Freestream

About: Freestream is a research topic. Over the lifetime, 3428 publications have been published within this topic receiving 56147 citations.

Transonic shockwave/turbulent boundary layer interactions on a porous surface

Abstract: Transonic shockwave/turbulent boundary layer interactions on a porous surface above a closed plenum chamber have been studied experimentally in the choked flow of a windtunnel test-section. The equivalent freestream Mach number is 0.76 and results were obtained for three shock strengths. Without the porous surface the Mach numbers ahead of the shock were 1.13, 1.18 and 1.26. The respective shock Mach numbers with the porous surface were 1.10, 1.11 and 1.19. Laser holographic interferometry results are used to measure the density flowfield and examine the nature of the interaction. These results show that the interaction on the porous surface is modified by a thin shear layer adjacent to the surface and the weakening of the Shockwave is attributed to this. The interaction was also studied by solving the two-dimensional Reynolds-averaged Navier-Stokes equations together with the two-layer algebraic eddy-viscosity model of Baldwin-Lomax modified with appropriate corrections for surface transpiration. The computed results show excellent agreement with the experimental data. The examination of these numerical results shows that the surface transpiration occurs at a low subsonic velocity and suggests that the effect of the transpiration through the porous surface on the interaction may be optimised.

Numerical Simulation of Combustion and Extinction of a Solid Cylinder in Low-Speed Cross Flow

Abstract: The combustion and extinction behavior of a diffusion flame around a solid fuel cylinder (PMMA) in low-speed forced flow in zero gravity was studied numerically using a quasi-steady gas phase model. This model includes two-dimensional continuity, full Navier Stokes' momentum, energy, and species equations with a one-step overall chemical reaction and second-order finite-rate Arrhenius kinetics. Surface radiation and Arrhenius pyrolysis kinetics are included on the solid fuel surface description and a parameter Phi, representing the percentage of gas-phase conductive heat flux going into the solid, is introduced into the interfacial energy balance boundary condition to complete the description for the quasi-steady gas-phase system. The model was solved numerically using a body-fitted coordinate transformation and the SIMPLE algorithm. The effects of varying freestream velocity and Phi were studied. These parameters have a significant effect on the flame structure and extinction limits. Two flame modes were identified: envelope flame and wake flame. Two kinds of flammability limits were found: quenching at low-flow speeds due to radiative loss and blow-off at high flow speeds due to insufficient gas residence time. A flammability map was constructed showing the existence of maximum Phi above which the solid is not flammable at any freestream velocity.

Effects of freestream nonequilibrium on convective heat transfer to a blunt body

Abstract: The axisymmetric Navier- Stokes equations are solved numerically for nonequilibrium airflows over a hemisphere. A formulation with a three-temperature thermochemical model has been employed to simulate vibrationally excited and partially dissociated airflow. A flow condition that has a total enthalpy of 25 MJ/kg and a surface pressure of 0.076 atm is studied. Computed stagnation point heat transfer using finite catalytic boundary conditions at the surface is compared with classical results. Departures from the classical heat transfer predictions caused by nonequilibrium effects are assessed for arcjet testing applications. A Damkohler number analysis is used to characterize the extent of flowfield nonequilibrium. It is shown that characterization of the thermodynamic state of the gas at the boundary-layer edge and within the boundary layer is needed to interpret the heat transfer measurements and to determine the surface catalytic efficiency.

Predicting Roof Pressures on a Low-Rise Structure From Freestream Turbulence Using Artificial Neural Networks

Abstract: This paper presents a generalized approach for predicting (i.e., interpolating) the magnitude and distribution of roof pressures near separated flow regions on a low-rise structure based on freestream turbulent flow conditions. A feed-forward multilayer artificial neural network (ANN) using a backpropagation (BP) training algorithm is employed to predict the mean, root-mean-square (RMS), and peak pressure coefficients on three geometrically scaled (1:50, 1:30, and 1:20) low-rise building models for a family of upwind approach flow conditions. A comprehensive dataset of recently published boundary layer wind tunnel (BLWT) pressure measurements was utilized for training, validation, and evaluation of the ANN model. On average, predicted ANN peak pressure coefficients for a group of pressure taps located near the roof corner were within 5.1, 6.9, and 7.7% of BLWT observations for the 1:50, 1:30, and 1:20 models, respectively. Further, very good agreement was found between predicted ANN mean and RMS pressure coefficients and BLWT data.

Numerical Investigation of Jet Interaction in a Supersonic Freestream

Abstract: The objective of this investigation is to numerically evaluate effects of a reaction control (divert) jet on the aerodynamic performance of a generic interceptor missile operating at supersonic flight conditions. These effects include transient operation, external chemical reactions, burning, and geometric scale (full scale versus subscale). Three-dimensional computations of the highly turbulent flow field produced by a pulsed, supersonic, lateral-jet control thruster interacting with the supersonic freestream and missile boundary layer of a generic interceptor missile are evaluated. A generic missile interceptor configuration consisting of a long, slender body containing fixed dorsal and tail fins is simulated in this study. Parametric computational fluid dynamic solutions are obtained at altitude conditions corresponding to 19.7 km for the following scenarios: 1) steady-state conditions with the lateral control jet turned off; 2) steady-state conditions with the lateral control jet turned on; 3) steady-state conditions with the lateral control jet turned off with 1/10 subscale; 4) steadystate conditions with the lateral control jet turned on with 1/10 subscale; 5) transient jet startup conditions; 6) transient jet shutdown conditions; 7) steady-state, finite-rate chemistry; and 8) steady-state, frozen calculations with the chemical reactions “turned off.” A thermally and calorically perfect gas with a specific heat ratio equal to 1.4 was assumed for both the transient and geometric scale calculations. Vehicle forces and moments are assessed from each solution by integrating the surface pressures and viscous shear stresses computed on the missile surfaces. These results are used to determine the influence of the jet interaction effects on the transient, external burning, and geometric scale aerodynamic performance of the missile. The analysis predicts strong transient influences, small external burning influences, and very small full-scale vs 1/10-subscale effects for the integrated normal force and pitching moment.