manualshive.com logo in svg

MIKE 21 BW, User Manual, Page: 68 / 196

Share
Download
MIKE 21 BW User Manual Download Page 68

Examples

68

MIKE 21 BW - © DHI

Figure 4.41

Comparison between modelled significant wave height using a phase-
resolving model (MIKE 21 BW) and a phase-averaged model (MIKE 21 
SW). Upper panel shows a comparison along y= 500 m (west-east 
direction) and lower panel shows a comparison along x= 500 m (south-
north direction)

Vector plot of the nearly steady state wave-induced velocity field (after 20 
minutes) is shown in Figure 4.42 for the two wave cases. For the MIKE 21 
BW simulation the velocity is computed as the time-average of the depth-
averaged velocity below the surface roller. As expected the current speeds 
are significant larger in case of regular waves. No data is available for qualita-
tive and quantitative comparisons. Sections 4.2.8 (Detached breakwater) and 
4.2.7 (Rip channel) deal with examples where experimental data is available.

Summary of Contents for 21 BW

Page 1: ...MIKE 2017 MIKE 21 BW Boussinesq Waves Module User Guide...

Page 2: ...2...

Page 3: ...EQUENTIAL DAMAGES OR DAMAGES FOR LOSS OF BUSINESS PROFITS OR SAVINGS BUSINESS INTERRUPTION LOSS OF BUSINESS INFORMA TION OR OTHER PECUNIARY LOSS ARISING OUT OF THE USE OF OR THE INABILITY TO USE THIS...

Page 4: ...4 MIKE 21 BW DHI...

Page 5: ...data 20 3 5 Calibrating and Verifying the Model 20 3 5 1 Purpose 20 3 5 2 Verification 21 3 5 3 Calibration parameters 21 3 6 Running the Production Simulations 21 3 7 Presenting the Results 21 3 8 A...

Page 6: ...ers 148 5 4 Output Parameters 152 5 4 1 Deterministic parameters 152 5 4 2 Phase averaged parameters 158 5 4 3 Wave disturbance parameters 164 5 4 4 Moving shoreline parameters 167 5 4 5 Hot start Par...

Page 7: ...eters are selected and how the results are presented Chapter 5 Reference Manual p 109 describes the parameters in the MIKE 21 BW dialogues It provides more details on specific aspects of the opera tio...

Page 8: ...types and file editors the Plot Composer the MIKE Zero Toolbox the MIKE 21 Toolbox and the Bathymetry Editor The documentation for these can be found from the MIKE Zero Documentation Index A step by s...

Page 9: ...ssinesq type equations originally derived by Madsen et al 1991 and Madsen and S rensen 1992 1 make the modules suitable for simulation of the propagation of directional wave trains travelling from dee...

Page 10: ...eneration of bound sub harmonics and super harmonics and near resonant triad interactions can also be modelled using MIKE 21 BW Thus details like the generation and release of low frequency oscillatio...

Page 11: ...IKE 21 BW also includes porosity for the simulation of partial reflection from and transmission through piers and breakwaters Sponge layers are applied when full absorption of wave energy is required...

Page 12: ...ew harbours is often performed using a combination of simulations with synthetic white noise spectra and simulations with natural wave conditions The purpose of the for mer type of simulations is to i...

Page 13: ...General Description 13 Figure 2 4 Simulation of wave penetration into Frederikshavn harbour Denmark...

Page 14: ...ications related to the 1DH module include computation of wave transformation for nonlinear waves from deep water through the surf zone and all the way up to the beach analysis of generation and relea...

Page 15: ...of low fre quency energy due to primary wave transformation can be computed This is of significant importance for harbour resonance seiching and coastal pro cesses Figure 2 6 Transformation of irregu...

Page 16: ...Introduction 16 MIKE 21 BW DHI...

Page 17: ...he follow ing sections For your particular study only some of the tasks might be rele vant Please note that whenever a word is written in italics it is included as an entry in the Online Help and in t...

Page 18: ...a under Bathymetry The choice of the grid spacing and time step depends on the wave conditions for which simulations are to be performed and the water depth in the area of interest The ratio of the ma...

Page 19: ...ke a long time if for example you have to initiate a monitor ing program Alternatively it may be carried out very quickly if you are able to use existing data which are immediately available In all ca...

Page 20: ...section 5 3 10 Porosity layers 3 4 5 Boundary data In most cases you will force the model by waves generated inside the model domain The internal wave generation of waves allows you to absorb all wav...

Page 21: ...ts to your measurements or other information you will in many cases see differences between the two The purpose of the calibration is then to tune the model so that these differences become negligible...

Page 22: ...tion can be accessed online via the Documentation index Providing MIKE21 BW with a suitable bathymetry dfs2 data file for 2DH applications and dfs1 data file for 1DH applications is essential for obta...

