COMPARISON OF TRIALS WITH INITIAL AND REVISED SIMULATIONS

In document COMPUTER SIMULATION OF THE PERFORMANCE OF LIFEJACKETS - A Feasibililty Study (Page 36-39)

A comparison between the trials and initial simulations is made in Section 6. It was found that the initial simulations were slow to right (the inherently buoyant simulation being particularly slow). The simulations were revised to improve buoyancy and density properties as described in Section 8.

The following sections compare the trials with the revised simulations and highlight improvements over the initial simulations.

10.1 INFLATABLE LIFEJACKET

The following comparisons can be made:

w In both the trials and revised simulation the jacket succeeded in self-righting.

w In the trial, self-righting was estimated to take 2 seconds. In the initial simulation self-righting took longer, about 3 seconds. However, in the revised simulation the self-righting time was only 1.6 seconds. This significant decrease in righting time was due to the increase in turning moment resulting from the increase in buoyancy force and the increased separation between the centres of gravity and buoyancy due to changes in DYNAMAN's mass distribution and repositioning of the larger lifejacket.

w Righting occurred in a single, smooth movement in both simulations. In the initial simulation this was dominated by the roll-over mode. However, in the revised simulation it was dominated by the sit-up mode. The reason for this is that in the revised simulation the jacket is positioned significantly further away from DYNAMAN's chest. As explained in Section 6, this increases the turning moment about the shoulder to shoulder axis compared to that about a head to hip axis and hence increases the ‘Sit-up’ mode in the righting action. w In the trials, the equilibrium position of the manikin in the water was

to lie at about 45° to the horizontal with the arms in line with and beside the torso. The legs were bent with the knees at about 90° to the thighs which were slightly raised relative to the fine of the torso. The knees stayed together.

In the initial simulation the torso was seen to take up a similar equilibrium position, however the limbs were spread and relaxed due to differing joint constraints between DYNAMAN and the manikin. In the revised simulation, DYNAMAN lay at about 60° to the horizontal with limbs in the same relaxed position. The increase in the angle of the body to the horizontal occurred due to the increased angle between the jacket and DYNAMAN in the revised

w In the trial some movement of the jacket relative to the manikin occurred. In both simulations such movement could not occur.

10.2 INHERENTLY BUOYANT LIFEJACKET

The following comparisons can be made:

w In both the trial and the simulations, the jacket succeeded in self-righting.

w The righting time in the trial was 2.4 - 3.6 seconds and in the initial simulation was 5.25 seconds. However, the righting time was reduced to 2.5 seconds in the revised simulation. As with the inflatable lifejacket, this reduction in righting time was due to the increased turning moment brought about by the revisions made to the DYNAMAN

w In the trial, the manikin righted in an apparently smooth movement. However, in the initial simulation righting was by no means

continuous. The body was seen to stay on its side for several seconds before righting was completed.

In the revised simulation the right motion was much improved in that it occurred in a smoother movement although a small delay still occurred when the body was on its side.

w The equilibrium position of the manikin in the trials wearing the inherently buoyant jacket was similar to its position when wearing the inflatable jacket described in Section 10.1.

In the initial simulation, DYNAMAN did not reach equilibrium after 8 seconds. In the revised simulation, equilibrium was reached about 6 seconds into the simulation. As for the inflatable jacket simulation the body lay at about 45° to the horizontal with relaxed limbs laying almost vertically. The relaxed final position of the limbs indicate the increased joint flexibility in DYNAMAN compared to a manikin. w In the trial the jacket moved around a great deal on the manikin. In

particular, the jacket had a noticeable tendency to slip sideways and move up towards the head. In both the initial and revised simulation this motion was prevented.

10.3 SUMMARY

The revised simulations have given much better righting times then the initial simulations. The results are summarised in Table 3. The inflatable jacket rights a little faster than measured (1.6 seconds as opposed to 2 seconds) and the inherently buoyant jacket rights within the range of observed time (2.5 seconds in the revised simulation as opposed to 2.4 to 3.6 seconds measured).

Table 3 Self-Righting Times 2.5 sees 1.6 secs Revised Simulation 5.25 secs 3.00 secs Initial Simulation 2.4 - 3.6 secs 1.9 - 2.00 secs Trial Inherently Buoyant Inflatable Jacket

The reduction in righting time is a direct consequence of the increase in buoyancy of the jackets and the adjustment in density and volume of DYNAMAN.

The predicted righting modes differ between the initial and revised simulations but it was apparent from the trials that a range of modes can occur and the one which occurs is very much dependent on the initial position of the lifejacket. This point is demonstrated most clearly by the change from roll-over to sit-up mode observed with the inflatable jacket in the initial and revised simulation.

11.

CONCLUSIONS, RECOMMENDATIONS AND

In document COMPUTER SIMULATION OF THE PERFORMANCE OF LIFEJACKETS - A Feasibililty Study (Page 36-39)