**6.1 Velocity field**

The direct route flow field with the velocity of the hybrid underwater glider PETREL at 0.5m/s、1m/s、1.5m/s and 2m/s was simulated by using CFD ways. The simulation results are shown in Figure 32.

Hydrodynamic Characteristics of the Main Parts of a Hybrid-Driven Underwater Glider PETREL 61

5.6×10-1 5.0×10-1 4.5×10-1 3.9×10-1 3.7×10-1 3.4×10-1 2.2×10-1 1.7×10-1 1.1×10-1 5.6×10-2 0.0×10-1

Unit:m/s

(a) Steady turning in longitudinal vertical plane (b) Steady turning in horizontal plane

The pressure distributions on the vehicle at the speeds 0.5m/s and 2m/s when the angle of

pressure on the vehicle gradual reduction from head to tail of the vehicle, a high pressure region on the head and a low pressure region on the tail, which induced the pressure drag on the vehicle. The pressure of the high pressure region become higher and the low pressure region become lower with the speed of the vehicle increasing, it means that the pressure drag on the vehicle increase with the speed increasing. It can be known from the pressure distribution on the propeller shroud that pressures drag act on the shroud because of there has higher pressure inside the shroud and lower pressure outside the shroud. The reason for thus pressure distribution is that the propeller doesn't rotating in the glide mode which makes the velocity of flow inside the shroud slower than the outside. So the shroud should

be removed or the profile changed to reduce the drag on the vehicle in glide mode.

 Fig. 34. Pressure distribution (*V* =0.5m/s) Fig. 35. Pressure distribution (*V* =2m/s) The pressure distributions on the vehicle at the speed 0.5m/s when the angle of attack

isn't zero are shown in Figure 36. The pressure distribution on the vehicle isn't symmetry, the pressure of front flow surface higher than back flow surface, when glide with an angle of attack. The wing has the biggest degree of asymmetry of the pressure distribution which

α

is zero are shown in Figure 34 and Figure 35. There has a tendency that the

2.45×10<sup>3</sup> 2.06×10<sup>3</sup> 1.66×10<sup>3</sup> 1.27×10<sup>3</sup> 8.75×10<sup>2</sup> 4.80×10<sup>2</sup> 8.61×10<sup>1</sup> -3.08×102 -7.02×102 -1.10×103 -1.49×103

Unit:Pascal

Fig. 33. The steady turning flow field

**6.2 Pressure distribution** 

attack

α

5.6×10-1 5.0×10-1 4.5×10-1 3.9×10-1 3.7×10-1 3.4×10-1 2.2×10-1 1.7×10-1 1.1×10-1 5.6×10-2 0.0×10-1

> 1.57×10<sup>2</sup> 1.34×10<sup>2</sup> 1.11×10<sup>2</sup> 8.77×10<sup>1</sup> 6.45×10<sup>1</sup> 4.12×10<sup>1</sup> 1.80×10<sup>1</sup> -5.19×100 -2.84×101 -5.16×101 -7.48×101

Unit:Pascal

(c) *V* =1.5m/s (d) *V* =2.0m/s

It is seen that the flow field patterns in the figures are nearly the same. There was high flow rate region near the abrupt curve surfaces of the vehicle head, ballast of the GPS, rudders, while there was also the low flow field domain on the front of those parts and near the tail of the vehicle. The high flow rate region area decreases as the velocity increases. The existence of the mast of GPS makes the flow field behind it disturbed, and makes the flow field asymmetrical. These changes will increase the drag and hydrodynamic moment on the vehicle.

The steady turning flow field in longitudinal vertical and horizontal plane with the velocity of vehicle at 0.5m/s, is shown in figure 33.

It is noted from Figure 33 that the pattern of the steady turning flow field in longitudinal vertical plane and in horizontal plane has notability difference. Due to the rotational speed, the flow field is obviously asymmetric and appears large scale high flow rate region and low flow rate region in the back of the field. An extra hydrodynamic moment is induced because of the asymmetry of the flow field.