Page 23: ...hment of a porosity map is described in this User Guide see Section 5 3 10 The porosity map is easily created using the MIKE 21 Toolbox Generate Sponge and Porosity Layer Map tool The porosity values...

Page 24: ...eight For harbour agitation studies also maps of the wave disturbance coefficient should be saved 7 It is recommended to view the results during the model run to make sure everything is as expected Th...

Page 25: ...directory is provided The directory names are as follows 2DH Boussinesq Wave Module Numerical flume test Examples MIKE_21 BW Flume Diffraction test Examples MIKE_21 BW Diffraction R nne Harbour Examp...

Page 26: ...d in Figure 4 1 The length in the model east west direction of the flume is 350 m and the width in the model north south direction is 55 m The uniform depth is 10 m Figure 4 1 Model layout The 10 poin...

Page 27: ...sponding to the spectral peak wave period In this example the thickness of the 10 point wide sponge layer is 0 5 0 6 times the wave length correspond ing to the spectral peak wave period However from...

Page 28: ...egular wave train Model setup The model setup is illustrated in Figure 4 3 The model size is 1200 m by 700 m with a uniform depth of 40 m Wave absorbing sponge layers are applied at the eastern and no...

Page 29: ...mulation requires the so called deep water terms to be included i e it is the enhanced Bouss inesq type equations which have to be solved in this case The grid spacing is chosen to be 10 m representin...

Page 30: ...the wave disturbance coefficient behind a fully reflective breakwater The model results are in good agreement with semi analytical solutions to the Helmholtz equation see e g Shore Protection Manual 1...

Page 31: ...in R nne harbour Den mark Left panel shows the harbour layout before the cruise terminal was constructed Model setup The model setup is illustrated in Figure 4 6 The model area is chosen based on foll...

Page 32: ...int wide sponge layer in order to absorb the waves here In reality the waves will break at a small beach here but this will not affect the waves in the central part of the harbour Figure 4 6 Model set...

Page 33: ...n the deepest part of the model The length of the simulation is 12 minutes corresponding to 1801 time steps Model results The model results are presented in Figure 4 7 showing a contour plot of the in...

Page 34: ...BW DHI Figure 4 7 Model results The upper panel shows the instantaneous surface eleva tion and the lower panel the wave disturbance Figure 4 8 3D picture of the simulated instantaneous surface elevati...

Page 35: ...ration lines for obliquely incident unidirectional waves Hanstholm har bour Denmark situated at the Danish west coast is used in this case see Figure 4 9 and 4 10 In this example the enhanced Boussine...

Page 36: ...Examples 36 MIKE 21 BW DHI Figure 4 9 Hanstholm harbour Denmark Figure 4 10 Location of Hanstholm harbour Model setup The model setup is illustrated in Figure 4 11...

Page 37: ...isation purposes oblique incident unidirectional irregular waves are used you can modify the specification if you like in order to see what hap pens if e g directional waves are applied The input data...

Page 38: ...ed wave disturbance coeffi cient marginally The length of the simulation is 25 minutes corresponding to 7501 time steps Model results The model results are presented in Figure 4 12 showing a contour p...

Page 39: ...sults Time series of simulated surface elevation at the main harbour entrance left panel and at the ferry terminal quay right panel see Figure 4 9 Figure 4 14 3D picture of the simulated instantaneous...

Page 40: ...ng period wave energy on wave periods 30 60 s in the harbour Name Layout dfs2 Description Bathymetry sponge layer and porosity coefficients Name Hanstholm bw Description Wave disturbance test Name Han...

Page 41: ...ow water lead to substantial cross spectral energy transfer over relatively short dis tances and during shoaling still more energy will be transferred into bound sub harmonics and super harmonics In a...

Page 42: ...he 20 40 points per wave length at depths 2 10 m where the pri mary part of the waves will break In most practical 2DH applications includ ing wave breaking a grid spacing of 1 2 m is used for peak wa...

Page 43: ...e coefficient map At the harbour entrance along the entrance channel and at northern groin a porosity map is specified for simulation of wave dissipation in the rubble mound structures see Figure 4 18...

Page 44: ...ype is set to 3 and the predominant wave direction is set to 270 N Hence we assume the breaking waves rollers mainly propagate in the positive x direction which is a good approximation in this case Th...

Page 45: ...ule Examples 45 most important issue is to resolved the breaking waves and you can check this in the section Check evaluation of selected Tmin dx and dt Figure 4 19 Numerical parameters Figure 4 20 Wa...

Page 46: ...uded in the Online Help Model run Before you execute the MIKE 21 BW model simulation you have to provide internal wave boundary data Simply drag and drop the Torsminde_Har bour 21t file into your MIKE...

