Difference between revisions of "Wave Forces on a Body"

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== Wave Forces on a Body ==
 
== Wave Forces on a Body ==
  
<center><math> U = \omega A \, </math></center>
+
{| border="0" align="center"
 
+
| <math> U = \omega A \, </math>
 
+
|-
<center><math> R_e = \frac{U\ell}{\nu} = \frac{\omega A \ell}{\nu} \, </math></center>
+
| <math> R_e = \frac{U\ell}{\nu} = \frac{\omega A \ell}{\nu} \, </math>
 
+
|-
 
+
| <math> K_C = \frac{UT}{\ell} = \frac{A\omega T}{\ell} = 2 \pi \frac{A}{\ell} \, </math>
<center><math> K_C = \frac{UT}{\ell} = \frac{A\omega T}{\ell} = 2 \pi \frac{A}{\ell} \, </math></center>
+
|}
 
 
  
 
{| border="0" align="center"
 
{| border="0" align="center"
 
|- align="center"
 
|- align="center"
| <math> D_F = \frac{F}{\rho g A \ell^2} = f \left( \frac{}{} \right. </math>
+
| <math> C_F = \frac{F}{\rho g A \ell^2} = f \left( \frac{}{} \right. </math>
 
| <math> \underbrace{\frac{A}{\lambda}} \,</math>,
 
| <math> \underbrace{\frac{A}{\lambda}} \,</math>,
 
| <math> \underbrace{\frac{\ell}{\lambda}} \, </math>,
 
| <math> \underbrace{\frac{\ell}{\lambda}} \, </math>,
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* ''Friction drag'' <math> ( C_F ) \, </math>
 
* ''Friction drag'' <math> ( C_F ) \, </math>
  
Associated with skin friction <math> \tau_w, \ i.e., \ \vec{F} \sim \iint_{\mbox{body (wetted surface)}} \tau_w dS \, </math>.
+
{| border="0"
 +
| Associated with skin friction <math> \tau_w, \ i.e., \ \, </math>
 +
| <math> \vec{F} \sim \iint \tau_w \, </math>
 +
| <math> dS \, </math>.
 +
|- align="center"
 +
|
 +
| body
 +
|- align="center"
 +
|
 +
| (wetted surface)
 +
|}
  
 
2. '''Inertial forces''' Froude-Krylov forces, diffraction forces, radiation forces.
 
2. '''Inertial forces''' Froude-Krylov forces, diffraction forces, radiation forces.
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Forces arising from potential flow wave theory,
 
Forces arising from potential flow wave theory,
  
<center><math> \vec{F} = \iint_{body (wetted surface)} p \hat{n} dS, \ \, </math> where <math> \ p = - \rho \left( \frac{\partial\phi}{\partial t} + g y + \frac{1}{2} \left| \nabla \phi \right|^2 \right) </math></center>
+
{| border="0" align="center"
 +
| <math> \vec{F} = \iint p \hat{n} \, </math>
 +
| <math> dS \, </math>,
 +
| where <math> \ p = - \rho \left( \frac{\partial\phi}{\partial t} + g y \right. \, </math>
 +
| <math> + \left. \underbrace{ \frac{1}{2} \left| \nabla \phi \right|^2} \right) \, </math>
 +
|- align="center"
 +
| body ||  ||  || <math> =0 \, </math> , for linear theory,
 +
|- align="center"
 +
| (wetted surface) ||  ||  || small amplitude waves
 +
|}
  
 
For linear theory, the velocity potential <math> \phi \, </math> and the pressure <math> p \, </math> can be decomposed to
 
For linear theory, the velocity potential <math> \phi \, </math> and the pressure <math> p \, </math> can be decomposed to
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| <math> \phi = \, </math> || <math> \underbrace{\phi_I} \, </math> || <math> + \, </math> || <math> \underbrace{\phi_D} \, </math> || <math> + \, </math> || <math> \underbrace{\phi_R} \, </math>
 
| <math> \phi = \, </math> || <math> \underbrace{\phi_I} \, </math> || <math> + \, </math> || <math> \underbrace{\phi_D} \, </math> || <math> + \, </math> || <math> \underbrace{\phi_R} \, </math>
 
|- align="center"
 
