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Industrial Aerodynamics. Mr.B.Navin Kumar Senior Lecturer Rajalakshmi Engineering College Thandalam-602 105. Unit - I. ATMOSPHERIC BOUNDARY LAYER. Atmospheric circulation.
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Industrial Aerodynamics Mr.B.Navin Kumar Senior Lecturer Rajalakshmi Engineering College Thandalam-602 105
Unit - I ATMOSPHERIC BOUNDARY LAYER
Atmospheric circulation • Atmospheric circulation is the large-scale movement of air, and the means (together with the smaller ocean circulation) by which thermal energy is distributed on the surface of the Earth. • The large-scale structure of the atmospheric circulation varies from year to year, but the basic climatological structure remains fairly constant. However, individual weather systems - mid-latitude depressions, or tropical convective cells - occur "randomly and it is accepted that weather cannot be predicted beyond a fairly short limit: perhaps a month in theory, or (currently) about ten days in practice (see Chaos theory and Butterfly effect). Nonetheless, as the climate is the average of these systems and patterns - where and when they tend to occur again and again -, it is stable over longer periods of time. • As a rule, the "cells" of Earth's atmosphere shift polewards in warmer climates (e.g. interglacials compared to glacial), but remain largely constant even due to continental drift. Tectonic uplift can significantly alter major elements of it, however - for example the jet stream -, and plate tectonics shift ocean currents. In the extremely hot climates of the Mesozoic, indications of a third desert belt at the Equator has been found; it was perhaps caused by convection. But even then, the overall latitudinal pattern of Earth's climate was not much different from the one today.
Local winds Local winds blow over a much smaller area than global winds and have a much shorter time span. Hot windsoriginate in vast anticyclones over hot deserts and include the Santa Ana (California), the Brickfielder (south-east Australia), the Sirocco (Mediterranean), the Haboob (Sudan), the Khamsin (Egypt), and the Harmattan (West Africa). • The causes of local winds are because of tempreture and air pressure. The greater one of these are, the stronger the winds are. A local wind is a little zephyr that one can get occasionally. A global wind would be the jet streams.
mean velocity distribution • The mean velocity distribution in a low-speed three-dimensional turbulent boundary-layer flow was investigated experimentally. The experiments were performed on a large-scale model which consisted of a flat plate on which secondary flow was generated by the pressure field introduced by a circular cylinder standing on the plate. The Reynolds number based on distance from the leading edge of the plate was about 6 x 106. • It was found that the wall-wake model of Coles does not apply for flow of this kind and the model breaks down in the case of conically divergent flow with rising pressure, for example, in the results of Kehl (1943). The triangular model for the yawed turbulent boundary layer proposed by Johnston (1960) was confirmed with good correlation. However, the value of yuτ/v which occurs at the vertex of the triangle was found to range up to 150 whereas Johnston gives the highest value as about 16 and hence assumes that the peak lies within the viscous sublayer. Much of his analysis is based on this assumption. • The dimensionless velocity-defect profile was found to lie in a fairly narrow band when plotted against y/δ for a wide variation of other parameters including the pressure gradient. The law of the wall was found to apply in the same form as for two-dimensional flow but for a more limited range of y.
Power law and logarithm law • Employing the Shannon entropy, this note derives the well-known power law and the Prandt–von Kármán universal (or logarithmic) velocity distribution equations for open channel flow. The Shannon entropy yields probability distributions underlying these velocity equations. With the use of this entropy, one obtains an expression for the power law exponent in physically measurable quantities (surface flow velocity and average logarithmic velocity) and an expression for shear flow depth in flow depth, surface flow velocity and shear velocity, thus obviating the need for fitting.
