Difference between revisions of "Radiation stress"
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==Notes== | ==Notes== | ||
− | The fact that spatial gradients in wave energy can induce a net force on the water body was first recognised by Longuet-Higgins and Stewart (1962<ref>Longuet-Higgins, M.S. and Stewart, R.W. 1962. Radiation stress and mass transport in gravity waves, with application to 'surf beats'. Journal of Fluid Mechanics 13: 481–504</ref>, 1964<ref>Longuet-Higgins, M.S. and Stewart, R.W. 1964. Radiation stresses in water waves; a physical discussion, with applications. Deep Sea Research 11: 529–562</ref>). The radiation stress is the momentum transferred through the water body per unit time (the flux of momentum) by wave orbital motion and by the dynamic pressure (the deviation from hydrostatic pressure) | + | The fact that spatial gradients in wave energy can induce a net force on the water body was first recognised by Longuet-Higgins and Stewart (1962<ref>Longuet-Higgins, M.S. and Stewart, R.W. 1962. Radiation stress and mass transport in gravity waves, with application to 'surf beats'. Journal of Fluid Mechanics 13: 481–504</ref>, 1964<ref>Longuet-Higgins, M.S. and Stewart, R.W. 1964. Radiation stresses in water waves; a physical discussion, with applications. Deep Sea Research 11: 529–562</ref>). The radiation stress is the momentum transferred through the water body per unit time (the flux of momentum) by horizontal wave orbital motion and by the dynamic pressure (the deviation from hydrostatic pressure) are given by (see [[Shallow-water wave theory#Radiation Stress (Momentum Flux)]]) |
+ | |||
+ | <math>S_{XX} =\Bigl\langle \int _{-h}^{\eta } (p+\rho u^{2} )dz \Bigr\rangle - \int _{-h}^{0} p_0 dz , \qquad S_{YY} =\Bigl\langle \int _{-h}^{\eta }(p+\rho v^{2} )dz \Bigr\rangle -\int _{-h}^{0}p_0 dz , \qquad S_{XY} =\Bigl\langle \int _{-h}^{\eta } \rho u v dz \Bigr\rangle , \qquad (1)</math> | ||
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+ | where <math>z=</math> vertical coordinate, <math>h=</math> water depth, <math>\eta =</math> wave surface elevation, <math>\rho = </math> water density, <math>p =</math> pressure, <math>p_0 =</math> hydrostatic pressure, <math>u=</math> horizontal orbital velocity in <math>x</math>-direction, <math>v=</math> horizontal orbital velocity in <math>y</math>-direction, and where <math> \bigl\langle … \bigr\rangle </math> represents the average over the wave period. | ||
For obliquely incident waves, cross-shore momentum is transferred by both cross-shore wave orbital motion and longshore wave orbital motion and longshore momentum is transferred by both longshore wave orbital motion and cross-shore wave orbital motion. For obliquely incident waves, a cross-shore gradient in the wave orbital motion, for example due to wave breaking, will exert a stress on the water mass in cross-shore direction as well as in longshore direction. A gradient in the stress in cross-shore direction generates a [[Wave set-up|water level set-up]] at the coast and a gradient in the stress in longshore direction generates a [[longshore current]]. Forcing by radiation stress gradients related to wave breaking is commonly an order of magnitude greater than forcing due to wind stress or other wave nonlinearities. | For obliquely incident waves, cross-shore momentum is transferred by both cross-shore wave orbital motion and longshore wave orbital motion and longshore momentum is transferred by both longshore wave orbital motion and cross-shore wave orbital motion. For obliquely incident waves, a cross-shore gradient in the wave orbital motion, for example due to wave breaking, will exert a stress on the water mass in cross-shore direction as well as in longshore direction. A gradient in the stress in cross-shore direction generates a [[Wave set-up|water level set-up]] at the coast and a gradient in the stress in longshore direction generates a [[longshore current]]. Forcing by radiation stress gradients related to wave breaking is commonly an order of magnitude greater than forcing due to wind stress or other wave nonlinearities. | ||
