资源描述
,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,单击此处编辑母版标题样式,Coastal Hydrodynamics,Chapter 2,WAVE THEORY,Stating description of wave motion,Stating basic equations of wave motion,Stating the small amplitude wave theory,Stating the finite amplitude wave theory,Stating wave theory limits of applicability,2/39,Chapter 3,WAVE TRANSFORMATIONS,Stating ocean wave characteristics,Stating transformations of waves,entering shallow water,3/39,Chapter 3,3.1,Ocean Wave Characteristics,Statistical analysis,Spectral analysis,4/39,Statistical characteristics of ocean,waves,Chapter 3,3.1,Ocean Wave Characteristics,2.,Wave height distribution and wave,period distribution,3.,Ocean wave energy spectra,4.,Deep-water wave propagation,5/39,Chapter 3,The actual sea surface is composed of a,large variety of waves moving in different,directions and with different frequencies,phases and amplitudes.,1.,Statistical characteristics,There is a great amount of randomness in,the sea,and statistical techniques need to,be brought to bear.,6/39,Chapter 3,The zero-up crossing method uses the time,when the surface wave profile crosses the,zero level in the upward direction.,Zero-up crossing method,7/39,Chapter 3,An individual wave height is defined by the,vertical difference between the maximum and,minimum levels with adjacent zero-up,crossing points,and the corresponding wave,period is defined by the interval of the two,crossing points.,8/39,Chapter 3,The maximum wave corresponds to the,maximum height in a given wave group.,Statistically representative waves,The one-tenth highest wave corresponds,to the average of the heights of the,one-tenth highest waves.,The significant wave corresponds to the,average of the heights of the one-third,highest waves.,9/39,Chapter 3,The mean wave corresponds to the mean,wave height in a given wave group.,The root-mean-square wave height for a,given wave group is defined as,10/39,Chapter 3,Example:,What are statistically representative waves?,11/39,Chapter 3,H,max,=4.89m,T,max,=8.0s;,H,1/10,=4.7m,T,1/10,=,7.5s;,H,1/3,=3.6m,T,1/3,=7.8s;,=,2.4m,=7.0s;,H,rms,=2.66m.,12/39,Chapter 3,The significant wave is the most frequently,used in the coastal engineering field.,The significant wave height corresponds,roughly to the wave height that is visually,observed and estimated.The significant,wave period has less physical meaning and,can lead to appreciable error if an attempt,is made to use it in calculations with the,theoretical wave equations.,13/39,Chapter 3,The probability that the wave height is,greater than or equal to an arbitrary wave,height can be obtained.Then we can obtain,the cumulative height curve.The statistically,representative waves defined by the excess,cumulative probability can be found.For,example,H,1%,represents a wave height,corresponding to the intersection of the 1,st,percentile with the cumulative height curve.,14/39,Chapter 3,2.,Statistical distribution,The wave height probability density function,follows from the Rayleigh probability,distribution.,15/39,Chapter 3,Rayleigh distribution curve,16/39,Chapter 3,From statistical theory,we can obtain important,relationships using the,distribution function,for the wave height.,17/39,Chapter 3,The distribution of the wave period of a fully,developed sea can be expressed as,The above equation means that the probability,density function of the square of the wave,period,follows the Rayleigh probability,distribution.,18/39,Chapter 3,For an adequate,description of,the sea surface,a large number,of waves must,be superimposed,to be realistic.,3.,Wave spectrum,19/39,Chapter 3,Broadly speaking,wave energy density spectra,are classified into frequency spectra and wave,number spectra.The former express the wave,energy density distribution as a function of,wave frequency,while the latter express it as,a function of wave number.In fact,it has,been shown that the wave number and the,frequency are uniquely correlated.,20/39,Chapter 3,If the amplitudes are plotted versus frequency,an,amplitude spectrum,results.,The,energy spectrum,is a plot of the square of,amplitudes versus frequency.,If the square of amplitudes over the frequency,interval are plotted versus frequency,an,energy density spectrum,which is more,popular,is obtained.,21/39,Chapter 3,Wave frequency spectrum,For a simple case of waves propagating in one,direction,the surface elevation measured at a,fixed point is expressed by,and the equation used to define the frequency,spectrum is,22/39,Chapter 3,PiersonMoskowitz spectrum,This expresses the spectrum of a fully arisen,sea due to a constant wind of sufficient,duration.The shape of the present spectrum,was derived on the basis of the wind wave,data obtained by pressure-type wave gages,attached to a ships body.,23/39,Chapter 3,Pierson-Moskowitz spectrum,24/39,Chapter 3,Bretschneider spectrum,This spectrum is applicable to wind waves under,finite fetch,and is valuable for estimating the,wave spectrum when the statistically defined,wave height and wave period are known.,25/39,Ch
展开阅读全文