HAL (Le Centre pour la Communication Scientifique Directe), Aug 27, 2007
Des simulations numériques directes d'écoulement turbulent en canal plan soumis à un forçage élec... more Des simulations numériques directes d'écoulement turbulent en canal plan soumis à un forçage électromagnétique transverse oscillant permettent de mieux comprendre les mécanismes de réduction de traînée. L'écoulement pariétal transverse généré par ce forçage rabat contre la paroi les replis des nappes de vorticité. Par ce fait, les
The possibility of generating and maintaining turbulence in an initially laminar channel flow is ... more The possibility of generating and maintaining turbulence in an initially laminar channel flow is investigated for two Reynolds numbers $Re_{b}(=u_{b}h/\unicode[STIX]{x1D708})=880$ and 2100 (based on bulk velocity, $u_{b}$ and half-height of the channel, $h$). The study is carried out through a direct numerical simulation based on the lattice Boltzmann method (LBM). The channel consists of two parallel walls separated by a distance $2h$ where the roughness elements are mounted on both walls. It was observed that when the transverse square bars span half the width of the channel and are mounted in a ‘staggered’ formation, the flow becomes fully turbulent with strong similarities to fully rough wall turbulent channel flows at much higher Reynolds number, as reported in the literature. For example, the rough wall mean velocity profile exhibits a significant downward shift when compared to the smooth wall one. Also, the turbulent kinetic energy budget is similar to its counterpart in rough wall turbulent channel flows at much higher Reynolds numbers than the present ones. It is further shown that the present velocity spectra compared very well with that obtained in a rough wall turbulent boundary layer. Finally, some elements of the possible physical mechanism allowing the generation, growth and sustainability of turbulence are proposed.
The effects of heat transfer into a substrate and axial diffusion are analyzed through numerical ... more The effects of heat transfer into a substrate and axial diffusion are analyzed through numerical simulations in order to elucidate the frequency response of wall hot-film gages. The ratio of the conductivities of the substrate and fluid plays an important role in steady flows when it is larger than 5 and the Péclet number is smaller than 50. The equivalent film length increases considerably with the conductivity ratio, and it decreases when a thin film of low conductivity is sandwiched between the hot-film gage and the substrate, showing a net improvement of the heat transfer conditions. The frequency response in unsteady flows is highly attenuated in the presence of heat transfer into the substrate. The cutoff frequency is strongly dependent on the conductivity ratio. Improved response is obtained with the two-layer substrate configuration. It is further shown that axial diffu-sion considerably affects the frequency response when the shear and/or the streamwise length of the film are small.
ABSTRACT A numerical simulation is undertaken to investigate the transport of momentum and a pass... more ABSTRACT A numerical simulation is undertaken to investigate the transport of momentum and a passive scalar in a localised turbulence in a channel with a contraction. The simulation is carried out using a hybrid method which combines the lattice Boltzmann method (LBM, for the velocity field) and the energy equation (for the temperature field). The localised turbulence is generated through pulsed jets issued in the Poiseuille flow developing in the channel at a Reynolds number of about 1000. The aim of the study is twofold : i) determine effect of the contraction on the localised turbulence, and ii) study how the passive scalar behaves in such contracted localised turbulence. The contraction increase the averaged vorticity in the channel flow, which is accompanied by an increase in the averaged kinetic energy. The contraction also tends to reduce the Reynolds stresses. These results are similar those obtained in turbulent pipe flow with an axisymmetric contraction and in a turbulent boundary layer subjected to a favourable pressure gradient. However, it is found that the heat transport in the normal to the wall direction is more dramatically affected (reduced) than that in the direction of the flow.
ABSTRACT The response of bursting to imposed oscillations in a two-dimensional water channel is s... more ABSTRACT The response of bursting to imposed oscillations in a two-dimensional water channel is studied. It is shown that oscillations imposed upon turbulent channel flow at a frequency such that the Stokes length equals 16 and the center-line amplitude equals 29 percent modulate the bursting process. In particular, the bursting frequency varies with an amplitude of 56 percent during the cycle and in phase with the turbulent intensity even if the detection threshold is modulated with this intensity. The time-mean bursting frequency is, however, the same as in the corresponding steady flow. The structure within the sweep itself is not strongly affected by the oscillations.
