Modeling momentum and scalar transport in a wall-bounded turbulent flow

A mildly-heated turbulent boundary layer was studied to characterize the relationship between velocity structures and the scalar field. Particle image velocimetry (PIV) and a Malley probe (Malley et al., 1992) were used to simultaneously measure the velocity field and the streamwise gradients of the scalar field (Gordeyev et al., 2014) respectively. Two distinct velocity scales were identified to be correlated to scalar mixing by conditionally averaging the velocity field on the existence of a scalar gradient. Resolvent analysis was used to create simple models of these velocity scales (McKeon and Sharma, 2010) and to probe their interaction. Using a combination of structural conditional averaging and conditional averaging on the scalar gradient, significant interaction was observed between the two scales of interest, with behavior consistent with the general scale interaction described by amplitude modulation (Hutchins and Marusic, 2007). The study constructed a model of the velocity field that was correlated to streamwise scalar gradients in the outer boundary layer

A constant-mass fuel delivery system for use in underwater autonomous vehicles

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 51).This thesis describes the design and assembly of two constant-mass fuel tanks to be used in autonomous underwater vehicles (AUVs). The fuel tanks are part of a power supply designed to increase AUV endurance without limiting maneuverability. The fuel tanks allow AUVs to burn liquid fuel while maintaining constant buoyancy, increasing vehicle endurance by a factor of four without sacrificing mission capabilities. The fuel tanks take air and water as ballast in proper proportions to replace the consumed mass of fuel. Active solenoid valves, a fuel pump, and a water pump control the mass flow through the system. The thesis covers the design of the mass flow in the system, the computer modeling of that system, and the prototyping of two constant-mass fuel tanks.by Theresa Ann Saxton-Fox.S.B

Coherent Structures, their Interactions, and their Effects on Passive Scalar Transport and Aero-Optic Distortion in a Turbulent Boundary Layer

This thesis focused on the characterization of coherent structures and their interactions in a turbulent boundary layer using data from particle image velocimetry (PIV) measurements performed at Caltech and from a direct numerical simulation (DNS) of Wu et al. (2017). Connections were identified between instantaneous and statistical descriptions of coherent velocity structures, through the analysis of representative models for their structures derived from the resolvent analysis of McKeon and Sharma (2010). The representative models were used in a novel conditional averaging technique to identify the average behavior of small scales about variations in the large-scale streamwise velocity field. Based upon the results of this analysis, a hypothesis for a scale interaction mechanism was proposed involving three-dimensional critical layers. The modeling and analysis methods were then applied to the aero-optic problem in which optical beams are observed to be distorted after passing through variable-density turbulent flows. Measurements using simultaneous PIV and an aero-optic sensor called a Malley probe (Malley, Sutton, and Kincheloe, 1992) were conducted in an incompressible, mildly-heated turbulent boundary layer with Prandtl number of 0.7. A conditional averaging analysis of the data identified that the nonlinear interaction of two scales was most correlated to the aero-optic distortion. The modeling of this interaction using resolvent modes led to new insights regarding the instantaneous relationship between the velocity and scalar fields over a range of Prandtl numbers.</p

Scale interactions and 3D critical layers in wall-bounded turbulent flows

Phenomena related to scale interaction in wall-bounded turbulent flows were considered through the lens of critical layer analysis. A 3D critical layer formulation was used, with the 3D critical layer associated with a particular structure defined as the height where the instantaneous velocity field composed of the large scales and the mean velocity matched the convection velocity of that structure. Characterization of the velocity field surrounding the 3D critical layer in wall-bounded turbulent flows led to conclusions consistent with previously observed phenomena including the shape of the interface defining uniform momentum zones (UMZs) [1] [2] and amplitude modulation of the small scales by the large scales [5]. The use of a 3D critical layer formulation in wall-bounded turbulent flows may lead to improved modeling of small scale activity in reduced order models and LES

Modeling momentum and scalar transport in a wall-bounded turbulent flow

A mildly-heated turbulent boundary layer was studied to characterize the relationship between velocity structures and the scalar field. Particle image velocimetry (PIV) and a Malley probe (Malley et al., 1992) were used to simultaneously measure the velocity field and the streamwise gradients of the scalar field (Gordeyev et al., 2014) respectively. Two distinct velocity scales were identified to be correlated to scalar mixing by conditionally averaging the velocity field on the existence of a scalar gradient. Resolvent analysis was used to create simple models of these velocity scales (McKeon and Sharma, 2010) and to probe their interaction. Using a combination of structural conditional averaging and conditional averaging on the scalar gradient, significant interaction was observed between the two scales of interest, with behavior consistent with the general scale interaction described by amplitude modulation (Hutchins and Marusic, 2007). The study constructed a model of the velocity field that was correlated to streamwise scalar gradients in the outer boundary layer

