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Wolfgang Schroder - One of the best experts on this subject based on the ideXlab platform.
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Measurements of the Wall-Shear Stress distribution in turbulent channel flow using the micro-pillar Shear Stress sensor MPS3
Experimental Thermal and Fluid Science, 2019Co-Authors: Michael Klaas, Wolfgang SchroderAbstract:Abstract This study investigates the Wall-Shear Stress (WSS) distribution in turbulent channel flow at friction Reynolds numbers of Re τ = 860 and 1300 using the micro-pillar Shear-Stress sensor (MPS3). The probability density functions and the joint probability density functions of the Wall-Shear Stress vectors indicate that the intermittency of the flow increases for higher Reynolds numbers. Extreme events occurring at the Wall, such as Wall-normal velocity spikes and backflow events, are detected and analyzed based on the Wall-Shear Stress patterns. The events representing the Wall-normal spikes are conditionally sampled from the Wall-normal velocity component, which is determined from the local Wall-Shear Stress gradients. The results show that the negative velocity spikes tend to co-occur with strong streamwise Wall-Shear Stress motions, and that the positive spikes are likely to accompany large spanwise motions. Rare backflow events are detected from the Wall-Shear Stress distribution for both Reynolds numbers, serving as an experimental evidence of near-Wall flow reversal events in turbulent channel flow. The diameter of the detected backflow region is approximately 20 viscous units. The data confirm the results from previous numerical studies. The Wall-normal velocity spikes and the backflow events are likely to be three-dimensional, suggesting an energy and momentum transfer between the viscous sublayer and the outer part of the turbulent channel flow.
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Wall Shear Stress measurements in an adverse pressure gradient turbulent boundary layer
AIAA Journal, 2014Co-Authors: Bernardo Nottebrock, Koen Geurts, Wolfgang SchroderAbstract:An adverse pressure gradient turbulent boundary layer has been established to show that the micro-pillar Shear-Stress sensor can be used in non-constant free-stream flows to measure the Wall-Shear Stress distribution. The quality of the reference has been ensured by additional measurements such as pressure and PIV measurements, and large-eddy simulations. The momentum based Reynolds number is Reθ = 4420 and the Clauser parameter characterizing the pressure gradient is β = 1.77. An introductory discussion on the sensitivity of the micro-pillar Shear-Stress sensor reveals only little effect of a pressure gradient on the Wall-Shear Stress results since the dependence of the drag force on the velocity distribution is damped. Hence, the sensor is only little sensitive to discrepancies from the linear law of the Wall due to a positive pressure gradient. Measurements of Wall-Shear Stress in an APG TBL using MPS are presented. The behavior and accuracy of the sensor is analyzed using the statistical moments. They coincide with the results of the numerical simulation. Furthermore, they reveal an effect of the pressure gradient on the Wall-Shear Stress. Turbulence intensity, skewness and kurtosis are smaller compared to the zero-pressure gradient turbulent boundary layer. This confirms the decrease in number and strength of the near-Wall high-speed streaks in the APG TBL which is in good agreement with the findings of Harun et al.
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Wall Shear Stress patterns of coherent structures in turbulent duct flow
Journal of Fluid Mechanics, 2009Co-Authors: Sebastian Grosse, Wolfgang SchroderAbstract:The Wall-Shear Stress distribution in turbulent duct flow has been assessed using the micro-pillar Shear-Stress sensor MPS 3 . The spatial resolution of the sensor line is 10.8 l + (viscous units) and the total field of view of 120 1 + along the spanwise direction allows to capture characteristic dimensions of the Wall-Shear Stress distribution at sufficiently high resolution. The results show the coexistence of low-Shear and high-Shear regions representing 'footprints' of near-Wall coherent structures. The regions of low Shear resemble long meandering bands locally interrupted by areas of higher Shear Stress. Conditional averages of the flow field indicate the existence of nearly streamwise counter-rotating vortices aligned in the streamwise direction. The results further show periods of very strong spanwise Wall-Shear Stress to be related to the occurrence of high streamwise Shear regions and momentum transfer towards the Wall. These events go along with a spanwise oscillation and a meandering of the low-Shear regions.
