A recent research interest for Professor Rosakis is Hypervelocity Impact. Hypervelocity impact is a rising concern in spacecraft missions where man-made 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.
We are interested in animal-fluid interactions that could inspire innovative designs for devices operating in air or water. The jellyfish represents an ideal model system for studying unsteady fluid transport in a flexible system at various scales. We are particularly interested in quantifying scaling effects, elucidating the relation between geometry and fluid transport, and dissecting the functional components of the flow field such as feeding currents and propulsive currents. Applications include the design of propulsion systems, and translation to the medical research for new perspectives on fluid transport in organs and vascular systems.
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) three-dimensional fields can be extracted. Quantitative tools such as the curvelet transform can then be applied to extracted fields to produce a multi-scale 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 structure-based models of turbulent flows.
A broad range of experimental and theoretical research efforts seek to exploit the directional amplification associated with the Navier-Stokes 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.
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 high-speed photography, full-field 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 pre-compression 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 co-workers, 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 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.
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.
To analyze aerodynamic interactions between vertical-axis wind turbines in detail, it is essential to be able to observe their flow fields. Quantitative in situ measurements pose a challenge because of the large spatial dimensions, high flow velocities and the remote locations of the VAWTs. This project implements Particle Image Velocimetry (PIV) in horizontal cross-sections of the VAWTs as well as in the regions between neighboring turbines in the wind farm. Novel methods for flow seeding and illumination are being developed, as well as the incorporation of high-speed, high-resolution cameras and optical sectioning techniques. PIV yields instantaneous, two-dimensional, two-component velocity fields together with the out of plane component of vorticity, and is therefore a considerable advantage over the single-point techniques that are currently available for field measurements of wind turbines.
As animals such as jellyfish or squid propel themselves through their environment they leave vortex rings in their wake. Recent experimental work in our group by (Ruiz et al., JFM 2011) has shown that these vortex rings significantly improve the efficiency of propulsion. These results were obtained using a self-propelled submarine which operated in two modes: steady and unsteady. The steady mode just used a propeller and created a jet like any most submarines. However, the unsteady mode used a variable fluid inlet that allowed for the creation of vortex rings in the wake of the vehicle, much like a swimming jellyfish. Using the unsteady mode, the submarine would be able to travel at the same speed yet have approximately 40% longer range than the steady mode. Currently, we are further researching this area by developing a new mechanism for generating the vortex rings, as shown in this photo.
Fluid motions created by organisms in the ocean are vital for both swimming and feeding. These organism-generated flows are likely to be eroded under realistic background turbulence conditions yet, studies of marine organisms are generally conducted in still water. We are designing and implementing experiments to understand how turbulence influences feeding and swimming activities. This work has application to both understanding the flow of energy through the ocean’s food web as well as understanding the performance of underwater vehicles or instruments under realistic flow conditions.
This research seeks to couple small amplitude, time-dependent 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 closed-loop control of high Reynolds number, non-canonical and applied wall-bounded flows using bulk actuation of discrete surface regions, with the potential for a significant contribution to future aerospace design methodologies and vehicle efficiency.
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 half-century. 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.
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.
In-field measurements at remote location present a challenge for the measurement systems involved. Not only do these systems have to be self-sufficient in regard to power supply and data acquisition but also robust and easy to handle. With the current level of miniaturization in electronics it becomes possible to construct PIV-systems, which meet these criteria and are even small enough to be used as hand held devices. The group has extensive experience in developing PIV-system, which are designed for SCUBA divers to take in-field measurements of the flow around marine organisms in daytime. The animals, which have been studied to date, are jelly fish, comb jelly and salps among others.