arXiv subjects

Boo Cheong Khoo

Publications and source records attributed to Boo Cheong Khoo.

24 records · Page 2Linked to original sources

arXiv2008

Incipient Separation in Shock Wave Boundary Layer Interactions as Induced by Sharp Fin

The incipient separation induced by the shock wave turbulent boundary layer interaction at the sharp fin is the subject of present study. Existing theories for the prediction of incipient separation, such as those put forward by McCabe (1966) and Dou and Deng (1992), can have thus far only predicting the direction of surface streamline and tend to over-predict the incipient separation condition based on the Stanbrook's criterion. In this paper, the incipient separation is firstly predicted with Dou and Deng (1992)'s theory and then compared with Lu and Settles (1990)' experimental data. The physical mechanism of the incipient separation as induced by the shock wave/turbulent boundary layer interactions at sharp fin is explained via the surface flow pattern analysis. Furthermore, the reason for the observed discrepancy between the predicted and experimental incipient separation conditions is clarified. It is found that when the wall limiting streamlines behind the shock wave becomes\ aligning with one ray from the virtual origin as the strength of shock wave increases, the incipient separation line is formed at which the wall limiting streamline becomes perpendicular to the local pressure gradient. The formation of this incipient separation line is the beginning of the separation process. The effects of Reynolds number and the Mach number on incipient separation are also discussed. Finally, a correlation for the correction of the incipient separation angle as predicted by the theory is also given.

physics.flu-dyn↗

arXiv2008

Simulations of Fibre Orientation in Dilute Suspensions with Front Moving in the Filling Process of a Rectangular Channel Using Level Set Method

The simulation of fibre orientation in dilute suspension with a front moving is carried out using the projection and level set methods. The motion of fibres is described using the Jeffery equation and the contribution of fibres to the flow is accounted for by the configuration field method. The governing Navier-Stokes equation for the fluid flow is solved using the projection method with finite difference scheme, while the fibre-related equations are directly solved with the Runge-Kutta method. Our findings indicate that the fibre motion has strong influence on the distributions of the streamwise and transverse velocities in the fountain flow. Fibre motion produces strong normal stress near the wall which leads to the reduction of transversal velocity as compared to the Newtonian flow without fibres and in turn the streamwise velocity near the wall is increased. Thus, the fibre addition to the flow weakens the strength of the fountain flow. It is also found that the fibre orientation is not always along the direction of velocity vector in the process of mold filling. In the region of the fountain flow, the fibre near the centerline is more oriented cross the streamwise direction comparing to that in the region far behind the flow front. This leads to that the fibre near the centreline in the region of fountain flow is more extended along the transverse direction. Since fibre orientation in the suspension flow and the shape of the flow front have great bearing on the quality of the product made from injection molding, this study has much implications for engineering applications. These results can also be useful in other field dealing with fibre suspensions.

physics.flu-dyn↗

arXiv2008

Numerical Simulation of Fibre Suspension Flow through an Axisymmetric Contraction and Expansion Passages by Brownian Configuration Field Method

In this paper, the finite element method is combined with the Brownian Configuration Field (BFC) method to simulate the fibre suspension flow in axisymmetric contraction and expansion passages. In order to solve for the high stress at high concentration, the Discrete Adaptive Viscoelastic Stress Splitting (DAVSS) method is employed. For the axisymmetric contraction and expansion passages with different geometry ratios, the results obtained are compared to available constitutive models and experiments. The predicted vortex length for dilute suspensions agrees well with experimental data in literature. Our numerical results show clearly the effect on vortex enhancement with increase of the volume fractions and the aspect ratios. Effect of aspect ratio of fibres on the vortex length is also studied. It is found that for the lower expansion ratio flows the vortex dimension in the corner region is fairly independent of fibre concentration and aspect ratio of fibres while the said vortex dimension increases with the increase of fibre concentration for contraction flows. The finding suggests that the aligned fibre approximation traditionally employed in previous work does not exactly describe the effect of fibre motion, and the present BFC method is deemed more suitable for the flow of dilute fibre suspensions. In terms of numerics, the employment of DAVSS enhances numerical stability in the presence of high concentration of fibre in the flow.

physics.flu-dyn↗

arXiv2008

Mechanism of Wall Turbulence in Boundary Layer Flow

The energy gradient method is used to analyze the turbulent generation in the transition boundary layer flow. It is found that the maximum of the energy gradient function occurs at the wall for the Blasius boundary layer flow. At this location under a sufficiently high Reynolds number, even a low level of free-stream disturbance can cause the turbulent transition and sustain the flow to be in a state of turbulence. This is an excellent explanation of the physics of self-sustenance of wall turbulence. The mechanism of receptivity for boundary layer flow can also be understood from the energy gradient criterion. That is, the free-stream disturbance can propagate towards the wall by the "energy gradient" process to cause turbulent transition, and the transition point in boundary layer can be moved forward towards the leading edge when the level of external disturbance increases.

physics.flu-dyn↗

arXiv2005

Threshold amplitudes for transition to turbulence in a pipe

Threshold amplitude of disturbance for transition to turbulence in a pipe Poiseuille flow is investigated. Based on the energy gradient theory, we argued that the transition to turbulence depends on magnitudes of the energy gradient of mean flow and the disturbance energy. Furthermore, the threshold disturbance energy required for turbulence transition is expressed as a function of the complementary angle to the energy angle which also characterizes the behaviour of the energy gradient of mean flow. With some mathematical treatments, the variation of the threshold energy of disturbance versus the Reynolds number is obtained. Then, it is found for a fixed disturbance frequency that the normalized amplitude of disturbance is scaled by Re with an exponent of -3/2 for the transition occurrence in a pipe. This value of exponent agrees well with several recent results reported in the literature. Finally, the mechanism of transition to turbulence is suggested for different disturbance levels.

physics.flu-dyn↗

arXiv2005

Shear Instabilities in Granular Flows down on Inclined Plane

Instabilities at interface of two stream granular flows have been reported in recent experiment [1] that breaking waves can form at the interface between two streams of identical grains flowing on an inclined plane downstream of a splitter plate. In this report, the theory of hydrodynamic instability is used to analyze the shear flow of granular materials. It is shown that the shear instability in two-stream granular flows actually comes from the competition between the energy gradients in transverse and streamwise directions as well as the interaction of two streams. We argue that the flow energy loss in the streamwise direction has a stabilizing effect, while the transverse component of the friction force formed by grain surface friction acts as the source of instabilities. An equation has been derived to characterize the transition between steady and wavy flows. Good qualitative agreement with the experimental data is obtained.

physics.class-ph↗