A closed container with volume V receives two input flows with velocities w1 ja w2. There is also an outlet flow with velocity w3. Which of the following equations would you use to solve the rate of change of water level in the container?

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Question restatement: A closed container with volume V receives two input flows with velocities w1 and w2. There is also an outlet flow with velocity w3. Which of the following equations would you use to solve the rate of change of water level in the container?

Option a: continuity equation
Reasoning: The continuity equation expresses conservation of mass in a control volume. It directly relates the net volumetric flow rate into the container to the rate of change of the fluid volume inside the container. If the inlets have cross-sectional areas A1 and A2 and the outlet has area A3, the rate of change of volume dV/dt equals A1 w1 + A2 w2 − A3 w3. Since the water level height is tied to the volume in a fixed container, this equation is the appropriate starting point for determining dh/dt (the rate of height change).

Option b: Bernoulli's equation
Reasoning: Bernoulli's equation relates pressure, velocity, and elevation along a streamline for incompressible, inviscid flow in steady conditions. It is not directly used to compute the time rate of change of the fluid height in a closed container, especially when the question concerns mass balance and transient storage rather than energy per se. Therefore, Bernoulli's equation is not the suitable tool for this specific rate-of-change calculation.

Option c: energy equation
Reasoning: The energy equation (or first law of thermodynamics for a control volume) accounts for energy balances, including kinetic, potential, and internal energy changes. While it is fundamental in many fluid problems, it does not directly yield the rate of change of liquid height from inlet and outlet flows unless you are explicitly tracking energy exchanges or turbine/pump work. For the objective stated—rate of change of water level—the mass balance via continuity is more direct.

Option d: linear momentum equation
Reasoning: The linear momentum equation (or momentum balance) is useful for analyzing forces and momentum fluxes on a control volume and can be engaged when solving for pressure distributions, forces on boundaries, or motion of the fluid as a whole. However, to obtain the rate of change of water level from inflows and outflows in a simple storage tank, the momentum equation is not the direct or simplest path; it introduces unnecessary complexity for this specific quantity unless coupled with additional force analyses.

Option e: Euler's equation
Reasoning: Euler's equation describes the motion of an inviscid fluid in a control volume, relating temporal and convective changes in velocity to pressure gradients. While related to momentum and flow dynamics, it is not the primary equation for determining the rate of change of stored fluid height in a tank via mass conservation. It’s not the most straightforward tool for this problem, making it an unsuitable choice for the asked quantity.

BH40A1401 Fluid Mechanics I - Contact teaching, Lappeenranta 6.1.2025-23.2.2025 Quiz 2

A closed container with volume V receives two input flows with velocities w1 ja w2. There is also an outlet flow with velocity w3. Which of the following equations would you use to solve the rate of change of water level in the container?

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