29-06-2013, 03:44 PM
Computational Hydraulics
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Basic Concepts
Open Channel flows deal with flow of water in open channels
Pressure is atmospheric at the water surface and the
pressure is equal to the depth of water at any section
Pressure head is the ratio of pressure and the specific weight
of water
Elevation head or the datum head is the height of the
section under consideration above a datum
Velocity head (= v2/2g /2g) is due to the average velocity of flow
) in that vertical section
Conservation of Mass
In any control volume consisting of the fluid ( water) under
consideration, the net change of mass in the control volume
due to inflow and out flow is equal to the the net rate of
change of mass in the control volume
This leads to the classical continuity equation balancing the
inflow, out flow and the storage change in the control
volume.
Since we are considering only water which is treated as
incompressible, the density effect can be ignored
Conservation of Energy
Mainly in open channels the energy will be in the form of potent potential energy
ial and kinetic energy
Potential energy is due to the elevation of the water parcel whi while the
le kinetic energy is due to its movement
In the context of open channel flow the total energy due these f factors actors
between any two sections is conserved
This conservation of energy principle leads to the classical Ber Bernoulli noulli’s s
equation
When used between two sections this equation has to account for the
energy loss between the two sections which is due to the resista resistance to the
nce flow by the bed shear etc.
Rapidly Varied Flow
This flow has very pronounced curvature of the streamlines
It is such that pressure distribution cannot be assumed to
be hydrostatic
The rapid variation in flow regime often take place in short
span
When rapidly varied flow occurs in a sudden sudden-transition
structure, the physical characteristics of the flow are
basically fixed by the boundary geometry of the structure as
well as by the state of the flow
Examples:
Channel expansion and cannel contraction
Sharp crested weirs
Broad crested weirs