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Flume or Open channel
The flume is supplied with a 5.0 metre long working section.
When studying Hydraulics, the fundamental concepts of energy and momentum are sometimes difficult to grasp, particularly where free surface flow is concerned. The Armfield Multi Purpose Teaching Flume has been developed to assist students to overcome this difficulty. It provides a basic but nonetheless comprehensive facility for student experiments in open channel flow.
Although small in comparison with the majority of flumes, the dimensions of the working section have been sized so that the various phenomena may be clearly seen and reasonably accurate results may be obtained from measurements taken.
The flume includes a service module which stores water for recirculation making the unit self contained.
The flume comprises a rectangular section of channel with inlet and discharge tanks, which is supported by a pair of rigid pedestals. A service module incorporating a sump tank and submersible pump provides a source of water which is continuously re-circulated through the channel section making the unit self contained.
The working section of the channel, which is open at the top, consists of clear acrylic sides which are bonded to a bed fabricated from painted aluminium alloy. The end tanks are constructed from glass reinforced plastic (GRP) with a smooth gel coat on the inside. Water enters the parallel working section via an inlet tank. The water pipe entering the inlet tank has diffused outlets and is covered by a perforated plate and glass marbles. The sides of the inlet tank are profiled with a smooth contraction towards the working section. The combined action of these features is to reduce the turbulence of the water entering the inlet tank and to produce a smooth flow of water into the working section of the channel.
The level in the working section of the flume may be controlled by an overshot weir arrangement at the exit consisting of stop logs in a slot. Stop logs are simply added or taken away to provide the required depth of water in the working section. Water exiting from the channel enters the discharge tank where it returns by gravity to the service module.
The service module is constructed from GRP. Water is drawn from a sump tank in the base of the service unit by a submersible pump. The water is delivered to the flume via a shunt type flow-meter and flow control valve. The flow-meter consists of a variable area flow-meter which is shunted across an orifice plate. This provides a direct reading of the volume flow-rate of the water passing through the working section.
Water returns from the flume discharge tank to a moulded channel on the top of the service module. The water then flows over a weir carrier and into a volumetric tank before returning to the sump tank under gravity. The weir carrier and volumetric tank provide alternative means of measuring the flow of water through the flume. A vee or rectangular notch weir can be installed in the weir carrier and a hook and point gauge can be mounted over the moulded channel. The measurement of water depth upstream of the weir allows the flow-rate to be calculated. Alternatively, flow-rates up to approximately 1 litre/sec can be measured using the volumetric tank and a stopwatch. In normal operation the dump valve in the base of the volumetric tank should be open to allow the water to re-circulate. Above 1 litre/sec the base of the volumetric tank will remain flooded and turbulence will restrict the accuracy of measurements taken.
A pair of hook and point gauges can be located along the length of the channel, on the top of the channel sides, to enable the height of the water above the bed to be measured. A scale attached to the top of the front channel side indicates the position along the length of the channel.
The flume has a jacking arrangement which permits the slope of the channel bed to be adjusted. The jack is operated by a hand-wheel and the mechanism incorporates a slope indicator calibrated directly in units of % slope. For normal operation the slope should be set to 0% (bed of channel level). The bed of the working section incorporates tappings with isolating valves and model mounting points. These features are described later.
Length of working section : 5.0m
Width of working section : 76mm
Depth of working section : 250mm
Max positive bed slope : +3.0 %
Max negative bed slope : -1.0 %
Flow range (shunt gap meter) : 0.5 - 2.5 litres/sec
Accuracy of flow measurement : ± 2.5% F.S.D.
GENERAL ASSEMBLY INSTRUCTIONS FOR MODELS
The models which sit on the channel bed (with the exception of the Venturi flume) are held in place by a clamping hook assembly. These are pushed through the channel bed from the top and are held in place by a gland located beneath the channel bed. Normally the hooks are up-ended and the non-hook end is pushed from under the channel through the gland until the tip is flush with the bed so as not to impede the water flow. There are two hooks per 2.5 metre length of channel section. The bed-mounted models are hooked in place via a retaining bar on the underside of the model. The appropriate hook is pushed upwards to clear the channel bed. The required model is placed over the hook and then slid along until its retaining bar is beneath the hook. The hook is then pulled from beneath and clamped by tightening the gland. The model will be held in place until the gland is released and the hook raised.
The Venturi flume is held in place by a simple stretcher screw. This is placed between the two sections of the Venturi and adjusted to clamp them against the side walls of the channel.
Use of Stop Logs
Stop logs are simple rectangular shapes which slot one after the other into the discharge end of the channel. Their main use is to raise the water level to different heights. A further use includes a simple overshot weir of varying height.
Plasticine is also provided with the flume. This is used on the leading edge of a model and is placed between the side wall of the channel and the side of the model. This is to ensure that water flows over the model and not around it.
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Last Edited : 10 March 2015 13:32:38