Page 47: ...surface elevation You can make a similar plot by loading drag and drop the PFS file MzPlot Surfaceelevation plc into the MIKE Zero shell during or after model execution From the figure is seen that t...

Page 48: ...for MIKE Animator Plus From Figure 4 23 and Figure 4 24 is seen that short period wave agitation inside the harbour is very limited Figure 4 23 2D visualisation of instantaneous surface elevation Figu...

Page 49: ...wave height and wave disturbance coefficient is presented in Figure 4 25 At the harbour entrance the wave height is reduced to approximately 2 m due to the wave breaking process Figure 4 25 Map of wa...

Page 50: ...line indicated in Figure 4 27 In general the agreement is good MIKE 21 BW pre dicts slightly higher waves along 600 900 m which is mainly due to the non linear shoaling At the harbour entrance 1100 m...

Page 51: ...ore clearly seen from the normal ised spectra lower panel At Point 3 most of the wave energy is on wave periods within the range 30 60 s which is in excellent agreement with field measurements Please...

Page 52: ...location map and the output section in DirectionalWaves BW file The lower panel shows the corresponding normalised frequency spectra calculated using the MIKE Zero WSWAT Linear Spectral Analysis Modu...

Page 53: ...WS Wave Analysis Tool included in MIKE Zero Figure 4 28 Calculated directional spectrum at the Point 1 and Point 2 see Figure 4 27 for location The spectrum is calculated using the MIKE Zero WSWAT Dir...

Page 54: ...ion The pro file plot shows the long waves along the vertical line depicted in Figure 4 27 The digital filtering is performed using the MIKE Zero WSWAT Digital Filtering Analysis Module List of data a...

Page 55: ...ame Sponge dfs2 Description Map of sponge coefficients Name Porosity dfs2 Description Map of porosity coefficients Name Tools Waves Generation Line 21t Description PFS file generation of internal wave...

Page 56: ...l waves PFS file RegularWaves bw H 2 m T 8 s and MWD 270 N Irregular directional waves PFS file DirectionalWaves bw Hm0 2 m Tp 8 s Tmin 4 5 s MWD 270 N standard JONSWAP spectrum Cos8 directional sprea...

Page 57: ...sponge layer is used for efficient wave absorption at the four model boundaries The thickness of the sponge layer is corresponding to one wave length or more The sponge layer map is generated by usin...

Page 58: ...ue to the smaller wave periods Tmin 4 5 s considered in the directional wave case the time step is reduce accordingly Figure 4 33 shows the wave breaking and moving shoreline parameters used in this a...

Page 59: ...e parameters Model run Before you execute the MIKE 21 BW model simulations you have to provide internal wave boundary data Simply drag and drop the file Wave_Genera tion 21t into your MIKE Zero shell...

Page 60: ...GHz and 1GB DDR RAM the required system time is approximately 2 hours the system CPS is approximately 190 000 points second for the regular wave simulation and approximately 5 hours the system CPS is...

Page 61: ...D visualisation of the simulated instantaneous surface elevation The surface rollers are shown in white You can make a similar plot by loading double click the PFS file Anim Regular Waves with Roller...

Page 62: ...urface elevation and corresponding wave spectrum are pre sented in Figure 4 37 regular waves and Figure 4 38 irregular waves The spectra are calculated by use of the WSWAT Linear Spectral Wave Analysi...

Page 63: ...4 37 Time series of surface elevation left and corresponding wave spec trum right Regular unidirectional wave case The data is extracted along the line y 500 m at water depth a 16m b 3m c 1m all upstr...

Page 64: ...ime series of surface elevation left and corresponding wave spec trum right Irregular directional wave case The data is extracted along the line y 500 m at water depth a 16m b 3m c 1m all upstream isl...

Page 65: ...ase note wave statistics are calculated only in water points which never dry out The uprush downrush area is shown by white colour in Figure 4 39 lower panel Figure 4 39 Map of wave heights Upper regu...

Page 66: ...ight along two cross sections An excellent agreement is seen at in wave breaking zone x 300 500m upper panel MIKE 21 BW predicts slightly higher waves immedi ately before breaking which is due to nonl...

Page 67: ...height and mean wave direction Upper panel shows MIKE 21 SW results and lower panel shows results for the MIKE 21 BW irregular wave case The directional analysis of the Boussinesq model results is mad...

Page 68: ...plot of the nearly steady state wave induced velocity field after 20 minutes is shown in Figure 4 42 for the two wave cases For the MIKE 21 BW simulation the velocity is computed as the time average o...

Page 69: ...2DH Boussinesq Wave Module Examples 69 Figure 4 42 Wave induced current fields Upper panel regular waves lower panel irregular waves...

