Mahmoud Ibrahim

The dynamic response of electrostatically driven resonators under mechanical shock

This paper presents a theoretical and experimental investigation of the response of electrostatically actuated parallel-plate resonators when subjected to mechanical shock. Resonators are commonly employed in resonant sensors, where they are operated at low pressure for enhanced sensitivity making their response to external disturbances such as shock a critical issue. A single-degree-of-freedom system is used to model a resonator, which is electrostatically driven by a dc load superimposed to an ac harmonic load. Simulation results are demonstrated in a series of shock spectra that help indicate the combined influence of shock, dc and ac loads. The effect of the shock duration coinciding with the ac harmonic frequency is investigated. It is concluded that accounting for electrostatic forces, especially the ac load, is crucial when addressing the reliability and performance of resonators against shock. It is found that for specific shock and ac excitation conditions, a resonator may experience early dynamic pull-in instability. Experimental work has been conducted on a capacitive sensor to verify the obtained theoretical results. The sensor is mounted on top of a small shaker and then both are placed inside a vacuum chamber. Acceleration pulses were applied on the sensor while powered by dc and ac loads. The response of the device was monitored using a laser-Doppler vibrometer. The experimental data were compared to the theoretical results and were found to be in good agreement.

Quantitative comparison of air containment systems

Data center cooling energy efficiency is critical to successful operation of modern large data centers. The current trend of deploying high heat load density cabinets in data centers necessitates the use of air containment systems. According to a recent poll (by Gartner), about 80% of the data centers already have or are planning to deploy a containment system. There are primarily two types of air containment systems: hot air containment and cold air containment. This paper compares the performance and cooling energy costs of different types of air containment systems. It also includes the set of guidelines that should be considered when selecting one containment type over the other. In order to compare the performance of standard Hot Aisle/Cold Aisle (HA/CA) arrangement vs. Containment Systems we have selected a comprehensive data center layout. This layout contains a mix of equipment (Server, Networking, and Storage) which is characteristic of a typical data center facility. In this paper we will first determine the cooling needs of the equipment and establish the associated set point conditions for the CRAH (Computer Room Air Handlers) units, and then later consider different types of chilled water systems to do the comparative analysis.

A Study for the Effect of the PCB Motion on the Dynamics of MEMS Devices Under Mechanical Shock

We present a theoretical and experimental investigation into the effect of the motion of a printed circuit board (PCB) on the response of microelectromechanical systems (MEMS) devices to shock loading. For the theoretical part, a 2-DOF model is used, where the first degree of freedom accounts for the PCB. The second degree of freedom represents the motion of the MEMS microstructure. Low-g acceleration pulses are applied to the MEMS-PCB assembly base to simulate shock pulses generated from a drop-table test. Simulation data are presented to show the effects of the natural frequency of the PCB, the natural frequency of the microstructure, and the shock pulse duration. Universal 3-D spectra representing the effect of these parameters are presented. It is found that neglecting the PCB effect on the design of MEMS devices under shock loads can lead to undesirable motion of their microstructures. The effects of electrostatic force and squeeze film damping are investigated. It is found that the amplification of motion due to the PCB can cause early pull-in instability for MEMS devices implementing electrostatic forces. The effect of higher order modes of a microbeam is studied through a continuous beam model coupled with a lumped model of the PCB. The limitations of the 2-DOF model are discussed. An experimental investigation is conducted to verify the theoretical results using a capacitive accelerometer. Experimental data for the response of the accelerometer while it is mounted on two representative PCBs due to different low-g shock conditions are shown.

Numerical modeling approach to dynamic data center cooling

Data centers are facilities that house large numbers of computer servers that dissipate high power. Due to varying operational loads their efficient thermal management is a big challenge that needs to be addressed. Computational modeling using a CFD code is a very useful technique for studying the cooling requirements for different data center power loading and room configuration. Much of the existing numerical modeling research literature focuses on simulating the data center thermal environment with constant operating conditions. In reality, data centers have highly time dependent operating conditions i.e. fluctuations in server power and A/C flow rates. Recent computational studies have shown that time dependent fluctuations in server rack power can lead to rapid fluctuations in rack inlet temperatures. For optimal data center performance, the cold air supply should also increase or decrease with the rack power. In this paper, a detailed numerical study of data center thermal performance is presented with time dependent power and cooling air supply conditions. Results are presented for average rack inlet temperatures as a function of time for different case studies. Fluctuations in inlet temperatures are explained by evaluation of the temperature and flow fields in a basic data center configuration.

Thermal mass characterization of a server at different fan speeds

Dynamic cooling has been proposed as one approach for enhancing the energy efficiency of data center facilities. It involves using sensors to closely monitor the data center environment with time and making real time decisions on how to allocate the cooling resources based on the location of hotspots and concentration of workloads. In order to effectively implement this approach, it is good to know the transient thermal response of the various systems comprising the data center must be determined, which is a function of thermal mass. Not only is thermal mass important in dynamic cooling, it also plays a major role in the temperature rise of a data center during power failure. A previous study concentrated on characterizing the thermal mass of a 2 RU server by running the server at different powers and a fixed fan speed. The fixed fan speed corresponds to one specific heat transfer coefficient value. This study is a continuation to the previous work, where the server fan speed is varied to deduce the change in heat transfer coefficient at different airflow rates. As expected, the heat transfer coefficient increases as the server airflow rate increases. The average thermal mass value obtained for the 2 RU server in this study was 12 kJ/K. A method of adopting the compact model developed in this study into a Computational Fluid Dynamics (CFD) code is proposed to cut down on the computational time of transient analysis.

