PowerPoint Presentation

Measurements in Fluid Mechanics058:180 (ME:5180)Time & Location: 2:30P - 3:20P MWF 3315 SCOffice Hours: 4:00P 5:00P MWF 223B-5 HL

Instructor: Lichuan Guilichuan-gui@uiowa.eduPhone: 319-384-0594 (Lab), 319-400-5985 (Cell) http://lcgui.net

2Lecture 4. Measurement systems and static response3Measuring systems and their componentsEssential systems in fluid mechanics experimentflowing fluids, flow-producing apparatus, test models etc. 1. Physical system:2. Measuring system:3. Experimenter(s):sensors, electric and electronic circuits, data acquisition and processing devices, and software person(s) who plans, executes, and interprets the measurements

Response of measuring system - relationship between values of an input and an output Inputs of measuring system 1. Desired inputs2. Undesirable inputsa. interfering inputs b. modifying inputs - add noise to desired inputs- change response to desired inputsExample: hot-wire anemometer used to measure air jet flow from a nozzle to lab room - The draft of air produced by ventilation system in lab acts as interfering inputs. - The room temperature change acts as modifying inputs. 4Filtering, compensation, and output correctionFilters- used to reduce or eliminate undesirable input effects- no-pass filters remove all fluctuations, permitting only a steady component- low-pass filters remove fluctuations with frequencies above a cut-off value- high-pass filters remove fluctuations with frequencies below a cut-off value- band-pass filters remove all fluctuations except those with frequencies within a certain band- band-reject filters remove all fluctuations with frequencies within a certain bandFilters classified according to frequency range: Filters classified in terms of physical operation - electrical-electronic filters, applied to electric signals;- mechanical filters, designed to filter motion or force fluctuations, e.g. shock absorbers used to reduce vibration of an apparatus; - thermal filters, designed to remove temperature fluctuations, e.g. thermal insulation- electromagnetic filters, designed to remove the interfering effects of electric and magnetic fields- digital filters, applied to recorded signals4

Rs1Measuring gaugeV0Filtering, compensation, and output correctionCompensation - introduce additional interfering or modifying inputs to partly or entirely cancel the original undesirable effects.Example:- sensitive to ambient-temperature fluctuations

Bonded strain gauge

V0Rs1Measuring gaugeRs2Reference gaugeAnalytical correction - remove undesirable effects & errors from outputaccording to knowledge of undesirable inputs & system responses 56Modes and functions of measuring system componentsOperation modes: Response modes:1. Passive output energy supplied by input 2. Active output energy supplied by external excitation source

Functions of measuring system components:1. Sensing used to produce output

2. Convection and conditioning used to transform output to a form, amplitude, or both more suitable for observation or further processing - analogue, discrete (digital, binary, etc.), or hybrid3. Transmission used to transfer signals or other information from one component to another 4. Processing and storage used display or store output7Static response and static calibrationStatic system - constant or slowly varying input and output Static calibration: 1. Zero drift parallel shift of primary calibration curve 2. Sensitivity drift a change in the slope of the primary calibration curve - determine input-output relationship (calibration curve) with standard system - performed separated for each desired input - accuracy depends on that of instruments used as standardEffects of undesirable inputs:

Static response 1. Theoretically determined by physical law e.g. liquid manometer for gas pressure difference measurement, Fig. (a)

Hydrostatic law: 2. More commonly determined by static calibration, e.g. variable-reluctance pressure transducer, Fig. (b) calibrated with liquid manometer:

8Static response and static calibrationStatic performance characteristics:Static sensitivity Scale readabilitySpan (input full-scale)Full-scale outputDynamic rangeNon-linearity Threshold Resolution Hysteresis - slope of input-output relationship - ratio of largest to smallest values of input - smallest input level for detectable output - difference between the output value corresponding to an input value reached from below and the output value corresponding to the same input value reached from above.

Elastic hysteresis of an bandInputOutput - constant in linear system linear - range of input to be measured with acceptable accuracyspan - range of output values measured from minimum to maximum input values full-scale output - local sensitivity varies over input range in non-linear system non-linear - minimum change in output can be observed - smallest input change for detectable output change - maximum deviation of actual response from straight line determined by least-square fit of calibration measurements 9Normality test and removal of outliersNormality test- to assess randomness of repeat measurement values (e.g. with probability graph paper) Rearrange a number of repeat values xi, i=1,2,,N , so that xi xi+1Compute percentage of repeat values that are not more than xi , i.e.Compute mean value and variance asNormalize the repeat values as

Plot yi vs. xi* on probability graph paper as rightAssess deviation of plottedpoints from the Gaussian line[passing through the points (0,50%) and (1, 84.1%)]10Normality tests and removal of outliersOutliersSpurious values due to(2) Temporary or intermittent undesirable input (1) human error, e.g. misreading of an instruments output Identified by application of Chauvenets criterionValue xi is a outlier if

Linear least-square fit (LLSF)

A set of calibration measurements:

LLSF line equation:11

Homework Questions and Problems: 1, 4 on page 41- Read textbook 2.1-2.2 on page 19-31 - Due on 08/31