wind speeds-Turbulence • Atmospheric turbulence causes vertical mixing of the atmosphere. it can be caused by convection (heating at the surface causing warm air parcels to lift) or by friction with objects on the earth's surface and shear stress between layers of the atmosphere moving at different wind speeds.Since the atmosphere is mixing, the temperature will be more or less constant with height (or at least closer to constant than it would be without turbulence). not sure what to tell you for wind speed, but in general the wind speed increases logarithmically with height due to friction with the earth's surface. Shear between layers with different wind speeds in the atmosphere causes turbulence
Roughness Parameters • Ra: Ra is the arithmetic average of the absolute values of the roughness profile ordinates. Also known as Arithmetic Average (AA), Center Line Average (CLA). The average roughness is the area between the roughness profile and its mean line, or the integral of the absolute value of the roughness profile height over the evaluation length • Rz: Rz is the arithmetic mean value of the single roughness depths of consecutive sampling lengths. Z is the sum of the height of the highest peaks and the lowest valley depth within a sampling length. • Cutoff λc: of a profile filter determines which wavelengths belong to roughness and which ones to waviness. • Sampling Length: is the reference for roughness evaluation. Its length is equal to the cutoff wavelength. • Traversing Length: is the overall length traveled by the stylus when acquiring the traced profile. It is the total of Pre-travel, evaluation length and post travel • Evaluation Length: is the part of the traversing length from where the values of the surface parameters are determined. • Pre-Travel: the first part of the traversing length. • Post-Travel: The last part of the traversing length
Aerodynamicists use wind tunnels to test models of proposed aircraft and engine components. During a test, the model is placed in the test section of the tunnel and air is made to flow past the model. Various types of instrumentation are used to determine the forces on the model. There are four main types of wind tunnel tests. • In some wind tunnel tests, the aerodynamic forces and moments on the model are measured directly. The model is mounted in the tunnel on a special machine called a force balance. The output from the balance is a signal that is related to the forces and moments on the model. Balances can be used to measure both the lift and drag forces. The balance must be calibrated against a known value of the force before, and sometimes during, the test. Force measurements usually require some data reduction or post-test processing to account for Reynolds number or Mach number effects on the model during testing. It is very important in data reports to always specify the reference value of variables used in data reduction. • In some wind tunnel tests, the model is instrumented with pressure taps and the component performance is calculated from the pressure data. Total pressure measurement is the normal procedure for determining aircraft inlet performance. Theoretically, the aerodynamic force on an aircraft model could be obtained using pressure instrumentation by integrating the pressure times an incremental area around the entire surface of the model. But, in practice, pressure integration is not used because of the large number of taps necessary to accurately resolve pressure variations. Airfoil drag can be determined by integrating the total pressure deficit in the wake created by a wing model. • In some wind tunnel tests, the model is instrumented to provide diagnostic information about the flow of air around the model. Diagnostic instrumentation includes static pressure taps, total pressure rakes, laser Doppler velocimetry, and hot-wire velocity probes. A diagnostic test does not provide overall aircraft performance, but helps the engineer to better understand how the fluid moves around and through the model. There are a variety of flow control devices that are employed to improve performance of the aircraft, if the local flow conditions are known. Depending on the type of instrumentation used in the experiment, steady state flow or unsteady, time-varying, flow information can be obtained. The engineer must use some experience when employing flow diagnostic instrumentation to properly place the instruments in regions of flow gradients or separations. • In some wind tunnel tests, flow visualization techniques are used to provide diagnostic information. Visualizaation techniques include free stream smoke, laser sheet, or surface oil flow. The assumption is made that the flow visualization medium moves exactly with the flow. Shadowgraphs or schlierin systems are used to visualize the shape and location of shock waves in compressible flows. For low speed flows, tufts or surface oil indicate the flow direction along the surface of a model.
