Core assumptions behind Bernoulli’s equation (inviscid, steady, along a streamline) Which set of statements best reflects the assumptions under which Bernoulli’s equation applies?

Civil Engineering Hydraulics Difficulty: Easy
Choose an option
  • A
    Velocity distribution across any section is uniform (lumped kinetic energy)
  • B
    No external body force acts except gravity
  • C
    There is no energy loss due to viscosity, heat, or shaft work between sections
  • D
    All of the above
  • E
    Only (b) and (c)

Answer

Correct Answer: All of the above

Explanation

Introduction / Context:Bernoulli’s equation is derived from the mechanical energy balance for steady, incompressible, inviscid flow along a streamline with no pumps/turbines and only gravity as a body force. Recognizing its assumptions prevents misuse in diffusers, across valves, or in highly viscous flows.

Given Data / Assumptions:

  • Steady flow (no local acceleration).
  • Incompressible fluid of constant density.
  • Negligible viscosity (inviscid), hence no dissipation between sections.
  • Body force is gravity only; no shaft work added or removed.
  • Uniform section-wise velocity is an engineering approximation used when applying average velocity with a kinetic energy correction factor near 1.

Concept / Approach:

Bernoulli states p/γ + v^2/(2 g) + z = constant along a streamline for the above conditions. In practice, a kinetic energy correction factor may be used if velocity is non-uniform; the “uniform velocity” statement reflects that simplified use without correction factors.

Step-by-Step Solution:

Identify inviscid assumption ⇒ no head loss.Restrict body forces to gravity ⇒ potential head term z.Steady, incompressible ⇒ no density/time variations.Uniform velocity (or correction factor near unity) allows use of average velocity in v^2/(2 g).

Verification / Alternative check:

Compare with the full energy equation including head loss and pump/turbine head; Bernoulli is the special case with h_L = 0 and h_pump = h_turbine = 0.

Why Other Options Are Wrong:

Options (a), (b), and (c) individually are partial statements; the correct comprehensive assumption set is captured by “All of the above”.

Common Pitfalls:

Applying Bernoulli across components with significant losses or work exchange; ignoring the need for correction factors when velocity is highly non-uniform.

Final Answer:

All of the above

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