Linear Wave Theory Is a Solution of the Laplace Equation

Total Page:16

File Type:pdf, Size:1020Kb

Linear Wave Theory Is a Solution of the Laplace Equation The University of the West Indies Organization of American States PROFESSIONAL DEVELOPMENT PROGRAMME: COASTAL INFRASTRUCTURE DESIGN, CONSTRUCTION AND MAINTENANCE A COURSE IN COASTAL DEFENSE SYSTEMS I CHAPTER 5 COASTAL PROCESSES: WAVES By PATRICK HOLMES, PhD Professor, Department of Civil and Environmental Engineering Imperial College, England Organized by Department of Civil Engineering, The University of the West Indies, in conjunction with Old Dominion University, Norfolk, VA, USA and Coastal Engineering Research Centre, US Army, Corps of Engineers, Vicksburg, MS, USA. St. Lucia, West Indies, July 18-21, 2001 Coastal Defense Systems 1 5-1 CDCM Professional Training Programme, 2001 5. Coastal Processes: WAVES P.Holmes, Imperial College, London 5.1 Introduction 5.2 Linear (Uniform) Waves. 5.3 Random Waves. 5.4 Waves in Shallow Water. 5.1 Introduction. Wave action is obviously a major factor in coastal engineering design. Much is known about wave mechanics when the wave height and period (or length) are known. In shallow water the properties of waves change; they change height and their direction of travel, which must be included in design calculations. However, waves generated by winds blowing over the ocean surface are not of the same height and period, they are random waves for which probability/statistical models have to be used. This section of the notes discusses these three topics: linear waves, random waves and waves in shallow water. 5.2 Linear (Uniform) Wave Theory. Figure 1 gives the general definitions for two-dimensional, linear water wave theory for which the following notation is needed: x,y are Cartesian co-ordinates with y = 0 at the still water level (positive upwards) η(x,t) = the free water surface; t = time u,v, = velocity components in the x,y directions, respectively φ(x,y,t) = the two-dimensional velocity potential ρ = the fluid density; g = gravitational acceleration a = wave amplitude = H/2; H = wave height k = wave number = 2π/L; L = wave length σ = wave frequency = 2π/T; T = wave period d = mean water depth; C = wave celerity = L/T Linear wave theory is a solution of the Laplace equation: δ2φ/δx2 + δ2φ/δy2 = 0 5.1 The particular flow in any condition is determined by the boundary conditions, in this case specific boundary conditions at the free surface of the fluid and at the bottom. (The details can be found in any good text on wave theories) Coastal Defense Systems 1 5-2 CDCM Professional Training Programme, 2001 y Direction of Propagation Wave Length Still W ater Level x Wave Height depth Particle Orbital M otion Bottom Figure 1. Definitions for Linear Water Wave Theory The solution for the potential function satisfying the Laplace equation (1) subject to the boundary conditions is: φ = (ga/σ). cosh k(y + d). sin (kx - σt) 5.2 cosh kd = (gHT/4π). cosh [(2π/L)( y + d)]. sin 2π(x/L – t/T) 5.3 cosh (2πd/L) η = a cos(kx - σt) = (H/2) cos 2π(x/L – t/T) 5.4 1/2 σ = (gk tanh kd) 5.5 or L = (gT2/2π) tanh (2πd/L) 5.6 or C = ((gL/2π) tanh (2πd/L))1/2 5.7 In “deep” water, d/L > 0.5, tanh (2πd/L) ≈ 1.0 Coastal Defense Systems 1 5-3 CDCM Professional Training Programme, 2001 2 2 ∴ Lo = gT /2π = 1.56T 5.8 where subscript “o” denotes deep water. In “shallow” water, d/L < 0.04, tanh (2πd/L) ≈ 2πd/L ∴ L = T (gd)1/2; C = √gd 5.9 For all depths the wave length, L can be found by iteration from: d/LO = d/L tanh (2πd/L) 5.10 Particle Velocities. From the derivation of the Laplace equation, for irrotational flow, u = δφ/δx and v = δφ/δy 5.11 so that from equation 2 the horizontal and vertical velocities of flow are given by: u = (πH/T). cosh [2π(y + d)/L]. cos 2π(x/L – t/T) 5.12 sinh 2πd/L v = (πH/T). sinh [2π(y + d)/L]. sin 2π(x/L – t/T) 5.13 sinh 2πd/L In “deep” water these simplify to: u = (πH/T). exp(2πy/LO). cos 2π(x/L – t/T) 5.14 v = (πH/T). exp(2πy/LO). sin 2π(x/L – t/T) 5.15 Note that y is measured positive upwards from the still water level. Pressure. In wave motion the pressure distribution in the vertical is no longer hydrostatic and is given by: p/ρg = cosh 2π[(y + d)/L] . η - y 5.16 cosh 2πd/L The cosh2π[(y + d)/L] / cosh 2πd/L term is known as the “pressure response factor” which tends to zero as y increases negatively, important when using pressure transducers for wave recording. Coastal Defense Systems 1 5-4 CDCM Professional Training Programme, 2001 Energy. The total