Browsing by Subject "Crossflow"

Now showing 1 - 4 of 4
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    Computational study of hypersonic boundary layer stability on cones
    (2012-12) Gronvall, Joel Edwin
    Due to the complex nature of boundary layer laminar-turbulent transition in hypersonic flows and the resultant effect on the design of re-entry vehicles, there remains considerable interest in developing a deeper understanding of the underlying physics. To that end, the use of experimental observations and computational analysis in a complementary manner will provide the greatest insights. It is the intent of this work to provide such an analysis for two ongoing experimental investigations. . The first focuses on the hypersonic boundary layer transition experiments for a slender cone that are being conducted at JAXA’s free-piston shock tunnel HIEST facility. Of particular interest are the measurements of disturbance frequencies associated with transition at high enthalpies. The computational analysis provided for these cases included two-dimensional CFD mean flow solutions for use in boundary layer stability analyses. The disturbances in the boundary layer were calculated using the linear parabolized stability equations. Estimates for transition locations, comparisons of measured disturbance frequencies and computed frequencies, and a determination of the type of disturbances present were made. It was found that for the cases where the disturbances were measured at locations where the flow was still laminar but nearly transitional, that the highly amplified disturbances showed reasonable agreement with the computations. Additionally, an investigation of the effects of finite-rate chemistry and vibrational excitation on flows over cones was conducted for a set of theoretical operational conditions at the HIEST facility. . The second study focuses on transition in three-dimensional hypersonic boundary layers, and for this the cone at angle of attack experiments being conducted at the Boeing/AFOSR Mach-6 quiet tunnel at Purdue University were examined. Specifically, the effect of surface roughness on the development of the stationary crossflow instability are investigated in this work. One standard mean flow solution and two direct numerical simulations of a slender cone at an angle of attack were computed. The direct numerical simulations included a digitally-filtered, randomly distributed surface roughness and were performed using a high-order, low-dissipation numerical scheme on appropriately resolved grids. Comparisons with experimental observations showed excellent qualitative agreement. Comparisons with similar previous computational work were also made and showed agreement in the wavenumber range of the most unstable crossflow modes.
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    Computational Study of Shock/Plume Interactions Between Multiple Jets in Supersonic Crossflow
    (2016-08) Tylczak, Erik
    The interaction of multiple jets in supersonic crossflow is simulated using hybrid Reynolds- Averaged Navier Stokes and Large Eddy Simulation turbulence models. The blockage of a jet generates a curved bow shock, and in multi-jet flows, each shock impinges on the other fuel plumes. The curved nature of each shock generates vorticity directly, and the impingement of each shock on the vortical structures within the adjacent fuel plumes strengthens vortical structures already present. These stirring motions are the major driver of fuel-air mixing, and so mixing enhancement is predicted to occur in multi-port configurations. The primary geometry considered is that of the combustion duct at the Calspan- University of Buffalo Research Center 48” Large Energy National Shock (LENS) tunnel. This geometry was developed to be representative of the geometry and flow physics of the Flight 2 test vehicle of the Hypersonic International Flight Research Experimenta- tion Program (HiFIRE-2). This geometry takes the form of a symmetric pair of external compression ramps that feed an isolator of approximately 4” × 1” cross-section. Nine interdigitated flush-wall injectors, four on one wall and five on the other, inject hydrogen at an angle of 30 degrees to the freestream. Two freestream flow conditions are consid- ered: approximately Mach 7.2 at a static temperature of 214K and a density of 0.039 kg/m3 for the five-injector case, and approximately Mach 8.9 at a static temperature of 167K and density of 0.014 kg/m3 for the nine-injector case. Validation computations are performed on a single-port experiment with an imposed shock wave. Unsteady calculations are performed on five-port and nine-port configura- tions, and the five-port configuration is compared to calculations performed with only a single active port on the same geometry. Analysis of statistical data demonstrates enhanced mixing in the multi-port configurations in regions where shock impingement occurs.
