To calibrate sharp crested rectangular and triangular weirs/notches

Objective

Determination of discharge coefficients of

EquipementA constant steady water supply tank ( notch tank) with baffle walls, pointer gauge, collecting tank, models

Introduction and theory:Different type of models are available to find discharge in an open channel as notch, venturiflume, weir etc. For calibration of either rectangular notch or V notch some flow is allowed in the flume. Once the flow becomes steady and uniform, discharge coefficients can be determined for any model.

In general, sharp crested notches are preferred where highly accurate discharge measurements are required, for example in hydraulic laboratories, industry and irrigation pilot schemes which do no carry debris and sedimetns.

Notches are those overflow structures whose length of crest in the direction of flow is accurately shaped. They may be rectangular, trapezoidal, V notch etc. The V- notch is one of the most precise discharge measuring devices suitable for a wide range of flow. The relationship between discharge and head over the weir can be developed by making the following assumptions as to the flow behavior.

1. Upstream of the weir, the flow is uniform and the pressure varies with depth according to the hydrostatic equation P = ρ gh
2. The free surface remains horizontal as far as the plane of the weir, and all particles passing over the weir move horizontally.
3. The pressure through out the sheet of liquid or nappe, which passes over the crest of the weir, is atmospheric.
4. The effect of viscosity and surface tension are negligible.
5. The velocity in the approach channel is negligible.

A triangular or V notch is having a triangular or V shaped opening provided in its body so that water is discharged through this opening only. The line which bisects the angle of the notch should be vertical and at the same distance both sides of the channel. The discharge coefficient Cd of a V notch may be determined by applying formula

Cd = Q/8/15 √2gH^{5/2 tan θ/2}

where Q is the discharge over a triangular notch, θ is the apex angle of notch, H is the head over the crest of the notch.

A rectangular notch, symmetrically located in a vertical thin plate which is placed perpendicular to the sides and bottom of a straight channel, is defined as a rectangular sharp crested weir. The discharge coefficient Cd of a rectangular notch may be determined by applying formula

Cd = Q/2/3 √2gh BH^3/2

where Q is the discharge over a rectangular notch, B is the width of notch, H is the head over the crest of the notch, and g is acceleration due to gravity,

Experimental Set Up:The experimental set up consist of tank whose inlet section is provided with 2 nos. of baffles for stream line flow. While at the downstream portion of the tank one can fix a notch of either rectangular notch or v-notch. A pointer gauge is used to measure the head of water over the model. A collecting tank is used to find the actual discharge through the notch.

Experimental procedure:The notch under test was positioned at the end of the tank, in a vertical plane, and with the sharp edge on the upstream side. The tank was filled with water up to the crest level and subsequently note down the crest level of the notch by the help of a pointer gauge.

The flow regulating value was adjusted to give the max. possible discharge without flooding the notch. Conditions were allowed to steady before the rate of discharge and head H were recorded. The flow rate is reduced in stages and the readings of discharge and H were taken. The procedures is repeated for other type of notch.

Observation and Computation sheet:

1. Triangular or V notch
2. Aapex angle of notch , θ=
3. Crest level of notch H ₁ =
4. Area of collecting tank, a =

Average Cd

(b) Rectangular notch

Width of notch, B =

Crest levelof notch H ₁=

Area of collecting tank, a =