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.

Flush-Mounted Hot Film Anemometer Accuracy in Pulsatile Flow

1986 ◽  
Vol 108 (3) ◽  
pp. 228-231 ◽  
Author(s):  
S. Nandy ◽  
J. M. Tarbell
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Pulsatile Flow ◽  
Straight Tube ◽  
Average Deviation ◽  
Pulsatile Flows ◽  
Measured Flow ◽  
Wall Shear ◽  
Hot Film ◽  
Low Shear

The accuracy of a flush-mounted hot film anemometer probe for wall shear stress measurements in physiological pulsatile flows was evaluated in fully developed pulsatile flow in a rigid straight tube. Measured wall shear stress waveform based on steady flow anemometer probe calibrations were compared to theoretical wall shear stress waveforms based on well-established theory and measured flow rate waveforms. The measured and theoretical waveforms were in close agreement during systole (average deviation of 14 percent at peak systole). As expected, agreement was poor during diastole because of flow reversal and diminished frequency response at low shear rate.

Derivation of Shear Rates From Near-Wall LDA Measurements Under Steady and Pulsatile Flow Conditions

1994 ◽  
Vol 116 (3) ◽  
pp. 361-368 ◽  
Author(s):  
Ray S. Fatemi ◽  
Stanley E. Rittgers
Keyword(s):  
Shear Rate ◽  
Linear Approximation ◽  
Curve Fitting ◽  
Pulsatile Flow ◽  
Wall Shear Rate ◽  
Measured Velocity ◽  
Shear Rates ◽  
Wall Shear ◽  
Flow Waveforms ◽  
Near Wall

Atherosclerosis, thrombosis, and intimal hyperplasia are major forms of cardiovascular diseases in the United States. Previous studies indicate a significant correlation between hemodynamics, in particular, wall shear rate, and pathology of the arterial walls. While results of these studies implicate morphologic and functional changes related to wall shear rate magnitude, a standard technique for wall shear rate measurement has not been established. In this study, theoretical and in-vitro experimental fully developed steady and physiologic pulsatile flow waveforms have been used to obtain velocity profiles in the near-wall region. The estimated wall shear rates from these results are compared to the theoretical value to assess the accuracy of the approximating technique. Experimentally obtained results from LDA suggest that in order to minimize the error in velocity data, and subsequently, the wall shear rate, the first measured velocity has to be 500 μm away from the wall. While a linear approximation did not produce errors larger than 16.4 percent at peak systole, these errors substantially increased as the velocity magnitudes decreased during late systole and diastole. Overall, a third degree polynomial curve fit using four points produced the most accurate estimation of wall shear rate through out the cardiac cycle. Results of higher degree curve-fitting functions can be unpredictable due to potential oscillations of the function near the wall. Hence, based on the results of this study, use of a linear approximation is not recommended; a third degree curve-fitting polynomial, using four points provided the most accurate approximation for these flow waveforms.

Experimental Study on Oscillatory Couette-Taylor Flows Behaviour

2013 ◽  
Author(s):  
Emna Berrich ◽  
Fethi Aloui ◽  
Jack Legrand
Keyword(s):  
Mass Transfer ◽  
Angular Velocity ◽  
Time Evolution ◽  
Vortex Flow ◽  
Outer Cylinder ◽  
Taylor Vortex ◽  
Shear Rates ◽  
Taylor Vortices ◽  
Taylor Vortex Flow ◽  
Wall Shear

In the simplest and original case of study of the Taylor–Couette TC problems, the fluid is contained between a fixed outer cylinder and a concentric inner cylinder which rotates at constant angular velocity. Much of the works done has been concerned on steady rotating cylinder(s) i.e. rotating cylinders with constant velocity and the various transitions that take place as the cylinder(s) velocity (ies) is (are) steadily increased. On this work, we concentrated our attention in the case in which the inner cylinder velocity is not constant, but oscillates harmonically (in time) clockwise and counter-clockwise while the outer cylinder is maintained fixed. Our aim is to attempt to answer the question if the modulation makes the flow more or less stable with respect to the vortices apparition than in the steady case. If the modulation amplitude is large enough to destabilise the circular Couette flow, two classes of axisymmetric Taylor vortex flow are possible: reversing Taylor Vortex Flow (RTVF) and Non-Reversing Taylor Vortex Flow (NRTVF) (Youd et al., 2003; Lopez and Marques, 2002). Our work presents an experimental investigation of the effect of oscillatory Couette-Taylor flow, i.e. both the oscillation frequency and amplitude on the apparition of RTVF and NRTVF by analysing the instantaneous and local mass transfer and wall shear rates evolutions, i.e. the impact of vortices at wall. The vortices may manifest themselves by the presence of time-oscillations of mass transfer and wall shear rates, this generally corresponds to an instability apparition even for steady rotating cylinder. On laminar CT flow, the time-evolution of wall shear rate is linear. It may be presented as a linear function of the angular velocity, i.e. the evolution is steady even if the angular velocity is not steady. At a “critical” frequency and amplitude, the laminar CT flow is disturbed and Taylor vortices appear. Comparing to a steady velocity case, oscillatory flow accelerate the instability apparition, i.e. the critical Taylor number corresponds to the transition is smaller than that of the steady case. For high oscillation amplitudes of the inner cylinder rotation, the mass transfer time-evolution has a sinusoidal evolution with non equal oscillation amplitudes. If the oscillation amplitude is large enough, it can destabilize the laminar Couette flow, Taylor vortices appears. The vortices direction can be deduced from the sign of the instantaneous wall shear rate time evolution.