Direct numerical simulations of turbulent channel flow subjected to spanwise wall oscillations in... more Direct numerical simulations of turbulent channel flow subjected to spanwise wall oscillations in the form of streamwise travelling waves (STW) were performed in an effort to elucidate the mechanism responsible for the observed drag reduction. We imposed large amplitudes to identify the proper effects of STW, while keeping the angular frequency and wavenumber fixed at a particular values. We primarily focus on the vorticity transport mechanism, to better understand the influence of STW actuation on the near-wall turbulence. We identify key terms appearing in the turbulent enstrophy transport equations that are directly linked to the STW actuation. The analysis reveals that the primary effect of the STW forcing is to attenuate the spanwise turbulent enstrophy at the wall, which is linked to the fluctuating wall shear stress. The suppression of the wall-normal turbulent enstrophy is deemed to be subordinate. To strengthen this point, we performed numerical experiments, where the streamwise fluctuating velocity, and consequently the spanwise vorticity, is artificially suppressed next to the wall. The anisotropic invariant maps show striking resemblance for large amplitude STW actuation and artificially forced cases. Detailed analysis of various structural features is provided, which includes the response of the near-wall streaks and shear layers of spanwise fluctuating velocity field. The quasistreamwise vortices, which play a key role in the regeneration mechanism, are shown to be pushed away from the wall, resulting in their weakened signature at the wall.
Experiments on the modulation characteristics of the wall shear stress s¢-longitudinal velocity u... more Experiments on the modulation characteristics of the wall shear stress s¢-longitudinal velocity u¢ and u¢Àu¢ space-time correlations are reported in a forced turbulent channel flow in a wide range of imposed frequencies. The resulting integral and Taylor scale properties are discussed in detail in the low buffer layer under steady and unsteady flow conditions. It is shown that the small-scale turbulence is sensitive to the imposed unsteadiness since the amplitude and phase of the Taylor length scale vary considerably in the imposed frequency range investigated here. The Taylor hypothesis is acceptably valid in steady and unsteady wall layers just above the low buffer layer. Production and instantaneous pressure gradients are mostly responsible for the deviation of the frozen turbulence-state in the viscous and low buffer sublayers.
The topological features dominating the different sublayers of the near wall turbulence are revis... more The topological features dominating the different sublayers of the near wall turbulence are revisited through direct numerical simulations performed in large computational domains up to the Karman number of 1100. The averages of the invariants of the velocity gradient tensor, conditioned by fixed magnitudes of the velocity components, are adequately performed. In the physical space, the dominating topology in the viscous sublayer is the stable focus stretching cohabiting with the unstable node/saddle/saddle points. The stable and unstable focuses mostly populate the inner layer with equal importance, while the former become predominant in the meso and outer layers. Eddies within the low speed streaks with intense streamwise negative velocity of very high shear stress are located near the zero-discriminant facets. The enstrophy and straining prevails the stable focus stretching and the unstable compressing, respectively. The Eulerian velocity field conditioned by a given topology seen by an observer moving with the local velocity is also considered. It is found that the wall normal velocity marks the stretching line of the unstable saddles and the compressing line of the stable nodes/saddles/saddles. The conditional velocity fields linked to the unstable focuses compressing and stable focuses stretching are closely similar.
International Journal of Heat and Fluid Flow, Dec 1, 2004
Spanwise space-time correlations of the wall shear stress and the longitudinal velocity fluctuati... more Spanwise space-time correlations of the wall shear stress and the longitudinal velocity fluctuations in the low buffer layer of an unsteady channel flow are reported. The imposed amplitude is 20% of the centerline velocity and the imposed frequency covers a large range going from the quasi-steady limit to the bursting frequency of the corresponding steady flow. The unsteady spanwise correlation coefficient is investigated both through its own modulation characteristics (amplitude and phase shifts) and those of the resulting streak spacing. A good correspondence is found between the modulation of the streak spacing and that of the ejection period. The data is further analyzed by temporal filtering of the wall shear stress and streamwise velocity fluctuations. It is shown that the large outer-layer structures play a ''passive'' role in the unsteady response of the near wall turbulence. The inner wall eddies, in return, are amply responsible for the unsteady reaction of both the turbulent wall shear stress and the streamwise velocity intensities in the buffer layer.
