Freestream

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

Near-field character of a jet discharged through a wall at 30 deg toa mainstream

Abstract: Measurements are reported of the flow created by the injection of a circular jet at 30° through a plane wall, past which an external stream is flowing. Attention has been focused on the region immediately surrounding the injection hole, which has received slight attention in earlier studies. The data include profiles of streamwise velocity and distributions of static pressure and film-cooling effectiveness on the surface, for ratios of average injection velocity: freestream velocity M of 0.1, 0.2, 0.5,1.0, and 1.5. Flow visualization photographs also are presented. Two modes of behavior are observed, according to the magnitude of M. For values of 0.3 and less, the injected jet remains attached to the surface, provoking considerable pressure and velocity disturbances, and giving rise to increasing effectiveness as M is increased. At values of M of 0.5 and above, the jet lifts off the surface, allowing penetration of the mainstream fluid beneath it. As a consequence, the effectiveness now diminishes as M is augmented, as are the surface pressure disturbances, and a complex flow structure is produced. Maximum effectiveness and coverage is observed when M is about 0.5, in conformity with earlier studies.

Suitability of the k–ω turbulence model for scramjet flowfield simulations

Abstract: The suitability of Wilcox's 2006 k ? turbulence model for scramjet flowfield simulations is demonstrated by validation against five test cases that have flowfields representative of those to be expected in scramjets. The five test cases include a 2D flat plate, an axisymmetric cylinder, a backward-facing step, the mixing of a pair of coaxial jets and the interaction between a shock wave and turbulent boundary layer. A generally good agreement between the numerical and experimental results is obtained for all test cases. These tests reveal that despite the turbulence model's sensitivity to freestream turbulence properties, the numerically predicted skin friction agrees with experimental data and theoretical correlations to their degree of uncertainty. The tests also confirm the importance of using a y+ value of less than 1 in getting accurate surface heat transfer distributions. In the coaxial jets case, the importance of matching the turbulence intensities at the inflow plane in improving the predictions of the turbulent mixing phenomena is also shown. A review of guidelines with regard to the setting up of grids and specification of freestream turbulence properties for turbulent Reynolds-averaged NavierStokes CFD simulations is also included in this paper. Copyright (C) 2011 John Wiley & Sons, Ltd.

Modal Analysis of Inclined Film Cooling Jet Flow

Abstract: Thermal and hydrodynamic flow field over a flat surface cooled with a single round inclined film cooling jet and fed by a plenum chamber is numerically investigated using large eddy simulation (LES) and validated with published measurements. The calculations are done for a freestream Reynolds number Re = 16,000, density ratio of coolant to freestream fluid ρj/ρ∞=2.0, and blowing ratio BR=ρjV/ρ∞V=1.0. A short delivery tube with aspect ratio l/D=1.75 and 35 deg inclination is considered. The evolution of the Kelvin–Helmholtz (K-H), hairpin and counterrotating vortex pair (CVP) vortical structures are discussed to identify their origins. Modal analysis of the complete 3D flow and temperature field is carried out using a dynamic mode decomposition (DMD) technique. The modal frequencies are identified, and the specific modal contribution toward the cooling wall temperature fluctuation is estimated on the film cooling wall. The low and intermediate frequency modes associated with streamwise and hairpin flow structures are found to have the largest contribution (in-excess of 28%) toward the wall temperature (or cooling effectiveness) fluctuations. The high frequency Kelvin–Helmholtz mode contributes toward initial mixing in the region of film cooling hole away from the wall. The individual modal temperature fluctuations on the wall and their corresponding hydrodynamic flow structures are presented and discussed.

Experimental Supersonic Hydrogen Combustion Employing Staged Injection Behind a Rearward-Facing Step

Abstract: An experimental investigation of a Mach 2 combustor has been conducted in order to characterize flow properties in a supersonic reacting flowfield. Hydrogen was injected transversely as staged, underexpanded jets behind a rearward-facing step into a ducted Mach 2 air freestream. The effects of the chemical reaction on the supersonic flowfield was investigated using shadowgraphs, broadband flame emission photography, and planar laser-induced fluorescence of OH. The shadowgraphs indicated that the wave pattern in the combustor along with flowfield unsteadiness was strongly affected by the heat release. The broadband flame emission photographs revealed large regions of no combustion in the vicinity of the fuel injectors where fuel/air mixing was insufficient to support combustion. These regions decreased in size as the freestream stagnation temperature was decreased for fixed hydrogen mass flow rate, consistent with an increase in the effective g-ratio with combustion. The size of the zones containing OH in the planar fluorescence images also increased as the main flow stagnation temperature was decreased. Reaction zones were found in the planar fluorescence images away from regions containing inject ant in a nonreacting study of the same geometry, indicating that the pressure rise associated with the reaction forced a large redistribution of the fuel.

Experimental Flow Structure Investigation of Compound Angled Film Cooling

Abstract: The experimental investigation of film-cooling flow structure provides reliable data for calibrating and validating a 3D feature based computational fluid dynamics (CFD) model being developed synchronously at the ETH Zurich. This paper reports on the flow structure of a film-cooling jet emanating from one hole in a row of holes angled 20 deg to the surface of a flat plate having a 45 deg lateral angle to the freestream flow in a steady flow, flat plate wind tunnel. This facility simulates a film-cooling row typically found on a turbine blade, giving engine representative nondimensionals in terms of geometry and operating conditions. The main flow is heated and the injected coolant is cooled strongly to obtain the requisite density ratio. All three velocity components were measured using a nonintrusive stereoscopic particle image velocimetry (PIV) system. The blowing ratio and density ratio are varied for a single compound angled geometry, and the complex three dimensional flow is investigated with special regard to vortical structure.