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Ursell number

Dimensionless number indicating the nonlinearity of long surface gravity waves on a fluid layer.

Influid dynamics, the Ursell number indicates the nonlinearity of long surface gravity waves on a fluid layer. This dimensionless parameter is named after Fritz Ursell, who discussed its significance in 1953.^[1]

The Ursell number is derived from the Stokes wave expansion, a perturbation series for nonlinear periodic waves, in the long-wave limitofshallow water – when the wavelength is much larger than the water depth. Then the Ursell number U is defined as:

${\displaystyle U={\frac {H}{h}}\left({\frac {\lambda }{h}}\right)^{2}\,=\,{\frac {H\,\lambda ^{2}}{h^{3}}},}$

which is, apart from a constant 3 / (32 π²), the ratio of the amplitudes of the second-order to the first-order term in the free surface elevation.^[2] The used parameters are:

• H : the wave height, i.e. the difference between the elevations of the wave crest and trough,
• h : the mean water depth, and
• λ : the wavelength, which has to be large compared to the depth, λ ≫ h.

So the Ursell parameter U is the relative wave height H / h times the relative wavelength λ / h squared.

For long waves (λ ≫ h) with small Ursell number, U ≪ 32 π² / 3 ≈ 100,^[3] linear wave theory is applicable. Otherwise (and most often) a non-linear theory for fairly long waves (λ > 7 h)^[4] – like the Korteweg–de Vries equationorBoussinesq equations – has to be used. The parameter, with different normalisation, was already introduced by George Gabriel Stokes in his historical paper on surface gravity waves of 1847.^[5]

Notes[edit]

References[edit]

• Dingemans, M. W. (1997). "Water wave propagation over uneven bottoms". Advanced Series on Ocean Engineering. 13. World Scientific: 25769. ISBN 978-981-02-0427-3. {{cite journal}}: Cite journal requires |journal= (help) In 2 parts, 967 pages.
• Svendsen, I. A. (2006). Introduction to nearshore hydrodynamics. Advanced Series on Ocean Engineering. Vol. 24. Singapore: World Scientific. ISBN 978-981-256-142-8. 722 pages.