Direct simulations and modelling of basic three-dimensional bifurcating tube flows

Three-dimensional bifurcating internal flow is studied for a single mother tube branching into two equal but diverging daughter tubes. The mother tube is straight and of circular cross-section, containing a fully developed incident motion, while the diverging daughters are straight and of semi-circular cross-section. This basic configuration is treated first by direct numerical simulation and secondly by slenderflow modelling, for a variety of Reynolds numbers and angles of divergence. The direct simulations and modelling highlight different forms of three-dimensional separation or flow reversal as well as enhanced upstream and downstream influence and pressure loss induced by the bifurcations especially at increased divergence angles. Comparisons between the results from the simulations and those from the slender-flow modelling show relatively close agreement at medium values of Reynolds number. In particular, as the angle of divergence increases for a given Reynolds number, there is generally first an increase in flow attachment on to the inner divider wall(s) and then, at higher angles, an increasing trend to flow reversal at the corners formed by the junctions of the outer wall with the divider; longitudinal vortex motion is also enhanced then. The agreement persists over a surprisingly wide range of divergence angles

Combination of Lighthill Acoustic Analogy and Stochastic Turbulence Modelling for Far-Field Acoustic Prediction

There are many approaches in determining the sound propagated from turbulent flows. Hybrid approaches, in which the turbulent noise source field is computed or modeled separately from the far-field calculation, are frequently used. To have a more feasible approach for basic estimation of sound propagation, cheaper methods can be developed using stochastic modeling of the turbulent fluctuations (turbulent noise source field). In this paper, a simple and easy to use stochastic model for the generation of turbulent velocity fluctuations called continuous filter white noise (CFWN) model is used. This method is based on the use of classical Langevian-equation to model the details of fluctuating field superimposed on averaged computed quantities. The sound propagation due to the generated unsteady field is evaluated using Lighthill's acoustic analogy. Our results are validated by comparing the directivity and the overall sound pressure level (OSPL) magnitudes with the available experimental results. Numerical results show reasonable agreement with the experiments, both in maximum directivity and magnitude of the OSPL

Retinal arteriolar geometry is associated with cerebral white matter hyperintensities on MRI

Background. Cerebral small vessel disease (lacunar stroke and cerebral white matter hyperintensities) is caused by vessel abnormalities of unknown aetiology. Retinal vessels show developmental and pathophysiological similarities to cerebral small vessels and microvessel geometry may influence vascular efficiency. Hypothesis. We hypothesized that retinal arteriolar branching angles or co-efficients (the ratio of the sum of the cross sectional areas of the two daughter vessels to the cross sectional area of the parent vessel at an arteriolar bifurcation) may be associated with cerebral small vessel disease. Methods. We performed a cross-sectional observational study in a tertiary referral hospital, United Kingdom. An experienced stroke physician recruited consecutive patients presenting with lacunar ischaemic stroke with a control group consisting of patients with minor cortical ischaemic stroke. We performed brain magnetic resonance imaging to assess the recent infarct and periventricular and deep white matter hyperintensities. We subtyped stroke with clinical and radiological findings. We took digital retinal photography to assess retinal arteriolar branching co-efficients and branching angles using a semi-automated technique. Results. We recruited 205 patients (104 lacunar stroke, 101 cortical stroke), mean age 68 (Standard Deviation 12) years. With multivariate analysis, increased branching coefficient was associated with periventricular white matter hyperintensities (p=0.006) and ischaemic heart disease (p<0.001); decreased branching co-efficient with deep white matter hyperintensities (p=0.003) but not with lacunar stroke subtype (p=0.96). We found no associations with retinal branching angles. Conclusions. Retinal arteriolar geometry differs between cerebral small vessel phenotypes. More research is needed to ascertain the clinical significance of these findings

Experimental study on the change of the orientation of high aspect ratio nozzle slit relative to the airflow

Abstract An experimental study to investigate the flow of liquid jet issued from a high aspect ratio nozzle slit into an incoming airflow by changing the orientation angle from the incoming free-stream was performed. A two-dimensional liquid sheet emerged from the narrow slit into the subsonic air crossflow. Different orientation angles between 0 and 90 degrees were studied. High-speed photography and shadowgraphy techniques were utilized to visualize the flow physics. The influence of the slit orientation angle on the flow morphology and the flow regimes of liquid sheets was investigated. Some fluid flow parameters were obtained by analyzing the images. The changes in breakup height of different orientations were measured. A model was offered for the breakup height of the liquid sheet based on the liquid-to-gas momentum ratio, gas Weber number, and a new non-dimensional parameter as a representation of the angle of slit orientation. Also, the defined sheet trajectory for each orientation angle was obtained, and the variations were examined. Empirical correlations for the defined trajectory of the sheet in terms of liquid to gas momentum ratio and gas Weber number for each orientation angle were proposed

Optimization of Flow Control Parameters Over SD7003 Airfoil With Synthetic Jet Actuator

Abstract An optimization study was conducted to find the optimum operational characteristics of a synthetic jet actuator (SJA) to postpone the static stall separation over an SD7003 airfoil at Reynolds number of 60,000. A genetic algorithm (GA) coupled with an artificial neural network (ANN) was employed. Aerodynamic performance (L/D) was chosen as the objective function. Both tangent to the boundary layer (TBL) and the cross to the boundary layer (CBL) configurations of SJA were used and their effectiveness in separation control were compared. The following design variables of the SJA were allowed to change within a predetermined range: location, the opening length, the injection velocity amplitude, the injection angle, and the nondimensional frequency. It was found that for location, opening length, and velocity amplitude ratio, a narrow range near the peak optimum values achieved the best performance. However, for the nondimensional frequency and jet injection angle, the optimum values providing highest performance were in a wider range of values. Activation of SJ actuator improved the aerodynamic performance of the airfoil significantly. However, TBL configuration of SJA produced superior improvement in aerodynamic performance. The optimum aerodynamic performance achieved by TBL-SJA was 34.4, in comparison to 25.3 for CBL-SJA and 5 for the uncontrolled stalled airfoil at 13 deg angle of attack.</jats:p