It has been recently discovered that spatially separated Herbig-Haro object
s, once considered unrelated, are linked within a chain that may extend for
parsecs in either direction of the embedded protostar, forming a giant Her
big-Haro jet. Presently, several dozen of these giant flows have been detec
ted, and the best documented example, the HH 34 system, shows a systematic
velocity decrease with distance on either side of the source. In this paper
, we have modeled giant jets by performing fully three-dimensional simulati
ons of overdense, radiatively cooling jets modulated with long-period (P si
milar to several hundred years) and large-amplitude sinusoidal velocity var
iability at injection (Delta upsilon similar to mean jet flow velocity). Al
lowing them to travel over a distance well beyond the source, we have found
that multiple traveling pulses develop, and their velocity indeed falls of
f smoothly and systematically with distance. This deceleration is fastest i
f the jet is pressure-confined, in which case the falloff in velocity is ro
ughly consistent with the observations. The deceleration occurs as momentum
is transferred by gas expelled sideways from the traveling pulses. The sim
ulation of a pressure-confined, steady state jet with similar initial condi
tions to those of the pulsed jet shows that the flow in this case experienc
es acceleration. This result is thus an additional indication that the prim
ary source of deceleration in the giant flows cannot be attributed to braki
ng of the jet head against the external medium.