Unsteady Entrance Flow Development in a Straight Tube

1994 ◽  
Vol 116 (3) ◽  
pp. 355-360 ◽  
Author(s):  
Xiaoyi He ◽  
David N. Ku
Keyword(s):  
Pulsatile Flow ◽  
Arterial System ◽  
Length Variation ◽  
Straight Tube ◽  
Phase Lag ◽  
Entrance Length ◽  
Flow Waveform ◽  
Mean Flow ◽  
Inlet Flow ◽  
Entrance Flow

The entrance conditions for pulsatile flow are important in the understanding blood flow out of the heart and in developing regions at branches. The pulsatile entrance flow was solved using a spectral element simulation of the full unsteady Navier- Stokes equations. A mean Reynolds number of 200 and a range of Womersley parameters from 1.8 to 12.5 was used for a sinusoidal inlet flow waveform 1+sin (ωt). Variations in the entrance length were observed during the pulsatile cycle. The amplitude of the entrance length variation decreased with an increase in the Womersley parameter. The phase lag between the entrance length and the inlet flow waveform increased for Womersley parameter α up to 5.0 and decreased for α larger than 5.0. For low α, the maximum entrance length during pulsatile flow was approximately the same as the steady entrance length for the peak flow. For high α, the pulsatile entrance length was more uniform during the cycle and tended to the entrance length for the mean flow. The wall shear rate reached its far downstream value after only about half of the entrance length and also exhibited a dependence on α. The results quantify the entrance conditions typically encountered in studies of the arterial system.

scholarly journals Effect of Pulsatile Flow Waveform and Womersley Number on the Flow in Stenosed Arterial Geometry

2012 ◽  
Vol 2012 ◽  
pp. 1-17 ◽  
Author(s):  
Moloy Kumar Banerjee ◽  
Ranjan Ganguly ◽  
Amitava Datta
Keyword(s):  
Pulsatile Flow ◽  
Physiological State ◽  
Circulatory System ◽  
The Body ◽  
Arterial System ◽  
Hemodynamic Parameters ◽  
Flow Waveform ◽  
Womersley Number ◽  
Flow Features ◽  
Hemodynamic Flow

The salient hemodynamic flow features in a stenosed artery depend not only on the degree of stenosis, but also on its location in the circulatory system and the physiological condition of the body. The nature of pulsatile flow waveform and local Womersley number vary in different regions of the arterial system and at different physiological state, which affects the local hemodynamic wall parameters, for example, the wall shear stress (WSS) and oscillatory shear index (OSI). Herein, we have numerically investigated the effects of different waveforms and Womersley numbers on the flow pattern and hemodynamic parameters in an axisymmetric stenosed arterial geometry with 50% diametral occlusion. Temporal evolution of the streamlines and hemodynamic parameters are investigated, and the time-averaged hemodynamic wall parameters are compared. Presence of the stenosis is found to increase the OSI of the flow even at the far-downstream side of the artery. At larger Womersley numbers, the instantaneous flow field in the stenosed region is found to have a stronger influence on the flow profiles of the previous time levels. The study delineates how an approximation in the assumption of inlet pulsatility profile may lead to significantly different prediction of hemodynamic wall parameters.

Abstract P323: Impact Of Diminished Pulsatility On Unravelling Of Von Willebrand Factor (VWF) In Patients With Continuous Flow Ventricular Assisted Devices (CF-VADs)

2021 ◽  
Vol 129 (Suppl_1) ◽  
Author(s):  
Khanh T Nguyen ◽  
Yi Wang ◽  
Guruprasad A Giridharan ◽  
Xuanhong Cheng ◽  
Palaniappan Sethu
Keyword(s):  
Von Willebrand Factor ◽  
Pulsatile Flow ◽  
Continuous Flow ◽  
Total Internal Reflection ◽  
Review Of Literature ◽  
Mean Flow ◽  
Shear Rates ◽  
Surgical Bleeding ◽  
Von Willebrand ◽  
Willebrand Factor