International Journal of Heat and Fluid Flow, Oct 1, 2017
The characteristics of the dissipation of the turbulent kinetic energy in wall-bounded flows are ... more The characteristics of the dissipation of the turbulent kinetic energy in wall-bounded flows are revisited through direct numerical simulations of turbulent channel flows realized in large computational domains up to the Reynolds number R e τ = h ū τ ν =1100 where ū τ is the friction velocity, ν and h stands for the kinematic viscosity and channel half width. It is shown that the local homogeneity assumption is acceptable in the whole layer, while the local isotropy is valid only in the far meso-layer and near the centerline. The mean dissipation in inner scale is Reynolds number dependent in the low-buffer and viscous sublayers. This is due to the local shear layers induced by the outer-layer irrotational eddies. A conceptual model is subsequently proposed to link the near wall dissipation to the large-scale structures. The dissipation statistics conditioned by fixed amplitudes of the velocity field are also analyzed in order to clarify whether the zero-crossings of the fluctuating velocity components contribute most to the energy dissipation or not. Incidentally, the occurrence of the Eulerian stagnation points is questioned. The largest contribution to the dissipation in the viscous sublayer comes from the level-crossings of the wall normal velocity v component in the spanwise direction, that peaks to 30% when v = 0 . The spanwise direction is the most active in terms of the level-crossing frequencies that are inversely proportional to the corresponding Taylor scales.
The effect of interacting vortices on the turbulent-like synthetic structures regeneration in a l... more The effect of interacting vortices on the turbulent-like synthetic structures regeneration in a low Reynolds number flow Re = 10 is investigated through direct numerical simulations. The configuration simulates two forced synthetic jets staggered in the lateral spanwise direction. The initial vortices create wall normal vorticity layers near the wall. The stagnation flow induced by the triggering eddy compresses the wall normal vorticity associated with the mother structure that consequently disappears rapidly. That breaks up the spanwise symmetry resulting in a shear layer transforming into a compact streamwise vorticity zone. The regeneration of coherent vortical structures is subsequently triggered resulting rapidly in a local turbulent-like spot. Main characteristics of the perturbed flow field are in close qualitative similarity with a localized turbulent spot that is in principle inconceivable at such a small Reynolds number. The analysis of the passive scalar transport shows that the method leads to efficient mixing at a Prandtl number Pr = 10.
ABSTRACT Spanwise electro-magnetic forcing is used to study turbulence control and drag reduction... more ABSTRACT Spanwise electro-magnetic forcing is used to study turbulence control and drag reduction in a numerical channel flow with a constant mass flow rate and low Reynolds number. The originality of this study comes from the computation of the force field from the geometry of the magnet and the electrode. It is shown that the tilt of the wall-normal component of the vorticity in the spanwise direction characterise the drag reduction caused by alternated spanwise forcing. Key wordsElectromagnetic turbulence control-spanwise oscillation
Recent data obtained in an unsteady turbulent channel flow is reviewed. Results concerning the mo... more Recent data obtained in an unsteady turbulent channel flow is reviewed. Results concerning the modulation characteristics of the Reynolds shear stresses, of the structural parameters and of the length scales inferred from unsteady spatial correlations are discussed. The close examination of both the amplitude and the phase shifts of the Reynolds shear stresses confirms the existence of three distinct inposed frequency regimes, namely the quasi-steady regime, the relaxation regime, in which the amplitudes decrease and which is accompanied by large time lags, and a subsequent third regime wherein the modulation characteristics change considerably. The fine structure of the near-wall turbulence response through quadrant analysis reveals large cyclic variations of the contributions of ejections and sweeps to the Reynolds shear stress. The reaction of the spanwise extent of the near-wall structures is investigated through the spanwise correlation coefficient between the wall shear stress and the streamwise velocity, and the resulting length scales. A temporal filtering of both signals shows that the inactive motions respond uniformly in the whole imposed frequency regime. A strong correlation is found between the modulation characteristics of the streak spacing and the ejection frequency.
