@misc{ZanounDewidarEgbers, author = {Zanoun, El-Sayed and Dewidar, Yasser and Egbers, Christoph}, title = {Reynolds number dependence of azimuthal and streamwise pipe flow structures}, series = {Journal of Fluid Mechanics}, volume = {Vol. 973}, journal = {Journal of Fluid Mechanics}, issn = {0022-1120}, doi = {10.1017/jfm.2023.700}, pages = {1 -- 36}, language = {en} } @misc{ELSheikhELBatshZanounetal., author = {EL-Sheikh, Mohamed and EL-Batsh, Hesham M. and Zanoun, El-Sayed and Attia, Ali M. A.}, title = {Numerical and experimental investigations of flow separation control through a linear compressor cascade}, series = {Discover applied sciences}, volume = {6}, journal = {Discover applied sciences}, number = {9}, publisher = {Springer International Publishing}, address = {Cham}, issn = {3004-9261}, doi = {10.1007/s42452-024-05982-3}, pages = {1 -- 24}, abstract = {Modern large-scale gas turbines are equipped with high-pressure ratio compressors to increase engine work and its overall efficiency. Flow separation and energy losses are also two interrelated phenomenon associated with changes in compressor loading level and performance. This paper examines therefore the control of flow separation using a passive-control technique. An arced divergent-convergent slot grooved from the blade pressure side to its suction side was adopted to control flow separation, reducing the losses through a linear compressor cascade. The spanwise location of the slot was selected based on CFD simulations where the corner separation was predicted. The slot height in the spanwise direction was selected to be 8\% of the blade height at the end-wall side. The present work was performed experimentally and numerically at an inlet Reynolds number, Rec=ρV∞C/μ=2.98×105, covering a wide range of incidence angles from +6∘to-6∘. The experimental work was carried out using a linear cascade test section consisting of six NACA 65-009 blade profiles integrated into a low-speed wind tunnel. A five-hole pressure probe system was used to obtain main flow parameters. Numerically, four turbulence models, including Spalart-Allmaras (S-A) model, Realizable (R k-ε) model, Shear-Stress Transport (SST k-ω) model, and Reynolds Stress model (RSM) were tested to predict the velocity and pressure fields. Good agreement between the experimental measurements and the numerical results, which were obtained using the RSM turbulence model in terms of velocity profiles and total pressure downstream of blades. It was observed also that the use of the arced-slotted blades for positive incident angles was more effective in reducing the separation than the negative and zero incident angles, approaching a maximum value of 33\% for 6° with enhanced blade loading reaching 17.6\%. It is to be concluded that, the use of arced slotted blade improves the compressor performance specially for positive incident angles.}, language = {en} } @misc{DurstZanoun, author = {Durst, Franz and Zanoun, El-Sayed}, title = {Laminar pipe flow instability : a theoretical-experimental perspective}, series = {Fluids}, volume = {10}, journal = {Fluids}, number = {8}, publisher = {MDPI}, address = {Basel}, issn = {2311-5521}, doi = {10.3390/fluids10080216}, pages = {1 -- 21}, abstract = {This paper revisits the theoretically predicted inherent stability of fully developed laminar pipe flow, which remains unconfirmed by experimental evidence. A recently developed theory of pipe-flow stability/instability addresses the gap between experimental observations and classical theoretical predictions by accounting for a parallel secondary flow through the pipe's roughness layer that accompanies the main stream. This secondary flow alters the near-wall velocity profile in the rough-wall region, creating an inflection point that promotes shear-driven instabilities and triggers the laminar-to-turbulent transition. A stability factor 𝑆=𝐷𝑐/𝐷 is introduced, where D is the nominal pipe diameter and 𝐷𝑐 refers to the critical pipe diameter. The pipe flow remains laminar and stable for 𝑆>1.0, and becomes unstable for 𝑆<1. Various experimental findings are theoretically derived, and the laminar-to-turbulent transition is identified at 𝑆=1.0. Particular attention is paid to the dependence of flow transition on both pipe diameter and pipe length. Rather than relying on a critical Reynolds number 𝑅𝑒𝑐, this study proposes the critical pipe diameter 𝐷𝑐 as the key parameter governing the laminar pipe flow instability, where 𝑅𝑒𝑐 refers here to the condition-dependent threshold at which laminar pipe flow becomes unstable and transition to turbulence occurs. The present analysis further suggests that instability arises only if the pipe length L exceeds a critical threshold 𝐿𝑐, that is, 𝐿>𝐿𝑐. The theoretical treatment presented provides deeper physical insights into the onset of laminar pipe flow instability including the phenomenon of reverse transition. It also distinguishes between natural and forced flow transitions, providing a refined understanding of the transition process. Finally, suggestions for future experimental work are made to further validate or challenge this new theoretical perspective on pipe flow instability.}, language = {en} } @misc{ZanounBauerWagneretal., author = {Zanoun, El-Sayed and Bauer, Christian and Wagner, Claus and Durst, Franz and Egbers, Christoph and Bellani, Gabriele and Talamelli, Alessandro}, title = {Cross-validation of numerical and experimental data in turbulent pipe flow with new scaling correlations}, series = {Journal of turbulence}, journal = {Journal of turbulence}, publisher = {Taylor \& Francis}, address = {London}, doi = {10.1080/14685248.2025.2560314}, pages = {1 -- 22}, abstract = {The dependence of turbulence statistics and wall friction on Reynolds number in fully developed turbulent pipe flow remains a fundamental subject in fluid mechanics. This paper cross-validates experimental and numerical results, focusing on the scaling of turbulence statistics at the pipe centerline and across the inner-outer flow region. Pipe flow experiments were reviewed for friction Reynolds numbers 810≤Re𝜏≤55×103, where Re𝜏=𝑢𝜏⁢𝑅/𝜈, 𝑢𝜏 is the wall friction velocity, 𝑅 the pipe radius, and 𝜈 the kinematic viscosity. Complementary DNS data for 180≤Re𝜏≤2880 provide detailed insight into near-wall turbulence. A novel friction correlation, Re𝜏=0.048Re0.923 c is introduced, predicting pipe-wall friction across a wide range of Re𝑐 with accuracy better than ±2.06\%, where Re𝑐 is the Reynolds number based on the centerline streamwise mean velocity component 𝑈zc. This correlation enables reliable friction estimates from centerline single-point measurements or DNS data without requiring near-wall or streamwise pressure-gradient information and is validated by consistent agreement with both experiments and DNS. The monotonic decrease in centerline turbulence intensity ⟨𝑢′𝑧2⟩1/2/𝑈zc with increasing Re𝑐 is explained using the streamwise mean momentum equation. Finally, azimuthal spatial filtering of DNS data highlights the limitations of hot-wire resolution near the wall. For Re𝜏≥2880, higher-order experimental statistics agree well with DNS for 𝑦+≥30 and into the logarithmic region, with both datasets equally well described by logarithmic or power-law correlations, while near-wall discrepancies remain due to resolution limits.}, language = {en} }