equation Of State (EOS) is considered. In order to do so, a hybrid lattice Boltzmann scheme is applied to solve the momentum and energy equations. Previously unreported phenomena are revealed as the temperature approaches the critical point. Fluctuations in pressure, density, and temperature increase, followed by changes in their respective probability density functions. Due to the non-linearity of the EOS, it is seen that variances of density and temperature and their respective covariance are equally important close to the critical point. Unlike the ideal EOS case, significant differences in the thermodynamic properties are also observed when the Reynolds number is increased. We also address issues related to the spectral behavior and scaling of density, pressure, temperature, and kinetic energy.
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| 11. |
- Alfredsson, P. Henrik, et al.
(författare)
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A new scaling for the streamwise turbulence intensity in wall-bounded turbulent flows and what it tells us about the "outer" peak
- 2011
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 23:4, s. 041702-
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Tidskriftsartikel (refereegranskat)abstract
- One recent focus of experimental studies of turbulence in high Reynolds number wall-bounded flows has been the scaling of the root mean square of the fluctuating streamwise velocity, but progress has largely been impaired by spatial resolution effects of hot-wire sensors. For the near-wall peak, recent results seem to have clarified the controversy; however, one of the remaining issues in this respect is the emergence of a second (so-called outer) peak at high Reynolds numbers. The present letter introduces a new scaling of the local turbulence intensity profile, based on the diagnostic plot by Alfredsson and Orlu [Eur. J. Mech. B/Fluids 42, 403 (2010)], which predicts the location and amplitude of the "outer" peak and suggests its presence as a question of sufficiently large scale separation.
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| 12. |
- Alkhabbaz, Mohammed, et al.
(författare)
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Impact of the Lewis number on finger flame acceleration at the early stage of burning in channels and tubes
- 2019
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 31:8
-
Tidskriftsartikel (refereegranskat)abstract
- For premixed combustion in channels and tubes with one end open, when a flame is ignited at the centerline at the closed end of the pipe and it propagates toward the open one, significant flame acceleration occurs at an early stage of the combustion process due to formation of a finger-shaped flame front. This scenario is tagged "finger flame acceleration" (FFA), involving an initially hemispherical flame kernel, which subsequently acquires a finger shape with increasing surface area of the flame front. Previous analytical and computational studies of FFA employed a conventional assumption of equidiffusivity when the thermal-to-mass-diffusivity ratio (the Lewis number) is unity (Le = 1). However, combustion is oftentimes nonequidiffusive (Le ≠ 1) in practice such that there has been a need to identify the role of Le in FFA. This demand is addressed in the present work. Specifically, the dynamics and morphology of the Le ≠ 1 flames in two-dimensional (2D) channels and cylindrical tubes are scrutinized by means of the computational simulations of the fully compressible reacting flow equations, and the role of Le is identified. Specifically, the Le > 1 flames accelerate slower as compared with the equidiffusive ones. In contrast, the Le < 1 flames acquire stronger distortion of the front, experience the diffusional-thermal combustion instability, and thereby accelerate much faster than the Le = 1 flames. In addition, combustion in a cylindrical configuration shows stronger FFA than that under the same burning conditions in a 2D planar geometry.
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| 13. |
- Altimira, Mireia, 1981-, et al.
(författare)
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Linear spatial instability of viscous flow of a liquid sheet through gas
- 2010
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 22:7
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Tidskriftsartikel (refereegranskat)abstract
- The present paper focuses on the linear spatial instability of a viscous two-dimensional liquid sheet bounded by two identical viscous gas streams. The Orr–Sommerfeld differential equations and the boundary conditions of the flow configuration are numerically solved using Chebyshev series expansions and the collocation method. The strong dependence of the instability parameters on the velocity profiles is proven by using both quadratic and error functions to define the base flow in the liquid sheet and the gas shear layer. The sensitivity of the spatial instability growth rate to changes in the dimensionless parameters of the problem is assessed. Regarding the liquid sheet Reynolds number, it has been observed that, when this parameter increases, both the most unstable growth rate and the corresponding wavenumber decrease, whereas the cutoff wavenumber increases. The results of this analysis are compared with temporal theory through Gaster transformation. The effects liquid and gas viscosity have on instability are studied by comparing the instability curves given by the presented model with those from an inviscid liquid sheet and a viscous liquid sheet in an inviscid gaseous medium. The model presented in this paper features a variation in the cutoff wavenumber with all the governing parameters of the problem, whereas that provided by cases that account for an inviscid surrounding gas depends only on the liquid sheet Weber number and the ratio of gas to liquid densities. Results provided by the presented model have been experimentally validated and show that quadratic profiles have a greater capacity to predict the disturbance wavelength.
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| 14. |
- Altintas, Atilla, 1979, et al.
(författare)
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A new approximation to modulation-effect analysis based on empirical mode decomposition
- 2019
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 31:2
-
Tidskriftsartikel (refereegranskat)abstract
- The modulation effect, namely, the amplification or attenuation of near-wall small-scale (SS) structures by outer large-scale (LS) structures, is one of two commonly accepted ways that outer LS turbulent fluctuations can influence near-wall ones. Mode decomposition based on filtering is widely used to analyze the modulation effect. In the present study, a new approximation is proposed based on empirical mode decomposition (EMD) to investigate the aforementioned amplitude modulation effect. Both methods are used, and their results are compared for two-point and single-point analyses. It has been shown that the LS and SS signals that are decomposed by filtering and EMD follow identical paths. Despite the similarities of the signals, the suggested method exhibits a slightly higher correlation coefficient R compared to the method based on filtering for the two-point analysis. For the one-point analysis, however, the suggested method gives a rational correlation coefficient for the one-point analysis compared to the two-point analysis, while the existing method seems far from giving a rational correlation coefficient value, which is too low compared to that of the two-point analysis. The suggested method is relevant to many recent studies that questioned the reliability of calculating the correlation coefficient with the existing method. The variation of R for identical signals extends the discussion of the correlation-coefficient calculations to the very first process, namely, obtaining LS and SS data from the original signal.