Page 70: ...n regular unidirectional waves Name Setup Directional Waves bw Description PFS file for setup of MIKE 21 BW run irregular directional waves Name Plot RegularWave Disturbance plc Description PFS for 2D...

Page 71: ...n according to laboratory experiments reported by Hamm 1992a b see reference in S rensen et al 1998 The size of model area is 1200 m x 1200 m and the bathymetry is shown in Figure 4 43 It con sists of...

Page 72: ...owpass filter is included to remove high frequency waves generated during uprush and downrush at the shoreline and to dissipate the wave energy in areas where the surface roller is not properly resolv...

Page 73: ...xamples considering wave breaking the deep water terms are included Also in this example we use Simple upwinding at steep gradients and near land for the space discretisation of the convective terms T...

Page 74: ...he speed of your PC For a laptop IBM T40 Pentium M Centrino 1 6 GHz and 1GB DDR RAM the required system time is approximately 7 hours the system CPS is approximately 180 000 points second You may redu...

Page 75: ...2DH Boussinesq Wave Module Examples 75 Figure 4 45 Visualisation 2D of instantaneous surface elevation...

Page 76: ...forcing a current towards the centre line The flow from the two sides join to form a rip current and two symmetrical circulation cells appear A combined vector and contour plot of the nearly steady s...

Page 77: ...8 Rip current along the rip channel y 300 grid points The rip current significantly affects the wave motion The large variation of the rip current causes an increase in the wave height which can be se...

Page 78: ...ulated only in water points which never dry out The uprush downrush area is own by white colour in Figure 4 47 and Figure 4 49 Figure 4 49 Map of relative wave height Please note only the lower half o...

Page 79: ...is considered as a prototype of the laboratory experiments cited above by use of a Froude scaling of 1 40 Model setup The size of model area is 1200 m x 1200 m and the bathymetry is shown in Figure 4...

Page 80: ...situations Regular unidirectional waves PFS file RegularWaves bw H 3 2 m T 10 7 s and MWD 270 N normal incidence Irregular directional waves PFS file DirectionalWaves bw Hm0 4 6 m Tp 10 7 s Tmin 4 s M...

Page 81: ...ponge layer offshore from the generation line see Figure 4 52 The absorbing beach on the offshore side of the detached break water is modelled using a sponge layer The thickness of the sponge layer is...

Page 82: ...stant Manning number of M 35 m1 3 s is used for the bottom friction dissipation Model parameters As in the other examples considering wave breaking the deep water terms are included In this example we...

Page 83: ...breakwater Hm0 4 6m Tp 10 7s h 13 2m for random wave generation and click on run button The length of the simulation is 42 minutes corresponding to 18001 time steps or approximately 235 peak wave per...

Page 84: ...averaged velocities under the surface roller obtained by cumulative averaging over approximately 8 peak wave periods starting 7 minutes after simulation start Wave statistics is com puted only in poin...

Page 85: ...igure 15 p 169 it is seen that the computed eddy structure agrees quite well with the measured structure with respect to the location of eddy the size of the quiescent area in the centre of the eddy a...

Page 86: ...water for the case of regular unidirectional waves upper panel and irregular directional waves lower panel Computed time averaged of the velocities beneath the surface rollers The time averaging is pe...

Page 87: ...partly wet i e also inside the swash zone Compared to the case with regular waves the main features of the eddy structure for random waves are that the quiescent area in the centre of the eddy and the...

Page 88: ...panel model results and lower panel photograph by Mory and Hamm 1997 see S rensen et al 1998 The temporal evolution of the wave induced velocity is shown in Figure 4 59 for the two considered wave si...

Page 89: ...59 Time series of wave induced current speed The time series is extracted at P 375 120 with reference to co ordinate system shown in Figure 4 56 The spatial variation of the relative wave height is pr...

Page 90: ...ve wave height in circulation cell for the case of regular unidirec tional waves upper panel and irregular directional waves lower panel List of data and parameter files All data required for this exa...

Page 91: ...on PFS file for setup of MIKE 21 BW run irregular directional waves Name Plot Regular Surface Elevation plc Description PFS file for 2D visualisation the simulated surface elevation using MIKE Zero Pl...

Page 92: ...sation the simulated wave disturbance coeffi cient using MIKE Zero Plot composer regular unidirectional waves Name Plot Directional Wave Disturbance plc Description PFS file for visualisation the simu...

Page 93: ...following data files and specification file within the folder of Demo Dif fraction are supplied with MIKE 21 Please note that in order not to overwrite the specification files you should copy them to...

Page 94: ...e waves are generated inside the model domain which is the most com mon method of forcing the 1DH Boussinesq Wave Module in practical appli cations The two model boundaries i e at j 0 and j jextr are...

