Water at a gauge pressure P₁ at street level (zero, reference height) flows into an office building through a pipe with the radius R₁ of the circular cross-section. The pipe tapers down to the radius R₂ of the circular cross-section by the top floor, located at the height h, where the faucet has been left open. Find the water speed V₂ in the pipe on the top floor in terms of P₁, R₁, R₂, h, water density ρ and g. Assume no branch pipes and ignore viscosity.
Sunday, November 27, 2016
Find the Water Speed V₂ in the Pipe on the Top Floor
Water at a gauge pressure P₁ at street level (zero, reference height) flows into an office building through a pipe with the radius R₁ of the circular cross-section. The pipe tapers down to the radius R₂ of the circular cross-section by the top floor, located at the height h, where the faucet has been left open. Find the water speed V₂ in the pipe on the top floor in terms of P₁, R₁, R₂, h, water density ρ and g. Assume no branch pipes and ignore viscosity.
t₀=0, x₀=0, v(x)=√(p+qx). x(t)=?
t₀=0
x₀=0
v(x)=√(p+qx)
x(t)=?
x₀=0
v(x)=√(p+qx)
x(t)=?
Calculus-based solution:
dx/dt=q⁰·⁵(p/q+x)⁰·⁵
(p/q+x)⁻⁰·⁵dx = q⁰·⁵dt
(p/q+x)⁻⁰·⁵d(p/q+x) = q⁰·⁵dt
2(p/q+x)⁰·⁵ = q⁰·⁵t + c
2(p/q+x)⁰·⁵ -2(p/q+x₀)⁰·⁵= q⁰·⁵(t-t₀)
2(p/q+x)⁰·⁵ -2(p/q)⁰·⁵= q⁰·⁵t
(p/q+x)⁰·⁵ -(p/q)⁰·⁵= ½q⁰·⁵t
(p/q+x)⁰·⁵ = ½q⁰·⁵t+(p/q)⁰·⁵
p/q+x = ¼qt² + q⁰·⁵t(p/q)⁰·⁵ + p/q
x = ¼qt² + q⁰·⁵t(p/q)⁰·⁵
x = p⁰·⁵t + ¼qt²
x = v₀ t + ½at²
v₀=p⁰·⁵
a=½q
v₀=p⁰·⁵
a=½q
Physics-based and algebra-based solution
If a=const then v²=v₀²+2a·Δx, so
v=√(v₀²+2a·Δx)
If a=const then v²=v₀²+2a·Δx, so
v=√(v₀²+2a·Δx)
As it is given:
v=√(p+qx)
so p=v₀², v₀=√p=p⁰·⁵
as x₀=0 then 2a·Δx=2ax
2ax=qx, 2a=q, a=½q
v=√(p+qx)
so p=v₀², v₀=√p=p⁰·⁵
as x₀=0 then 2a·Δx=2ax
2ax=qx, 2a=q, a=½q
as x = v₀ t + ½at²
then x = √p t + ¼qt²
then x = √p t + ¼qt²
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