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| 15. |
- Altintas, Atilla, 1979, et al.
(författare)
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Direct numerical simulation of drag reduction by spanwise oscillating dielectric barrier discharge plasma force
- 2020
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 32:7
-
Tidskriftsartikel (refereegranskat)abstract
- DBD (dielectric barrier discharge) plasma actuators have in recent years become increasingly attractive in studies of flow control due to their light structures and easy implementation, but the design of a series of actuators enabling drag reduction depends on many parameters (e.g., the length of the actuator, the space between actuators, and voltage applied) and remains a significant issue to address. In this study, velocities created by the DBD plasma actuators in stagnant flow obtained by the numerical model are compared with experimental results. Then, a DNS study is carried on, and spanwise oscillated DBD plasma actuators are examined to obtain a drag reduction in a fully developed turbulent channel flow. This study connects the conventional spanwise oscillated force in drag reduction studies with DBD plasma actuators. While the former is one of the most successful applications for the drag reduction, the latter is a most promising tool with its light and feasible structure.
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| 16. |
- Altintas, Atilla, 1979
(författare)
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Introducing the Rankine vortex method for drag reduction in wall-bounded turbulent flows at low Reynolds number through streamwise vortex manipulation
- 2024
-
Ingår i: Physics of Fluids. - 1070-6631 .- 1089-7666. ; 36:2
-
Tidskriftsartikel (refereegranskat)abstract
- It is well known that the near-wall streamwise vortices, together with the streaks, are the most important turbulent structures closely associated with drag reduction. Weakening or modifying the streamwise vortices are, thus, general approaches in near-wall turbulent control studies. In this study, a novel approach to manipulate the flow is introduced and applied to a turbulent channel flow in order to achieve drag reduction. The idea behind the “Rankine vortex method” is to generate a force based on the statistical and geometrical information of streamwise vortices. Direct numerical simulations of a turbulent channel flow at a frictional Reynolds number, R e τ , of 180 (based on the driving pressure gradient and channel half-width) are performed. The force is applied in the vicinity of the lower wall of the channel, and the results are comparatively analyzed for the cases with and without force implemented. A drag reduction of 10% is achieved. The theoretical flow control approach presented, along with the associated analysis, has the potential to enhance our current understanding of flow control mechanisms through the manipulation of near-wall turbulence.
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| 17. |
- Amini, Kasra, et al.
(författare)
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Scaling laws for near-wall flows of thixo-elasto-viscoplastic fluids in a millifluidic channel
- 2024
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 36:2
-
Tidskriftsartikel (refereegranskat)abstract
- Thixo-elasto-viscoplastic (TEVP) fluids are very complex fluids. In addition to elasticity and viscoplasticity, they exhibit thixotropy, i.e., time-dependent rheology due to breakdown and recovery of internal structures at different length- and timescales. General and consistent methods for a priori flow prediction of TEVP fluids based on rheological characteristics are yet to be developed. We report a combined study of the rheology and flow of 18 samples of different TEVP fluids (three yogurts and three concentrations of Laponite and Carbopol, respectively, in water in both the unstirred and a stirred state). The rheology is determined both with standard protocols and with an ex situ protocol aiming at reproducing the shear history of the fluid in the flow. Micrometer resolution flow measurements in a millimeter scale rectangular duct are performed with Doppler Optical Coherence Tomography (D-OCT). As expected, the results show the existence of a plug flow region for samples with sufficiently high yield stress. At low flow rates, the plug extends almost all the way to the wall and the extent of the plug decreases not only with increased flow rate but also with increased thixotropy. The ex situ rheology protocol enables estimation of the shear rate and shear stress close to the wall, making it possible to identify two scaling laws that relates four different non-dimensional groups quantifying the key properties wall-shear stress and slip velocity. The scaling laws are suggested as an ansatz for a priori prediction of the near-wall flow of TEVP fluids based on shear flow-curves obtained with a rheometer.
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| 18. |
- Amjadimanesh, Hossein, et al.
(författare)
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The effect of body position while coughing on the airborne transmission of pathogens
- 2022
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 34:4, s. 041902-
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Tidskriftsartikel (refereegranskat)abstract
- Given the recent acceptance of the central role of airborne transmission for SARS-CoV-2, increased attention has been paid to the dispersion of respiratory droplets in different scenarios. Studies including numerical simulations have been conducted on methods for breaking the chains of transmission. Here, we present the first such study on the impact of body position while coughing on the dispersion of respiratory droplets. Four scenarios are examined, including normal standing, bending the head at different angles, coughing into the elbow in still air, and a gentle breeze from the front and behind. The model showed that an uncovered cough is dangerous and causes many droplets to enter the environment, posing a cross-contamination threat to the others. Droplets with an initial diameter smaller than 62.5 mu m remain suspended in windless air for more than 3 min. In the presence of wind, these droplets move with the wind flow and may travel long distances greater than 3.5 m. The model showed that covering the mouth with the elbow while coughing is clearly the best strategy for reducing airborne transmission of exhaled pathogens. About 62% of the initial number of droplets deposit on the cougher's elbow immediately after the cough and have no chance of spreading through the air in both windless and windy conditions. Covering the cough in windless or light breeze conditions also causes the upward thermal plume around the body to expel many small droplets.
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| 19. |
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| 20. |
- Apazidis, Nicholas
(författare)
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Numerical investigation of shock induced bubble collapse in water
- 2016
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 28:4
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Tidskriftsartikel (refereegranskat)abstract
- A semi-conservative, stable, interphase-capturing numerical scheme for shock propagation in heterogeneous systems is applied to the problem of shock propagation in liquid-gas systems. The scheme is based on the volume-fraction formulation of the equations of motion for liquid and gas phases with separate equations of state. The semi-conservative formulation of the governing equations ensures the absence of spurious pressure oscillations at the material interphases between liquid and gas. Interaction of a planar shock in water with a single spherical bubble as well as twin adjacent bubbles is investigated. Several stages of the interaction process are considered, including focusing of the transmitted shock within the deformed bubble, creation of a water-hammer shock as well as generation of high-speed liquid jet in the later stages of the process.