Unit - II BLUFF BODY AERODYNAMICS
Boundary layers and separation- • In most situations it is inevitable that the boundary layer becomes detached from a solid body. This boundary layer separation results in a large increase in the drag on the body. We can understand this by returning to the flow of a nonviscous fluid around a cylinder. The pressure distribution is the same on the downstream side of the cylinder as on the upstream side; thus, there were no unbalanced forces on the cylinder and therefore no drag (d'Alembert's paradox again). If the flow of a viscous fluid about a body is such that the boundary layer remains attached, then we have almost the same result--we'll just have a small drag due to the skin friction. However, if the boundary layer separates from the cylinder, then the pressure on the downstream side of the cylinder is essentially constant, and equal to the low pressure on the top and bottom points of the cylinder. This pressure is much lower than the large pressure which occurs at the stagnation point on the upstream side of the cylinder, leading to a pressure imbalance and a large pressure drag on the cylinder. For instance, for a cylinder in a flow with a Reynolds number in the range , the boundary layer separates and the coefficient of drag is , much larger that the coefficient of drag due to skin friction, which we would estimate to be about .
Two dimensional wake and vortex formation- • A cylinder having mild variations in diameter along its span is subjected to controlled excitation at frequencies above and below the inherent shedding frequency from the corresponding two-dimensional cylinder. The response of the near wake is characterized in terms of timeline visualization and velocity traces, spectra, and phase plane representations. It is possible to generate several types of vortex formation, depending upon the excitation frequency. Globally locked-in, three-dimensional vortex formation can occur along the entire span of the flow. Regions of locally locked-in and period-doubled vortex formation can exist along different portions of the span provided the excitation frequency is properly tuned. Unlike the classical subharmonic instability in free shear flows, the occurrence of period-doubled vortex formation does not involve vortex coalescence; instead, the flow structure alternates between two different states. • An experimental study of the flow around a cylinder with a single straight perturbation was conducted in a wind tunnel. With this bluff body, positioned in a uniform crossflow, the vortex shedding frequency and other flow characteristics could be manipulated. The Strouhal number has been shown to be a function of the perturbation angular position, theta _{rm p}, as well as the perturbation size and Reynolds number. As much as a 50% change in Strouhal number could be achieved, simply by changing theta _{rm p} by 1^ circ. The perturbation size compared to the boundary-layer thickness, delta, was varied from approximately 1delta to about 20delta. The Reynolds number was varied from 10,000 to 40,000. A detailed investigation of the characteristic Strouhal number variation has shown that varying theta_{rm p} had a significant influence on the boundary -layer separation and transition to turbulence.
The Strouhal number St is a function of the Reynolds number Re (although a sufficiently varying one that it may be said that it is typically equal to 0.2, e.g. see figure below) and is proportional to the reciprocal of vortex spacing expressed as a number of obstacle diameter. It is used in the momentum transfer in general, and in both Von Karmann vortex streets and unsteady flow calculations in particular. It is normally defined in the following form : where : - n is the frequency of the observed phenomenon, - d is the characteristic length (which is the diameter of the cylinder in the case of vortex streets), - U is the velocity of the fluid.
The number of cars available on our planet is continuously increasing. But also other factors are important for the emissions and the energy consumption: How efficient is the motor of the car? How much fuel does it consume on a certain distance? How long are the distances the car owner goes per year in average? What driving style does the driver exhibit? What is the average speed? • The energy needed for optimal consumption of a car is not easy to calculate. The most simple physical equation for the description of accelerating a body of the mass m from the velocity v = 0 to the velocity v is the kinetic energy equation: • The energy required to accelerate a body increases with the mass of the body and with the square of the velocity gained. We can deduce this formula from four simple physical expressions: • a) work / energy = force × distance (E = F × s)b) force = mass × acceleration (F = m × a)c) acceleration = change of velocity with time (a = dv/dt)d) velocity = change of distance with time (v = ds/dt) • If we change the velocity from 0 to v the energy invested over the distance s is given according to: • The SI unit of an energy is 1 Joule [J] according to: • If the velocity of a mass is increasing continuously in time, which means 1 m/s after 1 second, 2 m/s after 2 seconds, we have a constant acceleration of 1 meter per second squared. The force needed for this acceleration depends on the mass itself. If the mass is 1 kg the required force is 1 Newton (N) = 1 kg × 1 m/s2. The total energy required or the work we have to carry out depends on over which distance we exacerbate the accelerating force. If it is one meter, the energy is 1 kg m2/s2 = 1 Joule.
