CTR events

A recent mean velocity transformation (Trettel and Larsson 2016) converts compressible mean velocities into equivalent incompressible mean velocities.  This transformation works properly for channel flows but does not work properly for boundary layers.  A two-dimensional extension of this transformation reveals the inherent limitations in the transformation as an extension of the incompressible law-of-the-wall.  The transformation cannot properly transform the outer layer coordinate in boundary layers, and the error associated with this correlates highly with the error in the transformation... Read More

The study of so called transmission problems in  [2] revealed that successful numerical filtering may include a delicate balance between the need to remove high frequency oscillations (filter often for accuracy) and the need to avoid possible growth  (filter seldom for stability).  In this talk we investigate this contradiction, and propose different avenues for improved functionality.

The filter operators derived in [1] are the basic building  blocks.  We demonstrate that explicit use of the basic filter operators guarantee accuracy but lead to instabilities, while an implicit imple... Read More

An unsupervised machine-learning algorithm, the self-organizing map (SOM), is used to identify the turbulent boundary layer (TBL) and non-TBL regions in bypass transition. The data employed for the analysis are from an archived direct simulation publicly available in the Johns Hopkins Turbulence Databases (JHTDB, http://turbulence.pha.jhu.edu). The data points in the entire flow domain are automatically classified into TBL and non-TBL regions by the SOM, based on their standardized velocity, velocity fluctuations, velocity gradients and their spatial locations. Thus the SOM identifies the t... Read More

Lu presents findings on the hydrodynamic stability of a premixed flame subjected to transverse shear. The problem configuration is a situation of interest for laminar and turbulent flames when they travel into a region of shear.

The linear stability problem is first analytically solved, and the dispersion relation is determined. The effects of the transverse shear and thermal expansion are examined. Lu’s research then carried out a weakly nonlinear analysis in the weak thermal expansion limit and derived the modified Michelson-Sivashinsky (MS) equation, which describes the evolution... Read More

One-Dimensional (1D) Reduced-Order Models (ROMs) are developed for a 3D highly turbulent Rayleigh Bènard convection (RBC) system with Rayleigh number Ra=106, which is nearly 600 times larger than its critical value, while three general objectives are pursued: 1) Predicting the time-mean response to external forcings, 2) Identification of underlying dynamics, and 3) Short-term prediction of spatiotemporal evolution of the flow. Towards the first aim, Linear Response Function (LRF) of the system is obtained via Green’s Function (GRF) method, which is an equation-dependent method for... Read More

No Abstract.

Unsteady flows over plunging and pitching airfoils with large excursions in effective angle of attack exhibit the phenomenon of dynamic stall. This process is characterized by unsteady separation and formation of a large leading-edge vortex that exerts high amplitude fluctuations in aerodynamic loads. Although several studies have been conducted for pitching airfoils at high Reynolds numbers, research on dynamic stall for plunging airfoils is more scarce, especially at low and moderate Reynolds numbers.

We employ large eddy simulations to study the flow physics of deep dynamic stall... Read More

Cavitation and bubble dynamics are of critical importance in various industrial systems. In ultrasound therapy, cloud cavitation that is nucleated in the human body is crucial for treatment outcomes; bubble clouds can coherently collapse to cause injury as well as scatter the wave energy to shield therapy targets. In this talk, I will present a method for multi-scale modeling and simulation of intense cloud cavitation in a compressible liquid, and its application for medical ultrasound. The method employs an Eulerian-Lagrangian approach to solve mixture- averaged equations of motion, in tha... Read More

Turbulence is an important example of chaotic dynamical systems, and sensitivity analysis is a powerful tool for design via computation.  However, traditional sensitivity methods explode for chaotic systems, posing the challenge we will address in this talk.  We first review how to compute meaningful derivatives of long-time-averaged objectives in chaotic systems via the shadowing method, which we then reformulate as a 'non-intrusive' minimization problem on the unstable subspace. Then we show a recent adjoint shadowing theorem, based on which we develop an adjoint sensitivity algorithm, NI... Read More

From oceanic breaking waves to atomization of liquid fuels for combustion, two-phase flows are omni-present in natural and industrial settings. Despite decades of numerical method development, due to the many challenges involved in simulation of realistic two-phase flows, no gold-standard has yet emerged in the literature. In this study, we present a novel diffuse interface method that addresses various challenges for simulation of incompressible, immiscible two-phase flows. The boundedness of this mass-conserving interface-capturing method is proven analytically. Then, a comparison of the... Read More