Page 95: ...height and wave period of 1 m and 8 s respectively The model domain is discretized using a structured mesh with 195 elements and 196 nodes The mesh size is 2 m and the integration time step is 0 1 s...

Page 96: ...lue is a little smaller than the expected value of about 0 40 cf Figure 4 65 which is mainly due to the wave non linearity The incoming waves are not perfectly linear as can be seen from the missing w...

Page 97: ...the running period from 5 minutes 3001 time steps to 20 minutes 12001 time steps you will obtain a variation of the wave height as shown in Figure 4 66 With an average value of the significant wave he...

Page 98: ...and a smoothing parameter of 100 default values in MIKE 21 BW A 50 point wide sponge layer is used in the slot in order to damp the oscillations in the slot see Figure 4 67 With respect to the parame...

Page 99: ...ure 4 68 the spatial variation of a number of phase averaged quantities are presented The maximum wave height is 0 121 m at the generation point and increases towards the break point see Figure 4 68 I...

Page 100: ...module also allows for an estimate of the depth aver aged undertow which is the return flow below the wave trough Outside the surf zone the undertow is small and equal to Stokes drift The calculation...

Page 101: ...vertical and horizontal run up height The still water shoreline is located at xr 23 3 m The simulated shoreline motion is converted into a vertical and a horizontal displacement as illustrated in Fig...

Page 102: ...e default installation Binary data files Parameter files 4 3 3 Torsminde barred beach Purpose of the example The purpose of this example is to illustrate how to model wave breaking in case of irregula...

Page 103: ...at spilling breakers would occur for see Madsen et al 1997a 1 p 295 The seaward boundary is treated as nonreflective using sponge layer 100 points Also an absorbing sponge layer is used at the shoreli...

Page 104: ...shows a line series of the simulated surface elevation on top of the bathymetry Spatial variation of the significant wave height and the maxi mum wave height is shown in Figure 4 75 In Figure 4 76 the...

Page 105: ...ation on top of the bathymetry Also the simulated extreme levels of the surface elevation taken throughout the entire simulation time is indicated The dashed line indicates the variation of the origin...

Page 106: ...75 Phase averaged model results Spatial variation of the significant and maximum wave height over a barred beach profile Figure 4 76 Phase averaged model results Spatial variation of the skewness and...

Page 107: ...ated using the WS Spectral Analysis module in MIKE 21 Figure 4 78 Model results 3D pictures of the simulated instantaneous surface ele vation Please note that this graph is based on a calculation incl...

Page 108: ...files Name Torsminde_mesh dfs1 Description Bathymetry Name Torsminde_model dfs1 Description sponge porosity resistance and filter coefficients Name H3 0Tp9Tm2 5 dfs0 Description Wave Boundary conditio...

Page 109: ...which they appear in the MIKE 21 BW editor Figure 5 1 Thereafter all other entries are arranged alphabetically in Section 5 5 Figure 5 1 MIKE 21 BW editor The Reference Manual entries are also availa...

Page 110: ...grid This module is typically selected for calculation of short and long period waves in ports harbours and coastal areas see Examples in Section 4 2 Figure 5 2 Example of a MIKE 21 BW 2DH application...

Page 111: ...n choosing this option you have to specify a 2D grid for 2DH applications type 2 data file with extension dfs2 or dt2 or a 1D profile for 1DH applications type 1 data file with exten sion dfs1 contain...

Page 112: ...Figure 5 4 Figure 5 4 Enclosed Harbour In many cases involving breakwaters what happens in part of the area between the outer face of a breakwater and an adjacent model boundary may have no influence...

Page 113: ...harbour may differ for dif ferent incoming wave directions This is described in Setup of internal gener ation lines section 5 3 4 In other applications such as semi enclosed harbours along exposed coa...

Page 114: ...5 6 Selection of a coastal profile for use in MIKE 21 BW 1DH The line on top of the 2D bathymetry plot indicates the location of the extracted profile It is noticed that the bathymetric profile includ...

Page 115: ...ed the minimum water depth is theoretically zero and a very small spatial resolution is required to resolve the wave field In practical applications it has been found that the most energetic waves sho...

Page 116: ...s water points are specified as negative values when they are below the bathymetry datum and positive values when they are above it Note that simulations are carried out using the still water level SW...

Page 117: ...occurs when the water particle velocity at the crest of the wave begins to exceed the speed of wave propagation Figure 5 9 Wave breaking Boussinesq simulations without wave breaking By comparison the...

Page 118: ...Sch ffer et al 1993 Madsen et al 1997a b and S rensen et al 2004 see references and scientific documentation Successful modelling of the wave breaking process requires an adequate resolution of the su...

