Wall shear stress in accelerating turbulent pipe flow

2011 ◽  
Vol 685 ◽  
pp. 440-460 ◽  
Author(s):  
S. He ◽  
C. Ariyaratne ◽  
A. E. Vardy
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Initial Period ◽  
Turbulent Pipe Flow ◽  
Shear Stresses ◽  
Acceleration Rate ◽  
The Mean ◽  
Wall Shear ◽  
Turbulent Wall

AbstractAn experimental study of wall shear stress in an accelerating flow of water in a pipe ramping between two steady turbulent flows has been undertaken in a large-scale experimental facility. Ensemble averaged mean and r.m.s. of the turbulent fluctuations of wall shear stresses have been derived from hot-film measurements from many repeated runs. The initial Reynolds number and the acceleration rate were varied systematically to give values of a non-dimensional acceleration parameter $k$ ranging from 0.16 to 14. The wall shear stress has been shown to follow a three-stage development. Stage 1 is associated with a period of minimal turbulence response; the measured turbulent wall shear stress remains largely unchanged except for a very slow increase which is readily associated with the stretching of existing turbulent eddies as a result of flow acceleration. In this condition of nearly ‘frozen’ turbulence, the unsteady wall shear stress is driven primarily by flow inertia, initially increasing rapidly and overshooting the pseudo-steady value, but then increasing more slowly and eventually falling below the pseudo-steady value. This variation is predicted by an analytical expression derived from a laminar flow formulation. The start of Stage 2 is marked by the generation of new turbulence causing both the mean and turbulent wall shear stress to increase rapidly, although there is a clear offset between the responses of these two quantities. The turbulent wall shear, reflecting local turbulent activities near the wall, responds first and the mean wall shear, reflecting conditions across the entire flow field, responds somewhat later. In Stage 3, the wall shear stress exhibits a quasi-steady variation. The duration of the initial period of nearly frozen turbulence response close to the wall increases with decreasing initial Reynolds number and with increasing acceleration. The latter is in contrast to the response of turbulence in the core of the flow, which previous measurements have shown to be independent of the rate of acceleration.

Wall friction and velocity measurements in a double-frequency pulsating turbulent flow

2016 ◽  
Vol 788 ◽  
pp. 521-548 ◽  
Author(s):  
L. R. Joel Sundstrom ◽  
Berhanu G. Mulu ◽  
Michel J. Cervantes
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Low Frequency ◽  
Turbulent Pipe Flow ◽  
Oscillating Flow ◽  
Base Case ◽  
Mean Flow ◽  
Double Frequency ◽  
The Mean ◽  
Wall Shear

Wall shear stress measurements employing a hot-film sensor along with laser Doppler velocimetry measurements of the axial and tangential velocity and turbulence profiles in a pulsating turbulent pipe flow are presented. Time-mean and phase-averaged results are derived from measurements performed at pulsation frequencies ${\it\omega}^{+}={\it\omega}{\it\nu}/\bar{u}_{{\it\tau}}^{2}$ over the range of 0.003–0.03, covering the low-frequency, intermediate and quasi-laminar regimes. In addition to the base case of a single pulsation imposed on the mean flow, the study also investigates the flow response when two pulsations are superimposed simultaneously. The measurements from the base case show that, when the pulsation belongs to the quasi-laminar regime, the oscillating flow tends towards a laminar state in which the velocity approaches the purely viscous Stokes solution with a low level of turbulence. For ${\it\omega}^{+}<0.006$, the oscillating flow is turbulent and exhibits a region with a logarithmic velocity distribution and a collapse of the turbulence intensities, similar to the time-averaged counterparts. In the low-frequency regime, the oscillating wall shear stress is shown to be directly proportional to the Stokes length normalized in wall units $l_{s}^{+}~(=\sqrt{2/{\it\omega}^{+}})$, as predicted by quasi-steady theory. The base case measurements are used as a reference when evaluating the data from the double-frequency case and the oscillating quantities are shown to be close to superpositions from the base case. The previously established view that the time-averaged quantities are unaffected by the imposition of small-amplitude pulsed unsteadiness is shown to hold also when two pulsations are superposed on the mean flow.

Wall Shear Stress in Turbulent Pipe Flow

2009 ◽  
Author(s):  
Roland Gårdhagen ◽  
Jonas Lantz ◽  
Fredrik Carlsson ◽  
Matts Karlsson
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Turbulent Flows ◽  
Intima Media Thickness ◽  
Elevated Risk ◽  
Laminar Flows ◽  
Turbulent Pipe Flow ◽  
Mean Flow ◽  
The Mean ◽  
Wall Shear

Low and/or oscillatory Wall Shear Stress (WSS) has been correlated with elevated risk for increased intima media thickness and atherosclerosis in several studies during the last decades [1, 2]. Most of the studies have addressed laminar flows, in which the oscillations mainly are due to the pulsating nature of blood flow. Turbulent flows however show significant spatial and temporal fluctuations although the mean flow is steady.

