The Younger Dryas (YD) cold reversal interrupts the warming climate of the deglaciation with glob... more The Younger Dryas (YD) cold reversal interrupts the warming climate of the deglaciation with global climatic impacts. The sudden cooling is typically linked to an abrupt slowdown of the Atlantic Meridional Overturning Circulation (AMOC) in response to meltwater discharges from ice sheets. However, inconsistencies regarding the YD-response of European summer temperatures have cast doubt whether the concept provides a sufficient explanation. Here we present results from a high-resolution global climate simulation together with a new July temperature compilation based on plant indicator species and show that European summers remain warm during the YD. Our climate simulation provides robust physical evidence that atmospheric blocking of cold westerly winds over Fennoscandia is a key mechanism counteracting the cooling impact of an AMOC-slowdown during summer. Despite the persistence of short warm summers, the YD is dominated by a shift to a continental climate with extreme winter to spr...
ABSTRACT The near-wall region of turbulent pipe and channel flows (as well as zero-pressure gradi... more ABSTRACT The near-wall region of turbulent pipe and channel flows (as well as zero-pressure gradient boundary layers) have been shown to exhibit a very high degree of similarity in terms of all statistical moments and many other features, while even the mean velocity profile in the two cases exhibits significant differences between in the outer region. The wake part of the profile, i.e. the deviation from the log-law, in the outer region is of substantially larger amplitude in pipe flow as compared to channel flow (although weaker than in boundary layer flow). This intriguing feature has been well known but has no simple explanation. Model predictions typically give identical results for the two flows. We have analyzed a new set of DNS for pipe and channel flows for friction Reynolds numbers up to 1 000 and made comparing calculations with differential Reynolds stress models (DRSM). We have strong indications that the key factor behind the difference in mean velocity in the outer region can be coupled to differences in the turbulent diffusion in this region. This is also supported by DRSM results, where interesting differences are seen depending on the sophistication of modeling the turbulent diffusion coefficient.
ABSTRACT Fully resolved direct numerical simulations (DNSs) have been performed with a high-order... more ABSTRACT Fully resolved direct numerical simulations (DNSs) have been performed with a high-order spectral element method to study the flow of an incompressible viscous fluid in a smooth circular pipe of radius R and axial length 25R in the turbulent flow regime at four different friction Reynolds numbers Re_{\tau} = 180, 360, 550 and 1,000. The new set of data is put into perspective with other simulation data sets, obtained in pipe, channel and boundary layer geometry. In particular, differences between different pipe DNS are highlighted. It turns out that the pressure is the variable which differs the most between pipes, channels and boundary layers, leading to significantly different mean and pressure fluctuations, potentially linked to a stronger wake region. In the buffer layer, the variation with Reynolds number of the inner peak of axial velocity fluctuation intensity is similar between channel and boundary layer flows, but lower for the pipe, while the inner peak of the pressure fluctuations show negligible differences between pipe and channel flows but is clearly lower than that for the boundary layer, which is the same behaviour as for the fluctuating wall shear stress. Finally, turbulent kinetic energy budgets are almost indistinguishable between the canonical flows close to the wall (up to y+ ≈ 100), while substantial differences are observed in production and dissipation in the outer layer. A clear Reynolds number dependency is documented for the three flow configurations.
Near-wall flow structures in turbulent shear flows are analysed, with particular emphasis on the ... more Near-wall flow structures in turbulent shear flows are analysed, with particular emphasis on the study of their space–time evolution and connection to turbulence production. The results are obtained from investigation of a database generated from direct numerical simulation of turbulent channel flow at a Reynolds number of 180 based on half-channel width and friction velocity. New light is shed on problems associated with conditional sampling techniques, together with methods to improve these techniques, for use both in physical and numerical experiments. The results clearly indicate that earlier conceptual models of the processes associated with near-wall turbulence production, based on flow visualization and probe measurements need to be modified. For instance, the development of asymmetry in the spanwise direction seems to be an important element in the evolution of near-wall structures in general, and for shear layers in particular. The inhibition of spanwise motion of the near-wall streaky pattern may be the primary reason for the ability of small longitudinal riblets to reduce turbulent skin friction below the value for a flat surface.
The probability density functions (PDFs) of the streamwise mean velocity in high Reynolds number ... more The probability density functions (PDFs) of the streamwise mean velocity in high Reynolds number turbulent boundary layers have been studied. The hypothesis of self-similar, normalized with the root mean square velocity, PDFs has been tested using the KTH database of high Reynolds number zero pressure-gradient turbulent boundary layer flow. The self-similarity was tested using the Kullback-Leibler divergence measure and it was found that the region of self-similar PDFs extends from about 160 viscous wall units to about 0.3 boundary layer thicknesses (in delta95). This region is somewhat larger than the universal overlap region. The PDF shape in the universal overlap region is close to Gaussian allowing for a Gram-Charlier expansion approximation of the measured PDFs. A remarkable collapse was found for 57 normalized PDF distributions for different positions within the universal overlap region and Reynolds numbers based on the momentum-loss thickness between 4300 and 12 600, strongly indicating a high degree of flow universality within the universal overlap region. Within the range studied, the Gram-Charlier expansion coefficients (related to the PDF moments) show no Reynolds number trend further supporting the self-similarity hypothesis.