Measurements in turbulent channel flow with forced oscillations covering a wide range of frequenc... more Measurements in turbulent channel flow with forced oscillations covering a wide range of frequencies (w' = 0.03-0.0005) and amplitudes (10-70 YO of centreline velocity) are presented and discussed. Phase averages of the velocity ( u ) across the flow, and of the wall shear stress ( T ) , as well as the turbulent fluctuations (u'u') and (7'7') are obtained with LDA and hot-film techniques. The time-mean quantities, except u", are only slightly affected by the imposed oscillations whatever their frequency and amplitude. It is shown that the appropriate similarity parameter for the oscillating quantities 21 and 'i is the non-dimensional Stokes length 1; (or the frequency w' = 2/13. In the regime of high-frequency forcing ( Z : < 10) the oscillating flow zl and .? are governed by purely viscous shear forces although the time-mean flow is fully turbulent. This behaviour may be explained by the physical significance of I, '. At lower frequency ZT > 10, the oscillating flow is influenced by the turbulence, in particular the amplitude of .? increases with respect to the Stokes amplitude and becomes proportional to 1;. The relative amplitude of (u'u') and (7'7') decreases sharply with increasing forcing frequency once IT < 25. This decay of the turbulence response is faster for the wall shear stress. For forcing frequencies such that 1, ' > 12, (u'u') and (7'7') lag behind ( u ) and (T) by respectively about 75 and 130 viscous time units. These lags decrease by a factor 2 at higher forcing frequencies. It is shown that in the log layer, the turbulence modulation diffuses away from the wall with a diffusivity equal to that of the timemean turbulence. The imposed oscillations are felt down to the small scales of the turbulence as may be evidenced from the cyclic modulation of the Taylor microscale, the skewness and the flatness factors of &'/at. The modulations of the skewness and the flatness go through a maximum around 1, ' = 12.
HAL (Le Centre pour la Communication Scientifique Directe), Aug 29, 2005
Un nouveau mécanisme de transition by-pass dans un écoulement de Poiseuille plan est proposé et a... more Un nouveau mécanisme de transition by-pass dans un écoulement de Poiseuille plan est proposé et analysé à travers les simulations numériques directes (SND) de haute résolution temporelle et spatiale. Il est basé sur l'interaction des perturbations localisées près d'une paroi, telles que celles engendrées par une distribution de rugosité. Nous montrons théoriquement qu'une telle distribution de perturbation peut engendrer sous certaines conditions la régénération de la vorticité longitudinale et causer une transition rapide, alors qu'individuellement chaque perturbation résulte en une configuration stable. Les résultats de SND confirment et renforcent cette hypothèse.
HAL (Le Centre pour la Communication Scientifique Directe), Aug 27, 2007
Des simulations numériques directes d'écoulement turbulent en canal plan soumis à un forçage élec... more Des simulations numériques directes d'écoulement turbulent en canal plan soumis à un forçage électromagnétique transverse oscillant permettent de mieux comprendre les mécanismes de réduction de traînée. L'écoulement pariétal transverse généré par ce forçage rabat contre la paroi les replis des nappes de vorticité. Par ce fait, les
The possibility of generating and maintaining turbulence in an initially laminar channel flow is ... more The possibility of generating and maintaining turbulence in an initially laminar channel flow is investigated for two Reynolds numbers $Re_{b}(=u_{b}h/\unicode[STIX]{x1D708})=880$ and 2100 (based on bulk velocity, $u_{b}$ and half-height of the channel, $h$). The study is carried out through a direct numerical simulation based on the lattice Boltzmann method (LBM). The channel consists of two parallel walls separated by a distance $2h$ where the roughness elements are mounted on both walls. It was observed that when the transverse square bars span half the width of the channel and are mounted in a ‘staggered’ formation, the flow becomes fully turbulent with strong similarities to fully rough wall turbulent channel flows at much higher Reynolds number, as reported in the literature. For example, the rough wall mean velocity profile exhibits a significant downward shift when compared to the smooth wall one. Also, the turbulent kinetic energy budget is similar to its counterpart in rough wall turbulent channel flows at much higher Reynolds numbers than the present ones. It is further shown that the present velocity spectra compared very well with that obtained in a rough wall turbulent boundary layer. Finally, some elements of the possible physical mechanism allowing the generation, growth and sustainability of turbulence are proposed.
The effects of heat transfer into a substrate and axial diffusion are analyzed through numerical ... more The effects of heat transfer into a substrate and axial diffusion are analyzed through numerical simulations in order to elucidate the frequency response of wall hot-film gages. The ratio of the conductivities of the substrate and fluid plays an important role in steady flows when it is larger than 5 and the Péclet number is smaller than 50. The equivalent film length increases considerably with the conductivity ratio, and it decreases when a thin film of low conductivity is sandwiched between the hot-film gage and the substrate, showing a net improvement of the heat transfer conditions. The frequency response in unsteady flows is highly attenuated in the presence of heat transfer into the substrate. The cutoff frequency is strongly dependent on the conductivity ratio. Improved response is obtained with the two-layer substrate configuration. It is further shown that axial diffu-sion considerably affects the frequency response when the shear and/or the streamwise length of the film are small.
