Hi! I am new to CFD. I am analyzing flow for exhaust manifolds. What are the typical inputs - mass flow at the inlets & pressure at the outlet?

What is the expected guideline to say design A or design B is better - Pressure drop and flow pattern or anything else?

Appreciate any help in this regard Venu

You don't mention what the manifolds will be connected to. If they'll be connected to a reciprocating engine, the manifolds can't be designed as stand-alone ducts. The behavior of the flow inside the manifold is affected both by what's happening inside of the cylinder, as well as what's happening inside of the rest of the exhaust system. A reciprocating engine is really a variable-volume Helmholz resonator. The cylinder, exhaust headers, exhaust pipes, intake runners, and intake plumbing all act together as a complete system.

While applying good aerodynamic practice to the design of the exhaust manifold (like avoiding shapes and bends that will cause separation) is sound reasoning, the only way to truly design a manifold and determine that design A is better than design B, is to model the complete system--the engine with all of its intake and exhaust plumbing connected. Remember that, in the case of reciprocating engines, the entire system is immersed in unsteady flow. Improving the steady-state flow characteristics of the manifold will often lead to unexpected results. Ask me how I know...

That said, for a well-posed solution, characteristic analysis would typically say that, on the inflow end of the manifold, density, temperature, and transverse velocities would be specified, and longitudinal velocity would be allowed to float. On the outflow, density, longitudinal, and lateral velocities would be allowed to float, and pressure, by way of temperature, would be specified.

should it be connected to reciprocating combustion engine, then A is better than B is it (A) helps/improves gas exchange utilizing wave and inertia effects... higher pressure drop can be sometimes useful in two-stroke engine...

Much exhaust (and for that matter, intake) manifold CFD is carried out using coupled 1D/3D models. That way the 1D code models most of the system (including the cylinders and valve flows), while the 3D code models the complex flow inside the manifold.