Coherent effects of flow- and pressure hull of a generic submarine on target scattering in an active sonar performance model

Transcription

1 Coheret effects of flow- ad pressure hull of a geeric submarie o target scatterig i a active soar performace model P. Schippers TNO-D&V-Uderwater Techology, Oude Waalsdorperweg 63, Post Box 96864, 2509 JG The Hague, Netherlads pieter.schippers@to.l 6521

2 Sice the late eighties the soar performace model ALMOST for active ad passive soar is uder developmet at TNO. For active detectio performace, iitially a poit target was used, with a sigle Target Stregth value depedet o parameters like aspect agle, based o measuremets or other sources. However to day there is a growig demad for TS of ships ad wakes with realistic dimesios ad characteristics. A geeric sub was modelled with additioal software routies, as a pixel file. A ewly developed time domai model for hull reflectio was implemeted, also usig scatterig pixels, assumig multiple scatterig with dampig i the metal hull layer. Some modellig results of Target Stregth computatios are show, for a geeric submarie with pressure hull, with aspect agle, frequecy ad badwidth as parameters. The modelled Target Stregth decreases towards low frequecies due to hull thickess, as kow from literature. 1. Itroductio Sice the late eighties the soar performace model ALMOST for active ad passive soar, of TNO Defece Safety ad Security, is used for aval operatios ad studies [1]. For active soar detectio performace, with the REACT module, a poit target with a sigle Target Stregth value is applied, based o measuremets or other kowledge. Target Stregth is depedet o type of ship, aspect agle, ad also the used frequecy bad. Withi the Torpedo Defece System Test Bed by TNO [2], which uses ALMOST as acoustic kerel, there is a demad for Target Stregth modellig of ships ad wakes. Therefore more precise modellig of target scatterig was ivestigated [3,4], where the target is represeted by reflectig pixels i the ew REATES module, withi ALMOST. Supposig a fictive ifiitesimally short emitted pulse for active soar, the reflectio cotributios from all pixels are take ito accout i the overall Target Impulse Respose fuctio, also icludig the geerally preset multi path propagatio betwee soar ad target. By meas of oly oe FFT, which is very efficiet i computatio time, this Target Impulse Respose fuctio ca be trasformed to the frequecy domai, where all specific soar sigal processig takes place, similar to real soar systems. So far, oly targets with a total reflectig outer surface were cosidered. O oe had, such target modellig will ofte yield realistic modellig results, as tested with the well-kow Target Stregth of a flat surface or a rigid sphere [5]. But o the other had, for frequecies below 1 or 2 khz, the ship walls will ot reflect totally aymore because of the ratio of wall thickess compared with wave legth. The wall mass per uit area will become too low for actig as a rigid reflector. Iside the wall there may be air or water, causig specific effects. Those wall effects are observed i practice towards lower frequecies. So it is iterestig to develop modellig that takes ito accout this wall structure effect of ships. The ew idea here is to cosider a extra pixel layer at the ier side of the wall. Moreover more ier layers of pixels ca be cosidered represetig multiple reflectios withi the wall. The beefit of such a method will be that agai oly oe FFT is eeded for the Target Impulse Respose fuctio of the target pixel file, but ow exteded i this way. Apart from savig computatio time, this method ca be cosidered as a time domai model, which could tur out to be much more computatioally effective tha frequecy depedet wall scatterig models, i particular for applicatio to the moder ultra wide bad soars. Before describig the method for additio of extra pixels represetig the iteral wall reflectios, first the formulas for a total reflectig target surface will be give as a itroductio. After the full describig the wall model, some modellig examples are show, which idicate the effect of wall thickess, for flow hull ad pressure hull of a geeric submarie. 2. Coheret echo modellig for total reflectig targets First the pixel model for total reflectig target surfaces will be described shortly here, see also [3]. After this the ewly developed modellig of soud peetratio ito the target hull, by addig extra pixels iwards the target surface, will be described. A fictive ifiitesimally short trasmitted pulse travels from the active soar positio towards the target, represeted by pixels, with idex i, formig the outer surface. Each pixel i scatters the icidet soud, after a travel time t, depedet o the src, i locatio of the pixel i the cloud, ad the target rage. The soar receiver gets the echo sigal after a travel time t rec, i. The complete arrival time for pixel i is: t = t + t (1) arrival, i, m, src, i, m rec, i, i = pixel idex for target represetatio 6522