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| 21. |
- Apazidis, Nicholas, et al.
(författare)
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Supersonic jet by blast wave focusing
- 2021
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:12, s. 126101-
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Tidskriftsartikel (refereegranskat)abstract
- A supersonic jet of Mach number M = 4.5 in air is produced experimentally at the apex of a miniature 150 x 50 x 5 mm converging section with a 2 x 5 mm opening by the principle of blast wave amplification through focusing. An initial plane blast wave of M = 2.4 in the convergence section is generated by the exploding wire technique. The profile of the convergence section is specially tailored to smoothly transform a plane blast wave into a perfectly cylindrical arc, imploding at the apex of the section. The cylindrical form of the imploding shock delivers maximum shock amplification in the two-dimensional test section and maximum subsequent jet flow velocity behind the shock front. Blast wave propagation in the convergence chamber as well as jet generation through a 2 mm opening at the apex into the adjacent exhaust chamber is optically captured by a high-speed camera using the shadowgraph method. Visualizing the flow provided a distinct advantage not only for obtaining detailed information on the flow characteristics but also for validating the numerical scheme which further enhanced the analysis. Experimental images together with the numerical analysis deliver detailed information on the blast wave propagation and focusing as well as subsequent jet initiation and development. One of the main advantages of the described method apart from being simple and robust is the effective focusing of low initial input energy levels of just around 500 Joules, resulting in production of supersonic jets in a small confined chamber.
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| 22. |
- Asmuth, Henrik, et al.
(författare)
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Wall-modeled lattice Boltzmann large-eddy simulation of neutral atmospheric boundary layers
- 2021
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 33:10, s. 105111-105111
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Tidskriftsartikel (refereegranskat)abstract
- The lattice Boltzmann method (LBM) sees a growing popularity in the field of atmospheric sciences and wind energy, largely due to itsexcellent computational performance. Still, LBM large-eddy simulation (LES) studies of canonical atmospheric boundary layer flows remainlimited. One reason for this is the early stage of development of LBM-specific wall models. In this work, we discuss LBM–LES of isothermalpressure-driven rough-wall boundary layers using a cumulant collision model. To that end, we also present a novel wall modeling approach,referred to as inverse momentum exchange method (iMEM). The iMEM enforces a wall shear stress at the off-wall grid points by adjustingthe slip velocity in bounce-back boundary schemes. In contrast to other methods, the approach does not rely on the eddy viscosity, nor doesit require the reconstruction of distribution functions. Initially, we investigate different aspects of the modeling of the wall shear stress, i.e.,an averaging of the input velocity as well as the wall-normal distance of its sampling location. Particularly, sampling locations above the firstoff-wall node are found to be an effective measure to reduce the occurring log-layer mismatch. Furthermore, we analyze the turbulence statis-tics at different grid resolutions. The results are compared to phenomenological scaling laws, experimental, and numerical references. Theanalysis demonstrates a satisfactory performance of the numerical model, specifically when compared to a well-established mixed pseudo-spectral finite difference (PSFD) solver. Generally, the study underlines the suitability of the LBM and particularly the cumulant LBM forcomputationally efficient LES of wall-modeled boundary layer flows.
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| 23. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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Investigations of Tip Vortex Mitigation By Using Roughness
- 2020
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1089-7666 .- 1070-6631. ; 32:6
-
Tidskriftsartikel (refereegranskat)abstract
- The application of artificial roughness to mitigate tip vortex cavitation inception is analyzed through numerical and experimental investigations carried out on an elliptical foil. Different roughness configurations and sizes are tested and effects on cavitation inception, drag, and lift, are studied. Implicit Large Eddy Simulation (ILES) is employed to conduct the simulation on a proper grid resolution having the tip vortex spatial resolution as fine as 0.062 mm. Two different approaches including using a rough wall function and resolving the flow around roughness elements are evaluated. New experiments, performed in the cavitation tunnel at Kongsberg Hydrodynamic Research Centre, for the rough foil are presented. The vortical structures and vorticity magnitude distributions are employed to demonstrate how different roughness patterns and configurations contribute to the vortex roll-up and consequently on the tip vortex strength. It is found that the application of roughness on the leading edge, tip region and trailing edge of the suction side are acceptable to mitigate the tip vortex and also to limit the performance degradation. This is regarded to be in close relation with the way that the tip vortex forms in the studied operating condition. The analysis of boundary layer characteristics shows a separation line caused by roughness is the reason for a more even distribution of vorticity over the tip compared to the smooth foil condition leading to a reduction in vortex strength. For the optimum roughness pattern, both the numerical results and experimental measurements show a decrease in the tip vortex cavitation inception as large as 33 % compared to the smooth foil condition with a drag force increase observed to be less than 2 %.
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| 24. |
- Bagheri, Shervin
(författare)
-
Effects of weak noise on oscillating flows : Linking quality factor, Floquet modes, and Koopman spectrum
- 2014
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 26:9, s. 094104-
-
Tidskriftsartikel (refereegranskat)abstract
- Many fluid flows, such as bluff body wakes, exhibit stable self-sustained oscillations for a wide range of parameters. Here we study the effect of weak noise on such flows. In the presence of noise, a flow with self-sustained oscillations is characterized not only by its period, but also by the quality factor. This measure gives an estimation of the number of oscillations over which periodicity is maintained. Using a recent theory [P. Gaspard, J. Stat. Phys. 106, 57 (2002)], we report on two observations. First, for weak noise the quality factor can be approximated using a linear Floquet analysis of the deterministic system; its size is inversely proportional to the inner-product between first direct and adjoint Floquet vectors. Second, the quality factor can readily be observed from the spectrum of evolution operators. This has consequences for Koopman/Dynamic mode decomposition analyses, which extract coherent structures associated with different frequencies from numerical or experimental flows. In particular, the presence of noise induces a damping on the eigenvalues, which increases quadratically with the frequency and linearly with the noise amplitude. (C) 2014 AIP Publishing LLC.