Introduction: Patients implanted with Continuous Flow Ventricular Assisted Devices (CF VADs) exhibit diminished pulsatility and are at a high risk for developing acquired von Willebrand Factor syndrome (AVWS) and non-surgical bleeding. This study aimed to understand how diminished pulsatility due to CF VAD impacts unravelling and patient plasma levels of von Willebrand Factor (vWF). A microfluidic approach was used to study unravelling of vWF under normal pulsatile flow and flow with diminished pulsatility. In addition, vWF levels in CF-VAD patients was measured to determine vWF levels in circulation. Hypothesis: We hypothesized that diminished pulsatility increases vWF unravelling, likely leading to increased vWF degradation and elevated levels of low MW vWF fragments in circulation, this in turn leads to decreased endothelial vWF production in CF-VAD patients. Methods: vWF molecules were immobilized in a microfluidic device and subjected to either normal pulsatile flow or flow with diminished pulsatility (same mean flow). vWF unravelling behavior was observed using total internal reflection fluorescence (TIRF) microscopy. Patient blood samples were collected 1-2 days pre CF-VAD implant and monthly post-implant. Patient plasma vWF levels were measured using an ELISA kit. Results: TIRF imaging showed that vWF molecules undergo unravelling and significantly greater elongation (p<0.05) under diminished pulsatility than with normal physiological pulsatility, despite higher levels of peak shear rates with normal pulsatility ( Fig.1A ). Evaluation of plasma vWF levels in patients (n=9) showed that vWF levels decreased progressively following CF-VAD placement ( Fig.1B ). These results suggest that diminished pulsatility increased unravelling of vWF and exposure of ADAMTS13 binding sites, potentially leading to enhanced cleavage of vWF into low molecular weight (MW) multimers. Review of literature suggests that both low MW multimers and diminished pulsatility cause endothelial dysfunction and decreased endothelial vWF production, which was evident in patient samples. Conclusion: Diminished pulsatility may independently promote vWF degradation and lead to decreased production of vWF, thus contributing to AVWS.

The onset of turbulence in physiological pulsatile flow in a straight tube

1998 ◽  
Vol 24 (1) ◽  
pp. 1-9 ◽  
Author(s):  
J. Peacock ◽  
T. Jones ◽  
C. Tock ◽  
R. Lutz
Keyword(s):  
Pulsatile Flow ◽  
Straight Tube ◽  
Onset Of Turbulence

scholarly journals Pulsatile Flow Measurement by a Speckle Triangle Assessment

2021 ◽  
Vol 9 ◽  
Author(s):  
Yuan Yuan ◽  
Yong Bi ◽  
Xiao Cao Gao ◽  
Wei Nan Gao
Keyword(s):  
Coronary Artery ◽  
Pulsatile Flow ◽  
Dynamic Range ◽  
Artery Bypass ◽  
High Dynamic Range ◽  
Laser Speckle ◽  
Flow Index ◽  
Mean Flow ◽  
Contrast Imaging ◽  
Phantom Experiment

The blood flow in the coronary artery (CA) is pulsatile and much higher than that measured in the brain, retina, and skin before. Its quantitative measurement is medically significant in the coronary artery bypass grafting (CABG). Here, to the best of our knowledge, we first detect the pulsatile flow using the laser speckle contrast imaging technique. Since the factors influencing the flow rate in the CA are complex, we developed a comprehensive model, a speckle triangle assessment (STA), to assess the characteristics of the flow: the speckle flow index (SFI), mean flow index (MFI), and pulsatility index (PI). The phantom experiment was performed and found that our customized setup possessed high dynamic range of the velocity measurement with good sensitivity. It also indicated that the pulsatile flow estimated by the speckle triangle assessment is promising to obtain a more accurate assessment of a coronary artery’s patency in the CABG.

The Frequency Response of Electrochemical Wall Shear Probes in Pulsatile Flow

1987 ◽  
Vol 109 (1) ◽  
pp. 60-64 ◽  
Author(s):  
L. Talbot ◽  
J. J. Steinert
Keyword(s):  
Mass Transfer ◽  
Frequency Response ◽  
Pulsatile Flow ◽  
Straight Tube ◽  
Asymptotic Results ◽  
Frequency Dependent ◽  
Shear Rates ◽  
Pulsatile Flows ◽  
Wall Shear ◽  
Good Agreement

The frequency response of surface-mounted electrochemical mass transfer probes used to deduce wall shear rates has been investigated experimentally for the case of fully developed laminar pulsatile flow in a straight tube. Generally good agreement is found with the asymptotic results obtained by Lighthill’s methods. The significance of the results with regard to the investigation of models of pulsatile flows of physiological interest is discussed. It is concluded that the frequency-dependent phase and amplitude corrections required to obtain accurate wall shear measurements are of such magnitudes as to render impractical the use of electrochemical probes to determine wall shear rates in these flows.

