Question Details

In a Lagrangian system, the position of a fluid particle in a flow is described as π‘₯ = π‘₯π‘œπ‘’ βˆ’π‘˜π‘‘ and 𝑦 = π‘¦π‘œπ‘’π‘˜π‘‘ where t is the time while π‘₯π‘œ, π‘¦π‘œ, and k are constants. The flow is

Options

A

unsteady and one-dimensional

B

steady and two-dimensional

C

steady and one-dimensional

D

unsteady and two-dimensional

Show Answer

Correct Answer :

Option B

steady and two-dimensional

Solution :

The correct answer is steady and two-dimensional.

To determine the nature of the flow, we need to find the velocity components of the fluid particle from its given position coordinates in the Lagrangian system and express them in terms of the spatial coordinates (Eulerian system).

The position of the fluid particle is given by:
x=x0e-kt
and
y=y0ekt

First, we find the velocity component in the x-direction, u, by differentiating x with respect to time t:
u=dxdt=-kx0e-kt

Substituting x=x0e-kt into the equation for u gives:
u=-kx

Next, we find the velocity component in the y-direction, v, by differentiating y with respect to time t:
v=dydt=ky0ekt

Substituting y=y0ekt into the equation for v gives:
v=ky

Now, let's analyze the velocity field:
1. Dimension of the flow: Since the velocity components exist in both the x and y directions (i.e., both u and v are non-zero and depend on the spatial coordinates x and y), the flow is two-dimensional.
2. Steadiness of the flow: A flow is steady if the velocity at any point does not change with time. The Eulerian velocity field components are:
u=-kx
and
v=ky
Since neither u nor v contains the time variable t explicitly, we have:
βˆ‚uβˆ‚t=0
and
βˆ‚vβˆ‚t=0
Thus, the velocity at any spatial position remains constant over time, making the flow steady.

Combining these two characteristics, the flow is steady and two-dimensional.

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