CFX Simulation of a Free Surface Water Chanel Flow over a...
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CFX Simulation of a Free Surface Water Chanel Flow over a Step
Colin Hartloper
December 3, 2010
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Presentation Outline
• Problem Background
• Theory
• Geometry and Meshing
• Boundary Conditions
• Results
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Problem Background
• VLHT – Low head turbine (less than 3.2m)
• CPL wants to install in North America, but has concerns.
• One such concern is the effect of an upstream geometry on the turbine performance.
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Free Surface Flow
• “Free Surface” refers to atm. pressure at the water surface
• Model water flowing in an open channel rather than through a duct
• Requires two materials in simulation– Air and water
• Requires slightly more sophisticated boundary conditions
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Governing Equations
Conservation of Mass
• Incompressible, steady state
• Two equations (one for water, one for air)
Conservation of Momentum
• Incompressible, steady state
• Three equations (x, y, and z momentum)
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Turbulence Model
• Proper modeling of turbulence requires very fine mesh– Very small relative length scale
• Therefore, use of k-epsilon model– k is magnitude of energy
– ε is length scale
– Use because it is useful for relatively small pressure gradients
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Critical Flow (theoretical verification)
• Governed by Froude number
• Fr < 1 is sub-critical, Fr > 1 is super-critical– Analogous to sub/supersonic gas flow
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Geometry – Side View
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Element Types
• Tetrahedral Element– Suitable for modeling general fluid flow
• Wedge Element (inflation)– Suitable for modeling fluid flow close to a wall,
where a boundary layer exists
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Meshing – Coarse
13812 Nodes, 55960 Elements
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Meshing – Coarse with Inflation
22818 Nodes, 74039 Elements
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Meshing – Fine with Inflation
27864 Nodes, 96271 Elements
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Boundary Conditions
• “Infinately wide channel”– Actual model thickness of 30mm
• Free surface conditions
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Results – Boundary Layer
Coarse Coarse with Inflation
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Velocity Profile Location
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Velocity Profile – Convergence?
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Swirling
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Validation – Froude Number
• For flow over step, Fr = 0.24 (sub-critical)
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Comparison to Super-Critical
• For flow over bump, Fr = 1.33 (super-critical)
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Questions?