Wind tunnel measurements of crosswind loads on high sided vehicles over long span bridges

J. Wind Eng. Ind. Aerodyn. 107–108 (2012) 214–224

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Wind tunnel measurements of crosswind loads on high sided vehicles over long span bridges F. Dorigatti a, M. Sterling a,n, D. Rocchi b, M. Belloli b, A.D. Quinn a, C.J. Baker a, E. Ozkan c a

School of Civil Engineering, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Politecnico di Milano, Department of Mechanical Engineering, via La Masa 1, 20156 Milan, Italy c Arup, 13 Fitzroy Street, London W1T 4QB, UK b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 June 2011 Received in revised form 2 April 2012 Accepted 14 April 2012 Available online 11 May 2012

This paper presents the results obtained from a series of wind tunnel experiments undertaken in order to assess (and improve) the operation of a generic long span bridge subjected to strong winds (the operation of the bridge is evaluated in terms of the aerodynamic loads on a number of vehicles—in all cases the mean crosswind is greater than or equal to 10 m/s). The aerodynamic loads, in terms of mean and peak side and lift force coefficients and rolling moment coefficient are presented for three 1:40 scale model vehicles placed on the bridge: a Van, a Bus and a Lorry. These vehicles were tested under static conditions (i.e., the motion between the vehicle and the bridge deck was not simulated), and subjected to a uniform turbulent crosswind at different incoming directions. Two separate bridge deck scenarios were examined—an ‘Ideal’ and a ‘Typical’ deck shape. The data show a good agreement with the findings of previous research with respect to the ‘Ideal’ bridge geometry. The Typical bridge section enables the differences between the idealised bridge and a more realistic deck geometry to be explored. The work indicates that the Lorry, rather than the Van or the Bus, is the critical vehicle in terms of overturning stability (as characterized by the highest magnitudes of the rolling moment coefficient). An approximately linear decrease of the side force and rolling moment coefficients is found on the Typical deck when the vehicle is positioned at a progressively larger distance from the windward edge. The research provides novel data and interpretation on the aerodynamic loads experienced by road vehicles. Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved.

Keywords: Crosswind High sided road vehicles Ideal bridge Typical bridge Vehicle aerodynamic load coefficients Wind tunnel tests

1. Introduction As discussed in Quinn et al. (2007), the stability of commercial vehicles to cross wind forces is a complex and challenging problem. Baker and Reynolds (1992) outlined the importance of weather events in terms of vehicle safety: in 1990 a single wind event in the UK was responsible for 400 accidents involving death or injury; the daily average for all road accidents in the UK in the same year was approximately 700. More general surveys (Edwards, 1994) have also supported this view. Baker (1986, 1987, 1988) undertook work which enabled the wind induced stability of vehicles running on an ‘open exposed ground’ (i.e., a conventional road) to be interpreted in a qualitative framework. Different and progressively more detailed models then ensued which incorporated the driver’s reaction and enabled the ‘safe envelope’ of the driving conditions to be evaluated (Baker, 1994). As a result of this research, a series of three different types of wind-induced road accidents was defined (Baker, 1986): overturning, sideslip and excessive rotation. Baker (1987) analysed a number of different types of vehicles (cars, coaches, large rigid vans

n

Corresponding author. Tel.: þ44 121 4145065; fax: þ44 121 4143675. E-mail address: [email protected] (M. Sterling).

and articulated tractor-trailers) with varying geometries (frontal area and wheelbase) and inertial properties (mass and rotational inertia). Based on this analysis, high sided vehicles were confirmed as the critical case. Additional analysis (Baker and Reynolds, 1992) established that the local topography and infrastructure can also influence the stability of high sided vehicles subject to cross winds. In some cases certain elements of the transportation network can be readily identified as the critical sections with respect to vehicle stability since they are normally built at exposed sites (e.g., viaducts and bridges). The natural winds to which such infrastructure are exposed not only determine the aerodynamic loads on the vehicles, but can also induce unsteady vibrations of the bridge deck which in turn can affect the dynamic behaviour of the vehicles. Chen and Cai (2004); Guo and Xu (2006) demonstrated that for the same value of wind velocity the risk of vehicle instability is higher if a vehicle is crossing a long span bridge as opposed to travelling on a road. Hence, the analysis of the aerodynamic behaviour of vehicles on bridges and viaducts warrants investigation. Research undertaken in the last decade has witnessed the development of numerical models capable of examining the vehicle–bridge–wind interaction (Cai and Chen, 2004; Guo and Xu, 2006). Such models enable the possibility of undertaking a

0167-6105/$ - see front matter Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jweia.2012.04.017

F. Dorigatti et al. / J. Wind Eng. Ind. Aerodyn. 107–108 (2012) 214–224

dedicated assessment of the stability of a vehicle crossing a long span bridge while subjected to a cross wind (Chen and Cai, 2004). In addition, they also allow a specific investigation of the dynamic behaviour of the bridge to be undertaken during realistic operating conditions, i.e., evaluating the loads on the bridge deck induced by the running vehicles (Chen and Wu, 2010). These techniques of analysis combine two subtopics that had been previously developed independently: the wind–bridge and the wind–vehicle interactions. It is interesting to note that the vehicle–wind components of these models, to a large extent build on the work undertaken by Baker (1991a,b), and assume a quasi-steady relationship between the cross winds and the aerodynamic forces on the vehicles. To apply such an approach, all the information relating to the interaction between the fluctuating nature of the wind field and the resulting complex flow field around the vehicle is expressed in terms of a series of nondimensional parameters, i.e., the aerodynamic force/moment coefficients. Their magnitudes depend on the geometrical features of the vehicle, on the characteristics of the impinging wind and also on the local topography; such coefficients are usually obtained empirically from wind tunnel (WT) simulations, numerical simulations (CFD), full-scale (FS) experiments or a combination of all three (Sterling et al., 2010). There is a wealth of data pertaining to the wind load coefficients for vehicles on standard ‘open ground’ scenarios (Baker, 1991a; Sterling et al., 2010). However, the corresponding data for bridge/ viaduct decks is sparse and as such recourse are often made to the work undertaken by Coleman and Baker (1994). Coleman and Baker (1994) undertook an extensive WT campaign which determined the load coefficients for an articulated lorry positioned on an idealised bridge deck model (Coleman, 1990). Although these data are valuable, it is rather limited since only one specific road vehicle geometry was examined. Furthermore, the investigated bridge profile represented an idealised situation, where no significant local variations in

