Venturi Flume — Typical Range of Coefficient of Discharge (Cd) For a properly calibrated Venturi flume operating under free-flow (submergence not excessive), the overall coefficient generally lies in which range?

Introduction:
Venturi flumes (including Parshall and other throat-control flumes) are open-channel primary devices that accelerate flow to near-critical conditions at a throat. Their rating curves are usually expressed as Q = C * H^n, where C embeds geometry and a discharge coefficient. Because losses are small and flow passes at atmospheric pressure, the overall coefficient often approaches unity, higher than for sharp-crested weirs or orifices.

Given Data / Assumptions:

• Properly constructed and installed Venturi/Parshall-type flume.
• Free-flow (not excessively submerged); head measured at the prescribed gage point.
• Reynolds number sufficiently high; approach is tranquil and well-conditioned with minimal sediment/debris.

Concept / Approach:
In a flume, energy loss between approach and throat is modest because there is no nappe contraction or enclosed jet as in weirs/orifices. The calibration therefore yields coefficients close to 1.00. Most practical tables show Cd clustered from about 0.97 to 1.00 depending on size and approach conditions.

Step-by-Step Solution:
Recognize that flumes operate under atmospheric pressure with streamlined contractions/expansions.Small head losses imply Cd near unity in rating equation Q = Cd * f(geometry, H).Hence the representative range is 0.95–1.00.

Verification / Alternative check:
Standard manuals list calibration constants that effectively correspond to Cd ≈ 1.0 for free flow. Submergence corrections reduce effective discharge, but under recommended submergence ratios the coefficient remains high.

Why Other Options Are Wrong:
0.30–0.45 and 0.50–0.75 are typical of highly lossy set-ups or sharp contractions, not Venturi flumes. 0.75–0.95 is closer but still low for well-designed free-flow flumes.

Common Pitfalls:
Applying free-flow calibration when submergence is significant; ignoring sediment build-up that alters throat geometry; measuring head at the wrong gage point.

Final Answer:
0.95 to 1.00