Research on Hydrodynamic Interference Suppression of Bottom-Mounted Monitoring Platform with Fairing Structure

In the disturbance of unsteady flow field under the sea, the monitoring accuracy and precision of the bottom-mounted acoustic monitoring platform will decrease. In order to reduce the hydrodynamic interference, the platform wrapped with fairing structure and separated from the retrieval unit is described. The suppression effect evaluation based on the correlation theory of sound pressure and particle velocity for spherical wave in infinite homogeneous medium is proposed and the difference value between them is used to evaluate the hydrodynamic restraining performance of the bottom-mounted platform under far field condition. Through the sea test, it is indicated that the platform with sparse layers fairing structure (there are two layers for the fairing, in which the inside layer is 6-layers sparse metal net, and the outside layer is 1-layer polyester cloth, and then it takes sparse layers for short) has no attenuation in the sound pressure response to the sound source signal, but obvious suppression in the velocity response to the hydrodynamic noise. The effective frequency of the fairing structure is decreased below 10 Hz, and the noise magnitude is reduced by 10 dB. With the comparison of different fairing structures, it is concluded that the tighter fairing structure can enhance the performance of sound transmission and flow restraining.


Introduction
Bottom-mounted acoustic monitoring platform is mainly used to detect the acoustic signal under the sea so that the marine environment information such as flow velocity, topography and depth can be obtained (Berteaux, 1976;Mavrakos and Chatjigeorgiou, 1997). With the special application features that it can work unmanned and silently in the severe environment, the bottom-mounted platform has become increasingly prevalent with the constantly development of marine exploration especially in ocean environmental monitoring, forecast and prevention of marine disaster, as well as military use etc. In order to obtain the marine environment profile data more accurately, the platform structure needs to be more steady and reliable, which has been an essential criterion in the design procedure (Roy, 1998;Morris, 1978;Lee, 1973;Cron, 1965). With the increasing application in some complex military areas, the low-noise requirement is proposed and the platform struc-ture is required to have higher sensitivity for detecting and receiving the weak signal from the hash marine background noise.
As the most critical sensor, the hydrophone is placed inside the platform body to avoid or reduce the outside flow interference. While the signal interference generated from the hydrodynamic coupling effect of hydrophone suspension system and platform structure becomes a serious technical restrict in the accuracy and stability of the system (Yang, 2003;Hawkes and Nehorai, 2001). Hence the structural vibration induced by flow excitation to the platform body needs to be suppressed. In order to decrease the flow influence, the platform body is generally designed into the streamline shape. There have been different shapes for the existing monitoring platform until now, which are the vertical cylinder, horizontal cylinder or spindle (Ghasemloonia et al., 2015). Meanwhile, the entire monitoring system should also satisfy the requirements of conveniently launch-ing and retrieving. Hence, the platform structure applied to underwater acoustic monitoring which needs the high adaptability and stability is necessary to be researched out.
Owing to the military sensibility of the underwater platform research, there are few relative reports and the main research works nearly focus on the statics analysis and structural design. Shonting et al. (1996) studied the emission undersea by the submerged system of single moored buoy. Wang researched the attitude of submerged buoy and the dynamic characteristics of mooring cable under tension to increase the anti-interference ability (Wang et al., 2012). Besides, many researches refer to the vortex-induced-vibration (VIV) analysis of marine offshore structures and semisubmersible platforms which can lead to the mechanical failures and low productivity efficiency. Sarpkaya (2004) gave a comprehensive review of the VIV research on the circular cylindrical structures. Kandasamy et al. (2016) concluded the vibration control methods for marine offshore structures subjected to the unsteady hydrodynamic forces. Oviedo-Tolentino et al. (2014) and Dewi et al. (1999) studied the VIV of a bottom fixed circular cylinder. Yuan et al. (2016) and Wu et al. (2014) analyzed the fluid and structure interaction of the AUV with different outline. Nevertheless, the relevant research on the hydrodynamic stability of bottom-mounted platform is really rare.
For the reduction of vibration influence, the modal frequencies of the platform body should be separated from the effective monitoring frequency range through the structure improvement and the vibration reduction treatments. The existing vibration evaluation methods are mainly carried out with the simulations and experiments, in which the sensors should be fixed on the structure (Kandasamy et al., 2016). Nevertheless, the additional test devices will generate some variances to the inherent vibration characteristics. Meanwhile, there are large differences for the modal parameters in different medium, and the difference value can reach 50%-70% (Kramer et al., 2013). Therefore, the evaluation process needs to be carried out in actual underwater environment by its own sensors which are the sound pressure and velocity sensors. This paper describes a novel platform structure applied for underwater acoustic monitoring in shallow sea environment, which can effectively reduce the flow interference.
The suppression effect evaluation based on the correlation theory of sound pressure and particle velocity for spherical wave in infinite homogeneous medium is proposed. Moreover, the cause of hydrodynamic noise is analyzed to better evaluate the flow restraining performance of the fairing structure. At last, the sea tests are carried out to compare the effect of different fairing structures. The sound pressure and velocity responses are analyzed in the frequency from 1 to 100 Hz, and the difference value of them is adopted to evaluate the flow restraining performance.