3 m,= multipath idices betwee soar source respectively receiver ad target. The above expressio is explicitly writte for multi path propagatio. The cotributio of pixel i for path t src, i, m, towards target, ad path t rec, i,, from target, is added to the Target Impulse Respose (TIR) fuctio, takig ito accout the total delay t arrival, i, m,, but also the stregth of the pulse-like cotributio. The latter depeds o the propagatio loss via path m, from soar to target ad path for the path back to the receiver. The pixel has the followig Target Stregth for a total reflectig target surface, see [5]: 2 10 d cosθ, TS, 20 log[ i m i m = ] (2) λ θ = agle betwee icidet soud ad local i,m ormal o reflectig surface, at pixel i Theoretically m for the icomig soud ca be iterchaged here with for the outgoig soud. The wall model is described ow below. The existig programmig for the TIR fuctio will be applied here with oly mior modificatios, by addig some extra pixels to each surface pixel, to model the wall characteristics. 3. Wall reflectio model A example of a pixel file for a geeric submarie is show i Figure 1. The iside pressure hull is show additioally here. A local part of the target surface is show i Figure 2, where the soud is supposed to hit this surface from above. Istead of assumig a total reflectig wall surface, i this ew model the wall is supposed to possess physical characteristics like material parameters. The desity, soud speed ad dampig per wave legth will play a mai role i the reflectivity, for istace for a iro ship wall of certai thickess, with air Figure 1 Represetatio of submarie Target pixel file or water behid it. The latter case will maily cocer submarie flow hulls. The upper pixel positios, see Figure 2, called level 0 pixels i the followig, are the pixels of the iitial target pixel file, to start with. So the level 0 pixels Scatterig pixel Soar side Physical layer (iro wall) modellig layer Figure 2 Scheme for ew wall pixel modellig i REATES, applied i geeric submarie with pressure hull basically describe the detailed ship costructio, show i Figure 1. I this target pixel file also a local wall thickess figure is added, so i additio to the already metioed local ormal vector per pixel. Mid here that the legth of the latter vector is equal to the (small) target area part A, represeted by the pixel. The iside wall boudary is show i Figure 2, as a so-called level 1 below the level 0 surface (the upper wall boudary). Physically, the part of the soud that is trasferred through level 0, will be reflected at level 1. Because of this trasfer of soud through level 0, also a partly reflectio at level 0 is supposed, this o the cotrary to the total reflectio assumptio i the simple modellig, see Eq.(2). The reflected soud at level 1, ca trasfer upward through level 0, ad farther back towards the soar. This mechaism is accouted for i the model by the extra pixel layer at level 1 see Figure 2. Because geerally the soud speed i the wall is differet from the water, therefore the positio of the level 1 pixels is adapted i a way that the travel time betwee level 0 ad level 1 remais valid, see below. Because of the partial reflectios at level 0 ad level 1, the Target Stregths of these pixels must be take lower tha for total reflectig pixels. A reflectio from level 1 back upwards ca also be reflected dowward at level 0. After a subsequet secod reflectio at level 1 upward it ca trasfer through level 0 back to the soar. This mechaism is accouted for by a secod extra pixel layer at level 2, placed below level 1, see Figure 2. Levels 0, 1 ad 2 are take equidistat. Other temporarily trapped reflectios iside the wall are take ito accout eve by more extra pixel layers, placed at levels 3, 4, 5, etc. It is plausible that the Target Stregth will geerally decrease for these lower levels. The TS for all extra pixels will be modelled i detail i the followig, as well as the origial level 0 pixel Target Stregths. 6523

