2.1. Preamble: Phase Velocity and Energy of Linear Gravity Waves at the Free Surface of a Shear Flow of Constant Vorticity
The intrinsic phase velocity,
, of a linear gravity wave of wavenumber
k, propagating in finite depth,
h, at the free surface of a vertically sheared flow of constant intensity,
, is
where
g is the gravity.
Note that the vorticity is .
In infinite depth the intrinsic phase velocity is
There is no loss of generality if the study is restricted to waves propagating with positive phase velocities so long as both positive and negative values of are considered.
Figure 1 shows the dimensionless intrinsic phase velocity deviation between finite and infinite depths as a function of the dispersive parameter
for several dimensionless values of
. For
we can see that the deviation is weak. Generally, in laboratory experiments one considers that gravity waves of wavelength
behave like waves in infinite depth.
Note that Teles da Silva and Peregrine [
10] introduced a measure of water depth which influences the wave properties,
.
The condition for the presence of a critical layer in the water flow is
which only occurs for
. Teles da Silva and Peregrine [
10] have shown that if there is a critical layer it is always at a depth
. Consequently, one can expect that the critical depth will not be in the layer of water which influences the wave if
.
The expressions of the excess of kinetic energy
T and potential energy
V are given by
and
In
Figure 2 is shown the dimensionless excess of total energy deviation between finite and infinite depths of a linear gravity wave of wave steepness
for several dimensionless values of
. For
the deviation is weak.
Finally, linear gravity waves in finite depth propagating at the surface of a flow of constant vorticity behave nearly like waves in infinite depth if .
2.2. Miles Theory in the Presence of Constant Vorticity in Water
The inviscid governing equations of the flow in air and water are the following
with
and where
is the fluid velocity,
is the fluid density,
p is the pressure and
is the acceleration due to gravity.
Equation (
1) corresponds to mass conservation, Equation (
2) is the Euler equation and Equation (
3) means incompressible fluids.
We consider the linear stability of the following solution of the system of Equations (
1)–(
3) which corresponds to a flat air–water interface
where
corresponds to the velocity in the air and in the water,
corresponds to atmospheric density and water density and
and
are unit vectors in the
x-direction and
z-direction, respectively.
where
is the wind velocity and
the flow velocity in the water.
where
and
are the atmospheric density and water density, respectively.
Let us perturb the equilibrium given by Equations (
4)–(
7) with an infinitesimal perturbation
1Substituting the expressions (
8)–(
10) into Equations (
1) and (
2) and linearising gives
where
is the vertical component of the velocity perturbation.
The solutions of the linearized problem are sought in the following form (normal modes)
where
k and
are the wavenumber and frequency of the perturbation, respectively.
Substituting the expressions (
14)–(
17) into the linearized equations gives the following Sturm–Liouville problem
where
,
and
The water flow is assumed to be vertically sheared with constant vorticity:
, where the shear
and
are constant. Without loss of generality we consider a frame of reference in which
. Hence, Equation (
18) reads
Let us assume
, then
First to avoid the existence of a critical layer in water, the study in deep water is restricted to waves with positive phase velocity and positive values of
(negative vorticity). Consequently,
and (
19) reads
The case corresponding to
(positive vorticity) will be discussed later in
Section 2.3.
Equation (
20) can be transformed to a reduced form
with the following change of variables
that is
The reduced form of (
20) is
Hence the general solution of Equation (
20) is
The solution satisfying the condition
as
is
Equation (
18) is integrated between two points below (
) and above (
) the air–water interface
with
where
is the Dirac delta function.
where
due to continuity of
.
Because we consider linear waves, without loss of generality we can set
The linear dispersion relation of gravity water waves on deep water in the presence of constant vorticity
is obtained from Equation (
23) when wind effect is ignored.
Let
and
the Taylor series in
in the presence of wind. Substituting the expansion of
c into Equation (
23) gives
Following Janssen [
8] and Thomas [
9], Equation (
18), in the atmospheric medium, is reduced to the following form
where
.
The growth rate
of wave amplitude is
where
and
denotes imaginary part.
Thomas [
9] has shown that
where
is the Wronskian given by
and
denotes the complex conjugate.
Let
be the normalised vertical component of air velocity. Then, Equation (
24) becomes the following Rayleigh equation
The Rayleigh equation has a singular point where the phase velocity, , of the waves equals the mean wind velocity . Consequently, the height, , of the critical layer in the atmosphere satisfies .
The growth rate can be rewritten as a function of the Wronskian of the solutions of the Rayleigh equation
One can show that
. Consequently, the Wronskian is constant for
and
as well and may show a jump
, with
, at the critical height. Due to the boundary condition at infinity,
as
,
. Finaly, the jump is equals to
and is given by the following expression
with
The result is
where
,
and
.
The expression of the Wronskien is
The normalised growth rate of surface wave amplitude is
Equation (
28) can be written differently
We assume the conservation of the momentum flux in the atmospheric boundary layer. Consequently, the wind profile is given by the folowing logarithmic law
where
is the friction velocity,
is the von Karman constant and
is the roughness length of the air–water interface given by the Charnock relation
.
Within the framework of a logarithmic law we obtain
To derive the expression of the growth rate of the wave amplitude as a function of the wave age
we use as reference velocity
and reference length
. Let
,
,
and
be the dimensionless variables and parameters. Note that
.
Using
the dimensionless growth rate is rewritten as follows
where
is the wave age and
the dimensionless vorticity.
Note that .
Using
, Equation (
29) reads
The dimensionless amplitude growth rate depends only on the wave age and vorticity.
The Rayleigh Equation (
26) is written in dimensionless form
where
The dimensionless unknown
is computed numerically by solving Equation (
34) with the method of [
11]. The dimensionless growth rate of the wave amplitude
is calculated once the critical value of
is known.
To check the validity of our approach we have compared our results in the absence of vorticity (
) with those of Beji and Nadaoka [
5], Stiassnie et al. [
6] and Kommen et al. [
12].
Figure 3 shows the dimensionless growth rate,
, defined by Miles [
1] as a function of the wave age
where
is the growth rate of the wave energy.
The agreement of our results with those of Beji and Nadaoka [
5] who used a different method is excellent.
Figure 4 displays the dimensionless growth rate of wave energy as a function of the inverse of wave age in the absence of vorticity. The agreement of our results with those of Stiassnie et al. [
6] is good whereas some deviation can be observed with those of Kommen et al. [
12].
Figure 5 shows the dimensionless growth rate of the wave amplitude as a function of the wave age for different values of the dimensionless vorticity. We can see that the growth rate of waves generated at the surface of a vertically sheared flow of constant negative vorticity decreases as the intensity
increases and vanishes when a limit to the wave energy growth is reached. The wave age corresponding to
can be determined easily as follows.
The dispersion relation is
As emphasized above we consider wind waves with
and
. Consequently,
Note that the limit wave age could be derived from Equation (
31). This theoretical result suggests the existence of fetch limited wind wave growth in the presence of vertically sheared flows of constant vorticity. As the waves become increasingly longer, the growth rate decreases and vanishes.
The role of shear flows, in water of infinite depth, on the behaviour of wind waves is comparable to the role of finite depth without vorticity. Young and Verhagen [
13] conducted field experiments showing depth limited wind wave growth. Later, Montalvo et al. [
14] found numerically that finite depth limited growth is reached with wave growth rates going to zero (see
Figure 1 of [
14]).