energy per wave per unit width of crest, E, is: E = ρgH2L/8 5.17 Note that wave energy is proportional to the SQUARE of the wave height. Group Velocity. Group velocity, CG, is defined as the velocity at which wave energy is transmitted. Physically this can be seen if a group of, say, five waves is generated in a laboratory channel. The leading wave will disappear but a new wave will be created at the rear of the group, so there will always be five waves. Thus the group travels at a slower speed than the individual waves within it. CG = nC, n = ½ [1 + (4πd/L)/( sinh 4πd/L)] 5.18 Power. The mean power transmitted per unit width of crest, P, is given by: 2 P = CG ρgH /8 5.19 Non-Linear Wave Theories. As noted above the boundary conditions used to obtain a solution for wave motion were linearised, that is, applied at y = 0 not on the free water surface, y = η, hence the term Linear (or Airy) Wave Theory. For a non-linear solution the free surface boundary conditions have to be applied at that free surface, η. But η is unknown! Therefore solutions have been developed, notably by Stokes, in series form for which the coefficients of the series can be derived. Thus, the free-surface is given by: η = a cos(θ) + b cos(2θ) + c cos (3θ) + d cos(4θ) + e cos(5θ) ……5.20 To obtain a solution to, say, third order, terms greater than order three are ignored. Taken to first order the solution is, of course, a linear wave. Another wave theory applicable in shallow water is Cnoidal Wave Theory. Solutions are given in terms of elliptic integrals of the first kind; the solution at one limit is identical with linear wave theory and at the other is identical to Solitary Wave Theory. As the name implies, the latter is a single wave with no trough and the mass of water moving entirely in the x direction. More recently, numerical solutions for wave motion have been established and in offshore engineering it is common to see numerical solutions up to 18th or 25th order being used to obtain velocity and acceleration information for the derivation of wave loads on offshore structures. Figure 2, based on the U.S. Army corps of Engineers “Shore Protection Manual” 1984, indicates the preferred theory for given wave parameters, but note that the figure is illustrative only. Coastal Defense Systems 1 5-5 CDCM Professional Training Programme, 2001 Shallow water Transitional Water Deep Water HO/LO = 0.14 Stokes’ 4th Order 0.01 BREAKING Stokes’ 3rd Order 2 H/d = 0.78 H/gT H = HB/4 NON-BREAKING Stokes’ 2nd Order Stream 0.001 Function L2H/d3 ≈ 26 Cnoidal Theory Linear Theory 0.0001 0.001 0.01 0.1 d/gT2 Figure 2. Parameter Space for Wave Theories - illustrative only. Coastal Defense Systems 1 5-6 CDCM Professional Training Programme, 2001 5.2. RANDOM WAVES 5.2.1. Introduction. Waves generated by winds blowing over the ocean have a range of wave periods, T (s), or wave frequency, ω (radians/s) and are clearly not of the same height, H Surface elevation (η (t)) Time (t) Figure 1 A Typical Wave Record (m). These variations arise initially from the random nature of pressure pulses acting on the water surface and the subsequent, complex growth of the perturbations in that free surface. Based on linear wave theory it is clear that waves with longer periods travel at a higher celerity that those with a shorter period. Thus, as the waves travel away from the area in which they are generated the longer period waves travel faster and the water surface elevation is more periodic, usually being termed "swell". Within or close to the generation area the sea surface elevation is a complex three-dimensional process with short-crested waves travelling in a range of directions relative to the wind direction. A record of such waves will look like Figure 1. The surface elevation is measured relative to still water level and a second record taken at some later time would have a similar appearance, i.e., its general properties would not change - it is a stationary process. Any linear derivatives of the surface elevation, for example, the water particle velocity or acceleration would have similar appearances. Coastal Defense Systems 1 5-7 CDCM Professional Training Programme, 2001 For design purposes it is necessary to quantify this in some way. This can be done in two ways: in terms of the probability properties of the record or in terms of its frequency content, specifically the distribution of energy as a function of frequency, called the wave spectrum. 5.2.1 Wind Generation of Waves and Wave Prediction.