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the wind field on the road level were induced by the geometry of the deck. Hence, in order to expand the database and enable further development of the numerical simulations, a need for a series of WT experiments examining a set of vehicle geometries for different bridge decks was established. The present work is concerned with extending the existing database regarding the aerodynamic properties of high sided vehicles over long span bridges. The presented data were measured during a WT experimental campaign. They are reported in terms of mean and peak coefficients for lift and side forces, and for the rolling moment with respect to three different high sided vehicle geometries: a ‘Van’, a (double deck) ‘Bus’, and an (articulated) ‘Lorry’. Each of these vehicles was tested in correspondence of two separate bridge deck scenarios, identified as ‘Ideal’ and ‘Typical’ bridge decks. All the measurements were undertaken in static conditions (i.e., with no vehicle movement) over a range of different orientations of the vehicles with respect to the oncoming mean wind (details of which are presented in what follows). The experimental set-up adopted for the wind tunnel tests is outlined in Section 2, which illustrates the bridge deck and vehicle geometries, the characteristics of the simulated wind, the instrumentation and testing procedure. Section 3 reports and examines the values of the mean and peak coefficients in correspondence of the variety of configurations tested: the two cases of an Ideal and a Typical bridge deck are presented and compared, and also a further comparison to the previous data by Coleman and Baker (1994) is included. Finally, some conclusions are drawn in Section 4.

2. Experimental set-up The results presented below were obtained from a series of experiments carried out as part of broader WT experimental

Fig. 1. Vehicle geometries at full scale (dimensions in metres): (a) commercial Van, (b) double-deck Bus and (c) articulated Lorry.

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study dealing with both the vehicles and bridge aerodynamic stability. The experiments were undertaken in the Politecnico di Milano Wind Tunnel (PMWT), which is a closed circuit facility and comprises of a boundary layer test section that is 36 m long, 14 m wide and 4 m high. For a more in-depth description of the facility the reader is referred to Bocciolone et al. (2008) and to http:// www.windtunnel.polimi.it.

2.1. Bridge deck and vehicle geometries Three different high-sided vehicle models (scale 1:40) were examined: a commercial Van, a double deck Bus and an articulated Lorry. The details and dimensions of these vehicles are given in Fig. 1 and Table 1 in terms of their length (L), width (W), and height (H). Each of these vehicles was tested on two different bridge sectional models (scale 1:40), i.e. the Ideal bridge deck section and the Typical bridge deck section illustrated in Fig. 2. The Ideal bridge model was developed with the view to minimising the local perturbations induced by the presence of the deck on the wind profile impinging the vehicles and enables comparisons with previously published data. As shown in Fig. 2a, it essentially consisted of a flat plate, reproducing a bridge section at full scale having a depth, chord and span of 1.56 m, 40 m and 240 m respectively. The relatively shallow deck and the 301 downward chamfer at the leading (and trailing) edge were specifically adopted to prevent the flow separation on the upper surface of the deck. The Typical bridge deck configuration, whose sketch is shown in Fig. 2b, offered a representation of a more ‘standard’ streamlined geometry akin to single-box girder cross section; such a design being of interest because it is similar to what frequently occurs on long span suspension and cable-stayed bridges. The full-scale equivalent depth, chord and span of this sectional bridge model were 4.92 m, 41.69 m and 240 m respectively. Thus, the Typical deck had a slightly greater chord and a considerably bigger depth than the flat plate, in addition to upward and downward chamfers at the leading and trailing edge. Furthermore, the Typical deck was also equipped with a set of crash barriers (1.25 m high at full scale, porosity  35%) denoting the edge of the carriageways. Again, such features are not uncommon in practice. As indicated, the same scale was adopted for the vehicle and bridge models. It is noted that previous works (Bocciolone et al., 2008; Sterling et al., 2010) using the same WT had adopted a larger scale. However, in order to ensure consistency with bridge aerodynamic experiments that were being undertaken as part of a larger project (and not reported here in order to maintain commercial

Table 1 Vehicle dimensions. Van

Model scale (mm) Full scale (m)

Double deck Bus

Articulated Lorry

W

L

H

W

L

H

W

L

H

52 2.08

157 6.28

67 2.68

63 2.52

253 10.12

110 4.4

60 2.40

414 16.56

96 3.84

confidence), the scale of 1:40 was adopted. In order to minimise the aeroelastic response of the deck, both of the bridge models were designed and built with relatively high stiffness and were rigidly supported by a series of eight vertical posts. The decks were positioned at approximately 0.9 m (at model scale) above the ground in order to ensure a reasonably uniform approaching velocity profile (see Section 2.2). In addition, two endplates were positioned at both the ends of each deck, which helped to minimise any unrealistic three-dimensional effects induced on the flow by the finite length of the sectional models and to provide additional flow stability around the vehicle models.

