Determination of Coefficient of Discharge of a Rectangular & Triangular sharp crested weir

Objective:

Objective of the experiment is to determine the (i) Coefficient of discharge of a triangular & a rectangular sharp crested weir. (ii) Calibration curve of height of upstream free surface above the crest of the weir & actual flow rate & (iii) Variation of coefficient of discharge with height of upstream free surface above the crest of the weir. 

Introduction:

A weir is an obstruction on a channel bottom over which the fluid must flow. It provides a convenient method of determining the flow rate in an open channel in terms of a single depth measurement. A sharp-crested weir is essentially a vertical sharp-edged flat plate placed across the channel in a way such that the fluid must flow across the sharp edge and drop into the pool downstream of the weir plate, as is shown in Fig. The specific shape of the flow area in the plane of the weir plate is used to designate the type of weir. Typical shapes include the rectangular weir, the triangular weir, and the trapezoidal weir, as indicated in Fig.

 

Apparatus used:

  • A channel with sharp crested rectangular weir.
  • Stop watch & collecting tank
  • Point gauge

Procedure:

  • The tank dimensions were measured.
  • The flow in the channel having rectangular notch was started.
  • The flow was kept constant.
  • The head of water in piezometer of constant time interval for collecting tank was noted
  • Open slightly the valve without increase the rotation suddenly after fixed time interval
  • Also note the height of upstream free surface above the crest of the weir.
  • Calculate the actual flow rate by 

Area of tank (A) = ______ × _______ = cm2

Time required to collect water to a depth ΔH = Δt = constant = _________ sec.

Observation Table 1: Rectangular weir

Width of rectangular Notch (b) = _______ cm

Observation Table 2: Triangular weir

Angle of triangular Notch (θ) = _______ degree

  • Draw the following curves for each of the weirs
  • Calibration curve of height(H) of upstream free surface above the crest of the weir & actual flow rate(Q) &
  • Variation of coefficient of discharge (Cd) with height (H) of upstream free surface above the crest of the weir.

Safety:

  • Reading is to be taken when the flow through the channel is almost steady.
  • Readings are to be taken carefully so that chances of errors (like parallax error, human error during stop watch reading etc.) are minimized.

 

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