An Acoustic Time-of-Flight Approach for Unsteady Temperature Measurements: Characterization of Entropy Waves in a Model Gas Turbine Combustor

2016 ◽  
Vol 139 (4) ◽  
Author(s):  
Dominik Wassmer ◽  
Bruno Schuermans ◽  
Christian Oliver Paschereit ◽  
Jonas P. Moeck
Keyword(s):  
Gas Turbine ◽  
Acoustic Waves ◽  
Fuel Injection ◽  
Acoustic Pulse ◽  
Time Of Flight ◽  
Phase Lag ◽  
Mean Flow ◽  
Unsteady Temperature ◽  
Arrival Times ◽  
Flow Velocities

Lean premixed combustion promotes the occurrence of thermoacoustic phenomena in gas turbine combustors. One mechanism that contributes to the flame–acoustic interaction is entropy noise. Fluctuations of the equivalence ratio in the mixing section cause the generation of hot spots in the flame. These so-called entropy waves are convectively transported to the first stage of the turbine and generate acoustic waves that travel back to the flame; a thermoacoustic loop is closed. However, due to the lack of experimental tools, a detailed investigation of entropy waves in gas turbine combustion systems has not been possible up to now. This work presents an acoustic time-of-flight based temperature measurement method which allows the measurement of temperature fluctuations in the relevant frequency range. A narrow acoustic pulse is generated with an electric spark discharge close to the combustor wall. The acoustic response is measured at the same axial location with an array of microphones circumferentially distributed around the combustion chamber. The delay in the pulse arrival times corresponds to the line-integrated inverse speed of sound. For the measurement of entropy waves in an atmospheric combustion test rig, fuel is periodically injected into the mixing tube of a premixed combustor. The subsequently generated entropy waves are measured for different forcing frequencies of the fuel injection and for different mean flow velocities in the combustor. The amplitude decay and phase lag of the entropy waves adhere well to a Strouhal number scaling for different mean flow velocities.

An Acoustic Time-of-Flight Approach for Unsteady Temperature Measurements: Characterization of Entropy Waves in a Model Gas Turbine Combustor

2016 ◽  
Author(s):  
Dominik Wassmer ◽  
Bruno Schuermans ◽  
Christian Oliver Paschereit ◽  
Jonas P. Moeck
Keyword(s):  
Temperature Measurement ◽  
Gas Turbine ◽  
Equivalence Ratio ◽  
Acoustic Pulse ◽  
Time Of Flight ◽  
Tunable Diode Laser ◽  
Phase Lag ◽  
Mean Flow ◽  
Unsteady Temperature ◽  
Flow Velocities

Lean premixed combustion promotes the occurrence of thermoacoustic phenomena in gas turbine combustors. One mechanism that contributes to the flame-acoustic interaction is entropy noise. Fluctuations of the equivalence ratio in the mixing section cause the generation of hot spots in the flame. These so called entropy waves are convectively transported to the first stage of the turbine and generate acoustic waves that travel back to the flame; a thermoacoustic loop is closed. However, due to the lack of experimental tools, a detailed investigation of entropy waves in gas turbine combustion systems has not been possible up to now. This work presents an acoustic time-of-flight based temperature measurement method which allows the detection of temperature fluctuations in the relevant frequency range. A narrow acoustic pulse is generated with an electric spark discharge close to the combustor wall. The acoustic response is measured at the same axial location with an array of microphones circumferentially distributed around the combustion chamber. The delay in the pulse arrival times corresponds to the line-integrated inverse speed of sound. For validation of this new method an experimental setup was developed capable of generating well defined entropy waves. As a reference temperature measurement technique a hot-wire anemometer is employed. For the measurement of entropy waves in an atmospheric combustion test rig, fuel is periodically injected into the mixing tube of a premixed combustor. The subsequently generated entropy waves are detected for different forcing frequencies of the fuel injection and for different mean flow velocities in the combustor. The amplitude decay and phase lag of the entropy waves adheres well to a Strouhal number scaling for different mean flow velocities. In addition, simultaneously to the entropy wave measurement, the equivalence ratio fluctuations in the mixing tube are detected using the Tunable Diode Laser Absorption Spectroscopy (TDLAS) technique.