2.2. Wind simulation A reference wind condition at full scale was assumed as a target in order to calibrate the flow simulation in the wind tunnel. A terrain roughness of z0 ¼ 0.003 m and a bridge deck height of 60 m above the mean sea height were assumed in order to match the actual full-scale location of the bridge. The characteristics of the reference wind for such site were then calculated in compliance with the ESDU methodology (ESDU, 2001) and consisted of a mean speed of 32 m/s in combination with a 12% streamwise turbulence intensity and a longitudinal turbulence length scale Lxu ¼266 m. The characteristics of the flow simulation performed in the wind tunnel are illustrated in Fig. 3, and were determined through a flow characterization performed using a set of four Series 100 cobra probes (Turbulent Flow Instrumentation). They are multi-hole pressure probes capable of resolving the three components of the mean and fluctuating velocities with a frequency response up to 2 kHz. The measurement accuracy for the velocity is of the order of 0.3 m/s with respect to the magnitude and approximately 711 in terms of direction of the vector. The four probes were positioned all at the same height and arranged in a rake, thus spanning a total distance of 300 mm transversally to the streamwise direction. In this way the variation of the vertical wind profiles was monitored at four different positions, with relative distances of 50, 100 and 150 mm one from the other. The vertical profiles of the approaching mean streamwise velocity are shown in Fig. 3a. U denotes the time averaged value of streamwise velocity at a height z above the ground, while Uref indicates the reference value of U, defined in the mid-span of the test section, at the height corresponding to the top surface of the deck (i.e. 0.9 m at model scale) and 7 m upwind of the bridge sectional model (where the wind profile is likely to be undisturbed by the presence of the model itself). The reference mean streamwise wind speed was measured by a Pitot tube in combination to an ultra-low-range differential pressure manometer (Furness FCO510). For all the experiments examined in this paper the Uref varied between 12 and 15 m/s. The velocity distribution can be considered to be uniform within the portion of height occupied by the deck and the vehicles, presenting a variation of approximately 72% in respect to the mean reference speed. Furthermore, the consistency outlined by the velocity profiles measured at four different positions confirmed a good spatial homogeneity of the mean flow, transversally to the incoming wind direction, within a distance approximately corresponding to

Fig. 2. Bridge deck geometries at full scale (dimensions in metres): (a) Ideal bridge deck and (b) Typical bridge deck.

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217

/ (std(U)2)

z model scale [m]

100

n*S

uu

10-1

-2

10 10-2

Experimental Data (WT) Von Karman Spectrum (Target FS) 10-1

100

101

x

U/Uref

n*Lu/Uref

Fig. 3. Wind simulation: streamwise velocity: (a) mean velocity vertical profile and (b) power spectral density at the deck height.

the length of the vehicles. The turbulence intensity at the deck height was approximately 6% in the streamwise direction and approximately 4.5% for the lateral and vertical velocity components. The longitudinal integral scale for the streamwise velocity (Lxu ) was an equivalent full-scale value of  30 m which is well above the vehicle height (or length or width); thus ensuring that the turbulent structures are well correlated over the vehicles—as would be expected in reality (Baker, 1991b). Fig. 3b indicates the power spectral density of the approaching streamwise velocity at deck height. To aid comparison between the wind tunnel simulation and the target spectrum at full scale (i.e., the Von Karman Spectrum as per ESDU (2001)) both the axis in Fig. 3b have been non-dimensionalized. The horizontal axis represents the reduced frequency (f¼nLxu / Uref) defined by the product between the frequency (n) and the turbulence length scale, divided by the reference mean wind speed. On the vertical axis the spectrum is multiplied by the frequency and divided by the mean velocity variance. The qualitatively good agreement outlined between the experimental and target spectra over a large proportion of the reduced frequency range suggests the turbulence simulation is sufficient for the current purposes. For the current work, the Reynolds number based on the vehicle height (H) and defined as Re¼Uref H/n (where n is the kinematic viscosity of air), varied in the interval between 5  104–105, considering the different dimensions of the three vehicles. It is acknowledged that the Reynolds numbers of the experiments presented in this paper are lower than the corresponding full-scale values. However, previous work (Sterling et al., 2010) has shown that such modelling can achieve appropriate results provided they are interpreted with care. Research undertaken by Coleman and Baker, conducted in similar turbulent flow conditions for a comparable range of values, found small Reynolds number effects on the measured values for the lift and side force coefficients. (A more in depth discussion can be found in Baker and Sterling (2011). 2.3. Force measurements In the results presented below only data relating to the side force, lift force and rolling moment are examined. This is in keeping with the approach adopted by Sterling et al. (2010), and considers that in terms of vehicle instability these components of force/moment are the most important. This has been supported by the results of Guo and Xu (2006) and Cheung and Chan (2010), which indicate overturning and sideslip as the governing instabilities for high sided vehicles over bridges in crosswinds.

The forces on the Bus, Van and Lorry were measured using Ruag SG-Balance 194-6, Ruag SG-Balance 194-6 and Ruag SG-Balance 187-6D force balances respectively. Such balances present different design loads associated to the different components of the forces and moments. The design loads corresponding to side force, lift force and rolling moment were 20 N, 10 N and 0.4 N m respectively for the balance SG 194-6, and 200 N, 100 N and 5 N m for the balance SG 187-6D. The accuracies can be estimated as 0.5% of the design loads. The results below indicate that the actual loads measured during the tests are vehicle dependant. For example, considering the side force, lift force and rolling moment, respectively, the range of variation for the magnitude of the measured loads were as follows: 0–1 N, 0–0.1 N and 0–0.025 N m for the Van; 0–5 N, 0–1.2 N and 0–0.3 N m for the Bus and the Lorry. In each case the force balance was installed inside the model by means of an aluminium frame. The model was then connected to the frame by screws on the top surface in case of the Bus, on the lateral surface in case of the Van and the Lorry. The entire system composed of the vehicle model and the internal balance was connected to the bridge deck by means of adjustable spacers (Fig. 4). They were specifically set in order to ensure the values of the clearance between the bottom of the vehicles and the road to correspond to their equivalent at full scale, which were 0.244 m, 0.2 m and 1.16 m for the Van, Bus and Lorry respectively. Furthermore, to minimise the interference on the underbody flow, the spacers presented a slender shape and, where possible, were positioned behind the wheels of the vehicles. In order to avoid vibrations, the entire aluminium structure connecting the vehicle model to the balance, and then to the deck, was specifically designed with a sufficient stiffness level. Thus, the natural frequencies of the model supporting system were shifted above 10 Hz. This arrangement enabled repeatable measurements to be made to an accuracy of 73%. Fig. 5 outlines the reference system used in the current work, which is the same for all the vehicles and is centred at the ground level. The side and lift forces are acting in the positive direction of the y and z axes, respectively, and the rolling moment is acting about the x-axis (following the ‘right-hand screw’ rule). It is worth noticing that the connection between the internal balances and the vehicle models, which varied depending on the vehicle model (as specified in the above), did not necessarily correspond to the location of the centre of mass of the vehicle. However, all the results for the rolling moment presented in this paper, for any vehicle, are referred to the same x-axis illustrated in Fig. 5, and

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Fig. 4. Installation of the internal force balance for the Bus on the Ideal deck.