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high reynolds number turbulent wind tunnel boundary layer Wall Shear Stress sensor
Journal of Turbulence, 2009Co-Authors: Sebastian Grose, Wolfgang SchroderAbstract:The fluctuating Wall-Shear Stress in a turbulent zero-pressure gradient wind tunnel boundary layer at high Reynolds numbers up to Re Θ = 11,400 is determined by the micro-pillar Shear-Stress sensor MPS3. The sensor concept has already successfully been applied to liquid fluid flows, where the sensor shows low-pass filter characteristics. The low-pass filter characteristic is favorable especially when turbulent frequencies larger than the damped eigenfrequency of the structure occur. The application in air, however, is critical since the dynamic transfer function of the sensor structures exhibits a strong resonance due to low damping. The current results demonstrate that as long as frequencies of turbulent Wall-Shear Stress fluctuations are below the damped eigenfrequency even a resonant sensor structure can be used to accurately assess the fluctuating Wall-Shear Stress, e.g., in turbulent wind tunnel boundary layers.
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dynamic Wall Shear Stress measurements in turbulent pipe flow using the micro pillar sensor mps3
International Journal of Heat and Fluid Flow, 2008Co-Authors: Sebastian Grose, Wolfgang SchroderAbstract:Abstract The micro-pillar Wall-Shear Stress sensor MPS3 has been used to measure the dynamic Wall-Shear Stress in turbulent pipe flow. The sensor device consists of a flexible micro-pillar which extends from the Wall into the viscous sublayer. The pillar-tip deflection caused by the exerting fluid forces serves as a measure for the local Wall-Shear Stress. The pillar is statically calibrated in linear Shear flow. A second-order estimate of the pillar dynamic response based on experimentally determined sensor characteristics shows the potential of the present sensor configuration to also measure the dynamic Wall-Shear Stress. The quality of the micro-pillar Shear Stress sensor MPS3 to correctly determine the skin friction will be shown by measuring the Wall friction in a well-defined fully developed turbulent pipe flow at Reynolds numbers Re b based on the bulk velocity U b and the pipe diameter D in the range of Re b = 10 , 000 – 20 , 000 . The results demonstrate a convincing agreement of the mean and dynamic Wall-Shear Stress obtained with the MPS3 sensor technique with analytical, experimental, and numerical results from the literature.
Arturo Pujia - One of the best experts on this subject based on the ideXlab platform.
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in vivo association between low Wall Shear Stress and plaque in subjects with asymmetrical carotid atherosclerosis
Stroke, 1997Co-Authors: Agostino Gnasso, Claudio Carallo, Concetta Irace, P L Mattioli, M S De Franceschi, C Motti, Arturo PujiaAbstract:Background and Purpose It is known that atherosclerosis does not involve both carotid arteries to the same extent. Pathological investigations have demonstrated that lesions develop in regions of low Wall Shear Stress. The aims of the present study were to verify the degree of carotid atherosclerosis asymmetry in a population-based study and to evaluate whether Wall Shear Stress is lower in carotids with atherosclerotic lesions than in carotids without lesions. Methods Participants in a cardiovascular disease prevention campaign (n=1166) were screened for carotid atherosclerosis by echo-Doppler examination. Of these, 23 subjects who presented plaque in the common carotid or bulb of one side and no plaque in the contralateral carotid tree were enrolled for common carotid Wall Shear Stress measurement. Shear Stress was calculated according to the following formula: Shear Stress=Blood Viscosity×Blood Velocity/Internal Diameter. Results Of the 1166 subjects screened, 400 (34%) had plaque and/or stenosis in the carotids. Ninety subjects had lesions exclusively in the right carotid, 111 had lesions exclusively in the left, 70 had lesions in both carotids but with different degrees of severity, and only 129 had similar lesions in both carotids. In the 23 subjects in whom Wall Shear Stress was measured, peak Shear Stress was 18.7±4.1 and 15.3±4.0 dynes·cm−2 (mean±SD) ( P <.0001) in the side without and the side with plaque, respectively. Mean Shear Stress yielded similar results. Conclusions The present results demonstrate that the atherosclerotic involvement of carotid arteries is usually asymmetrical and that Wall Shear Stress is lower in the carotid arteries where plaques are present than in plaque-free arteries. These findings provide in vivo evidence for a strong association between Shear Stress and atherosclerotic lesions.