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| 25. |
- Bagheri, Shervin, et al.
(författare)
-
The stabilizing effect of streaks on Tollmien-Schlichting and oblique waves : A parametric study
- 2007
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 19:7, s. 078103-1-078103-4
-
Tidskriftsartikel (refereegranskat)abstract
- The stabilizing effect of finite amplitude streaks on the linear growth of unstable perturbations [Tollmien-Schlichting (TS) and oblique waves] is numerically investigated by means of the nonlinear parabolized stability equations. We have found that for stabilization of a TS-wave, there exists an optimal spanwise spacing of the streaks. These streaks reach their maximum amplitudes close to the first neutral point of the TS-wave and induce the largest distortion of the mean flow in the unstable region of the TS-wave. For such a distribution, the required streak amplitude for complete stabilization of a given TS-wave is considerably lower than for beta=0.45, which is the optimal for streak growth and used in previous studies. We have also observed a damping effect of streaks on the growth rate of oblique waves in Blasius boundary layer and for TS-waves in Falkner-Skan boundary layers.
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| 26. |
- Bazesefidpar, Kazem, et al.
(författare)
-
The effect of contact angle hysteresis on a droplet in a viscoelastic two-phase system
- 2024
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 36:3
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the dynamic behavior of a two-dimensional droplet adhering to a wall in Poiseuille flow at low Reynolds numbers, in a system where one of the phases is viscoelastic represented by a Giesekus model. The Cahn-Hilliard Phase-Field method is used to capture the interface between the two phases. The presence of polymeric molecules alters the viscoelastic drop's deformation over time, categorizing it into two stages before contact line depinning. In the first stage, the viscoelastic droplet deforms faster, while in the second stage, the Newtonian counterpart accelerates and its deformation outpaces the viscoelastic droplet. The deformation of viscoelastic drop is retarded significantly in the second stage with increasing Deborah number De. The viscous bending of viscoelastic drop is enhanced on the receding side for small De, but it is weakened by further increase in De. On the advancing side, the viscous bending is decreased monotonically for Ca<0.25 with a non-monotonic behavior for Ca=0.25. The non-monotonic behavior on the receding side is attributed to the emergence of outward pulling stresses in the vicinity of the receding contact line and the inception of strain-hardening at higher De, while the reduction in the viscous bending at the advancing side is the result of just strain-hardening. Finally, when the medium is viscoelastic, the viscoelasticity suppresses the droplet deformation on both receding and advancing sides, and this effect becomes more pronounced with increasing De. Increasing the Giesekus mobility parameter enhances the weakening effect of viscous bending on the advancing side.
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| 27. |
- Becerra Garcia, Marley, 1979-, et al.
(författare)
-
Self-consistent modeling of laminar electrohydrodynamic plumes from ultra-sharp needles in cyclohexane
- 2017
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 29:12
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a self-consistent model of electrohydrodynamic (EHD) laminar plumes produced by electron injection from ultra-sharp needle tips in cyclohexane. Since the density of electrons injected into the liquid is well described by the Fowler-Nordheim field emission theory, the injection law is not assumed. Furthermore, the generation of electrons in cyclohexane and their conversion into negative ions is included in the analysis. Detailed steady-state characteristics of EHD plumes under weak injection and space-charge limited injection are studied. It is found that the plume characteristics far from both electrodes and under weak injection can be accurately described with an asymptotic simplified solution proposed by Vazquez et al. ["Dynamics of electrohydrodynamic laminar plumes: Scaling analysis and integral model," Phys. Fluids 12, 2809 (2000)] when the correct longitudinal electric field distribution and liquid velocity radial profile are used as input. However, this asymptotic solution deviates from the self-consistently calculated plume parameters under space-charge limited injection since it neglects the radial variations of the electric field produced by a high-density charged core. In addition, no significant differences in the model estimates of the plume are found when the simulations are obtained either with the finite element method or with a diffusion-free particle method. It is shown that the model also enables the calculation of the current-voltage characteristic of EHD laminar plumes produced by electron field emission, with good agreement with measured values reported in the literature.
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| 28. |
- Belson, Brandt A., et al.
(författare)
-
Feedback control of instabilities in the two-dimensional Blasius boundary layer : The role of sensors and actuators
- 2013
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 25:5, s. 054106-1-054106-17
-
Tidskriftsartikel (refereegranskat)abstract
- We analyze the effects of different types and positions of actuators and sensors on controllers' performance and robustness in the linearized 2D Blasius boundary layer. The investigation is carried out using direct numerical simulations (DNS). To facilitate controller design, we find reduced-order models from the DNS data using a system identification procedure called the Eigensystem Realization Algorithm. Due to the highly convective nature of the boundary layer and corresponding time delays, the relative position of the actuator and sensor has a strong influence on the closed-loop dynamics. We address this issue by considering two different configurations. When the sensor is upstream of the actuator, corresponding to disturbance-feedforward control, good performance is observed, as in previous work. However, feedforward control can be degraded by additional disturbances or uncertainties in the plant model, and we demonstrate this. We then examine feedback controllers in which the sensor is a short distance downstream of the actuator. Sensors farther downstream of the actuator cause inherent time delays that limit achievable performance. The performance of the resulting feedback controllers depends strongly on the form of actuation introduced, the quantities sensed, and the observability of the structures deformed by the controller's action. These aspects are addressed by varying the spatial distribution of actuator and sensor. We find an actuator-sensor pair that is well-suited for feedback control, and demonstrate that it has good performance and robustness, even in the presence of unmodeled disturbances.
