School of Mechanical Engineering Seminar

Wednesday, December 14, 2022 at 14:00
Wolfson Building of Mechanical Engineering, Room 206

Numerical analysis of the hemodynamics and performance of a miniature ventricular assist device

Yuval Gabso

           MSc of Moshe Rozenfeld

The use of axial percutaneous left ventricular assist devices (p-LVAD) in the clinical setting of acute heart failure has recently been significantly increased. Although these devices have improved the management, survival, and prognosis of many patients with acute of chronic heart failure, there is still high mortality and a wide burden of disease, mostly due to related thrombosis (blood clots) that can cause stroke, and hemolysis (rapture of blood cells) that can lead to anemia. p-LVADs designs with reduced blood trauma are likely to improve clinical outcomes and expand p-LVAD therapy to patients with less severe heart failure.

The present study investigated two configurations of p-LVAD designs, a floating impeller driven by magnetic levitation and a motor driven suction pump. The effects of the hub geometry on the hemodynamic characteristics, pump performance and blood damage were evaluated using CFD. It has been confirmed that assuming steady flow within a rotation frame approximates well the average flow of the transient case. Four different impellers (diameter of 5-6mm) were built and twenty cases were analyzed at different working conditions of pressure heads (60-80mmHg) and angular velocities (30-52kRPM). The numerical simulations used the Reynolds-Averaged Navier-Stokes approximation employing the SST turbulent model. Estimation of RBC hemolysis was conducted according to four different models (based on Kolmogorov length scale; and based on combinations of exposure time to shear stress above 425[Pa]). Probabilities for shear induced platelet activation (PDF) were estimated based on accumulated shear stresses along streamlines.

The results of the simulations reveal that the shape of the impeller’s hub has a great impact on the flow patterns, performance and the risk for blood damage. Impellers with a converging hub encourage recirculating flow and increase the residence time while impellers with an expanding hub have higher wall shear stress with less reversed flow regions. The suction pump has the best performance with efficiency of 19%, while the floating impeller reaches a maximal efficiency of 12% only. However, the high efficiency of the suction pump is achieved at the cost of increased risk of platelets activation. A strong correlation is also found between the angular velocity and the risk for hemolysis in all four models.

This study shed light on the effects of pump configuration on the performance and risk of blood damage, indicating the roles of the hub shape and angular velocity as dominant parameter. These findings can be utilized to improve future designs of p-LVAD.

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School of Mechanical Engineering Seminar
Wednesday, November 23, 2022 at 14:00
Wolfson Building of Mechanical Engineering, Room 206

Active control of Wing-Engine-Slat cut-out Region flow separation 

Maayan Possti

 MSc. of Avi Seifert

Fossil fuel is the most common energy source used to power aircraft and as a result, its engines emit polluting gases that also contribute to global warming. One possible remedy is increasing the bypass ratio and lowering the fan pressure ratio of turbofan aircraft engine, reducing the fuel consumption for the same thrust. Ultra-High Bypass Ratio (UHBR) engines are very effective from the fuel consumption aspect and therefore have great potential in civil aviation. On the other hand, the engine's very large diameter leads to a challenging integration with the aircraft wing. This assembly requires a slat-cut-out in the area where the engine is connected. This in turn causes local boundary layer flow separation. This study is focused on establishing the baseline flow of the small scaled model based on the DLR-F15 airfoil. Also, to apply active flow control (AFC) method in a form of steady suction in order to enhance lift and increase stall angle. The first part of the study includes the complex design and manufacturing of the small-scale model constructed out of the main element wing, two parts-slat, flap and an engine. The second part deals with investigating the baseline flow of the small-scale model and finding if it corresponds well to CFD simulations performed under the same conditions. This part was conducted at the TAU closed-loop low-speed wind-tunnel (WT), combining surface pressures and near-wake measurements. The last and final part of this study was the application of steady suction from two rows of suction holes located in the separation prone region. Testing was conducted at low Reynolds numbers with different configurations of the suction holes and suction magnitudes. All of the WT experiments were performed at a free-stream velocity of 25m/s correspond to Reynolds number of 640k, based on an average chord (including flap and slat) is 430 mm. It was found that applying steady suction leads to a significant improvement in terms of the lift coefficient.

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