Recommended publications
  • Glossary Physics (I-Introduction)
    1 Glossary Physics (I-introduction) - Efficiency: The percent of the work put into a machine that is converted into useful work output; = work done / energy used [-]. = eta In machines: The work output of any machine cannot exceed the work input (<=100%); in an ideal machine, where no energy is transformed into heat: work(input) = work(output), =100%. Energy: The property of a system that enables it to do work. Conservation o. E.: Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes. Equilibrium: The state of an object when not acted upon by a net force or net torque; an object in equilibrium may be at rest or moving at uniform velocity - not accelerating. Mechanical E.: The state of an object or system of objects for which any impressed forces cancels to zero and no acceleration occurs. Dynamic E.: Object is moving without experiencing acceleration. Static E.: Object is at rest.F Force: The influence that can cause an object to be accelerated or retarded; is always in the direction of the net force, hence a vector quantity; the four elementary forces are: Electromagnetic F.: Is an attraction or repulsion G, gravit. const.6.672E-11[Nm2/kg2] between electric charges: d, distance [m] 2 2 2 2 F = 1/(40) (q1q2/d ) [(CC/m )(Nm /C )] = [N] m,M, mass [kg] Gravitational F.: Is a mutual attraction between all masses: q, charge [As] [C] 2 2 2 2 F = GmM/d [Nm /kg kg 1/m ] = [N] 0, dielectric constant Strong F.: (nuclear force) Acts within the nuclei of atoms: 8.854E-12 [C2/Nm2] [F/m] 2 2 2 2 2 F = 1/(40) (e /d ) [(CC/m )(Nm /C )] = [N] , 3.14 [-] Weak F.: Manifests itself in special reactions among elementary e, 1.60210 E-19 [As] [C] particles, such as the reaction that occur in radioactive decay.
    [Show full text]
  • Fluid Mechanics
    FLUID MECHANICS PROF. DR. METİN GÜNER COMPILER ANKARA UNIVERSITY FACULTY OF AGRICULTURE DEPARTMENT OF AGRICULTURAL MACHINERY AND TECHNOLOGIES ENGINEERING 1 1. INTRODUCTION Mechanics is the oldest physical science that deals with both stationary and moving bodies under the influence of forces. Mechanics is divided into three groups: a) Mechanics of rigid bodies, b) Mechanics of deformable bodies, c) Fluid mechanics Fluid mechanics deals with the behavior of fluids at rest (fluid statics) or in motion (fluid dynamics), and the interaction of fluids with solids or other fluids at the boundaries (Fig.1.1.). Fluid mechanics is the branch of physics which involves the study of fluids (liquids, gases, and plasmas) and the forces on them. Fluid mechanics can be divided into two. a)Fluid Statics b)Fluid Dynamics Fluid statics or hydrostatics is the branch of fluid mechanics that studies fluids at rest. It embraces the study of the conditions under which fluids are at rest in stable equilibrium Hydrostatics is fundamental to hydraulics, the engineering of equipment for storing, transporting and using fluids. Hydrostatics offers physical explanations for many phenomena of everyday life, such as why atmospheric pressure changes with altitude, why wood and oil float on water, and why the surface of water is always flat and horizontal whatever the shape of its container. Fluid dynamics is a subdiscipline of fluid mechanics that deals with fluid flow— the natural science of fluids (liquids and gases) in motion. It has several subdisciplines itself, including aerodynamics (the study of air and other gases in motion) and hydrodynamics (the study of liquids in motion).