The Younger Dryas (YD) cold reversal interrupts the warming climate of the deglaciation with glob... more The Younger Dryas (YD) cold reversal interrupts the warming climate of the deglaciation with global climatic impacts. The sudden cooling is typically linked to an abrupt slowdown of the Atlantic Meridional Overturning Circulation (AMOC) in response to meltwater discharges from ice sheets. However, inconsistencies regarding the YD-response of European summer temperatures have cast doubt whether the concept provides a sufficient explanation. Here we present results from a high-resolution global climate simulation together with a new July temperature compilation based on plant indicator species and show that European summers remain warm during the YD. Our climate simulation provides robust physical evidence that atmospheric blocking of cold westerly winds over Fennoscandia is a key mechanism counteracting the cooling impact of an AMOC-slowdown during summer. Despite the persistence of short warm summers, the YD is dominated by a shift to a continental climate with extreme winter to spr...
ABSTRACT The near-wall region of turbulent pipe and channel flows (as well as zero-pressure gradi... more ABSTRACT The near-wall region of turbulent pipe and channel flows (as well as zero-pressure gradient boundary layers) have been shown to exhibit a very high degree of similarity in terms of all statistical moments and many other features, while even the mean velocity profile in the two cases exhibits significant differences between in the outer region. The wake part of the profile, i.e. the deviation from the log-law, in the outer region is of substantially larger amplitude in pipe flow as compared to channel flow (although weaker than in boundary layer flow). This intriguing feature has been well known but has no simple explanation. Model predictions typically give identical results for the two flows. We have analyzed a new set of DNS for pipe and channel flows for friction Reynolds numbers up to 1 000 and made comparing calculations with differential Reynolds stress models (DRSM). We have strong indications that the key factor behind the difference in mean velocity in the outer region can be coupled to differences in the turbulent diffusion in this region. This is also supported by DRSM results, where interesting differences are seen depending on the sophistication of modeling the turbulent diffusion coefficient.
ABSTRACT Fully resolved direct numerical simulations (DNSs) have been performed with a high-order... more ABSTRACT Fully resolved direct numerical simulations (DNSs) have been performed with a high-order spectral element method to study the flow of an incompressible viscous fluid in a smooth circular pipe of radius R and axial length 25R in the turbulent flow regime at four different friction Reynolds numbers Re_{\tau} = 180, 360, 550 and 1,000. The new set of data is put into perspective with other simulation data sets, obtained in pipe, channel and boundary layer geometry. In particular, differences between different pipe DNS are highlighted. It turns out that the pressure is the variable which differs the most between pipes, channels and boundary layers, leading to significantly different mean and pressure fluctuations, potentially linked to a stronger wake region. In the buffer layer, the variation with Reynolds number of the inner peak of axial velocity fluctuation intensity is similar between channel and boundary layer flows, but lower for the pipe, while the inner peak of the pressure fluctuations show negligible differences between pipe and channel flows but is clearly lower than that for the boundary layer, which is the same behaviour as for the fluctuating wall shear stress. Finally, turbulent kinetic energy budgets are almost indistinguishable between the canonical flows close to the wall (up to y+ ≈ 100), while substantial differences are observed in production and dissipation in the outer layer. A clear Reynolds number dependency is documented for the three flow configurations.
Near-wall flow structures in turbulent shear flows are analysed, with particular emphasis on the ... more Near-wall flow structures in turbulent shear flows are analysed, with particular emphasis on the study of their space–time evolution and connection to turbulence production. The results are obtained from investigation of a database generated from direct numerical simulation of turbulent channel flow at a Reynolds number of 180 based on half-channel width and friction velocity. New light is shed on problems associated with conditional sampling techniques, together with methods to improve these techniques, for use both in physical and numerical experiments. The results clearly indicate that earlier conceptual models of the processes associated with near-wall turbulence production, based on flow visualization and probe measurements need to be modified. For instance, the development of asymmetry in the spanwise direction seems to be an important element in the evolution of near-wall structures in general, and for shear layers in particular. The inhibition of spanwise motion of the near-wall streaky pattern may be the primary reason for the ability of small longitudinal riblets to reduce turbulent skin friction below the value for a flat surface.
The probability density functions (PDFs) of the streamwise mean velocity in high Reynolds number ... more The probability density functions (PDFs) of the streamwise mean velocity in high Reynolds number turbulent boundary layers have been studied. The hypothesis of self-similar, normalized with the root mean square velocity, PDFs has been tested using the KTH database of high Reynolds number zero pressure-gradient turbulent boundary layer flow. The self-similarity was tested using the Kullback-Leibler divergence measure and it was found that the region of self-similar PDFs extends from about 160 viscous wall units to about 0.3 boundary layer thicknesses (in delta95). This region is somewhat larger than the universal overlap region. The PDF shape in the universal overlap region is close to Gaussian allowing for a Gram-Charlier expansion approximation of the measured PDFs. A remarkable collapse was found for 57 normalized PDF distributions for different positions within the universal overlap region and Reynolds numbers based on the momentum-loss thickness between 4300 and 12 600, strongly indicating a high degree of flow universality within the universal overlap region. Within the range studied, the Gram-Charlier expansion coefficients (related to the PDF moments) show no Reynolds number trend further supporting the self-similarity hypothesis.
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