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    Control of jets in cross.
    (2010-07) Sau, Rajes
    We use direct numerical simulations to study control of jets in cross ow by axial pulsing. Our main idea is that pulsing generates vortex rings; the effect of pulsing on jets in crossflow can therefore be explained by studying the behavior of vortex rings in crossflow. A method is proposed that allows optimal values of pulsation frequency, modulation and energy to be estimated a priori. This is accomplished in the following three stages. First, direct numerical simulation is used to study the mixing of a passive scalar by a vortex ring issuing from a nozzle into stationary fluid. The ‘formation number’ (Gharib et al. 1998), is found to be 3.6. Simulations are performed for a range of stroke ratios encompassing the formation number, and the effect of stroke ratio on entrainment, and mixing is examined. When the stroke ratio is greater than the formation number, the resulting vortex ring with trailing column of fluid is shown to be less effective, at mixing and entrainment. As the ring forms, ambient fluid is entrained radially into the ring from the region outside the nozzle exit. This entrainment stops once the ring forms, and is absent in the trailing column. The rate of change of scalar containing fluid is studied for its dependence on stroke ratio. This rate varies linearly with stroke ratio until the formation number, and falls below the linear curve for stroke ratios greater than the formation number. This behavior is explained by considering the entrainment to be a combination of that due to the leading vortex ring, and that due to the trailing column. For stroke ratios less than the formation number, the trailing column is absent, and the size of the vortex ring increases with stroke ratio, resulting in increased mixing. For stroke ratios above the formation number, the leading vortex ring remains the same, and the length of the trailing column increases with stroke ratio. The overall entrainment decreases as a result. Next, direct numerical simulation is used to study the effect of crossflow on the dynamics, entrainment and mixing characteristics of vortex rings issuing from a circular nozzle. Three distinct regimes exist, depending on the velocity ratio and stroke ratio. Coherent vortex rings are not obtained at velocity ratios below approximately 2. At these low velocity ratios, the vorticity in the crossflow boundary layer inhibits roll–up of the nozzle boundary layer at the leading edge. As a result, a hairpin vortex forms instead of a vortex ring. For large stroke ratios and velocity ratio below 2, a series of hairpin vortices are shed downstream. The shedding is quite periodic for very low Reynolds numbers. For velocity ratios above 2, two regimes are obtained depending upon the stroke ratio. Lower stroke ratios yield a coherent asymmetric vortex ring, while higher stroke ratios yield an asymmetric vortex ring accompanied by a trailing column of vorticity. These two regimes are separated by a transition stroke ratio whose value decreases with decreasing velocity ratio. For very high values of the velocity ratio, the transition stroke ratio approaches the ‘formation number’ defined by Gharib et al. (1998). In the absence of trailing vorticity, the vortex ring tilts towards the upstream direction, while the presence of a trailing column causes it to tilt downstream. This behavior is explained. Then, we study the mixing behavior of pulsed jets in crossflow using direct numerical simulations. The pulse is a square wave and the simulations consider several jet velocity ratios and pulse conditions. We study the effects of pulsing, and explain the wide range of optimal pulsing conditions found in experimental studies of the problem. Vortex rings in crossflow exhibit three distinct flow regimes depending on stroke ratio and ring velocity ratio. The simulations of pulsed transverse jets show that at high velocity ratios, optimal pulse conditions correspond to the transition of the vortex rings produced by pulsing between the different regimes. At low velocity ratios, optimal pulsing conditions are related to the natural timescale on which hairpin vortices form. An optimal curve in the space of stroke ratio and velocity ratio is developed. Data from various experiments are interpreted in terms of the properties of the equivalent vortex rings and shown to collapse on the optimal curve. The proposed regime map allows the effects of experimental parameters such as pulse frequency, duty cycle, modulation, and pulse energy to all be predicted by determining their effect on the equivalent stroke and velocity ratios. The thesis also discusses work towards the development of Large Eddy Sim- ulation (LES) methodology to predict mixing in very high Reynolds number turbulent flows. We propose a novel estimation procedure to model the subgrid velocity for LES. The subgrid stress is obtained directly from the estimated subgrid velocity. The model coefficients for the subgrid velocity are obtained by imposing constraints on resulting ensemble-averaged subgrid dissipation and local subgrid kinetic energy. The subgrid dissipation may be obtained through either eddy–viscosity models or a new dynamic model for dissipation. The subgrid kinetic energy may be obtained either from the dynamic Yoshizawa model or a modeled transport equation. We also extend the estimation procedure to LES of passive scalar transport and propose an estimation model for subgrid scalar concentration. The subgrid flux is computed directly from the estimated subgrid velocity and estimated subgrid scalar. The model coefficient for the subgrid scalar is obtained by constraining mean scalar dissipation which is provided by an eddy–diffusivity approach. The velocity and scalar estimation models are applied to decaying isotropic turbulence with an uniform mean scalar gradient and good results are obtained. Realistic backscatter is also predicted. A dynamic model for subgrid scale dissipation is proposed. The dissipation is modeled using invariants of strain–rate tensor. The proposed dynamic approach uses a second level test filter and the model coefficient is obtained using two scalar and propose an estimation model for subgrid scalar concentration. The subgrid flux is computed directly from the estimated subgrid velocity and estimated subgrid scalar. The model coefficient for the subgrid scalar is obtained by constraining mean scalar dissipation which is provided by an eddy–diffusivity approach. The velocity and scalar estimation models are applied to decaying isotropic turbulence with an uniform mean scalar gradient and good results are obtained. Realistic backscatter is also predicted. A dynamic model for subgrid scale dissipation is proposed. The dissipation is modeled using invariants of strain–rate tensor. The proposed dynamic approach uses a second level test filter and the model coefficient is obtained using two scalar identities. We show that this approach can also be used to obtain the Smagorinsky model coefficient for subgrid stress. This is an alternative to Germano’s dynamic procedure where the single model constant is obtained by minimizing the error in a tensor identity, the Germano identity error
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    Fundamental Studies Of Crossflow Heat Exchangers For Laminar And Turbulent Flows
    (2017-08) Ahn, Jungwon
    The focus of this thesis research is heat transfer and fluid flow in heat exchangers where a fluid flows across heated cylindrical elements. Three unique devices are considered, and highly detailed and accurate solutions are obtained for each by making use of advanced numerical simulation techniques. As prerequisite to the implementation of these solutions, validation of the numerical procedure was obtained by comparing highly accurate and complete experimental data to the numerical predictions for a relevant test case. The first considered situation is a two-dimensional, in-line tube bank where the number of rows and the Reynolds number serve as parameters. New methods were devised to determine the prevailing flow regime in the tube bank, one based on the calculation of the turbulent viscosity and the other utilizing a comparison of heat transfer coefficients respectively determined from laminar and turbulent models. Array-based average heat transfer coefficients showed that shorter arrays gave rise to higher values of the transfer coefficient, in contrast to certain literature predictions. The second studied case is the simultaneous treatment of heat transfer in a pin-fin array and the fluid flow created by a conventional rotating fan which is delivered to the inlet of the array. The basic issue is the nature of the delivered flow. Even when a blower curve is used, it is assumed that the delivered flow is uniformly distributed across the heat exchanger. In reality, when blade rotation of the fluid mover are taken into account, the uniformity disappears. In fact, the delivered flow includes a swirl component superimposed on the main axial flow. The velocity of the delivered flow may be larger adjacent to the walls than it is in the core of the flow. In many cases, backflow occurs, driven by the rotation of the hub of the fan. The outcome of the work is that correct results require simultaneous treatment of the fluid mover and the heat exchanger. The final dealt-with case is the cylinder in crossflow and provides the most complete set of transient heat transfer results ever.