Computational fluid dynamics Questions With Complete Solutions

Why should you use CFD when we know empirical and semi-empirical models have worked before? correct answer: Because it is with CFD possible to model new equipment which has not yet been tested. Thus one can simulate performance of the equipment before spending tons of money to test in reality. What can you simulate with CFD? correct answer: CFD gives a detailed analysis of flows, heat transfer and mass transfer. Can study transport of chemical species, chemical reactors, combustion, evaporation, condensation and crystallization. What does a large variance and a low variance tell us? correct answer: A large variance tells us that the instantaneous concentration is far from the average and the probability that both reactants are present is low. A small variance tells us that the concentrations of both reactants are close to their average and there is a high probability that both are present simultaneously. How reliable are CFD simulations? correct answer: For laminar, single-phase flow the simulations are very accurate. For turbulent flow it is quite reliable also. For multiphase flow on the other hand traditional modelling is more accurate than the best CFD simulations. However, CFD can be used in this case to do rather accurate predictions. Also good for finding bottle necks in a simulation. Simulations of heat- and gas phase mass- transfer are accurate and straight forward. Diffusivities for liquids are three - four orders of magnitude lower and will thus require very dense grids. What steps should you follow when modelling with CFD? correct answer: 1) Define geometry and boundaries. Draw a 2D or 3D sketch of your system. 2) Generate the computational mesh. Check aspect ratio and skewness. 3) Define models for e.g. turbulence, chemical reactions etc. 4) Set fluid properties e.g. density, viscosity etc. 5) Set boundary conditions for outlet and the walls and set inlet conditions. 6) Choose a proper solver, whether it is transient or steady state, convergence critera etc. Upwind schemes, under-relaxation etc. 7) Analyze your results. Is the solution grid independent? Independent on BCs? Independent of model? How is a general balance equation formulated? correct answer: [Accumulation within system] = [net transport into system through boundaries] + [net generation within system]+[source terms] Describe how mass, heat and momentum is transported. correct answer: Mass, heat and momentum are transferred by convection of the mean flow and by random movement of molecules. In turbulent flows the tran