Fig. 5. Coordinate reference system and sign convention.

thus referred to a ‘pole’ located at the ground level. As outlined in Section 1, it is usual to evaluate forces and moments acting on a vehicle in non-dimensional form. Thus the instantaneous values of side (cy) and lift (cz) forces and rolling moment coefficients (cMx) were defined as cy ¼

cz ¼

0:5 r U 2ref A Fz

cMx ¼

Type of vehicle Yaw angle (deg.)

Fy

0:5 r

Table 2 Experimental configurations.

U 2ref

A

Mx 0:5 r U 2ref A H

ð1Þ

ð2Þ

Distance (d) of the vehicle from the windward edge of the deck (full scale)

Ideal bridge deck Van, Bus and Lorry

Typical bridge deck Van, Bus and Lorry

15, 30, 45, 55, 65, 75, 80, 75, 90, 105 and120 90, 100, 105, 115 and 125 (lanes 1 and 2) 60, 75, 90 and 105 (lanes 3 and 4) One position: Four positions: 8m Lane 1–5.7 m Lane 2–9.3 m Lane 3–28.6 m Lane 4–32.3 m

ð3Þ

where r is the air density, A denotes the nominal vehicle side area, defined as the product L  H (where L is the length and H is the height of the vehicle, as indicated in Section 2.1), Fy is the instantaneous side force, Fz is the instantaneous lift force and Mx is the instantaneous rolling moment. The time averaged values of the coefficients in Eqs. (1)–(3) are denoted by Cy, Cz and CMx, corresponding to the side force, lift force and rolling moment, respectively, whereas the peak values (C^ y, C^ z, C^ Mx) are simply obtained by replacing the instantaneous values of force or moment with the corresponding peak values averaged over the required time period. For the current work, time histories of 60 s were recorded at a sampling frequency of 500 Hz. The averaging period for the mean coefficients corresponds to the entire duration of the recorded time histories, while the averaging period for the peak coefficients is taken as 3 s.

2.4. Tested configurations and experimental procedure Table 2 outlines the experimental configurations examined in the current work with the definition of yaw angle given in Fig. 5.

The test procedure which was adopted was different depending on the deck model. For the experiments on the Ideal bridge, the three vehicles were tested at a single fixed distance from the windward edge, which was equivalent to 8 m at full scale. To vary the yaw angle, only the model vehicles were rotated to the incident wind, while the orientation of the deck was kept unchanged (with its longitudinal axis perpendicular to the streamwise velocity). This approach is consistent with the main purpose of the Ideal bridge deck test case (as stated in Section 2.1). Such an approach was adopted with the view of minimising the perturbation induced by the deck on the wind profile impinging the vehicles. The short distance of the vehicles from the windward edge was selected in order to minimise the development of the boundary layer on the upper surface of the deck. Maintaining the deck perpendicular to the mean wind, and rotate only the vehicles, was considered the most appropriate approach in order to keep minimum, at any yaw angle, the ‘interference’ of the deck on the wind profile at the road level. On the top surface of the Typical bridge, the local wind profile is expected to be modified as a result of the deck shape at the leading edge and of the presence of the crash barriers. In such a situation the orientation of the deck was considered to be significant in determining the wind

F. Dorigatti et al. / J. Wind Eng. Ind. Aerodyn. 107–108 (2012) 214–224

profile at the road level. To vary the yaw angle, the whole deck section was rotated with respect to the oncoming wind (the endplates were kept parallel the streamwise direction). Furthermore, to account for any changes in the wind profile on the top surface of the Typical deck which may change along the chord, four different positions of the vehicle models were investigated (Fig. 6). These positions coincided with the centre of the traffic lanes (two per direction of travel) whose distances from the deck windward edge are specified in Table 2. This not only simulated a different direction of travel but for each single deck orientation examined, two different values of yaw angles were obtained (this will be further explained in Section 3.2).

3. Results and discussion 3.1. Ideal bridge deck Fig. 7 illustrates the variation of the mean and peak side force coefficients, lift force coefficients and rolling moment coefficients

219

with respect to yaw angle. The mean side force (Fig. 7a) and rolling moment (Fig. 7b) coefficients follow a similar trend for the Bus and the Lorry, with lower values occurring at low yaw angles and tending to a maximum at about 901 yaw. Such a trend is similar to that shown in literature (Baker, 1991a; Sterling et al., 2010), although it is noted that the maximum values typically occur between yaw angles of 60–901. The corresponding coefficients for the Van outline a similar pattern for yaw angles greater than 451, with a different trend in the side force coefficient illustrated for low yaw angles, i.e. between 151 and 451. For yaw angles greater than 301 the Van appears to be the most stable vehicle with respect to side force and rolling moment. In terms of the side force coefficient the Bus and the Lorry have similar magnitudes with the latter vehicle tending to have slightly higher values for the rolling moment coefficient. The discrepancies in terms of aerodynamic coefficients between the three vehicles can be attributed to the different pressure distributions acting on the vehicles which in turn relate to the surrounding flow field which in turn is strongly influenced by the vehicle’s geometry. The Van is the smallest of all the vehicles

d wind 1

2

3

4

1

wind

2

3

4

Mean Coefficients

1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

40

60 80 100 120 140 Yaw angle

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1

20

40

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

60 80 100 120 140 Yaw angle

Ĉz

0

Cz

20

ĈMx

CMx

0

0

20

40

60 80 100 120 140 Yaw angle Van

Peak Coefficients

1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

Ĉy

Cy

Fig. 6. Roadway layout and vehicles positions on the Typical bridge deck.