Train Aerodynamics • Railway train aerodynamic problems are closely associated with the flows occurring around train. Much effort to speed up the train system has to date been paid on the improvement of electric motor power rather than understanding the flow around the train. This has led to larger energy losses and performance deterioration of the train system, since the flows around train are more disturbed due to turbulence of the increased speed of the train, and consequently the flow energies are converted to aerodynamic drag, noise and vibrations. With the speed-up of train, many engineering problems which have been neglected at low train speeds, are being raised with regard to aerodynamic noise and vibrations, impulse forces occurring as two trains intersect each other, impulse wave at the exit of tunnel, ear discomfort of passengers inside train, etc. These are of major limitation factors to the speed-up of train system. The present review addresses the state of the art on the aerodynamic and aero acoustic problems of high-speed railway train and highlights proper control strategies to alleviate undesirable aerodynamic problems of high-speed railway train system.
Unit - III WIND ENERGY COLLECTORS
Horizontal Axis Wind Turbines Most of the technology described on these pages is related to horizontal axis wind turbines (HAWTs, as some people like to call them). The reason is simple: All grid-connected commercial wind turbines today are built with a propeller-type rotor on a horizontal axis (i.e. a horizontal main shaft). The purpose of the rotor, of course, is to convert the linear motion of the wind into rotational energy that can be used to drive a generator. The same basic principle is used in a modern water turbine, where the flow of water is parallel to the rotational axis of the turbine blades. Vertical Axis Wind Turbines Eole C, a 4200 kW Vertical axis Darrieus wind turbine with 100 m rotor diameter at Cap Chat, Québec, Canada. The machine (which is the world's largest wind turbine) is no longer operational. Vertical axis wind turbines (VAWTs as some people call them) are a bit like water wheels in that sense. (Some vertical axis turbine types could actually work with a horizontal axis as well, but they would hardly be able to beat the efficiency of a propeller-type turbine). The only vertical axis turbine which has ever been manufactured commercially at any volume is the Darrieus machine, named after the French engineer Georges Darrieus who patented the design in 1931. (It was manufactured by the U.S. company FloWind which went bankrupt in 1997). The Darrieus machine is characterised by its C-shaped rotor blades which make it look a bit like an eggbeater. It is normally built with two or three blades
The Power Coefficient Very simply, we just divide the electrical power output by the wind energy input to measure how technically efficient a wind turbine is. In other words, we take the power curve , and divide it by the area of the rotor to get the power output per square metre of rotor area. For each wind speed, we then divide the result by the amount of power in the wind per square metre.
The graph shows a power coefficient curve for a typical Danish wind turbine. Although the average efficiency for these turbines is somewhat above 20 per cent, the efficiency varies very much with the wind speed. (If there are small kinks in the curve, they are usually due to measurement errors). • As you can see, the mechanical efficiency of the turbine is largest (in this case 44 per cent) at a wind speed around some 9 m/s. This is a deliberate choice by the engineers who designed the turbine. At low wind speeds efficiency is not so important, because there is not much energy to harvest. At high wind speeds the turbine must waste any excess energy above what the generator was designed for. Efficiency therefore matters most in the region of wind speeds where most of the energy is to be found.
