Chapter 4 has described the development and the use of numerical simulations to model hydroplaning on plane pavement surfaces with or without microtexture. The proposed model can also be applied to analyze the effects of pavement grooving on hydroplaning. This section shall discuss the verification of the model for simulating hydroplaning on transverse and longitudinal pavement grooving.
5.2.1 Verification against Experimental Data for Transverse Pavement Grooving
Verification of the simulation model in predicting the hydroplaning speed for transverse pavement grooving can be performed by comparing against experimental data reported by Horne and Tanner (1969). Four different pavement surfaces with transverse groove dimensions of 6 mm width by 6 mm depth by 25 mm spacing are tested and are shown in Table 5.1. They employed ASTM E-524 standard smooth tire of 165.5 kPa (24 psi) inflation pressure, and experiments were conducted on test surfaces flooded with water depth of 5.08 mm (0.2 in.) to 7.62 mm (0.3 in.). In the simulation model, pavement microtexture is
The predicted hydroplaning speeds and the friction factor at incipient hydroplaning calculated by the proposed model are plotted in Figure 5.1 together with the measured data by Horne and Tanner (1969). Next, the exponential friction-to-speed relationship proposed by Meyer (21) is applied to fit the data points. Meyer’s relationship is given in Equations (5.1) and (5.2).
(PNG v
v SN e
SN = 0 − 100) (5.1)
( )
⎟⎟⎠
⎜⎜ ⎞
⎝
= ⎛
v v
SN dv SN
PNG 100 d (5.2)
where SNv is the skid number at vehicle speed v, SN0 is a fictitious skid number at zero vehicle speed, and PNG is the percentage normalized gradient of the SN against v curve. Very high values of statistical coefficient of determination R2 of 0.938 and above are obtained for the four cases as shown in Figure 5.1. This confirms that the friction factors derived by the proposed simulation model are closely consistent with the experimentally measured data.
5.2.2 Verification against Experimental Data for Longitudinal Pavement Grooving For longitudinal pavement grooving, verification can be made by comparing against experimental data conducted by Horne (1969). The ASTM E-524 standard smooth tire of 165.5 kPa (24 psi) inflation pressure is used and experiments were conducted on longitudinally grooved surfaces with 6 mm width, 6 mm depth and 19 mm spacing. The pavement is flooded with water depth of 5.08 mm (0.2 in.) to 7.62 mm (0.3 in.). Figure 5.2 shows the comparison between the simulation results of the predicted hydroplaning speed and the friction coefficient at incipient hydroplaning and the experimental data points. Similar to the previous sub-section, the exponential friction-to-speed relationship can be developed and a very high value of statistical coefficient of determination R2 of 0.959 are obtained. This confirms that the proposed simulation model can predict friction factors that are close to the experimentally measured data.
As experimental studies on hydroplaning are often large in scale, costly to conduct and risky to personnel carrying out the experiments, the numerical approach proposed could be an efficient and cost effective mean to study the hydroplaning phenomenon and the effectiveness of pavement grooving designs. The previous sub-section has already verified the model for hydroplaning speed prediction on longitudinal and transverse pavement grooving. This section shall discuss on the use of the simulation model to study the effectiveness of longitudinal and transverse pavement grooving on hydroplaning control. Three particular designs are studied to understand how longitudinal and transverse pavement grooving can affect hydroplaning, as shown in Table 5.2.
5.3.1 Simulation Results for Transverse Pavement Grooving Designs
The ground hydrodynamic pressure distributions along selected profiles under a wheel of tire inflation pressure 186.2 kPa and sliding at a speed of 86.6 km/h for grooved pavement Designs A, B and C are shown in Figure 5.3. The average ground hydrodynamic pressure under the hydroplaning wheel is found to be 35.1 kPa for Design A, 57.3 kPa for Design B and 90.2 kPa for Design C. Therefore, hydroplaning does not occur at the NASA hydroplaning speed meant for the plane surface.
The model is next applied for speeds ranging from 0 km/h to 300 km/h to derive a tire pressure-hydroplaning speed relationship and to compare it with the NASA hydroplaning equation. It is found that recovery factors of 0.1190, 0.1981 and 0.3128 are obtained for Designs A, B and C respectively. The corresponding hydroplaning speeds are 199.5 km/h, 156.10 km/h and 124.4 km/h for a passenger car tire with tire inflation pressure of 186.2 kPa, against the predicted NASA hydroplaning speed of 86.9 km/h respectively. These significant increases in hydroplaning speed clearly demonstrate the benefits of applying transverse pavement grooving to reduce the occurrences of hydroplaning. The friction factors experienced by the wheel for a passenger car tire of inflation pressure of 186.2 kPa at incipient hydroplaning are found to be 0.442, 0.294 and 0.192 for Designs A, B and C respectively.
smooth surface (i.e. NASA hydroplaning relationship) and the three transverse grooving designs. It can be observed that the use of transverse pavement grooving increases the hydroplaning speed for a given tire inflation pressure and among the three grooving designs, Design A is most effective in guarding against hydroplaning.
5.3.2 Simulation Results for Longitudinal Pavement Grooving Designs
The ground hydrodynamic pressure distributions along selected profiles under a wheel of tire inflation pressure 186.2 kPa and sliding at a speed of 86.6 km/h for grooved pavement Designs A, B and C are shown in Figure 5.5. The average ground hydrodynamic pressure under the hydroplaning wheel is found to be 151.1 for Design A, 168.8 kPa for Design B and 173.6 kPa for Design C. Therefore, hydroplaning does not occur at the hydroplaning speed meant for the plane surface. Applying the model over speeds ranging from 0 km/h to 300 km/h, it is found that recovery factors of 0.4630, 0.5849 and 0.6014 are obtained for Designs B and C respectively. The corresponding hydroplaning speeds are 102.2 km/h, 90.9 km/h and 89.6 km/h for a passenger car tire with tire inflation pressure of 186.2 kPa, against the predicted NASA hydroplaning speed of 86.9 km/h, which correspond to a 17.6%, 4.6 % and a 3.1 % increase respectively. The increase in hydroplaning speed is small but nevertheless it shows that longitudinal pavement grooving does reduce the occurrences of hydroplaning. The friction factors experienced by the wheel for a passenger car tire of inflation pressure of 186.2 kPa at incipient hydroplaning are found to be 0.125, 0.111 and 0.108 for Designs A, B and C respectively. These are marginally higher than the friction coefficient of 0.0962 for the plane pavement surface, indicating that longitudinal pavement grooving of Design A, B and C offer little improvement in friction coefficient at incipient hydroplaning.
Figure 5.6 shows the relationship between hydroplaning speed and tire-pressure for a smooth surface (i.e. NASA hydroplaning relationship) and the three longitudinal grooving designs. It can be observed that the use of longitudinal pavement grooving increases the
pavement grooving design in combating hydroplaning.
5.3.3 Comparison between Transverse and Longitudinal Pavement Grooving for Designs A, B and C
Table 5.3 summarizes the simulation results for the three transverse and three longitudinal pavement grooving designs. It can be observed that consistently, the transverse pavement grooving of the three tested designs give a higher hydroplaning speed and a higher friction coefficient at incipient hydroplaning, compared to longitudinal pavement grooving of the same design. The numerical values of the friction coefficients at incipient hydroplaning for longitudinal pavement grooving are close to that of the plane pavement surface and the improvement in friction control is nearly negligible. However, somewhat more noticeable increases in hydroplaning speed are noted.