Abstracts - KTH Mechanics

22
Large eddy simulation of impinging jets with focus on the
inflow boundary conditions
T. Hällqvist ∗ and L. Fuchs †
This paper deals with Large Eddy Simulation (LES) of submerged circular impinging
jets. The main objective is to study the influence from the imposed inflow
conditions. The outcome of a numerical simulation depends strongly on the discretization
scheme, the computational grid and the type of modeling. However, in case of
unsteady simulations the inlet velocity field may be just as important for the quality
and relevance of the results. The benefits having an accurate scheme, a fine grid and a
sophisticated SGS model may not be sufficient if not the appropriate inlet conditions
are provided. In some applications, where the inflow is laminar or has limited influence
on the region of interest, the simplest type of velocity data may be considered,
i.e. a mean velocity profile with or without superimposed random perturbations. The
major drawback with random perturbations is that there are no correlations in time
or space. Furthermore, the energy distribution among the different scales is constant.
The consequence is that the applied disturbances will quickly dissipate.
The most correct method is to use the velocity field from a precursor simulation.
By doing this the correct correlations and spectrums are achieved. There are some
drawbacks with this method though, namely the needed amount of storage capacity
and the high cost for the precursor simulation. This method is also relatively inflexible
as an additional calculation must be performed if modifications to the velocity field is
wanted. Within the present work pipe flow simulations have been conducted with the
prescribed Reynolds number, which is Re D = 20000. The data from this simulation
has then been supplied at the inlet of the impinging jet. The impinging jet has a
nozzle-to-plate spacing (H/D) of 2 and 4 jet diameters. Results on mean velocities
and mean scalar concentration as well as higher order statistics are presented for
different type of inflow conditions (see figure 1).
∗ Scania CV AB, SE-151 87 Södertälje, Sweden.
† KTH Mechanics TR 8, SE-100 44 Stockholm, Sweden.
Figure 1: Influence from boundary conditions on the instantaneous scalar field for
H/D = 4. Left picture: randomly perturbed top-hat profile, middle picture: randomly
perturbed mollified profile, right picture: turbulent inflow conditions.