4 The pixels at level 1, see Figure 2, are used to model the ier boudary of the wall. Because the level 0 pixels are ot ay more cofiig a totally rigid reflector, the Target Stregth for these pixels is take lower tha for total reflectio. For coveiece, the Target Stregth TS per pixel is writte as itesity. I the case of a total reflective, so rigid surface, we have, [5]: 0.1* TS tr = A = target surface (part) of pixel = 10 [ λ /( Acosθ)] (3) For costitutig the TIR fuctio, the complex amplitude must be applied i REATES, where also the sig is of importace: 0.05* TS = 10 λ /( Acosθ ) (4) Ampl = tr The itesity reflectio coefficiet IR up at level 0, is take as the well-kow result for plae wave reflectio [6]. It is computed close to ormal icidece, as a practical choice, ad is depedet o mechaical ad acoustic material parameters for the wall, like desity, soud speed, ad dampig per wave legth, ad the same parameters for water. The TS for the level 0 pixels is decreased with this reflectio factor, as follows: 0 = tr IR up (5) =TS for level 0 pixel layer 0 IR up = itesity reflectio coefficiet; level 0 The itesity reflectio IR i at the iside of the wall, at level 1, is modelled agai with the plae wave result [6]. The extra pixel layer of level 1 of stregth i1 is as follows: = tr IRi δ τ τ =1 IR up Where: δ = sigle path itesity dampig i layer τ = trasmissio itesity ratio through wall boudary The trasmissio itesity ratio τ ito the wall is based o the eergy balace assumptio, which is quite valid for the propagatig soud cosidered here [6]. Mid that IR ca differ from IR, i because most ship costructios will be air-filled. A exceptio here is a submarie flow hull, where water is iside. Because the pixel scatterig must be able to describe the outer reflectio ad all 2 up iteral wall reflectios, the level 1 pixels are placed at a distace Δ p i the opposite directio of the local ormal vector (o the local surface): Δ = Δ c p water wallmaterial Where: Δ = the actual wall thickess This corrected thickess Δ p assures correctly modelled time delays for the level 1 pixels, for use i the TIR fuctio. Subsequetly level 2 pixels are costructed to represet double reflectio iside the wall, placed at Δ below level 1, see Figure 2: p = IR up IRi δ τ I the same way level 3 ad higher ca be costructed: c = R up Ri δ τ As metioed above, the TIR fuctio requires the complex amplitudes of the pixels, see Eq.(4), summarised ow from the above: Ampl0 = Ampl tr R up λ Ampl = ( ) 1 Ri δ τ Rup Ri δ A cosθ (=1,2,3..) By usig: R = ( j = up : level 0, or i : level 1) j IR j IR j, R j = itesity resp. complex amplitude reflectio coefficiet. Mid that all extra pixels, so level 1 ad higher, are actig i couter phase, idicated by a mius sig. This is due to the reflectios iside the wall. All amplitudes ca be added i phase: = 0 Ampl = ( R δ R )/( 1 δ R R ) up i up i (7) If R up ad R i are equal, which is the case for a flow hull of a submarie, where water is also at the iside, the for a wall thickess approachig zero, so with δ towards 1, o amplitude will remai i Eq.(7), which looks acoustically cosistet. I the followig part some examples are give for a geeric submarie where wall thickess, wall materials ad frequecies are varied. 6524

