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Viscous Flow and Boundary Layer
Published in Rose G. Davies, Aerodynamics Principles for Air Transport Pilots, 2020
Airflow separation does not only take place over an aerofoil, it also can happen behind different objects, or on other surfaces as well. The shapes of the objects and the quality of the surfaces can cause the passing airflow to leave its streamlining path and form vortices in the wake, as shown in Figure 3.13 (a). Rotating vortices produce a low-pressure zone and cause the fore–aft pressure difference along the object. The airflow around the object loses kinetic energy to form the vortices, and the object needs to overcome this fore–aft pressure difference to move forward. So this fore–aft pressure difference to the object is form drag. The form drag is also called separation drag. A blunt object or a sudden change of a surface in a fluid flow can disrupt the ability of the flow to follow the contour of the surface and separate, as shown in Figure 3.13 (b).
Wind Tunnels
Published in Ethirajan Rathakrishnan, Instrumentation, Measurements, and Experiments in Fluids, 2020
The presence of the lateral boundaries around the test-section (the walls of the test-section) produces the following. A lateral constraint to the flow pattern around a body is known as solid blocking. For closed throats solid blocking is the same as an increase in dynamic pressure, increasing all forces and moments at a given angle of attack. For open test-sections it is usually negligible, since the air stream is free to expand.A lateral constraint to flow pattern about the wake is known as wake blocking. This effect increases with increase of wake size. For closed test-section wake blocking increases the drag of the model. Wake blocking is usually negligible for open test-sections.An alteration to the local angle of attack along the span.
Wind Tunnels
Published in Ethirajan Rathakrishnan, Instrumentation, Measurements, and Experiments in Fluids, 2016
The presence of the lateral boundaries around the test-section (the walls of the test-section) produces the following. A lateral constraint to the flow pattern around a body is known as solid blocking. For closed throats solid blocking is the same as an increase in dynamic pressure, increasing all forces and moments at a given angle of attack. For open test-sections it is usually negligible, since the air stream is free to expand.A lateral constraint to flow pattern about the wake is known as wake blocking. This effect increases with increase of wake size. For closed test-section wake blocking increases the drag of the model. Wake blocking is usually negligible for open test-sections.An alteration to the local angle of attack along the span.
Study on boundary conditions of k-ε-fp turbulence model for wind turbine wake simulation
Published in International Journal of Green Energy, 2023
Tao Chen, Hang Meng, Li Li, Peng Wang, Dawei Li
In recent years, energy shortage and greenhouse effect have promoted the rapid development of wind energy. The global wind power market developed rapidly. By 2021, the cumulative installed global wind power capacity has reached 837 GW with an additional installed capacity of 93.6 GW, which is increasing by 12.80% over 2020. With increasing installation of wind turbines, the size of windfarm is getting larger. The problem of wake effect also follows: the upstream wind turbines absorb the kinetic energy and increase turbulence intensity of the incoming wind. The downstream wind of low-speed and high turbulence intensity is called “Wake” (Nash, Nouri, and Vasel-Be-Hagh 2021). It is obvious that the wake effect reduces the power generation of the downstream wind turbines. Furthermore, the high turbulence intensity of wake effect will cause fatigue load increase on wind turbines, which reduces the wind turbine longevity and brings risks to the wind farm. Due to the existence of wake, a certain distance must be kept between the wind turbines, not only for the purpose of improving the power generation, but also for making the wind turbines operate safely. The research on the wake effect of wind farm is the key to optimize the layout of wind turbines and to achieve the best benefit of wind farm (Gao et al. 2020). Therefore, accurate simulation of wind turbine wake is of great significance to the micro-sitting of wind farms (Zhao et al. 2020).
Scour around impermeable spur dikes: a review
Published in ISH Journal of Hydraulic Engineering, 2018
Manish Pandey, Z. Ahmad, P. K. Sharma
Primary vortex is shaped elliptically with an inner core of forced vortex whereas the outer core location is of the free vortex. In the scour hole near abutment or spur dike, the value of the maximum flow velocity and downflow elements are 1.35 and 0.75 times of the approach velocity, correspondingly. Another vortex with opposite direction of the primary vortex known as the secondary vortex, developing after the primary vortex was also perceived by them. It was believed that the scour capacity of primary vortex could be limited due to the effect of the secondary vortex. With the effect of the split of upstream and downstream flow near abutment or spur dike corners, wake vortices are formed at the downstream of the spur dike. Wake vortices can be stated as the unstable shear layers caused by the flow separation rolled up to form eddy structures. These wake vortex systems are slightly weak than the primary vortex system. Major flow components near a spur dike/vertical wall abutment are shown in Figure 2(a–b) after Kwan (1988).
Experimental study of turbulence intensity on the wake characteristics of a horizontal-axis wind turbine
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2022
Yuxia Han, Jianwen Wang, Xin Li, Kaining Jin, Bin Yang, Xueqing Dong, Caifeng Wen
In actual operation, wind turbines are inevitably influenced by ambient turbulence and the wake effect, and the wake region is accompanied by reduced wind speed and increased turbulence intensity. Thomsen et al. found a 5–25% increase in the wake turbulence intensity in wind turbines caused by the wake effect (Thomsen and Sorensen 1999), which in turn leads to the reduction of power output and turbine service life, and increase in fatigue load of downstream wind turbines (Vermeer, Sørensen, and Crespo 2003). Consequently, the design of wind turbines must take into account the effect of turbulence intensity on the structure of the turbine’s wake. The features of the local atmosphere’s turbulence must also be taken into account while selecting places for wind farms.