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Rapidly Varying Flow01:24

Rapidly Varying Flow

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Rapidly varying flow (RVF) in open channels is characterized by abrupt changes in flow depth over a short distance, with the rate of depth change relative to distance often approaching unity. These flows are inherently complex due to their transient and multi-dimensional nature, making exact analysis difficult. However, approximate solutions using simplified models provide valuable insights into their behavior.Key Features of Rapidly Varying FlowRVF is commonly observed in scenarios involving...
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Uniform depth channel flow keeps fluid depth consistent along channels such as irrigation canals. In natural channels, such as rivers, approximate uniform flow is often assumed. This condition occurs when the channel’s bottom slope matches the energy slope, balancing potential energy lost from gravity with head loss due to shear stress. This balance prevents depth changes along the channel length, resulting in a steady, uniform flow.Uniform flow in open channels with a constant cross-section...
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To calculate the flow rate for a trapezoidal channel, first, identify the bottom width, side slope, and flow depth of the channel. The cross-sectional area (A) corresponding to the depth of flow (y), channel bottom width (B), and side slope (θ) is determined by:Next, calculate the wetted perimeter, which includes the bottom width and the sloped side lengths in contact with the water. Using the values of the cross-sectional area and the wetted perimeter, determine the hydraulic radius by...
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Gradually Varying Flow01:29

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Gradually varying flow (GVF) in open channels describes situations where water depth changes slowly along the channel due to factors like non-uniform bed slope, channel shape variations, or obstructions. This flow type occurs when the depth adjusts gradually to balance gravitational forces, shear forces, and energy requirements, resulting in a low rate of depth change.Characteristics of Gradually Varying FlowGVF is commonly observed in natural streams, rivers, and canals, where flow depth...
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Total internal reflection fluorescence microscopy or TIRF is an advanced microscopic technique used to visualize fluorophores in samples close to a solid surface with a higher refractive index, such as a glass coverslip. TIRF only allows fluorophores in proximity to the solid surface to be excited. When light from a medium with a lower refractive index (such as air) hits the glass coverslip at a critical angle, the light undergoes total internal reflection stead of passing through the glass.
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Doppler Optical Coherence Tomography of Retinal Circulation
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Sub-diffusion flow velocimetry with number fluctuation optical coherence tomography.

Konstantine Cheishvili, Jeroen Kalkman

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    We developed a new optical coherence tomography (OCT) method to measure slow particle flows. This technique accurately measures flow velocity and particle concentration, even with diffusion present.

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    Area of Science:

    • Biophotonics
    • Fluid Dynamics
    • Optical Measurement Techniques

    Background:

    • Measuring slow or sub-diffusion flows in dilute suspensions is challenging for conventional methods.
    • Existing optical coherence tomography (OCT) techniques have limitations in minimum measurable velocity and Doppler angle dependency.

    Purpose of the Study:

    • To implement and validate a novel number fluctuation dynamic light scattering OCT method.
    • To enable accurate measurement of extremely slow, sub-diffusion flows in dilute particle suspensions.
    • To overcome limitations of conventional OCT methods regarding velocity sensitivity and Doppler angle.

    Main Methods:

    • Utilized number fluctuation dynamic light scattering with OCT.
    • Employed the second-order autocovariance function for analysis.
    • Developed a technique insensitive to particle diffusion for velocity estimation.
    • Demonstrated quantitative particle concentration determination.
    • Implemented 2D sub-diffusion flow imaging using a scanning OCT system and number fluctuation correlation analysis.

    Main Results:

    • Achieved a lower minimum measurable velocity compared to conventional correlation-based OCT and phase-resolved Doppler OCT.
    • Velocity estimation was not affected by particle diffusion, a key advantage.
    • The technique demonstrated independence from Doppler angle, similar to non-dilute correlation-based OCT.
    • Successfully quantified particle concentration in flowing suspensions.
    • Enabled high-rate 2D sub-diffusion flow imaging.

    Conclusions:

    • Number fluctuation dynamic light scattering OCT is a powerful tool for characterizing slow flows in dilute suspensions.
    • The method offers improved velocity sensitivity and broader applicability across Doppler angles.
    • This technique provides a new avenue for quantitative analysis of particle suspensions under flow.