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association between intima media thickness and Wall Shear Stress in common carotid arteries in healthy male subjects
Circulation, 1996Co-Authors: Agostino Gnasso, Claudio Carallo, Concetta Irace, Vitaliano Spagnuolo, Giuseppina De Novara, P L Mattioli, Arturo PujiaAbstract:Background Atherosclerotic lesions lie in regions of low Wall Shear Stress. No relationship between Wall Shear Stress and intima-media thickness in vivo has been reported. Aims of the present study were to verify the reproducibility of Wall Shear Stress measurement in vivo and to evaluate its association with intima-media thickness in the common carotid artery in healthy subjects. Methods and Results Wall Shear Stress was calculated according to the following formula: Shear Stress=Blood Viscosity×Blood Velocity/Internal Diameter. Blood viscosity was measured by use of a cone/plate viscometer. Blood velocity, internal diameter, and intima-media thickness were measured by high-resolution echo Doppler. Twenty-one healthy male subjects were investigated. Peak and mean Shear Stress values were 29.5±8.2 and 12.1±3.1 dynes/cm−2 (mean±SD), respectively. Peak Shear Stress was inversely related to intima-media thickness (r=.62), age (r=.77), systolic blood pressure (r=.61), and body mass index (r=.59) (P<.0001 for ...
N Hutchins - One of the best experts on this subject based on the ideXlab platform.
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recovery of Wall Shear Stress to equilibrium flow conditions after a rough to smooth step change in turbulent boundary layers
Journal of Fluid Mechanics, 2019Co-Authors: Charitha M De Silva, Ivan Marusic, Daniel Chung, Amirreza Rouhi, R Baidya, N HutchinsAbstract:This paper examines the recovery of the Wall-Shear Stress of a turbulent boundary layer that has undergone a sudden transition from a rough to a smooth surface. Early work of Antonia & Luxton (J. Fluid Mech., vol. 53, 1972, pp. 737–757) questioned the reliability of standard smooth-Wall methods for measuring Wall-Shear Stress in such conditions, and subsequent studies show significant disagreement depending on the approach used to determine the Wall-Shear Stress downstream. Here we address this by utilising a collection of experimental databases at Re≈4100 that have access to both ‘direct’ and ‘indirect’ measures of the Wall-Shear Stress to understand the recovery to equilibrium conditions of the new surface. Our results reveal that the viscous region ( z+≲4 ) recovers almost immediately to an equilibrium state with the new Wall conditions; however, the buffer region and beyond takes several boundary layer thicknesses before recovering to equilibrium conditions, which is longer than previously thought. A unique direct numerical simulation database of a Wall-bounded flow with a rough-to-smooth Wall transition is employed to confirm these findings. In doing so, we present evidence that any estimate of the Wall-Shear Stress from the mean velocity profile in the buffer region or further away from the Wall tends to underestimate its magnitude in the near vicinity of the rough-to-smooth transition, and this is likely to be partly responsible for the large scatter of recovery lengths to equilibrium conditions reported in the literature. Our results also reveal that smaller energetic scales in the near-Wall region recover to an equilibrium state associated with the new Wall conditions within one boundary layer thickness downstream of the transition, while larger energetic scales exhibit an over-energised state for several boundary layer thicknesses downstream of the transition. Based on these observations, an alternative approach to estimating the Wall-Shear Stress from the premultiplied energy spectrum is proposed.
