Research on numerical simulation of tandem thrusters flow-induced noise

A tandem propeller is a kind of propulsion device that is installed on the outer axis and the inner axis with the same axis respectively and rotates positively and negatively. Taking DTMB tandem thrusters as an example, which consists of propeller3689 and propeller3849, the hydrodynamic characteristic simulation of the tandem thrusters is completed, using sound analogy theory. The simulation results are consistent with the experimental results. The speed distribution of the tandem thrusters and the hydrodynamic wake of the tandem thrusters are analyzed. Through the analysis of acoustic radiation characteristics, it can be concluded that the noise characteristics of tandem thrusters are more complex than those of ordinary propellers, and there are more line spectrum characteristics in them.


Concept and connotation
A tandem propeller is a kind of propulsion device that is installed on the outer axis and the inner axis with the same axis respectively and rotates positively and negatively.Since the torque of the anterior and subsequent propeller can cancel each other, the yaw torque acting on the carrier can be effectively reduced.The subsequent propeller can absorb the rotational kinetic energy loss in the wake of the anterior paddle, making its propulsion efficiency much higher than that of the single paddle.Tandem thrusters are widely used in various underwater vehicles [1], such as torpedoes, submarines, tankers, merchant ships, and unmanned underwater vehicles.Table 1 shows the typical underwater vehicles using tandem thrusters as thrusters [1].In essence, the noise of the tandem propeller belongs to the fluid-induced acoustic radiation, which results from the non-uniform fluid field at the tail of the vehicle and the periodic unsteady force generated by the mutual influence between the multiple tandem propellers.The acoustic radiation feature mainly includes discrete spectrum and random broadband noise spectrum, which is richer than that of the traditional single propeller.Propeller noise is one of the three noise sources of underwater vehicles, and its unique radiation noise characteristics are the key basis for the detection and identification of targets.Therefore, it is very important to study the characteristic mechanism and modeling of tandem propeller noise for the development of detection equipment, target detection, and identification.

Overview of international and domestic progress
Due to the influence of non-uniform incoming flow and reverse motion of tandem thrusters, the flow field near the tandem propeller is very complicated, so its noise characteristics are more complex than that of the single propeller, and the hydrodynamic noise characteristics are usually discrete spectrum noise and random broadband spectrum noise.According to Hanson's prediction method for tandem thrusters [2] and Goldstein's acoustic analogy equation [3], Peters systematically summarized the mechanism of discrete spectral noise generation for tandem thrusters: the mutual influence between non-uniform fluid and the leading edge of the anterior paddle, the mutual influence between the nonuniform pressure distribution and the leading edge of the subsequent paddle in suction effect, the mutual influence between the shed wake vortex of the anterior paddle and the leading-edge of the subsequent paddle, and the mutual influence between the shed wake vortex of the anterior paddle hub and the boundary laminar flow and the subsequent paddle [4].Blandeau summarized the generating mechanism of wide-band spectrum noise of tandem thrusters [5]: the impact effect of anterior paddle wake on the guide edge of subsequent paddle, the mutual influence between blade boundary layer and blade accompanying edge, the impact effect of anterior tip vortex loss on the guide edge of subsequent paddle, the mutual influence between non-uniform incoming flow and anterior paddle, among which: the latter two points can be reduced by optimizing the design of the structure, so the main broadband noise comes from the influence of the first two factors.Zhu et al. carried out the calculation of the unsteady force on the blade surface based on the lifting surface theory and surface element method and studied the prediction of line spectrum noise.Wang and Yang [6] simulated the pulsating pressure in the flow field based on the CFD technology and combined with the acoustic analog equation to forecast the radiated noise of tandem thrusters in the wakefield.

Research on the noise mechanism of tandem thrusters
Because of the influence of high-speed rotation, peak slope, and multiple structural surfaces, the tandem propeller internal flow field has the characteristics of strong rotation, transient, pulsation, and nonlinear.At the same time, the rotation affects the velocity and direction of turbulent pulsation at the wall surface.The strength of circumferential internal flow and the anisotropy of internal flow of the tandem propeller are more distinct, the more likely it is to cause the fluid to separate, and a large range of shear flow will be generated near the blade wall.According to the classical hydrodynamic noise analogy theory of Lighthill [7], rotary blade noise can be roughly divided into two categories: the narrow-band feature in connection with the passing frequency of the blades of propellers and the broadband feature in connection with turbulent pressure pulsations on the blades.It can also be divided into two types: the monopole sound source formed by the periodic expansion and contraction of the surface of the blade thickness on the flow on the paddle disk, and the dipole sound source formed by the load on the blade acting periodically on the fluid on the paddle disk.The noise level depends on the size of the propeller, the motion state, and the load on the blade.The source of the broadband noise feature is usually the noise formed by the pulsating load caused by the turbulent boundary layer separation on the paddle disk.Meanwhile, the fluid vortex separated from the propellers and the circumambient fluid mixing also leads to the generation of a quadrupole sound source, and its noise size depends on the turbulent structure of the fluid.Among them, Figure 1 shows the noise-generation mechanism of rotating blades.

