Numerical investigation of PLIF gas seeding for hypersonic boundary layer flows

Abstract

Numerica l simula tions of gas-seeding strategies required for planar lase r-induced fluorescence ( PL.lF) in a Mach 10 air fl ow were performed. The work was performed to und ersta nd and qua ntify adve rse effects associated with gas seedin g a nd to compare different fl ow rates and differe nt t)"pes of seed gas. The gas was injec ted through a slot near the lea ding edge of a fl at plate wedge model used in NASA La ngl ey Resea rch Ce nter's 3 1-Inch Mach 10 Air Tunnel fac ility. Nitric ox ide, krypton, a nd iodin e gases were simulated at va rio us inj ection rates. Simulation results s howing the deflection of the velocity fi eld for each of the cases are presented. Streamwise distributions of veloc ity and concentration boundary layer thicknesses as well as ve rtical distributions of veloc ity, temperature, and mass distributions are presen ted for each of the cases. Rela tive me rits of the differe nt seedin g stra tegies are disc ussed. J. IntroductionT HE injection and dispersion ofa seed gas upstream of a fluid dynamic phenomenon is a common strategy when performing non-intrusive measurements. The use ofa seed gas as a fluid tracer is useful in flow visualization applications since the seeded gas convects through the velocity fie ld and marks fluid fl ow struclUres. Depending on the applicat ion, either uniform or loca lized seeding is required. In studies using planar laser-induced fluorescence (PLl F), for example, un iform seeding is preferred where quantitative measurements like temperature or velocity arc requ ired, while loca l seedi ng may be more advantageous when flow visualization is required, particu larly in applications in volving blowing. Although gas seeding can be avoided in some applications where the fluorescence species forms naturally in the flow, such as in combustion flows (hydroxyl radical, OH) or in high enthalpy wind tunn els (ni tric ox ide. NO), it is often unavoidable. Examples of when localized seed ing is required for PUF flow visualizmion include the study of fuel-air mixing for supersonic combustion ramjet (SCRAMJET) engines l and the study of reaction control system jet interactions wi th hypersonic crossflows associated spacecraft en tTy vehicles. . 3The strategy of seeding tracer gas into a flow to enable PUF measuremen ts has recently been used to study laminar-IO-Iurbulenttran si tion in hypersonic boundary layers. 4 -9 NASA's interest in this fluid dynamic problem is main ly motivated 10 further understand the increase in the surface heating that accompany the transit ion to turbulence du ring the atmospher ic entry of spacecraft vehicles. 10 Figure I shows a sample resul t from an NO PUF experiment where a 20 degree wedge is positioned in a hypersonic flow . I The wedge has a 0.81-mm thick, II -mm wide slot located approximately 29.4 mm downSlfeam of the leading edge where NO gas is injected. In thi s particu lar test, a Boundary Layer Transition Detailed Test Objective (BLT DTO) shaped-trip was used to cause the flow to tran sition to turbulence. Th e laser shee...

Nitric Oxide Chemistry Effects in Hypersonic Boundary Layers

“…Figures 6 and 7 show comparisons of vertical distributions of Mach number and temperature for a case without gas seeding. Consistent with the previous work, 11 the vertical (y) distributions are displayed at a streamwise position of x = 106 mm downstream of the leading edge. In general, there is good agreement between the rhoCentralReactingFoam code and ANSYS Fluent in predicting mean flow quantities.…”

“…The total temperature of the pipe inflow boundary was set to 321 K which is equal to that of the ANSYS Fluent simulations. 11 The mass flow rate was kept constant at 150 sccm.…”

“…The fixed temperature was determined from the experimental data at this condition. 11 The right outflow boundary is specified as zero normal gradient for the temperature, velocity and pressure boundary conditions. There was a concern that the zero gradient pressure boundary condition would result in unwanted wave reflections at the outlet.…”

“…11 The Roe-FDS scheme was used for shock capturing in the ANSYS Fluent simulations. Figures 6 and 7 show comparisons of vertical distributions of Mach number and temperature for a case without gas seeding.…”

Hypersonic Boundary Layer Measurements with Variable Blowing Rates Using Molecular Tagging Velocimetry

“…A numerical study of boundary layer behavior for relatively low blowing rates (≤ 3mg/s) performed in Ref. 16, discussed several aspects of blowing that may be used to expand on this last potential effect. American Institute of Aeronautics and Astronautics Considering all data at this location, even those which fall below the 10% rejection criteria, the farthest measurement from the wall surface at this location was obtained at 1.12 mm, which is within 8% of the estimated value.…”