ABSTRACT A numerical simulation is undertaken to investigate the transport of momentum and a pass... more ABSTRACT A numerical simulation is undertaken to investigate the transport of momentum and a passive scalar in a localised turbulence in a channel with a contraction. The simulation is carried out using a hybrid method which combines the lattice Boltzmann method (LBM, for the velocity field) and the energy equation (for the temperature field). The localised turbulence is generated through pulsed jets issued in the Poiseuille flow developing in the channel at a Reynolds number of about 1000. The aim of the study is twofold : i) determine effect of the contraction on the localised turbulence, and ii) study how the passive scalar behaves in such contracted localised turbulence. The contraction increase the averaged vorticity in the channel flow, which is accompanied by an increase in the averaged kinetic energy. The contraction also tends to reduce the Reynolds stresses. These results are similar those obtained in turbulent pipe flow with an axisymmetric contraction and in a turbulent boundary layer subjected to a favourable pressure gradient. However, it is found that the heat transport in the normal to the wall direction is more dramatically affected (reduced) than that in the direction of the flow.
ABSTRACT The response of bursting to imposed oscillations in a two-dimensional water channel is s... more ABSTRACT The response of bursting to imposed oscillations in a two-dimensional water channel is studied. It is shown that oscillations imposed upon turbulent channel flow at a frequency such that the Stokes length equals 16 and the center-line amplitude equals 29 percent modulate the bursting process. In particular, the bursting frequency varies with an amplitude of 56 percent during the cycle and in phase with the turbulent intensity even if the detection threshold is modulated with this intensity. The time-mean bursting frequency is, however, the same as in the corresponding steady flow. The structure within the sweep itself is not strongly affected by the oscillations.
Direct numerical simulations of turbulent channel flow subjected to spanwise wall oscillations in... more Direct numerical simulations of turbulent channel flow subjected to spanwise wall oscillations in the form of streamwise travelling waves (STW) were performed in an effort to elucidate the mechanism responsible for the observed drag reduction. We imposed large amplitudes to identify the proper effects of STW, while keeping the angular frequency and wavenumber fixed at a particular values. We primarily focus on the vorticity transport mechanism, to better understand the influence of STW actuation on the near-wall turbulence. We identify key terms appearing in the turbulent enstrophy transport equations that are directly linked to the STW actuation. The analysis reveals that the primary effect of the STW forcing is to attenuate the spanwise turbulent enstrophy at the wall, which is linked to the fluctuating wall shear stress. The suppression of the wall-normal turbulent enstrophy is deemed to be subordinate. To strengthen this point, we performed numerical experiments, where the streamwise fluctuating velocity, and consequently the spanwise vorticity, is artificially suppressed next to the wall. The anisotropic invariant maps show striking resemblance for large amplitude STW actuation and artificially forced cases. Detailed analysis of various structural features is provided, which includes the response of the near-wall streaks and shear layers of spanwise fluctuating velocity field. The quasistreamwise vortices, which play a key role in the regeneration mechanism, are shown to be pushed away from the wall, resulting in their weakened signature at the wall.
Experiments on the modulation characteristics of the wall shear stress s¢-longitudinal velocity u... more Experiments on the modulation characteristics of the wall shear stress s¢-longitudinal velocity u¢ and u¢Àu¢ space-time correlations are reported in a forced turbulent channel flow in a wide range of imposed frequencies. The resulting integral and Taylor scale properties are discussed in detail in the low buffer layer under steady and unsteady flow conditions. It is shown that the small-scale turbulence is sensitive to the imposed unsteadiness since the amplitude and phase of the Taylor length scale vary considerably in the imposed frequency range investigated here. The Taylor hypothesis is acceptably valid in steady and unsteady wall layers just above the low buffer layer. Production and instantaneous pressure gradients are mostly responsible for the deviation of the frozen turbulence-state in the viscous and low buffer sublayers.