Numerical simulation of noise characteristics of tandem propeller
This paper takes the DTMB combined tandem propeller model as the research object (anterior paddle DTMB3689 and subsequent paddle DTMB3849).According to the large eddy simulation method and sound analogy theory, the sound source term of the unsteady flow field is extracted by calculating the extraordinary flow field of the serial thruster.The noise characteristics of the model are calculated and analyzed, and the main characteristics and distribution rules of the discrete spectrum of the serial thruster are obtained.

Numerical simulation model
According to the geometric characteristics parameters of the propeller, the geometric model was completed.Figure 2 shows the geometric model of the tandem propeller.During the numerical modeling and analysis process, the topology structure, cell type, mesh quantity, and mesh quality of the calculated mesh will greatly affect the accuracy, convergence, and flow detail resolution of the numerical calculation results.Therefore, the consistency between the calculation area and the actual flow area should be kept as much as possible, especially in the wall boundary part, Figure 3 and Figure 4 show the grid model of the blade and the computing domain.

Numerical simulation
In the prediction of open water performance, the advance coefficient was changed by changing the incoming flow speed at a constant speed.The RNG-k-e equation was established for the numerical simulation, and the comparison between the calculated open water performance curve and the test value was shown in Figure 5.The error of the calculated value was less than 5%, which proved the rationality of numerical modeling such as boundary condition setting and calculation node grid division.As shown in the Figure 6, the induced rate distribution on each blade in the direction of incoming flow is similar, and the velocity is positive.The induced rate distribution on each blade gradually increases from the front to the back of the blade, which is opposite to the thrust, and the axial induced Simulation experiment Simulation experiment velocity is the smallest at the leaf tip, where the flow is more complicated.By analyzing the distribution characteristics of radial velocity, there is an obvious vortex region near the tip of the blade, and the induced velocity is positive and the other one is negative in the vortex region.The maximum and minimum radial velocity appear in this region, the isocontour of the induced velocity appears along the radial direction, and the induced velocity gradually decreases from inside to outside, forming a suction effect.The propeller wake is a network structure.Based on the unsteady calculation of the flow field of the tandem propeller, pulsating pressure on the surface of propeller blades at each moment is extracted, and by inputting the blade surface element, surface element area, etc. as the output into the FW-H acoustic integral equation, the FW-H equation was solved, and the acoustical signature of the above tandem propeller at the speed of n=10 r/s are calculated.The spectrum is shown in Figure 7. From the acoustical signature of the tandem thrusters, frequency distribution characteristics of the low-frequency line spectrum band conform to the following theoretical prediction Equation (1), that is:

Conclusions
In this paper, the hydrodynamic noise mechanism and prediction method of tandem thrusters are briefly described, and the generation mechanism of discrete spectrum noise of tandem thrusters is analyzed.The discrete spectral features not only reflect the multiplicative frequency characteristics (the integer multiple of the product of speed and blade number, that is, blade frequency and multiple blade frequency) but also show the combined frequency form characteristics, including the narrowband feature generated by the anterior paddle action, the narrow-band feature components generated by the subsequent paddle action, and the narrow-band feature generated by the joint action between the anterior paddle and subsequent paddle.

Figure 1 .
Figure 1.The noise generation mechanism of rotating blades.

Figure 2 .
Figure 2. Geometric model of the tandem propeller.

Figure 3 .
Figure 3. Grid model of the blade.

Figure 4 .
Figure 4. Schematic diagram of the overall grid model of the computing domain.

Figure 5 .
Figure 5. Open water characteristic curve of tandem thrusters.

Figure 6 .
Figure 6.The disk velocity and the wake distribution of the propellers.

Figure 7 .
Figure 7.The acoustical signature of the tandem thrusters.

Table 1 .
Application of counter-rotating propeller in underwater vehicle.