Betz Coefficient • Betz was able to develop an expression for Cp in terms of the induction factors. This is done by the velocity relations being substituted into power and power is substituted into the coefficient of power definition. The relationship Betz developed is given below: • Cp = 4a(1 − a)2 • The Betz limit is defined by the maximum value that can be given by the above formula. This is found by taking the derivative with respect to the axial induction factor, setting it to zero and solving for the axial induction factor. Betz was able to show that the optimum axial induction factor is one third. The optimum axial induction factor was then used to find the maximum coefficient of power. This maximum coefficient is the Betz limit. Betz was able to show that the maximum coefficient of power of a wind turbine is 16/27. Airflow operating at higher thrust will cause the axial induction factor to rise above the optimum value. Higher thrust cause more air to be deflected away from the turbine. When the axial induction factor falls below the optimum value the wind turbine is not extracting all the energy it can. This reduces pressure around the turbine and allows more air to pass through the turbine, but not enough to account for lack of energy being extracted. • The derivation of the Betz limit shows a simple analysis of wind turbine aerodynamics. In reality there is a lot more. A more rigorous analysis would include wake rotation, the effect of variable geometry. The effect of air foils on the flow is a major component of wind turbine aerodynamics. Within airfoils alone, the wind turbine aerodynamicist has to consider the effect of surface roughness, dynamic stall tip losses, solidity, among other problems.
Unit IV BUILDING AERODYNAMICS
Pressure distribution on low rise buildings • the conditionally sampled actual wind pressure distributions causing maximum quasi-static wind load effects at the base of low-rise building models with square and rectangular plans. The maximum normal stresses in column members were also examined to discuss the wind load combination of the along-wind, across-wind, uplift, and three moments. Then, it examines the actual wind pressure distributions causing the maximum quasi-static stresses in structural frames. These are compared with Kasperski’s load-response-correlation formula and the quasi-steady pressure distributions.
Wind loads on buildings • The design of buildings must account for wind loads, and these are affected by wind shear. For engineering purposes, a power law wind speed profile may be defined as follows: • Vz = speed of the wind at height ;Vg = gradient wind at gradient height = exponential coefficient Typically, buildings are designed to resist a strong wind with a very long return period, such as 50 years or more. The design wind speed is determined from historical records using Extreme value theory to predict future extreme wind speeds.
Building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the construction and occupancy of buildings and structures. The building code becomes law of a particular jurisdiction when formally enacted by the appropriate authority. • Building codes are generally intended to be applied by architects and engineers although this is not the case in the UK where Building Control Surveyors act as verifiers both in the public and private sector (Approved Inspectors), but are also used for various purposes by safety inspectors, environmental scientists, real estate developers, contractors and subcontractors, manufacturers of building products and materials, insurance companies, facility managers, tenants, and others. • There are often additional codes or sections of the same building code that have more specific requirements that apply to dwellings and special construction objects such as canopies, signs, pedestrian walkways, parking lots, and radio and television antennas.
Ventilating(the V in HVAC) is the process of "changing" or replacing air in any space to provide high indoor air quality (i.e. to control temperature, replenish oxygen, or remove moisture, odors, smoke, heat, dust, airborne bacteria, and carbon dioxide). Ventilation is used to remove unpleasant smells and excessive moisture, introduce outside air, to keep interior building air circulating, and to prevent stagnation of the interior air. • Ventilation includes both the exchange of air to the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings. Methods for ventilating a building may be divided into mechanical/forced and natural types.[ • "Mechanical" or "forced" ventilation is used to control indoor air quality. Excess humidity, odors, and contaminants can often be controlled via dilution or replacement with outside air. However, in humid climates much energy is required to remove excess moisture from ventilation air. • Natural ventilation is the ventilation of a building with outside air without the use of a fan or other mechanical system. It can be achieved with openable windows or trickle vents when the spaces to ventilate are small and the architecture permits. In more complex systems warm air in the building can be allowed to rise and flow out upper openings to the outside (stack effect) thus forcing cool outside air to be drawn into the building naturally through openings in the lower areas. These systems use very little energy but care must be taken to ensure the occupants' comfort. In warm or humid months, in many climates, maintaining thermal comfort solely via natural ventilation may not be possible so conventional air conditioning systems are used as backups. Air-side economizers perform the same function as natural ventilation, but use mechanical systems' fans, ducts, dampers, and control systems to introduce and distribute cool outdoor air when appropriate.
