On the Structure of Jets from Highly Underexpanded Nozzles Into Still Air

Adamson, T. C.; Nicholls, J. A.

doi:10.2514/8.7912

Cited by 233 publications

(59 citation statements)

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“…This is analogous to the backpressure around a jet injected into the quiescent air [14,26]. The assumption of improved penetration to be achieved by the matched pressure condition p eb p j , where p eb and p j are the effective backpressure and jet static pressure, respectively, rather than overpressurized injection, was supported by a later experimental study [27], while little or no effect of the freestream or jet Mach numbers was observed.…”

Section: Introductionmentioning

confidence: 84%

Physical Insight into Fuel-Air Mixing for Upstream-Fuel-Injected Scramjets via Multi-Objective Design Optimization

Ogawa

2015

Journal of Propulsion and Power

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Fuel injection and mixing into air play a crucial role in the operation of hypersonic airbreathing propulsion systems, particularly scramjet engines featuring upstream fuel injection. This study applies an advanced design methodology combining computational fluid dynamics and evolutionary algorithms assisted by surrogate modeling to a multi-objective optimization for fuel injection in a Mach 5.7 crossflow after the initial compression in a scramjet intake operating at Mach 7.6. Optimization is performed for elliptical injector configurations defined by four design parameters (i.e., the injection angle, spanwise spacing, aspect ratio, and radius of the injector), simultaneously aiming to maximize three objectives, that is, fuel/air mixing, total pressure saving, and fuel penetration into the crossflow. Statistical methods based on global sensitivity analysis are employed to assess the optimization results in conjunction with surrogate models to identify key design factors with respect to the three design objectives and additional performance measures. Major effects of the injection angle and aspect ratio have been observed on all considered design criteria. The spanwise spacing has been found to have considerable influence on the total pressure recovery, fuel penetration, and lateral spread when the injection pressure is adjusted to maintain a constant fuel/air equivalence ratio. Low-angle fuel injection through a highly elliptic orifice with wide spanwise spacing demonstrated the most comprehensive advantages in overall aspects. NomenclatureA j = injector area on floor, m 2 A n j = injector area normal to jet, m 2 AR = injector aspect ratio a j = semimajor axis of injector, m c = mass fraction c s = stoichiometric mass fraction c H 2 = hydrogen mass fraction c O 2 = oxygen mass fraction D = effective injector diameter, m f = vector of objective functions H = vertical height of computational domain, m h f = fuel penetration height (normalized) J = jet-to-freestream momentum flux ratio L = streamwise length of computational domain, m k = number of clustering L 0 = distance to upstream end of injector, m _ m = mass flow rate, kg∕s N = size of population pool p = static pressure, Pa p eb = effective backpressure (normalized) p j = jet static pressure (normalized) p 02 = pitot pressure behind normal shock, Pa p 0 = total pressure, Pa r j = effective injector radius, m S i = first-order sensitivity index S T i = total-effect sensitivity index U = freestream velocity, m∕s u = velocity, m∕s W = injector spacing (spanwise domain width), m w f = fuel lateral spread (normalized) X i = input vector for decision variable x i X = input matrices comprising vectors X i x = streamwise coordinate, m x = vector of decision variables Y = input vector for output parameter y y = spanwise coordinate, m z = vertical coordinate, m α j = injection angle, deg Γ= streamwise circulation (normalized) Δp 0 = total pressure loss (normalized) Δt = time step, s Δx = spatial step, m Δμ χ = mesh sensitivity error of parameter χ η m = mixing efficie...