|- align="center"
|  || Incident wave ||  || Diffracted wave ||  || Radiated wave
+
|  || Incident wave ||  || Scattered wave ||  || Radiated wave
 
|- align="center"
 
|- align="center"
 
|  || potential <math> (a) \, </math> ||  || potential <math> (b.1) \, </math> ||  || potential <math> (b.2) \, </math>
 
|  || potential <math> (a) \, </math> ||  || potential <math> (b.1) \, </math> ||  || potential <math> (b.2) \, </math>
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| <math> \left. \begin{matrix} & \phi \approx \phi_I \\ & p \approx - \rho \left( \frac{\partial\phi_I}{\partial t} + g y \right) \end{matrix} \right\} </math>
 
| <math> \left. \begin{matrix} & \phi \approx \phi_I \\ & p \approx - \rho \left( \frac{\partial\phi_I}{\partial t} + g y \right) \end{matrix} \right\} </math>
 
| <math> \Rightarrow \vec{F}_{FK} = \, </math>
 
| <math> \Rightarrow \vec{F}_{FK} = \, </math>
| <math> \iint \,</math>
+
| <math> \iint \underbrace{- \rho \left( \frac{\partial\phi_I}{\partial t} + g y \right)} </math>
| <math> \underbrace{- \rho \left( \frac{\partial\phi_I}{\partial t} + g y \right)} </math>
 
 
| <math> \hat{n} dS \leftarrow \, </math>
 
| <math> \hat{n} dS \leftarrow \, </math>
 
| width="225" | can calculate knowing (incident) <br> wave kinematics (and body geometry)
 
| width="225" | can calculate knowing (incident) <br> wave kinematics (and body geometry)
 +
|-
 +
|
 +
|
 +
| body <math> . \qquad \equiv p_I \, </math>
 +
|-
 +
|
 +
|
 +
| width="25" | surface
 +
|}
 +
 +
* Mathematical approximation After applying the divergence theorem, the <math> \vec{F}_{FK} \, </math> can be rewritten as
 +
{| border'"0" align="center"
 +
| <math> \vec{F}_{FK} \, </math>
 +
| <math>  = - \iint p_I \hat{n} \, </math>
 +
| <math> dS = - \iiint \nabla p_I d\forall \, </math>
 
|- align="center"
 
|- align="center"
|  ||
+
|
| width="25" | body surface
+
| body <br> surface
| <math> \equiv p_I \, </math>
+
| body <br> volume
 
|}
 
|}
 
* Mathematical approximation After applying the divergence theorem, the <math> \vec{F}_{FK} \, </math> can be rewritten as <math> \vec{F}_{FK} = - \iint_{\mbox{body surface}} p_I \hat{n} dS = -
 
\iiint_{\mbox{body volume}} \nabla p_I d\forall </math>.
 
  
 
If the body dimensions are very small comparable to the wave length, we can assume that <math> \nabla_{p_I} \, </math> is approximately constant through the body volume <math> \forall \, </math> and 'pull' the <math> \nabla_{p_I} \, </math> out of the integral. Thus, the <math> \vec{F}_{FK} \, </math> can be approximated as
 
If the body dimensions are very small comparable to the wave length, we can assume that <math> \nabla_{p_I} \, </math> is approximately constant through the body volume <math> \forall \, </math> and 'pull' the <math> \nabla_{p_I} \, </math> out of the integral. Thus, the <math> \vec{F}_{FK} \, </math> can be approximated as
  
<center><math> \vec{F}_{FK} \cong \left( - \nabla_{p_I} \right) \left| {}_{\mbox{at body center}} \iiint_{\mbox{body volume}} d\forall = \underbrace{\forall}_{\mbox{body volume}} \left( - \nabla_{p_I} \right) \right| {}_{\mbox{at body center}} </math></center>
+
{| border="0" align="center"
 +
| <math> \vec{F}_{FK} \cong \left( - \nabla_{p_I} \right) \left. \frac{}{} \right| \, </math>
 +
| valign="bottom" align="center" | at body
 +
| <math> \iiint d\forall = \, </math>
 +
| <math> \underbrace{\forall} \, </math>
 +
| <math> \left( - \nabla_{p_I} \right) \left. \frac{}{} \right| \, </math>
 +
| valign="bottom" | at body
 +
|-
 +
|
 +
| align="right" valign="top" | center
 +
| body <br> volume
 +
| body <br> volume
 +
|
 +
| align="right" valign="top" | center
 +
|}
  
 
The last relation is particularly useful for small bodies of non-trivial geometry for 13.021, that is all bodies that do not have a rectangular cross section.
 