0

20

40

60 80 Yaw angle

100 120 140

0

20

40

60 80 100 120 140 Yaw angle

20

40

60 80 100 120 140 Yaw angle

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 0

Bus

Lorry

Fig. 7. Ideal bridge deck—mean aerodynamic load coefficients: Cy (a), CMx (b) and Cz (c); 3s-peak aerodynamic load coefficients: C^ y (d), C^ Mx (e) and C^ z (f).

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Cross Correlation Coefficient CMx - Cy

1 0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0

0

-0.2

-0.2 0

20

40

60 80 Yaw Angle

100

120

Cross Correlation Coefficient CMx - Cz

1

140 Van

0 Bus

20

40

Lorry

60 80 Yaw Angle

100

120

140

Fig. 8. Ideal bridge deck—correlation coefficients: cMx-cy (a) and cMx-cz (b).

(in terms of overall height and side area), and has a rather elongated shape on the front in addition to a ‘streamlined’ taper from the cab to the roof. Based on the data illustrated in Fig. 7a and b, it is likely that such features not only cause a reduction in the magnitude of the side force coefficient, but also lower its point of action (i.e., the centre of pressure), thus explaining the decreased rolling moment coefficient. The Bus is taller than the Lorry which would suggest that the centre of pressure would be located at two different heights for each vehicle. However, as previously indicated the centre of pressure is related to the overall pressure field acting on the vehicle, and it is likely that the larger gap between the bottom of the Lorry and the ground compared to the Bus would have more of an influence on the pressure field, thus partially explaining the difference in rolling moment observed between the two vehicles. Furthermore, noting that the upper portion of the Bus is slightly rounded, both on the sides and on the front when compared to the Lorry, it can be hypothesized such a detail may have an effect in mitigating the wind induced rolling moment. This idea is supported by preliminary findings of Coleman (1990) and more recent work of Petzall et al. (2008); the latter found an increased stability of high sided vehicles in crosswinds when the vehicles had rounded upper edges. The variation of the mean lift force coefficient with respect to yaw angle is illustrated in Fig. 7c, and shows that the magnitude of this parameter is significantly less than either of the other two coefficients. It is possible to identify a general trend in the results for the Bus and the Lorry: the lift coefficients tend towards a maximum value at about 301 yaw and then decrease reaching a minimum between the 70 and 901 yaw angle before starting to rise again. In contrast, the Van is characterized by a mean lift force coefficient which is approximately zero and shows minor fluctuations with respect to the yaw angle. The trends observed for the Bus and the Lorry are consistent with the results of Coleman and Baker (1990, 1994) who found that the magnitude and variation of the mean lift force coefficients for an articulated lorry were substantially influenced by a low pressure region induced on the vehicle roof by a pair of vortices originating from the top-front windward corner of the trailer roof (as discussed further in Section 3.3). Hence, it is perhaps not unreasonable to postulate that similar patterns could be present in the current setup for the Bus and Lorry and would thus explain the trends illustrated at low yaw angles (i.e., yaw angles smaller than 301). Similarly it could be argued that a different flow pattern occurs over the roof of the Van (which is stable within the entire yaw angle range). As stated in Section 2, the peak coefficients were calculated as the maximum values corresponding to a 3 s running average filtered applied to the instantaneous coefficient time histories. As would be expected, for all of the three components (side and lift force, and rolling moment) the peak coefficients curves present higher magnitudes (in absolute value) than the mean values and show the same general trend over the entire range of yaw angles (Fig. 7d–f). The reasons postulated above with respect

to the trend of the mean coefficients are considered valid for the peak coefficients. The data in Fig. 7 suggest that the coefficients for the side force and rolling moment are reasonably well correlated across a wide range of yaw angles. This interpretation is also supported by the additional analyses shown in Fig. 8a, which illustrates rather high values of the cross correlation coefficient between these two parameters for all the three vehicles. Fig. 8b shows the cross correlation index calculated between the lift force and the rolling moment coefficient and indicates a low level of correlation over the entire yaw angle range for both the Van and the Bus. However, the Lorry reveals a rather different trend, with the average magnitude of such index of approximately 0.45, and a maximum value similar in magnitude to the corresponding data in Fig. 8a. This behaviour may partially be attributed to the geometry of the Lorry and in particular the sharp edges over the roof leading to a relatively large zone of separated flow (i.e., a low pressure region). However, based on the information available, no conclusive explanation for this can be given. To further investigate the correlation between the different load coefficients, the instantaneous values of cMx are plotted versus the corresponding instantaneous values of cy and cz (Fig. 9a and b, respectively). The limited degree of scatter outlined in Fig. 9a illustrates that instantaneous values of side force and rolling moment are qualitatively well correlated. This is in contrast to the conclusions of Baker and Sterling (2009), who found that average values of cross correlations could hide significant variations particularly at the extreme values. However, such work is perhaps not directly comparable, since it was undertaken in the lower part of the atmospheric boundary layer, where the variation of velocity and turbulence with respect to height above the ground is significantly greater than that outlined in the current experiments. The reduced degree of scatter shown in Fig. 9b by the data of the Lorry is consistent with what observed previously in Fig. 8b. Based on the above, the fluctuations in the rolling moment coefficients appear to be largely due to corresponding fluctuations in the side force coefficients. 3.2. Typical bridge deck Fig. 10 illustrates the aerodynamic behaviour of the Van, the Bus and the Lorry over the Typical bridge deck with respect to yaw angle in terms of mean side force coefficient, mean rolling moment coefficient and mean lift force coefficient. Five different sets of data are included in each figure: the first dataset relates to the already presented values of for the Ideal bridge deck (Fig. 7), while the remaining four dataset relate to data obtained on the vehicles positioned on different lanes (see Section 2.4). As explained in Section 2.4, due to the adopted experimental setup, the actual yaw angles investigated depended on which lane the vehicles were located in. As illustrated in Table 2 the ranges of yaw angle were as follows: 751, 901, 1051 and 1201 corresponding to lanes 1 and 2 (windward side) and 601, 751, 901 and 1051

