AERODYNAMIC LOADS ON A VEHICLE EXPOSED TO CROSS-WIND GUSTS: AN EXPERIMENTAL STUDY

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1 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan AERODYNAMIC LOADS ON A VEHICLE EXPOSED TO CROSS-WIND GUSTS: AN EXPERIMENTAL STUDY Hrvoje Kozmar 1,2, Kyle Butler 1, Ahsan Kareem 1 1 Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, IN 46556, USA, hkozmar@nd.edu, kbutler3@nd.edu, Ahsan.Kareem.1@nd.edu 2 Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Ivana Lučića 5, Zagreb, Croatia, hkozmar@fsb.hr ABSTRACT This paper presents experimental results concerning transient aerodynamic loads experienced by a vehicle. Two vehicle models were tested, one with wheels and the other one without wheels. Both models were separately placed on a bridge deck section and exposed to an air flow which consisted of an intermittent gusting, alternating between two velocity thresholds. Experimental results indicate key features for future investigations: an increase in steady aerodynamic loads in periods of wind gusts and two different unsteady phenomena. Wind gusting affects both vehicle models, exhibited in the power spectra of wind loads fluctuations as a low frequency peak at 0.3 Hz, corresponding to switching between lower and higher velocity, observed in both configurations. Vortex shedding from the bridge predominantly influences unsteady loads on a vehicle without wheels, exhibited in the power spectra of unsteady aerodynamic forces and moments as a strong high frequency peak at 10 Hz due to vortex shedding. KEYWORDS: BLUFF BODY AERODYNAMICS, CROSS-WIND GUSTS, VEHICLE, BRIDGE, TRANSIENT AERODYNAMIC LOADS, EXPERIMENTS Introduction Numerous long span bridges and viaducts have been built in past decades to satisfy demands for safe and efficient transportation. The aerodynamic performance of road vehicles under the action of strong cross-wind gusts remains one of the major issues encountered during design and construction to improve ride comfort and safety of passengers on these structures. [Baker (1991a, 1991b, 1991c)] has made extensive studies on the performance of high-sided vehicles exposed to cross winds on roads and bridges. A few studies aimed at quantifying the effects of transient wind loads on vehicles have been conducted [Bearman and Mullarkey (1994), Ryan and Dominy (1998)]. Recently, efforts have been made to reduce wind-induced instability of vehicles on bridges using wind barriers [Štrukelj et al. (2005), Wang et al. (2007), Procino et al. (2008), Kozmar et al. (2009a, 2009b)]. In this study, experiments have been carried out to evaluate transient wind loads, simulating the periodicity of Bora type winds occurring along the Croatian coast, on a generic vehicle model placed on a generic bridge deck section. Characteristics of Croatian Bora winds A major threat to traffic in coastal Croatia is a strong, cold and gusty north-easterly wind blowing over the Dinaric Alps along the eastern Adriatic. Its most prominent feature is strong gustiness. In severe Bora cases mean hourly wind speeds rarely exceed 17 m/s [Koračin (1984), Ivančan-Picek and Tutiš (1996), Belušić et al. (2004a, 2004b)], while gusts

2 may reach values to 69 m/s [Bajić and Peroš (2005)]. Spectral analyses of Bora gusts [Petkovšek (1982, 1987), Belušić and Bencetić Klaić (2004c)] reveal spectral peaks between 3 and 11 minutes, often with doubling of periods [Koračin (1982), Petkovšek and Rakovec (1983), Petkovšek (1984)], and a possibility of cessation and even reappearance of the pulsations in the course of a single Bora episode [Watanabe (1976)]. The gusts usually start rather suddenly, then weaken followed by gaps or weak wind periods which last about a third of the whole period, as shown in Figure 1, after [Petkovšek (1987)]. Figure 1. Time series of Bora wind gusting [Petkovšek (1987)] Bora is often more intense and frequent in winter and in the night. Synoptic situations during Bora episodes are usually related to a strong pressure gradient between an anticyclone over middle and eastern Europe and a depression over the central Mediterranean [Poje (1992)]. This ensures a supply of cold air impinging on the mountain. Therefore, the Adriatic Sea responds in a complex way to the Bora forcing [Orlić et al. (1994), Beg Paklar et al. (2001)]. The main Bora gusts are not a consequence of turbulence in a general flow, but of cold air cylinders rolling down the slope on the warm side of the ridge [Petkovšek (1987)]. Main rolls together with atmospheric turbulence and partly sliding cold air develop a general flow with observed Bora characteristics, as shown in Figure 2, after [Petkovšek (1987)]. Figure 2. Development of Bora gusts due to rolling of cold air cylinders down the slope on the sea side of the mountain [Petkovšek (1987)]