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| 29. |
- Bergström, Lars
(författare)
-
Interactions of three components and subcritical self-sustained amplification of disturbances in plane Poiseuille flow
- 1999
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 11:3, s. 590-601
-
Tidskriftsartikel (refereegranskat)abstract
- A low-dimensional nonlinear model for the normal velocity (v) and normal vorticity (eta) disturbance development in plane Poiseuille flow is studied. The study is restricted to the interactions of a pair of oblique components of the form e(/(alpha x+/-beta z)) and the component of the form e(/2 beta z) where alpha and beta are streamwise and spanwise wave numbers, respectively. The disturbances considered are also assumed to be highly elongated in the streamwise direction. Owing to the non-normal properties of the basic equations, the eta disturbance is first transiently amplified. Then, if the Reynolds number (R) and the initial disturbance are sufficiently large, the nonlinear interactions lead to a self-sustained process of disturbance amplification at subcritical R. For large R (R greater than or similar to 5000). the threshold disturbance amplitude scales like R-3. The, results also strongly indicate that the nonlinear feedback from eta to v is crucial for the establishment of the self-sustained process.
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| 30. |
- Bergström, Lars
(författare)
-
Nonmodal growth of three-dimensional disturbances on plane Couette-Poiseuille flows
- 2005
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 17:1
-
Tidskriftsartikel (refereegranskat)abstract
- The time development of three-dimensional disturbances superimposed on a variety of mean flow profiles representing plane Couette-Poiseuille flow is investigated numerically. Specifically, with y representing the wall normal coordinate, the mean flow profiles U(y) are represented by U(y) = A(1-y2) + By, where B = 1 ......
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| 31. |
- Bergström, Lars, et al.
(författare)
-
Symmetry properties of developing three-dimensional laminar disturbances in plane Poiseuille flow
- 1994
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 6:4, s. 1618-1620
-
Tidskriftsartikel (refereegranskat)abstract
- The evolution of a finite-amplitude point-like laminar disturbance in plane Poiseuille flow is investigated using hot-wire anemometry. In contrast to earlier experiments, the initial disturbance was introduced simultaneously through both the upper and lower wall of the channel, resulting in an antisymmetric disturbance in the normal velocity. Although the initial disturbance mainly generated symmetrical streamwise velocity modes, the subsequent development of the perturbation showed a marked tendency for antisymmetry.
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| 32. |
- Bergström, Lars
(författare)
-
The effect of the Earth's rotation on the transient amplification of disturbances in pipe flow
- 2003
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 15:10, s. 3028-3035
-
Tidskriftsartikel (refereegranskat)abstract
- In pipe flow, for large Reynolds numbers, A. A. Draad and F. T. M. Nieuwstadt \ref[J. Fluid Mech. 361 (1998), 297--308 showed that the small Coriolis force due to the Earth's rotation may affect the mean flow profile of liquids substantially. In this paper, the development of small disturbances superimposed on a laminar mean flow affected by the Coriolis force is investigated analytically. The investigation is focused on the time development and the transient growth of streamwise-independent disturbances since they are the most amplified disturbances without the Coriolis effect included. The results show that the modification of the parabolic mean flow caused by the Coriolis force significantly affects the transient disturbance amplification when the Reynolds number $(R)$ is high. For example, with $R=9000$ and the Ekman number $\rm Ek=5.23$, due to the Coriolis effect, the peak value of the transient disturbance amplification becomes about two-thirds of the peak value obtained in the case where the mean flow is unaffected by the Coriolis force. When the Reynolds number is decreased, the reduction of the transient growth becomes smaller.
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| 33. |
- Bilgili, Serdar, et al.
(författare)
-
Impacts of the Lewis and Markstein numbers on premixed flame acceleration in channels due to wall friction
- 2022
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 34:1
-
Tidskriftsartikel (refereegranskat)abstract
- The effects of flame stretch as well as that of thermal and molecular diffusion on the scenario of flame acceleration in channels are quantified by means of computational and analytical endeavors. The analytical formulation incorporates the internal transport flame properties into the theory of flame acceleration due to wall friction by means of the Markstein number, which characterizes the flame response to curvature and stretch. Being a positive or negative quantity and a function of the thermal-chemical combustion parameters, such as the thermal expansion ratio as well as the Lewis and Zeldovich numbers, the Markstein number either moderates or promotes flame acceleration. While the Markstein number may provide a substantial impact on the flame acceleration rate in narrow channels, this effect diminishes with increase in the channel width. The analytical formulation is accompanied by extensive computational simulations of the reacting flow equations, which clarify the impact of the Lewis number on flame acceleration. It is noted that for Lewis numbers below a certain critical value, at the initial stage of flame acceleration, a globally convex flame front splits into two or more finger-like segments, accompanied by a drastic increase in the flame front surface area and associated enhancement of flame acceleration. Later, however, these segments of the flame front meet, promptly consuming cavities and pockets, which substantially decreases the flame surface area and moderates acceleration. Eventually, this dynamics results in a single, globally convex flame, which keeps accelerating. Overall, the thermal-diffusive effects substantially facilitate flame acceleration, thereby advancing a potential deflagration-to-detonation transition.
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| 34. |
- Bobylev, Alexander, 1947-, et al.
(författare)
-
Shock wave structure for generalized Burnett equations
- 2011
-
Ingår i: Physics of fluids. - New York : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 23:3, s. 030607-030607-10
-
Tidskriftsartikel (refereegranskat)abstract
- Stationary shock wave solutions for the generalized Burnett equations (GBE) [ A. V. Bobylev, Generalized Burnett hydrodynamics, J. Stat. Phys. 132, 569 (2008) ] are studied. Based on the results of Bisi et al. [Qualitative analysis of the generalized Burnett equations and applications to half-space problems, Kinet. Relat. Models 1, 295 (2008) ], we choose a unique (optimal) form of GBE and solve numerically the shock wave problem for various Mach numbers. The results are compared with the numerical solutions of NavierStokes equations and with the MottSmith approximation for the Boltzmann equation (all calculations are done for Maxwell molecules) since it is believed that the MottSmith approximation yields better results for strong shocks. The comparison shows that GBE yield certain improvement of the NavierStokes results for moderate Mach numbers
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| 35. |
- Borg, K. I., et al.