Page 119: ...tween the maxi mum water depth and the deep water wave length hmax L0 is less than 0 22 The enhanced Boussinesq equations include the so called deep water terms allowing to extend the model into deepe...

Page 120: ...g with simple upwinding at steep gradients and near land quadratic upwinding with simple upwinding at steep gradients and near land simple upwinding differencing All three schemes are characterised by...

Page 121: ...ency noise appears in the computational domain with the largest water depth typically near an internal generation line or at an open boundary As explained in Blow up after some time steps see link bel...

Page 122: ...simulation Number of time steps The number of time steps is the total simulation time divided by the time step The total simulation time should allow for waves to reach all model areas and from this...

Page 123: ...tep in combination with the grid spacing is selected so that the Cou rant number is less than or equal to 1 Including the deep water terms To avoid numerical damping and instability the minimum wave p...

Page 124: ...3 1 Bathymetric parameters On this dialog you specify information related to the bathymetry i e the defini tion of land points and shift of reference level if any Bathymetry value representing land Th...

Page 125: ...hange accordingly to the change in water level Often the safest thing to do is to make new copies of the bathymetry files and change the depths in the different bathymetry files Figure 5 12 Example of...

Page 126: ...dary conditions such as surface elevations can not be specified Absorbing boundary An absorbing boundary allows wave energy to pass out of the model area without reflections propagating back into the...

Page 127: ...eration line One of the advantages of using internal generation is that sponge layers can be placed behind the gen eration line to absorb waves leaving the model area The wave data needed depends on t...

Page 128: ...generation line The use of directional waves requires a reasonably wide model area and a generation line depending on the specified directional distribution This is illustrated in Figure 5 14 The mai...

Page 129: ...ases An example is shown in Figure 5 14 where it can also be seen that the area with artificial land on the right hand side of the harbour entrance has been enlarged compared to the example above In F...

Page 130: ...rner Grid spacing You must also specify the grid spacing in the x and y directions correspond ing to the bathymetry used for the Boussinesq wave simulation Figure 5 16 Definition of main wave directio...

Page 131: ...ve to the generation line open boundary In both cases a dfs0 data file is used as input An example is shown in Figure 5 19 Figure 5 19 Boussinesq wave simulation using one dimensional wave generation...

Page 132: ...inesq wave simulation using directional wave generation input dfs1 see the MIKE 21 Toolbox tool Random Wave Generation 5 3 5 Bottom friction Short periodic waves Usually the effects of bottom friction...

Page 133: ...Chezy number formulation Manning number formulation Chezy number The bottom friction formulation used in MIKE 21 BW has been set up using the Chezy bed friction rule With the Chezy bed friction rule t...

Page 134: ...some hydraulic text books 5 3 6 Eddy viscosity The decomposition of the prognostic variables into a mean quantity and a tur bulent fluctuation leads to additional stress terms in the governing equati...

Page 135: ...n 5 5 It is not recommended to use eddy viscosity to damp out numerical instabili ties in MIKE 21 BW simulations Remarks and hints Eddy viscosity is mainly introduced in MIKE 21 BW for modelling of wa...

Page 136: ...dertaken in the Grid Editor dfs2 or in the Profile Editor dfs1 To obtained a smooth transition between 0 and 0 25 it is recommended to delete some 0 s around the area with filter coef ficient 0 25 and...

Page 137: ...ac tor The default value is 1 5 For plunging breakers see section Type of breakers Ozanne et al 2000 suggest a value of about 2 0 Type of roller celerity The roller celerity is assumed to be proportio...

Page 138: ...waves relative to true North The default direction is 270 N corresponding to waves from West Type of breakers Depth induced wave breaking of individual waves starts when the wave height becomes great...

Page 139: ...pness induced wave breaking which occurs on deeper water when the wave height becomes to large compared to the wave length White caps can be observed in the figure below Figure 5 27 White caps in open...

Page 140: ...nd a dis turbance of the flow in the physical domain On the other hand the numerical solution becomes unstable for extreme values of the slot width In practice it turns out that the slot width should...

Page 141: ...ty values at open water grid points should be set to unity i e porosity 1 0 The porosity should only be set to less than unity along structures where you want to include the dissipation effect of poro...

Page 142: ...sired reflection coefficient can be determined by the use of the MIKE 21 Toolbox program Calculation of Reflection Coefficients Important The porosity should never be smaller than 0 2 For impermeable...

Page 143: ...porous struc ture In most practical cases the pore sizes are relatively large typically 0 1 m to 1 0 m and the turbulent losses will dominate The laminar loss term has also been included to allow the...

Page 144: ...is outlined below Step 1 Open your bathymetry file dfs2 file and change the land value e g 5 along all structures having partial reflective properties to another value code e g 3 This is illustrated i...