Studies of the large-scale structure in adiabatic and moderately-wall-heated subsonic boundary layers

Simultaneous velocity-optical measurements in subsonic boundary layers were conducted in order to investigate the relationship between the instantaneous 2-D wavefronts, measured by different optical sensors, the Malley probe and 2-D Shack-Hartmann sensors, and the instantaneous large-scale structure along a wall-normal plane, using PIV in both incompressible and compressible subsonic boundary layers. These systematic studies of the instantaneous relation between the large-scale boundary layer structure and its aero-optical signature provide additional understanding of the instantaneous dynamics of the large-scale structure at subsonic and transonic speeds

Studies of the large-scale structure in adiabatic and moderately-wall-heated subsonic boundary layers

Simultaneous velocity-optical measurements in subsonic boundary layers were conducted in order to investigate the relationship between the instantaneous 2-D wavefronts, measured by different optical sensors, the Malley probe and 2-D Shack-Hartmann sensors, and the instantaneous large-scale structure along a wall-normal plane, using PIV in both incompressible and compressible subsonic boundary layers. These systematic studies of the instantaneous relation between the large-scale boundary layer structure and its aero-optical signature provide additional understanding of the instantaneous dynamics of the large-scale structure at subsonic and transonic speeds

Coherent structures, uniform momentum zones and the streamwise energy spectrum in wall-bounded turbulent flows

Large-scale motions (LSMs) in wall-bounded turbulent flows have well-characterised instantaneous structural features (Kovasznay et al., J. Fluid Mech., vol. 41 (2), 1970, pp. 283–325; Meinhart & Adrian, Phys. Fluids, vol. 7 (4), 1995, pp. 694–696) and a known spectral signature (Monty et al., J. Fluid Mech., vol. 632, 2009, pp. 431–442). This work aims to connect these previous observations through the development and analysis of a representative model for LSMs. The model is constructed to be consistent with the streamwise energy spectrum (Monty et al. 2009) and amplification characteristics of the Navier–Stokes equations (McKeon & Sharma, J. Fluid Mech., vol. 658, 2010, pp. 336–382), and is found to naturally recreate characteristics of instantaneous turbulent structures, including a bulge shape (Kovasznay et al. 1970) and the presence of uniform momentum zones (Meinhart & Adrian 1995) in the streamwise velocity field. The observed structural similarity between the LSM representative model and instantaneous experimental data supports the use of travelling wave models to connect statistical and instantaneous descriptions of coherent structures, and clarifies a simple general equivalency between symmetry in a Reynolds-decomposed velocity field and asymmetry in the laboratory frame

Effect of Coherent Structures on Aero-Optic Distortion in a Turbulent Boundary Layer

The deflection of a small-aperture laser beam was studied as it passed through an incompressible turbulent boundary layer that was heated at the wall. The heating at the wall was sufficiently mild that the temperature and density fields acted as passive scalars with a Prandtl number of 0.71. Simultaneous particle image velocimetry and Malley probe laser deflection measurements were performed in overlapping regions of the boundary layer to identify correlations between coherent velocity structures, passive scalar transport, and optical beam deflection. Streamwise gradients in the streamwise and wall-normal velocity fields were observed to be correlated to the deflection of the optical beam and to streamwise density gradients. The passage of a large-scale motion through the beam path was shown to affect the statistics of the optical beam deflection as well as the local distribution of small-scale velocity features. The wall-normal small-scale velocity features were consistently correlated to the beam deflection, throughout different phases of the large-scale motion convection. The observations motivated a hypothesis that views the large scales as heat carriers, whereas the small scales modify the local sense of a velocity and density gradient toward a streamwise gradient that directly affects the optical beam deflection

Effect of Coherent Structures on Aero-Optic Distortion in a Turbulent Boundary Layer

The deflection of a small-aperture laser beam was studied as it passed through an incompressible turbulent boundary layer that was heated at the wall. The heating at the wall was sufficiently mild that the temperature and density fields acted as passive scalars with a Prandtl number of 0.71. Simultaneous particle image velocimetry and Malley probe laser deflection measurements were performed in overlapping regions of the boundary layer to identify correlations between coherent velocity structures, passive scalar transport, and optical beam deflection. Streamwise gradients in the streamwise and wall-normal velocity fields were observed to be correlated to the deflection of the optical beam and to streamwise density gradients. The passage of a large-scale motion through the beam path was shown to affect the statistics of the optical beam deflection as well as the local distribution of small-scale velocity features. The wall-normal small-scale velocity features were consistently correlated to the beam deflection, throughout different phases of the large-scale motion convection. The observations motivated a hypothesis that views the large scales as heat carriers, whereas the small scales modify the local sense of a velocity and density gradient toward a streamwise gradient that directly affects the optical beam deflection