For an incompressible flow field , $$\overrightarrow {V,} $$ which one of the following conditions must be satisfied?

A spherical balloon with a diameter of $$10m,$$ shown in the figure below is used for advertisements. The balloon is filled with helium $$\left( {{R_{He}} = 2.08\,\,\,\,kJ/kg.K} \right)$$ at ambient conditions of $${15^ \circ }C$$ and $$100\,\,kPa.$$ Assuming no disturbances due to wind , the maximum allowable weight (in Newton) of ballon material and rope required to avoid the fall of the balloon $$({R_{air}} = 0.289kJ/kg.K)$$ is _____________.

A material $$P$$ of thickness $$1$$ $$mm$$ is sandwiched between two steel slabs, as shown in the figure below. $$A$$ heat flux $$10\,kW/{m^2}$$ is supplied to one of the steel slabs as shown. The boundary temperatures of the slabs are indicated in the figure. Assume thermal conductivity of this steel is $$10W/m.K.$$. Considering one-dimensional steady state heat conduction for the configuration, the thermal conductivity ($$k$$, in $$W/m.K$$) of material $$P$$ is _____________

Biot number signifies the ratio of