    [Show full text]
  • Acoustic Nonlinearity in Dispersive Solids
    ACOUSTIC NONLINEARITY IN DISPERSIVE SOLIDS John H. Cantrell and William T. Yost NASA Langley Research Center Mail Stop 231 Hampton, VA 23665-5225 INTRODUCTION It is well known that the interatomic anharmonicity of the propagation medium gives rise to the generation of harmonics of an initially sinusoidal acoustic waveform. It is, perhaps, less well appreciated that the anharmonicity also gives rise to the generation of a static or "dc" component of the propagating waveform. We have previously shown [1,2] that the static component is intrinsically linked to the acoustic (Boussinesq) radiation stress in the material. For nondispersive solids theory predicts that a propagating gated continuous waveform (acoustic toneburst) generates a static displacement pulse having the shape of a right-angled triangle, the slope of which is linearly proportional to the magnitude and sign of the acoustic nonlinearity parameter of the propagation medium. Such static displacement pulses have been experimentally verified in single crystal silicon [3] and germanium [4]. The purpose of the present investigation is to consider the effects of dispersion on the generation of the static acoustic wave component. It is well known that an initial disturbance in media which have both sufficiently large dispersion and nonlinearity can evolve into a series of solitary waves or solitons [5]. We consider here that an acoustic tone burst may be modeled as a modulated continuous waveform and that the generated initial static displacement pulse may be viewed as a modulation-confined disturbance. In media with sufficiently large dispersion and nonlinearity the static displacement pulse may be expected to evolve into a series of modulation solitons.
    [Show full text]
  • Type-I Hyperbolic Metasurfaces for Highly-Squeezed Designer
    Type-I hyperbolic metasurfaces for highly-squeezed designer polaritons with negative group velocity Yihao Yang1,2,3,4, Pengfei Qin2, Xiao Lin3, Erping Li2, Zuojia Wang5,*, Baile Zhang3,4,*, and Hongsheng Chen1,2,* 1State Key Laboratory of Modern Optical Instrumentation, College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, China. 2Key Lab. of Advanced Micro/Nano Electronic Devices & Smart Systems of Zhejiang, The Electromagnetics Academy at Zhejiang University, Zhejiang University, Hangzhou 310027, China. 3Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore. 4Centre for Disruptive Photonic Technologies, The Photonics Institute, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. 5School of Information Science and Engineering, Shandong University, Qingdao 266237, China *[email protected] (Z.W.); [email protected] (B.Z.); [email protected] (H.C.) Abstract Hyperbolic polaritons in van der Waals materials and metamaterial heterostructures provide unprecedenced control over light-matter interaction at the extreme nanoscale. Here, we propose a concept of type-I hyperbolic metasurface supporting highly-squeezed magnetic designer polaritons, which act as magnetic analogues to hyperbolic polaritons in the hexagonal boron nitride (h-BN) in the first Reststrahlen band. Comparing with the natural h-BN, the size and spacing of the metasurface unit cell can be readily scaled up (or down), allowing for manipulating designer polaritons in frequency and in space at will. Experimental measurements display the cone-like hyperbolic dispersion in the momentum space, associating with an effective refractive index up to 60 and a group velocity down to 1/400 of the light speed in vacuum.