(författare)
-
Force on a spinning sphere moving in a rarefied gas
- 2003
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 15:3, s. 736-741
-
Tidskriftsartikel (refereegranskat)abstract
- The force acting on a spinning sphere moving in a rarefied gas is calculated. It is found to have three contributions with different directions. The transversal contribution is of opposite direction compared to the so-called Magnus force normally exerted on a sphere by a dense gas. It is given by F=-alpha(tau)xi2/3piR(3)mnomegaxv, where alpha(tau) is the accommodation coefficient of tangential momentum, R is the radius of the sphere, m is the mass of a gas molecule, n is the number density of the surrounding gas, omega is the angular velocity, and v is the velocity of the center of the sphere relative to the gas. The dimensionless factor xi is close to unity, but depends on omega and kappa, the heat conductivity of the body.
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| 36. |
|
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| 37. |
- Borodulin, V. I., et al.
(författare)
-
Experimental and theoretical study of swept-wing boundary-layer instabilities. Three-dimensional Tollmien-Schlichting instability
- 2019
-
Ingår i: Physics of fluids. - : AMER INST PHYSICS. - 1070-6631 .- 1089-7666. ; 31:11
-
Tidskriftsartikel (refereegranskat)abstract
- Extensive combined experimental and theoretical investigations of the linear evolution of three-dimensional (3D) Tollmien-Schlichting (TS) instability modes of 3D boundary layers developing on a swept airfoil section have been carried out. The flow under consideration is the boundary layer over an airfoil at 350 sweep and an angle of attack of +1.5 degrees. At these conditions, TS instability is found to be the predominant one. Perturbations with different frequencies and spanwise wavenumbers are generated in a controlled way using a row of elastic membranes. All experimental results are deeply processed and compared with results of calculations based on theoretical approaches. Very good quantitative agreement of all measured and calculated stability characteristics of swept-wing boundary layers is achieved.
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| 38. |
- Borodulin, V. I., et al.
(författare)
-
Experimental and theoretical study of swept-wing boundary-layer instabilities. Unsteady crossflow instability
- 2019
-
Ingår i: Physics of fluids. - : AMER INST PHYSICS. - 1070-6631 .- 1089-7666. ; 31:6
-
Tidskriftsartikel (refereegranskat)abstract
- Extensive combined experimental and theoretical investigations of the linear evolution of unsteady (in general) Cross-Flow (CF) and three-dimensional (3D) Tollmien-Schlichting (TS) instability modes of 3D boundary layers developing on a swept airfoil section have been carried out. CF-instability characteristics are investigated in detail at an angle of attack of -5 degrees when this kind of instability dominates in the laminar-turbulent transition process, while the 3D TS-instability characteristics are studied at an angle of attack of +1.5 degrees when this kind of instability is predominant in the transition process. All experimental results are deeply processed and compared with results of calculations based on several theoretical approaches. For the first time, very good quantitative agreement of all measured and calculated stability characteristics of swept-wing boundary layers is achieved both for unsteady CF- and 3D TS-instability modes for the case of a boundary layer developing on a real swept airfoil. The first part of the present study (this paper) is devoted to the description of the case of CF-dominated transition, while the TS-dominated case will be described in detail in a subsequent second part of this investigation.
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| 39. |
- Brandenburg, Axel, et al.
(författare)
-
Turbulent radiative diffusion and turbulent Newtonian cooling
- 2021
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:9
-
Tidskriftsartikel (refereegranskat)abstract
- Radiation transport plays an important role in stellar atmospheres, but the effects of turbulence are being obscured by other effects such as stratification. Using radiative hydrodynamic simulations of forced turbulence, we determine the decay rates of sinusoidal large-scale temperature perturbations of different wavenumbers in the optically thick and thin regimes. Increasing the wavenumber increases the rate of decay in both regimes, but this effect is much weaker than for the usual turbulent diffusion of passive scalars, where the increase is quadratic for small wavenumbers. The turbulent decay is well described by an enhanced Newtonian cooling process in the optically thin limit, which is found to show a weak increase proportional to the square root of the wavenumber. In the optically thick limit, the increase in turbulent decay is somewhat steeper for wavenumbers below the energy-carrying wavenumber of the turbulence, but levels off toward larger wavenumbers. In the presence of turbulence, the typical cooling time is comparable to the turbulent turnover time. We observe that the temperature takes a long time to reach equilibrium in both the optically thin and thick cases, but in the former, the temperature retains smaller scale structures for longer.
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| 40. |
- Brandt, Luca, et al.
(författare)
-
Streak interactions and breakdown in boundary layer flows
- 2008
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 20:2
-
Tidskriftsartikel (refereegranskat)abstract
- The objective of this paper is to show that the interaction of streamwise velocity streaks of finite length can lead to turbulent breakdown in the flat-plate boundary layer flow. The work is motivated by previous numerical and experimental studies of transitional flows where the high-frequency oscillations leading to turbulence are seen to form in the region of strongest shear induced by streaks in relative motion. Therefore, a model for the interaction of steady and unsteady (i.e., slowly moving in the spanwise direction) spanwise periodic streaks is proposed. The interaction of two subsequent streaks is investigated for varying collision parameters. In particular, the relative spanwise position and angle are considered. The results show that the interaction is able to produce both a symmetric and asymmetric breakdown without the need for additional random noise from the main stream. Velocity structures characteristic of both scenarios are analyzed. Hairpin and A vortices are found in the case of symmetric collision between a low-speed region and an incoming high-speed streak, when a region of strong wall-normal shear is induced. Alternatively, when the incoming high-momentum fluid is misaligned with the low-speed streak in front, single quasi-streamwise vortices are identified. Despite the different symmetry at the breakdown, the detrimental interaction involves for both cases the tail of a low-speed region and the head of a high-speed streak. Further, the breakdown appears in both scenarios as an instability of three-dimensional shear layers formed between the two streaks. The streak interaction scenario is suggested to be of relevance for turbulence production in wall-bounded flows.
