  Near Infrared (NIR) and optical photometry and spectroscopy are presented for
the nova V1831 Aquilae, covering the early decline and dust forming phases
during the first $\sim$90 days after its discovery. The nova is highly reddened
due to interstellar extinction. Based solely on the nature of NIR spectrum we
are able to classify the nova to be of the Fe II class. The distance and
extinction to the nova are estimated to be 6.1 $\pm$ 0.5 kpc and $A_{\rm v}$
$\sim$ 9.02 respectively. Lower limits of the electron density, emission
measure and ionized ejecta mass are made from a Case B analysis of the NIR
Brackett lines while the neutral gas mass is estimated from the optical [OI]
lines. We discuss the cause for a rapid strengthening of the He I 1.0830 $\mu$m
line during the early stages. V1831 Aql formed a modest amount of dust fairly
early ($\sim$ 19.2 days after discovery); the dust shell is not seen to be
optically thick. Estimates are made of the dust temperature, dust mass and
grain size. Dust formation commences around day 19.2 at a condensation
temperature of 1461 $\pm$ 15 K, suggestive of a carbon composition, following
which the temperature is seen to gradually decrease to 950K. The dust mass
shows a rapid initial increase which we interpret as being due to an increase
in the number of grains, followed by a period of constancy suggesting the
absence of grain destruction processes during this latter time. A discussion is
made of the evolution of these parameters, including certain peculiarities seen
in the grain radius evolution.