− | + | Approximate analytical expressions of the radiation stresses can be obtained from linear wave theory. Assuming a uniform wave field propagating with an angle <math>\theta</math> to the cross-shore <math>x</math>-axis the expressions are | |
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+ | <math>S_{XX} \approx \Big(n(1+\cos^2 \theta) - \large\frac{1}{2}\normalsize \Big) E , \qquad S_{YY} \approx \Big(n(1+\sin^2 \theta) - \large\frac{1}{2}\normalsize \Big) E , \qquad S_{XY} \approx nE \sin \theta \cos \theta , \qquad (2) </math> | ||
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+ | where <math>E = \large\frac{1}{8}\normalsize \rho g H^2 =</math> wave energy, <math>H=</math> wave height, <math>k=</math> wave number, <math>g=</math> gravitational acceleration, and <math>n = \large\frac{1}{2}\normalsize + \Large\frac{kh}{\sinh (2kh)}\normalsize =</math> ratio of group celerity and wave celerity. | ||
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+ | These expressions underestimate the radiation stresses when waves become skewed and asymmetric while propagating into shallow water - see [[Shallow-water wave theory#Finite amplitude waves]]. For example, calculations with 5th order nonlinear Stokes theory give a 17% higher value of <math>S_{XX}</math> in the case of very steep waves<ref>Gao, X., Ma, X., Li, P., Yuan, F., Wu, Y. and Dong, G. 2023. Nonlinear analytical solution for radiation stress of higher-order Stokes waves on a flat bottom. Ocean Engineering 286 (2023) 115622</ref>. | ||
For irrotational periodic (regular) gravity waves an exact expression of <math>S_{XX}</math> is given by<ref>Longuet-Higgins, M.S. 1975. Integral properties of periodic gravity waves of finite amplitude. Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences 342 (1629): 157–174</ref> | For irrotational periodic (regular) gravity waves an exact expression of <math>S_{XX}</math> is given by<ref>Longuet-Higgins, M.S. 1975. Integral properties of periodic gravity waves of finite amplitude. Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences 342 (1629): 157–174</ref> | ||
− | <math>S_{XX} = 4 E_k - 3 E_p + \rho h \bigl\langle u_b^2 \bigr\rangle , \quad E_k = | + | <math>S_{XX} = 4 E_k - 3 E_p + \rho h \bigl\langle u_b^2 \bigr\rangle , \quad E_k = \large\frac{1}{2}\normalsize \rho \Bigl\langle \int_{-h}^{\eta} (u^2+w^2)dz \Bigr\rangle , \quad E_p = \large\frac{1}{2}\normalsize \rho g \bigl\langle \eta^2 \bigr\rangle , </math> |
− | where <math>u=</math> cross-shore wave orbital velocity, where <math>u_b=</math> cross-shore wave orbital velocity at the bottom, <math>w=</math> vertical wave orbital velocity | + | where <math>u=</math> cross-shore wave orbital velocity, where <math>u_b=</math> cross-shore wave orbital velocity at the bottom, <math>w=</math> vertical wave orbital velocity. |
Revision as of 15:56, 20 April 2024
Definition of Radiation stress:
Radiation stress is the flux of momentum carried by ocean waves.
This is the common definition for Radiation stress, other definitions can be discussed in the article
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Notes
The fact that spatial gradients in wave energy can induce a net force on the water body was first recognised by Longuet-Higgins and Stewart (1962[1], 1964[2]). The radiation stress is the momentum transferred through the water body per unit time (the flux of momentum) by horizontal wave orbital motion and by the dynamic pressure (the deviation from hydrostatic pressure) are given by (see Shallow-water wave theory#Radiation Stress (Momentum Flux))
[math]S_{XX} =\Bigl\langle \int _{-h}^{\eta } (p+\rho u^{2} )dz \Bigr\rangle - \int _{-h}^{0} p_0 dz , \qquad S_{YY} =\Bigl\langle \int _{-h}^{\eta }(p+\rho v^{2} )dz \Bigr\rangle -\int _{-h}^{0}p_0 dz , \qquad S_{XY} =\Bigl\langle \int _{-h}^{\eta } \rho u v dz \Bigr\rangle , \qquad (1)[/math]
where [math]z=[/math] vertical coordinate, [math]h=[/math] water depth, [math]\eta =[/math] wave surface elevation, [math]\rho = [/math] water density, [math]p =[/math] pressure, [math]p_0 =[/math] hydrostatic pressure, [math]u=[/math] horizontal orbital velocity in [math]x[/math]-direction, [math]v=[/math] horizontal orbital velocity in [math]y[/math]-direction, and where [math] \bigl\langle … \bigr\rangle [/math] represents the average over the wave period.
