Engineering topics

Lee, Moon Joo

Publications and source records attributed to Lee, Moon Joo.

7 records

NASA NTRS1991

Direct numerical simulation of turbulent plane Couette flow

Turbulent plane Couette flow was numerically simulated at a Reynolds number (U(sub w)h/nu) of 6000, where U(sub w) is the relative wall speed and h is half the channel-height. Unlike in Poiseuille flow, where the mean shear rate changes its sign at the centerline, the sign of mean shear rate in plane Couette flow remains the same across the whole channel. This difference is expected to yield several differences between the two flows, especially in the core region. The most significant and dramatic difference observed was the existence of large-scale structures in the core region of the plane Couette flow. The large eddies are extremely long in the flow direction and fill the entire channel (i.e., their vertical extent is 2h). The large-scale structures have the largest contribution from the wavenumber (k(sub x)h,k(sub z)h) = (0, plus or minus 1.5), corresponding to a wavelength lambda(sub z)/h is approximately equal to 4. The secondary motion associated with the k(sub x)h = 0 mode consists of the large-scale vortices. The large eddies contribute about 30 percent of turbulent kinetic energy.

Lee, Moon Joo↗

NASA NTRS1990

Structure of turbulence at high shear rate

The structure of homogeneous turbulence subject to high shear rate has been investigated by using three-dimensional, time-dependent numerical simulations of the Navier-Stokes equations. This study indicates that high shear rate alone is sufficient for generation of the streaky structures, and that the presence of a solid boundary is not necessary. Evolution of the statistical correlations is examined to determine the effect of high shear rate on the development of anisotropy in turbulence. It is shown that the streamwise fluctuating motions are enhanced so profoundly that a highly anisotropic turbulence state with a 'one-component' velocity field and 'two-component' vorticity field develops asymptotically as total shear increases. Because of high-shear rate, rapid distortion theory predicts remarkably well the anisotropic behavior of the structural quantities.

Lee, Moon Joo↗

NASA NTRS1990

Effects of organized turbulence structures on the phase distortion in a coherent optical beam propagating through a turbulent shear flow

Phase distortion in a coherent optical beam propagating through a turbulent shear flow is studied. The instantaneous distribution of the index refraction is represented by a passive-scalar field in a computed homogeneous shear flow. The flow contains organized vortical structures (hairpin eddies), which are characteristic of turbulent shear flows. The phase distortion induced by turbulent fluctuations is calculated from the optical path difference through the flow. A conceptual model is proposed for the distribution of scalar fluctuations produced by the hairpin vortices in the shear flow. It is shown that the phase distortion of an optical beam can be minimized by propagating the beam at an angle approximately normal to the organized vortical structures in a turbulent shear flow.

Truman, C. Randall↗

NASA NTRS1990

A contribution toward rational modeling of the pressure-strain-rate correlation

A novel method of obtaining an analytical expression of the 'linear part' of the pressure-strain-rate tensor in terms of the anisotropy tensor of the Reynolds stresses has been developed, where the coefficients of the seven independent tensor terms are functions of the invariants of the Reynolds-stress anisotropy. The coefficients are evaluated up to fourth order in the anisotropy of the Reynolds stresses to provide guidance for development of a turbulence model.

Lee, Moon Joo↗

NASA NTRS1989

Distortion of homogeneous turbulence by axisymmetric strain and dilatation

Rapid distortion theory is applied to study distortion of homogeneous turbulence subject to two different axisymmetric strain modes: the axisymmetric contraction (AC, nozzle-type flow), and the axisymmetric expansion (AE, diffuser-type flow). The paper explores the differences in effects of the two axisymmetric strain modes on the anisotropy of correlations and structures of turbulence; examines the effect of dilatation on the distortion of turbulence; and provides a theoretical background for turbulence model development. It is found that velocity and vorticity fluctuations are enhanced more efficiently by contraction than by expansion; contraction produces much higher anisotropy in velocity and vorticity than expansion; root-mean-square pressure is slightly reduced during contraction, whereas it increases rapidly during expansion; and vortical structures of rodlike shape develop in a contraction flow, while disklike structures develop in an expansion flow. A simple model that reflects the dependence of turbulence evolution on structural parameters such as the Reynolds-stress anisotropy and total strain is proposed, and is shown to outperform all other models for all cases examined, regardless of the mean strain rate.

Lee, Moon Joo↗

NASA NTRS1989

The structure of sheared turbulence near a plane boundary

The effects of shear on the structure of turbulence near a plane boundary are examined. Inhomogeneous rapid distortion theory is applied to a uniform-shear boundary layer (USBL) with a slip velocity condition at the surface. The effects of blocking the vertical component of the turbulence by the boundary in a USBL are compared with the effects in a shear-free boundary layer. It is shown that the blocking is only slightly modified by the presence of shear. The shear does alter the way in which the boundary influences the horizontal components and the turbulent shear stress near the horizontal component.

Lee, Moon Joo↗

NASA NTRS1987

Turbulence structure at high shear rate

The structure of homogeneous turbulence in the presence of a high shear rate is studied using results obtained from three-dimensional time-dependent numerical simulations of the Navier-Stokes equations on a grid of 512 x 128 x 128 node points. It is shown that high shear rate enhances the streamwise fluctuating motion to such an extent that a highly anisotropic turbulence state with a one-dimensional velocity field and two-dimensional small-scale turbulence develops asymptotically as total shear increases. Instantaneous velocity fields show that high shear rate in homogeneous turbulent shear flow produces structures which are similar to the streaks present in the viscous sublayer of turbulent boundary layers.

Lee, Moon Joo↗