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Browsing by Author "Bowersox, Rodney"

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    Compressible turbulence in a high-speed high Reynolds number mixing layer
    Bowersox, Rodney (Virginia Tech, 1992-09-07)
    Compressible turbulence in a high-speed, high Reynolds number, supersonic free shear layer was studied. A two-dimensional free mixing layer was chosen to study turbulence rather than a wall bounded flow due to the experimental fact that the effects of compressibility become significant at lower Mach numbers. The mixing layer was generated by supersonic injection of air (Ms = 1.8, Pts = 0.5 atm. Tts= 295K. and Re/m = 7x10⁶) through a rearward facing tangential slot, into a supersonic free stream (M∞ = 4.0, Pt∞ = 12.5 atm, Tt∞ = 290K, and Re/m = 70x10⁶). Flow visualization was accomplished by nanosecond Shadowgraph photography. The overall flow structure was documented with the Shadowgraph and conventional mean flow probes (Pitot pressure, cone-static pressure, and thermocouple probes). The turbulent structure of the flow field was also clearly depicted in the Shadowgraphs. Image processing techniques were developed in order to determine root-mean-square index of refraction (density) fluctuation levels from the Shadowgraph plates. Multiple overheat normal and cross-wire techniques were developed and/or improved for this study. The present research concentrated on the Reynolds averaged form of the Navier-Stokes equations. where the effects of compressibility are manifested through "apparent mass" terms (i.e. p′u′i). These terms appear in all of the Reynolds averaged Navier-Stokes equations (continuity, momentum, and energy). A new turbulence transformation, coupled with innovative experimental methods. allowed the full compressible Reynolds shear stress (the typical incompressible term, pu′iu′j as well as the apparent mass terms) to be directly measured. The full compressible heat flux and apparent mass terms were also estimated from the cross-wire results. Profiles were obtained at four downstream stations which were strategically located to map different levels of development of the shear flow. The first station was very close to the injector, about one free stream boundary layer thickness downstream (x/δ∞ ≈ 1), hence, it is in the initial region. The second station was located at x/δ∞ ≈ 28, which was near the beginning of the fully developed zone. The third station, x/δ∞ = 83, was just prior the shear layer and floor boundary layer merging. The last station was positioned just aft of the layer merging, x/δ∞ = 106. Reynolds averaging of the compressible Navier-Stokes equations implies that the compressible turbulence affects all of the governing equations. It was found, experimentally, that the effects of compressibility on turbulence were more than significant accounting for about 75% of the total level of the Reynolds shear stress formulation for the present study (i.e. the apparent mass term multiplied by the axial velocity was about 3-4 times the typical incompressible shear term). For the present mean adiabatic flow, the compressible turbulence accounted for 100% of the turbulent heat flux. The apparent mass in the continuity equation was, by definition, only due to compressibility. These results led to the development of anew Compressible Apparent Mass Mixing Length Extension (CAMMLE) model that accounts for compressible turbulence in all of the governing equations (i.e. the turbulence terms in the continuity, momentum, and energy were all consistently formulated). The CAMMLE formulation is a generalization of the Situ-Schetz compressible mixing length formulation, which was developed to account for the apparent mass terms in the momentum equation. A total of seven turbulence models were experimentally evaluated, the CAMMLE model, the Prandtl incompressible and the Situ-Schetz compressible mixing length models, the Prandtl and Bradshaw turbulent kinetic energy (TKE) formulations, and two compressible TKE extensions that are based upon a newly defined compressible TKE formulation. The measured turbulence data was used to assess the various models, where the measured mean flow profiles were used in the model formulations. The incompressible formulations were generally successful in representing the measured incompressible part of the Reynolds shear stress. However, this term only accounted for about 25% of the total shear stress level. All of the compressible extensions provided accurate estimates of the full compressible Reynolds shear stress. In addition, the newly developed CAMMLE model was also successful in representing the apparent mass terms in the continuity equation. The CAMMLE model was also the only formulation to accurately predict the measured compressible turbulent heat flux in the energy equation. The CAMMLE, Situ-Schetz, and Prandtl incompressible mixing length models were all incorporated in to a 3-D finite volume Navier-Stokes code (GASP 2.0). The numerical simulations indicated that the new compressible apparent mass mixing length extension performed very well. The CFD results also enlightened a misuse with all of the current compressible turbulence models. With the exception of the new apparent mass formulation, all existing turbulence models neglect the compressible turbulence effects on the continuity equation and treat the energy equation in an ad hoc effective eddy viscosity and thermal conductivity fashion. The numerical and theoretical studies indicated that this led to poor prediction of the mixing layer width for cases where the free stream Mach number was significantly higher than the injection Mach number.