For obliquely incident waves, cross-shore momentum is transferred by both cross-shore wave orbital motion and longshore wave orbital motion and longshore momentum is transferred by both longshore wave orbital motion and cross-shore wave orbital motion. For obliquely incident waves, a cross-shore gradient in the wave orbital motion, for example due to wave breaking, will exert a stress on the water mass in cross-shore direction as well as in longshore direction. A gradient in the stress in cross-shore direction generates a water level set-up at the coast and a gradient in the stress in longshore direction generates a longshore current. Forcing by radiation stress gradients related to wave breaking is commonly an order of magnitude greater than forcing due to wind stress or other wave nonlinearities.
Approximate analytical expressions of the radiation stresses can be obtained from linear wave theory. Assuming a uniform wave field propagating with an angle [math]\theta[/math] to the cross-shore [math]x[/math]-axis the expressions are
[math]S_{XX} \approx \Big(n(1+\cos^2 \theta) - \large\frac{1}{2}\normalsize \Big) E , \qquad S_{YY} \approx \Big(n(1+\sin^2 \theta) - \large\frac{1}{2}\normalsize \Big) E , \qquad S_{XY} \approx nE \sin \theta \cos \theta , \qquad (2) [/math]
where [math]E = \large\frac{1}{8}\normalsize \rho g H^2 =[/math] wave energy, [math]H=[/math] wave height, [math]k=[/math] wave number, [math]g=[/math] gravitational acceleration, and [math]n = \large\frac{1}{2}\normalsize + \Large\frac{kh}{\sinh (2kh)}\normalsize =[/math] ratio of group celerity and wave celerity.
These expressions underestimate the radiation stresses when waves become skewed and asymmetric while propagating into shallow water - see Shallow-water wave theory#Finite amplitude waves. For example, calculations with 5th order nonlinear Stokes theory give a 17% higher value of [math]S_{XX}[/math] in the case of very steep waves[3].
For irrotational periodic (regular) gravity waves an exact expression of [math]S_{XX}[/math] is given by[4]
[math]S_{XX} = 4 E_k - 3 E_p + \rho h \bigl\langle u_b^2 \bigr\rangle , \quad E_k = \large\frac{1}{2}\normalsize \rho \Bigl\langle \int_{-h}^{\eta} (u^2+w^2)dz \Bigr\rangle , \quad E_p = \large\frac{1}{2}\normalsize \rho g \bigl\langle \eta^2 \bigr\rangle , [/math]
where [math]u=[/math] cross-shore wave orbital velocity, where [math]u_b=[/math] cross-shore wave orbital velocity at the bottom, [math]w=[/math] vertical wave orbital velocity.
References
- ↑ Longuet-Higgins, M.S. and Stewart, R.W. 1962. Radiation stress and mass transport in gravity waves, with application to 'surf beats'. Journal of Fluid Mechanics 13: 481–504
- ↑ Longuet-Higgins, M.S. and Stewart, R.W. 1964. Radiation stresses in water waves; a physical discussion, with applications. Deep Sea Research 11: 529–562
- ↑ Gao, X., Ma, X., Li, P., Yuan, F., Wu, Y. and Dong, G. 2023. Nonlinear analytical solution for radiation stress of higher-order Stokes waves on a flat bottom. Ocean Engineering 286 (2023) 115622
- ↑ Longuet-Higgins, M.S. 1975. Integral properties of periodic gravity waves of finite amplitude. Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences 342 (1629): 157–174