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0

1 0.8 0.6 0.4 0.2 0 0

20 40 60 80 100 120 140 Yaw angle

0

20 40 60 80 100 120 140 Yaw angle

0.5

0.5

0.5 Cz

0.7

Cz

0.7

0.3

-0.1

-0.1 0

20 40 60 80 100 120 140 Yaw angle Ideal Bridge Deck

150

0

20 40 60 80 100 120 140 Yaw angle

0

20 40 60 80 100 120 140 Yaw angle

0.3 0.1

0.1

0.1

50 100 Yaw angle

1 0.8 0.6 0.4 0.2 0

0.7

0.3

Lorry

1.8 1.5 1.2 0.9 0.6 0.3 0

20 40 60 80 100 120 140 Yaw angle

CMx

1 0.8 0.6 0.4 0.2 0 0

Cz

20 40 60 80 100 120 140 Yaw angle

CMx

CMx

0

Bus

1.8 1.5 1.2 0.9 0.6 0.3 0

Cy

Van

1.8 1.5 1.2 0.9 0.6 0.3 0

Cy

Cy

Fig. 9. Ideal bridge deck—instantaneous values correlations at 901 yaw angle: cMx-cy (a) and cMx-cz (b).

-0.1 0

20 40 60 80 100 120 140 Yaw angle

Typ. Deck Lane 1

Typ. Deck Lane 2

Typ. Deck Lane 3

Typ. Deck Lane 4

Fig. 10. Typical bridge deck—mean coefficients: Cy for the Van (a), Bus (b) and Lorry (c); CMx for the Van (d), Bus (e) and Lorry (f); Cz for the Van (g), Bus (h) and Lorry (i).

corresponding to lanes 3 and 4 (leeward side). As outlined above the experiments relating to the Typical deck are part of a wider test campaign and these particular yaw angles were examined because they were of specific interest relative to the actual wind conditions on the site considered for the bridge. Fig. 10 illustrates results consistent with some of the findings related to the Ideal bridge test case. For example, irrespective of the lane considered, all the three vehicles present a rather limited variation in side force and rolling moment coefficients for yaw angles between 601 and 1201 (Fig. 10a–f). In addition, Fig. 10g–i shows that the magnitudes of the lift force coefficients are lower than those of the side force and rolling moment coefficients. Comparing the three different vehicles, the Van still appears as the less critical over the examined interval of yaw angles. Furthermore, the Bus and the Lorry present comparable values of side force

coefficient (for lanes 1 and 2). However, it is interesting to note that for lanes 3 and 4 the Bus illustrates higher values for 601 yaw angles. Fig. 10 also enables a quantitative comparison with the data from the Ideal bridge deck to be made. All the three coefficients for the Bus and the Lorry on the Typical bridge on lane 1, reveal a remarkably good agreement with those obtained on the Ideal bridge deck. As the lane number increases the agreement tends to decrease (apart for the rolling moment coefficient on lane 2). Unlike the other vehicles, the Van shows a deviation from the results of the Ideal bridge even when located on lane 1: on the Typical bridge the side and rolling moment coefficients are lower than on the Ideal deck, while the lift force coefficient is higher. It is difficult to observe any noticeable trend in the lift coefficient data with respect to lane number.

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Bus

0

20 40 60 80 100 120 140

0

20 40 60 80 100 120 140

0

1 0.8 0.6 0.4 0.2 0

20 40 60 80 100 120 140

0

20 40 60 80 100 120 140

0

0.5

0.5

0.5 Ĉz

0.7

Ĉz

0.7

0.3

20 40 60 80 100 120 140

0

20 40 60 80 100 120 140

0

Typ. Deck Lane 1

Typ. Deck Lane 2

20 40 60 80 100 120 140 Yaw angle

Yaw angle

Yaw angle Ideal Bridge Deck

0.3

-0.1

-0.1 0

20 40 60 80 100 120 140

0.1

0.1

-0.1

150

Yaw angle

0.7

0.1

100

1 0.8 0.6 0.4 0.2 0

Yaw angle

0.3

50

Yaw angle

ĈMx

ĈMx

ĈMx

1 0.8 0.6 0.4 0.2 0 Yaw angle

Ĉz

1.8 1.5 1.2 0.9 0.6 0.3 0

Yaw angle

Yaw angle

0

Lorry

1.8 1.5 1.2 0.9 0.6 0.3 0

Ĉy

Ĉy

Ĉy

Van 1.8 1.5 1.2 0.9 0.6 0.3 0

Typ. Deck Lane 3

Typ. Deck Lane 4

Fig. 11. Typical bridge deck—3s-peak coefficients: C^ y for the Van (a), Bus (b) and Lorry (c); C^ Mx for the Van (d), Bus (e)and Lorry (f); C^ z for the Van (g), Bus (h) and Lorry (i).