3 Several previous studies [Smith (1987), Klemp and Durran (1987), Enger and Grisogono (1998)] reported similarity of Bora dynamics with downslope winds in other parts of the world (e.g. the Boulder downslope windstorm), which allows extending some Boulder results to Bora winds [Belušić and Bencetić Klaić (2004c)]. In the past, Bora winds have been studied theoretically [Klemp and Durran (1987), Glasnović and Jurčec (1990), Enger and Grisogono (1998)] and observationally [Jurčec (1981), Smith (1987), Ivančan-Picek and Tutiš (1995, 1996)]. It was also attempted to carry out numerical simulations [Lazić and Tošić (1998), Tošić and Lazić (1998), Qian and Giraud (2000)]. Peroš (1994) calculated the power spectra of Bora wind gusting using the ARMA method originally developed by Li and Kareem (1990), and he also suggested the spectrum of Bora gusts like other random processes depends on the averaging period. As it may have been seen throughout this section, an extensive body of data is available on meteorological observations of Croatian Bora winds and there were attempts to carry out numerical simulations as well. However, there is an obvious lack of engineering studies dealing with Bora effects on structural objects and vehicles. This study aims to add up knowledge on effects of Bora winds on vehicles. a) b) Figure 3: TFFS models (all quotes in mm): a) Generic vehicle without wheels (model A), b) Generic vehicle with wheels (model B); In both configurations generic vehicle model was placed on a generic bridge deck section

4 Experimental setup Tests were performed in the Transient Flow Field Simulator (TFFS) in the NatHaz Lab at the University of Notre Dame, which enables a precise generation of transient wind events. Preliminary tests carried out in this facility were previously presented in [Butler and Kareem (2007)]. In this study, instantaneous velocities in the x-direction were measured using a single hot-wire probe (DANTEC 54T30). SenSym transducers (model ASCX01DN) were employed for pressure measurements on the vehicle model surface. The 114 mm long, 57 mm high and 57 mm wide prism made of Plexiglas represents a generic van model at the scale of 1:40. Transient aerodynamic loads experienced by the vehicle with and without wheels are presented in form of side force, lift force and overturning (rolling) moment. Horizontal and vertical incidence angles were normal to the side of the vehicle model in both tests. The two tested van models are presented in Figure 3. A photograph of model A placed in the TFFS test section is shown in Figure 4. Figure 4: A photograph of model A placed in the TFFS test section Results Wind gusting of a vehicle was simulated by exposing the vehicle model to an air flow consisting of a velocity time history that alternates within a predefined velocity threshold. Each simulated time history consisted of fifty gust events. Examples of the velocity time series for two employed van models are presented in Figure 5. Figure 5: Parts of recorded velocity time series, simulating the periodic features of Bora winds: a) model A, b) model B Steady aerodynamic loads experienced by the vehicle increase as the wind velocity increases, as shown in Table 1, where u is average freestream wind velocity. Side force

5 coefficient C FS, lift force coefficient C FL and overturning moment coefficient C MR were determined by integrating static surface pressures, as obtained from the tapings, over the respective model surface and calculated using the van model frontal area. All forces and moments were calculated using the velocity u averaged during normal wind periods (u = 3.8 m/s). Table 1: Steady aerodynamic side force, lift force and overturning moment a) Low velocity period (normal winds) b) High velocity period (wind gusts) u, m/s u ' 2, m/s C FS C FL C MR u, m/s u ' 2, m/s C FS C FL C MR Model A Model B In the velocity spectra presented in Figure 6, there is a peak at approximately 0.3 Hz due to intermittent switching between lower and higher velocity each 3 s. Figure 6: Power spectral density of longitudinal velocity fluctuations: a) model A, b) model B Figure 7: Power spectral density of side force fluctuations: a) model A, b) model B

