[pt] A combinação de modelos numéricos com modelos computacionais tem contribuido muito para o melhor entendimento matemático de fluxos gravitacionais, porém esses modelos não podem substituir a análise através de trabalhos experimentais. O uso de modelos físicos em escala provou ser essencial na validação de equações para modelagem de correntes de turbidez. Com o objetivo de diminuir o nível de dificuldade em modelar numericamente essas correntes e de gerar modelos computacionais de alto desempenho, algumas simplificações foram feitas durante o desenvolvimento das equações de velocidade. Dessa forma, para provar que tais simplificações não iriam alterar os resultados numéricos do modelo, foram realizados inúmeros experimentos, coletando informações sobre a evolução espaço- temporal de velocidades das correntes de turbidez não- confinadas com e sem partículas. Comparando os resultados do modelo numérico com os do modelo físico, foi concluído que, infelizmente, as aproximações influenciaram os resultados. Contudo, os dados e a comparação visual entre as simulações também revelaram alguns resultados encorajadores, os quais estimularão pesquisas futuras para se melhorar a precisão da equação de velocidade utilizada no modelo numérico. / [en] The combination between numerical and computer models has improved dramatically the mathematical understanding of gravity currents; however, these models can not replace the analysis by experimental work. The use of scaled analogue models, or physical models, proved to be essential in validating velocity equations for turbidity currents. In order to reduce the level of difficulty to model mathematically these currents, some approximations were applied during the development of the velocity equation. Therefore, willing to prove that these approximations would not compromise the numerical results, innumerous experiments were performed to acquire a spatio-temporal velocity evolution database for both unconfined particle free and particulate turbidity flows. Comparing the results from the numerical and physical simulations, it was concluded that, unfortunately, the approximations have influenced the numerical results. Nevertheless, the data and visual comparisons between the simulations also revealed some encouraging results, which will stimulate some future research to improve the accuracy of the depth-averaging velocity equation.

Indiana University-Purdue University Indianapolis (IUPUI) / We numerically investigate the underlying physics of two peculiar phenomena, which are axis-switching and saddle-back velocity profile, in both laminar and turbulent rectangular jets using lattice Boltzmann method (LBM). Previously developed computation protocols based on single-relaxation-time (SRT) and multiple-relaxation-time (MRT) lattice Boltzmann equations are utilized to perform direct numerical simulation (DNS) and large eddy simulation (LES) respectively. In the first study, we systematically study the axis-switching behavior in low aspect-ratio (AR), defined as the ratio of width over height, laminar rectangular jets with <italic>AR=1</italic> (square jet), 1.5, 2, 2.5, and 3. Focuses are on various flow properties on transverse planes downstream to investigate the correlation between the streamwise velocity and secondary flow. Three distinct regions of jet development are identified in all the five jets. The <italic>45&deg</italic> and <italic>90&deg</italic> axis-switching occur in characteristic decay (CD) region consecutively at the early and late stage. The half-width contour (HWC) reveals that <italic>45&deg</italic> axis-switching is mainly contributed by the corner effect, whereas the aspect-ratio (elliptic) feature affects the shape of the jet when <italic>45&deg</italic> axis-switching occurs. The close examinations of flow pattern and vorticity contour, as well as the correlation between streamwise velocity and vorticity, indicate that <italic>90&deg</italic> axis-switching results from boundary effect. Specific flow patterns for <italic>45&deg</italic> and <italic>90&deg</italic> axis-switching reveal the mechanism of the two types of axis-switching respectively. In the second study we develop an algorithm to generate a turbulent velocity field for the boundary condition at jet inlet. The turbulent velocity field satisfies incompressible continuity equation with prescribed energy spectrum in wave space. Application study of the turbulent velocity profile is on two turbulent jets with <italic>Re=25900</italic>. In the jets with <italic>AR=1.5</italic>, axis-switching phenomenon driven by the turbulent inlet velocity is more profound and in better agreement with experimental examination over the laminar counterpart. Characteristic jet development driven by both laminar and turbulent inlet velocity profile in square jet (<italic>AR=1</italic>) is also examined. Overall agreement of selected jet features is good, while quantitative match for the turbulence intensity profiles is yet to be obtained in future study. In the third study, we analyze the saddle-back velocity profile phenomenon in turbulent rectangular jets with AR ranging from 2 to 6 driven by the developed turbulent inlet velocity profiles with different turbulence intensity (<italic>I</italic>). Saddle-back velocity profile is observed in all jets. It has been noted that the saddle-back's peak velocities are resulted from the local minimum mixing intensity. Peak-center difference <italic>&Delta_pc</italic> and profound saddle-back (PSB) range are defined to quantify the saddle-back level and the effects of AR and <italic>I</italic> on saddle-back profile. It is found that saddle-back is more profound with larger AR or slimmer rectangular jets, while its relation with <italic>I</italic> is to be further determined.