Ricardo Mejia-Alvarez

Low-order representations of irregular surface roughness and their impact on a turbulent boundary layer

The present effort explores the relative impact of various topographical scales present within irregular surface roughness on a turbulent boundary layer under both developing- and developed-flow conditions. Low-order representations of highly irregular surface roughness replicated from a turbine-blade damaged by deposition of foreign materials were generated using singular value decomposition to decompose the complex topography into a set of topographical basis functions of decreasing importance to the original (“full”) surface character. The low-order surface models were then formed by truncating the full set of basis functions at the first 5 and 16 modes (containing approximately 71% and 95% of the full surface content, respectively) so that only the most dominant and large-scale topographical features were included in the models, while the finer-scale surface details are excluded. Physical replications of the full surface and the two low-order models were created using rapid prototyping methods to gene...

Wall-parallel stereo particle-image velocimetry measurements in the roughness sublayer of turbulent flow overlying highly irregular roughness

Stereo particle-image velocimetry measurements were conducted in a streamwise–spanwise (x − z) plane deep within the roughness sublayer (y = 0.047δ; δ is the boundary-layer thickness) of a zero-pressure-gradient turbulent boundary layer overlying highly irregular surface roughness replicated from a turbine blade damaged by foreign-material deposition. The ensemble-averaged streamwise velocity defect revealed the tendency of the roughness to promote channeling of the flow in the form of low-momentum pathways (LMPs) and high-momentum pathways. Enhanced turbulent and vortical activity was observed both between and along the spanwise boundaries of these streamwise-elongated pathways. In particular, streamwise pathways of wall-normal vortex cores of opposing rotational sense were observed along the spanwise boundaries of the identified LMP in the rough-wall flow. Conditional averaging revealed that these counter-rotating vortical motions are associated with streamwise flow against the mean-flow direction and c...

Robust suppression of background reflections in PIV images

Strong background reflections in PIV images are known to bias velocity estimates and their concomitant statistical ensembles. Many methods have been developed to eliminate background reflections, with the common premise of generating a reference background intensity map that is then subtracted from each individual PIV image prior to interrogation. This reference background intensity map can be generated in several ways, including acquiring a background image without particles, calculating the average or minimum intensity map based on an ensemble of PIV images, generating a reference intensity map for each individual PIV realization by means of various local sliding filters or considering the second frame of any PIV realization as its reference intensity map. Motivated by the need to suppress background reflections in a PIV study of flow over highly irregular surface roughness that generated significant diffuse background reflections from the complex topography, the efficacy of these methods was studied. It was found that all failed to adequately suppress such reflections, rendering the resulting velocity fields biased. A local-median normalization algorithm was developed to further suppress background reflections and this note reports the performance of this modified algorithm compared to those previously reported in the literature.

Single-Interface Richtmyer–Meshkov Turbulent Mixing at the Los Alamos Vertical Shock Tube

Mach number and initial conditions effects on Richtmyer–Meshkov (RM) mixing are studied by the vertical shock tube (VST) at Los Alamos National Laboratory (LANL). At the VST, a perturbed stable light-to-heavy (air–SF6, A = 0.64) interface is impulsively accelerated with a shock wave to induce RM mixing. We investigate changes to both large and small scales of mixing caused by changing the incident Mach number (Ma = 1.3 and 1.45) and the three-dimensional (3D) perturbations on the interface. Simultaneous density (quantitative planar laser-induced fluorescence (PLIF)) and velocity (particle image velocimetry (PIV)) measurements are used to characterize preshock initial conditions and the dynamic shocked interface. Initial conditions and fluid properties are characterized before shock. Using two types of dynamic measurements, time series (N = 5 realizations at ten locations) and statistics (N = 100 realizations at a single location) of the density and velocity fields, we calculate several mixing quantities. Mix width, density-specific volume correlations, density–vorticity correlations, vorticity, enstrophy, strain, and instantaneous dissipation rate are examined at one downstream location. Results indicate that large-scale mixing, such as the mix width, is strongly dependent on Mach number, whereas small scales are strongly influenced by initial conditions. The enstrophy and strain show focused mixing activity in the spike regions.

Simultaneous PIV and PLIF Measurements of Mach Number Effects on Single-Interface Richtmyer-Meshkov Mixing

Introduction of favorable (e.g. combustion within scram-jet engines) or adverse (e.g. inertial confinement fusion) mixing conditions in engineering applications are intensified by the Richtmyer-Meshkov instability (RMI). Despite numerous experimental and numerical research, the mechanisms behind RMI-induced small-scale mixing, particularly late-time turbulent mixing and turbulent statistics are not well understood.

