Browsing by Subject "High speed"

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    Discrete roughness effects on high-speed boundary layers
    (2015-01) Iyer, Prahladh Satyanarayanan
    This dissertation studies the effects of a discrete roughness element on a high-speed boundary layer using Direct Numerical Simulations (DNS) on unstructured grids. Flow past a cylindrical roughness element placed perpendicular to the flow and a hemispherical bump is studied. A compressible linear stability theory (LST) solver for parallel flows is developed based on the algorithm by Malik [33] and validated for a range of Mach numbers ranging from incompressible to Mach 10. The evolution of the perturbations from DNS is validated with the linear stability solver making the DNS algorithm suitable to study transition problems. Flow past a cylindrical roughness element at Mach 8.12 is simulated using DNS and the velocity profiles in the symmetry and wall--parallel planes are compared to the experiments of Bathel et al. [7]. The flow remains steady and laminar, and does not transition. Overall, good agreement is observed between DNS and experiments, thus validating our algorithm to study effect of roughness on high-speed flows. However, differences are observed in the separation region upstream and recirculation region downstream of the roughness. The DNS results are used to quantify possible uncertainties in the measurement technique as suggested by Danehy [20]. The effect of upstream injection (5% of the free-stream velocity) is also simulated to quantify its effects on the velocity profiles to mimic the injection of NO into air in the experiment. While the boundary layer thickness of the flow increases downstream of the injection location, its effect on the velocity profiles is small when the profiles are scaled with the boundary layer thickness.Flow past a hemispherical bump at Mach 3.37, 5.26 and 8.23 are simulated using DNS with the flow conditions matching the experiments of Danehy et al. [19] to understand the different flow features associated with the flow and the physical mechanism that causes the flow to transition to turbulence. It is observed that the Mach 3.37 and 5.26 flows transition to turbulence while the Mach 8.23 flow remains laminar downstream of the roughness element. The roughness element used in this study is large since the boundary layer thickness of the laminar boundary layer at the location of the roughness is smaller than the roughness height.The Mach 3.37 flow undergoes transition closer to the bump when compared to Mach 5.26, in agreement with experimental observations. Transition is accompanied by an increase in C_f and C_h (Stanton number). Even for the case that did not undergo transition (Mach 8.23), streamwise vortices induced by the roughness cause a significant rise in C_f until 20D downstream. Mean Van-Driest transformed velocity and Reynolds stress for Mach 3.37 and 5.26 shows good agreement with available data. The transition process involves the following key elements - Upon interaction with the roughness element, the boundary layer separates to form a series of spanwise vortices upstream of the roughness, and a separation shear layer. The system of spanwise vortices wrap around the roughness element in the form of horseshoe/necklace vortices to yield a system of counter-rotating streamwise vortices downstream of the element. These vortices are located beneath the separation shear layer and perturb it, which results in the formation of trains of hairpin-shaped vortices further downstream of the roughness for the cases that undergo transition. These hairpins spread in the span with increasing downstream distance and the flow increasingly resembles a fully developed turbulent boundary layer. A local Reynolds number based on the wall properties is seen to correlate the onset of transition for the cases considered.To assess the effect of roughness height on transition, a Mach 3.37 flow past a hemispherical bump is studied by varying the boundary layer thickness (k/delta = 2.54, 1.0, 0.25 & 0.125) where k is the roughness height and delta is the laminar boundary layer thickness at the location of the roughness. Transition occurs in all cases, and the essential mechanism of transition appears to be similar. At smaller boundary layer thickness, multiple trains of hairpin vortices are observed immediately downstream of the roughness, while a single train of hairpin vortices is observed at larger delta. This behavior is explained by the influence of the boundary layer thickness on the separation vortices upstream of the roughness element. Also, hairpin vortices that form downstream of the roughness initially scale with the height of the roughness element and further downstream, begin to scale with the boundary layer thickness, thus causing the entire boundary layer to transition. Dynamic Mode Decomposition of the pressure field for k/delta= 1 and 0.125 is used to obtain the frequency of shedding of hairpin vortices.
