LBM Scaling help - beginner

hi everyone i have been searching and reading this site for a few days now but i am still a little stuck. i have been given a LBM code which i need to use to optimise a wind barrier, to start with i have been trying to model a 2D rectangular block with dimensions 25x100 mm. from this i have deduced some realy world properties for this obstacle: U_real = 13,000 mm/s (30MPH) L_real = 25mm and a kinematic viscosity of air i believe to be Nu_real = 8.33mm^2/s this in turn gives me a Re of approx 5,000

so then to input this data into my LBM solver i have chosen the properties:
U_LBM = 0.1
Re = 5,000
L_LBM =2.5

the program then seems to calculate:

however when choosing these parameters the programme fails to produce stable results.

From what i have read i need to find a way to increase the grid size and chose a larger? i would just like to check that i am on the right lines or whether i have missed something.

any help would be great and thanks in advance Kyle

(the grid size and time step size are set to 1 and cannot be changed)