Shock-driven mixing: Experimental design and initial conditions

A new Vertical Shock Tube (VST) has been designed to study shock-induced mixing due to the Richtmyer-Meshkov Instability (RMI) developing on a 3-D multi-mode interface between two gases. These studies characterize how interface contours, gas density difference, and Mach No. affect the ensuing mixing by using simultaneous measurements of velocity/density fields. The VST allows for the formation of a single stably-stratified interface, removing complexities of the dual interface used in prior RMI work. The VST also features a new diaphragmless driver, making feasible larger ensembles of data by reducing intra-shot time, and a larger viewing window allowing new observations of late-time mixing. The initial condition (IC) is formed by a co-flow system, chosen to minimize diffusion at the gas interface. To ensure statistically stationary ICs, a contoured nozzle has been manufactured to form repeatable co-flowing jets that are manipulated by a flapping splitter plate to generate perturbations that span the VST....

Flow of turbulent boundary layers over low-order representations of irregular surface roughness

The present effort explores the relative impact of various topographical scales present within irregular surface roughness on a turbulent boundary layer under both developing and developed flow conditions. Low-order representations of highly-irregular surface roughness replicated from a turbine blade damaged by deposition of foreign materials are generated using singular value decomposition (SVD) to decompose the complex topography into a set of topographical basis functions (383 total) of decreasing importance to the original (“full”) surface character. The low-order surface models are then formed by truncating the full set of basis functions at the first 5 and 16 modes (containing approximately 65% and 95% of the full surface content, respectively), so that only the most dominant, and large-scale, topographical features are included in the models while the finer-scale surface details are excluded. Physical replications of the full surface and the two low-order models are created using rapid prototyping methods to generate short and long streamwise fetches of roughness and particle-image velocimetry is used to acquire ensembles of instantaneous velocity fields in the streamwise–wall-normal plane for developing and developed flow conditions at moderate Reynolds number. Comparison of single-point statistics (mean velocity and Reynolds normal and shear stresses) as well as quadrant analysis of the instantaneous events contributing to the mean Reynolds shear stress indicate that a 16-mode model of the full surface faithfully reproduces the characteristics of flow over the full surface for both developing and developed flow conditions. For the latter scenario, both the 5- and 16-mode models reproduce the outer-layer characteristics for flow over the full surface in accordance with Townsend’s wall similarity hypothesis. However, both low-order surface representations fail to reproduce important details of the Reynolds-shear-stress-producing events within the roughness sublayer, particularly the contributions of the most intense ejection and sweep events.

An Optimization-Based Approach to Design a Complex Loading Pattern Using a Modified Split Hopkinson Pressure Bar

The split Hopkinson pressure bar (SHPB) technique is used to characterize the mechanical response of a material during impact loading when a single stress wave pulse passes through that material [1]. The SHPB setup consists of two long bars: an incident bar and a transmission bar. The specimen, which needs to be characterized, is placed between these two bars. A striker propelled from a gas gun hits the incident bar generating a stress wave that propagates through the incident bar. A part of this wave is transmitted to the specimen and the transmission bar while the remaining part of the wave reflects back into the incident bar. By measuring the incident, transmitted, and reflected waves, the mechanical properties of the specimen are determined for high-strain-rate deformations.

Experimental Study of the Mechanics of Blast-Induced Traumatic Brain Injury

Blast-induced traumatic brain injury (bTBI) has become a “signature wound” of modern combat due to the expansive use of improvised explosive devices (IED). bTBI is characterized as either primary, secondary, tertiary, or quanternary.

Simultaneous PIV/PLIF Measurements of Multi-mode Perturbed Initial Conditions of Single-Interface Richtmyer-Meshkov Mixing

A Richtmyer-Meshkov Instability (RMI) might occur when a shock wave interacts with the interface between two fluids of different density. An initial perturbation in the interface is an optimal condition for the RMI. This is so because local misalignments between the density gradient across the interface and the pressure gradient of the shock wave induce non-zero baroclinic vorticity that amplifies the initial perturbation. RMI is known to occur in supernovas, collapsing gas bubbles in liquids, supersonic and hypersonic combustion, interacting flame fronts and pressure waves, laser-matter interactions, and inertial confinement fusion (ICF). The effects of RMIinduced mixing are detrimental to energy conversion efficiency in ICF, but can be advantageous in combustion processes.