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    Flow characterization on a thin film spinning apparatus
    (2014-09) Alvarado, Alonso Antonio
    In industrial milling operations that use comminution and wet-comminution techniques, the reduction of the particle size is usually achieved through crushing the sample with a material harder than the product. These methods are convenient when the required median particle size is above 400 um. However, to obtain post-milling particle distributions with 85% sub-micron particles (in number) is both energy intensive, and time consuming. For conventional milling machines, to have the required output in several ton/hr of a product, having a large number of particles in the micron or sub-micron sizes at an affordable rate is cumbersome.Here, a wet-comminution machine that has shown to achieve the aforementioned milestones in the laboratory scale is studied. However, when the machine is scaled to industrial processes, it was recorded that some of the product variables are difficult to scale. In these studies, we attempt to understand the mechanisms by which this machine operates in order to achieve successful scaling. The apparatus operates completely on fluid mechanics principles, it consists of two concentric cylinders, the inner cylinder that has a smaller radius than the outer, rotates while the larger is held stationary. The inner cylinder is also shorter in length than outer, hollow in the inside and has transversal holes where the shaft attaches to the apparatus. The apparatus can operate in batch condition, where the liquid volume is much less than the volume of the apparatus, typically 0.3Vt, 0.42Vt and 0.54Vt. In addition, the apparatus can operate with throughflow, which the upper plate covering the apparatus is reduced in radius.Two component Laser Doppler Velocimetry (LDV) was used to obtain even-time averaged statistics of the azimuthal and axial velocities, in the gap and underneath the impeller. Also, Flow Visualization using Kalliroscopic particles was performed as means of observing large scale structures in the gap. Moreover, single plane Particle Image Velocimetry (PIV) was used to acquire statistics of the axial and radial velocities in the gap, and both underneath as well as above the inner cylinder.It was found that at both throughflow conditions, the topology of the apparatus creates a free spinning boundary both at the bottom and above the inner cylinder. Near the bottom, the thickness of the boundary was found to decrease with Reynolds number to a limiting value, where Re; is based on gap thickness and inner cylinder tip speed. For Re > 2546, the liquid/air interface thickness is constant for a given holding volume. In the regions above and underneath the inner cylinder, corner vortices were detected; if viewing the left-hand-side, the lower one rotating counter clockwise, while the upper rotates clockwise. The thickness of these vortices was found to be constant for various axial flows at Re = 1110 and 2230. The radial length scale of the stationary vortices was found to be ~2.5d;.The flow generated inside of the gap was characterized to have Taylor vortex signatures. It was found that the length scale of the Taylor vortices in the gap is rather insensitive to Reynolds number or holding volume ratio. The average vortex pair wavelength; was found to be 3.6d. Average flow statistics in batch condition indicate that in the gap, at Re = 1110 and 2230, the azimuthal velocity is 0.5U over much of the length. Similarly, it was found that the net axial flow through the gap is close to zero.
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    Hypersonic Boundary Layer Stability Analysis Using Momentum Potential Theory
    (2020-09) Houston, Mary
    Linear Stability Theory (LST) and the Parabolized Stability Equations (PSE) have provided valuable tools for analysis and prediction of laminar to turbulent transition for plates, sharp cones, and geometries for which parallel-flow or a slowly-varying boundary layer can be assumed. However, these techniques struggle to capture the complex flow-physics present near the tip of blunt-cones. Input-output analysis has been used in conjunction with direct numerical simulation to capture the effects of nose bluntness on downstream stability. Using the results of the input-output analysis we apply momentum potential theory (MPT) to preform fluid-thermodynamic (FT) decomposition, separating disturbances into their vortical, thermal and acoustic components. A reference case of Mach 6 flow over a flat-plate is computed and output responses are compared to the results for Mach 6 flow over a blunt-cone of $7^{o}$ half angle. Perturbation eigenfunctions and structures are examined in the areas of second-mode amplification. For both the flat-plate and blunt-cone the vortical components are the largest, followed by the thermal then acoustic components. Fluid-thermodynamic structures in the second-mode amplification region of blunt-cone show wall-normal stretching above the critical layer. Fluid-thermodynamic decomposition of full-domain input and output results for the blunt-cone geometry are considered. It is found that input sensitivity is highest at the top of the entropy layer and along the boundary layer edge for the fore-half of the cone. Output response in the streamwise direction is highest in the regions between the generalized inflection point (GIP) and the boundary layer edge and dissipates near the surface, whereas wall-normal response extends to the surface and shows a local minimum between the GIP and boundary layer edge. To compliment existing studies on hypersonic boundary layer response to surface roughness/ vibration we look at input sensitivity and output response at the surface. It is found that there is greater sensitivity to wall-normal forcing than streamwise forcing at the surface and among the three FT components in this direction the vortical had the highest relative output amplitude. Finally, total fluctuating enthalpy (TFE) is considered for both the flat-plate and blunt-cone, in both cases the thermal terms provides the strongest source of TFE.