6 Intermittent switching between lower and higher velocity is a typical feature of Bora winds. Power spectra of longitudinal velocity fluctuations in two tested configurations are nearly the same. For model A there is a peak in side force fluctuations at 10 Hz possibly due to periodic shedding of vortices from the bridge and model, as shown in Figure 7. The unsteady aerodynamic side force for model B is manly influenced by the wind gusting phenomena, as there is a strong peak around 0.3 Hz, and a less intense peak at 10 Hz. The passage of flow beneath the vehicle model is in part responsible for this feature. Lift force fluctuations show a similar pattern for both models, i.e. strong low frequency peak at 0.3 Hz and a weaker peak at 10 Hz, as shown in Figure 8. The overturning moment for model A is influenced both by the wind gusting frequency, around 0.3 Hz, and by the shed vortices from the bridge around 10 Hz, while the intensity of the low frequency peak at 0.3 Hz is comparatively stronger for model B, as shown in Figure 9. Figure 8: Power spectral density of lift force fluctuations: a) model A, b) model B Figure 9: Power spectral density of overturning moment fluctuations: a) model A, b) model B

7 Conclusions Transient aerodynamic loads on a vehicle exposed to cross-wind gusts were experimentally simulated in the NatHaz Laboratory at the University of Notre Dame. Steady aerodynamic loads were larger during the simulated wind gusts, characterizing Bora winds, compared to the normal wind periods. Wind gusting effects on unsteady aerodynamic loads can be clearly seen for both models, as the low frequency peak at 0.3 Hz is due to the simulated Bora event. However, for model A (wheels omitted) there is a strong influence of shed vortices from the bridge, and their interactions with the van model, on the side force and overturning moment fluctuations, as a high frequency peak at 10 Hz, corresponding to shed vortices, was recorded. Acknowledgements The first author acknowledges the support of the Fulbright Visiting Scholar Grant. Support for the remaining authors was provided in part by the Global Center of Excellence, Tokyo Polytechnic University funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) under the project on load effects in transient flow conditions. References Bajić, A. and Peroš, B. (2005), Meteorological basis for wind loads calculation in Croatia, Wind and Structures, 8(6), Baker, C. J. (1991a), Ground vehicles in high cross winds. 2. Unsteady aerodynamic forces, Journal of Fluids and Structures, 5(1), Baker, C. J. (1991b), Ground vehicles in high cross winds. 3. The interaction of aerodynamic forces and the vehicle system, Journal of Fluids and Structures, 5(2), Baker, C. J. (1991c), Ground vehicles in high cross winds. 1. Steady aerodynamic forces, Journal of Fluids and Structures, 5, Bearman, P. W. and Mullarkey, S. P. (1994), Aerodynamic Forces on Road Vehicles due to Steady Side Winds and Gusts, Road Vehicle Aerodynamics, Royal Aeronautical Society Conference, Beg Paklar, G., Isakov, V., Koračin, D., Kourafalou, V., Orlić, M. (2001), A case study of bora-driven flow and density changes on the Adriatic Shelf (January 1987), Continental Shelf Research, 21(16-17), Belušić, D., Pasarić, M., Pasarić, Z., Orlić, M. (2004a), The dynamics of pulsations in the bora flow, Proceedings of the 11 th Conference on Mountain Meteorology & Annual Mesoscale Alpine Program (MAP). Belušić, D., Pasarić, M., Pasarić, Z., Orlić, M. (2004b), Quasi-periodic bora gusts related to the structure of troposphere, The Quarterly Journal of the Royal Meteorological Society, 130, Belušić, D. and Bencetić Klaić, Z. (2004c), Estimation of bora wind gusts using a limited area model, Tellus Series A - Dynamic Meteorology and Oceanography, 56(4), Butler, K. and Kareem, A. (2007), Physical and numerical modeling of downburst generated gust fronts, Proceedings of the 12 th International Conference on Wind Engineering, Cairns, Australia, Enger, L. and Grisogono, B. (1998), The response of bora-type flow to sea surface temperature, The Quarterly Journal of the Royal Meteorological Society, 124(548), Glasnović, D. and Jurčec, V. (1990), Determination of upstream bora layer depth, Meteorology and Atmospheric Physics, 43(1-4), Ivančan-Picek, B. and Tutiš, V. (1995), Mesoscale bora flow and mountain pressure drag, Meteorologische Zeitschrift, 4, Ivančan-Picek, B. and Tutiš, V. (1996), A case study of a severe Adriatic bora on 28th December 1992, Tellus, 44A,

