Al Khalid Othman

TCP performance measurement in different GPRS network scenarios

This paper presents the results of measured TCP performance in one of the commercially deployed live GPRS network in Malaysia under various network scenarios based on userspsila physical locations. End-to-end evaluation of FTP application is used for the assessment. Tracing at the GPRS air interface is done simultaneously, using a proprietary GSM/GPRS air interface testing tool and software. The results show that TCP performance is stable in all scenarios and can adapt fairly well to GPRS mobility and bad channel condition, although at the expense of reduced throughput. TCP imposes its slow start/congestion avoidance mechanism in such situations where packet loss is common. Tuning the TCP parameters to optimise its performance also proves to be beneficial.

End Suction Centrifugal Pump Operating in Turbine Mode for Microhydro Applications

This paper reviews the current research works on the end suction centrifugal pump coupled with induction generator running in turbine mode for microhydro application. The information can be used by practicing engineers, researchers, and plant managers to understand the potential of pump running as turbine. Review on experimental and simulation works was carried out encompassing end suction single stage low specific speed which is less than 10 kW. This is followed by review of their efficiency improvement through modifications. The results show that centrifugal pump can operate in turbine modes without any modification on mechanical components. However, to achieve the best efficiency point (BEP), it requires higher flow rate and head than pumping rating. Efficiency improvement is viable through geometric modification to improve hydraulic characteristic. The studies also show that pump as turbine (PAT) can be directly coupled with modified induction motor as generator by adding capacitor and electric control system, regulating voltage and frequency at the output terminal. It was found that PAT offers the best low cost solution for microhydro application especially for third world countries that do not have local microhydro manufacturer.

Numerical Simulation on End Suction Centrifugal Pump Running in Inverse Flow for Microhydro Applications

The initial capital cost for most microhydro projects has always been an overriding issue for self-funded remote communities. The cost will escalate significantly in the absence of local microhydro electromechanical manufacturers. The application of end suction centrifugal pump as turbine will reduce the overall cost, which renders microhydro systems feasible for self-funded projects and are therefore suitable for rural communities. The goal of this study is to design and develop a pump as turbine (PAT) which serves as a substitute to commercial electromechanical components. Numerical analysis of an inverse flow for an end suction centrifugal pump is presented in this paper, which includes the performance curves and hydraulic characteristics of the pump. ANSYS CFX, a commercial CFD software is used to simulate the performance of the pump with specific speed, Ns of 70 units (Euroflo EU50-20). The computational flow domain inside the pump is comprises of impeller, volute and draft tube. Unstructured tetrahedral mesh is used to maintain good surface mesh due to complex flow domain geometries. The governing equations used in the simulations are three-dimensional, incompressible Navier-Stokes and k-ϵ turbulence model under steady-state condition. The simulation results are compared with pump performance curve supplied by the pump manufacturer. The verification results show good agreement for flow rates between 0.7 and 1.3 QBEP. The best efficient point (BEP) for inverse flow is attained at a higher head and flow rate compared to pump mode, whereby the value is found to be 21.55 m and 14.0 l/s, respectively. It is believed that the findings of this study will be useful to predict hydraulic characteristics and performance curves of PAT and the model may be used to identify poor flow characteristics inside the pump. It is recommended that optimization process is carried out using CFD tools in future studies.

A simple and cost effective data acquisition system for mobile satellite experimental work under tree-shadowed and building-shadowed

Signal degradation in Mobile Satellite (MS) is due to several factors such as ionospheric effect, multipath fading, tree-shadowed and building-shadowed. This affects the Mobile Satellite Service (MSS) to provide continuous communication service to the users. Since there is not adequate data available on the signal performance of the MS in the Asian continent, thus, an experimental work is essential to be carried out to investigate the effect of factors such as the tree-shadowed and building-shadowed on the signal strength. This paper discusses the cost effective set up. It can be used to measure the effect of trees and buildings on the received mobile satellite signal operating in the L-band. The methodology uses to carry up the experimental works is discussed in detail. The measurements of the signal performance are performed under different MS environments such as tree-shadowed and building-shadowed and hence the results are compared to the data obtained for open space environment. The analysis of the signal performance under different MS environments is performed with respect to the elevation and azimuth degree at different time under a clear sky condition. The analysis produced forms an important part in the studies of the signal performance. From this research work, we would like to propose a simple and low cost data acquisition system in order to encourage more experimental works to be carried out especially in the less developed countries.

Open space experimental work for L-band mobile satellite (MS) using a simple and low cost data acquisition system

Mobile satellite (MS) signal performance is affected by many factors such as ionospheric effect, multipath fading and shadowing effect. Satellites transmit signals at the same strength, but things like trees and buildings can obscure a signal beyond recognition. Some experimental works have been done using the existing geo-satellite system or airborne platform in the developed countries which is quite expensive and complicated. Very little data is represents less developed countries such as Africa and Asia. Therefore, a simple and low cost data acquisition system that can be used to carry out measurement of the MS signal performance is proposed. The experiment is carried out in open area with no obstruction of the direct wave. In addition, NMEA sentences will be used to obtain the required signal propagation parameters. The measurement is then performed to determine the relationship between the signal performance with respect to the elevation and azimuth angles. Results obtained from analysis will be used to determine the characteristics of the open space environment. This method can be used to encourage more experimental works in the less develop countries. Propagation data for the open space environment can be used as a reference to determine the MS signal quality for shadowing environments.

Numerical Investigation of Rotational Speed on Pump as Turbine for Microhydro Applications

Pump as Turbine (PAT) always has been a favourable solution to generate electricity in rural areas when there is a potential microhydro site. Such systems have lower capital cost, and they are easier to maintain than commercially-available microhydro turbines. Normally, PAT is designed to run at a rated rotational speed so it can directly couple with an induction generator in order to match the synchronous speed. In an actual scenario, the PAT’s rotational speed changes and fluctuates with respect to flow rate due to the absence of a hydraulic control mechanism. It is essential to understand how the PAT behaves under different rotational speeds in order to design good microhydro systems. The aim of this study was to conduct simulation analysis of the effect of rotational speed on PAT’s performance curve over a range of flow rates. ANSYS CFX software was used as the Computational Fluid Dynamic (CFD) simulation tool in this study. Three distinct flow domains was modelled by Computer Aided Design (CAD) software and assembled as the computational fluid domains. Mesh independence analysis and convergence criteria were set to ensure the accuracy of the model. The torque generated by the impeller was collected from the simulation data and presented in the PAT performance curve. It was observed that the pressure head and torque generated increased at higher rotational speeds, thus maintaining the efficiency value. The results showed that the efficiency of the PAT was maintained around 76.5% for rotational speeds between 1350 to 1650 RPM, but the best efficiency point shifted to lower flow rate for lower rotational speed. The outcomes of this study will be useful for turbomachinery researchers, microhydro users, and project engineers for predicting the PAT performance for designing microhydro systems.

TCP performance analysis in dynamic GPRS LA

This paper presents the findings on TCP performance measurements in the LA process during the deployment of GPRS CS1 and CS2 coding schemes and after the activation of two more coding schemes, CS3 and CS4. The measurements are done under various network scenarios based on userspsila physical locations in one of Malaysiapsilas commercially deployed live GPRS networks. End-to-end evaluation of FTP application is used for the assessment together with tracing at the GPRS air interface. The results show that TCP works well in the LA process and can cope with the frequent switching between the coding schemes without any problem. Throughput performance is observed to be stable in all scenarios. The activation of higher coding schemes aided by TCP tuning also proves to be beneficial by the 23% increase in average throughput seen for urban areas.