Bottom-mounted monitoring platform description
The platform body is designed as a bottom-mounted form on the seafloor, and the entire system is divided into instrument cabin part, retrieval unit and main body part, which is shown in Fig. 1.
The entire system includes the main monitoring part and retrieval unit, which are shown in Fig. 2.
The main body consists of the instrument cabins and hydrophone frame. The instrument cabins include the electronic cabin and battery cabin which are used to acquire data and supply power. The hydrophone frame is used to fix the hydrophone with flexibility suspension, and the external surface is wrapped with fairing to reduce the flow interference. The fairing is comprised of the steel wire gauze and nylon fabric cloth which has better sound transmission and flow restraining.
The fairing is shown in Fig. 3.  The retrieval part is used to launch and retrieve the main body. There are two bins in the retrieval part which are used to store the cables and the mooring balls connected with it. The cables are tightened up when the platform on monitoring is set to release as the recovery command emitted with acoustic releaser.
To decrease the interference between different parts, the main body and the retrieval part are connected by the cable which is at least two times longer than the depth. Several saddle weights are tied to the cable to prevent the flutter. The arrangement is shown in Fig. 4.
The vector hydrophone is fixed on the balance bracket and then the outer body is wrapped by the fairing. To maintain the attitude stability of the hydrophone, the two axes of the balance bracket are arranged orthogonally, as shown in Fig. 5.
The main technical parameters of this platform are listed in Table 1.

Relation between the sound pressure and velocity in far field
In infinite homogeneous fluid medium, the wave equation for small amplitude oscillation is where c is the sound propagation velocity, p is the sound pressure, t is the time, and is the Laplace operator.
v With Euler equation, the relation of the particle velocity and p is ∇ where is the gradient operator, and ρ is the density.
For the spherical wave in infinite homogeneous medium, the sound pressure is where A is a constant, ω is the radian frequency, k is the wave number, k=ω/r, and r is the distance.    where n is the unit vector, and n = cos θ cos αi + sin θ cos α j + sin αk.
Hence, the wave impedance of the spherical wave Z 0 (kr) is The far field condition is satisfied for the distance between the hydrophone and the underwater sound source more than a few kilometers, which has . Therefore, the spherical wave can be approximated as the plane wave of which the real part is the main composition of the wave impedance, as well p and are in-phase (Piggott, 1964;Von Winkle, 1979). Eq. (6) is derived as: Suppose that the distance between the hydrophone and the sound source is 1-5 km, and the signal frequency range is 10-100 Hz, then the variation of the phase difference between the sound pressure and velocity is shown in Fig. 6.
As shown in Fig. 6, the phase difference value decreases with the increase of the distance and frequency. The value of difference is smaller than 2° when the distance is 1 km and the frequency is 10 Hz. For the long distance monitoring, the phase difference can be approximated as 0°, hence Eq. (7) is available to evaluate the performance of the bottom-mounted platform.