The topological features dominating the different sublayers of the near wall turbulence are revis... more The topological features dominating the different sublayers of the near wall turbulence are revisited through direct numerical simulations performed in large computational domains up to the Karman number of 1100. The averages of the invariants of the velocity gradient tensor, conditioned by fixed magnitudes of the velocity components, are adequately performed. In the physical space, the dominating topology in the viscous sublayer is the stable focus stretching cohabiting with the unstable node/saddle/saddle points. The stable and unstable focuses mostly populate the inner layer with equal importance, while the former become predominant in the meso and outer layers. Eddies within the low speed streaks with intense streamwise negative velocity of very high shear stress are located near the zero-discriminant facets. The enstrophy and straining prevails the stable focus stretching and the unstable compressing, respectively. The Eulerian velocity field conditioned by a given topology seen by an observer moving with the local velocity is also considered. It is found that the wall normal velocity marks the stretching line of the unstable saddles and the compressing line of the stable nodes/saddles/saddles. The conditional velocity fields linked to the unstable focuses compressing and stable focuses stretching are closely similar.
International Journal of Heat and Fluid Flow, Dec 1, 2004
Spanwise space-time correlations of the wall shear stress and the longitudinal velocity fluctuati... more Spanwise space-time correlations of the wall shear stress and the longitudinal velocity fluctuations in the low buffer layer of an unsteady channel flow are reported. The imposed amplitude is 20% of the centerline velocity and the imposed frequency covers a large range going from the quasi-steady limit to the bursting frequency of the corresponding steady flow. The unsteady spanwise correlation coefficient is investigated both through its own modulation characteristics (amplitude and phase shifts) and those of the resulting streak spacing. A good correspondence is found between the modulation of the streak spacing and that of the ejection period. The data is further analyzed by temporal filtering of the wall shear stress and streamwise velocity fluctuations. It is shown that the large outer-layer structures play a ''passive'' role in the unsteady response of the near wall turbulence. The inner wall eddies, in return, are amply responsible for the unsteady reaction of both the turbulent wall shear stress and the streamwise velocity intensities in the buffer layer.
International Journal of Heat and Fluid Flow, Oct 1, 2017
The characteristics of the dissipation of the turbulent kinetic energy in wall-bounded flows are ... more The characteristics of the dissipation of the turbulent kinetic energy in wall-bounded flows are revisited through direct numerical simulations of turbulent channel flows realized in large computational domains up to the Reynolds number R e τ = h ū τ ν =1100 where ū τ is the friction velocity, ν and h stands for the kinematic viscosity and channel half width. It is shown that the local homogeneity assumption is acceptable in the whole layer, while the local isotropy is valid only in the far meso-layer and near the centerline. The mean dissipation in inner scale is Reynolds number dependent in the low-buffer and viscous sublayers. This is due to the local shear layers induced by the outer-layer irrotational eddies. A conceptual model is subsequently proposed to link the near wall dissipation to the large-scale structures. The dissipation statistics conditioned by fixed amplitudes of the velocity field are also analyzed in order to clarify whether the zero-crossings of the fluctuating velocity components contribute most to the energy dissipation or not. Incidentally, the occurrence of the Eulerian stagnation points is questioned. The largest contribution to the dissipation in the viscous sublayer comes from the level-crossings of the wall normal velocity v component in the spanwise direction, that peaks to 30% when v = 0 . The spanwise direction is the most active in terms of the level-crossing frequencies that are inversely proportional to the corresponding Taylor scales.
The effect of interacting vortices on the turbulent-like synthetic structures regeneration in a l... more The effect of interacting vortices on the turbulent-like synthetic structures regeneration in a low Reynolds number flow Re = 10 is investigated through direct numerical simulations. The configuration simulates two forced synthetic jets staggered in the lateral spanwise direction. The initial vortices create wall normal vorticity layers near the wall. The stagnation flow induced by the triggering eddy compresses the wall normal vorticity associated with the mother structure that consequently disappears rapidly. That breaks up the spanwise symmetry resulting in a shear layer transforming into a compact streamwise vorticity zone. The regeneration of coherent vortical structures is subsequently triggered resulting rapidly in a local turbulent-like spot. Main characteristics of the perturbed flow field are in close qualitative similarity with a localized turbulent spot that is in principle inconceivable at such a small Reynolds number. The analysis of the passive scalar transport shows that the method leads to efficient mixing at a Prandtl number Pr = 10.