The last relation is particularly useful for small bodies of non-trivial geometry for 13.021, that is all bodies that do not have a rectangular cross section.
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* If <math> \frac{\ell}{\lambda} \gg 1/5 \, </math>, must consider wave diffraction, radiation <math> \left( \frac{A}{\ell} \leq \frac{0.07}{\ell / \lambda} \leq 0.035 \right) \, </math>.
 
* If <math> \frac{\ell}{\lambda} \gg 1/5 \, </math>, must consider wave diffraction, radiation <math> \left( \frac{A}{\ell} \leq \frac{0.07}{\ell / \lambda} \leq 0.035 \right) \, </math>.
  
* If <math> K_C \gg 1 \, </math>: separation important, viscous forces can not be neglected. Further on if <math> \frac{\ell}{lambda} \leq \frac{0.07}{A/\ell} \, </math> so <math>
+
* If <math> K_C \gg 1 \, </math>: separation important, viscous forces can not be neglected. Further on if <math> \frac{\ell}{\lambda} \leq \frac{0.07}{A/\ell} \, </math> so <math>
 
\frac{\ell}{\lambda} \ll 1 \, </math> ignore diffraction, i.e., the Froude-Krylov approximation is valid.
 
\frac{\ell}{\lambda} \ll 1 \, </math> ignore diffraction, i.e., the Froude-Krylov approximation is valid.
  
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III. <math> C D \, </math> is not important and <math> F - K \, </math> approximation is not valid.
 
III. <math> C D \, </math> is not important and <math> F - K \, </math> approximation is not valid.
 +
 +
 +
-----
 +
 +
This article is based on the MIT open course notes and the original article can be found [http://ocw.mit.edu/NR/rdonlyres/Mechanical-Engineering/2-20Spring-2005/E5599747-6768-4532-9601-8E4297E8DAC2/0/lecture22.pdf here].
 +
 +
[[Marine Hydrodynamics]]

Latest revision as of 09:46, 1 July 2009

Wave Forces on a Body

[math]\displaystyle{ U = \omega A \, }[/math]
[math]\displaystyle{ R_e = \frac{U\ell}{\nu} = \frac{\omega A \ell}{\nu} \, }[/math]
[math]\displaystyle{ K_C = \frac{UT}{\ell} = \frac{A\omega T}{\ell} = 2 \pi \frac{A}{\ell} \, }[/math]
[math]\displaystyle{ C_F = \frac{F}{\rho g A \ell^2} = f \left( \frac{}{} \right. }[/math] [math]\displaystyle{ \underbrace{\frac{A}{\lambda}} \, }[/math], [math]\displaystyle{ \underbrace{\frac{\ell}{\lambda}} \, }[/math], [math]\displaystyle{ R_e \, }[/math], [math]\displaystyle{ \frac{h}{\lambda} \, }[/math], roughness, [math]\displaystyle{ \ldots \left. \frac{}{} \right) \, }[/math]
Wave Diffraction
steepness parameter

Type of Forces

1. Viscous forces Form drag, viscous drag [math]\displaystyle{ = f ( R_e, K_c, \, }[/math] roughness, [math]\displaystyle{ \ldots ) }[/math].

  • Form drag [math]\displaystyle{ ( C_D ) \, }[/math]

Associated primarily with flow separation -normal stresses.

  • Friction drag [math]\displaystyle{ ( C_F ) \, }[/math]
Associated with skin friction [math]\displaystyle{ \tau_w, \ i.e., \ \, }[/math] [math]\displaystyle{ \vec{F} \sim \iint \tau_w \, }[/math] [math]\displaystyle{ dS \, }[/math].
body
(wetted surface)

2. Inertial forces Froude-Krylov forces, diffraction forces, radiation forces.

Forces arising from potential flow wave theory,

[math]\displaystyle{ \vec{F} = \iint p \hat{n} \, }[/math] [math]\displaystyle{ dS \, }[/math], where [math]\displaystyle{ \ p = - \rho \left( \frac{\partial\phi}{\partial t} + g y \right. \, }[/math] [math]\displaystyle{ + \left. \underbrace{ \frac{1}{2} \left| \nabla \phi \right|^2} \right) \, }[/math]
body [math]\displaystyle{ =0 \, }[/math] , for linear theory,
(wetted surface) small amplitude waves