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similarity and structure of Wall turbulence with lateral Wall Shear Stress variations
Journal of Fluid Mechanics, 2018Co-Authors: Daniel Chung, J P Monty, N HutchinsAbstract:Wall-bounded turbulence, where it occurs in engineering or nature, is commonly subjected to spatial variations in Wall Shear Stress. A prime example is spatially varying roughness. Here, we investigate the configuration where the Wall Shear Stress varies only in the lateral direction. The investigation is idealised in order to focus on one aspect, namely, the similarity and structure of turbulent inertial motion over an imposed scale of Stress variation. To this end, we analyse data from direct numerical simulation (DNS) of pressure-driven turbulent flow through a channel bounded by Walls of laterally alternating patches of high and low Wall Shear Stress. The Wall Shear Stress is imposed as a Neumann boundary condition such that the Wall Shear Stress ratio is fixed at 3 while the lateral spacing $s$ of the uniform-Stress patches is varied from 0.39 to 6.28 of the half-channel height $\unicode[STIX]{x1D6FF}$ . We find that global outer-layer similarity is maintained when $s$ is less than approximately $0.39\unicode[STIX]{x1D6FF}$ while local outer-layer similarity is recovered when $s$ is greater than approximately $6.28\unicode[STIX]{x1D6FF}$ . However, the transition between the two regimes through $s\approx \unicode[STIX]{x1D6FF}$ is not monotonic owing to the presence of secondary roll motions that extend across the whole cross-section of the flow. Importantly, these secondary roll motions are associated with an amplified skin-friction coefficient relative to both the small- and large- $s/\unicode[STIX]{x1D6FF}$ limits. It is found that the relationship between the secondary roll motions and the mean isovels is reversed through this transition from low longitudinal velocity over low Stress at small $s/\unicode[STIX]{x1D6FF}$ to high longitudinal velocity over low Stress at large $s/\unicode[STIX]{x1D6FF}$ .
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estimating Wall Shear Stress fluctuations given an outer region input
Journal of Fluid Mechanics, 2013Co-Authors: Romain Mathis, Ivan Marusic, Sergei Chernyshenko, N HutchinsAbstract:A model for the instantaneous Wall-Shear-Stress distribution is presented for zero-pressure-gradient turbulent boundary layers. The model, based on empirical and theoretical considerations, is able to reconstruct a statistically representative fluctuating Wall-Shear-Stress time-series, ${ \tau }_{w}^{\ensuremath{\prime} } (t)$ , using only the low-frequency content of the streamwise velocity measured in the logarithmic region, away from the Wall. Results, including spectra and second-order moments, show that the model is capable of successfully capturing Reynolds number trends as observed in other studies.
Alexander Assmann - One of the best experts on this subject based on the ideXlab platform.
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dispersive aortic cannulas reduce aortic Wall Shear Stress affecting atherosclerotic plaque embolization
Artificial Organs, 2015Co-Authors: Alexander Assmann, Ali Cemal Benim, Payam Akhyari, Franz Joos, Fethi Gul, Artur LichtenbergAbstract:Neurologic complications during on-pump cardiovascular surgery are often induced by mobilization of atherosclerotic plaques, which is directly related to enhanced Wall Shear Stress. In the present study, we numerically evaluated the impact of dispersive aortic cannulas on aortic blood flow characteristics, with special regard to the resulting Wall Shear Stress profiles. An idealized numerical model of the human aorta and its branches was created and used to model straight as well as bent dispersive aortic cannulas with meshlike tips inserted in the distal ascending aorta. Standard cannulas with straight beveled or bent tips served as controls. Using a recently optimized computing method, simulations of pulsatile and nonpulsatile extracorporeal circulation were performed. Dispersive aortic cannulas reduced the maximum and average aortic Wall Shear Stress values to approximately 50% of those with control cannulas, while the difference in local values was even larger. Moreover, under pulsatile circulation, dispersive cannulas shortened the time period during which Wall Shear Stress values were increased. The turbulent kinetic energy was also diminished by utilizing dispersive cannulas, reducing the risk of hemolysis. In summary, dispersive aortic cannulas decrease aortic Wall Shear Stress and turbulence during extracorporeal circulation and may therefore reduce the risk of endothelial and blood cell damage as well as that of neurologic complications caused by atherosclerotic plaque mobilization.
