Open Channel Flow Report Lab

 PART  PART A : THE HYDRAULIC JUMPS JUMPS

1.0

INTRODUCTION

The concept of the hydraulic jump when the hydraulic drop that occurs at a sudden drop in the bottom of a channel, and the free surface flow around obstructions like  bridge piers. A hydraulic jump forms when a supercritical flow changes into a subcritical flow flow.. The The chan change ge in the the flow flow regi regime me occur occurss with with a sudde sudden n rise rise in wate waterr surf surfac ace. e. Considerable turbulence, energy loss and air entrainment are produced in the hydraulic  jump. A hydraulic is used for mixing chemicals in water supply systems, for dissipating energy energy below below artifi artificia ciall channel channel controls controls,, and as an aerati aeration on device device to increa increase se the dissolved oxygen in water. In a hydraulic jump there occurs a sudden change in liuid depth from less! thancri thancritic tical al to greate greater! r!tha than!c n!crit ritica icall depth. depth. The veloci velocity ty of the flow flow changes changes from from supercritical to subcritical as a result of the jump. This transition takes place over a relatively short distance, usually less than " times the depth of flow after the jump, over  which the height of the liuid increase rapidly, incurring a considerable loss of energy. An example of a hydraulic jump can be observed when a jet of water from a faucet strikes the hori#ontal surface of the kitchen sink. The water flows rapidly outward and a circular   jump occurs. $e shall restrict the derivation of the basic euation of the hydraulic jump to rectangular hori#ontal channels. %irst, we shall determine the downstream depth of the  jump by using the momentum and continuity euations for one!dimensional flow. Then the energy loss due to the jump will be evaluated, using the energy euation.

2.0

OBJECTIVE

To investigate the characteristic a standing wave &the hydraulic jump' produced when waters beneath an undershot weir and to observe the flow patterns obtained.

4.0

THEORY

$hen water flowing rapidly changes to slower tranuil flow, a hydraulic jump or  standing wave is produced. This phenomenon can be seen where water shooting under a sluice gate mixes with deeper water downstream. It occurs when a depth less than critical changes to a depth which is greater than critical and must be accompanied by loss of  energy. An undular jump occurs when the change in depth is small. The surface of the water undulates in a series of oscillations, which gradually decay to a region of smooth tranuil flow. A direct jump occurs when the change in depth is great. The large amount of energy loss produces a #one of extremely turbulent water before it settles to smooth tranuil flow.

(y considering the forces acting with the fluid on either side of a hydraulic jump of unit width it can be shown that ) 2

∆*

+

d a  

va  ! 2g

da ¿

2



vb 2g

'

$here -* is the total head loss across jump &energy dissipated' &m'. v a  is the mean velocity before jump &ms', d a is the depth of flow before hydraulic jump &m'. v b is the mean velocity after hydraulic jump &m' and d b is the depth of flow after hhydraulic jump &m'. (ecause the working section is short, d  a  / d b and d b / d0. Therefore, simplifying the above euation, d a  ! d 1 '10  4 d 1 d 3 . ∆* ≈ &

5.0

EQUIPMENT

2' 3elf! contained 4lass 3ided Tilting %lume.

5' Adjustable 6ndershot $eir Tilting %lume. 0' Instrument Carrier.

%igure 0) Control 7anel

8' *ook and point gauge.

6.0

PROCEDURES

2. 9nsure the flume is level, with the downstream tilting overshot weir, E at the bottom of  its travel. :easure and record the actual breadth b &m' of the undershot weir. Install the undershot weir towards the inlet end of the flume and ensure that it is securely clamped in  position. 5. Adjust the undershot weir to position the sharp edge of the weir 5;mm above the bed of  the channel. Increase the height of the tilting overshot weir until the downstream level  just stars to rise. 0. 4radually open the flow control valve and adjust the flow until an undular jump is created with small ripple decaying towards the discharge end of the working section.