Cfd Validation Issues for Boundary-layer Stability and Transition

When there is some knowledge of transition location, mean laminar and turbulent heat transfer can usually be computed to better than 25% accuracy. However laminar-turbulent transition in the boundary layer is usually estimated from crude algebraic correlations. The scatter in these correlations can be a factor of 2 or more. Because transition causes heat transfer to rise by a factor of about 5 in high-speed flows, this uncertainty in transition location dominates overall uncertainty in heat-transfer predictions. Clearly, knowledge of the transition process is crucial for accurate vehicle heating and drag predictions over the whole flight regime. Laminar-turbulent transition is highly initialand operating-condition dependent. Finding careful, archival experiments for comparison is the main validation issue; few exist. The CFD formulations validated to date demonstrate that if the environment and operating conditions can be modelled and input correctly, the computations (nonlinear Parabolized Stability Equations and Direct Numerical Simulations) agree quantitatively with the experiments. A review of validation and verification, challenges of validation, and recent results for 3-D boundary layers and receptivity problems are presented. Also, the efficacy of transition control depends largely on the physics of the transition process. Whether one desires to delay transition by the various techniques of Laminar Flow Control (LFC; Pfenninger, 1965, 1977; Reshotko, 1984, 1985; Collier 1993; Joslin 1998) or to encourage transition for enhanced mixing or separation delay, the most effective strategy is to capitalize on the most unstable disturbances. Again, knowledge of the transition process is key to efficient application. REVIEW OF ROADMAP TO TRANSITION