Characterization of a Sudden Expansion Flow Chamber to Study the Response of Endothelium to Flow Recirculation

1995 ◽  
Vol 117 (2) ◽  
pp. 203-210 ◽  
Author(s):  
George A. Truskey ◽  
Kevin M. Barber ◽  
Thomas C. Robey ◽  
Lauri A. Olivier ◽  
Marty P. Combs
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Recirculation Zone ◽  
Numerical Solutions ◽  
Flow Chamber ◽  
Sudden Expansion ◽  
Shear Stresses ◽  
Stress Gradients ◽  
Wall Shear

In order to simulate regions of flow separation observed in vivo, a conventional parallel plate flow chamber was modified to produce an asymmetric sudden expansion. The flow field was visualized using light reflecting particles and the size of the recirculation zone was measured by image analysis of the particles. Finite element numerical solutions of the two and three-dimensional forms of the Navier-Stokes equation were used to determine the wall shear stress distribution and predict the location of reattachment. For two different size expansions, numerical estimates of the reattachment point along the centerline of the flow chamber agreed well with experimental values for Reynolds numbers below 473. Even at a Reynolds number of 473, the flow could be approximated as two-dimensional for 80 percent of the chamber width. Peak shear stresses in the recirculation zone as high as 80 dyne/cm2 and shear stress gradients of 2500 (dyne/cm2)/cm were produced. As an application of this flow chamber, subconfluent bovine aortic endothelial cell shape and orientation were examined in the zone of recirculation during a 24 h exposure to flow at a Reynolds number of 267. After 24 h, gradients in cell orientation and shape were observed within the recirculation zone. At the location of reattachment, where the wall shear stress was zero but the shear stress gradients were large, cells plated at low density were still aligned with the direction of flow. No preferred orientation was observed at the gasket edge where the wall shear stress and shear stress gradients were zero. At higher cell densities, no alignment was observed at the separation point. The results suggest that endothelial cells can respond to spatial gradients of wall shear stress.

Calculations of Wall Shear Stress in Harmonically Oscillated Turbulent Pipe Flow Using a Low-Reynolds-Number k-ε Model

1996 ◽  
Vol 118 (1) ◽  
pp. 189-194 ◽  
Author(s):  
J. O. Ismael ◽  
M. A. Cotton
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Pipe Flow ◽  
Low Reynolds Number ◽  
Turbulent Pipe Flow ◽  
Frequency Model ◽  
Wide Range ◽  
Wall Shear ◽  
Low Reynolds

The low-Reynolds-number k-ε turbulence model of Launder and Sharma is applied to the calculation of wall shear stress in spatially fully-developed turbulent pipe flow oscillated at small amplitudes. It is believed that the present study represents the first systematic evaluation of the turbulence closure under consideration over a wide range of frequency. Model results are well correlated in terms of the parameter ω+ = ωv/Uτ2 at high frequencies, whereas at low frequencies there is an additional Reynolds number dependence. Comparison is made with the experimental data of Finnicum and Hanratty.

Turbulent combined oscillatory flow and current in a pipe

1998 ◽  
Vol 373 ◽  
pp. 313-348 ◽  
Author(s):  
C. R. LODAHL ◽  
B. M. SUMER ◽  
J. FREDSØE
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Cross Section ◽  
Oscillatory Flow ◽  
Combined Flow ◽  
The Mean ◽  
Wall Shear ◽  
A Current

This work concerns the combined oscillatory flow and current in a circular, smooth pipe. The study comprises wall shear stress measurements, and laser-Doppler-anemometer velocity and turbulence measurements. Three kinds of pipes were used, with diameters D=19 cm, 9 cm, and 1.1 cm, enabling the influence of the parameter R/δ to be studied in the investigation (R/δ ranging from about 3 to 53), where R is the radius of the pipe, and δ is the Stokes layer thickness. The ranges of the two other parameters of the combined flow processes, namely the current Reynolds number, Rec, and the oscillatory-flow boundary-layer (i.e. the wave–boundary layer) Reynolds number, Rew, are: Rec=0−1.6×105, and Rew=0−7×106. The transition to turbulence in the combined flow case occurs at a current Reynolds number larger than the conventional value, ca. 2×103, depending on Rew, and R/δ. A turbulent current can be laminarized by superimposing an oscillatory flow. The overall average value of the wall shear stress (the mean wall shear stress) may retain its steady-current value, it may decrease, or it may increase, depending on the flow regime. The increase (which can be as much as a factor of 4) occurs when the combined flow is in the wave-dominated regime, while the oscillatory-flow component of the flow is in the turbulent regime. The component of the wall shear stress oscillating around the mean wall shear stress can also increase with respect to its oscillatory-flow-alone value. For this to occur, the originally laminar oscillatory boundary layer needs to become a fully developed turbulent boundary layer, when a turbulent current is superimposed. This increase can be as much as O(3–4). The velocity profiles across the cross-section of the pipe change near the wall when an oscillatory flow is superimposed on a current, in agreement with the results of the wall shear stress measurements. The period-averaged turbulence profiles across the cross-section of the pipe behave differently for different flow regimes. When the two components of the flow are equally significant, the turbulence profile appears to be different from those corresponding to the fundamental cases; the level of turbulence increases (only slightly) with respect to those experienced in the fundamental cases.