5 4. REATES results of modelled echoes for a geeric submarie REATES was ru for a shallow water sceario with a geeric submarie, already preseted above, see Figure 2. The above described wall model is used i this submarie, applied with water behid the flow hull, ad air iside the pressure hull. The target rage is 10 km, with a target aspect agle of A geeric active towed array soar is applied i the REATES modellig. Two pulses are chose, with cetre frequecies 1500 ad 500 Hz, ad with bad widths of 1000 ad 200 Hz, respectively. Computatios are made icludig multi path propagatio, for realistic hull thickess values of the submarie, see Figure 3. The upper part shows a complicated echo structure, which is grace to the wide bad of 1000 Hz. The horizotal axis is active soar rage, beig a time scale coverted i a liear way usig half the average soud speed. The lower part shows a similar echo structure but less detailed due to the smaller bad width of 200 Hz. Figure 3 REATES result with echo structure ad reverberatio backgroud, icludig multi path propagatio; left) soar pulse 1500 Hz with 1000 Hz bad; right) pulse 500 Hz with 200 Hz bad This same submarie ca also be modelled with the assumptio of total reflectig pixels for a total reflectig target surface. Comparisos of results for this total reflective submarie ad the submarie with realistic wall thickess are show i Figure 4 ad Figure 5. For simplicity oly the strogest propagatio path is take here. Figure 5 REATES result with echo structure ad reverberatio backgroud; soar pulse 500 Hz with 200 Hz bad; left) realistic hull; right) total reflectig target structure Obviously the total reflective (fictive) submarie turs out to show higher echo levels. For the above scearios, a effective Target Stregth figure ca be computed: The real submarie target is replaced by a poit target with kow Target Stregth TS. The modelled echo output is compared with the echo output for the submarie target, by meas of a maximum filter applied to both modellig results. The the effective Target Stregth is simply computed from the db differece of both maximum filter outputs. The Target Stregth is computed i this way, for oe sigle propagatio path as well as for 5 paths i the sceario, see Table 1. TS values for the realistic submarie with wall thickess tur out to be aroud 10 db lower, compared with the simple assumptio of a total reflective target surface. Of course this differece will be cosiderably depedet o frequecy ad aspect agle. F=1500/B=1000 Hz/ 1 path F=1500/B=1000 Hz/ 5 paths F=500/B=200 Hz/ 1 path F=500/B=200 Hz/ 5 paths Realistic Total hull reflectig 2 db Table 1 Target Stregth of geeric submarie based o maximum detectio filter Figure 4 REATES result with echo structure ad reverberatio backgroud; soar pulse 1500 Hz with 1000 Hz bad; left) realistic hull; right) total reflectig target structure Fially the Target Stregth is computed versus aspect agle, for both pulses, see Figure 6. The 500 Hz pulse shows cosiderably lower TS levels compared with the 1500 Hz pulse. This effect is ascribed to the effect of the hull thickess. 6525

6 aroud 10 db dow, compared with the simple assumptio of a total reflective submarie target. Further validatio of the modellig results versus Target Stregth measuremets, however, remais a importat item i the future. Refereces Figure 6 Target Stregth versus aspect; upper) pulse 1500 bad 1000 Hz; lower) pulse 500 bad 200 Hz Fially it must be remarked that validatio of these modellig results with measured Target Stregth data remais a importat item for future work. 5. Coclusios This paper presets a ewly developed hull reflectio model, applicable to techically specified submaries. Dimesios ad thickess of all submarie parts is iput for the model. The model is implemeted as a ew feature i the ALMOST REATES target echo model of TNO. REATES models detailed target echo structures, basically versus arrival time, ad was built i the last years as a further developmet of REACT, the active soar performace modellig i ALMOST, usig the programmed active soar equatio. REATES is able to model Target Stregth values for extesive specified ship targets, where these figures ca be used operatioally o board of ships i ALMOST/REACT. The preseted hull reflectio model yields more realistic ad reliable results from REATES, i particular for the low frequecy active (LFAS) soars. The effect o Target Stregth of the local hull thickess of submaries i geeral, is expected to be much depedet o aspect agle ad frequecy. This study shows Target Stregth differeces [1] P. Schippers, The ALMOST PC model for propagatio ad reverberatio i rage depedet eviromets, Proceedigs of UDT Europe, 1995 [2] F.P.A. Beders, Torpedo ad coutermeasures modellig i the torpedo defece system testbed, Proc. of the Europea UDT 2004 Coferece [3] P. Schippers, Coheret target scatterig i the active soar performace model ALMOST/REACT, Proceedigs of ECUA 2004 [4] P. Schippers ad S.P. Beeres, Echo structures ad Target Stregth modellig for a sythetic submarie, Proc. of the Europea UDT 2007 Coferece [5] R.J. Urick, Priciples of Uderwater Soud, McGraw-Hill Book Compay, New York, 2 d ed [6] M.A. Aislie ad P.W. Burs, Eergycoservig reflectio ad trasmissio coefficiets for a solid-solid boudary, J. Acoustic Soc. Am. 98 (5), (1995) 6526