Flow interference between two tripped cylinders

Alam, Md. Mahbub,Kim, Sangil,Maiti, Dilip Kumar Techno-Press 2016 Wind and Structures, An International Journal (WAS Vol.23 No.2

Flow interference is investigated between two tripped cylinders of identical diameter D at stagger angle ${\alpha}=0^{\circ}{\sim}180^{\circ}$ and gap spacing ratio $P^*$ (= P/D) = 0.1 ~ 5, where ${\alpha}$ is the angle between the freestream velocity and the line connecting the cylinder centers, and P is the gap width between the cylinders. Two tripwires, each of diameter 0.1D, were attached on each cylinder at azimuthal angle ${\beta}={\pm}30^{\circ}$, respectively. Time-mean drag coefficient ($C_D$) and fluctuating drag ($C_{Df}$) and lift ($C_{Lf}$) coefficients on the two tripped cylinders were measured and compared with those on plain cylinders. We also conducted surface pressure measurements to assimilate the fluid dynamics around the cylinders. $C_D$, $C_{Df}$ and $C_{Lf}$ all for the plain cylinders are strong function of ${\alpha}$ and $P^*$ due to strong mutual interference between the cylinders, connected to six interactions (Alam and Meyer 2011), namely boundary layer and cylinder, shear-layer/wake and cylinder, shear layer and shear layer, vortex and cylinder, vortex and shear layer, and vortex and vortex interactions. $C_D$, $C_{Df}$ and $C_{Lf}$ are very large for vortex and cylinder, vortex and shear layer, and vortex and vortex interactions, i.e., the interactions where vortex is involved. On the other hand, the interference as well as the strong interactions involving vortices is suppressed for the tripped cylinders, resulting in insignificant variations in $C_D$, $C_{Df}$ and $C_{Lf}$ with ${\alpha}$ and $P^*$. In most of the (${\alpha}$, $P^*$ ) region, the suppressions in $C_D$, $C_{Df}$ and $C_{Lf}$ are about 58%, 65% and 85%, respectively, with maximum suppressions 60%, 80% and 90%.

Flow interference between two tripped cylinders

Md. Mahbub Alam,김상일,Dilip Kumar Maiti 한국풍공학회 2016 Wind and Structures, An International Journal (WAS Vol.23 No.2

Flow interference is investigated between two tripped cylinders of identical diameter D at stagger angle α= 0° ~ 180° and gap spacing ratio P* (= P/D) = 0.1 ~ 5, where α is the angle between the freestream velocity and the line connecting the cylinder centers, and P is the gap width between the cylinders. Two tripwires, each of diameter 0.1D, were attached on each cylinder at azimuthal angle b= ±30°, respectively. Time-mean drag coefficient (CD) and fluctuating drag (CDf) and lift (CLf) coefficients on the two tripped cylinders were measured and compared with those on plain cylinders. We also conducted surface pressure measurements to assimilate the fluid dynamics around the cylinders. CD, CDf and CLf all for the plain cylinders are strong function of α and P* due to strong mutual interference between the cylinders, connected to six interactions (Alam and Meyer 2011), namely boundary layer and cylinder, shear-layer/wake and cylinder, shear layer and shear layer, vortex and cylinder, vortex and shear layer, and vortex and vortex interactions. CD, CDf and CLf are very large for vortex and cylinder, vortex and shear layer, and vortex and vortex interactions, i.e., the interactions where vortex is involved. On the other hand, the interference as well as the strong interactions involving vortices is suppressed for the tripped cylinders, resulting in insignificant variations in CD, CDf and CLf with α and P*. In most of the (α, P* ) region, the suppressions in CD, CDf and CLf are about 58%, 65% and 85%, respectively, with maximum suppressions 60%, 80% and 90%.

Fair Scheduling and Throughput Maximization for IEEE 802.16 Mesh Mode Broadband Wireless Access Networks

ALAM, Muhammad Mahbub,HAMID, Md. Abdul,RAZZAQUE, Md. Abdur,HONG, Choong Seon The Institute of Electronics, Information and Comm 2010 IEICE TRANSACTIONS ON COMMUNICATIONS - Vol.93 No.6

<P>Broadband wireless access networks are promising technology for providing better end user services. For such networks, designing a scheduling algorithm that fairly allocates the available bandwidth to the end users and maximizes the overall network throughput is a challenging task. In this paper, we develop a centralized fair scheduling algorithm for IEEE 802.16 mesh networks that exploits the <I>spatio-temporal bandwidth reuse</I> to further enhance the network throughput. The proposed mechanism reduces the length of a transmission round by increasing the number of non-contending links that can be scheduled simultaneously. We also propose a greedy algorithm that runs in polynomial time. Performance of the proposed algorithms is evaluated by extensive simulations. Results show that our algorithms achieve higher throughput than that of the existing ones and reduce the computational complexity.</P>

Variety of Nanopatterns on Different Substrates Using PS-b-PMMA and Their Applications

Alam, Md. Mahbub,Lee, Yu-Rim,Kim, Jin-Yeol,Jung, Woo-Gwang American Scientific Publishers 2012 Journal of Nanoscience and Nanotechnology Vol.12 No.2

eMCCA: An Enhanced Mesh Coordinated Channel Access Mechanism for IEEE 802.11s Wireless Mesh Networks

Islam, Md. Shariful,Alam, Muhammad Mahbub,Hong, Choong-Seon,Lee, Sung-Won The Korea Institute of Information and Commucation 2011 Journal of communications and networks Vol.13 No.6

