Fluid mechanics - Linearized shallow water equations

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kmot
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Hi,

In a text describing solution to linearized shallow water equations, I am not able to move forward.

It's a 1 dimensional shallow water setup. There is a steady state
Small image showing an overline bar above the letter U, a mathematical notation for a mean or average value.
(velocity) and
Small black letter h in a sans-serif font on a white background, with a thin black line along the top of the letter.
(height of free surface). On top of this steady state there are u' and h' as disturbances. The goal is to solve for h'(x,t)

Diagram of a one-dimensional free surface: a wave-shaped surface profile with a horizontal mean level h-bar, a labeled height disturbance h' and velocity disturbance u', and a mean flow arrow u-bar to the right.


The governing equations are:
(1)
Linearized momentum equation: u'_t + ū u'_x = −g (δρ/ρ₁) h'_x, with primes marking disturbances and subscripts denoting partial derivatives.
(momentum)
(2)
Linearized equation h'_t + ū h'_x + u'_x h̄ = 0, with primes on h and u, overbars on the steady-state values ū and h̄, and subscripts denoting partial derivatives.
(mass continuity)

Subscripts denote partial derivatives.

The text says these two can be combined to arrive at wave equation, without providing further details of how:
(3)
Equation: (∂/∂t + ū ∂/∂x)² h′ − (g h̄ δρ / ρ₁) h′ₓₓ = 0


But with anything I tried I wasn't able to eliminate u' completely. I'm always left with a term including u' in (3). Perhaps the term that can't be eliminated can be neglected, but I don't see why.

Would somebody be willing to provide a step by step guide how to go from (1) & (2) to (3), including reasoning of why something can be neglected (if neglecting is needed)?

Thank you very much
 
Last edited:
I found the solution. Here are the intermediate steps
Handwritten derivation on lined paper of a linearized shallow water wave equation, combining u and h equations and rearranging into (D/Dt)^2 h - g h̄ (δρ/ρ1) h_xx = 0, with the operator form (∂/∂t + ū ∂/∂x)^2 h - g h̄ (δρ/ρ1) h_xx = 0.
 
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