Getting more from flow data: assimilating experimental measurements
In a recent paper
Symon et al, Expts. in Fluids, 58(5) 2017,
Professor McKeon and colleagues provided an experimental demonstration of the assimilation of experimental mean velocity data to estimate turbulent stresses in the flow over an idealized airfoil. The attached figure shows a comparison of model forcing from (noisy) experiment on the left and the assimilated (clean) estimate on the right. The study highlights the utility of combining even relatively poorlyresolved and noisy experimental data with a model that obeys the governing equations in order to obtain information that either was not or could not be directly measured.
Research Area:
Physics of Fluids

Faculty:
McKeon
Hypervelocity Impact
A recent research interest for Professor Rosakis is Hypervelocity Impact. Hypervelocity impact is a rising concern in spacecraft missions where manmade debris in low Earth orbit (LEO) and meteoroids are capable of compromising or depleting the structural integrity of spacecraft. To address these concerns, the goal of current research is to experimentally investigate the underlying mechanisms responsible for deformation and damage evolution during hypervelocity impact utilizing Caltech/JPL's Small Particle Hypervelocity Impact Range (SPHIR) facility. By combining high speed photography, optical techniques, including Coherent Gradient Sensing (CGS) interferometry, the dynamic perforation behavior involving crater morphology, debris and ejecta formation and solid/fluid/plasma transitions and interactions have been examined.
Click image to view video.
Research Area:
Physics of Solids and Mechanics of Materials

Faculty:
Rosakis
Lagrangian and Vorticityfield Geometry of Turbulent Flows
The aims of this project are to develop a quantitative framework for investigating and characterizing the geometry of "eddies" in turbulent flows. This is done by the numerical simulation of turbulent fluid flows from which both Eulerian (time instantaneous) and Lagrangian (time evolving) threedimensional fields can be extracted.
Quantitative tools such as the curvelet transform can then be applied to extracted fields to produce a multiscale decomposition from which the statistical geometry of turbulent eddy structure over various length scales can be studied. The results of the research can provide support for structurebased models of turbulent flows.
Image of vortex isosurfaces in a viscous flow starting from TaylorGreen initial conditions. The vorticity vector is everywhere tangent to these isosurfaces which are extracted from a continuous vortexsurface field. Some sample vortex lines are shown.
The vortex surfaces, colored by vorticity intensity (yellow highest), depict a tangle of vorticity tubes and vorticity sheets.
Research Area:
Physics of Fluids,
Computational & Theoretical Mechanics

Faculty:
Pullin
Modeling of Wall Turbulence
A broad range of experimental and theoretical research efforts seek to exploit the directional amplification associated with the NavierStokes equations to illuminate the dominant mechanisms behind observed flow physics in wall turbulence. A particular topic of interest is the exploitation of flow receptivity to stochastic or optimized small disturbances to reconcile the statistical and structural pictures of wall turbulence and expand current modeling capabilities.
The modeling framework of McKeon & Sharma (J. Fluid Mech., 2010) can be used to predict hairpin vortex structure in turbulent pipe flow identified by isocontours of swirl, the imaginary part of the complex eigenvalues of the velocity gradient.
The shape of the streamwise velocity mode associated with nearwall turbulence activity: prediction from the model of McKeon & Sharma (J. Fluid Mech., 2010). Red and blue isocontours represent positive and negative velocity fluctuations relative to the mean velocity.
Research Area:
Physics of Fluids,
Computational & Theoretical Mechanics

Faculty:
McKeon
Laboratory Earthquakes
In the late eighties, Rosakis introduced the concept of "Laboratory Earthquakes" and since then his research interests have mainly focused on the mechanics of seismology, the physics of dynamic shear rupture and frictional sliding and on laboratory seismology. The goal of this body of work is to create, in a controlled and repeatable environment, surrogate laboratory earthquake scenarios mimicking various dynamic shear rupture process occurring in natural earthquake events. Such, highly instrumented, experiments are used to observe new physical phenomena and to also create benchmark comparisons with existing analysis and field observations. The experiments use highspeed photography, fullfield photoelasticity, and laser velocimetry as diagnostics. The fault systems are simulated using two photoelastic plates held together in frictional contact. The far field tectonic loading is simulated by precompression while the triggering of dynamic rupture (spontaneous nucleation) is achieved by suddenly dropping the normal stress in a small region along the interface. The frictional interface (fault) forms various angles with the compression axis to provide the shear driving force necessary for continued rupturing. Rosakis and his coworkers, investigate the characteristics of rupture, such as rupture speed, rupture mode, associated ground motion under various conditions such as tectonic load, interface complexity and roughness. Both homogeneous and bimaterial interfaces (abutted by various elastic and damaged media) are investigated. Rosakis and his coworkers have been credited with the experimental discovery of the "intersonic" or "supershear rupture" phenomenon. Indeed they have investigated this new phenomenon in various engineering and geophysical settings involving shear dominated rupture in the presence of weak interfaces or faults. Their experimental discoveries of supershear rupture has refocused the attention of the geophysics community to the study of supershear earthquakes.
The fastest supershear cracks on Earth: Bimaterial delamination; shear rupture of a graphite composite; and rupture of a frictional interface simulating a laboratory earthquake.
Click image to view video.
Research Area:
Physics of Solids and Mechanics of Materials