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| 41. |
- Brandt, Luca, et al.
(författare)
-
Weakly nonlinear analysis of boundary layer receptivity to free-stream disturbances
- 2002
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 14:4, s. 1426-1441
-
Tidskriftsartikel (refereegranskat)abstract
- The intent of the present paper is to study the receptivity of a zero pressure gradient boundary layer to free-stream disturbances with the aim to isolate the essential features involved in the generation of streamwise streaks. A weakly nonlinear formulation based on a perturbation expansion in the amplitude of the disturbance truncated at second order is used. It is shown that the perturbation model provide an efficient tool able to disentangle the sequence of events in the receptivity process. Two types of solutions are investigated: the first case amounts to the receptivity to oblique waves generated by a wave-like external forcing term oscillating in the free stream, the second the receptivity to free-stream turbulence-like disturbances, represented as a superposition of modes of the continuous spectrum of the Orr-Sommerfeld and Squire operators. A scaling property of the governing equations with the Reynolds number is also shown to be valid. The relation between this nonlinear receptivity process and previously investigated linear ones is also discussed.
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| 42. |
- Brearley, P, et al.
(författare)
-
Statistical behaviors of conditioned two-point second-order structure functions in turbulent premixed flames in different combustion regimes
- 2019
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 31:11
-
Tidskriftsartikel (refereegranskat)abstract
- The second-order structure functions and their components conditioned upon various events have been analyzed for unweighted and density-weighted velocities using a Direct Numerical Simulation database. The heat release due to combustion has been shown to have significant influences on the structure functions and their components conditioned on different mixture states. The use of density-weighted velocities changes the relative magnitudes of differently conditioned structure functions but does not reduce the scatter of these magnitudes. The structure functions conditioned to constant-density unburned reactants at both points and normalized using the root-mean-square velocity conditioned to the reactants are larger at higher values of mean reaction progress variables (deeper within the flame brush), with this trend being not weakened with increasing turbulence intensity u′/SL. These results indicate that, contrary to a common belief, combustion-induced thermal expansion can significantly affect the incoming constant-density turbulent flow of unburned reactants even at u′/SL and Karlovitz number Ka as large as 10 and 18, respectively. The statistical behaviors of the structure functions reveal that the magnitude of the flame normal gradient of the velocity component tangential to the local flame can be significant, and it increases with increasing turbulence intensity. Moreover, the structure functions conditioned on both points in the heat release zone bear the signature of the anisotropic effects induced by the baroclinic torque for the flames belonging to the wrinkled flamelet and corrugated flamelet regimes. These anisotropic effects weaken with increasing turbulence intensity in the thin reaction
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| 43. |
- Brunet, P., et al.
(författare)
-
Control of thermocapillary instabilities far from threshold
- 2005
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 17:10
-
Tidskriftsartikel (refereegranskat)abstract
- We report experiments on control of thermocapillary instabilities at high temperature differences, in an annular geometry. Previous studies [Phys. Fluids 14, 3039 (2002)] showed that a reasonable control of oscillatory instability could be achieved by optimizing a local heating feedback process. We conducted experiments with a basic flow converging from periphery to center. This constitutes a more unstable configuration than previously, and enables appearance of higher-order instabilities and chaos. Applying successfully local feedback control to the periodic state close to the threshold, we extend the process to higher temperature differences, where nonlinear as well as proportional/derivative control laws are necessary to obtain a significant decrease of the temperature fluctuations. Finally, proportional control allows us to synchronize a chaotic state, to a periodic one.
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| 44. |
- Byron, M., et al.
(författare)
-
Shape-dependence of particle rotation in isotropic turbulence
- 2015
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 27:3
-
Tidskriftsartikel (refereegranskat)abstract
- We consider the rotation of neutrally buoyant axisymmetric particles suspended in isotropic turbulence. Using laboratory experiments as well as numerical and analytical calculations, we explore how particle rotation depends upon particle shape. We find that shape strongly affects orientational trajectories, but that it has negligible effect on the variance of the particle angular velocity. Previous work has shown that shape significantly affects the variance of the tumbling rate of axisymmetric particles. It follows that shape affects the spinning rate in away that is, on average, complementary to the shape-dependence of the tumbling rate. We confirm this relationship using direct numerical simulations, showing how tumbling rate and spinning rate variances show complementary trends for rod-shaped and disk-shaped particles. We also consider a random but non-turbulent flow. This allows us to explore which of the features observed for rotation in turbulent flow are due to the effects of particle alignment in vortex tubes. (C) 2015 AIP Publishing LLC.
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| 45. |
- Bäbler, Matthäus, et al.
(författare)
-
Hydrodynamic interactions and orthokinetic collisions of porous aggregates in the Stokes regime
- 2006
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 18:1, s. 013302-
-
Tidskriftsartikel (refereegranskat)abstract
- The hydrodynamic interaction of two neutrally buoyant porous aggregates is investigated under creeping flow conditions for the case where the undisturbed velocity of the surrounding flow field is a linear function of position. In this framework, the relative velocity between two aggregates is given by the deformation of the undisturbed flow expressed through the rate of strain and the angular velocity of the flow field, and by two flow-independent hydrodynamic functions, typically referred to as A and B, which account for the disturbance of the flow field due to the presence of the particles [G. K. Batchelor and J. T. Green, J. Fluid Mech. 56, 375 (1972)]. In the present paper, the analysis of thehydrodynamic interaction that is known for the case of two impermeable, solid particles is extended to the case of porous aggregates by applying Brinkman's equation to describe the flow within the aggregates. A reflection scheme is applied to calculate A and B and the obtained expressions are applied to interpret the orthokinetic aggregation ofaggregates in diluted suspensions, where the collision frequency is computed using the method of relative trajectories of a pair of aggregates.
