7 research outputs found
A non-hybrid method for the PDF equations of turbulent flows on unstructured grids
In probability density function (PDF) methods of turbulent flows, the joint
PDF of several flow variables is computed by numerically integrating a system
of stochastic differential equations for Lagrangian particles. A set of
parallel algorithms is proposed to provide an efficient solution of the PDF
transport equation, modeling the joint PDF of turbulent velocity, frequency and
concentration of a passive scalar in geometrically complex configurations. An
unstructured Eulerian grid is employed to extract Eulerian statistics, to solve
for quantities represented at fixed locations of the domain (e.g. the mean
pressure) and to track particles. All three aspects regarding the grid make use
of the finite element method (FEM) employing the simplest linear FEM shape
functions. To model the small-scale mixing of the transported scalar, the
interaction by exchange with the conditional mean model is adopted. An adaptive
algorithm that computes the velocity-conditioned scalar mean is proposed that
homogenizes the statistical error over the sample space with no assumption on
the shape of the underlying velocity PDF. Compared to other hybrid
particle-in-cell approaches for the PDF equations, the current methodology is
consistent without the need for consistency conditions. The algorithm is tested
by computing the dispersion of passive scalars released from concentrated
sources in two different turbulent flows: the fully developed turbulent channel
flow and a street canyon (or cavity) flow. Algorithmic details on estimating
conditional and unconditional statistics, particle tracking and particle-number
control are presented in detail. Relevant aspects of performance and
parallelism on cache-based shared memory machines are discussed.Comment: Accepted in Journal of Computational Physics, Feb. 20, 200
Experimental investigations of one and two-phase pressure drop during flow in minichannels
W pracy przedstawiono wyniki bada艅 eksperymentalnych opor贸w przep艂ywu mieszaniny woda-powietrze w poziomych kwadratowych minikana艂ach o 艣rednicy hydraulicznej od 2 do 6 mm. Wyniki bada艅 por贸wnano z metod膮 Lockharta-Martinelliego i Chisholma, uzyskuj膮c rozrzut punkt贸w na poziomie 40%, kt贸ry jest zbli偶ony do warto艣ci otrzymanych dla kana艂贸w konwencjonalnych.Results of experimental investigations of two-phase air - water mixture flow in horizontal quadratic minichannels with the hydraulic diameter from 2 to 6 mm are presented in the paper. The measured values were compared with calculated ones using the Lockhart-Martineiii and Chisholm methods. Accuracy of calculations with the standard deviation about 40% is close that obtained in conventional channels
Novel approaches to estimating the turbulent kinetic energy dissipation rate from low- and moderate-resolution velocity fluctuation time series
In this paper we propose two approaches to estimating the turbulent kinetic
energy (TKE) dissipation rate, based on the zero-crossing method by Sreenivasan et al. (1983). The original formulation
requires a fine resolution of the measured signal, down to the smallest
dissipative scales. However, due to finite sampling frequency, as well as
measurement errors, velocity time series obtained from airborne experiments
are characterized by the presence of effective spectral cutoffs. In contrast
to the original formulation the new approaches are suitable for use with
signals originating from airborne experiments. The suitability of the new
approaches is tested using measurement data obtained during the Physics of
Stratocumulus Top (POST) airborne research campaign as well as synthetic
turbulence data. They appear useful and complementary to existing methods. We
show the number-of-crossings-based approaches respond differently to errors
due to finite sampling and finite averaging than the classical power spectral
method. Hence, their application for the case of short signals and small
sampling frequencies is particularly interesting, as it can increase the
robustness of turbulent kinetic energy dissipation rate retrieval