CTR events

Turbulent combustion is an extremely challenging “multi-multi” problem: multi-physics, multi-scale, and multi-species. Since not all scales of turbulence and combustion can be resolved in DNS for practical conditions of interest, models are required for the unresolved turbulent combustion processes in LES and RANS. However, the large number of thermochemical scalars required to describe combustion chemistry (potentially hundreds or thousands of chemical species) means that the unresolved state-space that needs to be modeled is extremely high-dimensional. Turbulent combustion models can gene... Read More

Swept shock-wave/turbulent-boundary-layer interactions (STBLIs) exhibit key dynamical differences from their spanwise-homogeneous counterparts, including the suppression of a dominant mechanism of low-frequency unsteadiness. An extensive database of wall-resolved simulations is constructed to examine the differences between the properties of spanwise-homogeneous, swept, and compound swept interaction classes. A novel method for calculating the dynamic linear response of these unsteady flows is also presented, which allows for the identification and characterization of absolute instabilities... Read More

The resolvent formulation of McKeon & Sharma (2010) is applied to supersonic turbulent boundary layer flows in order to study the validity of Morkovin’s hypothesis, which postulates that high-speed turbulence structure in zero pressure-gradient turbulent boundary layers remains largely the same as its incompressible counterpart. The resolvent analysis highlights two distinct regions of the supersonic turbulent boundary layer in the wave parameter space: the relatively supersonic region and the relatively subsonic region. The relatively supersonic region, where the flow is supersonic rel... Read More

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