Test 1 CFD Questions And Answers With Complete Solutions

Pros and Cons of Experimental correct answer: Pros: -Realistic results for flight conditions in tunnel -Good for finding trends and correlating variables Cons: -Unable to match all real flight conditions (e.g. Mach and Reynolds) -Costly (equipment, manufacturing time) -tunnel corrections -Difficult to measure Pros and Cons of Theoretical correct answer: Pros: -General analytical expressions -Generally easy to use and fast to analyze Cons: -Require significant simplifying assumptions -Limited to idealized situations Pros and Cons of Numerical Solutions correct answer: Pros: -Exact conditions of flight can be matched (e.g. Mach and Reynolds) -Complicated geometries can be analyzed -Can assess flows on and off the body where sensing equipment can't read -Cost effective -Can trade off between speed and accuracy depending on the circumstances Cons: -Numerical errors exist and can propagate -Simplified physics usually used --Particularly for chemically reacting flows OR --Turbulent behavior Three types of error that exist in numerical solutions correct answer: 1. Truncation Error 2. Round off error 3. Boundary condition error Final step of CA/CFD analysis? correct answer: Validation of results through experiments Rate of Computational Capability over the years correct answer: Order of magnitude ever 8yrs CA vs. CFD correct answer: CA (Computational Aerodynamics) -Strictly aerodynamics -Often use simplified sets of equations -Subsonic flight -Vortex lattice method -Panel method -Boundary layer methods CFD (Computational Fluid Dynamics) -Complex analysis using Navier-Stokes and maybe Euler's -Meteorology -Car CFD -Hypersonic flight -Massive flow separation Three phases of Aerospace Design correct answer: 1. Product of Conceptual Design-outline 2. Product of Preliminary design-fixed geometry 3. Product of Detail Design-rivet location Four steps of CFD correct answer: 1. Geometry modeling 2. Grid generation 3. Numerical analysis (solve) 4. Post-processing (pretty images) Equations in increasing steps of thoroughness correct answer: *Least Difficult* 1. Engineering Methods (cl=a alpha) -Often adjusted to match empirical data (semi- empirical) 2. Linear Potential 3. Non-linear Potential Small perturbation 4. Euler 5. Navier-Stokes -Reynolds Averaged -Large Eddy Simulation -Direct Simulation *Most Difficult* FLOPS and their prefixes correct answer: FLOPS (FLoating point Operations Per Second) Mega 10e6 Giga 10e9 Tera 10e12 Peta 10e15 Exo 10e18 Navier-Stokes assumptions correct answer: Newtonian Fluid-Stress is linearly proportional to rate of strain Continuum Fourier's Law for Heat Sutherland's Law for Viscosity as a function of Temp Euler assumptions correct answer: -Continuum -Newtonian Fluid -Inviscid Good for -Subsonic -Hypersonic (wave drag dominates) Bad for -Thermo -boundary layer effects -flow separation Strong Conservation Law Form correct answer: -Derivatives have constant coefficients --Derivatives of those coefficients don't appear in the equation -All spatial derivative terms can be identified as divergence terms Non-Linear (Full) Potential correct answer: Same as Euler --Continuum --Newtonian Fluid --Inviscid Plus --Irrotational --Isentropic (no shocks) --Calorically Perfect Gas Good For -Subsonic compressible flows -Flows with weak shocks M1n<1.3 Linearized Potential equation correct answer: -Small perturbations from freestream -Slender bodies (t<0.1c) -Small AoA -Small camber -Steady -Isentropic (no shocks) -No body forces -Irrotational Form 1 -Incompressible (M<0.3) Form 2 -Compressible -Subsonic and Supersonic Verification correct answer: Ensuring the code produces the correct answer for a given underlying physics -look for errors in code -incorrect equations and models -checking error Validation correct answer: The degree to which a code accurately represents the real world -Checks if code, models agree with real world Certification correct answer: Establishing the range of applicability of a code that has been verified and validated -seal of credibility on the code Grid Convergence study correct answer: Refining grid mesh until steady output is achieved How panel method works? Restrictions (no corrections)? correct answer: 1. Divide geometry into flat panels. 2. Place singularity (source, sink, doublet) on each panel of varying strength. 3. Solve Laplace Equation øxx+øyy+øzz=0 4. Pressure on each panel determined and summed for lift, moment and pressure drag. Restrictions: No circulation in flow so only valid for non-lifting bodies. How can Panel methods simulate viscous flows? Restriction? correct answer: Boundary layer solver Must be *attached* flow How can Panel methods simulate compressible flows? Restrictions? correct answer: Compressibility Correction Less than transonic (M<0.8) Vortex Panel method? correct answer: 1. Place simple vortex sheet on surface. 2. Solve for velocity potential at panel *i* caused by the vortex at panel *j* 3. Find perpendicular velocity at each point caused by all other vortices and free stream and set to zero (impermeable surface). 4. With normal=0 we now know vortex strength is equal to tangential velocity. 5. Solve for pressure in terms of velocity. 6. Integrate to derive aero forces. Kutta Condition correct answer: velocity TE=vortex strength TE=0 Good practices for grid development correct answer: 1. Pack grids tighter at areas of large geometry or flow condition gradients. 2. Use grid convergence studies to determine necessary grid density and don't use more than necessary. 3. Vary size of panels smoothly. Why are higher order discrete approximations generally desirable? correct answer: Usually arrive at a result faster because each step uses more information. (over-rides slow down caused by more complex equation)