    [Show full text]
  • NANOOS Asset List 1 National Oceanic and Atmospheric
    NANOOS Asset List 2008 NANOOS Asset List 1 National Oceanic and Atmospheric Administration 1.1 The CoastWatch West Coast Regional Node http://coastwatch.pfel.noaa.gov Daily – Monthly composites of satellite observations Sea Surface Temperature (GOES & POES) Ocean Color (MODIS and SeaWiFS) Ocean Winds (QuikSCAT) 1.2 The National Data Buoy Center http://seaboard.ndbc.noaa.gov/maps/Northwest.shtml 6 Minute – Hourly buoy observations Meteorological Observations (Air Temp., Pressure, Wind Speed and Direction) Ocean Observations (Water Temp., Wave Height, Period and Direction) 1.3 The Center for Operational Oceanographic Products and Services http://tidesandcurrents.noaa.gov http://opendap.co-ops.nos.noaa.gov/content 6 Minute near-shore station observations Meteorological Observations (Air Temp., Pressure, Wind Speed and Direction) Ocean Observations (Water Temp., Water Level) 1.4 NOAAWatch http://www.noaawatch.gov Information related to ongoing environmental events NOAAWatch themes include Air Quality, Droughts, Earthquakes, Excessive Heat, Fire, Flooding, Harmful Algal Blooms (HABs), Oil Spills, Rip Currents, Severe Weather, Space Weather, Tsunamis, and Volcanoes 1.5 National Weather Service http://www.weather.gov Environmental observations and forecasts Coastal and Marine Forecasts Weather Warnings 1 NANOOS Asset List 2008 Surface Pressure Maps Coastal and Marine Observations (Wind, Visibility, Sky Conditions, Temperature, Dew Point, Relative Humidity, Atmospheric Pressure, Pressure tendency) GOES Satellite Observations (Visible,
    [Show full text]
  • Fluid Viscosity and the Attenuation of Surface Waves: a Derivation Based on Conservation of Energy
    INSTITUTE OF PHYSICS PUBLISHING EUROPEAN JOURNAL OF PHYSICS Eur. J. Phys. 25 (2004) 115–122 PII: S0143-0807(04)64480-1 Fluid viscosity and the attenuation of surface waves: a derivation based on conservation of energy FBehroozi Department of Physics, University of Northern Iowa, Cedar Falls, IA 50614, USA Received 9 June 2003 Published 25 November 2003 Online at stacks.iop.org/EJP/25/115 (DOI: 10.1088/0143-0807/25/1/014) Abstract More than a century ago, Stokes (1819–1903)pointed out that the attenuation of surface waves could be exploited to measure viscosity. This paper provides the link between fluid viscosity and the attenuation of surface waves by invoking the conservation of energy. First we calculate the power loss per unit area due to viscous dissipation. Next we calculate the power loss per unit area as manifested in the decay of the wave amplitude. By equating these two quantities, we derive the relationship between the fluid viscosity and the decay coefficient of the surface waves in a transparent way. 1. Introduction Surface tension and gravity govern the propagation of surface waves on fluids while viscosity determines the wave attenuation. In this paper we focus on the relation between viscosity and attenuation of surface waves. More than a century ago, Stokes (1819–1903) pointed out that the attenuation of surface waves could be exploited to measure viscosity [1]. Since then, the determination of viscosity from the damping of surface waves has receivedmuch attention [2–10], particularly because the method presents the possibility of measuring viscosity noninvasively. In his attempt to obtain the functionalrelationship between viscosity and wave attenuation, Stokes observed that the harmonic solutions obtained by solving the Laplace equation for the velocity potential in the absence of viscosity also satisfy the linearized Navier–Stokes equation [11].
    [Show full text]
  • Downloaded 09/24/21 11:26 AM UTC 966 JOURNAL of PHYSICAL OCEANOGRAPHY VOLUME 46
    MARCH 2016 G R I M S H A W E T A L . 965 Modelling of Polarity Change in a Nonlinear Internal Wave Train in Laoshan Bay ROGER GRIMSHAW Department of Mathematical Sciences, Loughborough University, Loughborough, United Kingdom CAIXIA WANG AND LAN LI Physical Oceanography Laboratory, Ocean University of China, Qingdao, China (Manuscript received 23 July 2015, in final form 29 December 2015) ABSTRACT There are now several observations of internal solitary waves passing through a critical point where the coefficient of the quadratic nonlinear term in the variable coefficient Korteweg–de Vries equation changes sign, typically from negative to positive as the wave propagates shoreward. This causes a solitary wave of depression to transform into a train of solitary waves of elevation riding on a negative pedestal. However, recently a polarity change of a different kind was observed in Laoshan Bay, China, where a periodic wave train of elevation waves converted to a periodic wave train of depression waves as the thermocline rose on a rising tide. This paper describes the application of a newly developed theory for this phenomenon. The theory is based on the variable coefficient Korteweg–de Vries equation for the case when the coefficient of the quadratic nonlinear term undergoes a change of sign and predicts that a periodic wave train will pass through this critical point as a linear wave, where a phase change occurs that induces a change in the polarity of the wave, as observed. A two-layer model of the density stratification and background current shear is developed to make the theoretical predictions specific and quantitative.
