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.
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.
Streamwise space time correlations of the longitudinal fluctuating velocity at the beginning of t... more Streamwise space time correlations of the longitudinal fluctuating velocity at the beginning of the buffer layer in an unsteady channel flow are reported. It is shown that the small scale turbulence is sensitive to the imposed unsteadiness since the modulation characteristics of the Taylor lengthscale vary considerably in the imposed frequency range invetigated here. The modulation characteristics of the convection velocity coincide reasonably well with those of the local velocity showing that the Taylor hypothesis may still be used in unsteady flows.
A new method of identification of bursting events in a turbulent boundary layer is presented and ... more A new method of identification of bursting events in a turbulent boundary layer is presented and applied to VITA. The philosophy adapted here is similar to that of Tiederman (1988). It consists of the classification of individual events into groups corresponding to multiple breakups of the same streak (Bogard and Tiederman, 1986). The strategy is however different, in that it uses differences in the signatures of the events belonging to the same cluster. The iterative procedure finds out these differences and classifies events which have similar signatures. It is found that the distribution of the frequency of groups of events identified this way differs from the distribution of the solitary ones. The "multiple shear layer events" (MSLs) have strikingly different signatures compared to "single shear layer events" (SSLs). These differences are mostly present near the wall and disappear progressively at the beginning part of the constant shear layer. The contribution of the MSLs to the Reynolds shear stress is larger. The differences in the conditional averages of the streamwise and wall normal velocities are highly suggestive that the regeneration mechanism of the clusters of events near the wall is dynamically different.
Proceeding of Seventh International Symposium on Turbulence and Shear Flow Phenomena
... Sedat Tardu and Julien Barenzung Laboratoire des Ecoulements Géophysiques et Industriels (LEG... more ... Sedat Tardu and Julien Barenzung Laboratoire des Ecoulements Géophysiques et Industriels (LEGI) BP 53 X 38041 Grenoble, Cédex, France ... The critical Reynolds number under these circumstances is Rec = 150 according to (Alfredsson and Persson, 1989) and the flow is ...
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.
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.
Streamwise space time correlations of the longitudinal fluctuating velocity at the beginning of t... more Streamwise space time correlations of the longitudinal fluctuating velocity at the beginning of the buffer layer in an unsteady channel flow are reported. It is shown that the small scale turbulence is sensitive to the imposed unsteadiness since the modulation characteristics of the Taylor lengthscale vary considerably in the imposed frequency range invetigated here. The modulation characteristics of the convection velocity coincide reasonably well with those of the local velocity showing that the Taylor hypothesis may still be used in unsteady flows.
A new method of identification of bursting events in a turbulent boundary layer is presented and ... more A new method of identification of bursting events in a turbulent boundary layer is presented and applied to VITA. The philosophy adapted here is similar to that of Tiederman (1988). It consists of the classification of individual events into groups corresponding to multiple breakups of the same streak (Bogard and Tiederman, 1986). The strategy is however different, in that it uses differences in the signatures of the events belonging to the same cluster. The iterative procedure finds out these differences and classifies events which have similar signatures. It is found that the distribution of the frequency of groups of events identified this way differs from the distribution of the solitary ones. The "multiple shear layer events" (MSLs) have strikingly different signatures compared to "single shear layer events" (SSLs). These differences are mostly present near the wall and disappear progressively at the beginning part of the constant shear layer. The contribution of the MSLs to the Reynolds shear stress is larger. The differences in the conditional averages of the streamwise and wall normal velocities are highly suggestive that the regeneration mechanism of the clusters of events near the wall is dynamically different.
Proceeding of Seventh International Symposium on Turbulence and Shear Flow Phenomena
... Sedat Tardu and Julien Barenzung Laboratoire des Ecoulements Géophysiques et Industriels (LEG... more ... Sedat Tardu and Julien Barenzung Laboratoire des Ecoulements Géophysiques et Industriels (LEGI) BP 53 X 38041 Grenoble, Cédex, France ... The critical Reynolds number under these circumstances is Rec = 150 according to (Alfredsson and Persson, 1989) and the flow is ...
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