When there is no air left blow the nappe, it is known as
Adhering nappe
Understanding Nappe Types Over a Weir
When water flows over a weir, the sheet of water that falls is called a nappe. The behavior and shape of this nappe depend significantly on the pressure and presence of air underneath it, between the nappe and the downstream face of the weir or the crest.
Different Types of Nappe
Based on the conditions below the falling water sheet, the nappe can be classified into several types:
- Free Nappe: This occurs when there is free access of air to the space between the nappe and the weir crest/downstream face. The pressure below the nappe is atmospheric. The nappe separates completely from the weir face.
- Depressed Nappe: If the air supply below the nappe is restricted, the pressure in the space below drops below atmospheric pressure (becomes negative pressure or vacuum). This causes the nappe to be drawn downwards or depressed.
- Adhering (or Clinging) Nappe: This is an extreme case of restricted air supply. When there is virtually no air left below the nappe, the vacuum becomes strong enough that the nappe adheres or clings tightly to the downstream face of the weir. There is no air cushion between the nappe and the weir face.
Analysis of the Given Condition
The question specifically asks about the situation when "there is no air left below the nappe". Looking at the definitions above:
- A Free nappe has atmospheric pressure and plenty of air below.
- A Depressed nappe has below-atmospheric pressure due to restricted air, but usually some air is still present, causing the nappe to drop but not necessarily adhere completely.
- An Adhering nappe is characterized by the absence of air below, causing the nappe to stick to the weir face due to the significant vacuum (pressure much lower than atmospheric).
Therefore, the condition of having "no air left below the nappe" directly corresponds to the definition of an Adhering nappe.
Considering the options provided:
- Free nappe - Incorrect, as this has air below.
- Depressed nappe - Incorrect, as this usually implies restricted air, not complete absence leading to adherence.
- Adhering nappe - Correct, as this describes the state when there is no air below and the nappe clings to the weir.
- All options are correct - Incorrect, as only one specific condition is described.
Thus, when there is no air left below the nappe, it is known as an Adhering nappe.
Important Questions from Open Channel Flow
- A trapezoidal channel carries $5\text{ m}^3\text{.s}^{-1}$ of water under uniform flow condition. The channel has a bottom width of $2\text{ m}$, side slope of $2:1$ (horizontal: vertical), and a bed slope of $1\%$. If the Manning roughness coefficient is $0.03$, the conveyance of the channel, in $\text{m}^3\text{.s}^{-1}$, is
- Two rectangular channels, A and B, join to form a large rectangular channel, C. Each rectangular channel is lined with the same material and has the same bottom slope. The bottom widths of Channels A and B are $2\text{ m}$ and $1\text{ m}$, respectively. If the flow depth in each channel is $2\text{ m}$, the bottom width of Channel C (in m) is ________. (Rounded off to two decimal places)
- Water is flowing at the rate of 15 m$^3$/s through a rectangular channel of 5 m width.
If the acceleration due to gravity ($g$) is 9.81 m/s$^2$, the critical velocity for the flow is ______ m/s (rounded off to three decimal places). A Hydraulic Jump is said to be Oscillating Jump if its Froude number is ________.
The Francis formula for the discharge over cippoletti weir is