For linear theory, the velocity potential [math]\displaystyle{ \phi \, }[/math] and the pressure [math]\displaystyle{ p \, }[/math] can be decomposed to

[math]\displaystyle{ \phi = \, }[/math] [math]\displaystyle{ \underbrace{\phi_I} \, }[/math] [math]\displaystyle{ + \, }[/math] [math]\displaystyle{ \underbrace{\phi_D} \, }[/math] [math]\displaystyle{ + \, }[/math] [math]\displaystyle{ \underbrace{\phi_R} \, }[/math]
Incident wave Scattered wave Radiated wave
potential [math]\displaystyle{ (a) \, }[/math] potential [math]\displaystyle{ (b.1) \, }[/math] potential [math]\displaystyle{ (b.2) \, }[/math]
[math]\displaystyle{ - \frac{p}{\rho} = \, }[/math] [math]\displaystyle{ \frac{\partial\phi_I}{\partial t} \, }[/math] [math]\displaystyle{ + \, }[/math] [math]\displaystyle{ \frac{\partial\phi_D}{\partial t} \, }[/math] [math]\displaystyle{ + \, }[/math] [math]\displaystyle{ \frac{\partial\phi_R}{\partial t} \, }[/math] [math]\displaystyle{ + \, }[/math] [math]\displaystyle{ g y \, }[/math]

(a) Incident wave potential

  • Froude-Krylov Force approximation When [math]\displaystyle{ \ell \ll \lambda \, }[/math], the incident wave field is not significantly modified by the presence of the body, therefore ignore [math]\displaystyle{ \phi_D \, }[/math] and [math]\displaystyle{ \phi_R \, }[/math]. Froude-Krylov approximation:
[math]\displaystyle{ \left. \begin{matrix} & \phi \approx \phi_I \\ & p \approx - \rho \left( \frac{\partial\phi_I}{\partial t} + g y \right) \end{matrix} \right\} }[/math] [math]\displaystyle{ \Rightarrow \vec{F}_{FK} = \, }[/math] [math]\displaystyle{ \iint \underbrace{- \rho \left( \frac{\partial\phi_I}{\partial t} + g y \right)} }[/math] [math]\displaystyle{ \hat{n} dS \leftarrow \, }[/math] can calculate knowing (incident)
wave kinematics (and body geometry)
body [math]\displaystyle{ . \qquad \equiv p_I \, }[/math]
surface
  • Mathematical approximation After applying the divergence theorem, the [math]\displaystyle{ \vec{F}_{FK} \, }[/math] can be rewritten as
[math]\displaystyle{ \vec{F}_{FK} \, }[/math] [math]\displaystyle{ = - \iint p_I \hat{n} \, }[/math] [math]\displaystyle{ dS = - \iiint \nabla p_I d\forall \, }[/math]
body
surface
body
volume

If the body dimensions are very small comparable to the wave length, we can assume that [math]\displaystyle{ \nabla_{p_I} \, }[/math] is approximately constant through the body volume [math]\displaystyle{ \forall \, }[/math] and 'pull' the [math]\displaystyle{ \nabla_{p_I} \, }[/math] out of the integral. Thus, the [math]\displaystyle{ \vec{F}_{FK} \, }[/math] can be approximated as

[math]\displaystyle{ \vec{F}_{FK} \cong \left( - \nabla_{p_I} \right) \left. \frac{}{} \right| \, }[/math] at body [math]\displaystyle{ \iiint d\forall = \, }[/math] [math]\displaystyle{ \underbrace{\forall} \, }[/math] [math]\displaystyle{ \left( - \nabla_{p_I} \right) \left. \frac{}{} \right| \, }[/math] at body
center body
volume
body
volume
center

The last relation is particularly useful for small bodies of non-trivial geometry for 13.021, that is all bodies that do not have a rectangular cross section.

(b) Diffraction and Radiation Forces

(b.1) Diffraction or scattering force When [math]\displaystyle{ \ell \not\ll \lambda \, }[/math], the wave field near the body will be affected even if the body is stationary, so that no-flux B.C. is satisfied.

