  A model is developed to explain rise time of a spherical intruder placed in a
granular bed, which is considered as fluid. Phenomenon of rising intruder in a
granular bed is well known as Brazil nut effect. Radius of the intruder is
varied with $R_n = R_0 n, n = 1 .. 10$. An approximation for $t << \tau$ is
chosen in order to simplify the solution of second order differential equation
of intruder vertical position to obtain the rise time T. A non-physical
parameter $\alpha$ and also transformation from $T$ to $T'$ are needed to be
introduced in order to control the results to mimic reported experiment
qualitatively. Several forms of rise time $T_n$ and also transformed rise time
$T_n'$ against $n$ are presented and discussed.
