In a similar manner, it is usually recommended to leave a space of about 2 times the body width on each side to allow for local flow deviation. Therefore, it is recommended to have length of minimum 5 times the dimension of the body along the direction of the flow to allow enough space to the boundary condition imposed at the domain outlet.
Downstream, generally the body will shed a wake of lower energy flow, which is also convected by the flow. This dimension should be enough to allow the flow to adjust due to the presence of the geometry. Upstream of the body, a good starting point would be to have a minimum of roughly 2 times the length of the body itself. If the distance is too small, the numerical boundary conditions can affect the flow field around the body in a non-physical manner because in the solution algorithm the flow is always forced to comply with the conditions defined at the boundary. In general, it is necessary to allow enough space around the geometry of interest to so that the perturbations introduced by the presence of the geometry do not interfere with the boundaries themselves. Once the simulation has run, the results should be checked to see if the boundary of the domain are influencing the flow around the body and, if so, the size of the domain should be adjusted for any subsequent simulation. When previous results are not available or when carrying out a simulation with a very large domain is not feasible due to the computational cost, it is possible to use experience and best practises to estimate the domain dimensions. This can evaluated considering previous CFD simulations of similar geometries or by carrying out a simulation with an oversized domain. When deciding on the size of the computational domain for external aerodynamics problems, it would be beneficial to know in advance the effect of the body on the surrounding flow field. In internal aerodynamic problems instead, the geometry itself defines the computational domain and the boundary conditions are directly applied to the surfaces of the geometry. On the contrary, when no data is available and simple constant inlet velocity and constant pressure outlet boundary condition are used, the same principle of an external aerodynamic case should be used. If detailed data for setting up the inlet and outlet boundary conditions is known, these can be placed to match the experimental measurements. In these cases, the computational domain is defined by the geometry of the physical walls and generally only the position of the inlet and outlet boundaries need to be defined. Such distance should be large enough to make sure that the boundary conditions assigned to the outer domain do not alter the flow next to it an affect the quality of results.Īn external aerodynamics problem can involves the presents of physical boundaries, like the walls of a wind tunnel, or the ground in an open road simulation or aircraft landing condition. Therefore, the sides of the computational domain that do not represent physical geometries like walls, grounds and ceilings are generally placed as far away as possible from the geometry that needs to be modelled.
The streamlines deviates as they approach the aerofoil and, due to the high angle of attack, the flow over the top surface is separated with large recirculations zones. In most cases, the geometry of the body is such that it leaves a wake of low energy flow behind, as the flow separates from it.Īs an example, the picture below shows the flow around an aerofoil in a wind tunnel. Moreover, the presence of the body is felt upstream and the flow starts to divert before actually making contact with the body. In a low speed simulation (low Mach number/subsonic flow), the pressure increases in areas where the flow velocity reduces and decreases where the flow accelerates. Its shape and size of the depend mainly on the aerodynamic characteristics of the geometry.Ī body immersed in a flow field affects the flow around it as the flow needs to get around and past the body. In external aerodynamics, the computational domain, often referred to as outer domain, describes the region around the geometry of interest where the flow solution is required. The dimensions of a computational domain depend on the type of problem: whether it is an external aerodynamic problem or an internal flow problem.
0 kommentar(er)