Page 145: ...is to establish the porosity map Most often this is generated using the MIKE 21 Toolbox programme Generate Porosity Map as shown in Figure 5 31 In this tool you specify the calculated porosity value o...

Page 146: ...s a porosity value is determined using the MIKE 21 Toolbox programme Calculation of Reflection Coefficient For each of the defined sub areas the original porosity 0 5 in this example is replaced by th...

Page 147: ...e structures If no porosity values are specified the reflection will be 100 from land points As the porosity results in dissipation of energy the reflection may be con trolled by the porosity values a...

Page 148: ...s to repre sent a given transmission coefficient the MIKE21 Toolbox tool Calculation of Reflection Coefficients should be used 5 3 11 Sponge layers Sponge or absorbing layers can be used as efficient...

Page 149: ...eline see right hand figure below Figure 5 37 Example of sponge layer profiles The panel to the left shows a profile without moving shoreline and the right panel shows profile including moving shoreli...

Page 150: ...s above you are free to select your own values for the sponge layer damping coefficients Tool for generation of files including sponge layers Most often you will use the MIKE 21 Toolbox program Genera...

Page 151: ...and zero flux If you require a radiation boundary with another reference value you can specify the relevant level most often you only specify a surface elevation through an option Grid Point Sponge Va...

Page 152: ...such as skewness kurtosis and atiltness is calcu lated The temporal averaging can be performed on cumulative or subseries basis including or excluding the swash zone Wave disturbance parameters A comm...

Page 153: ...ies data dfs2 file For point series data type 0 you can choose a number of different points in each file You can view the results easily using the relevant MIKE Zero tools by clicking on the View butt...

Page 154: ...2DH module only Roller angle 2DH module only Roller celerity cx 2DH module only Roller celerity cy 2DH module only Water level roller 2DH module only Surface elevation The surface elevation is a basi...

Page 155: ...include this parameter in the output Auxiliary variable 1DH module only This prognostic variable is relevant to save in connection with debugging model results The auxiliary variable is defined as 5 9...

Page 156: ...e scientific documentation Usually you would not save this information Water level roller 2DH module only The water level including rollers is mainly used in connection with 3D visualis ation of the i...

Page 157: ...to a grid having the constant spatial resolution of Jlast Jfirst 1 Npoints In case you use a structured mesh of the bathymetry we recommend you to use the suggested default value of Npoints Jlast Jfi...

Page 158: ...a name and title of the file Figure 5 44 The list of phase averaged output parameters includes various varia bles for detailed analysis and presentation Wave statistics You can select between followin...

Page 159: ...r level first time Most often you should keep the default value Output type You can choose between following output types type 0 point series data dfs0 file type 1 line series data dfs1 file type 2 gr...

Page 160: ...zero crossing wave height within a specified time period Maximum surface elevation The maximum surface elevation max is defined as the maximum value of the surface elevation within a specified time p...

Page 161: ...ler thickness The mean roller thickness can be interpreted as a time averaged measure of the intensity of wave breaking Outside the surf zone the mean roller thick ness is zero The definition of the r...

Page 162: ...tiltness The atiltness A is defined by 5 13 where E denotes the mean operator The atiltness is a higher order integral measure of the lack of vertical wave profile symmetry The parameter takes a posit...

Page 163: ...Output Parameters 163 Figure 5 46 Example of phase averaged output showing the wave induced current upper The lower figure shows the instantaneous surface elevation on top of the bathymetry...

Page 164: ...ed meshes to save the entire profile of data i e keep the default values of Jfirst and Jlast 5 4 3 Wave disturbance parameters Figure 5 47 A number of wave disturbance parameters can be extracted from...

Page 165: ...e performed You must also specify the interval in number of time steps for which the 2D map is updated A typical value is 1 2 minutes Area statistics When wave dist coeff is set in your dialog setting...

Page 166: ...s a setup which you may use for reference Remarks and hints Start up period Regular waves Start up period For irregular waves in ports and harbours it is important to specify the simula tion time long...

Page 167: ...be started at arrival of e g wave No 30 and the calculation period should normally cover the time step range specified on the Simulation Period dialog The start up period should not be confused with...

Page 168: ...eous cross shore displace ment of the still water shoreline The initial value equals the x co ordinate of the still water shoreline Vertical run up The vertical run up is defined as the vertical displ...

Page 169: ...s a continuation of an earlier run For this to be possible details of the finishing conditions of the earlier simulation must have been saved in a hot data The information stored in the hot data file...

Page 170: ...zation APV Artificial land Batch mode Blow up Boussinesq cross terms Courant number Deep water terms First time model set up Hardware requirements High frequency noise Linear dispersion relation Mesh...