    [Show full text]
  • Part II-1 Water Wave Mechanics
    Chapter 1 EM 1110-2-1100 WATER WAVE MECHANICS (Part II) 1 August 2008 (Change 2) Table of Contents Page II-1-1. Introduction ............................................................II-1-1 II-1-2. Regular Waves .........................................................II-1-3 a. Introduction ...........................................................II-1-3 b. Definition of wave parameters .............................................II-1-4 c. Linear wave theory ......................................................II-1-5 (1) Introduction .......................................................II-1-5 (2) Wave celerity, length, and period.......................................II-1-6 (3) The sinusoidal wave profile...........................................II-1-9 (4) Some useful functions ...............................................II-1-9 (5) Local fluid velocities and accelerations .................................II-1-12 (6) Water particle displacements .........................................II-1-13 (7) Subsurface pressure ................................................II-1-21 (8) Group velocity ....................................................II-1-22 (9) Wave energy and power.............................................II-1-26 (10)Summary of linear wave theory.......................................II-1-29 d. Nonlinear wave theories .................................................II-1-30 (1) Introduction ......................................................II-1-30 (2) Stokes finite-amplitude wave theory ...................................II-1-32
    [Show full text]
  • Title Relation Between Wave Characteristics of Cnoidal Wave
    Relation between Wave Characteristics of Cnoidal Wave Title Theory Derived by Laitone and by Chappelear Author(s) YAMAGUCHI, Masataka; TSUCHIYA, Yoshito Bulletin of the Disaster Prevention Research Institute (1974), Citation 24(3): 217-231 Issue Date 1974-09 URL http://hdl.handle.net/2433/124843 Right Type Departmental Bulletin Paper Textversion publisher Kyoto University Bull. Disas. Prey. Res. Inst., Kyoto Univ., Vol. 24, Part 3, No. 225,September, 1974 217 Relation between Wave Characteristics of Cnoidal Wave Theory Derived by Laitone and by Chappelear By Masataka YAMAGUCHIand Yoshito TSUCHIYA (Manuscriptreceived October5, 1974) Abstract This paper presents the relation between wave characteristicsof the secondorder approxi- mate solutionof the cnoidal wave theory derived by Laitone and by Chappelear. If the expansionparameters Lo and L3in the Chappeleartheory are expanded in a series of the ratio of wave height to water depth and the expressionsfor wave characteristics of the secondorder approximatesolution of the cnoidalwave theory by Chappelearare rewritten in a series form to the second order of the ratio, the expressions for wave characteristics of the cnoidalwave theory derived by Chappelearagree exactly with the ones by Laitone, whichare convertedfrom the depth below the wave trough to the mean water depth. The limitingarea betweenthese theories for practical applicationis proposed,based on numerical comparison. In addition, somewave characteristicssuch as wave energy, energy flux in the cnoidal waves and so on are calculated. 1. Introduction In recent years, the various higher order solutions of finite amplitude waves based on the perturbation method have been extended with the progress of wave theories. For example, systematic deviations of the cnoidal wave theory, which is a nonlinear shallow water wave theory, have been made by Kellern, Laitone2), and Chappelears> respectively.
