# Unfrozen Skewed Turbulence for Wind Loading on Structures
## 4 Results  
### Flow Characteristics of Wind Storm Aina (2017)  
The Bjornafjord is a 5-km wide and 500-m deep fjord connecting some of the largest export regions of south-western Norway. To strengthen the economy of this area, the new E39 Coastal Highway Route [24] plans to replace the ferries crossing the Bjornafjord with a 5-km long bridge. The adopted design concept is a floating pontoon bridge, the dimensions of which make it highly sensitive to environmental loading, especially wind turbulence. The homogeneity of the mean and turbulence flow characteristics is a topic of ongoing research in the Bjornafjord. This fjord is rather flat and open to the sea, such that spatial variations of the flow characteristics remain smooth [76]. This allows for the application of the proposed wind field simulation algorithm. Nevertheless, the southern part of the bridge joining the sea and land will require a dedicated study. Indeed, this part will comprise a long-span cable-stayed bridge in the vicinity of a hill, where turbulence is unusually high and the flow is non-horizontal. The crossing location is in an open fjord characterized by islands and hills (Figure 2). Therefore, the flow is likely to be skewed along the deck, with a yaw angle up to 45\({}^{\circ}\). Also, the horizontal curvature of the future bridge deck implies that the yaw angle will vary along the bridge span. This motivates the adequate modelling of yawed turbulent flow conditions. Furthermore, the assumption of frozen turbulence may no longer be applicable for the range of frequencies associated with the dynamic motion of such a long bridge. Since 2015, four 50-m met-masts have been deployed in the Bjornafjord. Among them, the mast MW1 (Figure 2) was installed 23 m above sea level, on the west side of the fjord, on a small island with low vegetation. It was instrumented with three Gill WindMaster Pro  
Figure 2: Topography of the Bjornafjord sketching the future bridge crossing and the mast MW1 used in the analysis of the velocity data. 3-axis 3D sonic anemometers, mounted on 4-m long booms oriented with an angle of \(-2^{\circ}\) from North. Two of the anemometers were located at 49 m above ground but 8 m apart from each other. The third anemometer was 33 m above ground. The masts, supported by guyed wires, have a tubular structure with a diameter of ca. 25 cm to minimize flow distortion. The anemometers recorded the three velocity components at a sampling frequency of 10 Hz but the sonic temperature was not available. The influence of the local topography on the flow characteristics recorded on the mast MW1 under moderate wind conditions is given in [PERSON] et al. [76]. Generally, northerly or southerly wind directions prevail along the Norwegian coast. However, a westerly wind was recorded on the second day of storm Aina, which hit the southwestern coast of Norway on 7 and 8 December 2017 [77]. On that day, from 2:00 a.m. to 5:00 a.m., three hours of stationary records with a mean wind speed of 24 m s\({}^{-1}\) at 49 m above ground were recorded on MW1 (Figure 3). The velocity data did not show any clear trend and the mean wind direction increased by only 20\({}^{\circ}\) from 2:00 a.m. to 5:00 a.m. The flow was thus fairly stationary because the storm was moving toward the coast instead of along it. The dataset collected that day combines strong wind speed, stationary fluctuations and a duration of at least three hours. Therefore, it is a highly valuable record for the design of the Bjernafjord bridge. During storm Aina, two of the three sonic anemometers on MW1 displayed a higher-than-usual measurement noise in the high-frequency range, partly  
Figure 3: One-minute moving average time series corresponding to storm Aina (December 2017) from 2:00 a.m. to 5:00 a.m. on 8 December 2017 using one of two anemometers mounted 49 m above ground on MW1. from the use of a 10-Hz sampling frequency without an adequate anti-aliasing filter. In general, a value of 25 Hz is recommended [78].