Flow and Thermal Fields in a Pendant Droplet Moving on Lyophobic Surface

2010 ◽  
Author(s):  
Basant Singh Sikarwar ◽  
K. Muralidhar ◽  
Sameer Khandekar
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Reynolds Number ◽  
Shear Stress ◽  
Heat Transfer Coefficient ◽  
Wall Shear Stress ◽  
Transfer Coefficient ◽  
Maximum Shear Stress ◽  
Computational Domain ◽  
Wall Shear

Clusters of liquid drops growing and moving on physically or chemically textured lyophobic surfaces are encountered in drop-wise mode of vapor condensation. As opposed to film-wise condensation, drops permit a large heat transfer coefficient and are hence attractive. However, the temporal sustainability of drop formation on a surface is a challenging task, primarily because the sliding drops eventually leach away the lyophobicity promoter layer. Assuming that there is no chemical reaction between the promoter and the condensing liquid, the wall shear stress (viscous resistance) is the prime parameter for controlling physical leaching. The dynamic shape of individual droplets, as they form and roll/slide on such surfaces, determines the effective shear interaction at the wall. Given a shear stress distribution of an individual droplet, the net effect of droplet ensemble can be determined using the time averaged population density during condensation. In this paper, we solve the Navier-Stokes and the energy equation in three-dimensions on an unstructured tetrahedral grid representing the computational domain corresponding to an isolated pendant droplet sliding on a lyophobic substrate. We correlate the droplet Reynolds number (Re = 10–500, based on droplet hydraulic diameter), contact angle and shape of droplet with wall shear stress and heat transfer coefficient. The simulations presented here are for Prandtl Number (Pr) = 5.8. We see that, both Poiseuille number (Po) and Nusselt number (Nu), increase with increasing the droplet Reynolds number. The maximum shear stress as well as heat transfer occurs at the droplet corners. For a given droplet volume, increasing contact angle decreases the transport coefficients.

Preston-Static Tubes for the Measurement of Wall Shear Stress

1994 ◽  
Vol 116 (3) ◽  
pp. 645-649 ◽  
Author(s):  
Josef Daniel Ackerman ◽  
Louis Wong ◽  
C. Ross Ethier ◽  
D. Grant Allen ◽  
Jan K. Spelt
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Convenient Method ◽  
Pipe Flow ◽  
Total Pressure ◽  
Static Pressure ◽  
Shear Stresses ◽  
Streamline Curvature ◽  
Syringe Needle ◽  
Wall Shear

We present a Preston tube device that combines both total and static pressure readings for the measurement of wall shear stress. As such, the device facilitates the measurement of wall shear stress under conditions where there is streamline curvature and/or over surfaces on which it is difficult to either manufacture an array of static-pressure taps or to position a single tap. Our “Preston-static” device is easily and conveniently constructed from commercially available regular and side-bored syringe needles. The pressure difference between the total pressure measured in the regular syringe needle and the static pressure measured in the side-bored one is used to determine the wall shear stress. Wall shear stresses measured in pipe flow were consistent with independently determined values and values obtained using a conventional Preston tube. These results indicate that Preston-static tubes provide a reliable and convenient method of measuring wall shear stress.

Requirements for Mesh Resolution in 3D Computational Hemodynamics

2000 ◽  
Vol 123 (2) ◽  
pp. 134-144 ◽  
Author(s):  
Sujata Prakash ◽  
C. Ross Ethier
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Adaptive Mesh Refinement ◽  
Mesh Refinement ◽  
Adaptive Mesh ◽  
Velocity Fields ◽  
Shear Stresses ◽  
Large Artery ◽  
Mesh Independence ◽  
Wall Shear

Computational techniques are widely used for studying large artery hemodynamics. Current trends favor analyzing flow in more anatomically realistic arteries. A significant obstacle to such analyses is generation of computational meshes that accurately resolve both the complex geometry and the physiologically relevant flow features. Here we examine, for a single arterial geometry, how velocity and wall shear stress patterns depend on mesh characteristics. A well-validated Navier-Stokes solver was used to simulate flow in an anatomically realistic human right coronary artery (RCA) using unstructured high-order tetrahedral finite element meshes. Velocities, wall shear stresses (WSS), and wall shear stress gradients were computed on a conventional “high-resolution” mesh series (60,000 to 160,000 velocity nodes) generated with a commercial meshing package. Similar calculations were then performed in a series of meshes generated through an adaptive mesh refinement (AMR) methodology. Mesh-independent velocity fields were not very difficult to obtain for both the conventional and adaptive mesh series. However, wall shear stress fields, and, in particular, wall shear stress gradient fields, were much more difficult to accurately resolve. The conventional (nonadaptive) mesh series did not show a consistent trend towards mesh-independence of WSS results. For the adaptive series, it required approximately 190,000 velocity nodes to reach an r.m.s. error in normalized WSS of less than 10 percent. Achieving mesh-independence in computed WSS fields requires a surprisingly large number of nodes, and is best approached through a systematic solution-adaptive mesh refinement technique. Calculations of WSS, and particularly WSS gradients, show appreciable errors even on meshes that appear to produce mesh-independent velocity fields.

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