Caracterización de la entomofauna cadavérica

Lateral sides of the tractor semi-trailer combination are important in the overall flow field analysis. This is because these faces having the largest area of the vehicle’s external surface that exposed to wind actions. The windward side in particularly, has greater significance due to the fact that this side experiences the direct impact of the larger portion of the oncoming gust flow. The velocity contours at various time points during TSI gust scenario described in Section 5.6.2 show that the variation in gust speed causes increased interaction between the surfaces of the vehicle and the gust flow. In order to analyse this in terms of vehicle aerodynamic forces, contours of the total pressure distributions due to TSI gust conditions have been computed for the leeward and windward sides of the trailer in this section.

Figure 5-28 shows variations of the total pressure distribution on the leeward side (left) and windward side (right) of the trailer for three different instants of time during the TSI gust as described in Figure 5-23. As expected, the total pressure acting on windward side of the trailer was observed to be significantly higher than that of the leeward side. A very-low and sometimes

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negative pressure zones that were developed on the leeward side during the TSI gust event are due to the development of flow vortices and flow separation on the surface.

a) Pressure contour at monitor point 1 (i.e., at t5 sec): leeward side (right), windward side (left)

b) Pressure contour at monitor point 2 (i.e., at t13 sec): leeward side (right), windward side (left)

c) Pressure contour at monitor point 3 (i.e., at t15 sec): leeward side (right), windward side (left)

Figure 5-28: Snapshots of the total pressure distribution on sides of the trailer

Moreover, this pressure difference increased gradually over period of the TSI gust condition (i.e., from t= 5 second to t= 14 sec). As shown in Figure 5-28, at the mean speed of the TSI gust (i.e. from t=1 sec to t=10sec), only small high-pressure zones (i.e. 650 Pa≤ P ≤ 690 Pa) were developed on the windward side. The worst-cases of the scenario occurs when the vehicle

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experienced high speed gust at t = 14 sec. At this time, large stagnation area can be observed clearly on the windward surface of the vehicle on which high positive wind pressure was developed.

Over other surfaces of the vehicle, there are all low and negative wind pressures because of flow separations and wakes. The majority of the windward side is covered by high pressure (See Figure 5-28). The existence of the lower pressure region on the leeward side of the trailer explains the increase in aerodynamic side force and roll moment.

Figure 5-29: Distribution of total pressure in flow field of TSI gust .

In Figure 5-29, contours of total pressure distributions have also been computed at the middle- height plane of the domain. The pressure contour illustrates variation in the TSI gust pressure values over the vehicle’s body. Again, it is apparent from the figure that high-pressure region is concentrated near the vicinity of the windward side of the vehicle whereas on the other side, low pressure region is formed at the leeward side and back surfaces. As it will be discussed in the next section, high differences in pressure field generated by the wind gust plays an important role for the development of unsteady aerodynamic forces as well as vehicle dynamic instability.

For more details on TSI wind gust pressure, the pressure coefficient Cp acting on leeward and

windward sides of the trailer is also calculated in this investigation. Figure 5-30, depicts time- dependent pressure coefficients acting on leeward and windward sides of the trailer under the

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TSI wind scenario. The calculation of pressure coefficient was along the lines on the leeward and windward surfaces of the trailer. The lines are parallel to the ground at height equal to the height of trailer box centre. It can be seen from the figure that the variations in the pressure coefficients are significant large (i.e., from about Cp=0.03 to about Cp=5.8) with positive values on the

windward side of the trailer. The figure also demonstrate that the value of the pressure coefficients change monotonically and consistently with the size of the TSI wind gust speed. Therefore, effects of this parameters (pressure coefficient) on the vehicle roll stability will investigate further in Chapter 7.

Figure 5-30 : Time dependent pressure coefficients along a line on the leeward and windward sides of trailer in the TSI gust scenario

The average Cp on the windward and leeward faces of the tractor and semitrailer units under the TSI gust scenario were also computed (for the monitoring points shown in Figure 5-24) and are shown in Table 5-3. The resulting difference in pressure coefficient (ΔCp) between windward and leeward faces for each unite of the vehicle was calculated and is shown in Table 5-3. For all monitoring point, the differential pressure coefficient ΔCp was found to be dramatically higher for the trailer faces than those of the tractor. This conclusion proves that practically for

-6 -4 -2 0 2 4 6 8 0 5 10 15 20 Cp Time(sce) leeward windward

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developing vehicle crosswind stability/control system it is sufficient to consider wind pressure acting on a trailer unit.

Table 5-3: Pressure difference between windward side and leeward side of the tractor and semitrailer unites