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Selected Topic: Open Channel Flow

Selected Topic: Open Channel Flow. Reading: Munson, et al., Chapter 10. Introduction. Open channel flow - flow of a liquid in a channel (or conduit) that is not completely filled Main driving force is gravity; pressure force is only hydrostatic Applications river/stream flow

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Selected Topic: Open Channel Flow

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  1. Selected Topic: Open Channel Flow Reading: Munson, et al., Chapter 10 CE 150

  2. Introduction • Open channel flow - flow of a liquid in a channel (or conduit) that is not completely filled • Main driving force is gravity; pressure force is only hydrostatic • Applications • river/stream flow • flow in canals, ditches, gutters, sewers • ocean waves, tidal effects • roadway, parking lot runoff CE 150

  3. General Characteristics • Free surface represents an additional degree of freedom (and complexity) compared to pipe flows • New parameter: fluid depth (y) which may vary with time (t) and distance along channel (x) • Classifications based on depth • Uniform flow (UF): constant depth, or dy/dx = 0 • Gradually varying flow (GVF), dy/dx << 1 • Rapidly varying flow (RVF), dy/dx  1 CE 150

  4. General Characteristics • Classification based on flow condition • Laminar: Re  500 • Transitional: 500  Re  12,500 • Turbulent: Re  12,500 • where CE 150

  5. Surface Waves • Waves can occur when the channel boundaries are suddenly perturbed • Waves travel due to hydrostatic pressures differences • Small amplitude wave speed • Solitary waves: • Sinusoidal waves: CE 150

  6. Froude Number • Froude number is the ratio of fluid speed to wave speed: • Froude number determines the surface characteristics of open channel flow • Subcritical flow: Fr < 1 • Critical flow: Fr = 1 • Supercritical flow: Fr > 1 CE 150

  7. Channel Depth for GVF • Channel depth variation for steady, gradually varying flow (GVF): • If Sf > S0 and Fr < 1, channel depth will increase • If Sf < S0 and Fr < 1, channel depth will decrease • See Table 10.2 for all possibilities and classifications CE 150

  8. Manning Equation • For uniform channel flow (UF), the average velocity is given by the Manning equation: • where  = unit conversion constant n = Manning resistance coefficient (see Table 10.1) CE 150

  9. Hydraulic Jump • The hydraulic jump is an example of a rapidly varying flow (RVF), where a shallow high-speed flow suddenly changes to a deeper, low-speed flow • The jump occurs due to a conflict between upstream influences (supercritical flow, Fr > 1) and downstream influences (subcritical flow, Fr < 1) • The energy loss of a hydraulic jump is useful in spillway design CE 150

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