In this paper, we present a channel access mechanism, referred to as the enhanced mesh coordinated channel access (eMCCA) mechanism, for IEEE 802.11s-based wireless mesh networks. The current draft of IEEE 802.11s includes an optional medium access control (MAC), denoted as MCCA, which is designed to provide collision-free and guaranteed channel access during reserved periods. However, the MCCA mechanism fails to achieve the desired goal in the presence of contending non-MCCA nodes; this is because non-MCCA nodes are not aware of MCCA reservations and have equal access opportunities during reserved periods. We first present a probabilistic analysis that reveals the extent to which the performance of MCCA may be affected by contending non-MCCA nodes. We then propose eMCCA, which allows MCCA-enabled nodes to enjoy collision-free and guaranteed channel access during reserved periods by means of prioritized and preemptive access mechanisms. Finally, we evaluate the performance of eMCCA through extensive simulations under different network scenarios. The simulation results indicate that eMCCA outperforms other mechanisms in terms of success rate, network throughput, end-to-end delay, packet-loss rate, and mesh coordinated channel access opportunity-utilization.

Fair data collection in wireless sensor networks: analysis and protocol

Hamid, Md. Abdul,Alam, Muhammad Mahbub,Islam, Md. Shariful,Hong, Choong Seon,Lee, Sungwon Springer-Verlag 2010 Annales des télécommunications Vol.65 No.7

Aerodynamics of a cylinder in the wake of a V-shaped object

김상일,Md. Mahbub Alam,Mohammad Russel 한국풍공학회 2016 Wind and Structures, An International Journal (WAS Vol.23 No.2

The interaction between two different shaped structures is very important to be understood. Fluid-structure interactions and aerodynamics of a circular cylinder in the wake of a V-shaped cylinder are examined experimentally, including forces, shedding frequencies, lock-in process, etc., with the V-shaped cylinder width d varying from d/D = 0.6 to 2, where D is the circular cylinder diameter. While the streamwise separation between the circular cylinder and V-shaped cylinder was 10D fixed, the transverse distance T between them was varied from T/D = 0 to 1.5. While fluid force and shedding frequency of the circular cylinder were measured using a load cell installed in the circular cylinder, measurement of shedding frequency of the V-shaped cylinder was done by a hotwire. The major findings are: (i) a larger d begets a larger velocity deficit in the wake; (ii) with increase in d/D, the lock-in between the shedding from the two cylinders is centered at d/D = 1.1, occurring at d/D 0.95-1.35 depending on T/D; (iii) at a given T/D, when d/D is increased, the fluctuating lift grows and reaches a maximum before decaying; the d/D corresponding to the maximum fluctuating lift is dependent on T/D, and the relationship between them is linear, expressed as d/D=1.2+1/e T/D; that is, a larger d/D corresponds to a greater T/D for the maximum fluctuating lift.

EFT: a high throughput routing metric for IEEE 802.11s wireless mesh networks

Islam, Md. Shariful,Alam, Muhammad Mahbub,Hamid, Md. Abdul,Hong, Choong Seon,Lee, Sungwon Springer-Verlag 2010 Annales des télécommunications Vol.65 No.5

A review of wind-turbine structural stability, failure and alleviation

Shafiqur Rehman,Md. Mahbub Alam,Luai M. Alhems 한국풍공학회 2020 Wind and Structures, An International Journal (WAS Vol.30 No.5

Advancements in materialistic life styles and increasing awareness about adverse climatic changes and its negative effects on human life have been the driving force of finding new and clean sources of energy. Wind power has become technologically mature and commercially acceptable on global scale. However, fossil fuels have been the major sources of energy in most countries, renewable energy (particularly wind) is now booming worldwide. To cope with this wind energy technology, various related aspects have to be understood by the scientific, engineering, utility, and contracting communities. This study is an effort towards the understanding of the (i) wind turbine blade and tower structural stability issues, (ii) turbine blade and tower failures and remedial measures, (iii) weather and seismic effects on turbine blade and tower failures, (iv) gear box failures, and (v) turbine blade and tower failure analysis tools.

The effect of Reynolds number on the elliptical cylinder wake

Xiaoyu Shi,Md. Mahbub Alam,Honglei Bai,Hanfeng Wang 한국풍공학회 2020 Wind and Structures, An International Journal (WAS Vol.30 No.5

This work numerically investigates the effects of Reynolds number ReD (= 100 - 150), cross-sectional aspect ratio AR = ( 0.25 -1.0), and attack angle α (= 0 - 90) on the forces, Strouhal number, and wake of an elliptical cylinder, where ReD is based on the freestream velocity and cylinder cross-section height normal to the freestream flow, AR is the ratio of the minor axis to the major axis of the elliptical cylinder, and α is the angle between the cylinder major axis and the incoming flow. At ReD = 100, two distinct wake structures are identified, namely ‘Steady wake’ (pattern I) and ‘Karman wake followed by a steady wake (pattern II)’ when AR and α are varied in the ranges specified. When ReD is increased to 150, an additional wake pattern, ‘Karman wake followed by secondary wake (pattern III)’ materializes. Pattern I is characterized by two steady bubbles forming behind the cylinder. Pattern II features Karman vortex street immediately behind the cylinder, with the vortex street transmuting to two steady shear layers downstream. Inflection angle αi = 32, 37.5 and 45 are identified for AR = 0.25, 0.5 and 0.75, respectively, where the wake asymmetry is the greatest. The αi effectively distinguishes the dependence on α and AR of force and vortex shedding frequency at either ReD. In Pattern III, the Karman street forming behind the cylinder is modified to a secondary vortex street. At a given AR and α, ReD = 150 renders higher fluctuating lift and Strouhal number than ReD = 100.