- #1
danhall24
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Hi, I am a postgraduate environmental science student (NOT a mathematician!) struggling through some necessary maths. Any help with the following (which I suspect will be relatively straight foward) would be very much appreciated. Please ask questions if I have not made myself clear.
Given the dispersion relationship and intrinsic frequency of a 2-d internal gravity wave (in the horizontal (x dimension) and vertical (z dimension)), show that / explain why the group velocity vector is parallel to lines of constant phase and hence perpendicular to the phase speed. Note that this isn't a question I have been set, it is simply something I am struggling with from a textbook.
(I have checked and rechecked the equations below - they are definitely exactly the same as in the textbook.)
Dispersion relationship:
(ω - uk)2 (k2 + m2) - N2k2 = 0
where ω is frequency, u is flow speed in the x direction (which is constant, i.e. does not vary in the z direction), k is wavenumber in the x direction, m is wavenumber in the z direction and N is a constant.
Intrinsic frequency, v:
v = ω - uk = Nk / (k2 + m2)1/2
Horizontal phase speed, cx:
cx = v / k
Vertical phase speed, cz:
cz = v / m
Horizontal group velocity, cgx:
cgx = ∂v/∂k = u + (Nm2) / (k2 + m2) 3/2
Vertical group velocity, cgz:
cgz = ∂v/∂m = -Nkm / (k2 + m2) 3/2
In the textbook it simply says "it is easily shown from [the group velocity equations] that the group velocity vector is parallel to lines of constant phase."
It may be easy, but not for me. Any help much appreciated.
Homework Statement
Given the dispersion relationship and intrinsic frequency of a 2-d internal gravity wave (in the horizontal (x dimension) and vertical (z dimension)), show that / explain why the group velocity vector is parallel to lines of constant phase and hence perpendicular to the phase speed. Note that this isn't a question I have been set, it is simply something I am struggling with from a textbook.
(I have checked and rechecked the equations below - they are definitely exactly the same as in the textbook.)
Homework Equations
Dispersion relationship:
(ω - uk)2 (k2 + m2) - N2k2 = 0
where ω is frequency, u is flow speed in the x direction (which is constant, i.e. does not vary in the z direction), k is wavenumber in the x direction, m is wavenumber in the z direction and N is a constant.
Intrinsic frequency, v:
v = ω - uk = Nk / (k2 + m2)1/2
Horizontal phase speed, cx:
cx = v / k
Vertical phase speed, cz:
cz = v / m
Horizontal group velocity, cgx:
cgx = ∂v/∂k = u + (Nm2) / (k2 + m2) 3/2
Vertical group velocity, cgz:
cgz = ∂v/∂m = -Nkm / (k2 + m2) 3/2
In the textbook it simply says "it is easily shown from [the group velocity equations] that the group velocity vector is parallel to lines of constant phase."
It may be easy, but not for me. Any help much appreciated.