Modeling of mass transfer in biofilms in oscillatory flow conditions using k-ɛ turbulence model

2003 ◽  
Vol 3 (1-2) ◽  
pp. 201-207
Author(s):  
H. Nagaoka ◽  
T. Nakano ◽  
D. Akimoto
Keyword(s):  
Numerical Simulation ◽  
Mass Transfer ◽  
Turbulence Model ◽  
Uptake Rate ◽  
Oscillatory Flow ◽  
Substrate Uptake ◽  
Flow Conditions ◽  
Mass Transfer Mechanism ◽  
Substrate Uptake Rate ◽  
Good Agreement

The objective of this research is to investigate mass transfer mechanism in biofilms under oscillatory flow conditions. Numerical simulation of turbulence near a biofilm was conducted using the low Reynold’s number k-ɛ turbulence model. Substrate transfer in biofilms under oscillatory flow conditions was assumed to be carried out by turbulent diffusion caused by fluid movement and substrate concentration profile in biofilm was calculated. An experiment was carried out to measure velocity profile near a biofilm under oscillatory flow conditions and the influence of the turbulence on substrate uptake rate by the biofilm was also measured. Measured turbulence was in good agreement with the calculated one and the influence of the turbulence on the substrate uptake rate was well explained by the simulation.

The effect of two-phase flow regime on hydrodynamics and mass transfer in a horizontal-tube gas-liquid reactor

1985 ◽  
Vol 50 (3) ◽  
pp. 745-757 ◽  
Author(s):  
Andreas Zahn ◽  
Lothar Ebner ◽  
Kurt Winkler ◽  
Jan Kratochvíl ◽  
Jindřich Zahradník
Keyword(s):  
Mass Transfer ◽  
Pressure Drop ◽  
Flow Regime ◽  
Liquid Flow ◽  
Horizontal Tube ◽  
Liquid Holdup ◽  
Two Phase ◽  
Flow Rates ◽  
Type Relation ◽  
Good Agreement

The effect of two-phase flow regime on decisive hydrodynamic and mass transfer characteristics of horizontal-tube gas-liquid reactors (pressure drop, liquid holdup, kLaL) was determined in a cocurrent-flow experimental unit of the length 4.15 m and diameter 0.05 m with air-water system. An adjustable-height weir was installed in the separation chamber at the reactor outlet to simulate the effect of internal baffles on reactor hydrodynamics. Flow regime maps were developed in the whole range of experimental gas and liquid flow rates both for the weirless arrangement and for the weir height 0.05 m, the former being in good agreement with flow-pattern boundaries presented by Mandhane. In the whole range of experi-mental conditions pressure drop data could be well correlated as a function of gas and liquid flow rates by an empirical exponential-type relation with specific sets of coefficients obtained for individual flow regimes from experimental data. Good agreement was observed between values of pressure drop obtained for weirless arrangement and data calculated from the Lockhart-Martinelli correlation while the contribution of weir to the overall pressure drop was well described by a relation proposed for the pressure loss in closed-end tubes. In the region of negligible weir influence values of liquid holdup were again succesfully correlated by the Lockhart-Martinelli relation while the dependence of liquid holdup data on gas and liquid flow rates obtained under conditions of significant weir effect (i.e. at low flow rates of both phases) could be well described by an empirical exponential-type relation. Results of preliminary kLaL measurements confirmed the decisive effect of the rate of energy dissipation on the intensity of interfacial mass transfer in gas-liquid dispersions.

Measurement of wall shear stress in physiological pulsatile flows by flush-mounted hot film anemometry

1988 ◽  
Vol 16 (2) ◽  
pp. 235-238
Author(s):  
Subhashis Nandy ◽  
Alex Yefim Bekker ◽  
Gregory Allen Winchell ◽  
John Francis O'Riordan
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Pulsatile Flows ◽  
Wall Shear ◽  
Hot Film ◽  
Hot Film Anemometry

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.

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