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    Innovative Transverse Jet Interaction Arrangements in Supersonic Crossflow
    Wallis, Scott Evan (Virginia Tech, 2001-12-03)
    The experiments on this project proceeded on the premise that adding an array of auxiliary jets behind a main jet injector would alleviate the large region of low pressure typically found downstream of a normal, sonic injector in supersonic flow and also possibly increase in intensity of the upstream high-pressure region. The secondary jet would, in theory, "push" the primary jet further into the flow, increasing the size of the obstacle as seen by the flow. The resulting increased high pressure upstream of the flow would increase the force on the body. Also, the presence of secondary jets would reduce the intensity of the primary jet's low-pressure region. These results would be beneficial to increase the force and decrease the nose-down moment associated with sonic, normal injection into a supersonic crossflow. Therefore, in application to hypersonic, high-altitude missile maneuvering, the firing of a thruster with such an array would result in both added force and a reduction of the moment usually associated with the pressure field on the missile. Such an array could allow the missile to perform purely translational maneuvers with less fuel, all the while keeping the target in view. To accomplish this task, some modern missiles use a second injector far downstream from the primary injector. This second injector's primary function is to negate the nose-down moment, and it adds little to the overall jet effectiveness. To this end, two sets of experiments were conducted: one with low jet pressure ratio, Poj/P1 = 13.65, and low Mach number of 2.4 with Po,inf = 3.74 atm and To,inf = 293K for proof of concept and one at primary conditions Poj/P1 = 620, M1 = 4, Po,inf = 10.21 atm, To,inf = 293K. Spark Shadowgraphs were taken at both of these cases to study the structures present in the flow field and to qualitatively assess the effects of the secondary jet injectors. Placed under the Mach disk of the main jet, the secondary jets are hypothesized to push the plume of the main jet further up into the flow, increasing the force on the plate, and Shadowgraphs were used to test this hypothesis. Schlieren pictures were taken at the high M1, high-pressure ratio test case to further study the interaction of the secondary jets with the main jet. Pressure Sensitive Paint, PSP, was used in both cases to gain a greater understanding of the surface pressure near the injectors for different jet configurations. It was discovered that the addition of secondary jets could indeed both increase the force generated by the main jet and reduce the undesirable nose-down moment created by the main jet. In the low M1, low pressure ratio conditions, the addition of one pair of jets manipulated the surface pressure such that the force on the plate increased by 17% and the nose-down moment was increased by 9% over the main jet only case. The further addition of one more pair of injectors increased the surface pressure force on the plate by 34% and increased the nose-down moment on the plate by 3% when compared to the Main Jet Only case. It is important to note that, these increases are due solely to the manipulation of the surface pressure force field and not the thrust of the secondary jets. The added thrust would increase the force on the plate and their position would insure an increase of a nose-up moment. One pair of secondary jets increases the injectant mass flow by about 2.3%. Therefore, the effects reported above are seen to be disproportionate to the amount of added injectant. For the primary test conditions (M1 = 4, Poj/P1 = 620, Po = 10.21 atm, To = 293K) the addition of two pairs of secondary jets had a force increase of 62% and a nose-down moment decrease of 38% over that of the main jet only case. Three pairs increased the force 71% and the decreased the nose-down moment by 26% and four pairs increased the force 91% but increased the nose-down moment by 33%. These values do not account for the thrust of the secondary jets. Accounting for the beneficial effects of the thrust of the secondary jets, the force on the plate for two pairs of secondary jets increased the force 70% and decreased the moment 42%. Three pairs increased the force 83% and decreased the moment 35%. The increase of force for four pairs of secondary jets was 106% and the increase in nose-down moment was only 21%. A point of diminishing returns was reached. As more pairs of injectors are added further and further from the main injector, the beneficial force effects are offset by a growing moment penalty. By considering the locations of the secondary injectors to the main injector for both the low Mach number, low-pressure ratio tests and the main tests conditions, it can be surmised that the greatest benefit from the secondary jets can be extracted when the jets are placed within the main injector's downstream low-pressure region.
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    Meanflow and turbulence measurements in the wake of a supersonic through-flow cascade
    Bowersox, Rodney (Virginia Tech, 1990-01-07)
    Current emphasis on sustained supersonic and hypersonic cruise has sparked interest in more efficient power plants for this flight regime. Cycle studies have shown that the turbofan engine equipped with a supersonic through-flow fan, capable of accepting supersonic axial flow from the inlet, has the potential to be very efficient at the supersonic cruise condition.