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pulsatile extracorporeal circulation during on pump cardiac surgery enhances aortic Wall Shear Stress
Journal of Biomechanics, 2012Co-Authors: Alexander Assmann, Ali Cemal Benim, Payam Akhyari, U Boeken, Franz Joos, P Feindt, Artur LichtenbergAbstract:Abstract Controversy on superiority of pulsatile versus non-pulsatile extracorporeal circulation in cardiac surgery still continues. Stroke as one of the major adverse events during cardiopulmonary bypass is, in the majority of cases, caused by mobilization of aortic arteriosclerotic plaques that is inducible by pathologically elevated Wall Shear Stress values. The present study employs computational fluid dynamics to evaluate the aortic blood flow and Wall Shear Stress profiles under the influence of antegrade or retrograde perfusion with pulsatile versus non-pulsatile extracorporeal circulation. While, compared to physiological flow, a non-pulsatile perfusion resulted in generally decreased blood velocities and only moderately increased Shear forces (48 Pa versus 20 Pa antegradely and 127 Pa versus 30 Pa retrogradely), a pulsatile perfusion extensively enhanced the occurrence of turbulences, maximum blood flow speed and maximum Wall Shear Stress (1020 Pa versus 20 Pa antegradely and 1178 Pa versus 30 Pa retrogradely). Under these circumstances arteriosclerotic embolism has to be considered. Further simulations and experimental work are necessary to elucidate the impact of our findings on the scientific discourse of pulsatile versus non-pulsatile extracorporeal circulation.
Agostino Gnasso - One of the best experts on this subject based on the ideXlab platform.
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in vivo association between low Wall Shear Stress and plaque in subjects with asymmetrical carotid atherosclerosis
Stroke, 1997Co-Authors: Agostino Gnasso, Claudio Carallo, Concetta Irace, P L Mattioli, M S De Franceschi, C Motti, Arturo PujiaAbstract:Background and Purpose It is known that atherosclerosis does not involve both carotid arteries to the same extent. Pathological investigations have demonstrated that lesions develop in regions of low Wall Shear Stress. The aims of the present study were to verify the degree of carotid atherosclerosis asymmetry in a population-based study and to evaluate whether Wall Shear Stress is lower in carotids with atherosclerotic lesions than in carotids without lesions. Methods Participants in a cardiovascular disease prevention campaign (n=1166) were screened for carotid atherosclerosis by echo-Doppler examination. Of these, 23 subjects who presented plaque in the common carotid or bulb of one side and no plaque in the contralateral carotid tree were enrolled for common carotid Wall Shear Stress measurement. Shear Stress was calculated according to the following formula: Shear Stress=Blood Viscosity×Blood Velocity/Internal Diameter. Results Of the 1166 subjects screened, 400 (34%) had plaque and/or stenosis in the carotids. Ninety subjects had lesions exclusively in the right carotid, 111 had lesions exclusively in the left, 70 had lesions in both carotids but with different degrees of severity, and only 129 had similar lesions in both carotids. In the 23 subjects in whom Wall Shear Stress was measured, peak Shear Stress was 18.7±4.1 and 15.3±4.0 dynes·cm−2 (mean±SD) ( P <.0001) in the side without and the side with plaque, respectively. Mean Shear Stress yielded similar results. Conclusions The present results demonstrate that the atherosclerotic involvement of carotid arteries is usually asymmetrical and that Wall Shear Stress is lower in the carotid arteries where plaques are present than in plaque-free arteries. These findings provide in vivo evidence for a strong association between Shear Stress and atherosclerotic lesions.
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association between intima media thickness and Wall Shear Stress in common carotid arteries in healthy male subjects
Circulation, 1996Co-Authors: Agostino Gnasso, Claudio Carallo, Concetta Irace, Vitaliano Spagnuolo, Giuseppina De Novara, P L Mattioli, Arturo PujiaAbstract:Background Atherosclerotic lesions lie in regions of low Wall Shear Stress. No relationship between Wall Shear Stress and intima-media thickness in vivo has been reported. Aims of the present study were to verify the reproducibility of Wall Shear Stress measurement in vivo and to evaluate its association with intima-media thickness in the common carotid artery in healthy subjects. Methods and Results Wall Shear Stress was calculated according to the following formula: Shear Stress=Blood Viscosity×Blood Velocity/Internal Diameter. Blood viscosity was measured by use of a cone/plate viscometer. Blood velocity, internal diameter, and intima-media thickness were measured by high-resolution echo Doppler. Twenty-one healthy male subjects were investigated. Peak and mean Shear Stress values were 29.5±8.2 and 12.1±3.1 dynes/cm−2 (mean±SD), respectively. Peak Shear Stress was inversely related to intima-media thickness (r=.62), age (r=.77), systolic blood pressure (r=.61), and body mass index (r=.59) (P<.0001 for ...
