Print this datasheet and bring a copy to the lab.
Familiarize yourself with the procedure and theory below.
The purpose of this lab is to demonstrate flow in an open channel with no obstruction. The flow parameters and the properties and nature of the resulting hydraulic jump will be investigated.
Given a steady state flow in a smooth straight channel with a known flow velocity and depth of flow, we can calculate the change in water level (due to a hydraulic jump) as follows:
where:
y2 is the depth of flow after the jump (m)
y1 is the depth of flow prior to the jump (m)
Fr is the Froude Number for the flow (unitless)
The Froude number is related to the ratio of a fluid flow velocity, and the channel in which it flows through or around. It is a measure of flow resistance. Froude numbers allow for comparisons to be made between very different obstacles (dams, weirs, bridge supports etc...) in terms of the effect they will have on the flow around them. This may allow for the calculation of head rise/loss, flow velocity, and mass flow rate changes, forces exerted on submerged surfaces and other parameters that may be of interest to engineers designing submerged infrastructure.
The Froude number is calculated as:
Where:
Fr is the Froude Number for the flow (unitless)
V1 is the velocity of flow prior to the jump (m/s)
hm is the mean hydraulic depth (m) given by the CSA of the flow (prior to jump) / Chanel Width
g is the gravitational acceleration constant: 9.81 (m/s2)
A large Froude number (greater than 1.0) implies a supercritical flow, one where the flow is fast, well-defined, and moves parallel to the channel. This requires the least height to maintain steady-state flow.
A small Froude number (less than 1.0) implies a subcritical flow, one where the flow is slow and non-linear. This requires a greater height to maintain steady-state flow.
The transitional state where the Froude number is very close to 1.0 is referred to as critical flow and occurs immediately at the preceding edge of the hydraulic jump.
*** ENSURE THAT THE OPEN END OF THE TROUGH IS OVER THE WATER BARREL AT ALL TIMES, AVOID MOVING THE TROUGH ***
Measure the cross-sectional area (height and width) of the water outlet channel (the narrow slit near the hose attachment).
Ensure the trough is positioned to empty into the barrel.
Turn on the pump using the switch on the front of the lab bench, ensure that water flows through the trough.
If no water flows after a minute try turning the pump off/on again. Call the instructor in case of any difficulty.
Start the pDaq Flow-Meter software (if not already running) to measure the flow through the trough inlet.
Using the globe valve with the red handle, adjust the flow until the hydraulic jump is centred at the first clear crossbar (labeled "1") near the top.
Measure the distance from the outlet to the crossbar.
Record the flow reported by the flow meter.
Record the height of the water before and after the jump.
Repeat for the second crossbar (labeled "2").
To simulate a "Tidal Bore" as you might see several places in the Valley:
Turn the globe valve to reduce the flow, and watch as the wavefront travels in the opposite direction as the fluid flow.
Turn off the pump when you are finished with your data collection.
What flow rate positioned the wave at the first crossbar and at the second?
What was the velocity of the water as it exits the slot at the head of the trough for each of the two flows?
Note that for the two results below, large error margins are typical due to the non-ideality of the physical setup. The importance of the results is in generating a general idea of the resulting jump and flow parameters... knowing to within 5% is often not practical or useful when large factors of safety will be incorporated anyway. The theoretical to practical comparison only helps show that the results are in the right magnitude for the most part.
Consider the similar case of Reynolds numbers, where we are trying to identify which "regime" the flow pattern is in, and not a precise numerical result.
Our goal in the results section is to identify whether we are observing Subcritical, near critical or supercritical flow.
What was the measured height of the jump?
Compare this to theoretical calculation of the expected hydraulic jump.
What is the energy loss at the jump?
What is the calculated energy loss at the jump?
Describe (qualitatively) the following properties before and after the jump:
Fluid Velocity
Fluid Height
Type of Flow (Laminar, turbulent..etc)
Determine one other method that may be used to find the flow rate is the pDaq meters weren't available (procedure, not calculation).
Example: If you were testing a large channel exiting a dam or a river.
Which of the two methods do you think is more accurate?
Why might the turbulent transition area be a concern in an area like the tidal streams/rivers n the Annapolis Valley?