5.11 Recommendations for Current Influence Estimation
247
the presence of a current ("effective fetch") is decreased with wave development. In this case the wind speed value relative to the water surface becomes
decisive.
The wavelength dependence XjX0 for different values .:Y as a function of
the non-dimensional fetch X is shown in Fig. 5.39b. The character of this
dependence is similar to the height ratio li/fi0 . The wavelength is increased in
a countercurrent, while it is decreased in a fair current, whereas XjX0 ~ hjh0 .
The wave period is similarly influenced by the current (see Fig. 5.39c,d).
However, it is interesting to note that the mean wave period measured in the
movable coordinate system (see Fig. 5.39c), for example, on a ship drifting
with a current, is significantly less influenced by it (as long as ft/70 ~ ~)
in comparison with measurements in the immovable coordinate system (see
Fig. 5.39d), for example, made on a stationary platform.
The influence of current on waves is increased with increasing parameter
of the relative current velocity .:Y. It is necessary to point out that the current
velocity value V is not so important, as its value compared to the wind
speed U.
5.11 Some Practical Recommendations for Estimation
of Current Influence on Waves
Difference of the current influence on wave generation and transformation.
There is a necessity to estimate wind wave development in
the presence of a current. However the accurate corresponding practical recommendations are still absent (Building Standards and Rules, 1983). In this
case numerical models can be applied.
The results of this monograph can be used to obtain an estimation of current influence on waves. In order to do so, the nature of the aforementioned
prevailing mechanisms should be determined, whether it is a wave transformation or wave generation in a current. In the case of wave generation the
height and length are increased by the countercurrent due to increasing relative wind speed and wave interaction time with air flow at a fixed fetch. But,
in the other case, the wavelength is diminished with waves propagating in
a countercurrent being increased along the stream direction (see Sect. 5.4).
This means that the waves become "compressed" by length at a practically
constant mean wave period measured in the immovable coordinate system.
The processes of wave generation and transformation in a current are
interconnected in natural conditions. It should be noted that the separation
of these two mechanisms is rather conventional. Nevertheless, an attempt
should be undertaken to estimate the contribution of these two mechanisms
to the source function in the spectral density balance equation (5.1).
The wind energy input Gin is estimated by the formula (4.34) according
to the Miles mechanism (1960). The wind input occurs mainly in the highfrequency spectral range. Its energy is partially lost due to wave breaking
247
the presence of a current ("effective fetch") is decreased with wave development. In this case the wind speed value relative to the water surface becomes
decisive.
The wavelength dependence XjX0 for different values .:Y as a function of
the non-dimensional fetch X is shown in Fig. 5.39b. The character of this
dependence is similar to the height ratio li/fi0 . The wavelength is increased in
a countercurrent, while it is decreased in a fair current, whereas XjX0 ~ hjh0 .
The wave period is similarly influenced by the current (see Fig. 5.39c,d).
However, it is interesting to note that the mean wave period measured in the
movable coordinate system (see Fig. 5.39c), for example, on a ship drifting
with a current, is significantly less influenced by it (as long as ft/70 ~ ~)
in comparison with measurements in the immovable coordinate system (see
Fig. 5.39d), for example, made on a stationary platform.
The influence of current on waves is increased with increasing parameter
of the relative current velocity .:Y. It is necessary to point out that the current
velocity value V is not so important, as its value compared to the wind
speed U.
5.11 Some Practical Recommendations for Estimation
of Current Influence on Waves
Difference of the current influence on wave generation and transformation.
There is a necessity to estimate wind wave development in
the presence of a current. However the accurate corresponding practical recommendations are still absent (Building Standards and Rules, 1983). In this
case numerical models can be applied.
The results of this monograph can be used to obtain an estimation of current influence on waves. In order to do so, the nature of the aforementioned
prevailing mechanisms should be determined, whether it is a wave transformation or wave generation in a current. In the case of wave generation the
height and length are increased by the countercurrent due to increasing relative wind speed and wave interaction time with air flow at a fixed fetch. But,
in the other case, the wavelength is diminished with waves propagating in
a countercurrent being increased along the stream direction (see Sect. 5.4).
This means that the waves become "compressed" by length at a practically
constant mean wave period measured in the immovable coordinate system.
The processes of wave generation and transformation in a current are
interconnected in natural conditions. It should be noted that the separation
of these two mechanisms is rather conventional. Nevertheless, an attempt
should be undertaken to estimate the contribution of these two mechanisms
to the source function in the spectral density balance equation (5.1).
The wind energy input Gin is estimated by the formula (4.34) according
to the Miles mechanism (1960). The wind input occurs mainly in the highfrequency spectral range. Its energy is partially lost due to wave breaking