Faculty:
Rosakis
Eulerian Approach to High Strainrate Solid Mechanics
The calculation of strong waves in condensed
media remains a challenge. The prevasiling approach use a Lagrangian
formulation which follows material particles. We are examine the use of
Eulerian approaches where the mesh is fixed and the material flows
through the mesh. Our approach is based on m odern shock capturing
techniques that have been very successfully used for gas dynamics but
that have not received much attention for solid materials. The benefit
of an Eulerian approach is better resolution of vortical type flows which
mix multiple materials as well as the ability to experiment with subgrid
scale methods to resolve complex phenomena that cannot be captured at the
smallest length scale available to the computation. We are also
interested in the proper modeling of dissipation for solid (as well as
fluid) materials as it is known that current approaches which use
numerical dissipation can sometimes create artificial physical response
in the materials under study. We also make use of computational
techniques such as adaptive mesh refinement and parallel computation to
resolve to the fullest extent possible the relevant phenomena.
Mach reflection of a strong shock wave propagating in Aluminum
against a rigid inclined ramp. Wave propagation in shocked solids is more
complex than in fluids. In particular, an incoming shock wave can
bifurcate into an elastic prescusor and a trailing wave associated with
plastic work. Shown on the left of the figure are the contours of density
indicating the compression of the material. The two shock waves (elastic
and plastic) are clearly seen in addition to the reflected waves which
must appear when a shock interacts with an oblique boundary (known as
Mach reflection). The right figure shows the stress deviator contours
which indicate the material strength response of the solid material.
These calculations are performed using a new fully Eulerian formulation
of solid mechanics which is able to deal with strong shocks and the
complex waves that arise as a result of shock boundary interactions.
Joint work of Dale Pullin and Dan Meiron.
Research Area:
Physics of Solids and Mechanics of Materials,
Computational & Theoretical Mechanics

Faculty:
Pullin,
Meiron
Shock wave—Boundary Layer Interaction for Reflected Detonations
Bifurcation of reflected shock waves is often discussed as it pertains to shock tube performance. The related problem of detonation reflection is currently being studied in the Explosion Dynamics Laboratory under Prof. Joseph Shepherd. Shown are two schlieren images in very similar mixtures at identical initial pressure. The detonation case (left) is 90% nitrous oxide with 10% hydrogen to give detonation whereas the shockwave case (right) is 100% nitrous oxide. In both cases the Mach number of the reflected wave is 1.6, but the reflected shock wave has stronger interaction with the boundary layer than the reflected detonation wave. Preliminary results suggest this is due to the importance of the thermal boundary layer behind the detonation.
The fluid dynamics of detonation reflection (left) are drastically different from shock wave reflection (right) for similar initial mixtures and reflected mach numbers.
Research Area:
Physics of Fluids

Faculty:
Shepherd
Morphing Surfaces for Flow Control
This research seeks to couple small amplitude, timedependent perturbations to surface morphology with the controlled response of fluid systems ranging from turbulent boundary layers to bluff body separating flows. As such, experimental and analytical research is under way into both fluid response and the fabrication of smart, morphing surfaces. Objectives include drag reduction and force vector control via the implementation of closedloop control of high Reynolds number, noncanonical and applied wallbounded flows using bulk actuation of discrete surface regions, with the potential for a significant contribution to future aerospace design methodologies and vehicle efficiency.
Phaselocked reconstruction of a sphere wake manipulated by a mechanical approximation to a morphing surface, namely a roughness element (height 1% of the sphere diameter) driven around the sphere azimuth at constant streamwise angle and optimized angular frequency
Research Area:
Physics of Fluids

Faculty:
McKeon
Experiments in High Reynolds Number Wall Turbulence
Wall turbulence at high Reynolds numbers is a problem of extensive practical interest, but even the canonical configurations have continued to confound the accumulated wisdom acquired over the last halfcentury. There is a need for detailed measurements at high Reynolds number and further study of the new measurement issues that arise under these conditions. Ongoing experimental work involves laboratory and field campaigns designed to give insight into flows of practical interest, including the rough wall regime.
Reconstructed temporal variation of the streamwise velocity in the nearwall region of the atmospheric surface layer under near thermally neutral conditions. From top to bottom: sliding window mean (logarithmic and linear scales); sliding window root mean square fluctuations; overlay of select contours showing the complex interaction between large and small scale turbulent activity. Measurements were obtained using 31 single normal hotwires over the first five meters of the layer (Guala, Holmes & McKeon, J. Fluid Mech., 2011).
Research Area:
Physics of Fluids

Faculty:
McKeon
Theoretical Analyses of Animal Wake Vortex Stability
Swimming animals propel themselves by shedding vortex wakes which
range in complexity from the isolated vortex rings of jetting swimmers
(such as squid or jellyfish), to the chains of vortex rings formed by
most fish. In order to evaluate the performance of these swimmers, we
must assess the optimality of the vortex wakes they produce, which
requires an understanding of their stability. In this project, we
consider simple models for the vortex rings produced by swimming
animals, and study their stability under perturbations of the type
that might occur during the vortex formation process.
The wake of a simulated eel. The chains of vortex rings
which form in the wake of a swimming eel are revealed by the
forwardtime Lagrangian coherent structures (LCS), shown in blue. LCS
act as separatrices between regions of different flow kinematics, in
this case dividing the eel's vortex wake and the surrounding fluid.
Research Area:
Physics of Fluids,
Computational & Theoretical Mechanics