Calculation of the fairing's transmission loss
The hydrophone is mainly used to measure the characteristics of sound signals underwater. Nevertheless, the variation of acoustic impedance can be introduced by the fairing. Therefore the transmission loss of fairing should be calculated to evaluate the influence of fairing on the sound field.
ρc Assuming that the fairing is considered as the elastic spherical shell, the medium inside and outside of the fairing is all water and then the acoustic impendence is .
As shown in Fig. 7, the incident wave P 0 is supposed as plane wave along the radial direction of the fairing, and the magnitude is set to be 1, then , and is the frequency of the incident wave.
Expanding Eq. (8) along the sphere can obtain where is Bessel's equation, and is Legendre equation.
With the elastic spherical shell boundary conditions and Eular equation, the vector field in the center of sphere is expressed as: where is the sound pressure inside the fairing; is the radial velocity; is the tangential velocity; u is the wave size as ; is the first order Hanker function of the second kind; Z 0 and Z 1 are the impedances of the spherical shell in its zero-and first-order mode.
Assuming that the direction of incident wave is 0°, then ] . (11) Since the optimality of the spherical fairing is that the acoustic impedance is zero, which is also , then P 1 = ρcV 1 .
(12) To evaluate the influence of the fairing on the sound field inside, Eq. (12) can be used to compare the response of V 1 and P 1 , in which the difference between them closing to zero indicate that the fairing has relatively less influence.

54
WANG Zhen et al. China Ocean Eng., 2018, Vol. 32, No. 1, P. 51-61 sound pressure receivers Ocean ambient noise is the main interference in the underwater acoustic measurement. Fig. 8 gives the relation between the spectrum and the energy of the ocean ambient noise (Wenz, 1962).
As shown in Fig. 8, the turbulence noise caused by hydrodynamic force appears in the frequency range of 1-100 Hz. Under the dynamic force of flow around the platform, the vibration generated by the vector hydrophone will be transmitted to the sound receiver along the structure and hence cause the interference. Meanwhile, the flow around the hydrophone can also generate the vibration which is the pseudo noise in the response signal of the sound receiver.
According to the restriction of the sensor, the vector hydrophone is more sensitive to the hydrodynamic interference than the sound receiver. The primary reason for this phenomenon is the Karman vortex street caused by the flow around the hydrophone. The ratios of signal to noise of different receivers are expressed as: ω ρ where (SNR) V and (SNR) P are the signals to noise ratios of the vector and sound receiver, respectively; V 0 is the signal velocity; P 0 is the signal sound pressure; S is the surface area of the receiver; Q 0 is the volume velocity; is the signal frequency; is the water density; a is the radius of the receiver (Lee, 1973). In case of the same size, the ratio of (SNR) V to (SNR) P is . When , the value is -57 dB which corresponds to the experiment results from Keller (1977). In order to illustrate the different responses from the vector receiver and sound receiver to the ambient noise, Fig. 9 gives the comparison of response to ambient noise between the vector and sound receiver. The result is from the sea test in the South China Sea of which the depth is 100 m and the flow velocity is 0.5 m/s. It is shown that the vector response has the higher response magnitude below 40 Hz which is the frequency range of hydrodynamic noise. The difference is close to 10 dB.
In the measurement of the vector-phase signal below the frequency of 100 Hz, the interference is mainly caused by the vortex separation and wave oscillation, and the interference frequency has the great relation with the flow velocity. Fig. 10 gives the difference of flow induced noise from the vector receiver at different flow velocities. It is shown that the vector receiver has the higher noise response in the whole frequency range from 1 to 100 Hz and the magnitude increases with the flow velocity. As well as the vector receiver has the higher response magnitude.