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| 46. |
- Cannon, Ianto, et al.
(författare)
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The effect of droplet coalescence on drag in turbulent channel flows
- 2021
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:8
-
Tidskriftsartikel (refereegranskat)abstract
- We study the effect of droplet coalescence on turbulent wall-bounded flows by means of direct numerical simulations. In particular, the volume-of-fluid and front-tracking methods are used to simulate turbulent channel flows containing coalescing and non-coalescing droplets, respectively. We find that coalescing droplets have a negligible effect on the drag, whereas the non-coalescing ones steadily increase drag as the volume fraction of the dispersed phase increases: indeed, at 10% volume fraction, the non-coalescing droplets show a 30% increase in drag, whereas the coalescing droplets show less than 4% increase. We explain this by looking at the wall-normal location of droplets in the channel and show that non-coalescing droplets enter the viscous sublayer, generating an interfacial shear stress, which reduces the budget for viscous stress in the channel. On the other hand, coalescing droplets migrate toward the bulk of the channel forming large aggregates, which hardly affect the viscous shear stress while damping the Reynolds shear stress. We prove this by relating the mean viscous shear stress integrated in the wall-normal direction to the centerline velocity.
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| 47. |
- Carlson, Andreas, et al.
(författare)
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Modeling of dynamic wetting far from equilibrium
- 2009
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 21:12
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we present simulations of dynamic wetting far from equilibrium based on phase field theory. In direct simulations of recent experiments [J. C. Bird, S. Mandre, and H. A. Stone, Phys. Rev. Lett. 100, 234501 (2008)], we show that in order to correctly capture the dynamics of rapid wetting, it is crucial to account for nonequilibrium at the contact line, where the gas, liquid, and solid meet. A term in the boundary condition at the solid surface that naturally arises in the phase field theory is interpreted as allowing for the establishment of a local structure in the immediate vicinity of the contact line. A direct qualitative and quantitative match with experimental data of spontaneously wetting liquid droplets is shown.
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| 48. |
- Carlsson, Christian, et al.
(författare)
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Swirl switching in turbulent flow through 90 degrees pipe bends
- 2015
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 27:8
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Tidskriftsartikel (refereegranskat)abstract
- Turbulent flow through 90 degrees pipe bends, for four different curvatures, has been investigated using large eddy simulations. In particular, the origin of the so-called swirl switching phenomenon, which is a large scale oscillation of the flow after the bend, has been studied for different bend curvature ratios. A classification of the phenomenon into a high and a low frequency switching, with two distinct physical origins, is proposed. While the high frequency switching stems from modes formed at the bend, and becomes increasingly important for sharp curvatures, the low frequency switching originates from very-large-scale motions created in the upstream pipe flow.
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| 49. |
- Castanet, G., et al.
(författare)
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Transient evolution of the heat transfer and the vapor film thickness at the drop impact in the regime of film boiling
- 2018
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 30:12
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Tidskriftsartikel (refereegranskat)abstract
- When a drop impinges onto a wall heated above the Leidenfrost temperature, a very thin vapor film is formed at the interface between the liquid and the solid substrate. This vapor layer modifies the impact behavior of the drop and induces a significant decrease in heat transfer. A model is proposed for the growth of this vapor layer and its resistance to the heat transfer. The main assumptions are as follows: (i) a uniform but time varying thickness of the vapor film, (ii) a quasi-steady Poiseuille flow inside the vapor film, and (iii) a constant wall temperature. Heat energy and momentum balances are employed to obtain an ordinary differential equation describing the evolution of the vapor film thickness during the drop impact. For droplets injected at a temperature sufficiently lower than the saturation temperature, this equation predicts that the impact velocity has no influence on the thickness of the vapor film. This latter is solely governed by the local heat flux transferred to the liquid, which predominates over the heat flux used for liquid evaporation. An accurate description of the droplet heating is therefore required to complement this model. As an attempt, this description is based upon a one-dimensional analysis, which includes some effects due to the complex fluid flow inside the spreading droplet. Finally, the theoretical model is validated against experiments dealing with millimeter-sized ethanol droplets. Two optical measurement techniques, based on laser-induced fluorescence and infrared thermography, are combined to characterize the heat transfer as well as the thickness of the vapor film.
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| 50. |
- Chakraborty, N., et al.
(författare)
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Effects of Lewis Number on Conditional Fluid Velocity Statistics in Low Damköhler Number Turbulent Premixed Combustion: A Direct Numerical Simulation Analysis
- 2013
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 25:4, s. 045101-
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Tidskriftsartikel (refereegranskat)abstract
- The effects of global Lewis number Le on the statistics of fluid velocity components conditional in unburned reactants and fully burned products in the context of Reynolds Averaged Navier Stokes simulations have been analysed using a Direct Numerical Simulations (DNS) database of statistically planar turbulent premixed flames with a low Damkohler number and Lewis number ranging from 0.34 to 1.2. The conditional velocity statistics extracted from DNS data have been analysed with respect to the well-known Bray-Moss-Libby (BML) expressions which were derived based on bi-modal probability density function of reaction progress variable for high Damkohler number flames. It has been shown that the Lewis number substantially affects the mean velocity and the velocity fluctuation correlation conditional in products, with the effect being particularly pronounced for low Le. As far as the mean velocity and the velocity fluctuation correlation conditional in reactants are concerned, the BML expressions agree reasonably well with the DNS data reported in the present work. Based on a priori analysis of present and previously reported DNS data, the BML expressions have been empirically modified here in order to account for Lewis number effects, and the non-bimodal distribution of reaction progress variable. Moreover, it has been demonstrated for the first time that surface averaged velocity components and Reynolds stresses conditional in unburned reactants can be modelled without invoking expressions involving the Lewis number, as these surface averaged conditional quantities remain approximately equal to their conditionally averaged counterparts in the unburned mixture.
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