8 Jurčec, V. (1981), On mesoscale characteristics of bora conditions in Yugoslavia, Pure and Applied Geophysics, 119(3), Klemp, J. B. and Durran, D. R. (1987), Numerical modelling of bora winds, Meteorology and Atmospheric Physics, 36(1-4), Koračin, D. (1982), Spektralna analiza faktora mahovitosti bure na aerodromu Rijeka-Krk, Zbornik meteoroloških i hidroloških radova, 8, Koračin, D. (1984), Parameters of bora flow, Zbornik meteoroloških i hidroloških radova, 10, Kozmar, H., Procino, L., Borsani, A., Bartoli, G. (2009a), Testing aerodynamic properties of road windscreens, Građevinar, 61(3), Kozmar, H., Procino, L., Bartoli, G., Borsani, A. (2009b), Wind barriers on bridges: Effects of wind incidence angle on flow field characteristics, 7 th Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan. Lazić, L. and Tošić, I. (1998), A real data simulation of the Adriatic bora and the impact of mountain height on bora trajectories, Meteorology and Atmospheric Physics, 66(3-4), Li, Y. and Kareem, A. (1990), ARMA Systems in Wind Engineering, Probabilistic Engineering Mechanics, 5(2), Orlić, M., Kuzmić, M., Pasarić, Z. (1994), Response of the Adriatic Sea to the bora and sirocco forcing, Continental Shelf Research, 14(1), Peroš, B. (1994), Modeling of the Bora effects upon the lower layer, Engineering Modeling, 7(3-4), Petkovšek, Z. (1982), Gravity waves and bora gusts, Annalen für Meteorologie, 19, Petkovšek, Z. (1984), Some characteristics of bora gusts, Zbornik meteoroloških i hidroloških radova, 10, Petkovšek, Z. (1987), Main bora gusts a model explanation, Geofizika, 4, Petkovšek, Z. and Rakovec, J. (1983), Mjerenja i istraživanja mahovitosti bure, Proceedings of the ALPEX Conference 7, Poje, D. (1992), Wind persistence in Croatia, International Journal of Climatology, 12(6), Procino, L., Kozmar, H., Bartoli, G. and Borsani, A. (2008), Wind barriers on bridges: The effect of wall porosity, Proceedings of the 6 th Colloquium on Bluff Bodies Aerodynamics and Applications, Milan, Italy. Qian, M. W. and Giraud, C. (2000), A preliminary numerical simulation of bora wind with a limited area model of atmospheric circulation, Nuovo Cimento della Societa Italiana di Fisica C-Geophysics and Space Physics, 23(5), Ryan, A. and Dominy, R. G. (1998), Aerodynamic forces induced on a passenger vehicle in response to a transient cross-wind gust at a relative incidence of 30 degrees, Developments in Vehicle Aerodynamics, SAE Special Publication 1318, Smith, R. B. (1987), Aerial observations of the Yugoslavian bora, Journal of the Atmospheric Sciences, 44(2), Štrukelj, A., Ciglarič, I. and Pipenbaher, M. (2005), Analysis of a Bridge Structure and its Wind Barrier under Wind Loads, Structural Engineering International, 15(4), Tošić, I. and Lazić, L. (1998), Improved bora wind simulation using a nested Eta Model, Meteorology and Atmospheric Physics, 66(1-2), Wang, D. L., Chen, A. R. and Zhou, Z. Y. (2007), Aerodynamic characters of a trans-oceanic cable-stayed bridge with wind barrier, Proceedings of the 12 th International Conference on Wind Engineering, Cairns, Australia, Watanabe, K. (1976), Bora and Man: Weather Forecasts and Prognosis of Bora by the Fishermen's Traditional Way of Observation on the Croatian Coast, Local Wind Bora (ed. Yoshino, M.M.), University of Tokyo Press, Tokyo,