ABSTRACT Spanwise electro-magnetic forcing is used to study turbulence control and drag reduction... more ABSTRACT Spanwise electro-magnetic forcing is used to study turbulence control and drag reduction in a numerical channel flow with a constant mass flow rate and low Reynolds number. The originality of this study comes from the computation of the force field from the geometry of the magnet and the electrode. It is shown that the tilt of the wall-normal component of the vorticity in the spanwise direction characterise the drag reduction caused by alternated spanwise forcing. Key wordsElectromagnetic turbulence control-spanwise oscillation
Recent data obtained in an unsteady turbulent channel flow is reviewed. Results concerning the mo... more Recent data obtained in an unsteady turbulent channel flow is reviewed. Results concerning the modulation characteristics of the Reynolds shear stresses, of the structural parameters and of the length scales inferred from unsteady spatial correlations are discussed. The close examination of both the amplitude and the phase shifts of the Reynolds shear stresses confirms the existence of three distinct inposed frequency regimes, namely the quasi-steady regime, the relaxation regime, in which the amplitudes decrease and which is accompanied by large time lags, and a subsequent third regime wherein the modulation characteristics change considerably. The fine structure of the near-wall turbulence response through quadrant analysis reveals large cyclic variations of the contributions of ejections and sweeps to the Reynolds shear stress. The reaction of the spanwise extent of the near-wall structures is investigated through the spanwise correlation coefficient between the wall shear stress and the streamwise velocity, and the resulting length scales. A temporal filtering of both signals shows that the inactive motions respond uniformly in the whole imposed frequency regime. A strong correlation is found between the modulation characteristics of the streak spacing and the ejection frequency.
Measurements in turbulent channel flow with forced oscillations covering a wide range of frequenc... more Measurements in turbulent channel flow with forced oscillations covering a wide range of frequencies (w' = 0.03-0.0005) and amplitudes (10-70 YO of centreline velocity) are presented and discussed. Phase averages of the velocity ( u ) across the flow, and of the wall shear stress ( T ) , as well as the turbulent fluctuations (u'u') and (7'7') are obtained with LDA and hot-film techniques. The time-mean quantities, except u", are only slightly affected by the imposed oscillations whatever their frequency and amplitude. It is shown that the appropriate similarity parameter for the oscillating quantities 21 and 'i is the non-dimensional Stokes length 1; (or the frequency w' = 2/13. In the regime of high-frequency forcing ( Z : < 10) the oscillating flow zl and .? are governed by purely viscous shear forces although the time-mean flow is fully turbulent. This behaviour may be explained by the physical significance of I, '. At lower frequency ZT > 10, the oscillating flow is influenced by the turbulence, in particular the amplitude of .? increases with respect to the Stokes amplitude and becomes proportional to 1;. The relative amplitude of (u'u') and (7'7') decreases sharply with increasing forcing frequency once IT < 25. This decay of the turbulence response is faster for the wall shear stress. For forcing frequencies such that 1, ' > 12, (u'u') and (7'7') lag behind ( u ) and (T) by respectively about 75 and 130 viscous time units. These lags decrease by a factor 2 at higher forcing frequencies. It is shown that in the log layer, the turbulence modulation diffuses away from the wall with a diffusivity equal to that of the timemean turbulence. The imposed oscillations are felt down to the small scales of the turbulence as may be evidenced from the cyclic modulation of the Taylor microscale, the skewness and the flatness factors of &'/at. The modulations of the skewness and the flatness go through a maximum around 1, ' = 12.
HAL (Le Centre pour la Communication Scientifique Directe), Aug 29, 2005
Un nouveau mécanisme de transition by-pass dans un écoulement de Poiseuille plan est proposé et a... more Un nouveau mécanisme de transition by-pass dans un écoulement de Poiseuille plan est proposé et analysé à travers les simulations numériques directes (SND) de haute résolution temporelle et spatiale. Il est basé sur l'interaction des perturbations localisées près d'une paroi, telles que celles engendrées par une distribution de rugosité. Nous montrons théoriquement qu'une telle distribution de perturbation peut engendrer sous certaines conditions la régénération de la vorticité longitudinale et causer une transition rapide, alors qu'individuellement chaque perturbation résulte en une configuration stable. Les résultats de SND confirment et renforcent cette hypothèse.
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