[math]\displaystyle{ \vec{F}_D \ = \ }[/math] [math]\displaystyle{ \iint - \rho \left( \frac{\partial\phi_D}{\partial t} \right) \hat{n} dS }[/math]
body surface

(b.2) Radiation Force -added mass and damping coefficient Even in the absence of an incident wave, a body in motion creates waves and hence inertial wave forces.

[math]\displaystyle{ \vec{F}_R = \, }[/math] [math]\displaystyle{ \iint - \rho \left( \frac{\partial\phi_R}{\partial t} \right) \hat{n} dS = - }[/math] [math]\displaystyle{ \underbrace{m_{ij}} \, }[/math] [math]\displaystyle{ \dot{U}_j \ - \, }[/math] [math]\displaystyle{ \underbrace{d_{ij}} \, }[/math] [math]\displaystyle{ U_j \, }[/math]
body surface added mass wave radiation damping

Important parameters

[math]\displaystyle{ (1) K_C = \frac{UT}{\ell} = 2 \pi \frac{A}{\ell} \, }[/math] [math]\displaystyle{ \left. \begin{matrix} \\ \\ \\ \\ \\ \\ \end{matrix} \right\} \, }[/math] Interrelated through maximum wave steepness
[math]\displaystyle{ \frac{A}{\lambda} \leq 0.07 \, }[/math]
(2)diffraction parameter [math]\displaystyle{ \frac{\ell}{\lambda} \, }[/math] [math]\displaystyle{ \left( \frac{A}{\ell} \right) \left( \frac{\ell}{\lambda} \right) \leq 0.07 \, }[/math]
  • If [math]\displaystyle{ K_c \leq 1 \, }[/math]: no appreciable flow separation, viscous effect confined to boundary layer (hence small), solve problem via potential theory. In addition, depending on the value of the ratio [math]\displaystyle{ \frac{\ell}{\lambda} \, }[/math],
  • If [math]\displaystyle{ \frac{\ell}{\lambda} \ll 1 \, }[/math], ignore diffraction , wave effects in radiation problem (i.e., [math]\displaystyle{ d_{ij} \approx 0, \ m_{ij} \approx m_{ij} \, }[/math] infinite fluid added mass). F-K approximation might be used, calculate [math]\displaystyle{ \vec{F}_{FK} \, }[/math].
  • If [math]\displaystyle{ \frac{\ell}{\lambda} \gg 1/5 \, }[/math], must consider wave diffraction, radiation [math]\displaystyle{ \left( \frac{A}{\ell} \leq \frac{0.07}{\ell / \lambda} \leq 0.035 \right) \, }[/math].
  • If [math]\displaystyle{ K_C \gg 1 \, }[/math]: separation important, viscous forces can not be neglected. Further on if [math]\displaystyle{ \frac{\ell}{\lambda} \leq \frac{0.07}{A/\ell} \, }[/math] so [math]\displaystyle{ \frac{\ell}{\lambda} \ll 1 \, }[/math] ignore diffraction, i.e., the Froude-Krylov approximation is valid.
[math]\displaystyle{ F = \frac{1}{2} \rho \ell^2 \, }[/math] [math]\displaystyle{ \underbrace{U(t)} \, }[/math] [math]\displaystyle{ \left| U(t) \right| C_D \left( R_e \right) \, }[/math]
relative velocity
  • Intermediate [math]\displaystyle{ K_c - \, }[/math] both viscous and inertial effects important, use Morrison's formula.
[math]\displaystyle{ F= \frac{1}{2} \rho \ell^2 U(t) \left| U(t) \right| C_D \left( R_e \right) + \rho \ell^3 \dot{U} C_m \left( R_e, K_C \right) }[/math]
  • Summary

I. Use: [math]\displaystyle{ C D \, }[/math] and [math]\displaystyle{ F - K \, }[/math] approximation.

II. Use: [math]\displaystyle{ C F \, }[/math] and [math]\displaystyle{ F - K \, }[/math] approximation.

III. [math]\displaystyle{ C D \, }[/math] is not important and [math]\displaystyle{ F - K \, }[/math] approximation is not valid.



This article is based on the MIT open course notes and the original article can be found here.

Marine Hydrodynamics