Effect of Transient Boundary Conditions and Detailed Thermal Modeling of Data Center Rooms

With the ever increasing heat loads dissipated by computer hardware, the ability to accurately characterize the cooling requirements in data centers is becoming crucial. Computational fluid dynamics modeling has proven in many cases to be adequate in providing a general understanding of the thermal environment in data centers. However, almost all analyses of data centers to this day are conducted in steady state. The effect of changes in hardware configurations and cooling resources on server rack inlet temperatures and airflow through servers has not been adequately investigated. This paper introduces transient modeling of data centers with changing power dissipations and computer room air-conditioning (CRAC) airflow rates. Transient modeling proves advantageous in monitoring temperature fluctuations due to airflow changes, which in some cases leads to insufficient cooling. In addition, modeling server thermal mass is shown to be important for transient analysis, as it can lead to overshoots in temperatures. Another segment of this paper looks at the effect of introducing thermal characteristics curves into CRAC modeling on server inlet temperature. All the cases presented show that fixed CRAC supply temperature is a non-valid assumption.

Numerical modeling of data center with transient boundary conditions

Data centers are the facilities that house large number of computer servers that dissipate high power. Considering the dynamics of the data centers, their efficient thermal management is a big challenge that needs to be addressed. Computational analyses using a CFD code is very useful technique that helps the engineer to understand and solve the data center cooling problem. Several ongoing numerical modeling research efforts focus on simulating data center environment with constant boundary conditions. In reality, data centers have highly time dependent boundary conditions i.e. fluctuations in server powers and CRAC flow rate. In this paper, effect on time dependent boundary conditions on rack inlet temperatures will be discussed in detail with the transient modeling of data centers. Case studies will be presented to illustrate the transient fluctuations of rack inlet air temperature by the variation of rack power and CRAC flow rate.

Open and Contained Cold Aisle Experimentally Validated CFD Model Implementing CRAC and Server Fan Curves for a Data Center Test Laboratory

This paper presents the results of an experimentally validated Computational Fluid Dynamics (CFD) model for a data center with fully implemented fan curves on both the servers and the Computer Room Air Conditioner (CRAC). Open and contained cold aisle systems are considered experimentally and numerically. This work is divided into open (uncontained) cold aisle system calibration and validation, and fully contained cold aisle system calibration and leakage characterization.In the open system, the CRAC unit is calibrated using the manufacturer fan curve. Tiles flow measurements are used to calibrate the floor leakage. The fan curves of the load banks are generated experimentally. A full physics based model of the system is validated with two different CRAC fan speeds. The results showed a very good agreement with the tile flow measurements, with an approximate average error of 5%, indicating that the average model prediction of the tile flow is five percent lower that the measured values.In the fully contained cold aisle system, a detailed containment CFD model based on experimental measurements is developed. The model is validated by comparing the flow rate through the perforated floor tiles with the experimental measurements. The CFD results are in a good agreement with the experimental data. The average error is about 6.7%. Temperature measurements are used to calibrate other sources of containment and racks leaks including mounting rails and clearance between racks. The temperature measurements and the CFD results agree well with average error less than 2%.Detailed and equivalent modeling methods for the floor and containment system are investigated. It is found that the simple equivalent models are able to predict the flow rates however they did not succeed in providing detailed fluid flow information. While the detailed models succeeded in explaining the physical phenomena and predicting the flow rates with noticeable tradeoffs regarding the computational time.Important conclusions can be drawn from this study. In order to accurately model the containment system, both the CRAC and the load banks fan curves should be simulated in the numerical model. Unavoidable racks and containment leaks could cause inlet temperature increase even if the cold aisle is overprovisioned with cold air. It is also noted that heat conduction through the floor tiles causes a slight increase the inlet temperature of the cold aisles. Finally, it is noteworthy that using detailed modeling is necessary to understand the details of the thermal systems, however simpler and faster to compute equivalent models can be used in extended optimization studies that show relative rankings of different designs.Copyright

Benefit of Cold Aisle Containment During Cooling Failure

During a cooling equipment failure, temperatures within a data center can rise to the threshold limits of IT equipment very quickly. Data center operators want to know: how much ride-through time do I have? Does cold aisle containment increase or decrease the ride-through time? This study presents thermal transient data for tests conducted at a data center lab. The results reveal an interesting phenomenon that puts cold aisle containment at an advantage over the standard open hot aisle/cold aisle configuration.Copyright

Effect of Thermal Characteristics of Electronic Enclosures on Dynamic Data Center Performance

Data centers are the facilities that house large number of computer servers that dissipate high power. Considering the dynamics of the data centers, their efficient thermal management is a big challenge that needs to be addressed. Computational analysis using a CFD code is very useful technique that helps the engineer to understand and solve the data center cooling problem. Several ongoing numerical modeling research efforts assume the computer room air conditioning (CRAC) units as fixed flow devices with constant temperature boundary condition. In reality, CRAC supply temperature is governed by the thermal characteristic curve, as specified by vendor. In this paper, study is presented by incorporating the CRAC thermal characteristic curve in the numerical model. Case studies are presented to show how the segregated high and low powered clusters in a data center may affect the supply temperatures from the CRAC in their vicinity. Another concern that is crucial in analyzing data centers performance precisely is the effect of buoyancy and thermal mass on the facility environment. In some cases, the effect of thermal mass and buoyancy may cause unexpected behaviors such as temperature overshoot or rapid variations in temperature. Non-dimensional parameters are used to demonstrate the effects of thermal mass and buoyancy.Copyright