Analysis of the experiment for evaluating the flow restraining performance of fairing
In order to evaluate the effects of the platform structure, the sea test in the South China Sea is carried out between July 30, 2015 and August 10, 2015, as the period is in fishing off season and therefore the interference can be reduced. The sea floor in the test area is flat and composed of sediment, as well as the depth is 90 m.
The self-contained monitoring structure is applied in the monitoring system, which consists of the main body unit, mooring unit, and retrieval unit. The system is shown in Fig. 11.
Two platforms are used to compare the effects of fairing structure and signal direction on the system behavior. The distances between two platforms and the sound source are both 2000 m, and the two platforms are 200 m apart, as shown in Fig. 12. The fairing structures of two platforms are  dense layers and sparse layers respectively which are listed in Table 2. And there are two sound source depths which are 10 m and 50 m to compare the effects of the sound source positions near the surface and bottom of the ocean.
The signal sampling frequency is 40 Hz and the monitoring time is longer than 24 hours, hence the influences from flow and tide can be recorded. Meanwhile, the information including hydro meteorology, AIS and GPS are all acquired for the auxiliary analysis. The flow velocities in the period from July 30 to July 31 are shown in Fig. 13, of which the maximum value is 0.52 m/s and the average value is 0.31 m/s. The accuracy and precision of the signal sampling are related with the sensitivity of the hydrophone and the restraint effect of flow interference. According to our previous test results about the flow resisting and sound transmission effects, it is shown that: (1) The more close-knit the material, the better the flow resisting effect, and the worse the sound transmission effect, such as the polyester oxford cloth.
(2) The sparser the material, the worse the flow resisting effect, and the better the sound transmission effect, such as the metal net.
(3) The combination of polyester oxford cloth and met-al net is the optimal fairing form, of which the combination mode is the most critical part.
(4) The material with better flow resisting effect and worse sound transmission effect should be placed in the outermost layer, because the outermost layer has the more obvious effect on the flow resistance.
In order to evaluate the flow resisting effect and sound transmission, the fairing around the platform is designed as the multi-layer structure since the out layer uses the closeknit material and the inner layer uses the sparse material. The material and structure of the fairing are listed in Table 2. The material of the metal net is stainless steel, and the dense metal net has 800 wires on 100 mm of a grid, while the sparse net has 500 wires.
The structure of the fairing is shown in Fig. 14.
The evaluation procedure is as follows.
(1) Hydrodynamic interference evaluation (a) The evaluation analysis is carried out in the test period without sound source.
(b) Two record periods corresponding to the current ve- Triple layers, which the first layer inside is 3-layer dense metal net, the second layer is 3-layers sparse metal net and the layer outside is polyester oxford cloth (for short as dense layers) Dense metal, sparse metal and polyester oxford cloth #3 Double layers, which the layer inside is 6-layer sparse metal net, and the layer outside is polyester oxford cloth (for short as sparse layers) Sparse metal and polyester oxford cloth    (c) The responses of the sound pressure P and velocity V as well as the value of P-ρcV are compared respectively between different platforms in the frequency domain.
(d) The fairing structure with better flow resisting effect should have the lower value of the velocity and the value of the P-ρcV is near zero.
(2) Deterministic signal evaluation (a) The evaluation analysis is carried out in the test period with sound source.
(b) Two record periods corresponding to the sound source depth of 10 m and 50 m respectively (10 m means the sound source near the surface, 50 m means near the bot-tom) are analyzed to obtain the response in different signal directions while the signal frequency and current velocity are the same for different depths of the source.
(c) The responses of sound pressure and velocity as well as the value of the P-ρcV are compared respectively between different platforms in the frequency domain.
(d) The fairing structure with better sound transmission effect should have higher value of the sound pressure and the value of the P-ρcV is near zero.
The test period from July 30 12:00 to July 31 06:00 is analyzed, the sound source tests are carried out twice, and the signal frequencies are both 32 Hz and the depths are 10 m and 50 m, respectively. The sound source experiment configuration is listed in Table 3.
Firstly, the time-frequency spectrum of the values P, V and P-ρcV are shown in Fig. 15 to illustrate the test results.
The amplitudes of two signals are all 100 dB and the excitation time is 14:00 and 18:00, respectively, which are obvious in Figs. 15a-15d. In Figs. 15e and 15f, the values of P-ρcV are both near 0 dB for two platforms in the whole test period.
Then the evaluations are carried out.
6.1 Hydrodynamic interference evaluation (1) Test interval with the current velocity is 0.45 m/s The frequency spectrums of P and V from #2 and #3 are shown in Fig. 16.
In Fig. 16, the difference in P response is smaller than 2 dB while the V response of #2 is 5 dB higher especially when the frequency is below 20 Hz which is in the hydrodynamic frequency range. It means that the fairing structure of Platform #3 has better flow resisting effect but the same sound transmission effect to the ambient noise.
The frequency spectrum of P-ρcV is shown in Fig. 17. In Fig. 17, the amplitudes of both platform are minus when the frequency is smaller than 20 Hz, and the reason is that the excitation source is mainly the ambient sea noise and flow excitation which have greater influence on the vector sensor. When the frequency exceeds 20 Hz, the P-ρcV value of #3 is near 0 dB while #2 is -8 dB. It means that the acoustic impendence of #3 fairing structure is near zero and then it has relatively less influence according to Eq. (12).
(2) Test interval with the current velocity of 0.05 m/s The frequency spectrums of P and V from #2 and #3 are shown in Fig. 18.
In Fig. 18, the differences in P response and V response are all smaller than 2 dB which means that both fairing structures have almost the same flow resisting effect when the current velocity is 0.05 m/s or less.
The frequency spectrum of P-ρcV is shown in Fig. 19. In Fig. 19, the P-ρcV value of #3 oscillates around 0 dB in the whole frequency range while #2 is still smaller than 0 dB but the magnitude is -5 dB. It also means that the difference of the flow resisting effect between different fairings will decrease with the decrease of current velocity while #3  WANG Zhen et al. China Ocean Eng., 2018, Vol. 32, No. 1, P. 51-61 57 has better effect.
6.2 Deterministic signal evaluation (1) Test interval with the source depth of 10 m The frequency spectrums of P and V from #2 and #3 are shown in Fig. 20.
In Fig. 20, #3 has 5 dB higher magnitude of P response than #2 in the signal frequency of 32 Hz while V responses are both the same. As to other frequencies, the difference between two platforms in P response is smaller than 5 dB and in V response is near zero. It means that the fairing structure of Platform #3 has the better sound transmission effect on the deterministic signal.

