Liquid Surface Wave Theory And Its Frequency Characteristics

Surface acoustic wave(SAW) is a kind of elastics wave,the propagating energy of which concentrates on the medium(solid or liquid) surface.SAW travels along the medium surface,which carries a large number of surface information.Therefore the research of SAW has already brought to more and more attentions.Compared with the bulk,the liquid surface has much difference in composition,structure,force,energy state,and so on.It is the difference that makes the liquid surface takes on some characteristics.We can obtain some parameters of liquid by means of researching them.Optical method is an important technique to detect the substance properties,which possesses such good features:the lossless,non-contacted,real-time and prompt.Since laser was invented in the 1960s,it has been widely used in studying the surface waves.When the optical effects,including reflection,refraction,interference and diffraction,occur on the liquid surface,many fantastic phenomena will emerge,which may be used to measure some parameters of liquid,such as refraction index,contact angle,surface tension and so on.Therefore,optical methods have been adopted extensively in many fields,for example,Fluid Mechanics,Ocean Optics.In this thesis,optical method was used to study the liquid surface acoustic wave at low frequencies.The main research content and conclusions are as follows:(1) This thesis introduces the classification of Surface waves and the characteristics of the SAW technique.We emphatically introduces the current laser measure technique,analyzes the advantages and disadvantages of each technique.Then we put forward some suggestions on research approach of SAW in the future.(2) According to the hydrodynamic equations of SAW,we derived the general properties of SAW.The liquid dispersion relation was also obtained.For the SAW with frequency from more than ten Hz to few tens Hz,we analyzed the interference theory,and explained the special phenomenon occurring in the experiment.When frequency rising up from few tens Hz to several hundreds Hz,the SAW could be regarded as a phase grating.Then we discussed the diffraction field,fringe distribution,half-angular width and fringe intensity by Fourier Optics.Experimental result was in agreement with our theory.(3) The fluctuation process of the SAW was approximate to the string forced vibration in the one-dimensional space without boundary,by the methods of mathematical physics.In this way,we obtained the relationship between SAW frequency and excitation frequency,and measured the SAW frequency in real-time by means of photon counter.Experimental result was in agreement with our theory.For the SAW with frequency from few tens Hz to several hundreds Hz,excitation frequency should be considered to be the SAW frequency.The experiment system possesses such good features as real-time and non-contacted,which could be used to monitor the stability of frequency in any other SAW experiments.By discussing,we got to know that the influencing factors of the stability in our research were experiment environment,stability of the excitation and the amplitude of SAW.We wish this could be some useful reference for other research on SAW at low frequency.Finally we measured the wavelength,velocity of SAW and the surface tension of distilled water.The surface tension is 71.120×10^-3 N / m,which basically tallies with the general value.