Hydraulics Exam 2
Control point
Hydraulic structures and large changes in channel slop are sometimes called a ___________, where the flow characteristics change rapidly
Opening and closing valves, turning on pump
Hydraulic systems often experience sudden changes in flows when
Froude Number
A dimensionless number that represent the importance of the inertial forces to the field (gravity) forces in a flow field
Water hammer
Abrupt changes in dynamic closed conduit hydraulic systems create pressure waves that are known as
Compressibility of water and the pipe
Affects how quickly waves associated with water hammer are dissipated
Slows
As the compressibility of a fluid increases, the wave speed
Protect foundation lower water level
Bridge protection
Along the wetted perimeter
By what mechanism is energy dissipated in open channel flows
Intake of pump
Cavitation is likely to occur near the
Vapor pressure
Cavitation occurs when the absolute pressure in a hydraulic system drops below the _________________ of water
Trapezoids
Commonly employed in large-scale engineered channels
Along channel perimeter
Energy losses in open channel flow occurs
Static head
Energy needed to lift the water
Rivers, canals, sewer, treatment plants
Examples of open channel flow systems
Slope
Factors influencing open channel flow that can NOT be controlled is
Geometry and material
Factors influencing open channel flow that can be controlled are
Passes through critical depth
Flow measurement
Shape that has smallest perimeter for given area
For a given slope and material, what characteristics of the geometry will produce the biggest velocity
DeltaH(Qo + DeltaQ) ~ (2)(ki)(Qo)(DeltaQ) + (ki)(Qo^2)
Formula to approximate the headloss at Qo + DeltaQ based on a linear approximation of the function starting from a flow of Qo
Sub-critical
Fr < 1
Super-critical
Fr > 1
Dynamic head
Frictional losses that depend on flow rate
Geometry
In closed conduits, this is fixed In open channels, this varies in the direction of flow
Pressure
In closed conduits, this varies moving in the direction of flow. In open channels, this is fixed.
Maximum hydraulic radius
In rigid channels, the goal of the design is to maximize the velocity that is accomplished by
Downstream to upstream
In sub-critical flows, a disturbance in the flow goes from
Upstream to downstream
In super-critical flows, a disturbance in the flow goes from
Raise water level to ensure availablility
Irrigation
Runout
Maximum flow rate that can be delivered to a pump is its
Shut off head
Maximum head that can be supplied by a pump is known as the
Net positive suction head (NPSH)
Minimum head above the vapor pressure required to prevent cavitation within a given pump
Specific energy
Open channel flows are analyzed in terms of the
Gravity
Open channel flows are driven by
Turbulent, because big.
Open channel flows are primarily ___________. Why?
Centrifugal pumps
Operate at high pressures and low flows, most efficient kinds of pumps
Critical Depth
Over the top of the weir, the flow will pass through its
Head, power, and efficiency as a function of the flow rate
Pump curves typically depict
Energy
Pumps provide a mechanism to move water uphill by adding
Free board
Safety factor added to the minimum depth to ensure that the flow does not exceed the channel banks
Surge tanks
Structures that are open to the atmosphere that are placed near control points in hydraulic systems to reduce sudden changes in pressure due to valve closures
Bulk Modulus of Elasticity
The compressibility of a fluid is characterized by its
Critical depth
The depth associated with the minimum specific energy for a given discharge rate in a channel is known as
Normal depth
The depth associated with uniform, steady flows
Rated capacity
The flow rate corresponding to the point of highest efficiency is known as a pump's
Depth
The hydraulic radius of a very wide channel is best approximated by the
Surge tanks and closing the valve slowly
The two principal ways to mitigate the effects of water hammer:
Sound
The waves propagate through hydraulic systems at the speed of
Conservation of energy
Used to solve for flows in a network around loops
Conservation of mass
Used to solve for flows in a network at junctions
Fracture pipe walls and cause system failures
Water hammer can
Head (energy)
Water moves from a state of higher to lower
Axial flow pumps
Water on the suction side of the pump moves faster and experience lower pressures than on the discharge side, less efficient and provide less power
Flow, head average, capital cost of pump, operating costs for energy
What are major considerations that affect pump selection
Water temperature, elevation, height of pump above water, strainers
What are some factors that affect the propensity of cavitation for a hydraulic system
Semi-circle (complete)
What cross-section demonstrates flow with the greatest hydraulic efficiency
Normal depth
What do we get from Manning's equation
All energy losses are along the perimeter
What does it mean for the flow to vary "gradually"
Force gets big
What happens when velocity changes over a very short period of time
Pipe characteristics
When 1/Eb << Dk/Ep*e, the _________________ controls the wave speed
Fluid characteristics
When 1/Eb >> Dk/Ep*e, the _______________ controls the wave speed
Drawn down
When the flow transitions from sub-critical to super-critical this occurs
Hydraulic jump
When the flow transitions from super-critical to sub-critical this occurs
Flooding, erosion, structures
Why do engineers modify channel characteristics?
Energy losses inside the pump increase as flow/velocity increase
Why does the pump head curve slope downward