The data corresponding to vehicle peak coefficients relating to the Typical bridge deck are illustrated in Fig. 11, and are expressed in the same format as the corresponding mean coefficients (Fig. 10). The relationship between the peak and mean coefficients shown by the vehicles on the Typical deck is the same that was discussed for the Ideal bridge test case, i.e., they follow similar trends. The differences between the vehicle aerodynamic coefficients relating to the Ideal and Typical bridge deck suggest local modifications of the wind profile occurring in the latter case (particularly for lanes 3 and 4). These changes in the flow field may be attributed to the differences in geometry between the two bridges and, specifically, a region of relatively low streamwise wind speed is likely to occur on the top surface of the Typical bridge as a result of the crash barriers. If this is the case then the height of the Van would ensure that a relatively larger proportion of this vehicle was immersed in the low wind speed region compared to the Lorry and the Bus, thus explaining the larger reduction in side force/rolling moment observed for the Van in lane 1 compared to the other vehicles (Fig. 10a–f). Furthermore, if it is assumed that size (depth) of this low speed flow region increases from lanes 1 to 4, then this would result in a reduction of the aerodynamic loads on all of the vehicles as the distance from the leading edge of the deck increases. A further investigation on the dependence of the crosswind loads on the position of the vehicles for the Typical bridge is illustrated in Fig. 12 which shows the evolution of the mean side force and rolling moment coefficients for the Van, Bus and Lorry with respect to the four distances from the windward edge of the deck (i.e., the distances corresponding to lanes 1–4—see Section 2.4). It is interesting to note that a ‘quasi-linear’ trend can be inferred for all the vehicles for both the side force and rolling moment coefficients. The solid line in Fig. 12 represents a linear regression between the data points corresponding to an yaw angle of 901. It is also interesting to note that the gradient of the side force coefficient

lines are approximately 0.02, 0.03 and  0.03 for the Van, Bus and Lorry respectively, whereas the corresponding values for the rolling moment coefficients are approximately 0.01 for all vehicles. This agreement is somewhat remarkable. 3.3. Comparison to the results of Coleman and Baker (1994) As mentioned in Section 1, the work of Coleman and Baker (1994) is often taken as the default reference for cross wind aerodynamic data on vehicles over bridges. Coleman and Baker undertook a wind tunnel campaign on a 1:50 scale model of an articulated lorry tested in a single position over an ideal box girder type bridge. The experiments were carried out for two different wind conditions: low turbulence and in presence of a uniform turbulence simulation. For the former case the turbulence intensity was lower than 1%, while for the latter case a value of 12.5% turbulence intensity, with an associated longitudinal turbulence length scale (Lxu ) of 7.5 m (at full scale), was obtained. (Further details of the simulation can be found in Coleman (1990) and Coleman and Baker (1994)). The experiments performed by Coleman and Baker are similar to those reported above for the Ideal bridge deck. Both have a uniform turbulent wind field approaching the deck. The shape of the articulated lorry tested by Coleman and Baker is not too different from that of the Lorry illustrated in Fig. 1c: both these vehicles are characterized by sharp edges and similar dimensions of width and height (the length of the vehicles differs by 20%). Finally, the Reynolds numbers are of the same order of magnitude (ca. 105). There are differences in bridge geometry, however, these are not considered to be significant in aerodynamic terms. The results from Coleman and Baker, together with those for the Ideal bridge case are superimposed in Fig. 13 in terms of mean

1.4 1.2 1 0.8 0.6 0.4 0.2 0

CMx

Cy

Van

F. Dorigatti et al. / J. Wind Eng. Ind. Aerodyn. 107–108 (2012) 214–224

5

10 15 20 25 30 35 FS Distance (d) from the deck windward edge (m)

0

5

10

15

20

25

30

35

0

40

Cy

Lorry

CMx 5 10 15 20 25 30 35 FS Distance (d) from the deck windward edge (m) yaw 75

15

20

25

30

35

40

5

10

15

20

25

30

35

40

FS Distance (d) from the deck windward edge (m)

1.4 1.2 1 0.8 0.6 0.4 0.2 0

yaw 60

10

1.4 1.2 1 0.8 0.6 0.4 0.2 0

FS Distance (d) from the deck windward edge (m)

0

5

FS Distance (d) from the deck windward edge (m)

1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

1.4 1.2 1 0.8 0.6 0.4 0.2 0

40

CMx

Cy

Bus

0

223

yaw 90

1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

40

5

10

15

20

25

30

35

40

FS Distance (d) from the deck windward edge (m) yaw 105

yaw 120

Linear Trendline (90 yaw)

0

20

40

60 80 100 120 140 Yaw angle

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Cz

1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

CMx

Cy

Fig. 12. Typical bridge deck—mean coefficients variation with respect to distance along the deck from the windward egde: Cy (a) and CMx (b) for the Van; Cy (c) and CMx (d) for the Bus and Cy (e) and CMx (f) for the Lorry.

0

Coleman and Baker (1994) - Low Turb

20

40

60 80 100 120 140 Yaw angle

Coleman and Baker (1994) - Turb

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 0 Van

20

40

60 80 100 120 140 Yaw angle

Bus

Lorry

Fig. 13. Comparison: Ideal bridge deck versus Coleman and Baker (1994)—mean coefficients: Cy (a), CMx (b) and Cz (c).

coefficients for the side and lift force, and of the rolling moment with respect to the yaw angle. It is worth noting that the data of Coleman and Baker illustrated in Fig. 13 were extrapolated from those formerly published and reprocessed in order to be consistent with the definitions and the reference system specified in Section 2.3—Coleman and Baker originally used the area of the front of the vehicle in order to normalise the data rather than the nominal side

area. In addition, the translation of the axes reference system was required since Coleman and Baker originally positioned their reference system at the Centre of Gravity (located at 1.5 m height at full scale) rather than at the ground level. Among the three vehicles examined on the Ideal bridge, the Lorry is the one that presents the smallest deviations from the results of Coleman and Baker, which is perhaps not too surprising

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given the geometrical similarity of the two vehicles. The side force and rolling moment coefficients illustrate the best agreements for both the low turbulence and high turbulence simulations of Coleman and Baker. For the lift force coefficients, the data in Fig. 13c outline a better agreement of the low turbulence data to the trend of the Ideal bridge, whereas the turbulence conditions show relatively large deviations for yaw angles greater than 401. It is likely that the differences can be attributed to the different levels of turbulence between the two experiments (turbulence intensity¼6% for the Idealised case compared with 12.5% for Coleman and Baker) which may have resulted in differing degrees of flow separation over the roof of the vehicles, and hence different degrees of aerodynamic lift.