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Section: Introductionmentioning

confidence: 84%

Physical Insight into Fuel-Air Mixing for Upstream-Fuel-Injected Scramjets via Multi-Objective Design Optimization

Ogawa

2015

Journal of Propulsion and Power

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“…With increasing distance from the vent, the flow accelerates to elevated Mach numbers and decreases rapidly in both density and pressure (Kieffer, 1981;Ogden et al, 2008;. This pressure drop with distance from an underexpanded nozzle has been described in detail by Owen and Thornhill (1948) and Adamson and Nicholls (1959) using the method of characteristics (The analysis of the pressure variation in volcanic jets explored here makes showing the spectral differences between vent and plume lightning. While plume lightning manifests itself as discrete, impulsive signals (red, vertical lines), vent activity appears as a "continual" signal rising from the noise floor.…”

Section: Modelmentioning

confidence: 76%

Inferring Compressible Fluid Dynamics From Vent Discharges During Volcanic Eruptions

Harper

Cimarelli

Dufek

et al. 2018

Geophysical Research Letters

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Observations at numerous volcanoes reveal that eruptions are often accompanied by continual radio frequency (CRF) emissions. The source of this radiation, however, has remained elusive until now. Through experiments and the analysis of field data, we show that CRF originates from proximal discharges driven by the compressible fluid dynamics associated with individual volcanic explosions. Blasts produce flows that expand supersonically, generating regions of weakened dielectric strength in close proximity to the vent. As erupted material—charged through fragmentation, friction, or other electrification process—transits through such a region, pyroclasts remove charge from their surfaces in the form of small interparticle spark discharges or corona discharge. Discharge is maintained as long as overpressured conditions at the vent remain. Beyond describing the mechanism underlying CRF, we demonstrate that the magnitude of the overpressure at the vent as well as the structure of the supersonic jet can be inferred in real time by detecting and locating CRF sources.

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“…[10] These empirical relationships determined by the experimental studies suggest a 1D analytical solution for Mach disk height, prompting a number of analytical studies of underexpanded jet dynamics [e.g., Adamson and Nicholls, 1959;Young, 1975;Ewan and Moodie, 1986]. Using first principles and some assumptions of ideal behavior, these studies verified that, for a given mass flux at the vent, the Mach disk height is proportional to the vent radius and the square root of the overpressure (as well as a limited dependence on g).…”

Section: Introductionmentioning

confidence: 99%

Numerical simulations of volcanic jets: Importance of vent overpressure

et al. 2008

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[1] Explosive volcanic eruption columns are generally subdivided into a gas-thrust region and a convection-dominated plume. Where vents have greater than atmospheric pressure, the gas-thrust region is overpressured and develops a jet-like structure of standing shock waves. Using a pseudogas approximation for a mixture of tephra and gas, we numerically simulate the effects of shock waves on the gas-thrust region. These simulations are of free-jet decompression of a steady state high-pressure vent in the absence of gravity or a crater. Our results show that the strength and position of standing shock waves are strongly dependent on the vent pressure and vent radius. These factors control the gas-thrust region's dimensions and the character of vertical heat flux into the convective plume. With increased overpressure, the gas-thrust region becomes wider and develops an outer sheath in which the erupted mixture moves at higher speeds than it does near the column center. The radius of this sheath is linearly dependent on the vent radius and the square root of the overpressure. The sheath structure results in an annular vertical heat flux profile at the base of the convective plume, which is in stark contrast to the generally applied Gaussian or top-hat profile. We show that the magnitude of expansion is larger than that predicted from previous 1D analyses, resulting in much slower average vertical velocities after expansion. These new relationships between vent pressure and plume expansion may be used with observations of plume diameter to constrain the pressure at the vent.

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On the Structure of Jets from Highly Underexpanded Nozzles Into Still Air

Cited by 233 publications

References 1 publication

Physical Insight into Fuel-Air Mixing for Upstream-Fuel-Injected Scramjets via Multi-Objective Design Optimization

Physical Insight into Fuel-Air Mixing for Upstream-Fuel-Injected Scramjets via Multi-Objective Design Optimization

Inferring Compressible Fluid Dynamics From Vent Discharges During Volcanic Eruptions

Numerical simulations of volcanic jets: Importance of vent overpressure

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