    [Show full text]
  • Lecture 18 Ocean General Circulation Modeling
    Lecture 18 Ocean General Circulation Modeling 9.1 The equations of motion: Navier-Stokes The governing equations for a real fluid are the Navier-Stokes equations (con­ servation of linear momentum and mass mass) along with conservation of salt, conservation of heat (the first law of thermodynamics) and an equation of state. However, these equations support fast acoustic modes and involve nonlinearities in many terms that makes solving them both difficult and ex­ pensive and particularly ill suited for long time scale calculations. Instead we make a series of approximations to simplify the Navier-Stokes equations to yield the “primitive equations” which are the basis of most general circu­ lations models. In a rotating frame of reference and in the absence of sources and sinks of mass or salt the Navier-Stokes equations are @ �~v + �~v~v + 2�~ �~v + g�kˆ + p = ~ρ (9.1) t r · ^ r r · @ � + �~v = 0 (9.2) t r · @ �S + �S~v = 0 (9.3) t r · 1 @t �ζ + �ζ~v = ω (9.4) r · cpS r · F � = �(ζ; S; p) (9.5) Where � is the fluid density, ~v is the velocity, p is the pressure, S is the salinity and ζ is the potential temperature which add up to seven dependent variables. 115 12.950 Atmospheric and Oceanic Modeling, Spring '04 116 The constants are �~ the rotation vector of the sphere, g the gravitational acceleration and cp the specific heat capacity at constant pressure. ~ρ is the stress tensor and ω are non-advective heat fluxes (such as heat exchange across the sea-surface).F 9.2 Acoustic modes Notice that there is no prognostic equation for pressure, p, but there are two equations for density, �; one prognostic and one diagnostic.
    [Show full text]
  • Group Velocity
    Particle Waves and Group Velocity Particles with known energy Consider a particle with mass m, traveling in the +x direction and known velocity 1 vo and energy Eo = mvo . The wavefunction that represents this particle is: 2 !(x,t) = Ce jkxe" j#t [1] where C is a constant and 2! 1 ko = = 2mEo [2] "o ! Eo !o = 2"#o = [3] ! 2 The envelope !(x,t) of this wavefunction is 2 2 !(x,t) = C , which is a constant. This means that when a particle’s energy is known exactly, it’s position is completely unknown. This is consistent with the Heisenberg Uncertainty principle. Even though the magnitude of this wave function is a constant with respect to both position and time, its phase is not. As with any type of wavefunction, the phase velocity vp of this wavefunction is: ! E / ! v v = = o = E / 2m = v2 / 4 = o [4] p k 1 o o 2 2mEo ! At first glance, this result seems wrong, since we started with the assumption that the particle is moving at velocity vo. However, only the magnitude of a wavefunction contains measurable information, so there is no reason to believe that its phase velocity is the same as the particle’s velocity. Particles with uncertain energy A more realistic situation is when there is at least some uncertainly about the particle’s energy and momentum. For real situations, a particle’s energy will be known to lie only within some band of uncertainly. This can be handled by assuming that the particle’s wavefunction is the superposition of a range of constant-energy wavefunctions: jkn x # j$nt !(x,t) = "Cn (e e ) [5] n Here, each value of kn and ωn correspond to energy En, and Cn is the probability that the particle has energy En.
    [Show full text]
  • Particle Motions Caused by Seismic Interface Waves
    Prodeedings of the 37th Scandinavian Symposium on Physical Acoustics 2 - 5 February 2014 Particle motions caused by seismic interface waves Jens M. Hovem, [email protected] 37th Scandinavian Symposium on Physical Acoustics Geilo 2nd - 5th February 2014 Abstract Particle motion sensitivity has shown to be important for fish responding to low frequency anthropogenic such as sounds generated by piling and explosions. The purpose of this article is to discuss the particle motions of seismic interface waves generated by low frequency sources close to solid rigid bottoms. In such cases, interface waves, of the type known as ground roll, or Rayleigh, Stoneley and Scholte waves, may be excited. The interface waves are transversal waves with slow propagation speed and characterized with large particle movements, particularity in the vertical direction. The waves decay exponentially with distance from the bottom and the sea bottom absorption causes the waves to decay relative fast with range and frequency. The interface waves may be important to include in the discussion when studying impact of low frequency anthropogenic noise at generated by relative low frequencies, for instance by piling and explosion and other subsea construction works. 1 Introduction Particle motion sensitivity has shown to be important for fish responding to low frequency anthropogenic such as sounds generated by piling and explosions (Tasker et al. 2010). It is therefore surprising that studies of the impact of sounds generated by anthropogenic activities upon fish and invertebrates have usually focused on propagated sound pressure, rather than particle motion, see Popper, and Hastings (2009) for a summary and overview. Normally the sound pressure and particle velocity are simply related by a constant; the specific acoustic impedance Z=c, i.e.
    [Show full text]