Page 171: ...as land Artificial land should only be applied to water areas which have no influence on the wave conditions in areas of interest The idea of applying artificial land is to save computational time In...

Page 172: ...he extension bat in a text editor The bat file should include a path to the bin directory and lines for each specification file default 64bit installation path path C Program Files x86 DHI year bin x6...

Page 173: ...caused by an inconsistency in the simulation setup Therefore please check your model specifications carefully Near the end of the log file you get the message BLOW UP detected In other words you have...

Page 174: ...simulation may also be the result of inconsistencies in your input specifications This is because the Boussinesq wave model is relatively robust and it can take some time before the effects of inconsi...

Page 175: ...d absorb the breaking wave energy by introducing sponge layers You may also reconsider you model setup and include wave breaking As you have to resolve the individual breaking waves you would probably...

Page 176: ...ow in Figure 5 54 Then the time step should be reduced to a temporal resolution of 50 points per wave period If instabilities still appear then you may try using a partly lin ear time extrapolation of...

Page 177: ...u may also try to use a smaller number by trial and error If all your efforts fail you can contact you local DHI Software Support Centre 5 5 5 Boussinesq cross terms The representation of the Boussine...

Page 178: ...ction of the classical form of the Boussinesq equations is the shallow water limitation in terms of the water depth to deep water wave length ratio h L0 Celerity errors gradually increase for increasi...

Page 179: ...Setup Planner for the minimum wave period 4 Find the time step for the numerical time integration Beside fulfilling the CFL criterion first expression in bracket the minimum wave period has to be res...

Page 180: ...ram For a 4 500 MHz Pentium II processor the CPS is in order of 100000 points second for 2D applications and about 15000 points second for 1D applications including surf zone dynamics When you know th...

Page 181: ...times result in instabilities which eventually may cause a model blow up An example is illustrated in the figure below Figure 5 56 High frequency noise Often the instability high frequency noise appea...

Page 182: ...r gT2 2 T is the minimum wave period A comparison between the phase celerity derived for the Boussinesq equations and Stokes first order theory is shown in Madsen et al 1991 and Madsen and S rensen 19...

Page 183: ...57 Example of spatial resolution for a barred coastal profile Conditions Spectral peak period 8 s 40 nodes per wave length and minimum mesh spacing 0 2 m 5 5 13 Minimum wave period The minimum wave pe...

Page 184: ...esult of a simulation with time extrapolation default in MIKE 21 BW time extrapolation factor 1 On the second panel the time extrapolation has been omitted corresponding to a backward time centring of...

Page 185: ...Entries Arranged Alphabetically 185 Figure 5 58 Application of different time extrapolation factors...

Page 186: ...sion of an option parameter in a MIKE 21 BW PFS file 5 5 16 References The references listed below provide you with more basic information applica ble to the Boussinesq wave module Application verific...

Page 187: ...Waters editor M B Fischer Academic Press New York Abbott M B Petersen H M Skovgaard O 1978 On the Numerical Model ling of Short waves in Shallow Water Journal of Hydraulic Research 16 3 Boussinesq typ...

Page 188: ...E F H S Chen and L L Hadley 1996 Validation of numerical model for wind waves and swell in harbours Journal of Waterway Port Coastal and Ocean Engineering 122 5 245 257 Sponge Layers and Radiation Bou...

Page 189: ...ption and Modelling of Direc tional Seas Lyngby Paper No C 2 p 11 Mei C C 1983 The applied dynamics of ocean surface waves John Wiley Sons New York Svendsen I A and Jonsson I G 1980 Hydrodynamics of C...

Page 190: ...Reference Manual 190 MIKE 21 BW DHI...

Page 191: ...dsen P A S rensen O R and Sch ffer H A 1997a Surf zone dynam ics simulated by a Boussinesq type model Part I Model description and cross shore motion of regular waves Coastal Eng 32 255 288 Madsen P A...

Page 192: ...Scientific Documentation 192 MIKE 21 BW DHI...

Page 193: ...193 INDEX...

Page 194: ...e step 177 Cross momentum derivatives 120 Cross section of a breaking 137 Cruise terminal 31 D 1DH module 110 2DH module 110 Deep water terms 178 Deep water terms 37 Deterministic parameters 152 DHI s...

Page 195: ...d interactions 10 Non Darcy flow 143 Non Gaussian characteristics 162 Normal incident irregular waves 131 Numerical damping 184 Numerical dissipation 120 Numerical instabilities 140 Numerical instabil...

Page 196: ...onics 10 Super harmonics 10 Surf similarity parameter 103 Surface elevation 126 Surface roller concept 118 136 Surging 139 T Temporary file 111 Text books 188 Time of arrival 164 Time step interval 12...

Reviews:

No comments