Pressure surge

In marine structures, the pressure surge (wave surge), like the alternating forces that are generated by water waves in the rhythm of their movement, are part of the unsteady structure loads. A characteristic of the pressure surge is a non-uniform hydrodynamic pressure stress distribution with a peak value which momentarily acts on a stationary wall surface when a moving wave surface strikes this locally plane-parallel . If such peak pressure values are given as liquid pressure head , they can be up to a two-digit multiple ( ) of the wave height that generates them . ${\ displaystyle p_ {max}}$ ${\ displaystyle m}$ ${\ displaystyle H}$

{\ displaystyle p_ {max} / \ rho g = m \ cdot H}

.

Values of 500 kPa and more can be achieved as compressive stress .

A distinction must be made between this and the similar phenomenon known from shipbuilding, sea hammering (slamming), where there is a relative movement between the moving wave surface and the currently plane-parallel ship's skin .

Other comparable phenomena are the pressure surges that occur due to the formation of vapor bubbles and collapse of these bubbles under strongly fluctuating pressure conditions in centrifugal pumps and when hot steam is introduced into cold water (see cavitation ). These can also lead to material damage through erosion .

Characteristic

Pressure surge propagation on different embankment cover layers

As with the alternating forces generated by waves, the pressure hammer is not just a deterministically describable wave force . In nature, the physical and geometric boundary conditions cannot be precisely delimited when the pressure impact force arises and cannot be precisely defined in the formulation

{\ displaystyle F = \ int p \ dA = \ iint p (x, y) \ dxdy}

Both the compressive stress distribution and its effective area represent random variables . The stochastic character of the peak value is mainly determined by the air content in the water of the breaking wave. In general, the peak pressure is only effective locally on a very small area. In its development over time, the rise times (compression times) until the maximum value is reached are between a few milliseconds and several hundred milliseconds, with the greatest compressive stress maxima being reached within the smallest rise times. ${\ displaystyle p (x, y)}$ ${\ displaystyle A}$ ${\ displaystyle p_ {max}}$

A distinction must be made between pressure hammer effects

vertical (flat and curved) walls (e.g. breakwaters , piers , lighthouses),
vertical and inclined tubular elements (e.g. platform support elements),
inclined (flat) walls (e.g. embankments , breakwaters).

Maximum values of the pressure surge stresses in vertical structures can be a power of ten higher than in embankments with an incline less than 1: 3.

It is known that even large-volume monolithic building components can be moved by waves in the event of a pressure surge. In the event of defects or joints in the structure's surface, pressure surge stresses are transmitted into the structure in the form of a shock wave , which means that damage can be triggered locally from the inside.

Influencing the surf kinematics

In the case of embankments, the pressure hammer is generated solely by the crusher type fall crusher . Its occurrence can only be influenced within limits for a given set of value pairs for wave height and wave length by choosing a suitable slope . In hollow revetments, the lintel breaker does not appear in its distinctive form .

literature

A. Führböter: The pressure surge caused by breakers on embankments. In: Communications from the Franzius Institute of the Technical University of Hanover. Issue 27, 1966.
A. Führböter: Wave loads on dike and revetment slopes. In: Yearbook of the Port Construction Society. Volume 46, Schiffahrts-Verlag HANSA, Hamburg 1991, pp. 225-282.
G. Müller, P. Hull, W. Allsop, T. Bruce, M. Cooker, L. Franco: Wave Effects on Blockwork Structures: Model Tests. In: Journal of Hydraulic Research. Vol. 40, No. 2, 2002, pp. 117-124.