58
WANG Zhen et al. China Ocean Eng., 2018, Vol. 32, No. 1, P. 51-61 The frequency spectrum of P-ρcV is shown in Fig. 21. In Fig. 21, the P-ρcV value of #3 oscillates around 0 dB in the whole frequency range while #2 is in -7 dB. It also means that #3 fairing structure has relatively less influence on the deterministic sound signal penetration.
(2) Test interval with the source depth of 50 m The frequency spectrums of P and V from #2 and #3 are shown in Fig. 22.
In Fig. 22, the characteristics of both P response and V response in the frequency of 32 Hz are similar to the above. Nevertheless, the magnitude difference between two platforms in the signal frequency is enlarged to the value more than 8 dB, since the distance between the sound source and two platforms decreases as the depth of the sound source increases, and then #3 has more obvious response to the sound signal.
The frequency spectrum of P-ρcV is shown in Fig. 23.
In Fig. 23, the deviations from 0 dB are both larger than 5 dB when the frequency is smaller than 30 Hz for two platforms, because of the influence of the current on the vector hydrophone. The P-ρcV value of #3 oscillates around 0 dB while #2 is 5-8 dB lower which means that the effect of the sound transmission from #3 is better than that from #2.
According to the evaluation above, the fairing structure of sparse layers type in Platform #3 has better flow resisting and sound transmission effect. The reason is that the sparse layers type fairing structure has tighter wrapping pattern than that of #2, while the metal net used is sparser than that of #2. Hence the flow influence can be suppressed to    WANG Zhen et al. China Ocean Eng., 2018, Vol. 32, No. 1, P. 51-61 59 the lower magnitude especially when the current velocity is larger, while the sound signal can penetrate through the out fairing with smaller transmission loss. Meanwhile, from Figs. 20 and 22, the magnitude differences between two platforms in both P and V receivers increase with the increase of the sound source depth. This is because the platforms are bottom-mounted, and the distance between the platform and the sound source decreases with the increase of the sound source depth. In the closer distance, the signal intensity is larger and then the hydrophone response difference caused by the sound transmission performance of the fairing will decrease.

Conclusions
The bottom-mounted submersible platform structure wrapped with the fairing is designed to reduce the hydrodynamic influence. To evaluate the flow resisting and sound transmission effect of the fairing structure, the theoretical and experimental researches are carried out to obtain the following conclusions.
(1) A novel bottom-mounted submersible platform including the main monitoring part and retrieval unit is described.
(2) To evaluate the influence of the fairing on the sound field inside, the P-ρcV value close to zero indicates that the fairing has relatively less influence.
(3) Through the analysis of hydrodynamic noise, it is in-dicated that the vector hydrophone is more sensitive to the hydrodynamic interference than the sound receiver. The primary reason for this phenomenon is the Karman vortex street caused by the flow around the hydrophone.
(4) Through the sea test, the fairing structure of sparse layers type has better flow resisting and sound transmission effect. Besides, the flow resisting effect difference between different fairing structures will decrease as the distance between the sound source and the platform increases.