4. Conclusions The present work has investigated the variation of lift force coefficient, side force coefficient and rolling moment coefficient for three different vehicles (a Van, a double deck Bus and a Lorry) on two different bridge deck configurations. In addition, a comparison has been made with previously published data. Based on the above, the following conclusions can be drawn:

 Of the three vehicles tested, regardless the deck geometry, the

  





Van illustrates the lowest magnitudes for mean and peak force coefficients as well as of the rolling moment coefficient; the only exception to this relates to 15–301 yaw angle range, where the aerodynamic behaviour of the Van is comparable to that of the other two vehicles. The Lorry reveals greater magnitudes for the rolling moment coefficient thus appearing the most critical vehicle in terms of overturning stability. For the Ideal bridge deck, a good correlation is observed for all the vehicles between side force and rolling moment coefficients. There is a good agreement between the mean coefficients for the Lorry on the Ideal bridge and those previously obtained by Coleman and Baker (1994) for a similar vehicle geometry tested on an idealised bridge deck. Hence, the work on the Ideal bridge extends the current database of aerodynamic coefficients. The work on the Typical deck geometry provides novel data and interpretation: the decrease observed in side force and rolling moment coefficients with respect to the distance from the windward edge of the deck is approximately linear. The comparison between the aerodynamic behaviour of the three vehicles over the two different decks reveals only a limited agreement. The three aerodynamic coefficients measured on the Ideal bridge correspond to those obtained on the Typical bridge for the Bus and the Lorry (but not for the Van), as long as the two vehicles are positioned on the windward lane (lane 1). Clearly, the Typical bridge geometry analysed here represents only one specific example of deck design for long span bridges.

References Baker, C.J., 1986. A simplified analysis of various types of wind-induced road vehicle accidents. Journal of Wind Engineering and Industrial Aerodynamics 22, 69–85. Baker, C.J., 1987. Measures to control vehicle movement at exposed sites during windy periods. Journal of Wind Engineering and Industrial Aerodynamics 25, 151–161. Baker, C.J., 1988. High sided articulated road vehicles in strong cross winds. Journal of Wind Engineering and Industrial Aerodynamics 31, 67–85. Baker, C.J., 1991a. Ground vehicles in high cross winds Part I: steady aerodynamic forces. Journal of Fluids and Structures 5, 69–90. Baker, C.J., 1991b. Ground vehicles in high cross winds Part II: unsteady aerodynamic forces. Journal of Fluids and Structures 5, 91–111. Baker, C.J., 1994. The quantification of accident risk for road vehicles in cross winds. Journal of Wind Engineering and Industrial Aerodynamics 52, 93–107. Baker, C.J., Reynolds, S., 1992. Wind-induced accidents of road vehicles. Accident Analysis and Prevention 24, 559–575. Baker, C.J., Sterling, M., 2009. Aerodynamic forces on multiple unit trains in cross winds. Journal of Fluids Engineering—Transactions of the ASME, 131. Baker, C.J., Sterling, M., 2011. Design processes for wind effects around simple buildings. In: Proceedings of the 13th International Conference on Wind Engineering, Amsterdam, The Netherlands. Bocciolone, M., Cheli, F., Corradi, R., Muggiasca, S., Tomasini, G., 2008. Crosswind action on rail vehicles: wind tunnel experimental analyses. Journal of Wind Engineering and Industrial Aerodynamics 96, 584–610. Cai, C.S., Chen, S.R., 2004. Framework of vehicle–bridge–wind dynamic analysis. Journal of Wind Engineering and Industrial Aerodynamics 92, 579–607. Chen, S.R., Cai, C.S., 2004. Accident assessment of vehicles on long-span bridges in windy environments. Journal of Wind Engineering and Industrial Aerodynamics 92, 991–1024. Chen, S.R., Wu, J., 2010. Dynamic performance simulation of long-span bridge under combined loads of stochastic traffic and wind. Journal of Bridge Engineering 15, 219–230. Cheung, M.M.S., Chan, B.Y.B., 2010. Operational requirements for long-span bridges under strong wind events. Journal of Bridge Engineering 15, 131–143. Coleman, S.A., 1990. The Aerodynamics of Ground Vehicles in Cross Winds. University of Nottingham, Nottingham. Coleman, S.A., Baker, C.J., 1990. High sided road vehicles in cross winds. Journal of Wind Engineering and Industrial Aerodynamics 36, 1383–1392. Coleman, S.A., Baker, C.J., 1994. An experimental study of the aerodynamic behaviour of high sided lorries in cross winds. Journal of Wind Engineering and Industrial Aerodynamics 53, 401–429. Edwards, J.B., 1994. Wind-related road accidents in England and Wales 1980–1990. Journal of Wind Engineering and Industrial Aerodynamics 52, 293–303. ESDU, 2001. Characteristics of Atmospheric Turbulence Near the Ground, Part II: Single Point Data for Strong Winds (Neutral Atmosphere). Engineering Science Data Unit International plc, London. Guo, W.H., Xu, Y.L., 2006. Safety analysis of moving road vehicles on a long bridge under crosswind. Journal of Engineering Mechanics—ASCE 132, 438–446. Petzall, J., Torlund, P.A., Falkmer, T., Albertsson, P., Bjornstig, U., 2008. Aerodynamic design of high-sided coaches to reduce cross-wind sensitivity, based on wind tunnel tests. International Journal of Crashworthiness 13, 185–194. Quinn, A.D., Sterling, M., Robertson, A.P., Baker, C.I., 2007. An investigation of the wind-induced rolling moment on a commercial vehicle in the atmospheric boundary layer. Proceedings of the Institution of Mechanical Engineers Part D—Journal of Automobile Engineering 221, 1367–1379. Sterling, M., Quinn, A.D., Hargreaves, D.M., Cheli, F., Sabbioni, E., Tomasini, G., Delaunay, D., Baker, C.J., Morvan, H., 2010. A comparison of different methods to evaluate the wind induced forces on a high sided lorry. Journal of Wind Engineering and Industrial Aerodynamics 98, 10–20.