Dynamic accuracy of the electromagnetic flowmeter

1965 ◽  
Vol 20 (1) ◽  
pp. 142-147 ◽  
Author(s):  
M. F. O'Rourke
Keyword(s):  
Magnetic Field ◽  
Pulsatile Flow ◽  
Output Signal ◽  
Oscillatory Flow ◽  
Electromagnetic Flowmeter ◽  
Sine Wave ◽  
Dynamic Calibration ◽  
Phase Lag ◽  
Dynamic Accuracy ◽  
Sinusoidal Flow

Dynamic calibration of electromagnetic flowmeters was performed by resolving the output signal when sinusoidal flow of known characteristics was pumped through the probe. In two instruments amplitude and phase were found to be frequency dependent in the range 0–20 cycle/sec; the magnitude of these effects was not insignificant as many have assumed. In a sine wave instrument using a variety of probes, both amplitude and phase increased linearly with frequency: the former increasing by 0.57% per cycle per second, the latter by 3.6° per cycle per second. In the square wave flowmeter at 0 damping, amplitude decreased significantly above 5 cycle/sec, while phase lag increased by 4.9° per cycle per second. These effects are concluded to be due to the output filtering network. In the sine wave instrument it was demonstrated that magnetic field nonuniformity within the probe did not alter the accuracy in recording steady or oscillatory flow. It was thus possible to construct a small short flowmeter probe having narrow electromagnet coils, and it is expected that this probe should cause minimal interference to pulsatile flow patterns. magnetic fields flowmeter probes; frequency-response flowmeter Submitted on April 16, 1964

Laminar flow in a curved pipe with varying curvature

1976 ◽  
Vol 73 (4) ◽  
pp. 735-752 ◽  
Author(s):  
S. Murata ◽  
Y. Miyake ◽  
T. Inaba
Keyword(s):  
Reynolds Number ◽  
Flow Direction ◽  
Circular Cross Section ◽  
Outer Side ◽  
Large Reynolds Number ◽  
Phase Lag ◽  
Mean Flow ◽  
Curved Pipe ◽  
Curved Pipes ◽  
Centre Line

The steady laminar motion of fluid through pipes of circular cross-section, the curvature of whose centre-line varies locally, is analysed theoretically. The flow in three kinds of pipes whose centre-lines are specified by \[ \hat{y} = a(1+\kappa^2\hat{x}^2)^{\frac{1}{2}},\quad\hat{y} = a\tan h\kappa\hat{x}\quad{\rm and}\quad\hat{y} = a\sin\kappa\hat{x} \] are treated as the examples of once-, twice- and periodically-curved pipes, respectively. The analysis is valid for any other two-dimensionally curved pipes, when centre-line curvature is small. At very small Reynolds number, the position of maximum axial velocity shifts towards the inner side of the pipe section; at large Reynolds number, on the contrary, it tends to the outer side, owing to centrifugal force. Furthermore, in the latter case, adaptation of the flow follows the change of mean-flow direction, with a phase lag.

NUMERICAL STUDY OF PHYSIOLOGICAL TURBULENT FLOWS THROUGH STENOSED ARTERIES

2003 ◽  
Vol 14 (05) ◽  
pp. 635-659 ◽  
Author(s):  
WEI LIAO ◽  
T. S. LEE ◽  
H. T. LOW
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Turbulent Flow ◽  
Pulsatile Flow ◽  
Arterial Stenosis ◽  
Phase Lag ◽  
Womersley Number ◽  
Physiological Flow ◽  
Disturbance Intensity ◽  
Pulsatile Flows

A detailed analysis on the characteristics of transitional turbulent flow over a bell-shape stenosis for a physiological pulsatile flow is presented. The comparison of the numerical solutions to three types of pulsatile flows, including a physiological flow, an equivalent pulsatile flow and a simple pulsatile flow, are made in this work. Then the effects of the Reynolds number, Womersley number and constriction ratio of stenosis on the pulsatile turbulent flow fields for the physiological flow are considered. The comparison of the three pulsatile flows shows that the flow characteristics cannot be properly estimated if an equivalent or simple pulsatile inflow is used instead of actual physiological one in the study of the pulsatile flows through arterial stenosis. The equivalent or simple pulsatile inflow can lead to higher disturbance intensity in the vicinity of the stenosis than the physiological inflow. For a physiological flow, the recirculation zones with high disturbance intensity occur mainly in the distal of the stenosis. The larger Reynolds number and severer constriction ratio may result in more complex flow field and cause some important flow variables to increase dramatically near stenosis. The higher Womersley number leads to a larger phase lag between the imposed flow rate changes and the final converged flow field in one cycle. The turbulence intensity decreases with the increase of Womersley number for the same Reynolds number.

Sign in / Sign up

Export Citation Format

Share Document