(a) Steady turning in longitudinal vertical plane (b) Steady turning in horizontal plane

Fig. 33. The steady turning flow field

#### **6.2 Pressure distribution**

60 Autonomous Underwater Vehicles

5.6×10-1 5.0×10-1 4.5×10-1 3.9×10-1 3.7×10-1 3.4×10-1 2.2×10-1 1.7×10-1 1.1×10-1 5.6×10-2 0.0×10-2

Unit:m/s

(a) *V* =0.5m/s (b) *V* =1m/s

2.3×10-0 2.1×10-0 1.8×10-0 1.6×10-0 1.4×10-0 1.1×10-0 9.1×10-1 6.8×10-1 4.5×10-1 2.3×10-2 0.0×10-1

Unit:m/s

 (c) *V* =1.5m/s (d) *V* =2.0m/s

It is seen that the flow field patterns in the figures are nearly the same. There was high flow rate region near the abrupt curve surfaces of the vehicle head, ballast of the GPS, rudders, while there was also the low flow field domain on the front of those parts and near the tail of the vehicle. The high flow rate region area decreases as the velocity increases. The existence of the mast of GPS makes the flow field behind it disturbed, and makes the flow field asymmetrical. These changes will increase the drag and hydrodynamic moment on the

The steady turning flow field in longitudinal vertical and horizontal plane with the velocity

It is noted from Figure 33 that the pattern of the steady turning flow field in longitudinal vertical plane and in horizontal plane has notability difference. Due to the rotational speed, the flow field is obviously asymmetric and appears large scale high flow rate region and low flow rate region in the back of the field. An extra hydrodynamic moment is induced because

Fig. 32. The flow field at different velocity

of vehicle at 0.5m/s, is shown in figure 33.

of the asymmetry of the flow field.

vehicle.

1.7×10-0 1.5×10-0 1.4×10-0 1.2×10-0 1.0×10-0 8.5×10-1 6.8×10-1 5.1×10-1 3.4×10-1 1.7×10-2 0.0×10-1

1.1×1.0 1.0×10 9.1×10-1 7.9×10-1 6.8×10-1 5.7×10-1 4.5×10-1 3.4×10-1 2.3×10-1 1.1×10-12 2

Unit:m/s

Unit:m/s

The pressure distributions on the vehicle at the speeds 0.5m/s and 2m/s when the angle of attack α is zero are shown in Figure 34 and Figure 35. There has a tendency that the pressure on the vehicle gradual reduction from head to tail of the vehicle, a high pressure region on the head and a low pressure region on the tail, which induced the pressure drag on the vehicle. The pressure of the high pressure region become higher and the low pressure region become lower with the speed of the vehicle increasing, it means that the pressure drag on the vehicle increase with the speed increasing. It can be known from the pressure distribution on the propeller shroud that pressures drag act on the shroud because of there has higher pressure inside the shroud and lower pressure outside the shroud. The reason for thus pressure distribution is that the propeller doesn't rotating in the glide mode which makes the velocity of flow inside the shroud slower than the outside. So the shroud should be removed or the profile changed to reduce the drag on the vehicle in glide mode.

Fig. 34. Pressure distribution (*V* =0.5m/s) Fig. 35. Pressure distribution (*V* =2m/s)

The pressure distributions on the vehicle at the speed 0.5m/s when the angle of attack α isn't zero are shown in Figure 36. The pressure distribution on the vehicle isn't symmetry, the pressure of front flow surface higher than back flow surface, when glide with an angle of attack. The wing has the biggest degree of asymmetry of the pressure distribution which

Hydrodynamic Characteristics of the Main Parts of a Hybrid-Driven Underwater Glider PETREL 63

These findings indicate that the shroud of the underwater glider will increase drag, decrease the glide efficiency, but it improves the stability when the angle of attack is larger than 8º. Therefore, the shroud is not a successful design element for the HUG in glide mode, but it

Using CFD to analyze the shroud's hydrodynamic effects shows that the vehicle should only

Finally, the velocity field, pressure distribution of the hybrid glider PETREL were analyzed, which make us understand how those main parts effect on the hydrodynamic characteristic

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of the vehicle.

**8. References** 

452

pp. 48–59

38, pp. 45-51

makes the wings the main lift generating parts. The asymmetry of the pressure distribution on the vehicle also induces the hydrodynamic moment on the vehicle.

Fig. 36. Pressure distribution (*V* =0.5m/s, 6 α= )
